Mon, 25 Jan 2010 18:03:29 -0500
6919980: G1: remove +UseG1GC from under experimental options (second attempt)
Summary: Trying this again, as the original change was lost.
Reviewed-by: ysr, jmasa
1 /*
2 * Copyright 1998-2009 Sun Microsystems, Inc. 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
20 * CA 95054 USA or visit www.sun.com if you need additional information or
21 * have any questions.
22 *
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 ;
190 // Tunables ...
191 // The knob* variables are effectively final. Once set they should
192 // never be modified hence. Consider using __read_mostly with GCC.
194 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
195 static int Knob_HandOff = 0 ;
196 static int Knob_Verbose = 0 ;
197 static int Knob_ReportSettings = 0 ;
199 static int Knob_SpinLimit = 5000 ; // derived by an external tool -
200 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
201 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
202 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
203 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
204 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
205 static int Knob_SpinEarly = 1 ;
206 static int Knob_SuccEnabled = 1 ; // futile wake throttling
207 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
208 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
209 static int Knob_Bonus = 100 ; // spin success bonus
210 static int Knob_BonusB = 100 ; // spin success bonus
211 static int Knob_Penalty = 200 ; // spin failure penalty
212 static int Knob_Poverty = 1000 ;
213 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
214 static int Knob_FixedSpin = 0 ;
215 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
216 static int Knob_UsePause = 1 ;
217 static int Knob_ExitPolicy = 0 ;
218 static int Knob_PreSpin = 10 ; // 20-100 likely better
219 static int Knob_ResetEvent = 0 ;
220 static int BackOffMask = 0 ;
222 static int Knob_FastHSSEC = 0 ;
223 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
224 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
225 static volatile int InitDone = 0 ;
228 // hashCode() generation :
229 //
230 // Possibilities:
231 // * MD5Digest of {obj,stwRandom}
232 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
233 // * A DES- or AES-style SBox[] mechanism
234 // * One of the Phi-based schemes, such as:
235 // 2654435761 = 2^32 * Phi (golden ratio)
236 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
237 // * A variation of Marsaglia's shift-xor RNG scheme.
238 // * (obj ^ stwRandom) is appealing, but can result
239 // in undesirable regularity in the hashCode values of adjacent objects
240 // (objects allocated back-to-back, in particular). This could potentially
241 // result in hashtable collisions and reduced hashtable efficiency.
242 // There are simple ways to "diffuse" the middle address bits over the
243 // generated hashCode values:
244 //
246 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
247 intptr_t value = 0 ;
248 if (hashCode == 0) {
249 // This form uses an unguarded global Park-Miller RNG,
250 // so it's possible for two threads to race and generate the same RNG.
251 // On MP system we'll have lots of RW access to a global, so the
252 // mechanism induces lots of coherency traffic.
253 value = os::random() ;
254 } else
255 if (hashCode == 1) {
256 // This variation has the property of being stable (idempotent)
257 // between STW operations. This can be useful in some of the 1-0
258 // synchronization schemes.
259 intptr_t addrBits = intptr_t(obj) >> 3 ;
260 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
261 } else
262 if (hashCode == 2) {
263 value = 1 ; // for sensitivity testing
264 } else
265 if (hashCode == 3) {
266 value = ++GVars.hcSequence ;
267 } else
268 if (hashCode == 4) {
269 value = intptr_t(obj) ;
270 } else {
271 // Marsaglia's xor-shift scheme with thread-specific state
272 // This is probably the best overall implementation -- we'll
273 // likely make this the default in future releases.
274 unsigned t = Self->_hashStateX ;
275 t ^= (t << 11) ;
276 Self->_hashStateX = Self->_hashStateY ;
277 Self->_hashStateY = Self->_hashStateZ ;
278 Self->_hashStateZ = Self->_hashStateW ;
279 unsigned v = Self->_hashStateW ;
280 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
281 Self->_hashStateW = v ;
282 value = v ;
283 }
285 value &= markOopDesc::hash_mask;
286 if (value == 0) value = 0xBAD ;
287 assert (value != markOopDesc::no_hash, "invariant") ;
288 TEVENT (hashCode: GENERATE) ;
289 return value;
290 }
292 void BasicLock::print_on(outputStream* st) const {
293 st->print("monitor");
294 }
296 void BasicLock::move_to(oop obj, BasicLock* dest) {
297 // Check to see if we need to inflate the lock. This is only needed
298 // if an object is locked using "this" lightweight monitor. In that
299 // case, the displaced_header() is unlocked, because the
300 // displaced_header() contains the header for the originally unlocked
301 // object. However the object could have already been inflated. But it
302 // does not matter, the inflation will just a no-op. For other cases,
303 // the displaced header will be either 0x0 or 0x3, which are location
304 // independent, therefore the BasicLock is free to move.
305 //
306 // During OSR we may need to relocate a BasicLock (which contains a
307 // displaced word) from a location in an interpreter frame to a
308 // new location in a compiled frame. "this" refers to the source
309 // basiclock in the interpreter frame. "dest" refers to the destination
310 // basiclock in the new compiled frame. We *always* inflate in move_to().
311 // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
312 // cause performance problems in code that makes heavy use of a small # of
313 // uncontended locks. (We'd inflate during OSR, and then sync performance
314 // would subsequently plummet because the thread would be forced thru the slow-path).
315 // This problem has been made largely moot on IA32 by inlining the inflated fast-path
316 // operations in Fast_Lock and Fast_Unlock in i486.ad.
317 //
318 // Note that there is a way to safely swing the object's markword from
319 // one stack location to another. This avoids inflation. Obviously,
320 // we need to ensure that both locations refer to the current thread's stack.
321 // There are some subtle concurrency issues, however, and since the benefit is
322 // is small (given the support for inflated fast-path locking in the fast_lock, etc)
323 // we'll leave that optimization for another time.
325 if (displaced_header()->is_neutral()) {
326 ObjectSynchronizer::inflate_helper(obj);
327 // WARNING: We can not put check here, because the inflation
328 // will not update the displaced header. Once BasicLock is inflated,
329 // no one should ever look at its content.
330 } else {
331 // Typically the displaced header will be 0 (recursive stack lock) or
332 // unused_mark. Naively we'd like to assert that the displaced mark
333 // value is either 0, neutral, or 3. But with the advent of the
334 // store-before-CAS avoidance in fast_lock/compiler_lock_object
335 // we can find any flavor mark in the displaced mark.
336 }
337 // [RGV] The next line appears to do nothing!
338 intptr_t dh = (intptr_t) displaced_header();
339 dest->set_displaced_header(displaced_header());
340 }
342 // -----------------------------------------------------------------------------
344 // standard constructor, allows locking failures
345 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
346 _dolock = doLock;
347 _thread = thread;
348 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
349 _obj = obj;
351 if (_dolock) {
352 TEVENT (ObjectLocker) ;
354 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
355 }
356 }
358 ObjectLocker::~ObjectLocker() {
359 if (_dolock) {
360 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
361 }
362 }
364 // -----------------------------------------------------------------------------
367 PerfCounter * ObjectSynchronizer::_sync_Inflations = NULL ;
368 PerfCounter * ObjectSynchronizer::_sync_Deflations = NULL ;
369 PerfCounter * ObjectSynchronizer::_sync_ContendedLockAttempts = NULL ;
370 PerfCounter * ObjectSynchronizer::_sync_FutileWakeups = NULL ;
371 PerfCounter * ObjectSynchronizer::_sync_Parks = NULL ;
372 PerfCounter * ObjectSynchronizer::_sync_EmptyNotifications = NULL ;
373 PerfCounter * ObjectSynchronizer::_sync_Notifications = NULL ;
374 PerfCounter * ObjectSynchronizer::_sync_PrivateA = NULL ;
375 PerfCounter * ObjectSynchronizer::_sync_PrivateB = NULL ;
376 PerfCounter * ObjectSynchronizer::_sync_SlowExit = NULL ;
377 PerfCounter * ObjectSynchronizer::_sync_SlowEnter = NULL ;
378 PerfCounter * ObjectSynchronizer::_sync_SlowNotify = NULL ;
379 PerfCounter * ObjectSynchronizer::_sync_SlowNotifyAll = NULL ;
380 PerfCounter * ObjectSynchronizer::_sync_FailedSpins = NULL ;
381 PerfCounter * ObjectSynchronizer::_sync_SuccessfulSpins = NULL ;
382 PerfCounter * ObjectSynchronizer::_sync_MonInCirculation = NULL ;
383 PerfCounter * ObjectSynchronizer::_sync_MonScavenged = NULL ;
384 PerfLongVariable * ObjectSynchronizer::_sync_MonExtant = NULL ;
386 // One-shot global initialization for the sync subsystem.
387 // We could also defer initialization and initialize on-demand
388 // the first time we call inflate(). Initialization would
389 // be protected - like so many things - by the MonitorCache_lock.
391 void ObjectSynchronizer::Initialize () {
392 static int InitializationCompleted = 0 ;
393 assert (InitializationCompleted == 0, "invariant") ;
394 InitializationCompleted = 1 ;
395 if (UsePerfData) {
396 EXCEPTION_MARK ;
397 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
398 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
399 NEWPERFCOUNTER(_sync_Inflations) ;
400 NEWPERFCOUNTER(_sync_Deflations) ;
401 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
402 NEWPERFCOUNTER(_sync_FutileWakeups) ;
403 NEWPERFCOUNTER(_sync_Parks) ;
404 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
405 NEWPERFCOUNTER(_sync_Notifications) ;
406 NEWPERFCOUNTER(_sync_SlowEnter) ;
407 NEWPERFCOUNTER(_sync_SlowExit) ;
408 NEWPERFCOUNTER(_sync_SlowNotify) ;
409 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
410 NEWPERFCOUNTER(_sync_FailedSpins) ;
411 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
412 NEWPERFCOUNTER(_sync_PrivateA) ;
413 NEWPERFCOUNTER(_sync_PrivateB) ;
414 NEWPERFCOUNTER(_sync_MonInCirculation) ;
415 NEWPERFCOUNTER(_sync_MonScavenged) ;
416 NEWPERFVARIABLE(_sync_MonExtant) ;
417 #undef NEWPERFCOUNTER
418 }
419 }
421 // Compile-time asserts
422 // When possible, it's better to catch errors deterministically at
423 // compile-time than at runtime. The down-side to using compile-time
424 // asserts is that error message -- often something about negative array
425 // indices -- is opaque.
427 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
429 void ObjectMonitor::ctAsserts() {
430 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
431 }
433 static int Adjust (volatile int * adr, int dx) {
434 int v ;
435 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
436 return v ;
437 }
439 // Ad-hoc mutual exclusion primitives: SpinLock and Mux
440 //
441 // We employ SpinLocks _only for low-contention, fixed-length
442 // short-duration critical sections where we're concerned
443 // about native mutex_t or HotSpot Mutex:: latency.
444 // The mux construct provides a spin-then-block mutual exclusion
445 // mechanism.
446 //
447 // Testing has shown that contention on the ListLock guarding gFreeList
448 // is common. If we implement ListLock as a simple SpinLock it's common
449 // for the JVM to devolve to yielding with little progress. This is true
450 // despite the fact that the critical sections protected by ListLock are
451 // extremely short.
452 //
453 // TODO-FIXME: ListLock should be of type SpinLock.
454 // We should make this a 1st-class type, integrated into the lock
455 // hierarchy as leaf-locks. Critically, the SpinLock structure
456 // should have sufficient padding to avoid false-sharing and excessive
457 // cache-coherency traffic.
460 typedef volatile int SpinLockT ;
462 void Thread::SpinAcquire (volatile int * adr, const char * LockName) {
463 if (Atomic::cmpxchg (1, adr, 0) == 0) {
464 return ; // normal fast-path return
465 }
467 // Slow-path : We've encountered contention -- Spin/Yield/Block strategy.
468 TEVENT (SpinAcquire - ctx) ;
469 int ctr = 0 ;
470 int Yields = 0 ;
471 for (;;) {
472 while (*adr != 0) {
473 ++ctr ;
474 if ((ctr & 0xFFF) == 0 || !os::is_MP()) {
475 if (Yields > 5) {
476 // Consider using a simple NakedSleep() instead.
477 // Then SpinAcquire could be called by non-JVM threads
478 Thread::current()->_ParkEvent->park(1) ;
479 } else {
480 os::NakedYield() ;
481 ++Yields ;
482 }
483 } else {
484 SpinPause() ;
485 }
486 }
487 if (Atomic::cmpxchg (1, adr, 0) == 0) return ;
488 }
489 }
491 void Thread::SpinRelease (volatile int * adr) {
492 assert (*adr != 0, "invariant") ;
493 OrderAccess::fence() ; // guarantee at least release consistency.
494 // Roach-motel semantics.
495 // It's safe if subsequent LDs and STs float "up" into the critical section,
496 // but prior LDs and STs within the critical section can't be allowed
497 // to reorder or float past the ST that releases the lock.
498 *adr = 0 ;
499 }
501 // muxAcquire and muxRelease:
502 //
503 // * muxAcquire and muxRelease support a single-word lock-word construct.
504 // The LSB of the word is set IFF the lock is held.
505 // The remainder of the word points to the head of a singly-linked list
506 // of threads blocked on the lock.
507 //
508 // * The current implementation of muxAcquire-muxRelease uses its own
509 // dedicated Thread._MuxEvent instance. If we're interested in
510 // minimizing the peak number of extant ParkEvent instances then
511 // we could eliminate _MuxEvent and "borrow" _ParkEvent as long
512 // as certain invariants were satisfied. Specifically, care would need
513 // to be taken with regards to consuming unpark() "permits".
514 // A safe rule of thumb is that a thread would never call muxAcquire()
515 // if it's enqueued (cxq, EntryList, WaitList, etc) and will subsequently
516 // park(). Otherwise the _ParkEvent park() operation in muxAcquire() could
517 // consume an unpark() permit intended for monitorenter, for instance.
518 // One way around this would be to widen the restricted-range semaphore
519 // implemented in park(). Another alternative would be to provide
520 // multiple instances of the PlatformEvent() for each thread. One
521 // instance would be dedicated to muxAcquire-muxRelease, for instance.
522 //
523 // * Usage:
524 // -- Only as leaf locks
525 // -- for short-term locking only as muxAcquire does not perform
526 // thread state transitions.
527 //
528 // Alternatives:
529 // * We could implement muxAcquire and muxRelease with MCS or CLH locks
530 // but with parking or spin-then-park instead of pure spinning.
531 // * Use Taura-Oyama-Yonenzawa locks.
532 // * It's possible to construct a 1-0 lock if we encode the lockword as
533 // (List,LockByte). Acquire will CAS the full lockword while Release
534 // will STB 0 into the LockByte. The 1-0 scheme admits stranding, so
535 // acquiring threads use timers (ParkTimed) to detect and recover from
536 // the stranding window. Thread/Node structures must be aligned on 256-byte
537 // boundaries by using placement-new.
538 // * Augment MCS with advisory back-link fields maintained with CAS().
539 // Pictorially: LockWord -> T1 <-> T2 <-> T3 <-> ... <-> Tn <-> Owner.
540 // The validity of the backlinks must be ratified before we trust the value.
541 // If the backlinks are invalid the exiting thread must back-track through the
542 // the forward links, which are always trustworthy.
543 // * Add a successor indication. The LockWord is currently encoded as
544 // (List, LOCKBIT:1). We could also add a SUCCBIT or an explicit _succ variable
545 // to provide the usual futile-wakeup optimization.
546 // See RTStt for details.
547 // * Consider schedctl.sc_nopreempt to cover the critical section.
548 //
551 typedef volatile intptr_t MutexT ; // Mux Lock-word
552 enum MuxBits { LOCKBIT = 1 } ;
554 void Thread::muxAcquire (volatile intptr_t * Lock, const char * LockName) {
555 intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
556 if (w == 0) return ;
557 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
558 return ;
559 }
561 TEVENT (muxAcquire - Contention) ;
562 ParkEvent * const Self = Thread::current()->_MuxEvent ;
563 assert ((intptr_t(Self) & LOCKBIT) == 0, "invariant") ;
564 for (;;) {
565 int its = (os::is_MP() ? 100 : 0) + 1 ;
567 // Optional spin phase: spin-then-park strategy
568 while (--its >= 0) {
569 w = *Lock ;
570 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
571 return ;
572 }
573 }
575 Self->reset() ;
576 Self->OnList = intptr_t(Lock) ;
577 // The following fence() isn't _strictly necessary as the subsequent
578 // CAS() both serializes execution and ratifies the fetched *Lock value.
579 OrderAccess::fence();
580 for (;;) {
581 w = *Lock ;
582 if ((w & LOCKBIT) == 0) {
583 if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
584 Self->OnList = 0 ; // hygiene - allows stronger asserts
585 return ;
586 }
587 continue ; // Interference -- *Lock changed -- Just retry
588 }
589 assert (w & LOCKBIT, "invariant") ;
590 Self->ListNext = (ParkEvent *) (w & ~LOCKBIT );
591 if (Atomic::cmpxchg_ptr (intptr_t(Self)|LOCKBIT, Lock, w) == w) break ;
592 }
594 while (Self->OnList != 0) {
595 Self->park() ;
596 }
597 }
598 }
600 void Thread::muxAcquireW (volatile intptr_t * Lock, ParkEvent * ev) {
601 intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
602 if (w == 0) return ;
603 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
604 return ;
605 }
607 TEVENT (muxAcquire - Contention) ;
608 ParkEvent * ReleaseAfter = NULL ;
609 if (ev == NULL) {
610 ev = ReleaseAfter = ParkEvent::Allocate (NULL) ;
611 }
612 assert ((intptr_t(ev) & LOCKBIT) == 0, "invariant") ;
613 for (;;) {
614 guarantee (ev->OnList == 0, "invariant") ;
615 int its = (os::is_MP() ? 100 : 0) + 1 ;
617 // Optional spin phase: spin-then-park strategy
618 while (--its >= 0) {
619 w = *Lock ;
620 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
621 if (ReleaseAfter != NULL) {
622 ParkEvent::Release (ReleaseAfter) ;
623 }
624 return ;
625 }
626 }
628 ev->reset() ;
629 ev->OnList = intptr_t(Lock) ;
630 // The following fence() isn't _strictly necessary as the subsequent
631 // CAS() both serializes execution and ratifies the fetched *Lock value.
