Fri, 02 Jul 2010 17:23:43 -0400
6964164: MonitorInUseLists leak of contended objects
Summary: fix MonitorInUseLists memory leak and MonitorBound now works
Reviewed-by: chrisphi, dice
1 /*
2 * Copyright (c) 1998, 2009, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
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23 */
25 # include "incls/_precompiled.incl"
26 # include "incls/_synchronizer.cpp.incl"
28 #if defined(__GNUC__) && !defined(IA64)
29 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
30 #define ATTR __attribute__((noinline))
31 #else
32 #define ATTR
33 #endif
35 // Native markword accessors for synchronization and hashCode().
36 //
37 // The "core" versions of monitor enter and exit reside in this file.
38 // The interpreter and compilers contain specialized transliterated
39 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
40 // for instance. If you make changes here, make sure to modify the
41 // interpreter, and both C1 and C2 fast-path inline locking code emission.
42 //
43 // TODO: merge the objectMonitor and synchronizer classes.
44 //
45 // -----------------------------------------------------------------------------
47 #ifdef DTRACE_ENABLED
49 // Only bother with this argument setup if dtrace is available
50 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
52 HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,
53 jlong, uintptr_t, char*, int, long);
54 HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,
55 jlong, uintptr_t, char*, int);
56 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
57 jlong, uintptr_t, char*, int);
58 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
59 jlong, uintptr_t, char*, int);
60 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
61 jlong, uintptr_t, char*, int);
62 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
63 jlong, uintptr_t, char*, int);
64 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
65 jlong, uintptr_t, char*, int);
67 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \
68 char* bytes = NULL; \
69 int len = 0; \
70 jlong jtid = SharedRuntime::get_java_tid(thread); \
71 symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
72 if (klassname != NULL) { \
73 bytes = (char*)klassname->bytes(); \
74 len = klassname->utf8_length(); \
75 }
77 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
78 { \
79 if (DTraceMonitorProbes) { \
80 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
81 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
82 (monitor), bytes, len, (millis)); \
83 } \
84 }
86 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
87 { \
88 if (DTraceMonitorProbes) { \
89 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
90 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
91 (uintptr_t)(monitor), bytes, len); \
92 } \
93 }
95 #else // ndef DTRACE_ENABLED
97 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
98 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;}
100 #endif // ndef DTRACE_ENABLED
102 // ObjectWaiter serves as a "proxy" or surrogate thread.
103 // TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific
104 // ParkEvent instead. Beware, however, that the JVMTI code
105 // knows about ObjectWaiters, so we'll have to reconcile that code.
106 // See next_waiter(), first_waiter(), etc.
108 class ObjectWaiter : public StackObj {
109 public:
110 enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ } ;
111 enum Sorted { PREPEND, APPEND, SORTED } ;
112 ObjectWaiter * volatile _next;
113 ObjectWaiter * volatile _prev;
114 Thread* _thread;
115 ParkEvent * _event;
116 volatile int _notified ;
117 volatile TStates TState ;
118 Sorted _Sorted ; // List placement disposition
119 bool _active ; // Contention monitoring is enabled
120 public:
121 ObjectWaiter(Thread* thread) {
122 _next = NULL;
123 _prev = NULL;
124 _notified = 0;
125 TState = TS_RUN ;
126 _thread = thread;
127 _event = thread->_ParkEvent ;
128 _active = false;
129 assert (_event != NULL, "invariant") ;
130 }
132 void wait_reenter_begin(ObjectMonitor *mon) {
133 JavaThread *jt = (JavaThread *)this->_thread;
134 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
135 }
137 void wait_reenter_end(ObjectMonitor *mon) {
138 JavaThread *jt = (JavaThread *)this->_thread;
139 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
140 }
141 };
143 enum ManifestConstants {
144 ClearResponsibleAtSTW = 0,
145 MaximumRecheckInterval = 1000
146 } ;
149 #undef TEVENT
150 #define TEVENT(nom) {if (SyncVerbose) FEVENT(nom); }
152 #define FEVENT(nom) { static volatile int ctr = 0 ; int v = ++ctr ; if ((v & (v-1)) == 0) { ::printf (#nom " : %d \n", v); ::fflush(stdout); }}
154 #undef TEVENT
155 #define TEVENT(nom) {;}
157 // Performance concern:
158 // OrderAccess::storestore() calls release() which STs 0 into the global volatile
159 // OrderAccess::Dummy variable. This store is unnecessary for correctness.
160 // Many threads STing into a common location causes considerable cache migration
161 // or "sloshing" on large SMP system. As such, I avoid using OrderAccess::storestore()
162 // until it's repaired. In some cases OrderAccess::fence() -- which incurs local
163 // latency on the executing processor -- is a better choice as it scales on SMP
164 // systems. See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a
165 // discussion of coherency costs. Note that all our current reference platforms
166 // provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.
167 //
168 // As a general policy we use "volatile" to control compiler-based reordering
169 // and explicit fences (barriers) to control for architectural reordering performed
170 // by the CPU(s) or platform.
172 static int MBFence (int x) { OrderAccess::fence(); return x; }
174 struct SharedGlobals {
175 // These are highly shared mostly-read variables.
176 // To avoid false-sharing they need to be the sole occupants of a $ line.
177 double padPrefix [8];
178 volatile int stwRandom ;
179 volatile int stwCycle ;
181 // Hot RW variables -- Sequester to avoid false-sharing
182 double padSuffix [16];
183 volatile int hcSequence ;
184 double padFinal [8] ;
185 } ;
187 static SharedGlobals GVars ;
188 static int MonitorScavengeThreshold = 1000000 ;
189 static volatile int ForceMonitorScavenge = 0 ; // Scavenge required and pending
192 // Tunables ...
193 // The knob* variables are effectively final. Once set they should
194 // never be modified hence. Consider using __read_mostly with GCC.
196 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
197 static int Knob_HandOff = 0 ;
198 static int Knob_Verbose = 0 ;
199 static int Knob_ReportSettings = 0 ;
201 static int Knob_SpinLimit = 5000 ; // derived by an external tool -
202 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
203 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
204 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
205 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
206 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
207 static int Knob_SpinEarly = 1 ;
208 static int Knob_SuccEnabled = 1 ; // futile wake throttling
209 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
210 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
211 static int Knob_Bonus = 100 ; // spin success bonus
212 static int Knob_BonusB = 100 ; // spin success bonus
213 static int Knob_Penalty = 200 ; // spin failure penalty
214 static int Knob_Poverty = 1000 ;
215 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
216 static int Knob_FixedSpin = 0 ;
217 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
218 static int Knob_UsePause = 1 ;
219 static int Knob_ExitPolicy = 0 ;
220 static int Knob_PreSpin = 10 ; // 20-100 likely better
221 static int Knob_ResetEvent = 0 ;
222 static int BackOffMask = 0 ;
224 static int Knob_FastHSSEC = 0 ;
225 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
226 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
227 static volatile int InitDone = 0 ;
230 // hashCode() generation :
231 //
232 // Possibilities:
233 // * MD5Digest of {obj,stwRandom}
234 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
235 // * A DES- or AES-style SBox[] mechanism
236 // * One of the Phi-based schemes, such as:
237 // 2654435761 = 2^32 * Phi (golden ratio)
238 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
239 // * A variation of Marsaglia's shift-xor RNG scheme.
240 // * (obj ^ stwRandom) is appealing, but can result
241 // in undesirable regularity in the hashCode values of adjacent objects
242 // (objects allocated back-to-back, in particular). This could potentially
243 // result in hashtable collisions and reduced hashtable efficiency.
244 // There are simple ways to "diffuse" the middle address bits over the
245 // generated hashCode values:
246 //
248 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
249 intptr_t value = 0 ;
250 if (hashCode == 0) {
251 // This form uses an unguarded global Park-Miller RNG,
252 // so it's possible for two threads to race and generate the same RNG.
253 // On MP system we'll have lots of RW access to a global, so the
254 // mechanism induces lots of coherency traffic.
255 value = os::random() ;
256 } else
257 if (hashCode == 1) {
258 // This variation has the property of being stable (idempotent)
259 // between STW operations. This can be useful in some of the 1-0
260 // synchronization schemes.
261 intptr_t addrBits = intptr_t(obj) >> 3 ;
262 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
263 } else
264 if (hashCode == 2) {
265 value = 1 ; // for sensitivity testing
266 } else
267 if (hashCode == 3) {
268 value = ++GVars.hcSequence ;
269 } else
270 if (hashCode == 4) {
271 value = intptr_t(obj) ;
272 } else {
273 // Marsaglia's xor-shift scheme with thread-specific state
274 // This is probably the best overall implementation -- we'll
275 // likely make this the default in future releases.
276 unsigned t = Self->_hashStateX ;
277 t ^= (t << 11) ;
278 Self->_hashStateX = Self->_hashStateY ;
279 Self->_hashStateY = Self->_hashStateZ ;
280 Self->_hashStateZ = Self->_hashStateW ;
281 unsigned v = Self->_hashStateW ;
282 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
283 Self->_hashStateW = v ;
284 value = v ;
285 }
287 value &= markOopDesc::hash_mask;
288 if (value == 0) value = 0xBAD ;
289 assert (value != markOopDesc::no_hash, "invariant") ;
290 TEVENT (hashCode: GENERATE) ;
291 return value;
292 }
294 void BasicLock::print_on(outputStream* st) const {
295 st->print("monitor");
296 }
298 void BasicLock::move_to(oop obj, BasicLock* dest) {
299 // Check to see if we need to inflate the lock. This is only needed
300 // if an object is locked using "this" lightweight monitor. In that
301 // case, the displaced_header() is unlocked, because the
302 // displaced_header() contains the header for the originally unlocked
303 // object. However the object could have already been inflated. But it
304 // does not matter, the inflation will just a no-op. For other cases,
305 // the displaced header will be either 0x0 or 0x3, which are location
306 // independent, therefore the BasicLock is free to move.
307 //
308 // During OSR we may need to relocate a BasicLock (which contains a
309 // displaced word) from a location in an interpreter frame to a
310 // new location in a compiled frame. "this" refers to the source
311 // basiclock in the interpreter frame. "dest" refers to the destination
312 // basiclock in the new compiled frame. We *always* inflate in move_to().
313 // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
314 // cause performance problems in code that makes heavy use of a small # of
315 // uncontended locks. (We'd inflate during OSR, and then sync performance
316 // would subsequently plummet because the thread would be forced thru the slow-path).
317 // This problem has been made largely moot on IA32 by inlining the inflated fast-path
318 // operations in Fast_Lock and Fast_Unlock in i486.ad.
319 //
320 // Note that there is a way to safely swing the object's markword from
321 // one stack location to another. This avoids inflation. Obviously,
322 // we need to ensure that both locations refer to the current thread's stack.
323 // There are some subtle concurrency issues, however, and since the benefit is
324 // is small (given the support for inflated fast-path locking in the fast_lock, etc)
325 // we'll leave that optimization for another time.
327 if (displaced_header()->is_neutral()) {
328 ObjectSynchronizer::inflate_helper(obj);
329 // WARNING: We can not put check here, because the inflation
330 // will not update the displaced header. Once BasicLock is inflated,
331 // no one should ever look at its content.
332 } else {
333 // Typically the displaced header will be 0 (recursive stack lock) or
334 // unused_mark. Naively we'd like to assert that the displaced mark
335 // value is either 0, neutral, or 3. But with the advent of the
336 // store-before-CAS avoidance in fast_lock/compiler_lock_object
337 // we can find any flavor mark in the displaced mark.
338 }
339 // [RGV] The next line appears to do nothing!
340 intptr_t dh = (intptr_t) displaced_header();
341 dest->set_displaced_header(displaced_header());
342 }
344 // -----------------------------------------------------------------------------
346 // standard constructor, allows locking failures
347 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
348 _dolock = doLock;
349 _thread = thread;
350 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
351 _obj = obj;
353 if (_dolock) {
354 TEVENT (ObjectLocker) ;
356 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
357 }
358 }
360 ObjectLocker::~ObjectLocker() {
361 if (_dolock) {
362 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
363 }
364 }
366 // -----------------------------------------------------------------------------
369 PerfCounter * ObjectSynchronizer::_sync_Inflations = NULL ;
370 PerfCounter * ObjectSynchronizer::_sync_Deflations = NULL ;
371 PerfCounter * ObjectSynchronizer::_sync_ContendedLockAttempts = NULL ;
372 PerfCounter * ObjectSynchronizer::_sync_FutileWakeups = NULL ;
373 PerfCounter * ObjectSynchronizer::_sync_Parks = NULL ;
374 PerfCounter * ObjectSynchronizer::_sync_EmptyNotifications = NULL ;
375 PerfCounter * ObjectSynchronizer::_sync_Notifications = NULL ;
376 PerfCounter * ObjectSynchronizer::_sync_PrivateA = NULL ;
377 PerfCounter * ObjectSynchronizer::_sync_PrivateB = NULL ;
378 PerfCounter * ObjectSynchronizer::_sync_SlowExit = NULL ;
379 PerfCounter * ObjectSynchronizer::_sync_SlowEnter = NULL ;
380 PerfCounter * ObjectSynchronizer::_sync_SlowNotify = NULL ;
381 PerfCounter * ObjectSynchronizer::_sync_SlowNotifyAll = NULL ;
382 PerfCounter * ObjectSynchronizer::_sync_FailedSpins = NULL ;
383 PerfCounter * ObjectSynchronizer::_sync_SuccessfulSpins = NULL ;
384 PerfCounter * ObjectSynchronizer::_sync_MonInCirculation = NULL ;
385 PerfCounter * ObjectSynchronizer::_sync_MonScavenged = NULL ;
386 PerfLongVariable * ObjectSynchronizer::_sync_MonExtant = NULL ;
388 // One-shot global initialization for the sync subsystem.
389 // We could also defer initialization and initialize on-demand
390 // the first time we call inflate(). Initialization would
391 // be protected - like so many things - by the MonitorCache_lock.
393 void ObjectSynchronizer::Initialize () {
394 static int InitializationCompleted = 0 ;
395 assert (InitializationCompleted == 0, "invariant") ;
396 InitializationCompleted = 1 ;
397 if (UsePerfData) {
398 EXCEPTION_MARK ;
399 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
400 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
401 NEWPERFCOUNTER(_sync_Inflations) ;
402 NEWPERFCOUNTER(_sync_Deflations) ;
403 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
404 NEWPERFCOUNTER(_sync_FutileWakeups) ;
405 NEWPERFCOUNTER(_sync_Parks) ;
406 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
407 NEWPERFCOUNTER(_sync_Notifications) ;
408 NEWPERFCOUNTER(_sync_SlowEnter) ;
409 NEWPERFCOUNTER(_sync_SlowExit) ;
410 NEWPERFCOUNTER(_sync_SlowNotify) ;
411 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
412 NEWPERFCOUNTER(_sync_FailedSpins) ;
413 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
414 NEWPERFCOUNTER(_sync_PrivateA) ;
415 NEWPERFCOUNTER(_sync_PrivateB) ;
416 NEWPERFCOUNTER(_sync_MonInCirculation) ;
417 NEWPERFCOUNTER(_sync_MonScavenged) ;
418 NEWPERFVARIABLE(_sync_MonExtant) ;
419 #undef NEWPERFCOUNTER
420 }
421 }
423 // Compile-time asserts
424 // When possible, it's better to catch errors deterministically at
425 // compile-time than at runtime. The down-side to using compile-time
426 // asserts is that error message -- often something about negative array
427 // indices -- is opaque.
429 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
431 void ObjectMonitor::ctAsserts() {
432 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
433 }
435 static int Adjust (volatile int * adr, int dx) {
436 int v ;
437 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
438 return v ;
439 }
441 // Ad-hoc mutual exclusion primitives: SpinLock and Mux
442 //
443 // We employ SpinLocks _only for low-contention, fixed-length
444 // short-duration critical sections where we're concerned
445 // about native mutex_t or HotSpot Mutex:: latency.
446 // The mux construct provides a spin-then-block mutual exclusion
447 // mechanism.
448 //
449 // Testing has shown that contention on the ListLock guarding gFreeList
450 // is common. If we implement ListLock as a simple SpinLock it's common
451 // for the JVM to devolve to yielding with little progress. This is true
452 // despite the fact that the critical sections protected by ListLock are
453 // extremely short.
454 //
455 // TODO-FIXME: ListLock should be of type SpinLock.
456 // We should make this a 1st-class type, integrated into the lock
457 // hierarchy as leaf-locks. Critically, the SpinLock structure
458 // should have sufficient padding to avoid false-sharing and excessive
459 // cache-coherency traffic.
462 typedef volatile int SpinLockT ;
464 void Thread::SpinAcquire (volatile int * adr, const char * LockName) {
465 if (Atomic::cmpxchg (1, adr, 0) == 0) {
466 return ; // normal fast-path return
467 }
469 // Slow-path : We've encountered contention -- Spin/Yield/Block strategy.
470 TEVENT (SpinAcquire - ctx) ;
471 int ctr = 0 ;
472 int Yields = 0 ;
473 for (;;) {
474 while (*adr != 0) {
475 ++ctr ;
476 if ((ctr & 0xFFF) == 0 || !os::is_MP()) {
477 if (Yields > 5) {
478 // Consider using a simple NakedSleep() instead.
479 // Then SpinAcquire could be called by non-JVM threads
480 Thread::current()->_ParkEvent->park(1) ;
481 } else {
482 os::NakedYield() ;
483 ++Yields ;
484 }
485 } else {
486 SpinPause() ;
487 }
488 }
489 if (Atomic::cmpxchg (1, adr, 0) == 0) return ;
490 }
491 }
493 void Thread::SpinRelease (volatile int * adr) {
494 assert (*adr != 0, "invariant") ;
495 OrderAccess::fence() ; // guarantee at least release consistency.
496 // Roach-motel semantics.
497 // It's safe if subsequent LDs and STs float "up" into the critical section,
498 // but prior LDs and STs within the critical section can't be allowed
499 // to reorder or float past the ST that releases the lock.
500 *adr = 0 ;
501 }
503 // muxAcquire and muxRelease:
504 //
505 // * muxAcquire and muxRelease support a single-word lock-word construct.
506 // The LSB of the word is set IFF the lock is held.
507 // The remainder of the word points to the head of a singly-linked list
508 // of threads blocked on the lock.
509 //
510 // * The current implementation of muxAcquire-muxRelease uses its own
511 // dedicated Thread._MuxEvent instance. If we're interested in
512 // minimizing the peak number of extant ParkEvent instances then
513 // we could eliminate _MuxEvent and "borrow" _ParkEvent as long
514 // as certain invariants were satisfied. Specifically, care would need
515 // to be taken with regards to consuming unpark() "permits".
516 // A safe rule of thumb is that a thread would never call muxAcquire()
517 // if it's enqueued (cxq, EntryList, WaitList, etc) and will subsequently
518 // park(). Otherwise the _ParkEvent park() operation in muxAcquire() could
519 // consume an unpark() permit intended for monitorenter, for instance.
520 // One way around this would be to widen the restricted-range semaphore
521 // implemented in park(). Another alternative would be to provide
522 // multiple instances of the PlatformEvent() for each thread. One
523 // instance would be dedicated to muxAcquire-muxRelease, for instance.
524 //
525 // * Usage:
526 // -- Only as leaf locks
527 // -- for short-term locking only as muxAcquire does not perform
528 // thread state transitions.
529 //
530 // Alternatives:
531 // * We could implement muxAcquire and muxRelease with MCS or CLH locks
532 // but with parking or spin-then-park instead of pure spinning.
533 // * Use Taura-Oyama-Yonenzawa locks.
534 // * It's possible to construct a 1-0 lock if we encode the lockword as
535 // (List,LockByte). Acquire will CAS the full lockword while Release
536 // will STB 0 into the LockByte. The 1-0 scheme admits stranding, so
537 // acquiring threads use timers (ParkTimed) to detect and recover from
538 // the stranding window. Thread/Node structures must be aligned on 256-byte
539 // boundaries by using placement-new.
540 // * Augment MCS with advisory back-link fields maintained with CAS().
541 // Pictorially: LockWord -> T1 <-> T2 <-> T3 <-> ... <-> Tn <-> Owner.
542 // The validity of the backlinks must be ratified before we trust the value.
543 // If the backlinks are invalid the exiting thread must back-track through the
544 // the forward links, which are always trustworthy.
545 // * Add a successor indication. The LockWord is currently encoded as
546 // (List, LOCKBIT:1). We could also add a SUCCBIT or an explicit _succ variable
547 // to provide the usual futile-wakeup optimization.
548 // See RTStt for details.
549 // * Consider schedctl.sc_nopreempt to cover the critical section.
550 //
553 typedef volatile intptr_t MutexT ; // Mux Lock-word
554 enum MuxBits { LOCKBIT = 1 } ;
556 void Thread::muxAcquire (volatile intptr_t * Lock, const char * LockName) {
557 intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
558 if (w == 0) return ;
559 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
560 return ;
561 }
563 TEVENT (muxAcquire - Contention) ;
564 ParkEvent * const Self = Thread::current()->_MuxEvent ;
565 assert ((intptr_t(Self) & LOCKBIT) == 0, "invariant") ;
566 for (;;) {
567 int its = (os::is_MP() ? 100 : 0) + 1 ;
569 // Optional spin phase: spin-then-park strategy
570 while (--its >= 0) {
571 w = *Lock ;
572 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
573 return ;
574 }
575 }
577 Self->reset() ;
578 Self->OnList = intptr_t(Lock) ;
579 // The following fence() isn't _strictly necessary as the subsequent
580 // CAS() both serializes execution and ratifies the fetched *Lock value.
581 OrderAccess::fence();
582 for (;;) {
583 w = *Lock ;
584 if ((w & LOCKBIT) == 0) {
585 if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
586 Self->OnList = 0 ; // hygiene - allows stronger asserts
587 return ;
588 }
589 continue ; // Interference -- *Lock changed -- Just retry
590 }
591 assert (w & LOCKBIT, "invariant") ;
592 Self->ListNext = (ParkEvent *) (w & ~LOCKBIT );
593 if (Atomic::cmpxchg_ptr (intptr_t(Self)|LOCKBIT, Lock, w) == w) break ;
594 }
596 while (Self->OnList != 0) {
597 Self->park() ;
598 }
599 }
600 }
602 void Thread::muxAcquireW (volatile intptr_t * Lock, ParkEvent * ev) {
603 intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
604 if (w == 0) return ;
605 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
606 return ;
607 }
609 TEVENT (muxAcquire - Contention) ;
610 ParkEvent * ReleaseAfter = NULL ;
611 if (ev == NULL) {
612 ev = ReleaseAfter = ParkEvent::Allocate (NULL) ;
613 }
614 assert ((intptr_t(ev) & LOCKBIT) == 0, "invariant") ;
615 for (;;) {
616 guarantee (ev->OnList == 0, "invariant") ;
617 int its = (os::is_MP() ? 100 : 0) + 1 ;
619 // Optional spin phase: spin-then-park strategy
620 while (--its >= 0) {
621 w = *Lock ;
622 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
623 if (ReleaseAfter != NULL) {
624 ParkEvent::Release (ReleaseAfter) ;
625 }
626 return ;
627 }
628 }
630 ev->reset() ;
631 ev->OnList = intptr_t(Lock) ;
632 // The following fence() isn't _strictly necessary as the subsequent
633 // CAS() both serializes execution and ratifies the fetched *Lock value.
634 OrderAccess::fence();
635 for (;;) {
636 w = *Lock ;
637 if ((w & LOCKBIT) == 0) {
638 if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
639 ev->OnList = 0 ;
640 // We call ::Release while holding the outer lock, thus
641 // artificially lengthening the critical section.
642 // Consider deferring the ::Release() until the subsequent unlock(),
643 // after we've dropped the outer lock.
644 if (ReleaseAfter != NULL) {
645 ParkEvent::Release (ReleaseAfter) ;
646 }
647 return ;
648 }
649 continue ; // Interference -- *Lock changed -- Just retry
650 }
651 assert (w & LOCKBIT, "invariant") ;
652 ev->ListNext = (ParkEvent *) (w & ~LOCKBIT );
653 if (Atomic::cmpxchg_ptr (intptr_t(ev)|LOCKBIT, Lock, w) == w) break ;
654 }
656 while (ev->OnList != 0) {
657 ev->park() ;
658 }
659 }
660 }
662 // Release() must extract a successor from the list and then wake that thread.
663 // It can "pop" the front of the list or use a detach-modify-reattach (DMR) scheme
664 // similar to that used by ParkEvent::Allocate() and ::Release(). DMR-based
665 // Release() would :
666 // (A) CAS() or swap() null to *Lock, releasing the lock and detaching the list.
667 // (B) Extract a successor from the private list "in-hand"
668 // (C) attempt to CAS() the residual back into *Lock over null.
669 // If there were any newly arrived threads and the CAS() would fail.
670 // In that case Release() would detach the RATs, re-merge the list in-hand
671 // with the RATs and repeat as needed. Alternately, Release() might
672 // detach and extract a successor, but then pass the residual list to the wakee.
673 // The wakee would be responsible for reattaching and remerging before it
674 // competed for the lock.
675 //
676 // Both "pop" and DMR are immune from ABA corruption -- there can be
677 // multiple concurrent pushers, but only one popper or detacher.
678 // This implementation pops from the head of the list. This is unfair,
679 // but tends to provide excellent throughput as hot threads remain hot.
680 // (We wake recently run threads first).
682 void Thread::muxRelease (volatile intptr_t * Lock) {
683 for (;;) {
684 const intptr_t w = Atomic::cmpxchg_ptr (0, Lock, LOCKBIT) ;
685 assert (w & LOCKBIT, "invariant") ;
686 if (w == LOCKBIT) return ;
687 ParkEvent * List = (ParkEvent *) (w & ~LOCKBIT) ;
688 assert (List != NULL, "invariant") ;
689 assert (List->OnList == intptr_t(Lock), "invariant") ;
690 ParkEvent * nxt = List->ListNext ;
692 // The following CAS() releases the lock and pops the head element.
693 if (Atomic::cmpxchg_ptr (intptr_t(nxt), Lock, w) != w) {
694 continue ;
695 }
696 List->OnList = 0 ;
697 OrderAccess::fence() ;
698 List->unpark () ;
699 return ;
700 }
701 }
703 // ObjectMonitor Lifecycle
704 // -----------------------
705 // Inflation unlinks monitors from the global gFreeList and
706 // associates them with objects. Deflation -- which occurs at
707 // STW-time -- disassociates idle monitors from objects. Such
708 // scavenged monitors are returned to the gFreeList.
709 //
710 // The global list is protected by ListLock. All the critical sections
711 // are short and operate in constant-time.
712 //
713 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
714 //
715 // Lifecycle:
716 // -- unassigned and on the global free list
717 // -- unassigned and on a thread's private omFreeList
718 // -- assigned to an object. The object is inflated and the mark refers
719 // to the objectmonitor.
720 //
721 // TODO-FIXME:
722 //
723 // * We currently protect the gFreeList with a simple lock.
724 // An alternate lock-free scheme would be to pop elements from the gFreeList
725 // with CAS. This would be safe from ABA corruption as long we only
726 // recycled previously appearing elements onto the list in deflate_idle_monitors()
727 // at STW-time. Completely new elements could always be pushed onto the gFreeList
728 // with CAS. Elements that appeared previously on the list could only
729 // be installed at STW-time.
730 //
731 // * For efficiency and to help reduce the store-before-CAS penalty
732 // the objectmonitors on gFreeList or local free lists should be ready to install
733 // with the exception of _header and _object. _object can be set after inflation.
734 // In particular, keep all objectMonitors on a thread's private list in ready-to-install
735 // state with m.Owner set properly.
736 //
737 // * We could all diffuse contention by using multiple global (FreeList, Lock)
738 // pairs -- threads could use trylock() and a cyclic-scan strategy to search for
739 // an unlocked free list.
740 //
741 // * Add lifecycle tags and assert()s.
742 //
743 // * Be more consistent about when we clear an objectmonitor's fields:
744 // A. After extracting the objectmonitor from a free list.
745 // B. After adding an objectmonitor to a free list.
746 //
748 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
749 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ;
750 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL ;
751 int ObjectSynchronizer::gOmInUseCount = 0;
752 static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache
753 static volatile int MonitorFreeCount = 0 ; // # on gFreeList
754 static volatile int MonitorPopulation = 0 ; // # Extant -- in circulation
755 #define CHAINMARKER ((oop)-1)
757 // Constraining monitor pool growth via MonitorBound ...
758 //
759 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
760 // the rate of scavenging is driven primarily by GC. As such, we can find
761 // an inordinate number of monitors in circulation.
762 // To avoid that scenario we can artificially induce a STW safepoint
763 // if the pool appears to be growing past some reasonable bound.
764 // Generally we favor time in space-time tradeoffs, but as there's no
765 // natural back-pressure on the # of extant monitors we need to impose some
766 // type of limit. Beware that if MonitorBound is set to too low a value
767 // we could just loop. In addition, if MonitorBound is set to a low value
768 // we'll incur more safepoints, which are harmful to performance.
769 // See also: GuaranteedSafepointInterval
770 //
771 // As noted elsewhere, the correct long-term solution is to deflate at
772 // monitorexit-time, in which case the number of inflated objects is bounded
773 // by the number of threads. That policy obviates the need for scavenging at
774 // STW safepoint time. As an aside, scavenging can be time-consuming when the
775 // # of extant monitors is large. Unfortunately there's a day-1 assumption baked
776 // into much HotSpot code that the object::monitor relationship, once established
777 // or observed, will remain stable except over potential safepoints.
778 //
779 // We can use either a blocking synchronous VM operation or an async VM operation.
780 // -- If we use a blocking VM operation :
781 // Calls to ScavengeCheck() should be inserted only into 'safe' locations in paths
782 // that lead to ::inflate() or ::omAlloc().
783 // Even though the safepoint will not directly induce GC, a GC might
784 // piggyback on the safepoint operation, so the caller should hold no naked oops.
785 // Furthermore, monitor::object relationships are NOT necessarily stable over this call
786 // unless the caller has made provisions to "pin" the object to the monitor, say
787 // by incrementing the monitor's _count field.
788 // -- If we use a non-blocking asynchronous VM operation :
789 // the constraints above don't apply. The safepoint will fire in the future
790 // at a more convenient time. On the other hand the latency between posting and
791 // running the safepoint introduces or admits "slop" or laxity during which the
792 // monitor population can climb further above the threshold. The monitor population,
793 // however, tends to converge asymptotically over time to a count that's slightly
794 // above the target value specified by MonitorBound. That is, we avoid unbounded
795 // growth, albeit with some imprecision.
796 //
797 // The current implementation uses asynchronous VM operations.
798 //
799 // Ideally we'd check if (MonitorPopulation > MonitorBound) in omAlloc()
800 // immediately before trying to grow the global list via allocation.
801 // If the predicate was true then we'd induce a synchronous safepoint, wait
802 // for the safepoint to complete, and then again to allocate from the global
803 // free list. This approach is much simpler and precise, admitting no "slop".
804 // Unfortunately we can't safely safepoint in the midst of omAlloc(), so
805 // instead we use asynchronous safepoints.
807 static void InduceScavenge (Thread * Self, const char * Whence) {
808 // Induce STW safepoint to trim monitors
809 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
810 // More precisely, trigger an asynchronous STW safepoint as the number
811 // of active monitors passes the specified threshold.
812 // TODO: assert thread state is reasonable
814 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
815 if (Knob_Verbose) {
816 ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ;
817 ::fflush(stdout) ;
818 }
819 // Induce a 'null' safepoint to scavenge monitors
820 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
821 // to the VMthread and have a lifespan longer than that of this activation record.
822 // The VMThread will delete the op when completed.
823 VMThread::execute (new VM_ForceAsyncSafepoint()) ;
825 if (Knob_Verbose) {
826 ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ;
827 ::fflush(stdout) ;
828 }
829 }
830 }
831 /* Too slow for general assert or debug
832 void ObjectSynchronizer::verifyInUse (Thread *Self) {
833 ObjectMonitor* mid;
834 int inusetally = 0;
835 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
836 inusetally ++;
837 }
838 assert(inusetally == Self->omInUseCount, "inuse count off");
840 int freetally = 0;
841 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
842 freetally ++;
843 }
844 assert(freetally == Self->omFreeCount, "free count off");
845 }
846 */
848 ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
849 // A large MAXPRIVATE value reduces both list lock contention
850 // and list coherency traffic, but also tends to increase the
851 // number of objectMonitors in circulation as well as the STW
852 // scavenge costs. As usual, we lean toward time in space-time
853 // tradeoffs.
854 const int MAXPRIVATE = 1024 ;
855 for (;;) {
856 ObjectMonitor * m ;
858 // 1: try to allocate from the thread's local omFreeList.
859 // Threads will attempt to allocate first from their local list, then
860 // from the global list, and only after those attempts fail will the thread
861 // attempt to instantiate new monitors. Thread-local free lists take
862 // heat off the ListLock and improve allocation latency, as well as reducing
863 // coherency traffic on the shared global list.
864 m = Self->omFreeList ;
865 if (m != NULL) {
866 Self->omFreeList = m->FreeNext ;
867 Self->omFreeCount -- ;
868 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
869 guarantee (m->object() == NULL, "invariant") ;
870 if (MonitorInUseLists) {
871 m->FreeNext = Self->omInUseList;
872 Self->omInUseList = m;
873 Self->omInUseCount ++;
874 // verifyInUse(Self);
875 } else {
876 m->FreeNext = NULL;
877 }
878 return m ;
879 }
881 // 2: try to allocate from the global gFreeList
882 // CONSIDER: use muxTry() instead of muxAcquire().
883 // If the muxTry() fails then drop immediately into case 3.
884 // If we're using thread-local free lists then try
885 // to reprovision the caller's free list.
886 if (gFreeList != NULL) {
887 // Reprovision the thread's omFreeList.
888 // Use bulk transfers to reduce the allocation rate and heat
889 // on various locks.
890 Thread::muxAcquire (&ListLock, "omAlloc") ;
891 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
892 MonitorFreeCount --;
893 ObjectMonitor * take = gFreeList ;
894 gFreeList = take->FreeNext ;
895 guarantee (take->object() == NULL, "invariant") ;
896 guarantee (!take->is_busy(), "invariant") ;
897 take->Recycle() ;
898 omRelease (Self, take, false) ;
899 }
900 Thread::muxRelease (&ListLock) ;
901 Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;
902 if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
903 TEVENT (omFirst - reprovision) ;
905 const int mx = MonitorBound ;
906 if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > mx) {
907 // We can't safely induce a STW safepoint from omAlloc() as our thread
908 // state may not be appropriate for such activities and callers may hold
909 // naked oops, so instead we defer the action.
910 InduceScavenge (Self, "omAlloc") ;
911 }
912 continue;
913 }
915 // 3: allocate a block of new ObjectMonitors
916 // Both the local and global free lists are empty -- resort to malloc().
917 // In the current implementation objectMonitors are TSM - immortal.
918 assert (_BLOCKSIZE > 1, "invariant") ;
919 ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];
921 // NOTE: (almost) no way to recover if allocation failed.
922 // We might be able to induce a STW safepoint and scavenge enough
923 // objectMonitors to permit progress.
924 if (temp == NULL) {
925 vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ;
926 }
928 // Format the block.
929 // initialize the linked list, each monitor points to its next
930 // forming the single linked free list, the very first monitor
931 // will points to next block, which forms the block list.
932 // The trick of using the 1st element in the block as gBlockList
933 // linkage should be reconsidered. A better implementation would
934 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
936 for (int i = 1; i < _BLOCKSIZE ; i++) {
937 temp[i].FreeNext = &temp[i+1];
938 }
940 // terminate the last monitor as the end of list
941 temp[_BLOCKSIZE - 1].FreeNext = NULL ;
943 // Element [0] is reserved for global list linkage
944 temp[0].set_object(CHAINMARKER);
946 // Consider carving out this thread's current request from the
947 // block in hand. This avoids some lock traffic and redundant
948 // list activity.
950 // Acquire the ListLock to manipulate BlockList and FreeList.
951 // An Oyama-Taura-Yonezawa scheme might be more efficient.
952 Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
953 MonitorPopulation += _BLOCKSIZE-1;
954 MonitorFreeCount += _BLOCKSIZE-1;
956 // Add the new block to the list of extant blocks (gBlockList).
957 // The very first objectMonitor in a block is reserved and dedicated.
958 // It serves as blocklist "next" linkage.
959 temp[0].FreeNext = gBlockList;
960 gBlockList = temp;
962 // Add the new string of objectMonitors to the global free list
963 temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;
964 gFreeList = temp + 1;
965 Thread::muxRelease (&ListLock) ;
966 TEVENT (Allocate block of monitors) ;
967 }
968 }
970 // Place "m" on the caller's private per-thread omFreeList.
971 // In practice there's no need to clamp or limit the number of
972 // monitors on a thread's omFreeList as the only time we'll call
973 // omRelease is to return a monitor to the free list after a CAS
974 // attempt failed. This doesn't allow unbounded #s of monitors to
975 // accumulate on a thread's free list.
976 //
977 // In the future the usage of omRelease() might change and monitors
978 // could migrate between free lists. In that case to avoid excessive
979 // accumulation we could limit omCount to (omProvision*2), otherwise return
980 // the objectMonitor to the global list. We should drain (return) in reasonable chunks.
981 // That is, *not* one-at-a-time.
984 void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m, bool fromPerThreadAlloc) {
985 guarantee (m->object() == NULL, "invariant") ;
987 // Remove from omInUseList
988 if (MonitorInUseLists && fromPerThreadAlloc) {
989 ObjectMonitor* curmidinuse = NULL;
990 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; ) {
991 if (m == mid) {
992 // extract from per-thread in-use-list
993 if (mid == Self->omInUseList) {
994 Self->omInUseList = mid->FreeNext;
995 } else if (curmidinuse != NULL) {
996 curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
997 }
998 Self->omInUseCount --;
999 // verifyInUse(Self);
1000 break;
1001 } else {
1002 curmidinuse = mid;
1003 mid = mid->FreeNext;
1004 }
1005 }
1006 }
1008 // FreeNext is used for both onInUseList and omFreeList, so clear old before setting new
1009 m->FreeNext = Self->omFreeList ;
1010 Self->omFreeList = m ;
1011 Self->omFreeCount ++ ;
1012 }
1014 // Return the monitors of a moribund thread's local free list to
1015 // the global free list. Typically a thread calls omFlush() when
1016 // it's dying. We could also consider having the VM thread steal
1017 // monitors from threads that have not run java code over a few
1018 // consecutive STW safepoints. Relatedly, we might decay
1019 // omFreeProvision at STW safepoints.
1020 //
1021 // Also return the monitors of a moribund thread"s omInUseList to
1022 // a global gOmInUseList under the global list lock so these
1023 // will continue to be scanned.
1024 //
1025 // We currently call omFlush() from the Thread:: dtor _after the thread
1026 // has been excised from the thread list and is no longer a mutator.
1027 // That means that omFlush() can run concurrently with a safepoint and
1028 // the scavenge operator. Calling omFlush() from JavaThread::exit() might
1029 // be a better choice as we could safely reason that that the JVM is
1030 // not at a safepoint at the time of the call, and thus there could
1031 // be not inopportune interleavings between omFlush() and the scavenge
1032 // operator.
1034 void ObjectSynchronizer::omFlush (Thread * Self) {
1035 ObjectMonitor * List = Self->omFreeList ; // Null-terminated SLL
1036 Self->omFreeList = NULL ;
1037 ObjectMonitor * Tail = NULL ;
1038 int Tally = 0;
1039 if (List != NULL) {
1040 ObjectMonitor * s ;
1041 for (s = List ; s != NULL ; s = s->FreeNext) {
1042 Tally ++ ;
1043 Tail = s ;
1044 guarantee (s->object() == NULL, "invariant") ;
1045 guarantee (!s->is_busy(), "invariant") ;
1046 s->set_owner (NULL) ; // redundant but good hygiene
1047 TEVENT (omFlush - Move one) ;
1048 }
1049 guarantee (Tail != NULL && List != NULL, "invariant") ;
1050 }
1052 ObjectMonitor * InUseList = Self->omInUseList;
1053 ObjectMonitor * InUseTail = NULL ;
1054 int InUseTally = 0;
1055 if (InUseList != NULL) {
1056 Self->omInUseList = NULL;
1057 ObjectMonitor *curom;
1058 for (curom = InUseList; curom != NULL; curom = curom->FreeNext) {
1059 InUseTail = curom;
1060 InUseTally++;
1061 }
1062 // TODO debug
1063 assert(Self->omInUseCount == InUseTally, "inuse count off");
1064 Self->omInUseCount = 0;
1065 guarantee (InUseTail != NULL && InUseList != NULL, "invariant");
1066 }
1068 Thread::muxAcquire (&ListLock, "omFlush") ;
1069 if (Tail != NULL) {
1070 Tail->FreeNext = gFreeList ;
1071 gFreeList = List ;
1072 MonitorFreeCount += Tally;
1073 }
1075 if (InUseTail != NULL) {
1076 InUseTail->FreeNext = gOmInUseList;
1077 gOmInUseList = InUseList;
1078 gOmInUseCount += InUseTally;
1079 }
1081 Thread::muxRelease (&ListLock) ;
1082 TEVENT (omFlush) ;
1083 }
1086 // Get the next block in the block list.
1087 static inline ObjectMonitor* next(ObjectMonitor* block) {
1088 assert(block->object() == CHAINMARKER, "must be a block header");
1089 block = block->FreeNext ;
1090 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
1091 return block;
1092 }
1094 // Fast path code shared by multiple functions
1095 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1096 markOop mark = obj->mark();
1097 if (mark->has_monitor()) {
1098 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1099 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1100 return mark->monitor();
1101 }
1102 return ObjectSynchronizer::inflate(Thread::current(), obj);
1103 }
1105 // Note that we could encounter some performance loss through false-sharing as
1106 // multiple locks occupy the same $ line. Padding might be appropriate.
1108 #define NINFLATIONLOCKS 256
1109 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;
1111 static markOop ReadStableMark (oop obj) {
1112 markOop mark = obj->mark() ;
1113 if (!mark->is_being_inflated()) {
1114 return mark ; // normal fast-path return
1115 }
1117 int its = 0 ;
1118 for (;;) {
1119 markOop mark = obj->mark() ;
1120 if (!mark->is_being_inflated()) {
1121 return mark ; // normal fast-path return
1122 }
1124 // The object is being inflated by some other thread.
1125 // The caller of ReadStableMark() must wait for inflation to complete.
1126 // Avoid live-lock
1127 // TODO: consider calling SafepointSynchronize::do_call_back() while
1128 // spinning to see if there's a safepoint pending. If so, immediately
1129 // yielding or blocking would be appropriate. Avoid spinning while
1130 // there is a safepoint pending.
1131 // TODO: add inflation contention performance counters.
1132 // TODO: restrict the aggregate number of spinners.
1134 ++its ;
1135 if (its > 10000 || !os::is_MP()) {
1136 if (its & 1) {
1137 os::NakedYield() ;
1138 TEVENT (Inflate: INFLATING - yield) ;
1139 } else {
1140 // Note that the following code attenuates the livelock problem but is not
1141 // a complete remedy. A more complete solution would require that the inflating
1142 // thread hold the associated inflation lock. The following code simply restricts
1143 // the number of spinners to at most one. We'll have N-2 threads blocked
1144 // on the inflationlock, 1 thread holding the inflation lock and using
1145 // a yield/park strategy, and 1 thread in the midst of inflation.
1146 // A more refined approach would be to change the encoding of INFLATING
1147 // to allow encapsulation of a native thread pointer. Threads waiting for
1148 // inflation to complete would use CAS to push themselves onto a singly linked
1149 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
1150 // and calling park(). When inflation was complete the thread that accomplished inflation
1151 // would detach the list and set the markword to inflated with a single CAS and
1152 // then for each thread on the list, set the flag and unpark() the thread.
1153 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
1154 // wakes at most one thread whereas we need to wake the entire list.
1155 int ix = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;
1156 int YieldThenBlock = 0 ;
1157 assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
1158 assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
1159 Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
1160 while (obj->mark() == markOopDesc::INFLATING()) {
1161 // Beware: NakedYield() is advisory and has almost no effect on some platforms
1162 // so we periodically call Self->_ParkEvent->park(1).
1163 // We use a mixed spin/yield/block mechanism.
1164 if ((YieldThenBlock++) >= 16) {
1165 Thread::current()->_ParkEvent->park(1) ;
1166 } else {
1167 os::NakedYield() ;
1168 }
1169 }
1170 Thread::muxRelease (InflationLocks + ix ) ;
1171 TEVENT (Inflate: INFLATING - yield/park) ;
1172 }
1173 } else {
1174 SpinPause() ; // SMP-polite spinning
1175 }
1176 }
1177 }
1179 ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
1180 // Inflate mutates the heap ...
1181 // Relaxing assertion for bug 6320749.
1182 assert (Universe::verify_in_progress() ||
1183 !SafepointSynchronize::is_at_safepoint(), "invariant") ;
1185 for (;;) {
1186 const markOop mark = object->mark() ;
1187 assert (!mark->has_bias_pattern(), "invariant") ;
1189 // The mark can be in one of the following states:
1190 // * Inflated - just return
1191 // * Stack-locked - coerce it to inflated
1192 // * INFLATING - busy wait for conversion to complete
1193 // * Neutral - aggressively inflate the object.
1194 // * BIASED - Illegal. We should never see this
1196 // CASE: inflated
1197 if (mark->has_monitor()) {
1198 ObjectMonitor * inf = mark->monitor() ;
1199 assert (inf->header()->is_neutral(), "invariant");
1200 assert (inf->object() == object, "invariant") ;
1201 assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1202 return inf ;
1203 }
1205 // CASE: inflation in progress - inflating over a stack-lock.
1206 // Some other thread is converting from stack-locked to inflated.
1207 // Only that thread can complete inflation -- other threads must wait.
1208 // The INFLATING value is transient.
1209 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1210 // We could always eliminate polling by parking the thread on some auxiliary list.
1211 if (mark == markOopDesc::INFLATING()) {
1212 TEVENT (Inflate: spin while INFLATING) ;
1213 ReadStableMark(object) ;
1214 continue ;
1215 }
1217 // CASE: stack-locked
1218 // Could be stack-locked either by this thread or by some other thread.