632 OrderAccess::fence();
633 for (;;) {
634 w = *Lock ;
635 if ((w & LOCKBIT) == 0) {
636 if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
637 ev->OnList = 0 ;
638 // We call ::Release while holding the outer lock, thus
639 // artificially lengthening the critical section.
640 // Consider deferring the ::Release() until the subsequent unlock(),
641 // after we've dropped the outer lock.
642 if (ReleaseAfter != NULL) {
643 ParkEvent::Release (ReleaseAfter) ;
644 }
645 return ;
646 }
647 continue ; // Interference -- *Lock changed -- Just retry
648 }
649 assert (w & LOCKBIT, "invariant") ;
650 ev->ListNext = (ParkEvent *) (w & ~LOCKBIT );
651 if (Atomic::cmpxchg_ptr (intptr_t(ev)|LOCKBIT, Lock, w) == w) break ;
652 }
654 while (ev->OnList != 0) {
655 ev->park() ;
656 }
657 }
658 }
660 // Release() must extract a successor from the list and then wake that thread.
661 // It can "pop" the front of the list or use a detach-modify-reattach (DMR) scheme
662 // similar to that used by ParkEvent::Allocate() and ::Release(). DMR-based
663 // Release() would :
664 // (A) CAS() or swap() null to *Lock, releasing the lock and detaching the list.
665 // (B) Extract a successor from the private list "in-hand"
666 // (C) attempt to CAS() the residual back into *Lock over null.
667 // If there were any newly arrived threads and the CAS() would fail.
668 // In that case Release() would detach the RATs, re-merge the list in-hand
669 // with the RATs and repeat as needed. Alternately, Release() might
670 // detach and extract a successor, but then pass the residual list to the wakee.
671 // The wakee would be responsible for reattaching and remerging before it
672 // competed for the lock.
673 //
674 // Both "pop" and DMR are immune from ABA corruption -- there can be
675 // multiple concurrent pushers, but only one popper or detacher.
676 // This implementation pops from the head of the list. This is unfair,
677 // but tends to provide excellent throughput as hot threads remain hot.
678 // (We wake recently run threads first).
680 void Thread::muxRelease (volatile intptr_t * Lock) {
681 for (;;) {
682 const intptr_t w = Atomic::cmpxchg_ptr (0, Lock, LOCKBIT) ;
683 assert (w & LOCKBIT, "invariant") ;
684 if (w == LOCKBIT) return ;
685 ParkEvent * List = (ParkEvent *) (w & ~LOCKBIT) ;
686 assert (List != NULL, "invariant") ;
687 assert (List->OnList == intptr_t(Lock), "invariant") ;
688 ParkEvent * nxt = List->ListNext ;
690 // The following CAS() releases the lock and pops the head element.
691 if (Atomic::cmpxchg_ptr (intptr_t(nxt), Lock, w) != w) {
692 continue ;
693 }
694 List->OnList = 0 ;
695 OrderAccess::fence() ;
696 List->unpark () ;
697 return ;
698 }
699 }
701 // ObjectMonitor Lifecycle
702 // -----------------------
703 // Inflation unlinks monitors from the global gFreeList and
704 // associates them with objects. Deflation -- which occurs at
705 // STW-time -- disassociates idle monitors from objects. Such
706 // scavenged monitors are returned to the gFreeList.
707 //
708 // The global list is protected by ListLock. All the critical sections
709 // are short and operate in constant-time.
710 //
711 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
712 //
713 // Lifecycle:
714 // -- unassigned and on the global free list
715 // -- unassigned and on a thread's private omFreeList
716 // -- assigned to an object. The object is inflated and the mark refers
717 // to the objectmonitor.
718 //
719 // TODO-FIXME:
720 //
721 // * We currently protect the gFreeList with a simple lock.
722 // An alternate lock-free scheme would be to pop elements from the gFreeList
723 // with CAS. This would be safe from ABA corruption as long we only
724 // recycled previously appearing elements onto the list in deflate_idle_monitors()
725 // at STW-time. Completely new elements could always be pushed onto the gFreeList
726 // with CAS. Elements that appeared previously on the list could only
727 // be installed at STW-time.
728 //
729 // * For efficiency and to help reduce the store-before-CAS penalty
730 // the objectmonitors on gFreeList or local free lists should be ready to install
731 // with the exception of _header and _object. _object can be set after inflation.
732 // In particular, keep all objectMonitors on a thread's private list in ready-to-install
733 // state with m.Owner set properly.
734 //
735 // * We could all diffuse contention by using multiple global (FreeList, Lock)
736 // pairs -- threads could use trylock() and a cyclic-scan strategy to search for
737 // an unlocked free list.
738 //
739 // * Add lifecycle tags and assert()s.
740 //
741 // * Be more consistent about when we clear an objectmonitor's fields:
742 // A. After extracting the objectmonitor from a free list.
743 // B. After adding an objectmonitor to a free list.
744 //
746 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
747 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ;
748 static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache
749 #define CHAINMARKER ((oop)-1)
751 ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
752 // A large MAXPRIVATE value reduces both list lock contention
753 // and list coherency traffic, but also tends to increase the
754 // number of objectMonitors in circulation as well as the STW
755 // scavenge costs. As usual, we lean toward time in space-time
756 // tradeoffs.
757 const int MAXPRIVATE = 1024 ;
758 for (;;) {
759 ObjectMonitor * m ;
761 // 1: try to allocate from the thread's local omFreeList.
762 // Threads will attempt to allocate first from their local list, then
763 // from the global list, and only after those attempts fail will the thread
764 // attempt to instantiate new monitors. Thread-local free lists take
765 // heat off the ListLock and improve allocation latency, as well as reducing
766 // coherency traffic on the shared global list.
767 m = Self->omFreeList ;
768 if (m != NULL) {
769 Self->omFreeList = m->FreeNext ;
770 Self->omFreeCount -- ;
771 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
772 guarantee (m->object() == NULL, "invariant") ;
773 return m ;
774 }
776 // 2: try to allocate from the global gFreeList
777 // CONSIDER: use muxTry() instead of muxAcquire().
778 // If the muxTry() fails then drop immediately into case 3.
779 // If we're using thread-local free lists then try
780 // to reprovision the caller's free list.
781 if (gFreeList != NULL) {
782 // Reprovision the thread's omFreeList.
783 // Use bulk transfers to reduce the allocation rate and heat
784 // on various locks.
785 Thread::muxAcquire (&ListLock, "omAlloc") ;
786 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
787 ObjectMonitor * take = gFreeList ;
788 gFreeList = take->FreeNext ;
789 guarantee (take->object() == NULL, "invariant") ;
790 guarantee (!take->is_busy(), "invariant") ;
791 take->Recycle() ;
792 omRelease (Self, take) ;
793 }
794 Thread::muxRelease (&ListLock) ;
795 Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;
796 if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
797 TEVENT (omFirst - reprovision) ;
798 continue ;
799 }
801 // 3: allocate a block of new ObjectMonitors
802 // Both the local and global free lists are empty -- resort to malloc().
803 // In the current implementation objectMonitors are TSM - immortal.
804 assert (_BLOCKSIZE > 1, "invariant") ;
805 ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];
807 // NOTE: (almost) no way to recover if allocation failed.
808 // We might be able to induce a STW safepoint and scavenge enough
809 // objectMonitors to permit progress.
810 if (temp == NULL) {
811 vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ;
812 }
814 // Format the block.
815 // initialize the linked list, each monitor points to its next
816 // forming the single linked free list, the very first monitor
817 // will points to next block, which forms the block list.
818 // The trick of using the 1st element in the block as gBlockList
819 // linkage should be reconsidered. A better implementation would
820 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
822 for (int i = 1; i < _BLOCKSIZE ; i++) {
823 temp[i].FreeNext = &temp[i+1];
824 }
826 // terminate the last monitor as the end of list
827 temp[_BLOCKSIZE - 1].FreeNext = NULL ;
829 // Element [0] is reserved for global list linkage
830 temp[0].set_object(CHAINMARKER);
832 // Consider carving out this thread's current request from the
833 // block in hand. This avoids some lock traffic and redundant
834 // list activity.
836 // Acquire the ListLock to manipulate BlockList and FreeList.
837 // An Oyama-Taura-Yonezawa scheme might be more efficient.
838 Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
840 // Add the new block to the list of extant blocks (gBlockList).
841 // The very first objectMonitor in a block is reserved and dedicated.
842 // It serves as blocklist "next" linkage.
843 temp[0].FreeNext = gBlockList;
844 gBlockList = temp;
846 // Add the new string of objectMonitors to the global free list
847 temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;
848 gFreeList = temp + 1;
849 Thread::muxRelease (&ListLock) ;
850 TEVENT (Allocate block of monitors) ;
851 }
852 }
854 // Place "m" on the caller's private per-thread omFreeList.
855 // In practice there's no need to clamp or limit the number of
856 // monitors on a thread's omFreeList as the only time we'll call
857 // omRelease is to return a monitor to the free list after a CAS
858 // attempt failed. This doesn't allow unbounded #s of monitors to
859 // accumulate on a thread's free list.
860 //
861 // In the future the usage of omRelease() might change and monitors
862 // could migrate between free lists. In that case to avoid excessive
863 // accumulation we could limit omCount to (omProvision*2), otherwise return
864 // the objectMonitor to the global list. We should drain (return) in reasonable chunks.
865 // That is, *not* one-at-a-time.
868 void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m) {
869 guarantee (m->object() == NULL, "invariant") ;
870 m->FreeNext = Self->omFreeList ;
871 Self->omFreeList = m ;
872 Self->omFreeCount ++ ;
873 }
875 // Return the monitors of a moribund thread's local free list to
876 // the global free list. Typically a thread calls omFlush() when
877 // it's dying. We could also consider having the VM thread steal
878 // monitors from threads that have not run java code over a few
879 // consecutive STW safepoints. Relatedly, we might decay
880 // omFreeProvision at STW safepoints.
881 //
882 // We currently call omFlush() from the Thread:: dtor _after the thread
883 // has been excised from the thread list and is no longer a mutator.
884 // That means that omFlush() can run concurrently with a safepoint and
885 // the scavenge operator. Calling omFlush() from JavaThread::exit() might
886 // be a better choice as we could safely reason that that the JVM is
887 // not at a safepoint at the time of the call, and thus there could
888 // be not inopportune interleavings between omFlush() and the scavenge
889 // operator.
891 void ObjectSynchronizer::omFlush (Thread * Self) {
892 ObjectMonitor * List = Self->omFreeList ; // Null-terminated SLL
893 Self->omFreeList = NULL ;
894 if (List == NULL) return ;
895 ObjectMonitor * Tail = NULL ;
896 ObjectMonitor * s ;
897 for (s = List ; s != NULL ; s = s->FreeNext) {
898 Tail = s ;
899 guarantee (s->object() == NULL, "invariant") ;
900 guarantee (!s->is_busy(), "invariant") ;
901 s->set_owner (NULL) ; // redundant but good hygiene
902 TEVENT (omFlush - Move one) ;
903 }
905 guarantee (Tail != NULL && List != NULL, "invariant") ;
906 Thread::muxAcquire (&ListLock, "omFlush") ;
907 Tail->FreeNext = gFreeList ;
908 gFreeList = List ;
909 Thread::muxRelease (&ListLock) ;
910 TEVENT (omFlush) ;
911 }
914 // Get the next block in the block list.
915 static inline ObjectMonitor* next(ObjectMonitor* block) {
916 assert(block->object() == CHAINMARKER, "must be a block header");
917 block = block->FreeNext ;
918 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
919 return block;
920 }
922 // Fast path code shared by multiple functions
923 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
924 markOop mark = obj->mark();
925 if (mark->has_monitor()) {
926 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
927 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
928 return mark->monitor();
929 }
930 return ObjectSynchronizer::inflate(Thread::current(), obj);
931 }
933 // Note that we could encounter some performance loss through false-sharing as
934 // multiple locks occupy the same $ line. Padding might be appropriate.
936 #define NINFLATIONLOCKS 256
937 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;
939 static markOop ReadStableMark (oop obj) {
940 markOop mark = obj->mark() ;
941 if (!mark->is_being_inflated()) {
942 return mark ; // normal fast-path return
943 }
945 int its = 0 ;
946 for (;;) {
947 markOop mark = obj->mark() ;
948 if (!mark->is_being_inflated()) {
949 return mark ; // normal fast-path return
950 }
952 // The object is being inflated by some other thread.
953 // The caller of ReadStableMark() must wait for inflation to complete.
954 // Avoid live-lock
955 // TODO: consider calling SafepointSynchronize::do_call_back() while
956 // spinning to see if there's a safepoint pending. If so, immediately
957 // yielding or blocking would be appropriate. Avoid spinning while
958 // there is a safepoint pending.
959 // TODO: add inflation contention performance counters.
960 // TODO: restrict the aggregate number of spinners.
962 ++its ;
963 if (its > 10000 || !os::is_MP()) {
964 if (its & 1) {
965 os::NakedYield() ;
966 TEVENT (Inflate: INFLATING - yield) ;
967 } else {
968 // Note that the following code attenuates the livelock problem but is not
969 // a complete remedy. A more complete solution would require that the inflating
970 // thread hold the associated inflation lock. The following code simply restricts
971 // the number of spinners to at most one. We'll have N-2 threads blocked
972 // on the inflationlock, 1 thread holding the inflation lock and using
973 // a yield/park strategy, and 1 thread in the midst of inflation.
974 // A more refined approach would be to change the encoding of INFLATING
975 // to allow encapsulation of a native thread pointer. Threads waiting for
976 // inflation to complete would use CAS to push themselves onto a singly linked
977 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
978 // and calling park(). When inflation was complete the thread that accomplished inflation
979 // would detach the list and set the markword to inflated with a single CAS and
980 // then for each thread on the list, set the flag and unpark() the thread.
981 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
982 // wakes at most one thread whereas we need to wake the entire list.
983 int ix = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;
984 int YieldThenBlock = 0 ;
985 assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
986 assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
987 Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
988 while (obj->mark() == markOopDesc::INFLATING()) {
989 // Beware: NakedYield() is advisory and has almost no effect on some platforms
990 // so we periodically call Self->_ParkEvent->park(1).
991 // We use a mixed spin/yield/block mechanism.
992 if ((YieldThenBlock++) >= 16) {
993 Thread::current()->_ParkEvent->park(1) ;
994 } else {
995 os::NakedYield() ;
996 }
997 }
998 Thread::muxRelease (InflationLocks + ix ) ;
999 TEVENT (Inflate: INFLATING - yield/park) ;
1000 }
1001 } else {
1002 SpinPause() ; // SMP-polite spinning
1003 }
1004 }
1005 }
1007 ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
1008 // Inflate mutates the heap ...
1009 // Relaxing assertion for bug 6320749.
1010 assert (Universe::verify_in_progress() ||
1011 !SafepointSynchronize::is_at_safepoint(), "invariant") ;
1013 for (;;) {
1014 const markOop mark = object->mark() ;
1015 assert (!mark->has_bias_pattern(), "invariant") ;
1017 // The mark can be in one of the following states:
1018 // * Inflated - just return
1019 // * Stack-locked - coerce it to inflated
1020 // * INFLATING - busy wait for conversion to complete
1021 // * Neutral - aggressively inflate the object.
1022 // * BIASED - Illegal. We should never see this
1024 // CASE: inflated
1025 if (mark->has_monitor()) {
1026 ObjectMonitor * inf = mark->monitor() ;
1027 assert (inf->header()->is_neutral(), "invariant");
1028 assert (inf->object() == object, "invariant") ;
1029 assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1030 return inf ;
1031 }
1033 // CASE: inflation in progress - inflating over a stack-lock.
1034 // Some other thread is converting from stack-locked to inflated.
1035 // Only that thread can complete inflation -- other threads must wait.
1036 // The INFLATING value is transient.
1037 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1038 // We could always eliminate polling by parking the thread on some auxiliary list.
1039 if (mark == markOopDesc::INFLATING()) {
1040 TEVENT (Inflate: spin while INFLATING) ;
1041 ReadStableMark(object) ;
1042 continue ;
1043 }
1045 // CASE: stack-locked
1046 // Could be stack-locked either by this thread or by some other thread.
1047 //
1048 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1049 // to install INFLATING into the mark word. We originally installed INFLATING,
1050 // allocated the objectmonitor, and then finally STed the address of the
1051 // objectmonitor into the mark. This was correct, but artificially lengthened
1052 // the interval in which INFLATED appeared in the mark, thus increasing
1053 // the odds of inflation contention.
1054 //
1055 // We now use per-thread private objectmonitor free lists.
1056 // These list are reprovisioned from the global free list outside the
1057 // critical INFLATING...ST interval. A thread can transfer
1058 // multiple objectmonitors en-mass from the global free list to its local free list.
1059 // This reduces coherency traffic and lock contention on the global free list.
1060 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1061 // before or after the CAS(INFLATING) operation.
1062 // See the comments in omAlloc().
1064 if (mark->has_locker()) {
1065 ObjectMonitor * m = omAlloc (Self) ;
1066 // Optimistically prepare the objectmonitor - anticipate successful CAS
1067 // We do this before the CAS in order to minimize the length of time
1068 // in which INFLATING appears in the mark.
1069 m->Recycle();
1070 m->FreeNext = NULL ;
1071 m->_Responsible = NULL ;
1072 m->OwnerIsThread = 0 ;
1073 m->_recursions = 0 ;
1074 m->_SpinDuration = Knob_SpinLimit ; // Consider: maintain by type/class
1076 markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
1077 if (cmp != mark) {
1078 omRelease (Self, m) ;
1079 continue ; // Interference -- just retry
1080 }
1082 // We've successfully installed INFLATING (0) into the mark-word.
1083 // This is the only case where 0 will appear in a mark-work.
1084 // Only the singular thread that successfully swings the mark-word
1085 // to 0 can perform (or more precisely, complete) inflation.
1086 //
1087 // Why do we CAS a 0 into the mark-word instead of just CASing the
1088 // mark-word from the stack-locked value directly to the new inflated state?