1219 //
1220 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1221 // to install INFLATING into the mark word. We originally installed INFLATING,
1222 // allocated the objectmonitor, and then finally STed the address of the
1223 // objectmonitor into the mark. This was correct, but artificially lengthened
1224 // the interval in which INFLATED appeared in the mark, thus increasing
1225 // the odds of inflation contention.
1226 //
1227 // We now use per-thread private objectmonitor free lists.
1228 // These list are reprovisioned from the global free list outside the
1229 // critical INFLATING...ST interval. A thread can transfer
1230 // multiple objectmonitors en-mass from the global free list to its local free list.
1231 // This reduces coherency traffic and lock contention on the global free list.
1232 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1233 // before or after the CAS(INFLATING) operation.
1234 // See the comments in omAlloc().
1236 if (mark->has_locker()) {
1237 ObjectMonitor * m = omAlloc (Self) ;
1238 // Optimistically prepare the objectmonitor - anticipate successful CAS
1239 // We do this before the CAS in order to minimize the length of time
1240 // in which INFLATING appears in the mark.
1241 m->Recycle();
1242 m->_Responsible = NULL ;
1243 m->OwnerIsThread = 0 ;
1244 m->_recursions = 0 ;
1245 m->_SpinDuration = Knob_SpinLimit ; // Consider: maintain by type/class
1247 markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
1248 if (cmp != mark) {
1249 omRelease (Self, m, true) ;
1250 continue ; // Interference -- just retry
1251 }
1253 // We've successfully installed INFLATING (0) into the mark-word.
1254 // This is the only case where 0 will appear in a mark-work.
1255 // Only the singular thread that successfully swings the mark-word
1256 // to 0 can perform (or more precisely, complete) inflation.
1257 //
1258 // Why do we CAS a 0 into the mark-word instead of just CASing the
1259 // mark-word from the stack-locked value directly to the new inflated state?
1260 // Consider what happens when a thread unlocks a stack-locked object.
1261 // It attempts to use CAS to swing the displaced header value from the
1262 // on-stack basiclock back into the object header. Recall also that the
1263 // header value (hashcode, etc) can reside in (a) the object header, or
1264 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1265 // header in an objectMonitor. The inflate() routine must copy the header
1266 // value from the basiclock on the owner's stack to the objectMonitor, all
1267 // the while preserving the hashCode stability invariants. If the owner
1268 // decides to release the lock while the value is 0, the unlock will fail
1269 // and control will eventually pass from slow_exit() to inflate. The owner
1270 // will then spin, waiting for the 0 value to disappear. Put another way,
1271 // the 0 causes the owner to stall if the owner happens to try to
1272 // drop the lock (restoring the header from the basiclock to the object)
1273 // while inflation is in-progress. This protocol avoids races that might
1274 // would otherwise permit hashCode values to change or "flicker" for an object.
1275 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1276 // 0 serves as a "BUSY" inflate-in-progress indicator.
1279 // fetch the displaced mark from the owner's stack.
1280 // The owner can't die or unwind past the lock while our INFLATING
1281 // object is in the mark. Furthermore the owner can't complete
1282 // an unlock on the object, either.
1283 markOop dmw = mark->displaced_mark_helper() ;
1284 assert (dmw->is_neutral(), "invariant") ;
1286 // Setup monitor fields to proper values -- prepare the monitor
1287 m->set_header(dmw) ;
1289 // Optimization: if the mark->locker stack address is associated
1290 // with this thread we could simply set m->_owner = Self and
1291 // m->OwnerIsThread = 1. Note that a thread can inflate an object
1292 // that it has stack-locked -- as might happen in wait() -- directly
1293 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1294 m->set_owner(mark->locker());
1295 m->set_object(object);
1296 // TODO-FIXME: assert BasicLock->dhw != 0.
1298 // Must preserve store ordering. The monitor state must
1299 // be stable at the time of publishing the monitor address.
1300 guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
1301 object->release_set_mark(markOopDesc::encode(m));
1303 // Hopefully the performance counters are allocated on distinct cache lines
1304 // to avoid false sharing on MP systems ...
1305 if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
1306 TEVENT(Inflate: overwrite stacklock) ;
1307 if (TraceMonitorInflation) {
1308 if (object->is_instance()) {
1309 ResourceMark rm;
1310 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1311 (intptr_t) object, (intptr_t) object->mark(),
1312 Klass::cast(object->klass())->external_name());
1313 }
1314 }
1315 return m ;
1316 }
1318 // CASE: neutral
1319 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1320 // If we know we're inflating for entry it's better to inflate by swinging a
1321 // pre-locked objectMonitor pointer into the object header. A successful
1322 // CAS inflates the object *and* confers ownership to the inflating thread.
1323 // In the current implementation we use a 2-step mechanism where we CAS()
1324 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1325 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1326 // would be useful.
1328 assert (mark->is_neutral(), "invariant");
1329 ObjectMonitor * m = omAlloc (Self) ;
1330 // prepare m for installation - set monitor to initial state
1331 m->Recycle();
1332 m->set_header(mark);
1333 m->set_owner(NULL);
1334 m->set_object(object);
1335 m->OwnerIsThread = 1 ;
1336 m->_recursions = 0 ;
1337 m->_Responsible = NULL ;
1338 m->_SpinDuration = Knob_SpinLimit ; // consider: keep metastats by type/class
1340 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1341 m->set_object (NULL) ;
1342 m->set_owner (NULL) ;
1343 m->OwnerIsThread = 0 ;
1344 m->Recycle() ;
1345 omRelease (Self, m, true) ;
1346 m = NULL ;
1347 continue ;
1348 // interference - the markword changed - just retry.
1349 // The state-transitions are one-way, so there's no chance of
1350 // live-lock -- "Inflated" is an absorbing state.
1351 }
1353 // Hopefully the performance counters are allocated on distinct
1354 // cache lines to avoid false sharing on MP systems ...
1355 if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
1356 TEVENT(Inflate: overwrite neutral) ;
1357 if (TraceMonitorInflation) {
1358 if (object->is_instance()) {
1359 ResourceMark rm;
1360 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1361 (intptr_t) object, (intptr_t) object->mark(),
1362 Klass::cast(object->klass())->external_name());
1363 }
1364 }
1365 return m ;
1366 }
1367 }
1370 // This the fast monitor enter. The interpreter and compiler use
1371 // some assembly copies of this code. Make sure update those code
1372 // if the following function is changed. The implementation is
1373 // extremely sensitive to race condition. Be careful.
1375 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
1376 if (UseBiasedLocking) {
1377 if (!SafepointSynchronize::is_at_safepoint()) {
1378 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
1379 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
1380 return;
1381 }
1382 } else {
1383 assert(!attempt_rebias, "can not rebias toward VM thread");
1384 BiasedLocking::revoke_at_safepoint(obj);
1385 }
1386 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1387 }
1389 slow_enter (obj, lock, THREAD) ;
1390 }
1392 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
1393 assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
1394 // if displaced header is null, the previous enter is recursive enter, no-op
1395 markOop dhw = lock->displaced_header();
1396 markOop mark ;
1397 if (dhw == NULL) {
1398 // Recursive stack-lock.
1399 // Diagnostics -- Could be: stack-locked, inflating, inflated.
1400 mark = object->mark() ;
1401 assert (!mark->is_neutral(), "invariant") ;
1402 if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
1403 assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
1404 }
1405 if (mark->has_monitor()) {
1406 ObjectMonitor * m = mark->monitor() ;
1407 assert(((oop)(m->object()))->mark() == mark, "invariant") ;
1408 assert(m->is_entered(THREAD), "invariant") ;
1409 }
1410 return ;
1411 }
1413 mark = object->mark() ;
1415 // If the object is stack-locked by the current thread, try to
1416 // swing the displaced header from the box back to the mark.
1417 if (mark == (markOop) lock) {
1418 assert (dhw->is_neutral(), "invariant") ;
1419 if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
1420 TEVENT (fast_exit: release stacklock) ;
1421 return;
1422 }
1423 }
1425 ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ;
1426 }
1428 // This routine is used to handle interpreter/compiler slow case
1429 // We don't need to use fast path here, because it must have been
1430 // failed in the interpreter/compiler code.
1431 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
1432 markOop mark = obj->mark();
1433 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
1435 if (mark->is_neutral()) {
1436 // Anticipate successful CAS -- the ST of the displaced mark must
1437 // be visible <= the ST performed by the CAS.
1438 lock->set_displaced_header(mark);
1439 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
1440 TEVENT (slow_enter: release stacklock) ;
1441 return ;
1442 }
1443 // Fall through to inflate() ...
1444 } else
1445 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1446 assert(lock != mark->locker(), "must not re-lock the same lock");
1447 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
1448 lock->set_displaced_header(NULL);
1449 return;
1450 }
1452 #if 0
1453 // The following optimization isn't particularly useful.
1454 if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
1455 lock->set_displaced_header (NULL) ;
1456 return ;
1457 }
1458 #endif
1460 // The object header will never be displaced to this lock,
1461 // so it does not matter what the value is, except that it
1462 // must be non-zero to avoid looking like a re-entrant lock,
1463 // and must not look locked either.
1464 lock->set_displaced_header(markOopDesc::unused_mark());
1465 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
1466 }
1468 // This routine is used to handle interpreter/compiler slow case
1469 // We don't need to use fast path here, because it must have
1470 // failed in the interpreter/compiler code. Simply use the heavy
1471 // weight monitor should be ok, unless someone find otherwise.
1472 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
1473 fast_exit (object, lock, THREAD) ;
1474 }
1476 // NOTE: must use heavy weight monitor to handle jni monitor enter
1477 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter
1478 // the current locking is from JNI instead of Java code
1479 TEVENT (jni_enter) ;
1480 if (UseBiasedLocking) {
1481 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1482 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1483 }
1484 THREAD->set_current_pending_monitor_is_from_java(false);
1485 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
1486 THREAD->set_current_pending_monitor_is_from_java(true);
1487 }
1489 // NOTE: must use heavy weight monitor to handle jni monitor enter
1490 bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {
1491 if (UseBiasedLocking) {
1492 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1493 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1494 }
1496 ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj());
1497 return monitor->try_enter(THREAD);
1498 }
1501 // NOTE: must use heavy weight monitor to handle jni monitor exit
1502 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
1503 TEVENT (jni_exit) ;
1504 if (UseBiasedLocking) {
1505 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1506 }
1507 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1509 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
1510 // If this thread has locked the object, exit the monitor. Note: can't use
1511 // monitor->check(CHECK); must exit even if an exception is pending.
1512 if (monitor->check(THREAD)) {
1513 monitor->exit(THREAD);
1514 }
1515 }
1517 // complete_exit()/reenter() are used to wait on a nested lock
1518 // i.e. to give up an outer lock completely and then re-enter
1519 // Used when holding nested locks - lock acquisition order: lock1 then lock2
1520 // 1) complete_exit lock1 - saving recursion count
1521 // 2) wait on lock2
1522 // 3) when notified on lock2, unlock lock2
1523 // 4) reenter lock1 with original recursion count
1524 // 5) lock lock2
1525 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
1526 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
1527 TEVENT (complete_exit) ;
1528 if (UseBiasedLocking) {
1529 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1530 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1531 }
1533 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1535 return monitor->complete_exit(THREAD);
1536 }
1538 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
1539 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
1540 TEVENT (reenter) ;
1541 if (UseBiasedLocking) {
1542 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1543 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1544 }
1546 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1548 monitor->reenter(recursion, THREAD);
1549 }
1551 // This exists only as a workaround of dtrace bug 6254741
1552 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
1553 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
1554 return 0;
1555 }
1557 // NOTE: must use heavy weight monitor to handle wait()
1558 void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
1559 if (UseBiasedLocking) {
1560 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1561 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1562 }
1563 if (millis < 0) {
1564 TEVENT (wait - throw IAX) ;
1565 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
1566 }
1567 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1568 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
1569 monitor->wait(millis, true, THREAD);
1571 /* This dummy call is in place to get around dtrace bug 6254741. Once
1572 that's fixed we can uncomment the following line and remove the call */
1573 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
1574 dtrace_waited_probe(monitor, obj, THREAD);
1575 }
1577 void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {
1578 if (UseBiasedLocking) {
1579 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1580 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1581 }
1582 if (millis < 0) {
1583 TEVENT (wait - throw IAX) ;
1584 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
1585 }
1586 ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;
1587 }
1589 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
1590 if (UseBiasedLocking) {
1591 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1592 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1593 }
1595 markOop mark = obj->mark();
1596 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1597 return;
1598 }
1599 ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
1600 }
1602 // NOTE: see comment of notify()
1603 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
1604 if (UseBiasedLocking) {
1605 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1606 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1607 }
1609 markOop mark = obj->mark();
1610 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1611 return;
1612 }
1613 ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
1614 }
1616 intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
1617 if (UseBiasedLocking) {
1618 // NOTE: many places throughout the JVM do not expect a safepoint
1619 // to be taken here, in particular most operations on perm gen
1620 // objects. However, we only ever bias Java instances and all of
1621 // the call sites of identity_hash that might revoke biases have
1622 // been checked to make sure they can handle a safepoint. The
1623 // added check of the bias pattern is to avoid useless calls to
1624 // thread-local storage.
1625 if (obj->mark()->has_bias_pattern()) {
1626 // Box and unbox the raw reference just in case we cause a STW safepoint.
1627 Handle hobj (Self, obj) ;
1628 // Relaxing assertion for bug 6320749.
1629 assert (Universe::verify_in_progress() ||
1630 !SafepointSynchronize::is_at_safepoint(),
1631 "biases should not be seen by VM thread here");
1632 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
1633 obj = hobj() ;
1634 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1635 }
1636 }
1638 // hashCode() is a heap mutator ...
1639 // Relaxing assertion for bug 6320749.
1640 assert (Universe::verify_in_progress() ||
1641 !SafepointSynchronize::is_at_safepoint(), "invariant") ;
1642 assert (Universe::verify_in_progress() ||
1643 Self->is_Java_thread() , "invariant") ;
1644 assert (Universe::verify_in_progress() ||
1645 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
1647 ObjectMonitor* monitor = NULL;
1648 markOop temp, test;
1649 intptr_t hash;
1650 markOop mark = ReadStableMark (obj);
1652 // object should remain ineligible for biased locking
1653 assert (!mark->has_bias_pattern(), "invariant") ;
1655 if (mark->is_neutral()) {
1656 hash = mark->hash(); // this is a normal header
1657 if (hash) { // if it has hash, just return it
1658 return hash;
1659 }
1660 hash = get_next_hash(Self, obj); // allocate a new hash code
1661 temp = mark->copy_set_hash(hash); // merge the hash code into header
1662 // use (machine word version) atomic operation to install the hash
1663 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
1664 if (test == mark) {
1665 return hash;
1666 }
1667 // If atomic operation failed, we must inflate the header
1668 // into heavy weight monitor. We could add more code here
1669 // for fast path, but it does not worth the complexity.
1670 } else if (mark->has_monitor()) {
1671 monitor = mark->monitor();
1672 temp = monitor->header();
1673 assert (temp->is_neutral(), "invariant") ;
1674 hash = temp->hash();
1675 if (hash) {
1676 return hash;
1677 }
1678 // Skip to the following code to reduce code size
1679 } else if (Self->is_lock_owned((address)mark->locker())) {
1680 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
1681 assert (temp->is_neutral(), "invariant") ;
1682 hash = temp->hash(); // by current thread, check if the displaced
1683 if (hash) { // header contains hash code
1684 return hash;
1685 }
1686 // WARNING:
1687 // The displaced header is strictly immutable.
1688 // It can NOT be changed in ANY cases. So we have
1689 // to inflate the header into heavyweight monitor
1690 // even the current thread owns the lock. The reason
1691 // is the BasicLock (stack slot) will be asynchronously
1692 // read by other threads during the inflate() function.
1693 // Any change to stack may not propagate to other threads
1694 // correctly.
1695 }
1697 // Inflate the monitor to set hash code
1698 monitor = ObjectSynchronizer::inflate(Self, obj);
1699 // Load displaced header and check it has hash code
1700 mark = monitor->header();
1701 assert (mark->is_neutral(), "invariant") ;
1702 hash = mark->hash();
1703 if (hash == 0) {
1704 hash = get_next_hash(Self, obj);
1705 temp = mark->copy_set_hash(hash); // merge hash code into header
1706 assert (temp->is_neutral(), "invariant") ;
1707 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
1708 if (test != mark) {
1709 // The only update to the header in the monitor (outside GC)
1710 // is install the hash code. If someone add new usage of
1711 // displaced header, please update this code
1712 hash = test->hash();
1713 assert (test->is_neutral(), "invariant") ;
1714 assert (hash != 0, "Trivial unexpected object/monitor header usage.");
1715 }
1716 }
1717 // We finally get the hash
1718 return hash;
1719 }
1721 // Deprecated -- use FastHashCode() instead.
1723 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
1724 return FastHashCode (Thread::current(), obj()) ;
1725 }
1727 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
1728 Handle h_obj) {
1729 if (UseBiasedLocking) {
1730 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
1731 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1732 }
1734 assert(thread == JavaThread::current(), "Can only be called on current thread");
1735 oop obj = h_obj();
1737 markOop mark = ReadStableMark (obj) ;
1739 // Uncontended case, header points to stack
1740 if (mark->has_locker()) {
1741 return thread->is_lock_owned((address)mark->locker());
1742 }
1743 // Contended case, header points to ObjectMonitor (tagged pointer)
1744 if (mark->has_monitor()) {
1745 ObjectMonitor* monitor = mark->monitor();
1746 return monitor->is_entered(thread) != 0 ;
1747 }
1748 // Unlocked case, header in place
1749 assert(mark->is_neutral(), "sanity check");
1750 return false;
1751 }
1753 // Be aware of this method could revoke bias of the lock object.
1754 // This method querys the ownership of the lock handle specified by 'h_obj'.
1755 // If the current thread owns the lock, it returns owner_self. If no
1756 // thread owns the lock, it returns owner_none. Otherwise, it will return
1757 // ower_other.
1758 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
1759 (JavaThread *self, Handle h_obj) {
1760 // The caller must beware this method can revoke bias, and
1761 // revocation can result in a safepoint.
1762 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
1763 assert (self->thread_state() != _thread_blocked , "invariant") ;
1765 // Possible mark states: neutral, biased, stack-locked, inflated
1767 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
1768 // CASE: biased
1769 BiasedLocking::revoke_and_rebias(h_obj, false, self);
1770 assert(!h_obj->mark()->has_bias_pattern(),
1771 "biases should be revoked by now");
1772 }
1774 assert(self == JavaThread::current(), "Can only be called on current thread");
1775 oop obj = h_obj();
1776 markOop mark = ReadStableMark (obj) ;
1778 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
1779 if (mark->has_locker()) {
1780 return self->is_lock_owned((address)mark->locker()) ?
1781 owner_self : owner_other;
1782 }
1784 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
1785 // The Object:ObjectMonitor relationship is stable as long as we're
1786 // not at a safepoint.
1787 if (mark->has_monitor()) {
1788 void * owner = mark->monitor()->_owner ;
1789 if (owner == NULL) return owner_none ;
1790 return (owner == self ||
1791 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
1792 }
1794 // CASE: neutral
1795 assert(mark->is_neutral(), "sanity check");
1796 return owner_none ; // it's unlocked
1797 }
1799 // FIXME: jvmti should call this
1800 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
1801 if (UseBiasedLocking) {
1802 if (SafepointSynchronize::is_at_safepoint()) {
1803 BiasedLocking::revoke_at_safepoint(h_obj);
1804 } else {
1805 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
1806 }
1807 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1808 }
1810 oop obj = h_obj();
1811 address owner = NULL;
1813 markOop mark = ReadStableMark (obj) ;
1815 // Uncontended case, header points to stack
1816 if (mark->has_locker()) {
1817 owner = (address) mark->locker();
1818 }
1820 // Contended case, header points to ObjectMonitor (tagged pointer)
1821 if (mark->has_monitor()) {
1822 ObjectMonitor* monitor = mark->monitor();
1823 assert(monitor != NULL, "monitor should be non-null");
1824 owner = (address) monitor->owner();
1825 }
1827 if (owner != NULL) {
1828 return Threads::owning_thread_from_monitor_owner(owner, doLock);
1829 }
1831 // Unlocked case, header in place
1832 // Cannot have assertion since this object may have been
1833 // locked by another thread when reaching here.
1834 // assert(mark->is_neutral(), "sanity check");
1836 return NULL;
1837 }
1839 // Iterate through monitor cache and attempt to release thread's monitors
1840 // Gives up on a particular monitor if an exception occurs, but continues
1841 // the overall iteration, swallowing the exception.
1842 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1843 private:
1844 TRAPS;
1846 public:
1847 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1848 void do_monitor(ObjectMonitor* mid) {
1849 if (mid->owner() == THREAD) {
1850 (void)mid->complete_exit(CHECK);
1851 }
1852 }
1853 };
1855 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1856 // ignored. This is meant to be called during JNI thread detach which assumes
1857 // all remaining monitors are heavyweight. All exceptions are swallowed.
1858 // Scanning the extant monitor list can be time consuming.
1859 // A simple optimization is to add a per-thread flag that indicates a thread
1860 // called jni_monitorenter() during its lifetime.
1861 //
1862 // Instead of No_Savepoint_Verifier it might be cheaper to
1863 // use an idiom of the form:
1864 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1865 // <code that must not run at safepoint>
1866 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1867 // Since the tests are extremely cheap we could leave them enabled
1868 // for normal product builds.
1870 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1871 assert(THREAD == JavaThread::current(), "must be current Java thread");
1872 No_Safepoint_Verifier nsv ;
1873 ReleaseJavaMonitorsClosure rjmc(THREAD);
1874 Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
1875 ObjectSynchronizer::monitors_iterate(&rjmc);
1876 Thread::muxRelease(&ListLock);
1877 THREAD->clear_pending_exception();
1878 }
1880 // Visitors ...
1882 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
1883 ObjectMonitor* block = gBlockList;
1884 ObjectMonitor* mid;
1885 while (block) {
1886 assert(block->object() == CHAINMARKER, "must be a block header");
1887 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
1888 mid = block + i;
1889 oop object = (oop) mid->object();
1890 if (object != NULL) {
1891 closure->do_monitor(mid);
1892 }
1893 }
1894 block = (ObjectMonitor*) block->FreeNext;
1895 }
1896 }
1898 void ObjectSynchronizer::oops_do(OopClosure* f) {
1899 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1900 for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
1901 assert(block->object() == CHAINMARKER, "must be a block header");
1902 for (int i = 1; i < _BLOCKSIZE; i++) {
1903 ObjectMonitor* mid = &block[i];
1904 if (mid->object() != NULL) {
1905 f->do_oop((oop*)mid->object_addr());
1906 }
1907 }
1908 }
1909 }
1911 // Deflate_idle_monitors() is called at all safepoints, immediately
1912 // after all mutators are stopped, but before any objects have moved.
1913 // It traverses the list of known monitors, deflating where possible.
1914 // The scavenged monitor are returned to the monitor free list.
1915 //
1916 // Beware that we scavenge at *every* stop-the-world point.
1917 // Having a large number of monitors in-circulation negatively
1918 // impacts the performance of some applications (e.g., PointBase).
1919 // Broadly, we want to minimize the # of monitors in circulation.