1089 // Consider what happens when a thread unlocks a stack-locked object.
1090 // It attempts to use CAS to swing the displaced header value from the
1091 // on-stack basiclock back into the object header. Recall also that the
1092 // header value (hashcode, etc) can reside in (a) the object header, or
1093 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1094 // header in an objectMonitor. The inflate() routine must copy the header
1095 // value from the basiclock on the owner's stack to the objectMonitor, all
1096 // the while preserving the hashCode stability invariants. If the owner
1097 // decides to release the lock while the value is 0, the unlock will fail
1098 // and control will eventually pass from slow_exit() to inflate. The owner
1099 // will then spin, waiting for the 0 value to disappear. Put another way,
1100 // the 0 causes the owner to stall if the owner happens to try to
1101 // drop the lock (restoring the header from the basiclock to the object)
1102 // while inflation is in-progress. This protocol avoids races that might
1103 // would otherwise permit hashCode values to change or "flicker" for an object.
1104 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1105 // 0 serves as a "BUSY" inflate-in-progress indicator.
1108 // fetch the displaced mark from the owner's stack.
1109 // The owner can't die or unwind past the lock while our INFLATING
1110 // object is in the mark. Furthermore the owner can't complete
1111 // an unlock on the object, either.
1112 markOop dmw = mark->displaced_mark_helper() ;
1113 assert (dmw->is_neutral(), "invariant") ;
1115 // Setup monitor fields to proper values -- prepare the monitor
1116 m->set_header(dmw) ;
1118 // Optimization: if the mark->locker stack address is associated
1119 // with this thread we could simply set m->_owner = Self and
1120 // m->OwnerIsThread = 1. Note that a thread can inflate an object
1121 // that it has stack-locked -- as might happen in wait() -- directly
1122 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1123 m->set_owner(mark->locker());
1124 m->set_object(object);
1125 // TODO-FIXME: assert BasicLock->dhw != 0.
1127 // Must preserve store ordering. The monitor state must
1128 // be stable at the time of publishing the monitor address.
1129 guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
1130 object->release_set_mark(markOopDesc::encode(m));
1132 // Hopefully the performance counters are allocated on distinct cache lines
1133 // to avoid false sharing on MP systems ...
1134 if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
1135 TEVENT(Inflate: overwrite stacklock) ;
1136 if (TraceMonitorInflation) {
1137 if (object->is_instance()) {
1138 ResourceMark rm;
1139 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1140 (intptr_t) object, (intptr_t) object->mark(),
1141 Klass::cast(object->klass())->external_name());
1142 }
1143 }
1144 return m ;
1145 }
1147 // CASE: neutral
1148 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1149 // If we know we're inflating for entry it's better to inflate by swinging a
1150 // pre-locked objectMonitor pointer into the object header. A successful
1151 // CAS inflates the object *and* confers ownership to the inflating thread.
1152 // In the current implementation we use a 2-step mechanism where we CAS()
1153 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1154 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1155 // would be useful.
1157 assert (mark->is_neutral(), "invariant");
1158 ObjectMonitor * m = omAlloc (Self) ;
1159 // prepare m for installation - set monitor to initial state
1160 m->Recycle();
1161 m->set_header(mark);
1162 m->set_owner(NULL);
1163 m->set_object(object);
1164 m->OwnerIsThread = 1 ;
1165 m->_recursions = 0 ;
1166 m->FreeNext = NULL ;
1167 m->_Responsible = NULL ;
1168 m->_SpinDuration = Knob_SpinLimit ; // consider: keep metastats by type/class
1170 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1171 m->set_object (NULL) ;
1172 m->set_owner (NULL) ;
1173 m->OwnerIsThread = 0 ;
1174 m->Recycle() ;
1175 omRelease (Self, m) ;
1176 m = NULL ;
1177 continue ;
1178 // interference - the markword changed - just retry.
1179 // The state-transitions are one-way, so there's no chance of
1180 // live-lock -- "Inflated" is an absorbing state.
1181 }
1183 // Hopefully the performance counters are allocated on distinct
1184 // cache lines to avoid false sharing on MP systems ...
1185 if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
1186 TEVENT(Inflate: overwrite neutral) ;
1187 if (TraceMonitorInflation) {
1188 if (object->is_instance()) {
1189 ResourceMark rm;
1190 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1191 (intptr_t) object, (intptr_t) object->mark(),
1192 Klass::cast(object->klass())->external_name());
1193 }
1194 }
1195 return m ;
1196 }
1197 }
1200 // This the fast monitor enter. The interpreter and compiler use
1201 // some assembly copies of this code. Make sure update those code
1202 // if the following function is changed. The implementation is
1203 // extremely sensitive to race condition. Be careful.
1205 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
1206 if (UseBiasedLocking) {
1207 if (!SafepointSynchronize::is_at_safepoint()) {
1208 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
1209 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
1210 return;
1211 }
1212 } else {
1213 assert(!attempt_rebias, "can not rebias toward VM thread");
1214 BiasedLocking::revoke_at_safepoint(obj);
1215 }
1216 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1217 }
1219 slow_enter (obj, lock, THREAD) ;
1220 }
1222 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
1223 assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
1224 // if displaced header is null, the previous enter is recursive enter, no-op
1225 markOop dhw = lock->displaced_header();
1226 markOop mark ;
1227 if (dhw == NULL) {
1228 // Recursive stack-lock.
1229 // Diagnostics -- Could be: stack-locked, inflating, inflated.
1230 mark = object->mark() ;
1231 assert (!mark->is_neutral(), "invariant") ;
1232 if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
1233 assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
1234 }
1235 if (mark->has_monitor()) {
1236 ObjectMonitor * m = mark->monitor() ;
1237 assert(((oop)(m->object()))->mark() == mark, "invariant") ;
1238 assert(m->is_entered(THREAD), "invariant") ;
1239 }
1240 return ;
1241 }
1243 mark = object->mark() ;
1245 // If the object is stack-locked by the current thread, try to
1246 // swing the displaced header from the box back to the mark.
1247 if (mark == (markOop) lock) {
1248 assert (dhw->is_neutral(), "invariant") ;
1249 if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
1250 TEVENT (fast_exit: release stacklock) ;
1251 return;
1252 }
1253 }
1255 ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ;
1256 }
1258 // This routine is used to handle interpreter/compiler slow case
1259 // We don't need to use fast path here, because it must have been
1260 // failed in the interpreter/compiler code.
1261 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
1262 markOop mark = obj->mark();
1263 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
1265 if (mark->is_neutral()) {
1266 // Anticipate successful CAS -- the ST of the displaced mark must
1267 // be visible <= the ST performed by the CAS.
1268 lock->set_displaced_header(mark);
1269 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
1270 TEVENT (slow_enter: release stacklock) ;
1271 return ;
1272 }
1273 // Fall through to inflate() ...
1274 } else
1275 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1276 assert(lock != mark->locker(), "must not re-lock the same lock");
1277 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
1278 lock->set_displaced_header(NULL);
1279 return;
1280 }
1282 #if 0
1283 // The following optimization isn't particularly useful.
1284 if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
1285 lock->set_displaced_header (NULL) ;
1286 return ;
1287 }
1288 #endif
1290 // The object header will never be displaced to this lock,
1291 // so it does not matter what the value is, except that it
1292 // must be non-zero to avoid looking like a re-entrant lock,
1293 // and must not look locked either.
1294 lock->set_displaced_header(markOopDesc::unused_mark());
1295 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
1296 }
1298 // This routine is used to handle interpreter/compiler slow case
1299 // We don't need to use fast path here, because it must have
1300 // failed in the interpreter/compiler code. Simply use the heavy
1301 // weight monitor should be ok, unless someone find otherwise.
1302 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
1303 fast_exit (object, lock, THREAD) ;
1304 }
1306 // NOTE: must use heavy weight monitor to handle jni monitor enter
1307 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter
1308 // the current locking is from JNI instead of Java code
1309 TEVENT (jni_enter) ;
1310 if (UseBiasedLocking) {
1311 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1312 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1313 }
1314 THREAD->set_current_pending_monitor_is_from_java(false);
1315 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
1316 THREAD->set_current_pending_monitor_is_from_java(true);
1317 }
1319 // NOTE: must use heavy weight monitor to handle jni monitor enter
1320 bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {
1321 if (UseBiasedLocking) {
1322 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1323 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1324 }
1326 ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj());
1327 return monitor->try_enter(THREAD);
1328 }
1331 // NOTE: must use heavy weight monitor to handle jni monitor exit
1332 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
1333 TEVENT (jni_exit) ;
1334 if (UseBiasedLocking) {
1335 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1336 }
1337 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1339 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
1340 // If this thread has locked the object, exit the monitor. Note: can't use
1341 // monitor->check(CHECK); must exit even if an exception is pending.
1342 if (monitor->check(THREAD)) {
1343 monitor->exit(THREAD);
1344 }
1345 }
1347 // complete_exit()/reenter() are used to wait on a nested lock
1348 // i.e. to give up an outer lock completely and then re-enter
1349 // Used when holding nested locks - lock acquisition order: lock1 then lock2
1350 // 1) complete_exit lock1 - saving recursion count
1351 // 2) wait on lock2
1352 // 3) when notified on lock2, unlock lock2
1353 // 4) reenter lock1 with original recursion count
1354 // 5) lock lock2
1355 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
1356 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
1357 TEVENT (complete_exit) ;
1358 if (UseBiasedLocking) {
1359 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1360 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1361 }
1363 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1365 return monitor->complete_exit(THREAD);
1366 }
1368 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
1369 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
1370 TEVENT (reenter) ;
1371 if (UseBiasedLocking) {
1372 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1373 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1374 }
1376 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1378 monitor->reenter(recursion, THREAD);
1379 }
1381 // This exists only as a workaround of dtrace bug 6254741
1382 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
1383 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
1384 return 0;
1385 }
1387 // NOTE: must use heavy weight monitor to handle wait()
1388 void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
1389 if (UseBiasedLocking) {
1390 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1391 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1392 }
1393 if (millis < 0) {
1394 TEVENT (wait - throw IAX) ;
1395 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
1396 }
1397 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1398 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
1399 monitor->wait(millis, true, THREAD);
1401 /* This dummy call is in place to get around dtrace bug 6254741. Once
1402 that's fixed we can uncomment the following line and remove the call */
1403 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
1404 dtrace_waited_probe(monitor, obj, THREAD);
1405 }
1407 void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {
1408 if (UseBiasedLocking) {
1409 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1410 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1411 }
1412 if (millis < 0) {
1413 TEVENT (wait - throw IAX) ;
1414 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
1415 }
1416 ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;
1417 }
1419 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
1420 if (UseBiasedLocking) {
1421 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1422 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1423 }
1425 markOop mark = obj->mark();
1426 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1427 return;
1428 }
1429 ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
1430 }
1432 // NOTE: see comment of notify()
1433 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
1434 if (UseBiasedLocking) {
1435 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1436 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1437 }
1439 markOop mark = obj->mark();
1440 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1441 return;
1442 }
1443 ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
1444 }
1446 intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
1447 if (UseBiasedLocking) {
1448 // NOTE: many places throughout the JVM do not expect a safepoint
1449 // to be taken here, in particular most operations on perm gen
1450 // objects. However, we only ever bias Java instances and all of
1451 // the call sites of identity_hash that might revoke biases have
1452 // been checked to make sure they can handle a safepoint. The
1453 // added check of the bias pattern is to avoid useless calls to
1454 // thread-local storage.
1455 if (obj->mark()->has_bias_pattern()) {
1456 // Box and unbox the raw reference just in case we cause a STW safepoint.
1457 Handle hobj (Self, obj) ;
1458 // Relaxing assertion for bug 6320749.
1459 assert (Universe::verify_in_progress() ||
1460 !SafepointSynchronize::is_at_safepoint(),
1461 "biases should not be seen by VM thread here");
1462 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
1463 obj = hobj() ;
1464 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1465 }
1466 }
1468 // hashCode() is a heap mutator ...
1469 // Relaxing assertion for bug 6320749.
1470 assert (Universe::verify_in_progress() ||
1471 !SafepointSynchronize::is_at_safepoint(), "invariant") ;
1472 assert (Universe::verify_in_progress() ||
1473 Self->is_Java_thread() , "invariant") ;
1474 assert (Universe::verify_in_progress() ||
1475 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
1477 ObjectMonitor* monitor = NULL;
1478 markOop temp, test;
1479 intptr_t hash;
1480 markOop mark = ReadStableMark (obj);
1482 // object should remain ineligible for biased locking
1483 assert (!mark->has_bias_pattern(), "invariant") ;
1485 if (mark->is_neutral()) {
1486 hash = mark->hash(); // this is a normal header
1487 if (hash) { // if it has hash, just return it
1488 return hash;
1489 }
1490 hash = get_next_hash(Self, obj); // allocate a new hash code
1491 temp = mark->copy_set_hash(hash); // merge the hash code into header
1492 // use (machine word version) atomic operation to install the hash
1493 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
1494 if (test == mark) {
1495 return hash;
1496 }
1497 // If atomic operation failed, we must inflate the header
1498 // into heavy weight monitor. We could add more code here
1499 // for fast path, but it does not worth the complexity.
1500 } else if (mark->has_monitor()) {
1501 monitor = mark->monitor();
1502 temp = monitor->header();
1503 assert (temp->is_neutral(), "invariant") ;
1504 hash = temp->hash();
1505 if (hash) {
1506 return hash;
1507 }
1508 // Skip to the following code to reduce code size
1509 } else if (Self->is_lock_owned((address)mark->locker())) {
1510 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
1511 assert (temp->is_neutral(), "invariant") ;
1512 hash = temp->hash(); // by current thread, check if the displaced
1513 if (hash) { // header contains hash code
1514 return hash;
1515 }
1516 // WARNING:
1517 // The displaced header is strictly immutable.
1518 // It can NOT be changed in ANY cases. So we have
1519 // to inflate the header into heavyweight monitor
1520 // even the current thread owns the lock. The reason
1521 // is the BasicLock (stack slot) will be asynchronously
1522 // read by other threads during the inflate() function.
1523 // Any change to stack may not propagate to other threads
1524 // correctly.
1525 }
1527 // Inflate the monitor to set hash code
1528 monitor = ObjectSynchronizer::inflate(Self, obj);
1529 // Load displaced header and check it has hash code
1530 mark = monitor->header();
1531 assert (mark->is_neutral(), "invariant") ;
1532 hash = mark->hash();
1533 if (hash == 0) {
1534 hash = get_next_hash(Self, obj);
1535 temp = mark->copy_set_hash(hash); // merge hash code into header
1536 assert (temp->is_neutral(), "invariant") ;
1537 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
1538 if (test != mark) {
1539 // The only update to the header in the monitor (outside GC)
1540 // is install the hash code. If someone add new usage of
1541 // displaced header, please update this code
1542 hash = test->hash();
1543 assert (test->is_neutral(), "invariant") ;
1544 assert (hash != 0, "Trivial unexpected object/monitor header usage.");
1545 }
1546 }
1547 // We finally get the hash
1548 return hash;
1549 }
1551 // Deprecated -- use FastHashCode() instead.
1553 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
1554 return FastHashCode (Thread::current(), obj()) ;
1555 }
1557 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
1558 Handle h_obj) {
1559 if (UseBiasedLocking) {
1560 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
1561 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1562 }
1564 assert(thread == JavaThread::current(), "Can only be called on current thread");
1565 oop obj = h_obj();
1567 markOop mark = ReadStableMark (obj) ;
1569 // Uncontended case, header points to stack
1570 if (mark->has_locker()) {
1571 return thread->is_lock_owned((address)mark->locker());
1572 }
1573 // Contended case, header points to ObjectMonitor (tagged pointer)
1574 if (mark->has_monitor()) {
1575 ObjectMonitor* monitor = mark->monitor();
1576 return monitor->is_entered(thread) != 0 ;
1577 }
1578 // Unlocked case, header in place
1579 assert(mark->is_neutral(), "sanity check");
1580 return false;
1581 }
1583 // Be aware of this method could revoke bias of the lock object.
1584 // This method querys the ownership of the lock handle specified by 'h_obj'.
1585 // If the current thread owns the lock, it returns owner_self. If no
1586 // thread owns the lock, it returns owner_none. Otherwise, it will return
1587 // ower_other.
1588 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
1589 (JavaThread *self, Handle h_obj) {
1590 // The caller must beware this method can revoke bias, and
1591 // revocation can result in a safepoint.
1592 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
1593 assert (self->thread_state() != _thread_blocked , "invariant") ;
1595 // Possible mark states: neutral, biased, stack-locked, inflated
1597 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
1598 // CASE: biased
1599 BiasedLocking::revoke_and_rebias(h_obj, false, self);
1600 assert(!h_obj->mark()->has_bias_pattern(),
1601 "biases should be revoked by now");
1602 }
1604 assert(self == JavaThread::current(), "Can only be called on current thread");
1605 oop obj = h_obj();
1606 markOop mark = ReadStableMark (obj) ;
1608 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
1609 if (mark->has_locker()) {
1610 return self->is_lock_owned((address)mark->locker()) ?
1611 owner_self : owner_other;
1612 }
1614 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
1615 // The Object:ObjectMonitor relationship is stable as long as we're
1616 // not at a safepoint.
1617 if (mark->has_monitor()) {
1618 void * owner = mark->monitor()->_owner ;
1619 if (owner == NULL) return owner_none ;
1620 return (owner == self ||
1621 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
1622 }
1624 // CASE: neutral
1625 assert(mark->is_neutral(), "sanity check");
1626 return owner_none ; // it's unlocked
1627 }
1629 // FIXME: jvmti should call this
1630 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
1631 if (UseBiasedLocking) {
1632 if (SafepointSynchronize::is_at_safepoint()) {
1633 BiasedLocking::revoke_at_safepoint(h_obj);
1634 } else {
1635 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
1636 }
1637 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1638 }
1640 oop obj = h_obj();
1641 address owner = NULL;
1643 markOop mark = ReadStableMark (obj) ;
1645 // Uncontended case, header points to stack
1646 if (mark->has_locker()) {
1647 owner = (address) mark->locker();
1648 }
1650 // Contended case, header points to ObjectMonitor (tagged pointer)
1651 if (mark->has_monitor()) {
1652 ObjectMonitor* monitor = mark->monitor();
1653 assert(monitor != NULL, "monitor should be non-null");
1654 owner = (address) monitor->owner();
1655 }
1657 if (owner != NULL) {
1658 return Threads::owning_thread_from_monitor_owner(owner, doLock);
1659 }
1661 // Unlocked case, header in place
1662 // Cannot have assertion since this object may have been
1663 // locked by another thread when reaching here.
1664 // assert(mark->is_neutral(), "sanity check");
1666 return NULL;
1667 }
1669 // Iterate through monitor cache and attempt to release thread's monitors
1670 // Gives up on a particular monitor if an exception occurs, but continues
1671 // the overall iteration, swallowing the exception.
1672 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1673 private:
1674 TRAPS;
1676 public:
1677 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1678 void do_monitor(ObjectMonitor* mid) {
1679 if (mid->owner() == THREAD) {
1680 (void)mid->complete_exit(CHECK);
1681 }
1682 }
1683 };
1685 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1686 // ignored. This is meant to be called during JNI thread detach which assumes
1687 // all remaining monitors are heavyweight. All exceptions are swallowed.
1688 // Scanning the extant monitor list can be time consuming.
1689 // A simple optimization is to add a per-thread flag that indicates a thread
1690 // called jni_monitorenter() during its lifetime.
1691 //
1692 // Instead of No_Savepoint_Verifier it might be cheaper to
1693 // use an idiom of the form:
1694 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1695 // <code that must not run at safepoint>
1696 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1697 // Since the tests are extremely cheap we could leave them enabled
1698 // for normal product builds.
1700 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1701 assert(THREAD == JavaThread::current(), "must be current Java thread");
1702 No_Safepoint_Verifier nsv ;
1703 ReleaseJavaMonitorsClosure rjmc(THREAD);
1704 Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
1705 ObjectSynchronizer::monitors_iterate(&rjmc);
1706 Thread::muxRelease(&ListLock);
1707 THREAD->clear_pending_exception();
1708 }
1710 // Visitors ...
1712 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
1713 ObjectMonitor* block = gBlockList;
1714 ObjectMonitor* mid;
1715 while (block) {
1716 assert(block->object() == CHAINMARKER, "must be a block header");
1717 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
1718 mid = block + i;
1719 oop object = (oop) mid->object();
1720 if (object != NULL) {
1721 closure->do_monitor(mid);
1722 }
1723 }
1724 block = (ObjectMonitor*) block->FreeNext;
1725 }
1726 }
1728 void ObjectSynchronizer::oops_do(OopClosure* f) {
1729 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1730 for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
1731 assert(block->object() == CHAINMARKER, "must be a block header");
1732 for (int i = 1; i < _BLOCKSIZE; i++) {
1733 ObjectMonitor* mid = &block[i];
1734 if (mid->object() != NULL) {
1735 f->do_oop((oop*)mid->object_addr());
1736 }
1737 }
1738 }
1739 }
1741 // Deflate_idle_monitors() is called at all safepoints, immediately
1742 // after all mutators are stopped, but before any objects have moved.
1743 // It traverses the list of known monitors, deflating where possible.
1744 // The scavenged monitor are returned to the monitor free list.
1745 //
1746 // Beware that we scavenge at *every* stop-the-world point.
1747 // Having a large number of monitors in-circulation negatively
1748 // impacts the performance of some applications (e.g., PointBase).
1749 // Broadly, we want to minimize the # of monitors in circulation.
1750 // Alternately, we could partition the active monitors into sub-lists
1751 // of those that need scanning and those that do not.
1752 // Specifically, we would add a new sub-list of objectmonitors
1753 // that are in-circulation and potentially active. deflate_idle_monitors()
1754 // would scan only that list. Other monitors could reside on a quiescent
1755 // list. Such sequestered monitors wouldn't need to be scanned by
1756 // deflate_idle_monitors(). omAlloc() would first check the global free list,
1757 // then the quiescent list, and, failing those, would allocate a new block.
1758 // Deflate_idle_monitors() would scavenge and move monitors to the
1759 // quiescent list.
1760 //
1761 // Perversely, the heap size -- and thus the STW safepoint rate --
1762 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1763 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1764 // This is an unfortunate aspect of this design.
1765 //
1766 // Another refinement would be to refrain from calling deflate_idle_monitors()
1767 // except at stop-the-world points associated with garbage collections.
1768 //
1769 // An even better solution would be to deflate on-the-fly, aggressively,
1770 // at monitorexit-time as is done in EVM's metalock or Relaxed Locks.
1772 void ObjectSynchronizer::deflate_idle_monitors() {
1773 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1774 int nInuse = 0 ; // currently associated with objects
1775 int nInCirculation = 0 ; // extant
1776 int nScavenged = 0 ; // reclaimed
1778 ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors
1779 ObjectMonitor * FreeTail = NULL ;
1781 // Iterate over all extant monitors - Scavenge all idle monitors.
1782 TEVENT (deflate_idle_monitors) ;
1783 for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
1784 assert(block->object() == CHAINMARKER, "must be a block header");
1785 nInCirculation += _BLOCKSIZE ;
1786 for (int i = 1 ; i < _BLOCKSIZE; i++) {
1787 ObjectMonitor* mid = &block[i];
1788 oop obj = (oop) mid->object();
1790 if (obj == NULL) {
1791 // The monitor is not associated with an object.
1792 // The monitor should either be a thread-specific private
1793 // free list or the global free list.
1794 // obj == NULL IMPLIES mid->is_busy() == 0
1795 guarantee (!mid->is_busy(), "invariant") ;
1796 continue ;
1797 }
1799 // Normal case ... The monitor is associated with obj.
1800 guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
1801 guarantee (mid == obj->mark()->monitor(), "invariant");
1802 guarantee (mid->header()->is_neutral(), "invariant");
1804 if (mid->is_busy()) {
1805 if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
1806 nInuse ++ ;
1807 } else {
1808 // Deflate the monitor if it is no longer being used
1809 // It's idle - scavenge and return to the global free list
1810 // plain old deflation ...
1811 TEVENT (deflate_idle_monitors - scavenge1) ;
1812 if (TraceMonitorInflation) {
1813 if (obj->is_instance()) {
1814 ResourceMark rm;
1815 tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1816 (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());
1817 }
1818 }
1820 // Restore the header back to obj
1821 obj->release_set_mark(mid->header());
1822 mid->clear();
1824 assert (mid->object() == NULL, "invariant") ;
1826 // Move the object to the working free list defined by FreeHead,FreeTail.
1827 mid->FreeNext = NULL ;
1828 if (FreeHead == NULL) FreeHead = mid ;
1829 if (FreeTail != NULL) FreeTail->FreeNext = mid ;
1830 FreeTail = mid ;
1831 nScavenged ++ ;
1832 }
1833 }
1834 }
1836 // Move the scavenged monitors back to the global free list.
1837 // In theory we don't need the freelist lock as we're at a STW safepoint.
1838 // omAlloc() and omFree() can only be called while a thread is _not in safepoint state.
1839 // But it's remotely possible that omFlush() or release_monitors_owned_by_thread()
1840 // might be called while not at a global STW safepoint. In the interest of
1841 // safety we protect the following access with ListLock.
1842 // An even more conservative and prudent approach would be to guard
1843 // the main loop in scavenge_idle_monitors() with ListLock.
1844 if (FreeHead != NULL) {
1845 guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
1846 assert (FreeTail->FreeNext == NULL, "invariant") ;
1847 // constant-time list splice - prepend scavenged segment to gFreeList
1848 Thread::muxAcquire (&ListLock, "scavenge - return") ;
1849 FreeTail->FreeNext = gFreeList ;
1850 gFreeList = FreeHead ;
1851 Thread::muxRelease (&ListLock) ;
1852 }
1854 if (_sync_Deflations != NULL) _sync_Deflations->inc(nScavenged) ;
1855 if (_sync_MonExtant != NULL) _sync_MonExtant ->set_value(nInCirculation);
1857 // TODO: Add objectMonitor leak detection.
1858 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1859 GVars.stwRandom = os::random() ;
1860 GVars.stwCycle ++ ;
1861 }
1863 // A macro is used below because there may already be a pending
1864 // exception which should not abort the execution of the routines
1865 // which use this (which is why we don't put this into check_slow and
1866 // call it with a CHECK argument).
1868 #define CHECK_OWNER() \
1869 do { \
1870 if (THREAD != _owner) { \
1871 if (THREAD->is_lock_owned((address) _owner)) { \
1872 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1873 _recursions = 0; \
1874 OwnerIsThread = 1 ; \
1875 } else { \
1876 TEVENT (Throw IMSX) ; \
1877 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1878 } \
1879 } \
1880 } while (false)
1882 // TODO-FIXME: eliminate ObjectWaiters. Replace this visitor/enumerator
1883 // interface with a simple FirstWaitingThread(), NextWaitingThread() interface.
1885 ObjectWaiter* ObjectMonitor::first_waiter() {
1886 return _WaitSet;
1887 }
1889 ObjectWaiter* ObjectMonitor::next_waiter(ObjectWaiter* o) {
1890 return o->_next;
1891 }
1893 Thread* ObjectMonitor::thread_of_waiter(ObjectWaiter* o) {
1894 return o->_thread;
1895 }
1897 // initialize the monitor, exception the semaphore, all other fields
1898 // are simple integers or pointers
1899 ObjectMonitor::ObjectMonitor() {
1900 _header = NULL;
1901 _count = 0;
1902 _waiters = 0,
1903 _recursions = 0;
1904 _object = NULL;
1905 _owner = NULL;
1906 _WaitSet = NULL;
1907 _WaitSetLock = 0 ;
1908 _Responsible = NULL ;
1909 _succ = NULL ;
1910 _cxq = NULL ;
1911 FreeNext = NULL ;
1912 _EntryList = NULL ;
1913 _SpinFreq = 0 ;
1914 _SpinClock = 0 ;
1915 OwnerIsThread = 0 ;
1916 }
1918 ObjectMonitor::~ObjectMonitor() {
1919 // TODO: Add asserts ...
1920 // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
1921 // _count == 0 _EntryList == NULL etc
1922 }
1924 intptr_t ObjectMonitor::is_busy() const {
1925 // TODO-FIXME: merge _count and _waiters.
1926 // TODO-FIXME: assert _owner == null implies _recursions = 0
1927 // TODO-FIXME: assert _WaitSet != null implies _count > 0
1928 return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ;
1929 }
1931 void ObjectMonitor::Recycle () {
1932 // TODO: add stronger asserts ...
1933 // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
1934 // _count == 0 EntryList == NULL
1935 // _recursions == 0 _WaitSet == NULL
1936 // TODO: assert (is_busy()|_recursions) == 0
1937 _succ = NULL ;
1938 _EntryList = NULL ;
1939 _cxq = NULL ;
1940 _WaitSet = NULL ;
1941 _recursions = 0 ;
1942 _SpinFreq = 0 ;
1943 _SpinClock = 0 ;
1944 OwnerIsThread = 0 ;
1945 }
1947 // WaitSet management ...
1949 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1950 assert(node != NULL, "should not dequeue NULL node");
1951 assert(node->_prev == NULL, "node already in list");
1952 assert(node->_next == NULL, "node already in list");
1953 // put node at end of queue (circular doubly linked list)
1954 if (_WaitSet == NULL) {
1955 _WaitSet = node;
1956 node->_prev = node;
1957 node->_next = node;
1958 } else {
1959 ObjectWaiter* head = _WaitSet ;
1960 ObjectWaiter* tail = head->_prev;
1961 assert(tail->_next == head, "invariant check");
1962 tail->_next = node;
1963 head->_prev = node;
1964 node->_next = head;
1965 node->_prev = tail;
1966 }
1967 }
1969 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1970 // dequeue the very first waiter
1971 ObjectWaiter* waiter = _WaitSet;
1972 if (waiter) {
1973 DequeueSpecificWaiter(waiter);
1974 }
1975 return waiter;
1976 }
1978 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1979 assert(node != NULL, "should not dequeue NULL node");
1980 assert(node->_prev != NULL, "node already removed from list");
1981 assert(node->_next != NULL, "node already removed from list");
1982 // when the waiter has woken up because of interrupt,
1983 // timeout or other spurious wake-up, dequeue the
1984 // waiter from waiting list
1985 ObjectWaiter* next = node->_next;
1986 if (next == node) {
1987 assert(node->_prev == node, "invariant check");
1988 _WaitSet = NULL;
1989 } else {
1990 ObjectWaiter* prev = node->_prev;
1991 assert(prev->_next == node, "invariant check");
1992 assert(next->_prev == node, "invariant check");
1993 next->_prev = prev;
1994 prev->_next = next;
1995 if (_WaitSet == node) {
1996 _WaitSet = next;
1997 }
1998 }
1999 node->_next = NULL;
2000 node->_prev = NULL;
2001 }
2003 static char * kvGet (char * kvList, const char * Key) {
2004 if (kvList == NULL) return NULL ;
2005 size_t n = strlen (Key) ;
2006 char * Search ;
2007 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2008 if (strncmp (Search, Key, n) == 0) {
2009 if (Search[n] == '=') return Search + n + 1 ;
2010 if (Search[n] == 0) return (char *) "1" ;
2011 }
2012 }
2013 return NULL ;
2014 }
2016 static int kvGetInt (char * kvList, const char * Key, int Default) {
2017 char * v = kvGet (kvList, Key) ;
2018 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2019 if (Knob_ReportSettings && v != NULL) {
2020 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2021 ::fflush (stdout) ;
2022 }
2023 return rslt ;
2024 }
2026 // By convention we unlink a contending thread from EntryList|cxq immediately
2027 // after the thread acquires the lock in ::enter(). Equally, we could defer
2028 // unlinking the thread until ::exit()-time.
2030 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
2031 {
2032 assert (_owner == Self, "invariant") ;
2033 assert (SelfNode->_thread == Self, "invariant") ;
2035 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
2036 // Normal case: remove Self from the DLL EntryList .
2037 // This is a constant-time operation.
2038 ObjectWaiter * nxt = SelfNode->_next ;
2039 ObjectWaiter * prv = SelfNode->_prev ;
2040 if (nxt != NULL) nxt->_prev = prv ;
2041 if (prv != NULL) prv->_next = nxt ;
2042 if (SelfNode == _EntryList ) _EntryList = nxt ;
2043 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
2044 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
2045 TEVENT (Unlink from EntryList) ;
2046 } else {
2047 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
2048 // Inopportune interleaving -- Self is still on the cxq.
2049 // This usually means the enqueue of self raced an exiting thread.
2050 // Normally we'll find Self near the front of the cxq, so
2051 // dequeueing is typically fast. If needbe we can accelerate
2052 // this with some MCS/CHL-like bidirectional list hints and advisory
2053 // back-links so dequeueing from the interior will normally operate
2054 // in constant-time.
2055 // Dequeue Self from either the head (with CAS) or from the interior
2056 // with a linear-time scan and normal non-atomic memory operations.
2057 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
2058 // and then unlink Self from EntryList. We have to drain eventually,
2059 // so it might as well be now.
2061 ObjectWaiter * v = _cxq ;
2062 assert (v != NULL, "invariant") ;
2063 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
2064 // The CAS above can fail from interference IFF a "RAT" arrived.
2065 // In that case Self must be in the interior and can no longer be
2066 // at the head of cxq.
2067 if (v == SelfNode) {
2068 assert (_cxq != v, "invariant") ;
2069 v = _cxq ; // CAS above failed - start scan at head of list
2070 }
2071 ObjectWaiter * p ;
2072 ObjectWaiter * q = NULL ;
2073 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
2074 q = p ;
2075 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
2076 }
2077 assert (v != SelfNode, "invariant") ;
2078 assert (p == SelfNode, "Node not found on cxq") ;
2079 assert (p != _cxq, "invariant") ;
2080 assert (q != NULL, "invariant") ;
2081 assert (q->_next == p, "invariant") ;
2082 q->_next = p->_next ;
2083 }
2084 TEVENT (Unlink from cxq) ;
2085 }
2087 // Diagnostic hygiene ...
2088 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
2089 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
2090 SelfNode->TState = ObjectWaiter::TS_RUN ;
2091 }
2093 // Caveat: TryLock() is not necessarily serializing if it returns failure.
2094 // Callers must compensate as needed.
2096 int ObjectMonitor::TryLock (Thread * Self) {
2097 for (;;) {
2098 void * own = _owner ;
2099 if (own != NULL) return 0 ;
2100 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
2101 // Either guarantee _recursions == 0 or set _recursions = 0.
2102 assert (_recursions == 0, "invariant") ;
2103 assert (_owner == Self, "invariant") ;
2104 // CONSIDER: set or assert that OwnerIsThread == 1
2105 return 1 ;
2106 }
2107 // The lock had been free momentarily, but we lost the race to the lock.
2108 // Interference -- the CAS failed.
2109 // We can either return -1 or retry.
2110 // Retry doesn't make as much sense because the lock was just acquired.
2111 if (true) return -1 ;
2112 }
2113 }
2115 // NotRunnable() -- informed spinning
2116 //
2117 // Don't bother spinning if the owner is not eligible to drop the lock.
2118 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2119 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2120 // The thread must be runnable in order to drop the lock in timely fashion.
2121 // If the _owner is not runnable then spinning will not likely be
2122 // successful (profitable).
2123 //
2124 // Beware -- the thread referenced by _owner could have died
2125 // so a simply fetch from _owner->_thread_state might trap.
2126 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2127 // Because of the lifecycle issues the schedctl and _thread_state values
2128 // observed by NotRunnable() might be garbage. NotRunnable must
2129 // tolerate this and consider the observed _thread_state value
2130 // as advisory.
2131 //
2132 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2133 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2134 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2135 // with the LSB of _owner. Another option would be to probablistically probe
2136 // the putative _owner->TypeTag value.
2137 //
2138 // Checking _thread_state isn't perfect. Even if the thread is
2139 // in_java it might be blocked on a page-fault or have been preempted
2140 // and sitting on a ready/dispatch queue. _thread state in conjunction
2141 // with schedctl.sc_state gives us a good picture of what the
2142 // thread is doing, however.