1920 //
1921 // We have added a flag, MonitorInUseLists, which creates a list
1922 // of active monitors for each thread. deflate_idle_monitors()
1923 // only scans the per-thread inuse lists. omAlloc() puts all
1924 // assigned monitors on the per-thread list. deflate_idle_monitors()
1925 // returns the non-busy monitors to the global free list.
1926 // When a thread dies, omFlush() adds the list of active monitors for
1927 // that thread to a global gOmInUseList acquiring the
1928 // global list lock. deflate_idle_monitors() acquires the global
1929 // list lock to scan for non-busy monitors to the global free list.
1930 // An alternative could have used a single global inuse list. The
1931 // downside would have been the additional cost of acquiring the global list lock
1932 // for every omAlloc().
1933 //
1934 // Perversely, the heap size -- and thus the STW safepoint rate --
1935 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1936 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1937 // This is an unfortunate aspect of this design.
1938 //
1939 // Another refinement would be to refrain from calling deflate_idle_monitors()
1940 // except at stop-the-world points associated with garbage collections.
1941 //
1942 // An even better solution would be to deflate on-the-fly, aggressively,
1943 // at monitorexit-time as is done in EVM's metalock or Relaxed Locks.
1946 // Deflate a single monitor if not in use
1947 // Return true if deflated, false if in use
1948 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1949 ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
1950 bool deflated;
1951 // Normal case ... The monitor is associated with obj.
1952 guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
1953 guarantee (mid == obj->mark()->monitor(), "invariant");
1954 guarantee (mid->header()->is_neutral(), "invariant");
1956 if (mid->is_busy()) {
1957 if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
1958 deflated = false;
1959 } else {
1960 // Deflate the monitor if it is no longer being used
1961 // It's idle - scavenge and return to the global free list
1962 // plain old deflation ...
1963 TEVENT (deflate_idle_monitors - scavenge1) ;
1964 if (TraceMonitorInflation) {
1965 if (obj->is_instance()) {
1966 ResourceMark rm;
1967 tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1968 (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());
1969 }
1970 }
1972 // Restore the header back to obj
1973 obj->release_set_mark(mid->header());
1974 mid->clear();
1976 assert (mid->object() == NULL, "invariant") ;
1978 // Move the object to the working free list defined by FreeHead,FreeTail.
1979 if (*FreeHeadp == NULL) *FreeHeadp = mid;
1980 if (*FreeTailp != NULL) {
1981 ObjectMonitor * prevtail = *FreeTailp;
1982 assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); // TODO KK
1983 prevtail->FreeNext = mid;
1984 }
1985 *FreeTailp = mid;
1986 deflated = true;
1987 }
1988 return deflated;
1989 }
1991 // Caller acquires ListLock
1992 int ObjectSynchronizer::walk_monitor_list(ObjectMonitor** listheadp,
1993 ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
1994 ObjectMonitor* mid;
1995 ObjectMonitor* next;
1996 ObjectMonitor* curmidinuse = NULL;
1997 int deflatedcount = 0;
1999 for (mid = *listheadp; mid != NULL; ) {
2000 oop obj = (oop) mid->object();
2001 bool deflated = false;
2002 if (obj != NULL) {
2003 deflated = deflate_monitor(mid, obj, FreeHeadp, FreeTailp);
2004 }
2005 if (deflated) {
2006 // extract from per-thread in-use-list
2007 if (mid == *listheadp) {
2008 *listheadp = mid->FreeNext;
2009 } else if (curmidinuse != NULL) {
2010 curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
2011 }
2012 next = mid->FreeNext;
2013 mid->FreeNext = NULL; // This mid is current tail in the FreeHead list
2014 mid = next;
2015 deflatedcount++;
2016 } else {
2017 curmidinuse = mid;
2018 mid = mid->FreeNext;
2019 }
2020 }
2021 return deflatedcount;
2022 }
2024 void ObjectSynchronizer::deflate_idle_monitors() {
2025 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
2026 int nInuse = 0 ; // currently associated with objects
2027 int nInCirculation = 0 ; // extant
2028 int nScavenged = 0 ; // reclaimed
2029 bool deflated = false;
2031 ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors
2032 ObjectMonitor * FreeTail = NULL ;
2034 TEVENT (deflate_idle_monitors) ;
2035 // Prevent omFlush from changing mids in Thread dtor's during deflation
2036 // And in case the vm thread is acquiring a lock during a safepoint
2037 // See e.g. 6320749
2038 Thread::muxAcquire (&ListLock, "scavenge - return") ;
2040 if (MonitorInUseLists) {
2041 int inUse = 0;
2042 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
2043 nInCirculation+= cur->omInUseCount;
2044 int deflatedcount = walk_monitor_list(cur->omInUseList_addr(), &FreeHead, &FreeTail);
2045 cur->omInUseCount-= deflatedcount;
2046 // verifyInUse(cur);
2047 nScavenged += deflatedcount;
2048 nInuse += cur->omInUseCount;
2049 }
2051 // For moribund threads, scan gOmInUseList
2052 if (gOmInUseList) {
2053 nInCirculation += gOmInUseCount;
2054 int deflatedcount = walk_monitor_list((ObjectMonitor **)&gOmInUseList, &FreeHead, &FreeTail);
2055 gOmInUseCount-= deflatedcount;
2056 nScavenged += deflatedcount;
2057 nInuse += gOmInUseCount;
2058 }
2060 } else for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
2061 // Iterate over all extant monitors - Scavenge all idle monitors.
2062 assert(block->object() == CHAINMARKER, "must be a block header");
2063 nInCirculation += _BLOCKSIZE ;
2064 for (int i = 1 ; i < _BLOCKSIZE; i++) {
2065 ObjectMonitor* mid = &block[i];
2066 oop obj = (oop) mid->object();
2068 if (obj == NULL) {
2069 // The monitor is not associated with an object.
2070 // The monitor should either be a thread-specific private
2071 // free list or the global free list.
2072 // obj == NULL IMPLIES mid->is_busy() == 0
2073 guarantee (!mid->is_busy(), "invariant") ;
2074 continue ;
2075 }
2076 deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);
2078 if (deflated) {
2079 mid->FreeNext = NULL ;
2080 nScavenged ++ ;
2081 } else {
2082 nInuse ++;
2083 }
2084 }
2085 }
2087 MonitorFreeCount += nScavenged;
2089 // Consider: audit gFreeList to ensure that MonitorFreeCount and list agree.
2091 if (Knob_Verbose) {
2092 ::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n",
2093 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
2094 MonitorPopulation, MonitorFreeCount) ;
2095 ::fflush(stdout) ;
2096 }
2098 ForceMonitorScavenge = 0; // Reset
2100 // Move the scavenged monitors back to the global free list.
2101 if (FreeHead != NULL) {
2102 guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
2103 assert (FreeTail->FreeNext == NULL, "invariant") ;
2104 // constant-time list splice - prepend scavenged segment to gFreeList
2105 FreeTail->FreeNext = gFreeList ;
2106 gFreeList = FreeHead ;
2107 }
2108 Thread::muxRelease (&ListLock) ;
2110 if (_sync_Deflations != NULL) _sync_Deflations->inc(nScavenged) ;
2111 if (_sync_MonExtant != NULL) _sync_MonExtant ->set_value(nInCirculation);
2113 // TODO: Add objectMonitor leak detection.
2114 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
2115 GVars.stwRandom = os::random() ;
2116 GVars.stwCycle ++ ;
2117 }
2119 // A macro is used below because there may already be a pending
2120 // exception which should not abort the execution of the routines
2121 // which use this (which is why we don't put this into check_slow and
2122 // call it with a CHECK argument).
2124 #define CHECK_OWNER() \
2125 do { \
2126 if (THREAD != _owner) { \
2127 if (THREAD->is_lock_owned((address) _owner)) { \
2128 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
2129 _recursions = 0; \
2130 OwnerIsThread = 1 ; \
2131 } else { \
2132 TEVENT (Throw IMSX) ; \
2133 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
2134 } \
2135 } \
2136 } while (false)
2138 // TODO-FIXME: eliminate ObjectWaiters. Replace this visitor/enumerator
2139 // interface with a simple FirstWaitingThread(), NextWaitingThread() interface.
2141 ObjectWaiter* ObjectMonitor::first_waiter() {
2142 return _WaitSet;
2143 }
2145 ObjectWaiter* ObjectMonitor::next_waiter(ObjectWaiter* o) {
2146 return o->_next;
2147 }
2149 Thread* ObjectMonitor::thread_of_waiter(ObjectWaiter* o) {
2150 return o->_thread;
2151 }
2153 // initialize the monitor, exception the semaphore, all other fields
2154 // are simple integers or pointers
2155 ObjectMonitor::ObjectMonitor() {
2156 _header = NULL;
2157 _count = 0;
2158 _waiters = 0,
2159 _recursions = 0;
2160 _object = NULL;
2161 _owner = NULL;
2162 _WaitSet = NULL;
2163 _WaitSetLock = 0 ;
2164 _Responsible = NULL ;
2165 _succ = NULL ;
2166 _cxq = NULL ;
2167 FreeNext = NULL ;
2168 _EntryList = NULL ;
2169 _SpinFreq = 0 ;
2170 _SpinClock = 0 ;
2171 OwnerIsThread = 0 ;
2172 }
2174 ObjectMonitor::~ObjectMonitor() {
2175 // TODO: Add asserts ...
2176 // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
2177 // _count == 0 _EntryList == NULL etc
2178 }
2180 intptr_t ObjectMonitor::is_busy() const {
2181 // TODO-FIXME: merge _count and _waiters.
2182 // TODO-FIXME: assert _owner == null implies _recursions = 0
2183 // TODO-FIXME: assert _WaitSet != null implies _count > 0
2184 return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ;
2185 }
2187 void ObjectMonitor::Recycle () {
2188 // TODO: add stronger asserts ...
2189 // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
2190 // _count == 0 EntryList == NULL
2191 // _recursions == 0 _WaitSet == NULL
2192 // TODO: assert (is_busy()|_recursions) == 0
2193 _succ = NULL ;
2194 _EntryList = NULL ;
2195 _cxq = NULL ;
2196 _WaitSet = NULL ;
2197 _recursions = 0 ;
2198 _SpinFreq = 0 ;
2199 _SpinClock = 0 ;
2200 OwnerIsThread = 0 ;
2201 }
2203 // WaitSet management ...
2205 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2206 assert(node != NULL, "should not dequeue NULL node");
2207 assert(node->_prev == NULL, "node already in list");
2208 assert(node->_next == NULL, "node already in list");
2209 // put node at end of queue (circular doubly linked list)
2210 if (_WaitSet == NULL) {
2211 _WaitSet = node;
2212 node->_prev = node;
2213 node->_next = node;
2214 } else {
2215 ObjectWaiter* head = _WaitSet ;
2216 ObjectWaiter* tail = head->_prev;
2217 assert(tail->_next == head, "invariant check");
2218 tail->_next = node;
2219 head->_prev = node;
2220 node->_next = head;
2221 node->_prev = tail;
2222 }
2223 }
2225 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2226 // dequeue the very first waiter
2227 ObjectWaiter* waiter = _WaitSet;
2228 if (waiter) {
2229 DequeueSpecificWaiter(waiter);
2230 }
2231 return waiter;
2232 }
2234 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2235 assert(node != NULL, "should not dequeue NULL node");
2236 assert(node->_prev != NULL, "node already removed from list");
2237 assert(node->_next != NULL, "node already removed from list");
2238 // when the waiter has woken up because of interrupt,
2239 // timeout or other spurious wake-up, dequeue the
2240 // waiter from waiting list
2241 ObjectWaiter* next = node->_next;
2242 if (next == node) {
2243 assert(node->_prev == node, "invariant check");
2244 _WaitSet = NULL;
2245 } else {
2246 ObjectWaiter* prev = node->_prev;
2247 assert(prev->_next == node, "invariant check");
2248 assert(next->_prev == node, "invariant check");
2249 next->_prev = prev;
2250 prev->_next = next;
2251 if (_WaitSet == node) {
2252 _WaitSet = next;
2253 }
2254 }
2255 node->_next = NULL;
2256 node->_prev = NULL;
2257 }
2259 static char * kvGet (char * kvList, const char * Key) {
2260 if (kvList == NULL) return NULL ;
2261 size_t n = strlen (Key) ;
2262 char * Search ;
2263 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2264 if (strncmp (Search, Key, n) == 0) {
2265 if (Search[n] == '=') return Search + n + 1 ;
2266 if (Search[n] == 0) return (char *) "1" ;
2267 }
2268 }
2269 return NULL ;
2270 }
2272 static int kvGetInt (char * kvList, const char * Key, int Default) {
2273 char * v = kvGet (kvList, Key) ;
2274 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2275 if (Knob_ReportSettings && v != NULL) {
2276 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2277 ::fflush (stdout) ;
2278 }
2279 return rslt ;
2280 }
2282 // By convention we unlink a contending thread from EntryList|cxq immediately
2283 // after the thread acquires the lock in ::enter(). Equally, we could defer
2284 // unlinking the thread until ::exit()-time.
2286 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
2287 {
2288 assert (_owner == Self, "invariant") ;
2289 assert (SelfNode->_thread == Self, "invariant") ;
2291 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
2292 // Normal case: remove Self from the DLL EntryList .
2293 // This is a constant-time operation.
2294 ObjectWaiter * nxt = SelfNode->_next ;
2295 ObjectWaiter * prv = SelfNode->_prev ;
2296 if (nxt != NULL) nxt->_prev = prv ;
2297 if (prv != NULL) prv->_next = nxt ;
2298 if (SelfNode == _EntryList ) _EntryList = nxt ;
2299 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
2300 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
2301 TEVENT (Unlink from EntryList) ;
2302 } else {
2303 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
2304 // Inopportune interleaving -- Self is still on the cxq.
2305 // This usually means the enqueue of self raced an exiting thread.
2306 // Normally we'll find Self near the front of the cxq, so
2307 // dequeueing is typically fast. If needbe we can accelerate
2308 // this with some MCS/CHL-like bidirectional list hints and advisory
2309 // back-links so dequeueing from the interior will normally operate
2310 // in constant-time.
2311 // Dequeue Self from either the head (with CAS) or from the interior
2312 // with a linear-time scan and normal non-atomic memory operations.
2313 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
2314 // and then unlink Self from EntryList. We have to drain eventually,
2315 // so it might as well be now.
2317 ObjectWaiter * v = _cxq ;
2318 assert (v != NULL, "invariant") ;
2319 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
2320 // The CAS above can fail from interference IFF a "RAT" arrived.
2321 // In that case Self must be in the interior and can no longer be
2322 // at the head of cxq.
2323 if (v == SelfNode) {
2324 assert (_cxq != v, "invariant") ;
2325 v = _cxq ; // CAS above failed - start scan at head of list
2326 }
2327 ObjectWaiter * p ;
2328 ObjectWaiter * q = NULL ;
2329 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
2330 q = p ;
2331 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
2332 }
2333 assert (v != SelfNode, "invariant") ;
2334 assert (p == SelfNode, "Node not found on cxq") ;
2335 assert (p != _cxq, "invariant") ;
2336 assert (q != NULL, "invariant") ;
2337 assert (q->_next == p, "invariant") ;
2338 q->_next = p->_next ;
2339 }
2340 TEVENT (Unlink from cxq) ;
2341 }
2343 // Diagnostic hygiene ...
2344 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
2345 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
2346 SelfNode->TState = ObjectWaiter::TS_RUN ;
2347 }
2349 // Caveat: TryLock() is not necessarily serializing if it returns failure.
2350 // Callers must compensate as needed.
2352 int ObjectMonitor::TryLock (Thread * Self) {
2353 for (;;) {
2354 void * own = _owner ;
2355 if (own != NULL) return 0 ;
2356 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
2357 // Either guarantee _recursions == 0 or set _recursions = 0.
2358 assert (_recursions == 0, "invariant") ;
2359 assert (_owner == Self, "invariant") ;
2360 // CONSIDER: set or assert that OwnerIsThread == 1
2361 return 1 ;
2362 }
2363 // The lock had been free momentarily, but we lost the race to the lock.
2364 // Interference -- the CAS failed.
2365 // We can either return -1 or retry.
2366 // Retry doesn't make as much sense because the lock was just acquired.
2367 if (true) return -1 ;
2368 }
2369 }
2371 // NotRunnable() -- informed spinning
2372 //
2373 // Don't bother spinning if the owner is not eligible to drop the lock.
2374 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2375 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2376 // The thread must be runnable in order to drop the lock in timely fashion.
2377 // If the _owner is not runnable then spinning will not likely be
2378 // successful (profitable).
2379 //
2380 // Beware -- the thread referenced by _owner could have died
2381 // so a simply fetch from _owner->_thread_state might trap.
2382 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2383 // Because of the lifecycle issues the schedctl and _thread_state values
2384 // observed by NotRunnable() might be garbage. NotRunnable must
2385 // tolerate this and consider the observed _thread_state value
2386 // as advisory.
2387 //
2388 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2389 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2390 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2391 // with the LSB of _owner. Another option would be to probablistically probe
2392 // the putative _owner->TypeTag value.
2393 //
2394 // Checking _thread_state isn't perfect. Even if the thread is
2395 // in_java it might be blocked on a page-fault or have been preempted
2396 // and sitting on a ready/dispatch queue. _thread state in conjunction
2397 // with schedctl.sc_state gives us a good picture of what the
2398 // thread is doing, however.
2399 //
2400 // TODO: check schedctl.sc_state.
2401 // We'll need to use SafeFetch32() to read from the schedctl block.
2402 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2403 //
2404 // The return value from NotRunnable() is *advisory* -- the
2405 // result is based on sampling and is not necessarily coherent.
2406 // The caller must tolerate false-negative and false-positive errors.
2407 // Spinning, in general, is probabilistic anyway.
2410 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2411 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2412 if (!OwnerIsThread) return 0 ;
2414 if (ox == NULL) return 0 ;
2416 // Avoid transitive spinning ...
2417 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2418 // Immediately after T1 acquires L it's possible that T2, also
2419 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2420 // This occurs transiently after T1 acquired L but before
2421 // T1 managed to clear T1.Stalled. T2 does not need to abort
2422 // its spin in this circumstance.
2423 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2425 if (BlockedOn == 1) return 1 ;
2426 if (BlockedOn != 0) {
2427 return BlockedOn != intptr_t(this) && _owner == ox ;
2428 }
2430 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2431 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2432 // consider also: jst != _thread_in_Java -- but that's overspecific.
2433 return jst == _thread_blocked || jst == _thread_in_native ;
2434 }
2437 // Adaptive spin-then-block - rational spinning
2438 //
2439 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2440 // algorithm. On high order SMP systems it would be better to start with
2441 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
2442 // a contending thread could enqueue itself on the cxq and then spin locally
2443 // on a thread-specific variable such as its ParkEvent._Event flag.
2444 // That's left as an exercise for the reader. Note that global spinning is
2445 // not problematic on Niagara, as the L2$ serves the interconnect and has both
2446 // low latency and massive bandwidth.
2447 //
2448 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2449 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2450 // (duration) or we can fix the count at approximately the duration of
2451 // a context switch and vary the frequency. Of course we could also
2452 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2453 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
2454 //
2455 // This implementation varies the duration "D", where D varies with
2456 // the success rate of recent spin attempts. (D is capped at approximately
2457 // length of a round-trip context switch). The success rate for recent
2458 // spin attempts is a good predictor of the success rate of future spin
2459 // attempts. The mechanism adapts automatically to varying critical
2460 // section length (lock modality), system load and degree of parallelism.
2461 // D is maintained per-monitor in _SpinDuration and is initialized
2462 // optimistically. Spin frequency is fixed at 100%.
2463 //
2464 // Note that _SpinDuration is volatile, but we update it without locks
2465 // or atomics. The code is designed so that _SpinDuration stays within
2466 // a reasonable range even in the presence of races. The arithmetic
2467 // operations on _SpinDuration are closed over the domain of legal values,
2468 // so at worst a race will install and older but still legal value.
2469 // At the very worst this introduces some apparent non-determinism.
2470 // We might spin when we shouldn't or vice-versa, but since the spin
2471 // count are relatively short, even in the worst case, the effect is harmless.
2472 //
2473 // Care must be taken that a low "D" value does not become an
2474 // an absorbing state. Transient spinning failures -- when spinning
2475 // is overall profitable -- should not cause the system to converge
2476 // on low "D" values. We want spinning to be stable and predictable
2477 // and fairly responsive to change and at the same time we don't want
2478 // it to oscillate, become metastable, be "too" non-deterministic,
2479 // or converge on or enter undesirable stable absorbing states.
2480 //
2481 // We implement a feedback-based control system -- using past behavior
2482 // to predict future behavior. We face two issues: (a) if the
2483 // input signal is random then the spin predictor won't provide optimal
2484 // results, and (b) if the signal frequency is too high then the control
2485 // system, which has some natural response lag, will "chase" the signal.
2486 // (b) can arise from multimodal lock hold times. Transient preemption
2487 // can also result in apparent bimodal lock hold times.
2488 // Although sub-optimal, neither condition is particularly harmful, as
2489 // in the worst-case we'll spin when we shouldn't or vice-versa.
2490 // The maximum spin duration is rather short so the failure modes aren't bad.
2491 // To be conservative, I've tuned the gain in system to bias toward
2492 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2493 // "rings" or oscillates between spinning and not spinning. This happens
2494 // when spinning is just on the cusp of profitability, however, so the
2495 // situation is not dire. The state is benign -- there's no need to add
2496 // hysteresis control to damp the transition rate between spinning and
2497 // not spinning.
2498 //
2499 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
2500 //
2501 // Spin-then-block strategies ...
2502 //
2503 // Thoughts on ways to improve spinning :
2504 //
2505 // * Periodically call {psr_}getloadavg() while spinning, and
2506 // permit unbounded spinning if the load average is <
2507 // the number of processors. Beware, however, that getloadavg()
2508 // is exceptionally fast on solaris (about 1/10 the cost of a full
2509 // spin cycle, but quite expensive on linux. Beware also, that
2510 // multiple JVMs could "ring" or oscillate in a feedback loop.
2511 // Sufficient damping would solve that problem.
2512 //
2513 // * We currently use spin loops with iteration counters to approximate
2514 // spinning for some interval. Given the availability of high-precision
2515 // time sources such as gethrtime(), %TICK, %STICK, RDTSC, etc., we should
2516 // someday reimplement the spin loops to duration-based instead of iteration-based.
2517 //
2518 // * Don't spin if there are more than N = (CPUs/2) threads
2519 // currently spinning on the monitor (or globally).
2520 // That is, limit the number of concurrent spinners.
2521 // We might also limit the # of spinners in the JVM, globally.
2522 //
2523 // * If a spinning thread observes _owner change hands it should
2524 // abort the spin (and park immediately) or at least debit
2525 // the spin counter by a large "penalty".
2526 //
2527 // * Classically, the spin count is either K*(CPUs-1) or is a
2528 // simple constant that approximates the length of a context switch.
2529 // We currently use a value -- computed by a special utility -- that
2530 // approximates round-trip context switch times.
2531 //
2532 // * Normally schedctl_start()/_stop() is used to advise the kernel
2533 // to avoid preempting threads that are running in short, bounded
2534 // critical sections. We could use the schedctl hooks in an inverted
2535 // sense -- spinners would set the nopreempt flag, but poll the preempt
2536 // pending flag. If a spinner observed a pending preemption it'd immediately
2537 // abort the spin and park. As such, the schedctl service acts as
2538 // a preemption warning mechanism.