2143 //
2144 // TODO: check schedctl.sc_state.
2145 // We'll need to use SafeFetch32() to read from the schedctl block.
2146 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2147 //
2148 // The return value from NotRunnable() is *advisory* -- the
2149 // result is based on sampling and is not necessarily coherent.
2150 // The caller must tolerate false-negative and false-positive errors.
2151 // Spinning, in general, is probabilistic anyway.
2154 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2155 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2156 if (!OwnerIsThread) return 0 ;
2158 if (ox == NULL) return 0 ;
2160 // Avoid transitive spinning ...
2161 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2162 // Immediately after T1 acquires L it's possible that T2, also
2163 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2164 // This occurs transiently after T1 acquired L but before
2165 // T1 managed to clear T1.Stalled. T2 does not need to abort
2166 // its spin in this circumstance.
2167 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2169 if (BlockedOn == 1) return 1 ;
2170 if (BlockedOn != 0) {
2171 return BlockedOn != intptr_t(this) && _owner == ox ;
2172 }
2174 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2175 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2176 // consider also: jst != _thread_in_Java -- but that's overspecific.
2177 return jst == _thread_blocked || jst == _thread_in_native ;
2178 }
2181 // Adaptive spin-then-block - rational spinning
2182 //
2183 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2184 // algorithm. On high order SMP systems it would be better to start with
2185 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
2186 // a contending thread could enqueue itself on the cxq and then spin locally
2187 // on a thread-specific variable such as its ParkEvent._Event flag.
2188 // That's left as an exercise for the reader. Note that global spinning is
2189 // not problematic on Niagara, as the L2$ serves the interconnect and has both
2190 // low latency and massive bandwidth.
2191 //
2192 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2193 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2194 // (duration) or we can fix the count at approximately the duration of
2195 // a context switch and vary the frequency. Of course we could also
2196 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2197 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
2198 //
2199 // This implementation varies the duration "D", where D varies with
2200 // the success rate of recent spin attempts. (D is capped at approximately
2201 // length of a round-trip context switch). The success rate for recent
2202 // spin attempts is a good predictor of the success rate of future spin
2203 // attempts. The mechanism adapts automatically to varying critical
2204 // section length (lock modality), system load and degree of parallelism.
2205 // D is maintained per-monitor in _SpinDuration and is initialized
2206 // optimistically. Spin frequency is fixed at 100%.
2207 //
2208 // Note that _SpinDuration is volatile, but we update it without locks
2209 // or atomics. The code is designed so that _SpinDuration stays within
2210 // a reasonable range even in the presence of races. The arithmetic
2211 // operations on _SpinDuration are closed over the domain of legal values,
2212 // so at worst a race will install and older but still legal value.
2213 // At the very worst this introduces some apparent non-determinism.
2214 // We might spin when we shouldn't or vice-versa, but since the spin
2215 // count are relatively short, even in the worst case, the effect is harmless.
2216 //
2217 // Care must be taken that a low "D" value does not become an
2218 // an absorbing state. Transient spinning failures -- when spinning
2219 // is overall profitable -- should not cause the system to converge
2220 // on low "D" values. We want spinning to be stable and predictable
2221 // and fairly responsive to change and at the same time we don't want
2222 // it to oscillate, become metastable, be "too" non-deterministic,
2223 // or converge on or enter undesirable stable absorbing states.
2224 //
2225 // We implement a feedback-based control system -- using past behavior
2226 // to predict future behavior. We face two issues: (a) if the
2227 // input signal is random then the spin predictor won't provide optimal
2228 // results, and (b) if the signal frequency is too high then the control
2229 // system, which has some natural response lag, will "chase" the signal.
2230 // (b) can arise from multimodal lock hold times. Transient preemption
2231 // can also result in apparent bimodal lock hold times.
2232 // Although sub-optimal, neither condition is particularly harmful, as
2233 // in the worst-case we'll spin when we shouldn't or vice-versa.
2234 // The maximum spin duration is rather short so the failure modes aren't bad.
2235 // To be conservative, I've tuned the gain in system to bias toward
2236 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2237 // "rings" or oscillates between spinning and not spinning. This happens
2238 // when spinning is just on the cusp of profitability, however, so the
2239 // situation is not dire. The state is benign -- there's no need to add
2240 // hysteresis control to damp the transition rate between spinning and
2241 // not spinning.
2242 //
2243 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
2244 //
2245 // Spin-then-block strategies ...
2246 //
2247 // Thoughts on ways to improve spinning :
2248 //
2249 // * Periodically call {psr_}getloadavg() while spinning, and
2250 // permit unbounded spinning if the load average is <
2251 // the number of processors. Beware, however, that getloadavg()
2252 // is exceptionally fast on solaris (about 1/10 the cost of a full
2253 // spin cycle, but quite expensive on linux. Beware also, that
2254 // multiple JVMs could "ring" or oscillate in a feedback loop.
2255 // Sufficient damping would solve that problem.
2256 //
2257 // * We currently use spin loops with iteration counters to approximate
2258 // spinning for some interval. Given the availability of high-precision
2259 // time sources such as gethrtime(), %TICK, %STICK, RDTSC, etc., we should
2260 // someday reimplement the spin loops to duration-based instead of iteration-based.
2261 //
2262 // * Don't spin if there are more than N = (CPUs/2) threads
2263 // currently spinning on the monitor (or globally).
2264 // That is, limit the number of concurrent spinners.
2265 // We might also limit the # of spinners in the JVM, globally.
2266 //
2267 // * If a spinning thread observes _owner change hands it should
2268 // abort the spin (and park immediately) or at least debit
2269 // the spin counter by a large "penalty".
2270 //
2271 // * Classically, the spin count is either K*(CPUs-1) or is a
2272 // simple constant that approximates the length of a context switch.
2273 // We currently use a value -- computed by a special utility -- that
2274 // approximates round-trip context switch times.
2275 //
2276 // * Normally schedctl_start()/_stop() is used to advise the kernel
2277 // to avoid preempting threads that are running in short, bounded
2278 // critical sections. We could use the schedctl hooks in an inverted
2279 // sense -- spinners would set the nopreempt flag, but poll the preempt
2280 // pending flag. If a spinner observed a pending preemption it'd immediately
2281 // abort the spin and park. As such, the schedctl service acts as
2282 // a preemption warning mechanism.
2283 //
2284 // * In lieu of spinning, if the system is running below saturation
2285 // (that is, loadavg() << #cpus), we can instead suppress futile
2286 // wakeup throttling, or even wake more than one successor at exit-time.
2287 // The net effect is largely equivalent to spinning. In both cases,
2288 // contending threads go ONPROC and opportunistically attempt to acquire
2289 // the lock, decreasing lock handover latency at the expense of wasted
2290 // cycles and context switching.
2291 //
2292 // * We might to spin less after we've parked as the thread will
2293 // have less $ and TLB affinity with the processor.
2294 // Likewise, we might spin less if we come ONPROC on a different
2295 // processor or after a long period (>> rechose_interval).
2296 //
2297 // * A table-driven state machine similar to Solaris' dispadmin scheduling
2298 // tables might be a better design. Instead of encoding information in
2299 // _SpinDuration, _SpinFreq and _SpinClock we'd just use explicit,
2300 // discrete states. Success or failure during a spin would drive
2301 // state transitions, and each state node would contain a spin count.
2302 //
2303 // * If the processor is operating in a mode intended to conserve power
2304 // (such as Intel's SpeedStep) or to reduce thermal output (thermal
2305 // step-down mode) then the Java synchronization subsystem should
2306 // forgo spinning.
2307 //
2308 // * The minimum spin duration should be approximately the worst-case
2309 // store propagation latency on the platform. That is, the time
2310 // it takes a store on CPU A to become visible on CPU B, where A and
2311 // B are "distant".
2312 //
2313 // * We might want to factor a thread's priority in the spin policy.
2314 // Threads with a higher priority might spin for slightly longer.
2315 // Similarly, if we use back-off in the TATAS loop, lower priority
2316 // threads might back-off longer. We don't currently use a
2317 // thread's priority when placing it on the entry queue. We may
2318 // want to consider doing so in future releases.
2319 //
2320 // * We might transiently drop a thread's scheduling priority while it spins.
2321 // SCHED_BATCH on linux and FX scheduling class at priority=0 on Solaris
2322 // would suffice. We could even consider letting the thread spin indefinitely at
2323 // a depressed or "idle" priority. This brings up fairness issues, however --
2324 // in a saturated system a thread would with a reduced priority could languish
2325 // for extended periods on the ready queue.
2326 //
2327 // * While spinning try to use the otherwise wasted time to help the VM make
2328 // progress:
2329 //
2330 // -- YieldTo() the owner, if the owner is OFFPROC but ready
2331 // Done our remaining quantum directly to the ready thread.
2332 // This helps "push" the lock owner through the critical section.
2333 // It also tends to improve affinity/locality as the lock
2334 // "migrates" less frequently between CPUs.
2335 // -- Walk our own stack in anticipation of blocking. Memoize the roots.
2336 // -- Perform strand checking for other thread. Unpark potential strandees.
2337 // -- Help GC: trace or mark -- this would need to be a bounded unit of work.
2338 // Unfortunately this will pollute our $ and TLBs. Recall that we
2339 // spin to avoid context switching -- context switching has an
2340 // immediate cost in latency, a disruptive cost to other strands on a CMT
2341 // processor, and an amortized cost because of the D$ and TLB cache
2342 // reload transient when the thread comes back ONPROC and repopulates
2343 // $s and TLBs.
2344 // -- call getloadavg() to see if the system is saturated. It'd probably
2345 // make sense to call getloadavg() half way through the spin.
2346 // If the system isn't at full capacity the we'd simply reset
2347 // the spin counter to and extend the spin attempt.
2348 // -- Doug points out that we should use the same "helping" policy
2349 // in thread.yield().
2350 //
2351 // * Try MONITOR-MWAIT on systems that support those instructions.
2352 //
2353 // * The spin statistics that drive spin decisions & frequency are
2354 // maintained in the objectmonitor structure so if we deflate and reinflate
2355 // we lose spin state. In practice this is not usually a concern
2356 // as the default spin state after inflation is aggressive (optimistic)
2357 // and tends toward spinning. So in the worst case for a lock where
2358 // spinning is not profitable we may spin unnecessarily for a brief
2359 // period. But then again, if a lock is contended it'll tend not to deflate
2360 // in the first place.
2363 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
2364 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
2366 // Spinning: Fixed frequency (100%), vary duration
2368 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
2370 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2371 int ctr = Knob_FixedSpin ;
2372 if (ctr != 0) {
2373 while (--ctr >= 0) {
2374 if (TryLock (Self) > 0) return 1 ;
2375 SpinPause () ;
2376 }
2377 return 0 ;
2378 }
2380 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
2381 if (TryLock(Self) > 0) {
2382 // Increase _SpinDuration ...
2383 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2384 // Raising _SpurDuration to the poverty line is key.
2385 int x = _SpinDuration ;
2386 if (x < Knob_SpinLimit) {
2387 if (x < Knob_Poverty) x = Knob_Poverty ;
2388 _SpinDuration = x + Knob_BonusB ;
2389 }
2390 return 1 ;
2391 }
2392 SpinPause () ;
2393 }
2395 // Admission control - verify preconditions for spinning
2396 //
2397 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2398 // becoming an absorbing state. Put another way, we spin briefly to
2399 // sample, just in case the system load, parallelism, contention, or lock
2400 // modality changed.
2401 //
2402 // Consider the following alternative:
2403 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2404 // spin attempt. "Periodically" might mean after a tally of
2405 // the # of failed spin attempts (or iterations) reaches some threshold.
2406 // This takes us into the realm of 1-out-of-N spinning, where we
2407 // hold the duration constant but vary the frequency.
2409 ctr = _SpinDuration ;
2410 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2411 if (ctr <= 0) return 0 ;
2413 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2414 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2415 TEVENT (Spin abort - notrunnable [TOP]);
2416 return 0 ;
2417 }
2419 int MaxSpin = Knob_MaxSpinners ;
2420 if (MaxSpin >= 0) {
2421 if (_Spinner > MaxSpin) {
2422 TEVENT (Spin abort -- too many spinners) ;
2423 return 0 ;
2424 }
2425 // Slighty racy, but benign ...
2426 Adjust (&_Spinner, 1) ;
2427 }
2429 // We're good to spin ... spin ingress.
2430 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2431 // when preparing to LD...CAS _owner, etc and the CAS is likely
2432 // to succeed.
2433 int hits = 0 ;
2434 int msk = 0 ;
2435 int caspty = Knob_CASPenalty ;
2436 int oxpty = Knob_OXPenalty ;
2437 int sss = Knob_SpinSetSucc ;
2438 if (sss && _succ == NULL ) _succ = Self ;
2439 Thread * prv = NULL ;
2441 // There are three ways to exit the following loop:
2442 // 1. A successful spin where this thread has acquired the lock.
2443 // 2. Spin failure with prejudice
2444 // 3. Spin failure without prejudice
2446 while (--ctr >= 0) {
2448 // Periodic polling -- Check for pending GC
2449 // Threads may spin while they're unsafe.
2450 // We don't want spinning threads to delay the JVM from reaching
2451 // a stop-the-world safepoint or to steal cycles from GC.
2452 // If we detect a pending safepoint we abort in order that
2453 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2454 // this thread, if safe, doesn't steal cycles from GC.
2455 // This is in keeping with the "no loitering in runtime" rule.
2456 // We periodically check to see if there's a safepoint pending.
2457 if ((ctr & 0xFF) == 0) {
2458 if (SafepointSynchronize::do_call_back()) {
2459 TEVENT (Spin: safepoint) ;
2460 goto Abort ; // abrupt spin egress
2461 }
2462 if (Knob_UsePause & 1) SpinPause () ;
2464 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2465 if (hits > 50 && scb != NULL) {
2466 int abend = (*scb)(SpinCallbackArgument, 0) ;
2467 }
2468 }
2470 if (Knob_UsePause & 2) SpinPause() ;
2472 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2473 // This is useful on classic SMP systems, but is of less utility on
2474 // N1-style CMT platforms.
2475 //
2476 // Trade-off: lock acquisition latency vs coherency bandwidth.
2477 // Lock hold times are typically short. A histogram
2478 // of successful spin attempts shows that we usually acquire
2479 // the lock early in the spin. That suggests we want to
2480 // sample _owner frequently in the early phase of the spin,
2481 // but then back-off and sample less frequently as the spin
2482 // progresses. The back-off makes a good citizen on SMP big
2483 // SMP systems. Oversampling _owner can consume excessive
2484 // coherency bandwidth. Relatedly, if we _oversample _owner we
2485 // can inadvertently interfere with the the ST m->owner=null.
2486 // executed by the lock owner.
2487 if (ctr & msk) continue ;
2488 ++hits ;
2489 if ((hits & 0xF) == 0) {
2490 // The 0xF, above, corresponds to the exponent.
2491 // Consider: (msk+1)|msk
2492 msk = ((msk << 2)|3) & BackOffMask ;
2493 }
2495 // Probe _owner with TATAS
2496 // If this thread observes the monitor transition or flicker
2497 // from locked to unlocked to locked, then the odds that this
2498 // thread will acquire the lock in this spin attempt go down
2499 // considerably. The same argument applies if the CAS fails
2500 // or if we observe _owner change from one non-null value to
2501 // another non-null value. In such cases we might abort
2502 // the spin without prejudice or apply a "penalty" to the
2503 // spin count-down variable "ctr", reducing it by 100, say.
2505 Thread * ox = (Thread *) _owner ;
2506 if (ox == NULL) {
2507 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2508 if (ox == NULL) {
2509 // The CAS succeeded -- this thread acquired ownership
2510 // Take care of some bookkeeping to exit spin state.
2511 if (sss && _succ == Self) {
2512 _succ = NULL ;
2513 }
2514 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2516 // Increase _SpinDuration :
2517 // The spin was successful (profitable) so we tend toward
2518 // longer spin attempts in the future.
2519 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2520 // If we acquired the lock early in the spin cycle it
2521 // makes sense to increase _SpinDuration proportionally.
2522 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2523 int x = _SpinDuration ;
2524 if (x < Knob_SpinLimit) {
2525 if (x < Knob_Poverty) x = Knob_Poverty ;
2526 _SpinDuration = x + Knob_Bonus ;
2527 }
2528 return 1 ;
2529 }
2531 // The CAS failed ... we can take any of the following actions:
2532 // * penalize: ctr -= Knob_CASPenalty
2533 // * exit spin with prejudice -- goto Abort;
2534 // * exit spin without prejudice.
2535 // * Since CAS is high-latency, retry again immediately.
2536 prv = ox ;
2537 TEVENT (Spin: cas failed) ;
2538 if (caspty == -2) break ;
2539 if (caspty == -1) goto Abort ;
2540 ctr -= caspty ;
2541 continue ;
2542 }
2544 // Did lock ownership change hands ?
2545 if (ox != prv && prv != NULL ) {
2546 TEVENT (spin: Owner changed)
2547 if (oxpty == -2) break ;
2548 if (oxpty == -1) goto Abort ;
2549 ctr -= oxpty ;
2550 }
2551 prv = ox ;
2553 // Abort the spin if the owner is not executing.
2554 // The owner must be executing in order to drop the lock.
2555 // Spinning while the owner is OFFPROC is idiocy.
2556 // Consider: ctr -= RunnablePenalty ;
2557 if (Knob_OState && NotRunnable (Self, ox)) {
2558 TEVENT (Spin abort - notrunnable);
2559 goto Abort ;
2560 }
2561 if (sss && _succ == NULL ) _succ = Self ;
2562 }
2564 // Spin failed with prejudice -- reduce _SpinDuration.
2565 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2566 // AIMD is globally stable.
2567 TEVENT (Spin failure) ;
2568 {
2569 int x = _SpinDuration ;
2570 if (x > 0) {
2571 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2572 // This is globally sample and tends to damp the response.
2573 x -= Knob_Penalty ;
2574 if (x < 0) x = 0 ;
2575 _SpinDuration = x ;
2576 }
2577 }
2579 Abort:
2580 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2581 if (sss && _succ == Self) {
2582 _succ = NULL ;
2583 // Invariant: after setting succ=null a contending thread
2584 // must recheck-retry _owner before parking. This usually happens
2585 // in the normal usage of TrySpin(), but it's safest
2586 // to make TrySpin() as foolproof as possible.
2587 OrderAccess::fence() ;
2588 if (TryLock(Self) > 0) return 1 ;
2589 }
2590 return 0 ;
2591 }
2593 #define TrySpin TrySpin_VaryDuration
2595 static void DeferredInitialize () {
2596 if (InitDone > 0) return ;
2597 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2598 while (InitDone != 1) ;
2599 return ;
2600 }
2602 // One-shot global initialization ...
2603 // The initialization is idempotent, so we don't need locks.
2604 // In the future consider doing this via os::init_2().
2605 // SyncKnobs consist of <Key>=<Value> pairs in the style
2606 // of environment variables. Start by converting ':' to NUL.
2608 if (SyncKnobs == NULL) SyncKnobs = "" ;
2610 size_t sz = strlen (SyncKnobs) ;
2611 char * knobs = (char *) malloc (sz + 2) ;
2612 if (knobs == NULL) {
2613 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2614 guarantee (0, "invariant") ;
2615 }
2616 strcpy (knobs, SyncKnobs) ;
2617 knobs[sz+1] = 0 ;
2618 for (char * p = knobs ; *p ; p++) {
2619 if (*p == ':') *p = 0 ;
2620 }
2622 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2623 SETKNOB(ReportSettings) ;
2624 SETKNOB(Verbose) ;
2625 SETKNOB(FixedSpin) ;
2626 SETKNOB(SpinLimit) ;
2627 SETKNOB(SpinBase) ;
2628 SETKNOB(SpinBackOff);
2629 SETKNOB(CASPenalty) ;
2630 SETKNOB(OXPenalty) ;
2631 SETKNOB(LogSpins) ;
2632 SETKNOB(SpinSetSucc) ;
2633 SETKNOB(SuccEnabled) ;
2634 SETKNOB(SuccRestrict) ;
2635 SETKNOB(Penalty) ;
2636 SETKNOB(Bonus) ;
2637 SETKNOB(BonusB) ;
2638 SETKNOB(Poverty) ;
2639 SETKNOB(SpinAfterFutile) ;
2640 SETKNOB(UsePause) ;
2641 SETKNOB(SpinEarly) ;
2642 SETKNOB(OState) ;
2643 SETKNOB(MaxSpinners) ;
2644 SETKNOB(PreSpin) ;
2645 SETKNOB(ExitPolicy) ;
2646 SETKNOB(QMode);
2647 SETKNOB(ResetEvent) ;
2648 SETKNOB(MoveNotifyee) ;
2649 SETKNOB(FastHSSEC) ;
2650 #undef SETKNOB
2652 if (os::is_MP()) {
2653 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2654 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2655 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2656 } else {
2657 Knob_SpinLimit = 0 ;
2658 Knob_SpinBase = 0 ;
2659 Knob_PreSpin = 0 ;
2660 Knob_FixedSpin = -1 ;
2661 }
2663 if (Knob_LogSpins == 0) {
2664 ObjectSynchronizer::_sync_FailedSpins = NULL ;
2665 }
2667 free (knobs) ;
2668 OrderAccess::fence() ;
2669 InitDone = 1 ;
2670 }
2672 // Theory of operations -- Monitors lists, thread residency, etc:
2673 //
2674 // * A thread acquires ownership of a monitor by successfully
2675 // CAS()ing the _owner field from null to non-null.
2676 //
2677 // * Invariant: A thread appears on at most one monitor list --
2678 // cxq, EntryList or WaitSet -- at any one time.
2679 //
2680 // * Contending threads "push" themselves onto the cxq with CAS
2681 // and then spin/park.
2682 //
2683 // * After a contending thread eventually acquires the lock it must
2684 // dequeue itself from either the EntryList or the cxq.
2685 //
2686 // * The exiting thread identifies and unparks an "heir presumptive"
2687 // tentative successor thread on the EntryList. Critically, the
2688 // exiting thread doesn't unlink the successor thread from the EntryList.
2689 // After having been unparked, the wakee will recontend for ownership of
2690 // the monitor. The successor (wakee) will either acquire the lock or
2691 // re-park itself.
2692 //
2693 // Succession is provided for by a policy of competitive handoff.
2694 // The exiting thread does _not_ grant or pass ownership to the
2695 // successor thread. (This is also referred to as "handoff" succession").
2696 // Instead the exiting thread releases ownership and possibly wakes
2697 // a successor, so the successor can (re)compete for ownership of the lock.
2698 // If the EntryList is empty but the cxq is populated the exiting
2699 // thread will drain the cxq into the EntryList. It does so by
2700 // by detaching the cxq (installing null with CAS) and folding
2701 // the threads from the cxq into the EntryList. The EntryList is
2702 // doubly linked, while the cxq is singly linked because of the
2703 // CAS-based "push" used to enqueue recently arrived threads (RATs).
2704 //
2705 // * Concurrency invariants:
2706 //
2707 // -- only the monitor owner may access or mutate the EntryList.
2708 // The mutex property of the monitor itself protects the EntryList
2709 // from concurrent interference.
2710 // -- Only the monitor owner may detach the cxq.
2711 //
2712 // * The monitor entry list operations avoid locks, but strictly speaking
2713 // they're not lock-free. Enter is lock-free, exit is not.
2714 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
2715 //
2716 // * The cxq can have multiple concurrent "pushers" but only one concurrent
2717 // detaching thread. This mechanism is immune from the ABA corruption.
2718 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
2719 //
2720 // * Taken together, the cxq and the EntryList constitute or form a
2721 // single logical queue of threads stalled trying to acquire the lock.
2722 // We use two distinct lists to improve the odds of a constant-time
2723 // dequeue operation after acquisition (in the ::enter() epilog) and
2724 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
2725 // A key desideratum is to minimize queue & monitor metadata manipulation
2726 // that occurs while holding the monitor lock -- that is, we want to
2727 // minimize monitor lock holds times. Note that even a small amount of
2728 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
2729 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
2730 // locks and monitor metadata.
2731 //
2732 // Cxq points to the the set of Recently Arrived Threads attempting entry.
2733 // Because we push threads onto _cxq with CAS, the RATs must take the form of
2734 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
2735 // the unlocking thread notices that EntryList is null but _cxq is != null.
2736 //
2737 // The EntryList is ordered by the prevailing queue discipline and
2738 // can be organized in any convenient fashion, such as a doubly-linked list or
2739 // a circular doubly-linked list. Critically, we want insert and delete operations
2740 // to operate in constant-time. If we need a priority queue then something akin
2741 // to Solaris' sleepq would work nicely. Viz.,
2742 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
2743 // Queue discipline is enforced at ::exit() time, when the unlocking thread
2744 // drains the cxq into the EntryList, and orders or reorders the threads on the
2745 // EntryList accordingly.
2746 //
2747 // Barring "lock barging", this mechanism provides fair cyclic ordering,
2748 // somewhat similar to an elevator-scan.
2749 //
2750 // * The monitor synchronization subsystem avoids the use of native
2751 // synchronization primitives except for the narrow platform-specific
2752 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
2753 // the semantics of park-unpark. Put another way, this monitor implementation
2754 // depends only on atomic operations and park-unpark. The monitor subsystem
2755 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
2756 // underlying OS manages the READY<->RUN transitions.
2757 //
2758 // * Waiting threads reside on the WaitSet list -- wait() puts
2759 // the caller onto the WaitSet.
2760 //
2761 // * notify() or notifyAll() simply transfers threads from the WaitSet to
2762 // either the EntryList or cxq. Subsequent exit() operations will
2763 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
2764 // it's likely the notifyee would simply impale itself on the lock held
2765 // by the notifier.
2766 //
2767 // * An interesting alternative is to encode cxq as (List,LockByte) where
2768 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
2769 // variable, like _recursions, in the scheme. The threads or Events that form
2770 // the list would have to be aligned in 256-byte addresses. A thread would
2771 // try to acquire the lock or enqueue itself with CAS, but exiting threads
2772 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
2773 // Note that is is *not* word-tearing, but it does presume that full-word
2774 // CAS operations are coherent with intermix with STB operations. That's true
2775 // on most common processors.
2776 //
2777 // * See also http://blogs.sun.com/dave
2780 void ATTR ObjectMonitor::EnterI (TRAPS) {
2781 Thread * Self = THREAD ;
2782 assert (Self->is_Java_thread(), "invariant") ;
2783 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
2785 // Try the lock - TATAS
2786 if (TryLock (Self) > 0) {
2787 assert (_succ != Self , "invariant") ;
2788 assert (_owner == Self , "invariant") ;
2789 assert (_Responsible != Self , "invariant") ;
2790 return ;
2791 }
2793 DeferredInitialize () ;
2795 // We try one round of spinning *before* enqueueing Self.
2796 //
2797 // If the _owner is ready but OFFPROC we could use a YieldTo()
2798 // operation to donate the remainder of this thread's quantum
2799 // to the owner. This has subtle but beneficial affinity
2800 // effects.
2802 if (TrySpin (Self) > 0) {
2803 assert (_owner == Self , "invariant") ;
2804 assert (_succ != Self , "invariant") ;
2805 assert (_Responsible != Self , "invariant") ;
2806 return ;
2807 }
2809 // The Spin failed -- Enqueue and park the thread ...
2810 assert (_succ != Self , "invariant") ;
2811 assert (_owner != Self , "invariant") ;
2812 assert (_Responsible != Self , "invariant") ;
2814 // Enqueue "Self" on ObjectMonitor's _cxq.
2815 //
2816 // Node acts as a proxy for Self.
2817 // As an aside, if were to ever rewrite the synchronization code mostly
2818 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
2819 // Java objects. This would avoid awkward lifecycle and liveness issues,
2820 // as well as eliminate a subset of ABA issues.
2821 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
2822 //
2824 ObjectWaiter node(Self) ;
2825 Self->_ParkEvent->reset() ;
2826 node._prev = (ObjectWaiter *) 0xBAD ;
2827 node.TState = ObjectWaiter::TS_CXQ ;
2829 // Push "Self" onto the front of the _cxq.
2830 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
2831 // Note that spinning tends to reduce the rate at which threads
2832 // enqueue and dequeue on EntryList|cxq.
2833 ObjectWaiter * nxt ;
2834 for (;;) {
2835 node._next = nxt = _cxq ;
2836 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
2838 // Interference - the CAS failed because _cxq changed. Just retry.
2839 // As an optional optimization we retry the lock.
2840 if (TryLock (Self) > 0) {
2841 assert (_succ != Self , "invariant") ;
2842 assert (_owner == Self , "invariant") ;
2843 assert (_Responsible != Self , "invariant") ;
2844 return ;
2845 }
2846 }
2848 // Check for cxq|EntryList edge transition to non-null. This indicates
2849 // the onset of contention. While contention persists exiting threads
2850 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
2851 // operations revert to the faster 1-0 mode. This enter operation may interleave
2852 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
2853 // arrange for one of the contending thread to use a timed park() operations
2854 // to detect and recover from the race. (Stranding is form of progress failure
2855 // where the monitor is unlocked but all the contending threads remain parked).
2856 // That is, at least one of the contended threads will periodically poll _owner.
2857 // One of the contending threads will become the designated "Responsible" thread.
2858 // The Responsible thread uses a timed park instead of a normal indefinite park
2859 // operation -- it periodically wakes and checks for and recovers from potential
2860 // strandings admitted by 1-0 exit operations. We need at most one Responsible
2861 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
2862 // be responsible for a monitor.
2863 //
2864 // Currently, one of the contended threads takes on the added role of "Responsible".
2865 // A viable alternative would be to use a dedicated "stranding checker" thread
2866 // that periodically iterated over all the threads (or active monitors) and unparked
2867 // successors where there was risk of stranding. This would help eliminate the
2868 // timer scalability issues we see on some platforms as we'd only have one thread
2869 // -- the checker -- parked on a timer.
2871 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
2872 // Try to assume the role of responsible thread for the monitor.
2873 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
2874 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
2875 }
2877 // The lock have been released while this thread was occupied queueing
2878 // itself onto _cxq. To close the race and avoid "stranding" and
2879 // progress-liveness failure we must resample-retry _owner before parking.
2880 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
2881 // In this case the ST-MEMBAR is accomplished with CAS().
2882 //
2883 // TODO: Defer all thread state transitions until park-time.
2884 // Since state transitions are heavy and inefficient we'd like
2885 // to defer the state transitions until absolutely necessary,
2886 // and in doing so avoid some transitions ...
2888 TEVENT (Inflated enter - Contention) ;
2889 int nWakeups = 0 ;
2890 int RecheckInterval = 1 ;
2892 for (;;) {
2894 if (TryLock (Self) > 0) break ;
2895 assert (_owner != Self, "invariant") ;
2897 if ((SyncFlags & 2) && _Responsible == NULL) {
2898 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
2899 }
2901 // park self
2902 if (_Responsible == Self || (SyncFlags & 1)) {
2903 TEVENT (Inflated enter - park TIMED) ;
2904 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
2905 // Increase the RecheckInterval, but clamp the value.
2906 RecheckInterval *= 8 ;
2907 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
2908 } else {
2909 TEVENT (Inflated enter - park UNTIMED) ;
2910 Self->_ParkEvent->park() ;
2911 }
2913 if (TryLock(Self) > 0) break ;
2915 // The lock is still contested.
2916 // Keep a tally of the # of futile wakeups.
2917 // Note that the counter is not protected by a lock or updated by atomics.
2918 // That is by design - we trade "lossy" counters which are exposed to
2919 // races during updates for a lower probe effect.
2920 TEVENT (Inflated enter - Futile wakeup) ;
2921 if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
2922 ObjectSynchronizer::_sync_FutileWakeups->inc() ;
2923 }
2924 ++ nWakeups ;
2926 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
2927 // We can defer clearing _succ until after the spin completes
2928 // TrySpin() must tolerate being called with _succ == Self.
2929 // Try yet another round of adaptive spinning.
2930 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
2932 // We can find that we were unpark()ed and redesignated _succ while
2933 // we were spinning. That's harmless. If we iterate and call park(),
2934 // park() will consume the event and return immediately and we'll
2935 // just spin again. This pattern can repeat, leaving _succ to simply
2936 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
2937 // Alternately, we can sample fired() here, and if set, forgo spinning
2938 // in the next iteration.
2940 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
2941 Self->_ParkEvent->reset() ;
2942 OrderAccess::fence() ;
2943 }
2944 if (_succ == Self) _succ = NULL ;
2946 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
2947 OrderAccess::fence() ;
2948 }
2950 // Egress :
2951 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
2952 // Normally we'll find Self on the EntryList .
2953 // From the perspective of the lock owner (this thread), the
2954 // EntryList is stable and cxq is prepend-only.
2955 // The head of cxq is volatile but the interior is stable.
2956 // In addition, Self.TState is stable.
2958 assert (_owner == Self , "invariant") ;
2959 assert (object() != NULL , "invariant") ;
2960 // I'd like to write:
2961 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
2962 // but as we're at a safepoint that's not safe.
2964 UnlinkAfterAcquire (Self, &node) ;
2965 if (_succ == Self) _succ = NULL ;
2967 assert (_succ != Self, "invariant") ;
2968 if (_Responsible == Self) {
2969 _Responsible = NULL ;
2970 // Dekker pivot-point.
2971 // Consider OrderAccess::storeload() here
2973 // We may leave threads on cxq|EntryList without a designated
2974 // "Responsible" thread. This is benign. When this thread subsequently
2975 // exits the monitor it can "see" such preexisting "old" threads --
2976 // threads that arrived on the cxq|EntryList before the fence, above --
2977 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
2978 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
2979 // non-null and elect a new "Responsible" timer thread.
2980 //
2981 // This thread executes:
2982 // ST Responsible=null; MEMBAR (in enter epilog - here)
2983 // LD cxq|EntryList (in subsequent exit)
2984 //
2985 // Entering threads in the slow/contended path execute:
2986 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
2987 // The (ST cxq; MEMBAR) is accomplished with CAS().
2988 //
2989 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
2990 // exit operation from floating above the ST Responsible=null.
2991 //
2992 // In *practice* however, EnterI() is always followed by some atomic
2993 // operation such as the decrement of _count in ::enter(). Those atomics
2994 // obviate the need for the explicit MEMBAR, above.
2995 }
2997 // We've acquired ownership with CAS().
2998 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
2999 // But since the CAS() this thread may have also stored into _succ,
3000 // EntryList, cxq or Responsible. These meta-data updates must be
3001 // visible __before this thread subsequently drops the lock.
3002 // Consider what could occur if we didn't enforce this constraint --
3003 // STs to monitor meta-data and user-data could reorder with (become
3004 // visible after) the ST in exit that drops ownership of the lock.
3005 // Some other thread could then acquire the lock, but observe inconsistent
3006 // or old monitor meta-data and heap data. That violates the JMM.
3007 // To that end, the 1-0 exit() operation must have at least STST|LDST
3008 // "release" barrier semantics. Specifically, there must be at least a
3009 // STST|LDST barrier in exit() before the ST of null into _owner that drops
3010 // the lock. The barrier ensures that changes to monitor meta-data and data
3011 // protected by the lock will be visible before we release the lock, and
3012 // therefore before some other thread (CPU) has a chance to acquire the lock.
3013 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
3014 //
3015 // Critically, any prior STs to _succ or EntryList must be visible before
3016 // the ST of null into _owner in the *subsequent* (following) corresponding
3017 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
3018 // execute a serializing instruction.
3020 if (SyncFlags & 8) {
3021 OrderAccess::fence() ;
3022 }
3023 return ;
3024 }
3026 // ExitSuspendEquivalent:
3027 // A faster alternate to handle_special_suspend_equivalent_condition()
3028 //
3029 // handle_special_suspend_equivalent_condition() unconditionally
3030 // acquires the SR_lock. On some platforms uncontended MutexLocker()
3031 // operations have high latency. Note that in ::enter() we call HSSEC
3032 // while holding the monitor, so we effectively lengthen the critical sections.