2539 //
2540 // * In lieu of spinning, if the system is running below saturation
2541 // (that is, loadavg() << #cpus), we can instead suppress futile
2542 // wakeup throttling, or even wake more than one successor at exit-time.
2543 // The net effect is largely equivalent to spinning. In both cases,
2544 // contending threads go ONPROC and opportunistically attempt to acquire
2545 // the lock, decreasing lock handover latency at the expense of wasted
2546 // cycles and context switching.
2547 //
2548 // * We might to spin less after we've parked as the thread will
2549 // have less $ and TLB affinity with the processor.
2550 // Likewise, we might spin less if we come ONPROC on a different
2551 // processor or after a long period (>> rechose_interval).
2552 //
2553 // * A table-driven state machine similar to Solaris' dispadmin scheduling
2554 // tables might be a better design. Instead of encoding information in
2555 // _SpinDuration, _SpinFreq and _SpinClock we'd just use explicit,
2556 // discrete states. Success or failure during a spin would drive
2557 // state transitions, and each state node would contain a spin count.
2558 //
2559 // * If the processor is operating in a mode intended to conserve power
2560 // (such as Intel's SpeedStep) or to reduce thermal output (thermal
2561 // step-down mode) then the Java synchronization subsystem should
2562 // forgo spinning.
2563 //
2564 // * The minimum spin duration should be approximately the worst-case
2565 // store propagation latency on the platform. That is, the time
2566 // it takes a store on CPU A to become visible on CPU B, where A and
2567 // B are "distant".
2568 //
2569 // * We might want to factor a thread's priority in the spin policy.
2570 // Threads with a higher priority might spin for slightly longer.
2571 // Similarly, if we use back-off in the TATAS loop, lower priority
2572 // threads might back-off longer. We don't currently use a
2573 // thread's priority when placing it on the entry queue. We may
2574 // want to consider doing so in future releases.
2575 //
2576 // * We might transiently drop a thread's scheduling priority while it spins.
2577 // SCHED_BATCH on linux and FX scheduling class at priority=0 on Solaris
2578 // would suffice. We could even consider letting the thread spin indefinitely at
2579 // a depressed or "idle" priority. This brings up fairness issues, however --
2580 // in a saturated system a thread would with a reduced priority could languish
2581 // for extended periods on the ready queue.
2582 //
2583 // * While spinning try to use the otherwise wasted time to help the VM make
2584 // progress:
2585 //
2586 // -- YieldTo() the owner, if the owner is OFFPROC but ready
2587 // Done our remaining quantum directly to the ready thread.
2588 // This helps "push" the lock owner through the critical section.
2589 // It also tends to improve affinity/locality as the lock
2590 // "migrates" less frequently between CPUs.
2591 // -- Walk our own stack in anticipation of blocking. Memoize the roots.
2592 // -- Perform strand checking for other thread. Unpark potential strandees.
2593 // -- Help GC: trace or mark -- this would need to be a bounded unit of work.
2594 // Unfortunately this will pollute our $ and TLBs. Recall that we
2595 // spin to avoid context switching -- context switching has an
2596 // immediate cost in latency, a disruptive cost to other strands on a CMT
2597 // processor, and an amortized cost because of the D$ and TLB cache
2598 // reload transient when the thread comes back ONPROC and repopulates
2599 // $s and TLBs.
2600 // -- call getloadavg() to see if the system is saturated. It'd probably
2601 // make sense to call getloadavg() half way through the spin.
2602 // If the system isn't at full capacity the we'd simply reset
2603 // the spin counter to and extend the spin attempt.
2604 // -- Doug points out that we should use the same "helping" policy
2605 // in thread.yield().
2606 //
2607 // * Try MONITOR-MWAIT on systems that support those instructions.
2608 //
2609 // * The spin statistics that drive spin decisions & frequency are
2610 // maintained in the objectmonitor structure so if we deflate and reinflate
2611 // we lose spin state. In practice this is not usually a concern
2612 // as the default spin state after inflation is aggressive (optimistic)
2613 // and tends toward spinning. So in the worst case for a lock where
2614 // spinning is not profitable we may spin unnecessarily for a brief
2615 // period. But then again, if a lock is contended it'll tend not to deflate
2616 // in the first place.
2619 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
2620 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
2622 // Spinning: Fixed frequency (100%), vary duration
2624 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
2626 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2627 int ctr = Knob_FixedSpin ;
2628 if (ctr != 0) {
2629 while (--ctr >= 0) {
2630 if (TryLock (Self) > 0) return 1 ;
2631 SpinPause () ;
2632 }
2633 return 0 ;
2634 }
2636 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
2637 if (TryLock(Self) > 0) {
2638 // Increase _SpinDuration ...
2639 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2640 // Raising _SpurDuration to the poverty line is key.
2641 int x = _SpinDuration ;
2642 if (x < Knob_SpinLimit) {
2643 if (x < Knob_Poverty) x = Knob_Poverty ;
2644 _SpinDuration = x + Knob_BonusB ;
2645 }
2646 return 1 ;
2647 }
2648 SpinPause () ;
2649 }
2651 // Admission control - verify preconditions for spinning
2652 //
2653 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2654 // becoming an absorbing state. Put another way, we spin briefly to
2655 // sample, just in case the system load, parallelism, contention, or lock
2656 // modality changed.
2657 //
2658 // Consider the following alternative:
2659 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2660 // spin attempt. "Periodically" might mean after a tally of
2661 // the # of failed spin attempts (or iterations) reaches some threshold.
2662 // This takes us into the realm of 1-out-of-N spinning, where we
2663 // hold the duration constant but vary the frequency.
2665 ctr = _SpinDuration ;
2666 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2667 if (ctr <= 0) return 0 ;
2669 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2670 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2671 TEVENT (Spin abort - notrunnable [TOP]);
2672 return 0 ;
2673 }
2675 int MaxSpin = Knob_MaxSpinners ;
2676 if (MaxSpin >= 0) {
2677 if (_Spinner > MaxSpin) {
2678 TEVENT (Spin abort -- too many spinners) ;
2679 return 0 ;
2680 }
2681 // Slighty racy, but benign ...
2682 Adjust (&_Spinner, 1) ;
2683 }
2685 // We're good to spin ... spin ingress.
2686 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2687 // when preparing to LD...CAS _owner, etc and the CAS is likely
2688 // to succeed.
2689 int hits = 0 ;
2690 int msk = 0 ;
2691 int caspty = Knob_CASPenalty ;
2692 int oxpty = Knob_OXPenalty ;
2693 int sss = Knob_SpinSetSucc ;
2694 if (sss && _succ == NULL ) _succ = Self ;
2695 Thread * prv = NULL ;
2697 // There are three ways to exit the following loop:
2698 // 1. A successful spin where this thread has acquired the lock.
2699 // 2. Spin failure with prejudice
2700 // 3. Spin failure without prejudice
2702 while (--ctr >= 0) {
2704 // Periodic polling -- Check for pending GC
2705 // Threads may spin while they're unsafe.
2706 // We don't want spinning threads to delay the JVM from reaching
2707 // a stop-the-world safepoint or to steal cycles from GC.
2708 // If we detect a pending safepoint we abort in order that
2709 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2710 // this thread, if safe, doesn't steal cycles from GC.
2711 // This is in keeping with the "no loitering in runtime" rule.
2712 // We periodically check to see if there's a safepoint pending.
2713 if ((ctr & 0xFF) == 0) {
2714 if (SafepointSynchronize::do_call_back()) {
2715 TEVENT (Spin: safepoint) ;
2716 goto Abort ; // abrupt spin egress
2717 }
2718 if (Knob_UsePause & 1) SpinPause () ;
2720 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2721 if (hits > 50 && scb != NULL) {
2722 int abend = (*scb)(SpinCallbackArgument, 0) ;
2723 }
2724 }
2726 if (Knob_UsePause & 2) SpinPause() ;
2728 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2729 // This is useful on classic SMP systems, but is of less utility on
2730 // N1-style CMT platforms.
2731 //
2732 // Trade-off: lock acquisition latency vs coherency bandwidth.
2733 // Lock hold times are typically short. A histogram
2734 // of successful spin attempts shows that we usually acquire
2735 // the lock early in the spin. That suggests we want to
2736 // sample _owner frequently in the early phase of the spin,
2737 // but then back-off and sample less frequently as the spin
2738 // progresses. The back-off makes a good citizen on SMP big
2739 // SMP systems. Oversampling _owner can consume excessive
2740 // coherency bandwidth. Relatedly, if we _oversample _owner we
2741 // can inadvertently interfere with the the ST m->owner=null.
2742 // executed by the lock owner.
2743 if (ctr & msk) continue ;
2744 ++hits ;
2745 if ((hits & 0xF) == 0) {
2746 // The 0xF, above, corresponds to the exponent.
2747 // Consider: (msk+1)|msk
2748 msk = ((msk << 2)|3) & BackOffMask ;
2749 }
2751 // Probe _owner with TATAS
2752 // If this thread observes the monitor transition or flicker
2753 // from locked to unlocked to locked, then the odds that this
2754 // thread will acquire the lock in this spin attempt go down
2755 // considerably. The same argument applies if the CAS fails
2756 // or if we observe _owner change from one non-null value to
2757 // another non-null value. In such cases we might abort
2758 // the spin without prejudice or apply a "penalty" to the
2759 // spin count-down variable "ctr", reducing it by 100, say.
2761 Thread * ox = (Thread *) _owner ;
2762 if (ox == NULL) {
2763 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2764 if (ox == NULL) {
2765 // The CAS succeeded -- this thread acquired ownership
2766 // Take care of some bookkeeping to exit spin state.
2767 if (sss && _succ == Self) {
2768 _succ = NULL ;
2769 }
2770 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2772 // Increase _SpinDuration :
2773 // The spin was successful (profitable) so we tend toward
2774 // longer spin attempts in the future.
2775 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2776 // If we acquired the lock early in the spin cycle it
2777 // makes sense to increase _SpinDuration proportionally.
2778 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2779 int x = _SpinDuration ;
2780 if (x < Knob_SpinLimit) {
2781 if (x < Knob_Poverty) x = Knob_Poverty ;
2782 _SpinDuration = x + Knob_Bonus ;
2783 }
2784 return 1 ;
2785 }
2787 // The CAS failed ... we can take any of the following actions:
2788 // * penalize: ctr -= Knob_CASPenalty
2789 // * exit spin with prejudice -- goto Abort;
2790 // * exit spin without prejudice.
2791 // * Since CAS is high-latency, retry again immediately.
2792 prv = ox ;
2793 TEVENT (Spin: cas failed) ;
2794 if (caspty == -2) break ;
2795 if (caspty == -1) goto Abort ;
2796 ctr -= caspty ;
2797 continue ;
2798 }
2800 // Did lock ownership change hands ?
2801 if (ox != prv && prv != NULL ) {
2802 TEVENT (spin: Owner changed)
2803 if (oxpty == -2) break ;
2804 if (oxpty == -1) goto Abort ;
2805 ctr -= oxpty ;
2806 }
2807 prv = ox ;
2809 // Abort the spin if the owner is not executing.
2810 // The owner must be executing in order to drop the lock.
2811 // Spinning while the owner is OFFPROC is idiocy.
2812 // Consider: ctr -= RunnablePenalty ;
2813 if (Knob_OState && NotRunnable (Self, ox)) {
2814 TEVENT (Spin abort - notrunnable);
2815 goto Abort ;
2816 }
2817 if (sss && _succ == NULL ) _succ = Self ;
2818 }
2820 // Spin failed with prejudice -- reduce _SpinDuration.
2821 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2822 // AIMD is globally stable.
2823 TEVENT (Spin failure) ;
2824 {
2825 int x = _SpinDuration ;
2826 if (x > 0) {
2827 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2828 // This is globally sample and tends to damp the response.
2829 x -= Knob_Penalty ;
2830 if (x < 0) x = 0 ;
2831 _SpinDuration = x ;
2832 }
2833 }
2835 Abort:
2836 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2837 if (sss && _succ == Self) {
2838 _succ = NULL ;
2839 // Invariant: after setting succ=null a contending thread
2840 // must recheck-retry _owner before parking. This usually happens
2841 // in the normal usage of TrySpin(), but it's safest
2842 // to make TrySpin() as foolproof as possible.
2843 OrderAccess::fence() ;
2844 if (TryLock(Self) > 0) return 1 ;
2845 }
2846 return 0 ;
2847 }
2849 #define TrySpin TrySpin_VaryDuration
2851 static void DeferredInitialize () {
2852 if (InitDone > 0) return ;
2853 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2854 while (InitDone != 1) ;
2855 return ;
2856 }
2858 // One-shot global initialization ...
2859 // The initialization is idempotent, so we don't need locks.
2860 // In the future consider doing this via os::init_2().
2861 // SyncKnobs consist of <Key>=<Value> pairs in the style
2862 // of environment variables. Start by converting ':' to NUL.
2864 if (SyncKnobs == NULL) SyncKnobs = "" ;
2866 size_t sz = strlen (SyncKnobs) ;
2867 char * knobs = (char *) malloc (sz + 2) ;
2868 if (knobs == NULL) {
2869 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2870 guarantee (0, "invariant") ;
2871 }
2872 strcpy (knobs, SyncKnobs) ;
2873 knobs[sz+1] = 0 ;
2874 for (char * p = knobs ; *p ; p++) {
2875 if (*p == ':') *p = 0 ;
2876 }
2878 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2879 SETKNOB(ReportSettings) ;
2880 SETKNOB(Verbose) ;
2881 SETKNOB(FixedSpin) ;
2882 SETKNOB(SpinLimit) ;
2883 SETKNOB(SpinBase) ;
2884 SETKNOB(SpinBackOff);
2885 SETKNOB(CASPenalty) ;
2886 SETKNOB(OXPenalty) ;
2887 SETKNOB(LogSpins) ;
2888 SETKNOB(SpinSetSucc) ;
2889 SETKNOB(SuccEnabled) ;
2890 SETKNOB(SuccRestrict) ;
2891 SETKNOB(Penalty) ;
2892 SETKNOB(Bonus) ;
2893 SETKNOB(BonusB) ;
2894 SETKNOB(Poverty) ;
2895 SETKNOB(SpinAfterFutile) ;
2896 SETKNOB(UsePause) ;
2897 SETKNOB(SpinEarly) ;
2898 SETKNOB(OState) ;
2899 SETKNOB(MaxSpinners) ;
2900 SETKNOB(PreSpin) ;
2901 SETKNOB(ExitPolicy) ;
2902 SETKNOB(QMode);
2903 SETKNOB(ResetEvent) ;
2904 SETKNOB(MoveNotifyee) ;
2905 SETKNOB(FastHSSEC) ;
2906 #undef SETKNOB
2908 if (os::is_MP()) {
2909 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2910 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2911 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2912 } else {
2913 Knob_SpinLimit = 0 ;
2914 Knob_SpinBase = 0 ;
2915 Knob_PreSpin = 0 ;
2916 Knob_FixedSpin = -1 ;
2917 }
2919 if (Knob_LogSpins == 0) {
2920 ObjectSynchronizer::_sync_FailedSpins = NULL ;
2921 }
2923 free (knobs) ;
2924 OrderAccess::fence() ;
2925 InitDone = 1 ;
2926 }
2928 // Theory of operations -- Monitors lists, thread residency, etc:
2929 //
2930 // * A thread acquires ownership of a monitor by successfully
2931 // CAS()ing the _owner field from null to non-null.
2932 //
2933 // * Invariant: A thread appears on at most one monitor list --
2934 // cxq, EntryList or WaitSet -- at any one time.
2935 //
2936 // * Contending threads "push" themselves onto the cxq with CAS
2937 // and then spin/park.
2938 //
2939 // * After a contending thread eventually acquires the lock it must
2940 // dequeue itself from either the EntryList or the cxq.
2941 //
2942 // * The exiting thread identifies and unparks an "heir presumptive"
2943 // tentative successor thread on the EntryList. Critically, the
2944 // exiting thread doesn't unlink the successor thread from the EntryList.
2945 // After having been unparked, the wakee will recontend for ownership of
2946 // the monitor. The successor (wakee) will either acquire the lock or
2947 // re-park itself.
2948 //
2949 // Succession is provided for by a policy of competitive handoff.
2950 // The exiting thread does _not_ grant or pass ownership to the
2951 // successor thread. (This is also referred to as "handoff" succession").
2952 // Instead the exiting thread releases ownership and possibly wakes
2953 // a successor, so the successor can (re)compete for ownership of the lock.
2954 // If the EntryList is empty but the cxq is populated the exiting
2955 // thread will drain the cxq into the EntryList. It does so by
2956 // by detaching the cxq (installing null with CAS) and folding
2957 // the threads from the cxq into the EntryList. The EntryList is
2958 // doubly linked, while the cxq is singly linked because of the
2959 // CAS-based "push" used to enqueue recently arrived threads (RATs).
2960 //
2961 // * Concurrency invariants:
2962 //
2963 // -- only the monitor owner may access or mutate the EntryList.
2964 // The mutex property of the monitor itself protects the EntryList
2965 // from concurrent interference.
2966 // -- Only the monitor owner may detach the cxq.
2967 //
2968 // * The monitor entry list operations avoid locks, but strictly speaking
2969 // they're not lock-free. Enter is lock-free, exit is not.
2970 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
2971 //
2972 // * The cxq can have multiple concurrent "pushers" but only one concurrent
2973 // detaching thread. This mechanism is immune from the ABA corruption.
2974 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
2975 //
2976 // * Taken together, the cxq and the EntryList constitute or form a
2977 // single logical queue of threads stalled trying to acquire the lock.
2978 // We use two distinct lists to improve the odds of a constant-time
2979 // dequeue operation after acquisition (in the ::enter() epilog) and
2980 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
2981 // A key desideratum is to minimize queue & monitor metadata manipulation
2982 // that occurs while holding the monitor lock -- that is, we want to
2983 // minimize monitor lock holds times. Note that even a small amount of
2984 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
2985 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
2986 // locks and monitor metadata.
2987 //
2988 // Cxq points to the the set of Recently Arrived Threads attempting entry.
2989 // Because we push threads onto _cxq with CAS, the RATs must take the form of
2990 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
2991 // the unlocking thread notices that EntryList is null but _cxq is != null.
2992 //
2993 // The EntryList is ordered by the prevailing queue discipline and
2994 // can be organized in any convenient fashion, such as a doubly-linked list or
2995 // a circular doubly-linked list. Critically, we want insert and delete operations
2996 // to operate in constant-time. If we need a priority queue then something akin
2997 // to Solaris' sleepq would work nicely. Viz.,
2998 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
2999 // Queue discipline is enforced at ::exit() time, when the unlocking thread
3000 // drains the cxq into the EntryList, and orders or reorders the threads on the
3001 // EntryList accordingly.
3002 //
3003 // Barring "lock barging", this mechanism provides fair cyclic ordering,
3004 // somewhat similar to an elevator-scan.
3005 //
3006 // * The monitor synchronization subsystem avoids the use of native
3007 // synchronization primitives except for the narrow platform-specific
3008 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
3009 // the semantics of park-unpark. Put another way, this monitor implementation
3010 // depends only on atomic operations and park-unpark. The monitor subsystem
3011 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
3012 // underlying OS manages the READY<->RUN transitions.
3013 //
3014 // * Waiting threads reside on the WaitSet list -- wait() puts
3015 // the caller onto the WaitSet.
3016 //
3017 // * notify() or notifyAll() simply transfers threads from the WaitSet to
3018 // either the EntryList or cxq. Subsequent exit() operations will
3019 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
3020 // it's likely the notifyee would simply impale itself on the lock held
3021 // by the notifier.
3022 //
3023 // * An interesting alternative is to encode cxq as (List,LockByte) where
3024 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
3025 // variable, like _recursions, in the scheme. The threads or Events that form
3026 // the list would have to be aligned in 256-byte addresses. A thread would
3027 // try to acquire the lock or enqueue itself with CAS, but exiting threads
3028 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
3029 // Note that is is *not* word-tearing, but it does presume that full-word
3030 // CAS operations are coherent with intermix with STB operations. That's true
3031 // on most common processors.
3032 //
3033 // * See also http://blogs.sun.com/dave
3036 void ATTR ObjectMonitor::EnterI (TRAPS) {
3037 Thread * Self = THREAD ;
3038 assert (Self->is_Java_thread(), "invariant") ;
3039 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
3041 // Try the lock - TATAS
3042 if (TryLock (Self) > 0) {
3043 assert (_succ != Self , "invariant") ;
3044 assert (_owner == Self , "invariant") ;
3045 assert (_Responsible != Self , "invariant") ;
3046 return ;
3047 }
3049 DeferredInitialize () ;
3051 // We try one round of spinning *before* enqueueing Self.
3052 //
3053 // If the _owner is ready but OFFPROC we could use a YieldTo()
3054 // operation to donate the remainder of this thread's quantum
3055 // to the owner. This has subtle but beneficial affinity
3056 // effects.
3058 if (TrySpin (Self) > 0) {
3059 assert (_owner == Self , "invariant") ;
3060 assert (_succ != Self , "invariant") ;
3061 assert (_Responsible != Self , "invariant") ;
3062 return ;
3063 }
3065 // The Spin failed -- Enqueue and park the thread ...
3066 assert (_succ != Self , "invariant") ;
3067 assert (_owner != Self , "invariant") ;
3068 assert (_Responsible != Self , "invariant") ;
3070 // Enqueue "Self" on ObjectMonitor's _cxq.
3071 //
3072 // Node acts as a proxy for Self.
3073 // As an aside, if were to ever rewrite the synchronization code mostly
3074 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
3075 // Java objects. This would avoid awkward lifecycle and liveness issues,
3076 // as well as eliminate a subset of ABA issues.
3077 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
3078 //
3080 ObjectWaiter node(Self) ;
3081 Self->_ParkEvent->reset() ;
3082 node._prev = (ObjectWaiter *) 0xBAD ;
3083 node.TState = ObjectWaiter::TS_CXQ ;
3085 // Push "Self" onto the front of the _cxq.
3086 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
3087 // Note that spinning tends to reduce the rate at which threads
3088 // enqueue and dequeue on EntryList|cxq.
3089 ObjectWaiter * nxt ;
3090 for (;;) {
3091 node._next = nxt = _cxq ;
3092 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
3094 // Interference - the CAS failed because _cxq changed. Just retry.
3095 // As an optional optimization we retry the lock.
3096 if (TryLock (Self) > 0) {
3097 assert (_succ != Self , "invariant") ;
3098 assert (_owner == Self , "invariant") ;
3099 assert (_Responsible != Self , "invariant") ;
3100 return ;
3101 }
3102 }
3104 // Check for cxq|EntryList edge transition to non-null. This indicates
3105 // the onset of contention. While contention persists exiting threads
3106 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
3107 // operations revert to the faster 1-0 mode. This enter operation may interleave
3108 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
3109 // arrange for one of the contending thread to use a timed park() operations
3110 // to detect and recover from the race. (Stranding is form of progress failure
3111 // where the monitor is unlocked but all the contending threads remain parked).
3112 // That is, at least one of the contended threads will periodically poll _owner.
3113 // One of the contending threads will become the designated "Responsible" thread.
3114 // The Responsible thread uses a timed park instead of a normal indefinite park
3115 // operation -- it periodically wakes and checks for and recovers from potential
3116 // strandings admitted by 1-0 exit operations. We need at most one Responsible
3117 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
3118 // be responsible for a monitor.