3033 //
3034 // There are a number of possible solutions:
3035 //
3036 // A. To ameliorate the problem we might also defer state transitions
3037 // to as late as possible -- just prior to parking.
3038 // Given that, we'd call HSSEC after having returned from park(),
3039 // but before attempting to acquire the monitor. This is only a
3040 // partial solution. It avoids calling HSSEC while holding the
3041 // monitor (good), but it still increases successor reacquisition latency --
3042 // the interval between unparking a successor and the time the successor
3043 // resumes and retries the lock. See ReenterI(), which defers state transitions.
3044 // If we use this technique we can also avoid EnterI()-exit() loop
3045 // in ::enter() where we iteratively drop the lock and then attempt
3046 // to reacquire it after suspending.
3047 //
3048 // B. In the future we might fold all the suspend bits into a
3049 // composite per-thread suspend flag and then update it with CAS().
3050 // Alternately, a Dekker-like mechanism with multiple variables
3051 // would suffice:
3052 // ST Self->_suspend_equivalent = false
3053 // MEMBAR
3054 // LD Self_>_suspend_flags
3055 //
3058 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
3059 int Mode = Knob_FastHSSEC ;
3060 if (Mode && !jSelf->is_external_suspend()) {
3061 assert (jSelf->is_suspend_equivalent(), "invariant") ;
3062 jSelf->clear_suspend_equivalent() ;
3063 if (2 == Mode) OrderAccess::storeload() ;
3064 if (!jSelf->is_external_suspend()) return false ;
3065 // We raced a suspension -- fall thru into the slow path
3066 TEVENT (ExitSuspendEquivalent - raced) ;
3067 jSelf->set_suspend_equivalent() ;
3068 }
3069 return jSelf->handle_special_suspend_equivalent_condition() ;
3070 }
3073 // ReenterI() is a specialized inline form of the latter half of the
3074 // contended slow-path from EnterI(). We use ReenterI() only for
3075 // monitor reentry in wait().
3076 //
3077 // In the future we should reconcile EnterI() and ReenterI(), adding
3078 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
3079 // loop accordingly.
3081 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
3082 assert (Self != NULL , "invariant") ;
3083 assert (SelfNode != NULL , "invariant") ;
3084 assert (SelfNode->_thread == Self , "invariant") ;
3085 assert (_waiters > 0 , "invariant") ;
3086 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
3087 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
3088 JavaThread * jt = (JavaThread *) Self ;
3090 int nWakeups = 0 ;
3091 for (;;) {
3092 ObjectWaiter::TStates v = SelfNode->TState ;
3093 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
3094 assert (_owner != Self, "invariant") ;
3096 if (TryLock (Self) > 0) break ;
3097 if (TrySpin (Self) > 0) break ;
3099 TEVENT (Wait Reentry - parking) ;
3101 // State transition wrappers around park() ...
3102 // ReenterI() wisely defers state transitions until
3103 // it's clear we must park the thread.
3104 {
3105 OSThreadContendState osts(Self->osthread());
3106 ThreadBlockInVM tbivm(jt);
3108 // cleared by handle_special_suspend_equivalent_condition()
3109 // or java_suspend_self()
3110 jt->set_suspend_equivalent();
3111 if (SyncFlags & 1) {
3112 Self->_ParkEvent->park ((jlong)1000) ;
3113 } else {
3114 Self->_ParkEvent->park () ;
3115 }
3117 // were we externally suspended while we were waiting?
3118 for (;;) {
3119 if (!ExitSuspendEquivalent (jt)) break ;
3120 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
3121 jt->java_suspend_self();
3122 jt->set_suspend_equivalent();
3123 }
3124 }
3126 // Try again, but just so we distinguish between futile wakeups and
3127 // successful wakeups. The following test isn't algorithmically
3128 // necessary, but it helps us maintain sensible statistics.
3129 if (TryLock(Self) > 0) break ;
3131 // The lock is still contested.
3132 // Keep a tally of the # of futile wakeups.
3133 // Note that the counter is not protected by a lock or updated by atomics.
3134 // That is by design - we trade "lossy" counters which are exposed to
3135 // races during updates for a lower probe effect.
3136 TEVENT (Wait Reentry - futile wakeup) ;
3137 ++ nWakeups ;
3139 // Assuming this is not a spurious wakeup we'll normally
3140 // find that _succ == Self.
3141 if (_succ == Self) _succ = NULL ;
3143 // Invariant: after clearing _succ a contending thread
3144 // *must* retry _owner before parking.
3145 OrderAccess::fence() ;
3147 if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
3148 ObjectSynchronizer::_sync_FutileWakeups->inc() ;
3149 }
3150 }
3152 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
3153 // Normally we'll find Self on the EntryList.
3154 // Unlinking from the EntryList is constant-time and atomic-free.
3155 // From the perspective of the lock owner (this thread), the
3156 // EntryList is stable and cxq is prepend-only.
3157 // The head of cxq is volatile but the interior is stable.
3158 // In addition, Self.TState is stable.
3160 assert (_owner == Self, "invariant") ;
3161 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3162 UnlinkAfterAcquire (Self, SelfNode) ;
3163 if (_succ == Self) _succ = NULL ;
3164 assert (_succ != Self, "invariant") ;
3165 SelfNode->TState = ObjectWaiter::TS_RUN ;
3166 OrderAccess::fence() ; // see comments at the end of EnterI()
3167 }
3169 bool ObjectMonitor::try_enter(Thread* THREAD) {
3170 if (THREAD != _owner) {
3171 if (THREAD->is_lock_owned ((address)_owner)) {
3172 assert(_recursions == 0, "internal state error");
3173 _owner = THREAD ;
3174 _recursions = 1 ;
3175 OwnerIsThread = 1 ;
3176 return true;
3177 }
3178 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3179 return false;
3180 }
3181 return true;
3182 } else {
3183 _recursions++;
3184 return true;
3185 }
3186 }
3188 void ATTR ObjectMonitor::enter(TRAPS) {
3189 // The following code is ordered to check the most common cases first
3190 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
3191 Thread * const Self = THREAD ;
3192 void * cur ;
3194 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
3195 if (cur == NULL) {
3196 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
3197 assert (_recursions == 0 , "invariant") ;
3198 assert (_owner == Self, "invariant") ;
3199 // CONSIDER: set or assert OwnerIsThread == 1
3200 return ;
3201 }
3203 if (cur == Self) {
3204 // TODO-FIXME: check for integer overflow! BUGID 6557169.
3205 _recursions ++ ;
3206 return ;
3207 }
3209 if (Self->is_lock_owned ((address)cur)) {
3210 assert (_recursions == 0, "internal state error");
3211 _recursions = 1 ;
3212 // Commute owner from a thread-specific on-stack BasicLockObject address to
3213 // a full-fledged "Thread *".
3214 _owner = Self ;
3215 OwnerIsThread = 1 ;
3216 return ;
3217 }
3219 // We've encountered genuine contention.
3220 assert (Self->_Stalled == 0, "invariant") ;
3221 Self->_Stalled = intptr_t(this) ;
3223 // Try one round of spinning *before* enqueueing Self
3224 // and before going through the awkward and expensive state
3225 // transitions. The following spin is strictly optional ...
3226 // Note that if we acquire the monitor from an initial spin
3227 // we forgo posting JVMTI events and firing DTRACE probes.
3228 if (Knob_SpinEarly && TrySpin (Self) > 0) {
3229 assert (_owner == Self , "invariant") ;
3230 assert (_recursions == 0 , "invariant") ;
3231 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3232 Self->_Stalled = 0 ;
3233 return ;
3234 }
3236 assert (_owner != Self , "invariant") ;
3237 assert (_succ != Self , "invariant") ;
3238 assert (Self->is_Java_thread() , "invariant") ;
3239 JavaThread * jt = (JavaThread *) Self ;
3240 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
3241 assert (jt->thread_state() != _thread_blocked , "invariant") ;
3242 assert (this->object() != NULL , "invariant") ;
3243 assert (_count >= 0, "invariant") ;
3245 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
3246 // Ensure the object-monitor relationship remains stable while there's contention.
3247 Atomic::inc_ptr(&_count);
3249 { // Change java thread status to indicate blocked on monitor enter.
3250 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
3252 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
3253 if (JvmtiExport::should_post_monitor_contended_enter()) {
3254 JvmtiExport::post_monitor_contended_enter(jt, this);
3255 }
3257 OSThreadContendState osts(Self->osthread());
3258 ThreadBlockInVM tbivm(jt);
3260 Self->set_current_pending_monitor(this);
3262 // TODO-FIXME: change the following for(;;) loop to straight-line code.
3263 for (;;) {
3264 jt->set_suspend_equivalent();
3265 // cleared by handle_special_suspend_equivalent_condition()
3266 // or java_suspend_self()
3268 EnterI (THREAD) ;
3270 if (!ExitSuspendEquivalent(jt)) break ;
3272 //
3273 // We have acquired the contended monitor, but while we were
3274 // waiting another thread suspended us. We don't want to enter
3275 // the monitor while suspended because that would surprise the
3276 // thread that suspended us.
3277 //
3278 _recursions = 0 ;
3279 _succ = NULL ;
3280 exit (Self) ;
3282 jt->java_suspend_self();
3283 }
3284 Self->set_current_pending_monitor(NULL);
3285 }
3287 Atomic::dec_ptr(&_count);
3288 assert (_count >= 0, "invariant") ;
3289 Self->_Stalled = 0 ;
3291 // Must either set _recursions = 0 or ASSERT _recursions == 0.
3292 assert (_recursions == 0 , "invariant") ;
3293 assert (_owner == Self , "invariant") ;
3294 assert (_succ != Self , "invariant") ;
3295 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3297 // The thread -- now the owner -- is back in vm mode.
3298 // Report the glorious news via TI,DTrace and jvmstat.
3299 // The probe effect is non-trivial. All the reportage occurs
3300 // while we hold the monitor, increasing the length of the critical
3301 // section. Amdahl's parallel speedup law comes vividly into play.
3302 //
3303 // Another option might be to aggregate the events (thread local or
3304 // per-monitor aggregation) and defer reporting until a more opportune
3305 // time -- such as next time some thread encounters contention but has
3306 // yet to acquire the lock. While spinning that thread could
3307 // spinning we could increment JVMStat counters, etc.
3309 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
3310 if (JvmtiExport::should_post_monitor_contended_entered()) {
3311 JvmtiExport::post_monitor_contended_entered(jt, this);
3312 }
3313 if (ObjectSynchronizer::_sync_ContendedLockAttempts != NULL) {
3314 ObjectSynchronizer::_sync_ContendedLockAttempts->inc() ;
3315 }
3316 }
3318 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
3319 assert (_owner == Self, "invariant") ;
3321 // Exit protocol:
3322 // 1. ST _succ = wakee
3323 // 2. membar #loadstore|#storestore;
3324 // 2. ST _owner = NULL
3325 // 3. unpark(wakee)
3327 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
3328 ParkEvent * Trigger = Wakee->_event ;
3330 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
3331 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
3332 // out-of-scope (non-extant).
3333 Wakee = NULL ;
3335 // Drop the lock
3336 OrderAccess::release_store_ptr (&_owner, NULL) ;
3337 OrderAccess::fence() ; // ST _owner vs LD in unpark()
3339 // TODO-FIXME:
3340 // If there's a safepoint pending the best policy would be to
3341 // get _this thread to a safepoint and only wake the successor
3342 // after the safepoint completed. monitorexit uses a "leaf"
3343 // state transition, however, so this thread can't become
3344 // safe at this point in time. (Its stack isn't walkable).
3345 // The next best thing is to defer waking the successor by
3346 // adding to a list of thread to be unparked after at the
3347 // end of the forthcoming STW).
3348 if (SafepointSynchronize::do_call_back()) {
3349 TEVENT (unpark before SAFEPOINT) ;
3350 }
3352 // Possible optimizations ...
3353 //
3354 // * Consider: set Wakee->UnparkTime = timeNow()
3355 // When the thread wakes up it'll compute (timeNow() - Self->UnparkTime()).
3356 // By measuring recent ONPROC latency we can approximate the
3357 // system load. In turn, we can feed that information back
3358 // into the spinning & succession policies.
3359 // (ONPROC latency correlates strongly with load).
3360 //
3361 // * Pull affinity:
3362 // If the wakee is cold then transiently setting it's affinity
3363 // to the current CPU is a good idea.
3364 // See http://j2se.east/~dice/PERSIST/050624-PullAffinity.txt
3365 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
3366 Trigger->unpark() ;
3368 // Maintain stats and report events to JVMTI
3369 if (ObjectSynchronizer::_sync_Parks != NULL) {
3370 ObjectSynchronizer::_sync_Parks->inc() ;
3371 }
3372 }
3375 // exit()
3376 // ~~~~~~
3377 // Note that the collector can't reclaim the objectMonitor or deflate
3378 // the object out from underneath the thread calling ::exit() as the
3379 // thread calling ::exit() never transitions to a stable state.
3380 // This inhibits GC, which in turn inhibits asynchronous (and
3381 // inopportune) reclamation of "this".
3382 //
3383 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
3384 // There's one exception to the claim above, however. EnterI() can call
3385 // exit() to drop a lock if the acquirer has been externally suspended.
3386 // In that case exit() is called with _thread_state as _thread_blocked,
3387 // but the monitor's _count field is > 0, which inhibits reclamation.
3388 //
3389 // 1-0 exit
3390 // ~~~~~~~~
3391 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
3392 // the fast-path operators have been optimized so the common ::exit()
3393 // operation is 1-0. See i486.ad fast_unlock(), for instance.
3394 // The code emitted by fast_unlock() elides the usual MEMBAR. This
3395 // greatly improves latency -- MEMBAR and CAS having considerable local
3396 // latency on modern processors -- but at the cost of "stranding". Absent the
3397 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
3398 // ::enter() path, resulting in the entering thread being stranding
3399 // and a progress-liveness failure. Stranding is extremely rare.
3400 // We use timers (timed park operations) & periodic polling to detect
3401 // and recover from stranding. Potentially stranded threads periodically
3402 // wake up and poll the lock. See the usage of the _Responsible variable.
3403 //
3404 // The CAS() in enter provides for safety and exclusion, while the CAS or
3405 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
3406 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
3407 // We detect and recover from stranding with timers.
3408 //
3409 // If a thread transiently strands it'll park until (a) another
3410 // thread acquires the lock and then drops the lock, at which time the
3411 // exiting thread will notice and unpark the stranded thread, or, (b)
3412 // the timer expires. If the lock is high traffic then the stranding latency
3413 // will be low due to (a). If the lock is low traffic then the odds of
3414 // stranding are lower, although the worst-case stranding latency
3415 // is longer. Critically, we don't want to put excessive load in the
3416 // platform's timer subsystem. We want to minimize both the timer injection
3417 // rate (timers created/sec) as well as the number of timers active at
3418 // any one time. (more precisely, we want to minimize timer-seconds, which is
3419 // the integral of the # of active timers at any instant over time).
3420 // Both impinge on OS scalability. Given that, at most one thread parked on
3421 // a monitor will use a timer.
3423 void ATTR ObjectMonitor::exit(TRAPS) {
3424 Thread * Self = THREAD ;
3425 if (THREAD != _owner) {
3426 if (THREAD->is_lock_owned((address) _owner)) {
3427 // Transmute _owner from a BasicLock pointer to a Thread address.
3428 // We don't need to hold _mutex for this transition.
3429 // Non-null to Non-null is safe as long as all readers can
3430 // tolerate either flavor.
3431 assert (_recursions == 0, "invariant") ;
3432 _owner = THREAD ;
3433 _recursions = 0 ;
3434 OwnerIsThread = 1 ;
3435 } else {
3436 // NOTE: we need to handle unbalanced monitor enter/exit
3437 // in native code by throwing an exception.
3438 // TODO: Throw an IllegalMonitorStateException ?
3439 TEVENT (Exit - Throw IMSX) ;
3440 assert(false, "Non-balanced monitor enter/exit!");
3441 if (false) {
3442 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
3443 }
3444 return;
3445 }
3446 }
3448 if (_recursions != 0) {
3449 _recursions--; // this is simple recursive enter
3450 TEVENT (Inflated exit - recursive) ;
3451 return ;
3452 }
3454 // Invariant: after setting Responsible=null an thread must execute
3455 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
3456 if ((SyncFlags & 4) == 0) {
3457 _Responsible = NULL ;
3458 }
3460 for (;;) {
3461 assert (THREAD == _owner, "invariant") ;
3463 // Fast-path monitor exit:
3464 //
3465 // Observe the Dekker/Lamport duality:
3466 // A thread in ::exit() executes:
3467 // ST Owner=null; MEMBAR; LD EntryList|cxq.
3468 // A thread in the contended ::enter() path executes the complementary:
3469 // ST EntryList|cxq = nonnull; MEMBAR; LD Owner.
3470 //
3471 // Note that there's a benign race in the exit path. We can drop the
3472 // lock, another thread can reacquire the lock immediately, and we can
3473 // then wake a thread unnecessarily (yet another flavor of futile wakeup).
3474 // This is benign, and we've structured the code so the windows are short
3475 // and the frequency of such futile wakeups is low.
3476 //
3477 // We could eliminate the race by encoding both the "LOCKED" state and
3478 // the queue head in a single word. Exit would then use either CAS to
3479 // clear the LOCKED bit/byte. This precludes the desirable 1-0 optimization,
3480 // however.
3481 //
3482 // Possible fast-path ::exit() optimization:
3483 // The current fast-path exit implementation fetches both cxq and EntryList.
3484 // See also i486.ad fast_unlock(). Testing has shown that two LDs
3485 // isn't measurably slower than a single LD on any platforms.
3486 // Still, we could reduce the 2 LDs to one or zero by one of the following:
3487 //
3488 // - Use _count instead of cxq|EntryList
3489 // We intend to eliminate _count, however, when we switch
3490 // to on-the-fly deflation in ::exit() as is used in
3491 // Metalocks and RelaxedLocks.
3492 //
3493 // - Establish the invariant that cxq == null implies EntryList == null.