3119 //
3120 // Currently, one of the contended threads takes on the added role of "Responsible".
3121 // A viable alternative would be to use a dedicated "stranding checker" thread
3122 // that periodically iterated over all the threads (or active monitors) and unparked
3123 // successors where there was risk of stranding. This would help eliminate the
3124 // timer scalability issues we see on some platforms as we'd only have one thread
3125 // -- the checker -- parked on a timer.
3127 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
3128 // Try to assume the role of responsible thread for the monitor.
3129 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
3130 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
3131 }
3133 // The lock have been released while this thread was occupied queueing
3134 // itself onto _cxq. To close the race and avoid "stranding" and
3135 // progress-liveness failure we must resample-retry _owner before parking.
3136 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
3137 // In this case the ST-MEMBAR is accomplished with CAS().
3138 //
3139 // TODO: Defer all thread state transitions until park-time.
3140 // Since state transitions are heavy and inefficient we'd like
3141 // to defer the state transitions until absolutely necessary,
3142 // and in doing so avoid some transitions ...
3144 TEVENT (Inflated enter - Contention) ;
3145 int nWakeups = 0 ;
3146 int RecheckInterval = 1 ;
3148 for (;;) {
3150 if (TryLock (Self) > 0) break ;
3151 assert (_owner != Self, "invariant") ;
3153 if ((SyncFlags & 2) && _Responsible == NULL) {
3154 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
3155 }
3157 // park self
3158 if (_Responsible == Self || (SyncFlags & 1)) {
3159 TEVENT (Inflated enter - park TIMED) ;
3160 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
3161 // Increase the RecheckInterval, but clamp the value.
3162 RecheckInterval *= 8 ;
3163 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
3164 } else {
3165 TEVENT (Inflated enter - park UNTIMED) ;
3166 Self->_ParkEvent->park() ;
3167 }
3169 if (TryLock(Self) > 0) break ;
3171 // The lock is still contested.
3172 // Keep a tally of the # of futile wakeups.
3173 // Note that the counter is not protected by a lock or updated by atomics.
3174 // That is by design - we trade "lossy" counters which are exposed to
3175 // races during updates for a lower probe effect.
3176 TEVENT (Inflated enter - Futile wakeup) ;
3177 if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
3178 ObjectSynchronizer::_sync_FutileWakeups->inc() ;
3179 }
3180 ++ nWakeups ;
3182 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
3183 // We can defer clearing _succ until after the spin completes
3184 // TrySpin() must tolerate being called with _succ == Self.
3185 // Try yet another round of adaptive spinning.
3186 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
3188 // We can find that we were unpark()ed and redesignated _succ while
3189 // we were spinning. That's harmless. If we iterate and call park(),
3190 // park() will consume the event and return immediately and we'll
3191 // just spin again. This pattern can repeat, leaving _succ to simply
3192 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
3193 // Alternately, we can sample fired() here, and if set, forgo spinning
3194 // in the next iteration.
3196 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
3197 Self->_ParkEvent->reset() ;
3198 OrderAccess::fence() ;
3199 }
3200 if (_succ == Self) _succ = NULL ;
3202 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
3203 OrderAccess::fence() ;
3204 }
3206 // Egress :
3207 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
3208 // Normally we'll find Self on the EntryList .
3209 // From the perspective of the lock owner (this thread), the
3210 // EntryList is stable and cxq is prepend-only.
3211 // The head of cxq is volatile but the interior is stable.
3212 // In addition, Self.TState is stable.
3214 assert (_owner == Self , "invariant") ;
3215 assert (object() != NULL , "invariant") ;
3216 // I'd like to write:
3217 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3218 // but as we're at a safepoint that's not safe.
3220 UnlinkAfterAcquire (Self, &node) ;
3221 if (_succ == Self) _succ = NULL ;
3223 assert (_succ != Self, "invariant") ;
3224 if (_Responsible == Self) {
3225 _Responsible = NULL ;
3226 // Dekker pivot-point.
3227 // Consider OrderAccess::storeload() here
3229 // We may leave threads on cxq|EntryList without a designated
3230 // "Responsible" thread. This is benign. When this thread subsequently
3231 // exits the monitor it can "see" such preexisting "old" threads --
3232 // threads that arrived on the cxq|EntryList before the fence, above --
3233 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
3234 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
3235 // non-null and elect a new "Responsible" timer thread.
3236 //
3237 // This thread executes:
3238 // ST Responsible=null; MEMBAR (in enter epilog - here)
3239 // LD cxq|EntryList (in subsequent exit)
3240 //
3241 // Entering threads in the slow/contended path execute:
3242 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
3243 // The (ST cxq; MEMBAR) is accomplished with CAS().
3244 //
3245 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
3246 // exit operation from floating above the ST Responsible=null.
3247 //
3248 // In *practice* however, EnterI() is always followed by some atomic
3249 // operation such as the decrement of _count in ::enter(). Those atomics
3250 // obviate the need for the explicit MEMBAR, above.
3251 }
3253 // We've acquired ownership with CAS().
3254 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
3255 // But since the CAS() this thread may have also stored into _succ,
3256 // EntryList, cxq or Responsible. These meta-data updates must be
3257 // visible __before this thread subsequently drops the lock.
3258 // Consider what could occur if we didn't enforce this constraint --
3259 // STs to monitor meta-data and user-data could reorder with (become
3260 // visible after) the ST in exit that drops ownership of the lock.
3261 // Some other thread could then acquire the lock, but observe inconsistent
3262 // or old monitor meta-data and heap data. That violates the JMM.
3263 // To that end, the 1-0 exit() operation must have at least STST|LDST
3264 // "release" barrier semantics. Specifically, there must be at least a
3265 // STST|LDST barrier in exit() before the ST of null into _owner that drops
3266 // the lock. The barrier ensures that changes to monitor meta-data and data
3267 // protected by the lock will be visible before we release the lock, and
3268 // therefore before some other thread (CPU) has a chance to acquire the lock.
3269 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
3270 //
3271 // Critically, any prior STs to _succ or EntryList must be visible before
3272 // the ST of null into _owner in the *subsequent* (following) corresponding
3273 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
3274 // execute a serializing instruction.
3276 if (SyncFlags & 8) {
3277 OrderAccess::fence() ;
3278 }
3279 return ;
3280 }
3282 // ExitSuspendEquivalent:
3283 // A faster alternate to handle_special_suspend_equivalent_condition()
3284 //
3285 // handle_special_suspend_equivalent_condition() unconditionally
3286 // acquires the SR_lock. On some platforms uncontended MutexLocker()
3287 // operations have high latency. Note that in ::enter() we call HSSEC
3288 // while holding the monitor, so we effectively lengthen the critical sections.
3289 //
3290 // There are a number of possible solutions:
3291 //
3292 // A. To ameliorate the problem we might also defer state transitions
3293 // to as late as possible -- just prior to parking.
3294 // Given that, we'd call HSSEC after having returned from park(),
3295 // but before attempting to acquire the monitor. This is only a
3296 // partial solution. It avoids calling HSSEC while holding the
3297 // monitor (good), but it still increases successor reacquisition latency --
3298 // the interval between unparking a successor and the time the successor
3299 // resumes and retries the lock. See ReenterI(), which defers state transitions.
3300 // If we use this technique we can also avoid EnterI()-exit() loop
3301 // in ::enter() where we iteratively drop the lock and then attempt
3302 // to reacquire it after suspending.
3303 //
3304 // B. In the future we might fold all the suspend bits into a
3305 // composite per-thread suspend flag and then update it with CAS().
3306 // Alternately, a Dekker-like mechanism with multiple variables
3307 // would suffice:
3308 // ST Self->_suspend_equivalent = false
3309 // MEMBAR
3310 // LD Self_>_suspend_flags
3311 //
3314 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
3315 int Mode = Knob_FastHSSEC ;
3316 if (Mode && !jSelf->is_external_suspend()) {
3317 assert (jSelf->is_suspend_equivalent(), "invariant") ;
3318 jSelf->clear_suspend_equivalent() ;
3319 if (2 == Mode) OrderAccess::storeload() ;
3320 if (!jSelf->is_external_suspend()) return false ;
3321 // We raced a suspension -- fall thru into the slow path
3322 TEVENT (ExitSuspendEquivalent - raced) ;
3323 jSelf->set_suspend_equivalent() ;
3324 }
3325 return jSelf->handle_special_suspend_equivalent_condition() ;
3326 }
3329 // ReenterI() is a specialized inline form of the latter half of the
3330 // contended slow-path from EnterI(). We use ReenterI() only for
3331 // monitor reentry in wait().
3332 //
3333 // In the future we should reconcile EnterI() and ReenterI(), adding
3334 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
3335 // loop accordingly.
3337 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
3338 assert (Self != NULL , "invariant") ;
3339 assert (SelfNode != NULL , "invariant") ;
3340 assert (SelfNode->_thread == Self , "invariant") ;
3341 assert (_waiters > 0 , "invariant") ;
3342 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
3343 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
3344 JavaThread * jt = (JavaThread *) Self ;
3346 int nWakeups = 0 ;
3347 for (;;) {
3348 ObjectWaiter::TStates v = SelfNode->TState ;
3349 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
3350 assert (_owner != Self, "invariant") ;
3352 if (TryLock (Self) > 0) break ;
3353 if (TrySpin (Self) > 0) break ;
3355 TEVENT (Wait Reentry - parking) ;
3357 // State transition wrappers around park() ...
3358 // ReenterI() wisely defers state transitions until
3359 // it's clear we must park the thread.
3360 {
3361 OSThreadContendState osts(Self->osthread());
3362 ThreadBlockInVM tbivm(jt);
3364 // cleared by handle_special_suspend_equivalent_condition()
3365 // or java_suspend_self()
3366 jt->set_suspend_equivalent();
3367 if (SyncFlags & 1) {
3368 Self->_ParkEvent->park ((jlong)1000) ;
3369 } else {
3370 Self->_ParkEvent->park () ;
3371 }
3373 // were we externally suspended while we were waiting?
3374 for (;;) {
3375 if (!ExitSuspendEquivalent (jt)) break ;
3376 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
3377 jt->java_suspend_self();
3378 jt->set_suspend_equivalent();
3379 }
3380 }
3382 // Try again, but just so we distinguish between futile wakeups and
3383 // successful wakeups. The following test isn't algorithmically
3384 // necessary, but it helps us maintain sensible statistics.
3385 if (TryLock(Self) > 0) break ;
3387 // The lock is still contested.
3388 // Keep a tally of the # of futile wakeups.
3389 // Note that the counter is not protected by a lock or updated by atomics.
3390 // That is by design - we trade "lossy" counters which are exposed to
3391 // races during updates for a lower probe effect.
3392 TEVENT (Wait Reentry - futile wakeup) ;
3393 ++ nWakeups ;
3395 // Assuming this is not a spurious wakeup we'll normally
3396 // find that _succ == Self.
3397 if (_succ == Self) _succ = NULL ;
3399 // Invariant: after clearing _succ a contending thread
3400 // *must* retry _owner before parking.
3401 OrderAccess::fence() ;
3403 if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
3404 ObjectSynchronizer::_sync_FutileWakeups->inc() ;
3405 }
3406 }
3408 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
3409 // Normally we'll find Self on the EntryList.
3410 // Unlinking from the EntryList is constant-time and atomic-free.
3411 // From the perspective of the lock owner (this thread), the
3412 // EntryList is stable and cxq is prepend-only.
3413 // The head of cxq is volatile but the interior is stable.
3414 // In addition, Self.TState is stable.
3416 assert (_owner == Self, "invariant") ;
3417 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3418 UnlinkAfterAcquire (Self, SelfNode) ;
3419 if (_succ == Self) _succ = NULL ;
3420 assert (_succ != Self, "invariant") ;
3421 SelfNode->TState = ObjectWaiter::TS_RUN ;
3422 OrderAccess::fence() ; // see comments at the end of EnterI()
3423 }
3425 bool ObjectMonitor::try_enter(Thread* THREAD) {
3426 if (THREAD != _owner) {
3427 if (THREAD->is_lock_owned ((address)_owner)) {
3428 assert(_recursions == 0, "internal state error");
3429 _owner = THREAD ;
3430 _recursions = 1 ;
3431 OwnerIsThread = 1 ;
3432 return true;
3433 }
3434 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3435 return false;
3436 }
3437 return true;
3438 } else {
3439 _recursions++;
3440 return true;
3441 }
3442 }
3444 void ATTR ObjectMonitor::enter(TRAPS) {
3445 // The following code is ordered to check the most common cases first
3446 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
3447 Thread * const Self = THREAD ;
3448 void * cur ;
3450 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
3451 if (cur == NULL) {
3452 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
3453 assert (_recursions == 0 , "invariant") ;
3454 assert (_owner == Self, "invariant") ;
3455 // CONSIDER: set or assert OwnerIsThread == 1
3456 return ;
3457 }
3459 if (cur == Self) {
3460 // TODO-FIXME: check for integer overflow! BUGID 6557169.
3461 _recursions ++ ;
3462 return ;
3463 }
3465 if (Self->is_lock_owned ((address)cur)) {
3466 assert (_recursions == 0, "internal state error");
3467 _recursions = 1 ;
3468 // Commute owner from a thread-specific on-stack BasicLockObject address to
3469 // a full-fledged "Thread *".
3470 _owner = Self ;
3471 OwnerIsThread = 1 ;
3472 return ;
3473 }
3475 // We've encountered genuine contention.
3476 assert (Self->_Stalled == 0, "invariant") ;
3477 Self->_Stalled = intptr_t(this) ;
3479 // Try one round of spinning *before* enqueueing Self
3480 // and before going through the awkward and expensive state
3481 // transitions. The following spin is strictly optional ...
3482 // Note that if we acquire the monitor from an initial spin
3483 // we forgo posting JVMTI events and firing DTRACE probes.
3484 if (Knob_SpinEarly && TrySpin (Self) > 0) {
3485 assert (_owner == Self , "invariant") ;
3486 assert (_recursions == 0 , "invariant") ;
3487 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3488 Self->_Stalled = 0 ;
3489 return ;
3490 }
3492 assert (_owner != Self , "invariant") ;
3493 assert (_succ != Self , "invariant") ;
3494 assert (Self->is_Java_thread() , "invariant") ;
3495 JavaThread * jt = (JavaThread *) Self ;
3496 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
3497 assert (jt->thread_state() != _thread_blocked , "invariant") ;
3498 assert (this->object() != NULL , "invariant") ;
3499 assert (_count >= 0, "invariant") ;
3501 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
3502 // Ensure the object-monitor relationship remains stable while there's contention.
3503 Atomic::inc_ptr(&_count);
3505 { // Change java thread status to indicate blocked on monitor enter.
3506 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
3508 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
3509 if (JvmtiExport::should_post_monitor_contended_enter()) {
3510 JvmtiExport::post_monitor_contended_enter(jt, this);
3511 }
3513 OSThreadContendState osts(Self->osthread());
3514 ThreadBlockInVM tbivm(jt);
3516 Self->set_current_pending_monitor(this);
3518 // TODO-FIXME: change the following for(;;) loop to straight-line code.
3519 for (;;) {
3520 jt->set_suspend_equivalent();
3521 // cleared by handle_special_suspend_equivalent_condition()
3522 // or java_suspend_self()
3524 EnterI (THREAD) ;
3526 if (!ExitSuspendEquivalent(jt)) break ;
3528 //
3529 // We have acquired the contended monitor, but while we were
3530 // waiting another thread suspended us. We don't want to enter
3531 // the monitor while suspended because that would surprise the
3532 // thread that suspended us.
3533 //
3534 _recursions = 0 ;
3535 _succ = NULL ;
3536 exit (Self) ;
3538 jt->java_suspend_self();
3539 }
3540 Self->set_current_pending_monitor(NULL);
3541 }
3543 Atomic::dec_ptr(&_count);
3544 assert (_count >= 0, "invariant") ;
3545 Self->_Stalled = 0 ;
3547 // Must either set _recursions = 0 or ASSERT _recursions == 0.
3548 assert (_recursions == 0 , "invariant") ;
3549 assert (_owner == Self , "invariant") ;
3550 assert (_succ != Self , "invariant") ;
3551 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3553 // The thread -- now the owner -- is back in vm mode.
3554 // Report the glorious news via TI,DTrace and jvmstat.
3555 // The probe effect is non-trivial. All the reportage occurs
3556 // while we hold the monitor, increasing the length of the critical
3557 // section. Amdahl's parallel speedup law comes vividly into play.
3558 //
3559 // Another option might be to aggregate the events (thread local or
3560 // per-monitor aggregation) and defer reporting until a more opportune
3561 // time -- such as next time some thread encounters contention but has
3562 // yet to acquire the lock. While spinning that thread could
3563 // spinning we could increment JVMStat counters, etc.
3565 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
3566 if (JvmtiExport::should_post_monitor_contended_entered()) {
3567 JvmtiExport::post_monitor_contended_entered(jt, this);
3568 }
3569 if (ObjectSynchronizer::_sync_ContendedLockAttempts != NULL) {
3570 ObjectSynchronizer::_sync_ContendedLockAttempts->inc() ;
3571 }
3572 }
3574 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
3575 assert (_owner == Self, "invariant") ;
3577 // Exit protocol:
3578 // 1. ST _succ = wakee
3579 // 2. membar #loadstore|#storestore;
3580 // 2. ST _owner = NULL
3581 // 3. unpark(wakee)
3583 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
3584 ParkEvent * Trigger = Wakee->_event ;
3586 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
3587 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
3588 // out-of-scope (non-extant).
3589 Wakee = NULL ;
3591 // Drop the lock
3592 OrderAccess::release_store_ptr (&_owner, NULL) ;
3593 OrderAccess::fence() ; // ST _owner vs LD in unpark()
3595 // TODO-FIXME:
3596 // If there's a safepoint pending the best policy would be to
3597 // get _this thread to a safepoint and only wake the successor
3598 // after the safepoint completed. monitorexit uses a "leaf"
3599 // state transition, however, so this thread can't become
3600 // safe at this point in time. (Its stack isn't walkable).
3601 // The next best thing is to defer waking the successor by
3602 // adding to a list of thread to be unparked after at the
3603 // end of the forthcoming STW).
3604 if (SafepointSynchronize::do_call_back()) {
3605 TEVENT (unpark before SAFEPOINT) ;
3606 }
3608 // Possible optimizations ...
3609 //
3610 // * Consider: set Wakee->UnparkTime = timeNow()
3611 // When the thread wakes up it'll compute (timeNow() - Self->UnparkTime()).
3612 // By measuring recent ONPROC latency we can approximate the
3613 // system load. In turn, we can feed that information back
3614 // into the spinning & succession policies.
3615 // (ONPROC latency correlates strongly with load).
3616 //
3617 // * Pull affinity:
3618 // If the wakee is cold then transiently setting it's affinity
3619 // to the current CPU is a good idea.
3620 // See http://j2se.east/~dice/PERSIST/050624-PullAffinity.txt
3621 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
3622 Trigger->unpark() ;
3624 // Maintain stats and report events to JVMTI
3625 if (ObjectSynchronizer::_sync_Parks != NULL) {
3626 ObjectSynchronizer::_sync_Parks->inc() ;
3627 }
3628 }
3631 // exit()
3632 // ~~~~~~
3633 // Note that the collector can't reclaim the objectMonitor or deflate
3634 // the object out from underneath the thread calling ::exit() as the
3635 // thread calling ::exit() never transitions to a stable state.
3636 // This inhibits GC, which in turn inhibits asynchronous (and
3637 // inopportune) reclamation of "this".
3638 //
3639 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
3640 // There's one exception to the claim above, however. EnterI() can call
3641 // exit() to drop a lock if the acquirer has been externally suspended.
3642 // In that case exit() is called with _thread_state as _thread_blocked,
3643 // but the monitor's _count field is > 0, which inhibits reclamation.
3644 //
3645 // 1-0 exit
3646 // ~~~~~~~~
3647 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
3648 // the fast-path operators have been optimized so the common ::exit()
3649 // operation is 1-0. See i486.ad fast_unlock(), for instance.
3650 // The code emitted by fast_unlock() elides the usual MEMBAR. This
3651 // greatly improves latency -- MEMBAR and CAS having considerable local
3652 // latency on modern processors -- but at the cost of "stranding". Absent the
3653 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
3654 // ::enter() path, resulting in the entering thread being stranding
3655 // and a progress-liveness failure. Stranding is extremely rare.
3656 // We use timers (timed park operations) & periodic polling to detect
3657 // and recover from stranding. Potentially stranded threads periodically
3658 // wake up and poll the lock. See the usage of the _Responsible variable.
3659 //
3660 // The CAS() in enter provides for safety and exclusion, while the CAS or
3661 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
3662 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
3663 // We detect and recover from stranding with timers.
3664 //
3665 // If a thread transiently strands it'll park until (a) another
3666 // thread acquires the lock and then drops the lock, at which time the
3667 // exiting thread will notice and unpark the stranded thread, or, (b)
3668 // the timer expires. If the lock is high traffic then the stranding latency
3669 // will be low due to (a). If the lock is low traffic then the odds of
3670 // stranding are lower, although the worst-case stranding latency
3671 // is longer. Critically, we don't want to put excessive load in the
3672 // platform's timer subsystem. We want to minimize both the timer injection
3673 // rate (timers created/sec) as well as the number of timers active at
3674 // any one time. (more precisely, we want to minimize timer-seconds, which is
3675 // the integral of the # of active timers at any instant over time).
3676 // Both impinge on OS scalability. Given that, at most one thread parked on
3677 // a monitor will use a timer.
3679 void ATTR ObjectMonitor::exit(TRAPS) {
3680 Thread * Self = THREAD ;
3681 if (THREAD != _owner) {
3682 if (THREAD->is_lock_owned((address) _owner)) {
3683 // Transmute _owner from a BasicLock pointer to a Thread address.
3684 // We don't need to hold _mutex for this transition.
3685 // Non-null to Non-null is safe as long as all readers can
3686 // tolerate either flavor.
3687 assert (_recursions == 0, "invariant") ;
3688 _owner = THREAD ;
3689 _recursions = 0 ;
3690 OwnerIsThread = 1 ;
3691 } else {
3692 // NOTE: we need to handle unbalanced monitor enter/exit
3693 // in native code by throwing an exception.
3694 // TODO: Throw an IllegalMonitorStateException ?
3695 TEVENT (Exit - Throw IMSX) ;
3696 assert(false, "Non-balanced monitor enter/exit!");
3697 if (false) {
3698 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
3699 }
3700 return;
3701 }
3702 }
3704 if (_recursions != 0) {
3705 _recursions--; // this is simple recursive enter
3706 TEVENT (Inflated exit - recursive) ;
3707 return ;
3708 }
3710 // Invariant: after setting Responsible=null an thread must execute
3711 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
3712 if ((SyncFlags & 4) == 0) {
3713 _Responsible = NULL ;
3714 }
3716 for (;;) {
3717 assert (THREAD == _owner, "invariant") ;
3719 // Fast-path monitor exit:
3720 //
3721 // Observe the Dekker/Lamport duality:
3722 // A thread in ::exit() executes:
3723 // ST Owner=null; MEMBAR; LD EntryList|cxq.
3724 // A thread in the contended ::enter() path executes the complementary:
3725 // ST EntryList|cxq = nonnull; MEMBAR; LD Owner.
3726 //
3727 // Note that there's a benign race in the exit path. We can drop the
3728 // lock, another thread can reacquire the lock immediately, and we can
3729 // then wake a thread unnecessarily (yet another flavor of futile wakeup).
3730 // This is benign, and we've structured the code so the windows are short
3731 // and the frequency of such futile wakeups is low.
3732 //
3733 // We could eliminate the race by encoding both the "LOCKED" state and
3734 // the queue head in a single word. Exit would then use either CAS to
3735 // clear the LOCKED bit/byte. This precludes the desirable 1-0 optimization,
3736 // however.
3737 //
3738 // Possible fast-path ::exit() optimization:
3739 // The current fast-path exit implementation fetches both cxq and EntryList.
3740 // See also i486.ad fast_unlock(). Testing has shown that two LDs
3741 // isn't measurably slower than a single LD on any platforms.
3742 // Still, we could reduce the 2 LDs to one or zero by one of the following:
3743 //
3744 // - Use _count instead of cxq|EntryList
3745 // We intend to eliminate _count, however, when we switch
3746 // to on-the-fly deflation in ::exit() as is used in
3747 // Metalocks and RelaxedLocks.
3748 //
3749 // - Establish the invariant that cxq == null implies EntryList == null.
3750 // set cxq == EMPTY (1) to encode the state where cxq is empty
3751 // by EntryList != null. EMPTY is a distinguished value.
3752 // The fast-path exit() would fetch cxq but not EntryList.
3753 //
3754 // - Encode succ as follows:
3755 // succ = t : Thread t is the successor -- t is ready or is spinning.
3756 // Exiting thread does not need to wake a successor.
3757 // succ = 0 : No successor required -> (EntryList|cxq) == null
3758 // Exiting thread does not need to wake a successor
3759 // succ = 1 : Successor required -> (EntryList|cxq) != null and
3760 // logically succ == null.
3761 // Exiting thread must wake a successor.
3762 //
3763 // The 1-1 fast-exit path would appear as :
3764 // _owner = null ; membar ;
3765 // if (_succ == 1 && CAS (&_owner, null, Self) == null) goto SlowPath
3766 // goto FastPathDone ;
3767 //
3768 // and the 1-0 fast-exit path would appear as:
3769 // if (_succ == 1) goto SlowPath
3770 // Owner = null ;
3771 // goto FastPathDone
3772 //
3773 // - Encode the LSB of _owner as 1 to indicate that exit()
3774 // must use the slow-path and make a successor ready.
3775 // (_owner & 1) == 0 IFF succ != null || (EntryList|cxq) == null
3776 // (_owner & 1) == 0 IFF succ == null && (EntryList|cxq) != null (obviously)
3777 // The 1-0 fast exit path would read:
3778 // if (_owner != Self) goto SlowPath
3779 // _owner = null
3780 // goto FastPathDone
3782 if (Knob_ExitPolicy == 0) {
3783 // release semantics: prior loads and stores from within the critical section
3784 // must not float (reorder) past the following store that drops the lock.
3785 // On SPARC that requires MEMBAR #loadstore|#storestore.
3786 // But of course in TSO #loadstore|#storestore is not required.
3787 // I'd like to write one of the following:
3788 // A. OrderAccess::release() ; _owner = NULL
3789 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
3790 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
3791 // store into a _dummy variable. That store is not needed, but can result
3792 // in massive wasteful coherency traffic on classic SMP systems.
3793 // Instead, I use release_store(), which is implemented as just a simple
3794 // ST on x64, x86 and SPARC.
3795 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
3796 OrderAccess::storeload() ; // See if we need to wake a successor
3797 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
3798 TEVENT (Inflated exit - simple egress) ;
3799 return ;
3800 }
3801 TEVENT (Inflated exit - complex egress) ;
3803 // Normally the exiting thread is responsible for ensuring succession,
3804 // but if other successors are ready or other entering threads are spinning
3805 // then this thread can simply store NULL into _owner and exit without
3806 // waking a successor. The existence of spinners or ready successors
3807 // guarantees proper succession (liveness). Responsibility passes to the
3808 // ready or running successors. The exiting thread delegates the duty.
3809 // More precisely, if a successor already exists this thread is absolved
3810 // of the responsibility of waking (unparking) one.
3811 //
3812 // The _succ variable is critical to reducing futile wakeup frequency.
3813 // _succ identifies the "heir presumptive" thread that has been made
3814 // ready (unparked) but that has not yet run. We need only one such
3815 // successor thread to guarantee progress.
3816 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
3817 // section 3.3 "Futile Wakeup Throttling" for details.
3818 //
3819 // Note that spinners in Enter() also set _succ non-null.
3820 // In the current implementation spinners opportunistically set
3821 // _succ so that exiting threads might avoid waking a successor.
3822 // Another less appealing alternative would be for the exiting thread
3823 // to drop the lock and then spin briefly to see if a spinner managed
3824 // to acquire the lock. If so, the exiting thread could exit
3825 // immediately without waking a successor, otherwise the exiting
3826 // thread would need to dequeue and wake a successor.
3827 // (Note that we'd need to make the post-drop spin short, but no
3828 // shorter than the worst-case round-trip cache-line migration time.
3829 // The dropped lock needs to become visible to the spinner, and then
3830 // the acquisition of the lock by the spinner must become visible to
3831 // the exiting thread).
3832 //
3834 // It appears that an heir-presumptive (successor) must be made ready.
3835 // Only the current lock owner can manipulate the EntryList or
3836 // drain _cxq, so we need to reacquire the lock. If we fail
3837 // to reacquire the lock the responsibility for ensuring succession
3838 // falls to the new owner.
3839 //
3840 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3841 return ;
3842 }
3843 TEVENT (Exit - Reacquired) ;
3844 } else {
3845 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
3846 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
3847 OrderAccess::storeload() ;
3848 // Ratify the previously observed values.
3849 if (_cxq == NULL || _succ != NULL) {
3850 TEVENT (Inflated exit - simple egress) ;
3851 return ;
3852 }
3854 // inopportune interleaving -- the exiting thread (this thread)
3855 // in the fast-exit path raced an entering thread in the slow-enter
3856 // path.
3857 // We have two choices:
3858 // A. Try to reacquire the lock.
3859 // If the CAS() fails return immediately, otherwise
3860 // we either restart/rerun the exit operation, or simply
3861 // fall-through into the code below which wakes a successor.
3862 // B. If the elements forming the EntryList|cxq are TSM
3863 // we could simply unpark() the lead thread and return
3864 // without having set _succ.
3865 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3866 TEVENT (Inflated exit - reacquired succeeded) ;
3867 return ;
3868 }
3869 TEVENT (Inflated exit - reacquired failed) ;
3870 } else {
3871 TEVENT (Inflated exit - complex egress) ;
3872 }
3873 }
3875 guarantee (_owner == THREAD, "invariant") ;
3877 // Select an appropriate successor ("heir presumptive") from the EntryList
3878 // and make it ready. Generally we just wake the head of EntryList .
3879 // There's no algorithmic constraint that we use the head - it's just
3880 // a policy decision. Note that the thread at head of the EntryList
3881 // remains at the head until it acquires the lock. This means we'll
3882 // repeatedly wake the same thread until it manages to grab the lock.
3883 // This is generally a good policy - if we're seeing lots of futile wakeups
3884 // at least we're waking/rewaking a thread that's like to be hot or warm
3885 // (have residual D$ and TLB affinity).
3886 //
3887 // "Wakeup locality" optimization:
3888 // http://j2se.east/~dice/PERSIST/040825-WakeLocality.txt
3889 // In the future we'll try to bias the selection mechanism
3890 // to preferentially pick a thread that recently ran on
3891 // a processor element that shares cache with the CPU on which
3892 // the exiting thread is running. We need access to Solaris'
3893 // schedctl.sc_cpu to make that work.
3894 //
3895 ObjectWaiter * w = NULL ;
3896 int QMode = Knob_QMode ;
3898 if (QMode == 2 && _cxq != NULL) {
3899 // QMode == 2 : cxq has precedence over EntryList.
3900 // Try to directly wake a successor from the cxq.
3901 // If successful, the successor will need to unlink itself from cxq.
3902 w = _cxq ;
3903 assert (w != NULL, "invariant") ;
3904 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3905 ExitEpilog (Self, w) ;
3906 return ;
3907 }
3909 if (QMode == 3 && _cxq != NULL) {
3910 // Aggressively drain cxq into EntryList at the first opportunity.
3911 // This policy ensure that recently-run threads live at the head of EntryList.
3912 // Drain _cxq into EntryList - bulk transfer.
3913 // First, detach _cxq.
3914 // The following loop is tantamount to: w = swap (&cxq, NULL)
3915 w = _cxq ;
3916 for (;;) {
3917 assert (w != NULL, "Invariant") ;
3918 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
3919 if (u == w) break ;
3920 w = u ;
3921 }
3922 assert (w != NULL , "invariant") ;
3924 ObjectWaiter * q = NULL ;
3925 ObjectWaiter * p ;
3926 for (p = w ; p != NULL ; p = p->_next) {
3927 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3928 p->TState = ObjectWaiter::TS_ENTER ;
3929 p->_prev = q ;
3930 q = p ;
3931 }
3933 // Append the RATs to the EntryList
3934 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
3935 ObjectWaiter * Tail ;
3936 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
3937 if (Tail == NULL) {
3938 _EntryList = w ;
3939 } else {
3940 Tail->_next = w ;
3941 w->_prev = Tail ;
3942 }
3944 // Fall thru into code that tries to wake a successor from EntryList
3945 }
3947 if (QMode == 4 && _cxq != NULL) {
3948 // Aggressively drain cxq into EntryList at the first opportunity.
3949 // This policy ensure that recently-run threads live at the head of EntryList.
3951 // Drain _cxq into EntryList - bulk transfer.
3952 // First, detach _cxq.
3953 // The following loop is tantamount to: w = swap (&cxq, NULL)
3954 w = _cxq ;
3955 for (;;) {
3956 assert (w != NULL, "Invariant") ;
3957 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
3958 if (u == w) break ;
3959 w = u ;
3960 }
3961 assert (w != NULL , "invariant") ;
3963 ObjectWaiter * q = NULL ;
3964 ObjectWaiter * p ;
3965 for (p = w ; p != NULL ; p = p->_next) {
3966 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3967 p->TState = ObjectWaiter::TS_ENTER ;
3968 p->_prev = q ;
3969 q = p ;
3970 }
3972 // Prepend the RATs to the EntryList
3973 if (_EntryList != NULL) {
3974 q->_next = _EntryList ;
3975 _EntryList->_prev = q ;
3976 }
3977 _EntryList = w ;
3979 // Fall thru into code that tries to wake a successor from EntryList
3980 }
3982 w = _EntryList ;
3983 if (w != NULL) {
3984 // I'd like to write: guarantee (w->_thread != Self).
3985 // But in practice an exiting thread may find itself on the EntryList.
3986 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
3987 // then calls exit(). Exit release the lock by setting O._owner to NULL.
3988 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
3989 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
3990 // release the lock "O". T2 resumes immediately after the ST of null into
3991 // _owner, above. T2 notices that the EntryList is populated, so it
3992 // reacquires the lock and then finds itself on the EntryList.
3993 // Given all that, we have to tolerate the circumstance where "w" is
3994 // associated with Self.
3995 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
3996 ExitEpilog (Self, w) ;
3997 return ;
3998 }
4000 // If we find that both _cxq and EntryList are null then just
4001 // re-run the exit protocol from the top.
4002 w = _cxq ;
4003 if (w == NULL) continue ;
4005 // Drain _cxq into EntryList - bulk transfer.
4006 // First, detach _cxq.
4007 // The following loop is tantamount to: w = swap (&cxq, NULL)
4008 for (;;) {
4009 assert (w != NULL, "Invariant") ;
4010 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
4011 if (u == w) break ;
4012 w = u ;
4013 }
4014 TEVENT (Inflated exit - drain cxq into EntryList) ;
4016 assert (w != NULL , "invariant") ;
4017 assert (_EntryList == NULL , "invariant") ;
4019 // Convert the LIFO SLL anchored by _cxq into a DLL.
4020 // The list reorganization step operates in O(LENGTH(w)) time.
4021 // It's critical that this step operate quickly as
4022 // "Self" still holds the outer-lock, restricting parallelism
4023 // and effectively lengthening the critical section.
4024 // Invariant: s chases t chases u.
4025 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
4026 // we have faster access to the tail.
4028 if (QMode == 1) {
4029 // QMode == 1 : drain cxq to EntryList, reversing order
4030 // We also reverse the order of the list.
4031 ObjectWaiter * s = NULL ;
4032 ObjectWaiter * t = w ;
4033 ObjectWaiter * u = NULL ;
4034 while (t != NULL) {
4035 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
4036 t->TState = ObjectWaiter::TS_ENTER ;
4037 u = t->_next ;
4038 t->_prev = u ;
4039 t->_next = s ;
4040 s = t;
4041 t = u ;
4042 }
4043 _EntryList = s ;
4044 assert (s != NULL, "invariant") ;
4045 } else {
4046 // QMode == 0 or QMode == 2
4047 _EntryList = w ;
4048 ObjectWaiter * q = NULL ;
4049 ObjectWaiter * p ;
4050 for (p = w ; p != NULL ; p = p->_next) {
4051 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
4052 p->TState = ObjectWaiter::TS_ENTER ;
4053 p->_prev = q ;
4054 q = p ;
4055 }
4056 }
4058 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
4059 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
4061 // See if we can abdicate to a spinner instead of waking a thread.
4062 // A primary goal of the implementation is to reduce the
4063 // context-switch rate.
4064 if (_succ != NULL) continue;
4066 w = _EntryList ;
4067 if (w != NULL) {
4068 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4069 ExitEpilog (Self, w) ;
4070 return ;
4071 }
4072 }
4073 }
4074 // complete_exit exits a lock returning recursion count
4075 // complete_exit/reenter operate as a wait without waiting
4076 // complete_exit requires an inflated monitor
4077 // The _owner field is not always the Thread addr even with an
4078 // inflated monitor, e.g. the monitor can be inflated by a non-owning
4079 // thread due to contention.
4080 intptr_t ObjectMonitor::complete_exit(TRAPS) {
4081 Thread * const Self = THREAD;
4082 assert(Self->is_Java_thread(), "Must be Java thread!");
4083 JavaThread *jt = (JavaThread *)THREAD;
4085 DeferredInitialize();
4087 if (THREAD != _owner) {
4088 if (THREAD->is_lock_owned ((address)_owner)) {
4089 assert(_recursions == 0, "internal state error");
4090 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
4091 _recursions = 0 ;
4092 OwnerIsThread = 1 ;
4093 }
4094 }
4096 guarantee(Self == _owner, "complete_exit not owner");
4097 intptr_t save = _recursions; // record the old recursion count
4098 _recursions = 0; // set the recursion level to be 0
4099 exit (Self) ; // exit the monitor
4100 guarantee (_owner != Self, "invariant");
4101 return save;
4102 }
4104 // reenter() enters a lock and sets recursion count
4105 // complete_exit/reenter operate as a wait without waiting
4106 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
4107 Thread * const Self = THREAD;
4108 assert(Self->is_Java_thread(), "Must be Java thread!");
4109 JavaThread *jt = (JavaThread *)THREAD;
4111 guarantee(_owner != Self, "reenter already owner");
4112 enter (THREAD); // enter the monitor
4113 guarantee (_recursions == 0, "reenter recursion");
4114 _recursions = recursions;
4115 return;
4116 }
4118 // Note: a subset of changes to ObjectMonitor::wait()
4119 // will need to be replicated in complete_exit above
4120 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
4121 Thread * const Self = THREAD ;
4122 assert(Self->is_Java_thread(), "Must be Java thread!");
4123 JavaThread *jt = (JavaThread *)THREAD;
4125 DeferredInitialize () ;
4127 // Throw IMSX or IEX.
4128 CHECK_OWNER();
4130 // check for a pending interrupt
4131 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
4132 // post monitor waited event. Note that this is past-tense, we are done waiting.
4133 if (JvmtiExport::should_post_monitor_waited()) {
4134 // Note: 'false' parameter is passed here because the
4135 // wait was not timed out due to thread interrupt.
4136 JvmtiExport::post_monitor_waited(jt, this, false);
4137 }
4138 TEVENT (Wait - Throw IEX) ;
4139 THROW(vmSymbols::java_lang_InterruptedException());
4140 return ;
4141 }
4142 TEVENT (Wait) ;
4144 assert (Self->_Stalled == 0, "invariant") ;
4145 Self->_Stalled = intptr_t(this) ;
4146 jt->set_current_waiting_monitor(this);
4148 // create a node to be put into the queue
4149 // Critically, after we reset() the event but prior to park(), we must check
4150 // for a pending interrupt.
4151 ObjectWaiter node(Self);
4152 node.TState = ObjectWaiter::TS_WAIT ;
4153 Self->_ParkEvent->reset() ;
4154 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
4156 // Enter the waiting queue, which is a circular doubly linked list in this case
4157 // but it could be a priority queue or any data structure.
4158 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
4159 // by the the owner of the monitor *except* in the case where park()
4160 // returns because of a timeout of interrupt. Contention is exceptionally rare
4161 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
4163 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
4164 AddWaiter (&node) ;
4165 Thread::SpinRelease (&_WaitSetLock) ;
4167 if ((SyncFlags & 4) == 0) {
4168 _Responsible = NULL ;
4169 }
4170 intptr_t save = _recursions; // record the old recursion count
4171 _waiters++; // increment the number of waiters
4172 _recursions = 0; // set the recursion level to be 1
4173 exit (Self) ; // exit the monitor
4174 guarantee (_owner != Self, "invariant") ;
4176 // As soon as the ObjectMonitor's ownership is dropped in the exit()
4177 // call above, another thread can enter() the ObjectMonitor, do the
4178 // notify(), and exit() the ObjectMonitor. If the other thread's
4179 // exit() call chooses this thread as the successor and the unpark()
4180 // call happens to occur while this thread is posting a
4181 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
4182 // handler using RawMonitors and consuming the unpark().
4183 //
4184 // To avoid the problem, we re-post the event. This does no harm
4185 // even if the original unpark() was not consumed because we are the
4186 // chosen successor for this monitor.
4187 if (node._notified != 0 && _succ == Self) {
4188 node._event->unpark();
4189 }
4191 // The thread is on the WaitSet list - now park() it.
4192 // On MP systems it's conceivable that a brief spin before we park
4193 // could be profitable.
4194 //
4195 // TODO-FIXME: change the following logic to a loop of the form
4196 // while (!timeout && !interrupted && _notified == 0) park()
4198 int ret = OS_OK ;
4199 int WasNotified = 0 ;
4200 { // State transition wrappers
4201 OSThread* osthread = Self->osthread();
4202 OSThreadWaitState osts(osthread, true);
4203 {
4204 ThreadBlockInVM tbivm(jt);
4205 // Thread is in thread_blocked state and oop access is unsafe.
4206 jt->set_suspend_equivalent();
4208 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
4209 // Intentionally empty
4210 } else
4211 if (node._notified == 0) {
4212 if (millis <= 0) {
4213 Self->_ParkEvent->park () ;
4214 } else {
4215 ret = Self->_ParkEvent->park (millis) ;
4216 }
4217 }
4219 // were we externally suspended while we were waiting?
4220 if (ExitSuspendEquivalent (jt)) {
4221 // TODO-FIXME: add -- if succ == Self then succ = null.
4222 jt->java_suspend_self();
4223 }
4225 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
4228 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
4229 // from the WaitSet to the EntryList.
4230 // See if we need to remove Node from the WaitSet.
4231 // We use double-checked locking to avoid grabbing _WaitSetLock
4232 // if the thread is not on the wait queue.
4233 //
4234 // Note that we don't need a fence before the fetch of TState.
4235 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
4236 // written by the is thread. (perhaps the fetch might even be satisfied
4237 // by a look-aside into the processor's own store buffer, although given
4238 // the length of the code path between the prior ST and this load that's
4239 // highly unlikely). If the following LD fetches a stale TS_WAIT value
4240 // then we'll acquire the lock and then re-fetch a fresh TState value.
4241 // That is, we fail toward safety.
4243 if (node.TState == ObjectWaiter::TS_WAIT) {
4244 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
4245 if (node.TState == ObjectWaiter::TS_WAIT) {
4246 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
4247 assert(node._notified == 0, "invariant");
4248 node.TState = ObjectWaiter::TS_RUN ;
4249 }
4250 Thread::SpinRelease (&_WaitSetLock) ;
4251 }
4253 // The thread is now either on off-list (TS_RUN),
4254 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
4255 // The Node's TState variable is stable from the perspective of this thread.
4256 // No other threads will asynchronously modify TState.
4257 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
4258 OrderAccess::loadload() ;
4259 if (_succ == Self) _succ = NULL ;
4260 WasNotified = node._notified ;
4262 // Reentry phase -- reacquire the monitor.
4263 // re-enter contended monitor after object.wait().
4264 // retain OBJECT_WAIT state until re-enter successfully completes
4265 // Thread state is thread_in_vm and oop access is again safe,
4266 // although the raw address of the object may have changed.
4267 // (Don't cache naked oops over safepoints, of course).
4269 // post monitor waited event. Note that this is past-tense, we are done waiting.
4270 if (JvmtiExport::should_post_monitor_waited()) {
4271 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
4272 }
4273 OrderAccess::fence() ;
4275 assert (Self->_Stalled != 0, "invariant") ;
4276 Self->_Stalled = 0 ;
4278 assert (_owner != Self, "invariant") ;
4279 ObjectWaiter::TStates v = node.TState ;
4280 if (v == ObjectWaiter::TS_RUN) {
4281 enter (Self) ;
4282 } else {
4283 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
4284 ReenterI (Self, &node) ;
4285 node.wait_reenter_end(this);
4286 }
4288 // Self has reacquired the lock.
4289 // Lifecycle - the node representing Self must not appear on any queues.
4290 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
4291 // want residual elements associated with this thread left on any lists.
4292 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
4293 assert (_owner == Self, "invariant") ;
4294 assert (_succ != Self , "invariant") ;
4295 } // OSThreadWaitState()
4297 jt->set_current_waiting_monitor(NULL);
4299 guarantee (_recursions == 0, "invariant") ;
4300 _recursions = save; // restore the old recursion count
4301 _waiters--; // decrement the number of waiters
4303 // Verify a few postconditions
4304 assert (_owner == Self , "invariant") ;
4305 assert (_succ != Self , "invariant") ;
4306 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
4308 if (SyncFlags & 32) {
4309 OrderAccess::fence() ;
4310 }
4312 // check if the notification happened
4313 if (!WasNotified) {
4314 // no, it could be timeout or Thread.interrupt() or both
4315 // check for interrupt event, otherwise it is timeout
4316 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
4317 TEVENT (Wait - throw IEX from epilog) ;
4318 THROW(vmSymbols::java_lang_InterruptedException());
4319 }
4320 }
4322 // NOTE: Spurious wake up will be consider as timeout.
4323 // Monitor notify has precedence over thread interrupt.
4324 }
4327 // Consider:
4328 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
4329 // then instead of transferring a thread from the WaitSet to the EntryList
4330 // we might just dequeue a thread from the WaitSet and directly unpark() it.
4332 void ObjectMonitor::notify(TRAPS) {
4333 CHECK_OWNER();
4334 if (_WaitSet == NULL) {
4335 TEVENT (Empty-Notify) ;
4336 return ;
4337 }
4338 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
4340 int Policy = Knob_MoveNotifyee ;
4342 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
4343 ObjectWaiter * iterator = DequeueWaiter() ;
4344 if (iterator != NULL) {
4345 TEVENT (Notify1 - Transfer) ;
4346 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
4347 guarantee (iterator->_notified == 0, "invariant") ;
4348 // Disposition - what might we do with iterator ?
4349 // a. add it directly to the EntryList - either tail or head.
4350 // b. push it onto the front of the _cxq.
4351 // For now we use (a).
4352 if (Policy != 4) {
4353 iterator->TState = ObjectWaiter::TS_ENTER ;
4354 }
4355 iterator->_notified = 1 ;
4357 ObjectWaiter * List = _EntryList ;
4358 if (List != NULL) {
4359 assert (List->_prev == NULL, "invariant") ;
4360 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4361 assert (List != iterator, "invariant") ;
4362 }
4364 if (Policy == 0) { // prepend to EntryList
4365 if (List == NULL) {
4366 iterator->_next = iterator->_prev = NULL ;
4367 _EntryList = iterator ;
4368 } else {
4369 List->_prev = iterator ;
4370 iterator->_next = List ;
4371 iterator->_prev = NULL ;
4372 _EntryList = iterator ;
4373 }
4374 } else
4375 if (Policy == 1) { // append to EntryList
4376 if (List == NULL) {
4377 iterator->_next = iterator->_prev = NULL ;
4378 _EntryList = iterator ;
4379 } else {
4380 // CONSIDER: finding the tail currently requires a linear-time walk of
4381 // the EntryList. We can make tail access constant-time by converting to
4382 // a CDLL instead of using our current DLL.
4383 ObjectWaiter * Tail ;
4384 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
4385 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
4386 Tail->_next = iterator ;
4387 iterator->_prev = Tail ;
4388 iterator->_next = NULL ;
4389 }
4390 } else
4391 if (Policy == 2) { // prepend to cxq
4392 // prepend to cxq
4393 if (List == NULL) {
4394 iterator->_next = iterator->_prev = NULL ;
4395 _EntryList = iterator ;
4396 } else {
4397 iterator->TState = ObjectWaiter::TS_CXQ ;
4398 for (;;) {
4399 ObjectWaiter * Front = _cxq ;
4400 iterator->_next = Front ;
4401 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
4402 break ;
4403 }
4404 }
4405 }
4406 } else
4407 if (Policy == 3) { // append to cxq
4408 iterator->TState = ObjectWaiter::TS_CXQ ;
4409 for (;;) {
4410 ObjectWaiter * Tail ;
4411 Tail = _cxq ;
4412 if (Tail == NULL) {
4413 iterator->_next = NULL ;
4414 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
4415 break ;
4416 }
4417 } else {
4418 while (Tail->_next != NULL) Tail = Tail->_next ;
4419 Tail->_next = iterator ;
4420 iterator->_prev = Tail ;
4421 iterator->_next = NULL ;
4422 break ;
4423 }
4424 }
4425 } else {
4426 ParkEvent * ev = iterator->_event ;
4427 iterator->TState = ObjectWaiter::TS_RUN ;
4428 OrderAccess::fence() ;
4429 ev->unpark() ;
4430 }
4432 if (Policy < 4) {
4433 iterator->wait_reenter_begin(this);
4434 }
4436 // _WaitSetLock protects the wait queue, not the EntryList. We could
4437 // move the add-to-EntryList operation, above, outside the critical section
4438 // protected by _WaitSetLock. In practice that's not useful. With the
4439 // exception of wait() timeouts and interrupts the monitor owner
4440 // is the only thread that grabs _WaitSetLock. There's almost no contention
4441 // on _WaitSetLock so it's not profitable to reduce the length of the
4442 // critical section.
4443 }
4445 Thread::SpinRelease (&_WaitSetLock) ;
4447 if (iterator != NULL && ObjectSynchronizer::_sync_Notifications != NULL) {
4448 ObjectSynchronizer::_sync_Notifications->inc() ;
4449 }
4450 }
4453 void ObjectMonitor::notifyAll(TRAPS) {
4454 CHECK_OWNER();
4455 ObjectWaiter* iterator;
4456 if (_WaitSet == NULL) {
4457 TEVENT (Empty-NotifyAll) ;
4458 return ;
4459 }
4460 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
4462 int Policy = Knob_MoveNotifyee ;
4463 int Tally = 0 ;
4464 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
4466 for (;;) {
4467 iterator = DequeueWaiter () ;
4468 if (iterator == NULL) break ;
4469 TEVENT (NotifyAll - Transfer1) ;
4470 ++Tally ;
4472 // Disposition - what might we do with iterator ?
4473 // a. add it directly to the EntryList - either tail or head.
4474 // b. push it onto the front of the _cxq.
4475 // For now we use (a).
4476 //
4477 // TODO-FIXME: currently notifyAll() transfers the waiters one-at-a-time from the waitset
4478 // to the EntryList. This could be done more efficiently with a single bulk transfer,
4479 // but in practice it's not time-critical. Beware too, that in prepend-mode we invert the
4480 // order of the waiters. Lets say that the waitset is "ABCD" and the EntryList is "XYZ".
4481 // After a notifyAll() in prepend mode the waitset will be empty and the EntryList will
4482 // be "DCBAXYZ".
4484 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
4485 guarantee (iterator->_notified == 0, "invariant") ;
4486 iterator->_notified = 1 ;
4487 if (Policy != 4) {
4488 iterator->TState = ObjectWaiter::TS_ENTER ;
4489 }
4491 ObjectWaiter * List = _EntryList ;
4492 if (List != NULL) {
4493 assert (List->_prev == NULL, "invariant") ;
4494 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4495 assert (List != iterator, "invariant") ;
4496 }
4498 if (Policy == 0) { // prepend to EntryList
4499 if (List == NULL) {
4500 iterator->_next = iterator->_prev = NULL ;
4501 _EntryList = iterator ;
4502 } else {
4503 List->_prev = iterator ;
4504 iterator->_next = List ;
4505 iterator->_prev = NULL ;
4506 _EntryList = iterator ;
4507 }
4508 } else
4509 if (Policy == 1) { // append to EntryList
4510 if (List == NULL) {
4511 iterator->_next = iterator->_prev = NULL ;
4512 _EntryList = iterator ;
4513 } else {
4514 // CONSIDER: finding the tail currently requires a linear-time walk of
4515 // the EntryList. We can make tail access constant-time by converting to
4516 // a CDLL instead of using our current DLL.
4517 ObjectWaiter * Tail ;
4518 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
4519 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
4520 Tail->_next = iterator ;
4521 iterator->_prev = Tail ;
4522 iterator->_next = NULL ;
4523 }
4524 } else
4525 if (Policy == 2) { // prepend to cxq
4526 // prepend to cxq
4527 iterator->TState = ObjectWaiter::TS_CXQ ;
4528 for (;;) {
4529 ObjectWaiter * Front = _cxq ;
4530 iterator->_next = Front ;
4531 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
4532 break ;
4533 }
4534 }
4535 } else
4536 if (Policy == 3) { // append to cxq
4537 iterator->TState = ObjectWaiter::TS_CXQ ;
4538 for (;;) {
4539 ObjectWaiter * Tail ;
4540 Tail = _cxq ;
4541 if (Tail == NULL) {
4542 iterator->_next = NULL ;
4543 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
4544 break ;
4545 }
4546 } else {
4547 while (Tail->_next != NULL) Tail = Tail->_next ;
4548 Tail->_next = iterator ;
4549 iterator->_prev = Tail ;
4550 iterator->_next = NULL ;
4551 break ;
4552 }
4553 }
4554 } else {
4555 ParkEvent * ev = iterator->_event ;
4556 iterator->TState = ObjectWaiter::TS_RUN ;
4557 OrderAccess::fence() ;
4558 ev->unpark() ;
4559 }
4561 if (Policy < 4) {
4562 iterator->wait_reenter_begin(this);
4563 }
4565 // _WaitSetLock protects the wait queue, not the EntryList. We could
4566 // move the add-to-EntryList operation, above, outside the critical section
4567 // protected by _WaitSetLock. In practice that's not useful. With the
4568 // exception of wait() timeouts and interrupts the monitor owner
4569 // is the only thread that grabs _WaitSetLock. There's almost no contention
4570 // on _WaitSetLock so it's not profitable to reduce the length of the
4571 // critical section.
4572 }
4574 Thread::SpinRelease (&_WaitSetLock) ;
4576 if (Tally != 0 && ObjectSynchronizer::_sync_Notifications != NULL) {
4577 ObjectSynchronizer::_sync_Notifications->inc(Tally) ;
4578 }
4579 }
4581 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
4582 // TODO-FIXME: remove check_slow() -- it's likely dead.
4584 void ObjectMonitor::check_slow(TRAPS) {
4585 TEVENT (check_slow - throw IMSX) ;
4586 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
4587 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
4588 }
4591 // -------------------------------------------------------------------------
4592 // The raw monitor subsystem is entirely distinct from normal
4593 // java-synchronization or jni-synchronization. raw monitors are not
4594 // associated with objects. They can be implemented in any manner
4595 // that makes sense. The original implementors decided to piggy-back
4596 // the raw-monitor implementation on the existing Java objectMonitor mechanism.
4597 // This flaw needs to fixed. We should reimplement raw monitors as sui-generis.
4598 // Specifically, we should not implement raw monitors via java monitors.
4599 // Time permitting, we should disentangle and deconvolve the two implementations
4600 // and move the resulting raw monitor implementation over to the JVMTI directories.
4601 // Ideally, the raw monitor implementation would be built on top of
4602 // park-unpark and nothing else.
4603 //
4604 // raw monitors are used mainly by JVMTI
4605 // The raw monitor implementation borrows the ObjectMonitor structure,
4606 // but the operators are degenerate and extremely simple.
4607 //
4608 // Mixed use of a single objectMonitor instance -- as both a raw monitor
4609 // and a normal java monitor -- is not permissible.
4610 //
4611 // Note that we use the single RawMonitor_lock to protect queue operations for
4612 // _all_ raw monitors. This is a scalability impediment, but since raw monitor usage
4613 // is deprecated and rare, this is not of concern. The RawMonitor_lock can not
4614 // be held indefinitely. The critical sections must be short and bounded.
4615 //
4616 // -------------------------------------------------------------------------
4618 int ObjectMonitor::SimpleEnter (Thread * Self) {
4619 for (;;) {
4620 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
4621 return OS_OK ;
4622 }
4624 ObjectWaiter Node (Self) ;
4625 Self->_ParkEvent->reset() ; // strictly optional
4626 Node.TState = ObjectWaiter::TS_ENTER ;
4628 RawMonitor_lock->lock_without_safepoint_check() ;
4629 Node._next = _EntryList ;
4630 _EntryList = &Node ;
4631 OrderAccess::fence() ;
4632 if (_owner == NULL && Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
4633 _EntryList = Node._next ;
4634 RawMonitor_lock->unlock() ;
4635 return OS_OK ;
4636 }
4637 RawMonitor_lock->unlock() ;
4638 while (Node.TState == ObjectWaiter::TS_ENTER) {
4639 Self->_ParkEvent->park() ;
4640 }
4641 }
4642 }
4644 int ObjectMonitor::SimpleExit (Thread * Self) {
4645 guarantee (_owner == Self, "invariant") ;
4646 OrderAccess::release_store_ptr (&_owner, NULL) ;
4647 OrderAccess::fence() ;
4648 if (_EntryList == NULL) return OS_OK ;
4649 ObjectWaiter * w ;
4651 RawMonitor_lock->lock_without_safepoint_check() ;
4652 w = _EntryList ;
4653 if (w != NULL) {
4654 _EntryList = w->_next ;
4655 }
4656 RawMonitor_lock->unlock() ;
4657 if (w != NULL) {
4658 guarantee (w ->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4659 ParkEvent * ev = w->_event ;
4660 w->TState = ObjectWaiter::TS_RUN ;
4661 OrderAccess::fence() ;
4662 ev->unpark() ;
4663 }
4664 return OS_OK ;
4665 }
4667 int ObjectMonitor::SimpleWait (Thread * Self, jlong millis) {
4668 guarantee (_owner == Self , "invariant") ;
4669 guarantee (_recursions == 0, "invariant") ;
4671 ObjectWaiter Node (Self) ;
4672 Node._notified = 0 ;
4673 Node.TState = ObjectWaiter::TS_WAIT ;
4675 RawMonitor_lock->lock_without_safepoint_check() ;
4676 Node._next = _WaitSet ;
4677 _WaitSet = &Node ;
4678 RawMonitor_lock->unlock() ;
4680 SimpleExit (Self) ;
4681 guarantee (_owner != Self, "invariant") ;
4683 int ret = OS_OK ;
4684 if (millis <= 0) {
4685 Self->_ParkEvent->park();
4686 } else {
4687 ret = Self->_ParkEvent->park(millis);
4688 }
4690 // If thread still resides on the waitset then unlink it.
4691 // Double-checked locking -- the usage is safe in this context
4692 // as we TState is volatile and the lock-unlock operators are
4693 // serializing (barrier-equivalent).
4695 if (Node.TState == ObjectWaiter::TS_WAIT) {
4696 RawMonitor_lock->lock_without_safepoint_check() ;
4697 if (Node.TState == ObjectWaiter::TS_WAIT) {
4698 // Simple O(n) unlink, but performance isn't critical here.
4699 ObjectWaiter * p ;
4700 ObjectWaiter * q = NULL ;
4701 for (p = _WaitSet ; p != &Node; p = p->_next) {
4702 q = p ;
4703 }
4704 guarantee (p == &Node, "invariant") ;
4705 if (q == NULL) {
4706 guarantee (p == _WaitSet, "invariant") ;
4707 _WaitSet = p->_next ;
4708 } else {
4709 guarantee (p == q->_next, "invariant") ;
4710 q->_next = p->_next ;
4711 }
4712 Node.TState = ObjectWaiter::TS_RUN ;
4713 }
4714 RawMonitor_lock->unlock() ;
4715 }
4717 guarantee (Node.TState == ObjectWaiter::TS_RUN, "invariant") ;
4718 SimpleEnter (Self) ;
4720 guarantee (_owner == Self, "invariant") ;
4721 guarantee (_recursions == 0, "invariant") ;
4722 return ret ;
4723 }
4725 int ObjectMonitor::SimpleNotify (Thread * Self, bool All) {
4726 guarantee (_owner == Self, "invariant") ;
4727 if (_WaitSet == NULL) return OS_OK ;
4729 // We have two options:
4730 // A. Transfer the threads from the WaitSet to the EntryList
4731 // B. Remove the thread from the WaitSet and unpark() it.
4732 //
4733 // We use (B), which is crude and results in lots of futile
4734 // context switching. In particular (B) induces lots of contention.
4736 ParkEvent * ev = NULL ; // consider using a small auto array ...
4737 RawMonitor_lock->lock_without_safepoint_check() ;
4738 for (;;) {
4739 ObjectWaiter * w = _WaitSet ;
4740 if (w == NULL) break ;
4741 _WaitSet = w->_next ;
4742 if (ev != NULL) { ev->unpark(); ev = NULL; }
4743 ev = w->_event ;
4744 OrderAccess::loadstore() ;
4745 w->TState = ObjectWaiter::TS_RUN ;
4746 OrderAccess::storeload();
4747 if (!All) break ;
4748 }
4749 RawMonitor_lock->unlock() ;
4750 if (ev != NULL) ev->unpark();
4751 return OS_OK ;
4752 }
4754 // Any JavaThread will enter here with state _thread_blocked
4755 int ObjectMonitor::raw_enter(TRAPS) {
4756 TEVENT (raw_enter) ;
4757 void * Contended ;
4759 // don't enter raw monitor if thread is being externally suspended, it will
4760 // surprise the suspender if a "suspended" thread can still enter monitor
4761 JavaThread * jt = (JavaThread *)THREAD;
4762 if (THREAD->is_Java_thread()) {
4763 jt->SR_lock()->lock_without_safepoint_check();
4764 while (jt->is_external_suspend()) {
4765 jt->SR_lock()->unlock();
4766 jt->java_suspend_self();
4767 jt->SR_lock()->lock_without_safepoint_check();
4768 }
4769 // guarded by SR_lock to avoid racing with new external suspend requests.
4770 Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
4771 jt->SR_lock()->unlock();
4772 } else {
4773 Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
4774 }
4776 if (Contended == THREAD) {
4777 _recursions ++ ;
4778 return OM_OK ;
4779 }
4781 if (Contended == NULL) {
4782 guarantee (_owner == THREAD, "invariant") ;
4783 guarantee (_recursions == 0, "invariant") ;
4784 return OM_OK ;
4785 }
4787 THREAD->set_current_pending_monitor(this);
4789 if (!THREAD->is_Java_thread()) {
4790 // No other non-Java threads besides VM thread would acquire
4791 // a raw monitor.
4792 assert(THREAD->is_VM_thread(), "must be VM thread");
4793 SimpleEnter (THREAD) ;
4794 } else {
4795 guarantee (jt->thread_state() == _thread_blocked, "invariant") ;
4796 for (;;) {
4797 jt->set_suspend_equivalent();
4798 // cleared by handle_special_suspend_equivalent_condition() or
4799 // java_suspend_self()
4800 SimpleEnter (THREAD) ;
4802 // were we externally suspended while we were waiting?
4803 if (!jt->handle_special_suspend_equivalent_condition()) break ;
4805 // This thread was externally suspended
4806 //
4807 // This logic isn't needed for JVMTI raw monitors,
4808 // but doesn't hurt just in case the suspend rules change. This
4809 // logic is needed for the ObjectMonitor.wait() reentry phase.
4810 // We have reentered the contended monitor, but while we were
4811 // waiting another thread suspended us. We don't want to reenter
4812 // the monitor while suspended because that would surprise the
4813 // thread that suspended us.
4814 //
4815 // Drop the lock -
4816 SimpleExit (THREAD) ;
4818 jt->java_suspend_self();
4819 }
4821 assert(_owner == THREAD, "Fatal error with monitor owner!");
4822 assert(_recursions == 0, "Fatal error with monitor recursions!");
4823 }
4825 THREAD->set_current_pending_monitor(NULL);
4826 guarantee (_recursions == 0, "invariant") ;
4827 return OM_OK;
4828 }
4830 // Used mainly for JVMTI raw monitor implementation
4831 // Also used for ObjectMonitor::wait().
4832 int ObjectMonitor::raw_exit(TRAPS) {
4833 TEVENT (raw_exit) ;
4834 if (THREAD != _owner) {
4835 return OM_ILLEGAL_MONITOR_STATE;
4836 }
4837 if (_recursions > 0) {
4838 --_recursions ;
4839 return OM_OK ;
4840 }
4842 void * List = _EntryList ;
4843 SimpleExit (THREAD) ;
4845 return OM_OK;
4846 }
4848 // Used for JVMTI raw monitor implementation.
4849 // All JavaThreads will enter here with state _thread_blocked
4851 int ObjectMonitor::raw_wait(jlong millis, bool interruptible, TRAPS) {
4852 TEVENT (raw_wait) ;
4853 if (THREAD != _owner) {
4854 return OM_ILLEGAL_MONITOR_STATE;
4855 }
4857 // To avoid spurious wakeups we reset the parkevent -- This is strictly optional.
4858 // The caller must be able to tolerate spurious returns from raw_wait().
4859 THREAD->_ParkEvent->reset() ;
4860 OrderAccess::fence() ;
4862 // check interrupt event
4863 if (interruptible && Thread::is_interrupted(THREAD, true)) {
4864 return OM_INTERRUPTED;
4865 }
4867 intptr_t save = _recursions ;
4868 _recursions = 0 ;
4869 _waiters ++ ;
4870 if (THREAD->is_Java_thread()) {
4871 guarantee (((JavaThread *) THREAD)->thread_state() == _thread_blocked, "invariant") ;
4872 ((JavaThread *)THREAD)->set_suspend_equivalent();
4873 }
4874 int rv = SimpleWait (THREAD, millis) ;
4875 _recursions = save ;
4876 _waiters -- ;
4878 guarantee (THREAD == _owner, "invariant") ;
4879 if (THREAD->is_Java_thread()) {
4880 JavaThread * jSelf = (JavaThread *) THREAD ;
4881 for (;;) {
4882 if (!jSelf->handle_special_suspend_equivalent_condition()) break ;
4883 SimpleExit (THREAD) ;
4884 jSelf->java_suspend_self();
4885 SimpleEnter (THREAD) ;
4886 jSelf->set_suspend_equivalent() ;
4887 }
4888 }
4889 guarantee (THREAD == _owner, "invariant") ;
4891 if (interruptible && Thread::is_interrupted(THREAD, true)) {
4892 return OM_INTERRUPTED;
4893 }
4894 return OM_OK ;
4895 }
4897 int ObjectMonitor::raw_notify(TRAPS) {
4898 TEVENT (raw_notify) ;
4899 if (THREAD != _owner) {
4900 return OM_ILLEGAL_MONITOR_STATE;
4901 }
4902 SimpleNotify (THREAD, false) ;
4903 return OM_OK;
4904 }
4906 int ObjectMonitor::raw_notifyAll(TRAPS) {
4907 TEVENT (raw_notifyAll) ;
4908 if (THREAD != _owner) {
4909 return OM_ILLEGAL_MONITOR_STATE;
4910 }
4911 SimpleNotify (THREAD, true) ;
4912 return OM_OK;
4913 }
4915 #ifndef PRODUCT
4916 void ObjectMonitor::verify() {
4917 }
4919 void ObjectMonitor::print() {
4920 }
4921 #endif
4923 //------------------------------------------------------------------------------
4924 // Non-product code
4926 #ifndef PRODUCT
4928 void ObjectSynchronizer::trace_locking(Handle locking_obj, bool is_compiled,
4929 bool is_method, bool is_locking) {
4930 // Don't know what to do here
4931 }
4933 // Verify all monitors in the monitor cache, the verification is weak.
4934 void ObjectSynchronizer::verify() {
4935 ObjectMonitor* block = gBlockList;
4936 ObjectMonitor* mid;
4937 while (block) {
4938 assert(block->object() == CHAINMARKER, "must be a block header");
4939 for (int i = 1; i < _BLOCKSIZE; i++) {
4940 mid = block + i;
4941 oop object = (oop) mid->object();
4942 if (object != NULL) {
4943 mid->verify();
4944 }
4945 }
4946 block = (ObjectMonitor*) block->FreeNext;
4947 }
4948 }
4950 // Check if monitor belongs to the monitor cache
4951 // The list is grow-only so it's *relatively* safe to traverse
4952 // the list of extant blocks without taking a lock.
4954 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
4955 ObjectMonitor* block = gBlockList;
4957 while (block) {
4958 assert(block->object() == CHAINMARKER, "must be a block header");
4959 if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
4960 address mon = (address) monitor;
4961 address blk = (address) block;
4962 size_t diff = mon - blk;
4963 assert((diff % sizeof(ObjectMonitor)) == 0, "check");
4964 return 1;
4965 }
4966 block = (ObjectMonitor*) block->FreeNext;
4967 }
4968 return 0;
4969 }
4971 #endif