3494 // set cxq == EMPTY (1) to encode the state where cxq is empty
3495 // by EntryList != null. EMPTY is a distinguished value.
3496 // The fast-path exit() would fetch cxq but not EntryList.
3497 //
3498 // - Encode succ as follows:
3499 // succ = t : Thread t is the successor -- t is ready or is spinning.
3500 // Exiting thread does not need to wake a successor.
3501 // succ = 0 : No successor required -> (EntryList|cxq) == null
3502 // Exiting thread does not need to wake a successor
3503 // succ = 1 : Successor required -> (EntryList|cxq) != null and
3504 // logically succ == null.
3505 // Exiting thread must wake a successor.
3506 //
3507 // The 1-1 fast-exit path would appear as :
3508 // _owner = null ; membar ;
3509 // if (_succ == 1 && CAS (&_owner, null, Self) == null) goto SlowPath
3510 // goto FastPathDone ;
3511 //
3512 // and the 1-0 fast-exit path would appear as:
3513 // if (_succ == 1) goto SlowPath
3514 // Owner = null ;
3515 // goto FastPathDone
3516 //
3517 // - Encode the LSB of _owner as 1 to indicate that exit()
3518 // must use the slow-path and make a successor ready.
3519 // (_owner & 1) == 0 IFF succ != null || (EntryList|cxq) == null
3520 // (_owner & 1) == 0 IFF succ == null && (EntryList|cxq) != null (obviously)
3521 // The 1-0 fast exit path would read:
3522 // if (_owner != Self) goto SlowPath
3523 // _owner = null
3524 // goto FastPathDone
3526 if (Knob_ExitPolicy == 0) {
3527 // release semantics: prior loads and stores from within the critical section
3528 // must not float (reorder) past the following store that drops the lock.
3529 // On SPARC that requires MEMBAR #loadstore|#storestore.
3530 // But of course in TSO #loadstore|#storestore is not required.
3531 // I'd like to write one of the following:
3532 // A. OrderAccess::release() ; _owner = NULL
3533 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
3534 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
3535 // store into a _dummy variable. That store is not needed, but can result
3536 // in massive wasteful coherency traffic on classic SMP systems.
3537 // Instead, I use release_store(), which is implemented as just a simple
3538 // ST on x64, x86 and SPARC.
3539 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
3540 OrderAccess::storeload() ; // See if we need to wake a successor
3541 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
3542 TEVENT (Inflated exit - simple egress) ;
3543 return ;
3544 }
3545 TEVENT (Inflated exit - complex egress) ;
3547 // Normally the exiting thread is responsible for ensuring succession,
3548 // but if other successors are ready or other entering threads are spinning
3549 // then this thread can simply store NULL into _owner and exit without
3550 // waking a successor. The existence of spinners or ready successors
3551 // guarantees proper succession (liveness). Responsibility passes to the
3552 // ready or running successors. The exiting thread delegates the duty.
3553 // More precisely, if a successor already exists this thread is absolved
3554 // of the responsibility of waking (unparking) one.
3555 //
3556 // The _succ variable is critical to reducing futile wakeup frequency.
3557 // _succ identifies the "heir presumptive" thread that has been made
3558 // ready (unparked) but that has not yet run. We need only one such
3559 // successor thread to guarantee progress.
3560 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
3561 // section 3.3 "Futile Wakeup Throttling" for details.
3562 //
3563 // Note that spinners in Enter() also set _succ non-null.
3564 // In the current implementation spinners opportunistically set
3565 // _succ so that exiting threads might avoid waking a successor.
3566 // Another less appealing alternative would be for the exiting thread
3567 // to drop the lock and then spin briefly to see if a spinner managed
3568 // to acquire the lock. If so, the exiting thread could exit
3569 // immediately without waking a successor, otherwise the exiting
3570 // thread would need to dequeue and wake a successor.
3571 // (Note that we'd need to make the post-drop spin short, but no
3572 // shorter than the worst-case round-trip cache-line migration time.
3573 // The dropped lock needs to become visible to the spinner, and then
3574 // the acquisition of the lock by the spinner must become visible to
3575 // the exiting thread).
3576 //
3578 // It appears that an heir-presumptive (successor) must be made ready.
3579 // Only the current lock owner can manipulate the EntryList or
3580 // drain _cxq, so we need to reacquire the lock. If we fail
3581 // to reacquire the lock the responsibility for ensuring succession
3582 // falls to the new owner.
3583 //
3584 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3585 return ;
3586 }
3587 TEVENT (Exit - Reacquired) ;
3588 } else {
3589 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
3590 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
3591 OrderAccess::storeload() ;
3592 // Ratify the previously observed values.
3593 if (_cxq == NULL || _succ != NULL) {
3594 TEVENT (Inflated exit - simple egress) ;
3595 return ;
3596 }
3598 // inopportune interleaving -- the exiting thread (this thread)
3599 // in the fast-exit path raced an entering thread in the slow-enter
3600 // path.
3601 // We have two choices:
3602 // A. Try to reacquire the lock.
3603 // If the CAS() fails return immediately, otherwise
3604 // we either restart/rerun the exit operation, or simply
3605 // fall-through into the code below which wakes a successor.
3606 // B. If the elements forming the EntryList|cxq are TSM
3607 // we could simply unpark() the lead thread and return
3608 // without having set _succ.
3609 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3610 TEVENT (Inflated exit - reacquired succeeded) ;
3611 return ;
3612 }
3613 TEVENT (Inflated exit - reacquired failed) ;
3614 } else {
3615 TEVENT (Inflated exit - complex egress) ;
3616 }
3617 }
3619 guarantee (_owner == THREAD, "invariant") ;
3621 // Select an appropriate successor ("heir presumptive") from the EntryList
3622 // and make it ready. Generally we just wake the head of EntryList .
3623 // There's no algorithmic constraint that we use the head - it's just
3624 // a policy decision. Note that the thread at head of the EntryList
3625 // remains at the head until it acquires the lock. This means we'll
3626 // repeatedly wake the same thread until it manages to grab the lock.
3627 // This is generally a good policy - if we're seeing lots of futile wakeups
3628 // at least we're waking/rewaking a thread that's like to be hot or warm
3629 // (have residual D$ and TLB affinity).
3630 //
3631 // "Wakeup locality" optimization:
3632 // http://j2se.east/~dice/PERSIST/040825-WakeLocality.txt
3633 // In the future we'll try to bias the selection mechanism
3634 // to preferentially pick a thread that recently ran on
3635 // a processor element that shares cache with the CPU on which
3636 // the exiting thread is running. We need access to Solaris'
3637 // schedctl.sc_cpu to make that work.
3638 //
3639 ObjectWaiter * w = NULL ;
3640 int QMode = Knob_QMode ;
3642 if (QMode == 2 && _cxq != NULL) {
3643 // QMode == 2 : cxq has precedence over EntryList.
3644 // Try to directly wake a successor from the cxq.
3645 // If successful, the successor will need to unlink itself from cxq.
3646 w = _cxq ;
3647 assert (w != NULL, "invariant") ;
3648 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3649 ExitEpilog (Self, w) ;
3650 return ;
3651 }
3653 if (QMode == 3 && _cxq != NULL) {
3654 // Aggressively drain cxq into EntryList at the first opportunity.
3655 // This policy ensure that recently-run threads live at the head of EntryList.
3656 // Drain _cxq into EntryList - bulk transfer.
3657 // First, detach _cxq.
3658 // The following loop is tantamount to: w = swap (&cxq, NULL)
3659 w = _cxq ;
3660 for (;;) {
3661 assert (w != NULL, "Invariant") ;
3662 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
3663 if (u == w) break ;
3664 w = u ;
3665 }
3666 assert (w != NULL , "invariant") ;
3668 ObjectWaiter * q = NULL ;
3669 ObjectWaiter * p ;
3670 for (p = w ; p != NULL ; p = p->_next) {
3671 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3672 p->TState = ObjectWaiter::TS_ENTER ;
3673 p->_prev = q ;
3674 q = p ;
3675 }
3677 // Append the RATs to the EntryList
3678 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
3679 ObjectWaiter * Tail ;
3680 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
3681 if (Tail == NULL) {
3682 _EntryList = w ;
3683 } else {
3684 Tail->_next = w ;
3685 w->_prev = Tail ;
3686 }
3688 // Fall thru into code that tries to wake a successor from EntryList
3689 }
3691 if (QMode == 4 && _cxq != NULL) {
3692 // Aggressively drain cxq into EntryList at the first opportunity.
3693 // This policy ensure that recently-run threads live at the head of EntryList.
3695 // Drain _cxq into EntryList - bulk transfer.
3696 // First, detach _cxq.
3697 // The following loop is tantamount to: w = swap (&cxq, NULL)
3698 w = _cxq ;
3699 for (;;) {
3700 assert (w != NULL, "Invariant") ;
3701 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
3702 if (u == w) break ;
3703 w = u ;
3704 }
3705 assert (w != NULL , "invariant") ;
3707 ObjectWaiter * q = NULL ;
3708 ObjectWaiter * p ;
3709 for (p = w ; p != NULL ; p = p->_next) {
3710 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3711 p->TState = ObjectWaiter::TS_ENTER ;
3712 p->_prev = q ;
3713 q = p ;
3714 }
3716 // Prepend the RATs to the EntryList
3717 if (_EntryList != NULL) {
3718 q->_next = _EntryList ;
3719 _EntryList->_prev = q ;
3720 }
3721 _EntryList = w ;
3723 // Fall thru into code that tries to wake a successor from EntryList
3724 }
3726 w = _EntryList ;
3727 if (w != NULL) {
3728 // I'd like to write: guarantee (w->_thread != Self).
3729 // But in practice an exiting thread may find itself on the EntryList.
3730 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
3731 // then calls exit(). Exit release the lock by setting O._owner to NULL.
3732 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
3733 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
3734 // release the lock "O". T2 resumes immediately after the ST of null into
3735 // _owner, above. T2 notices that the EntryList is populated, so it
3736 // reacquires the lock and then finds itself on the EntryList.
3737 // Given all that, we have to tolerate the circumstance where "w" is
3738 // associated with Self.
3739 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
3740 ExitEpilog (Self, w) ;
3741 return ;
3742 }
3744 // If we find that both _cxq and EntryList are null then just
3745 // re-run the exit protocol from the top.
3746 w = _cxq ;
3747 if (w == NULL) continue ;
3749 // Drain _cxq into EntryList - bulk transfer.
3750 // First, detach _cxq.
3751 // The following loop is tantamount to: w = swap (&cxq, NULL)
3752 for (;;) {
3753 assert (w != NULL, "Invariant") ;
3754 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
3755 if (u == w) break ;
3756 w = u ;
3757 }
3758 TEVENT (Inflated exit - drain cxq into EntryList) ;
3760 assert (w != NULL , "invariant") ;
3761 assert (_EntryList == NULL , "invariant") ;
3763 // Convert the LIFO SLL anchored by _cxq into a DLL.
3764 // The list reorganization step operates in O(LENGTH(w)) time.
3765 // It's critical that this step operate quickly as
3766 // "Self" still holds the outer-lock, restricting parallelism
3767 // and effectively lengthening the critical section.
3768 // Invariant: s chases t chases u.
3769 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
3770 // we have faster access to the tail.
3772 if (QMode == 1) {
3773 // QMode == 1 : drain cxq to EntryList, reversing order
3774 // We also reverse the order of the list.
3775 ObjectWaiter * s = NULL ;
3776 ObjectWaiter * t = w ;
3777 ObjectWaiter * u = NULL ;
3778 while (t != NULL) {
3779 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
3780 t->TState = ObjectWaiter::TS_ENTER ;
3781 u = t->_next ;
3782 t->_prev = u ;
3783 t->_next = s ;
3784 s = t;
3785 t = u ;
3786 }
3787 _EntryList = s ;
3788 assert (s != NULL, "invariant") ;
3789 } else {
3790 // QMode == 0 or QMode == 2
3791 _EntryList = w ;
3792 ObjectWaiter * q = NULL ;
3793 ObjectWaiter * p ;
3794 for (p = w ; p != NULL ; p = p->_next) {
3795 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3796 p->TState = ObjectWaiter::TS_ENTER ;
3797 p->_prev = q ;
3798 q = p ;
3799 }
3800 }
3802 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
3803 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
3805 // See if we can abdicate to a spinner instead of waking a thread.
3806 // A primary goal of the implementation is to reduce the
3807 // context-switch rate.
3808 if (_succ != NULL) continue;
3810 w = _EntryList ;
3811 if (w != NULL) {
3812 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
3813 ExitEpilog (Self, w) ;
3814 return ;
3815 }
3816 }
3817 }
3818 // complete_exit exits a lock returning recursion count
3819 // complete_exit/reenter operate as a wait without waiting
3820 // complete_exit requires an inflated monitor
3821 // The _owner field is not always the Thread addr even with an
3822 // inflated monitor, e.g. the monitor can be inflated by a non-owning
3823 // thread due to contention.
3824 intptr_t ObjectMonitor::complete_exit(TRAPS) {
3825 Thread * const Self = THREAD;
3826 assert(Self->is_Java_thread(), "Must be Java thread!");
3827 JavaThread *jt = (JavaThread *)THREAD;
3829 DeferredInitialize();
3831 if (THREAD != _owner) {
3832 if (THREAD->is_lock_owned ((address)_owner)) {
3833 assert(_recursions == 0, "internal state error");
3834 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
3835 _recursions = 0 ;
3836 OwnerIsThread = 1 ;
3837 }
3838 }
3840 guarantee(Self == _owner, "complete_exit not owner");
3841 intptr_t save = _recursions; // record the old recursion count
3842 _recursions = 0; // set the recursion level to be 0
3843 exit (Self) ; // exit the monitor
3844 guarantee (_owner != Self, "invariant");
3845 return save;
3846 }
3848 // reenter() enters a lock and sets recursion count
3849 // complete_exit/reenter operate as a wait without waiting
3850 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
3851 Thread * const Self = THREAD;
3852 assert(Self->is_Java_thread(), "Must be Java thread!");
3853 JavaThread *jt = (JavaThread *)THREAD;
3855 guarantee(_owner != Self, "reenter already owner");
3856 enter (THREAD); // enter the monitor
3857 guarantee (_recursions == 0, "reenter recursion");
3858 _recursions = recursions;
3859 return;
3860 }
3862 // Note: a subset of changes to ObjectMonitor::wait()
3863 // will need to be replicated in complete_exit above
3864 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
3865 Thread * const Self = THREAD ;
3866 assert(Self->is_Java_thread(), "Must be Java thread!");
3867 JavaThread *jt = (JavaThread *)THREAD;
3869 DeferredInitialize () ;
3871 // Throw IMSX or IEX.
3872 CHECK_OWNER();
3874 // check for a pending interrupt
3875 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
3876 // post monitor waited event. Note that this is past-tense, we are done waiting.
3877 if (JvmtiExport::should_post_monitor_waited()) {
3878 // Note: 'false' parameter is passed here because the
3879 // wait was not timed out due to thread interrupt.
3880 JvmtiExport::post_monitor_waited(jt, this, false);
3881 }
3882 TEVENT (Wait - Throw IEX) ;
3883 THROW(vmSymbols::java_lang_InterruptedException());
3884 return ;
3885 }
3886 TEVENT (Wait) ;
3888 assert (Self->_Stalled == 0, "invariant") ;
3889 Self->_Stalled = intptr_t(this) ;
3890 jt->set_current_waiting_monitor(this);
3892 // create a node to be put into the queue
3893 // Critically, after we reset() the event but prior to park(), we must check
3894 // for a pending interrupt.
3895 ObjectWaiter node(Self);
3896 node.TState = ObjectWaiter::TS_WAIT ;
3897 Self->_ParkEvent->reset() ;
3898 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
3900 // Enter the waiting queue, which is a circular doubly linked list in this case
3901 // but it could be a priority queue or any data structure.
3902 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
3903 // by the the owner of the monitor *except* in the case where park()
3904 // returns because of a timeout of interrupt. Contention is exceptionally rare
3905 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
3907 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
3908 AddWaiter (&node) ;
3909 Thread::SpinRelease (&_WaitSetLock) ;
3911 if ((SyncFlags & 4) == 0) {
3912 _Responsible = NULL ;
3913 }
3914 intptr_t save = _recursions; // record the old recursion count
3915 _waiters++; // increment the number of waiters
3916 _recursions = 0; // set the recursion level to be 1
3917 exit (Self) ; // exit the monitor
3918 guarantee (_owner != Self, "invariant") ;
3920 // As soon as the ObjectMonitor's ownership is dropped in the exit()
3921 // call above, another thread can enter() the ObjectMonitor, do the
3922 // notify(), and exit() the ObjectMonitor. If the other thread's
3923 // exit() call chooses this thread as the successor and the unpark()
3924 // call happens to occur while this thread is posting a
3925 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
3926 // handler using RawMonitors and consuming the unpark().
3927 //
3928 // To avoid the problem, we re-post the event. This does no harm
3929 // even if the original unpark() was not consumed because we are the
3930 // chosen successor for this monitor.
3931 if (node._notified != 0 && _succ == Self) {
3932 node._event->unpark();
3933 }
3935 // The thread is on the WaitSet list - now park() it.
3936 // On MP systems it's conceivable that a brief spin before we park
3937 // could be profitable.
3938 //
3939 // TODO-FIXME: change the following logic to a loop of the form
3940 // while (!timeout && !interrupted && _notified == 0) park()
3942 int ret = OS_OK ;
3943 int WasNotified = 0 ;
3944 { // State transition wrappers
3945 OSThread* osthread = Self->osthread();
3946 OSThreadWaitState osts(osthread, true);
3947 {
3948 ThreadBlockInVM tbivm(jt);
3949 // Thread is in thread_blocked state and oop access is unsafe.
3950 jt->set_suspend_equivalent();
3952 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
3953 // Intentionally empty
3954 } else
3955 if (node._notified == 0) {
3956 if (millis <= 0) {
3957 Self->_ParkEvent->park () ;
3958 } else {
3959 ret = Self->_ParkEvent->park (millis) ;
3960 }
3961 }
3963 // were we externally suspended while we were waiting?
3964 if (ExitSuspendEquivalent (jt)) {
3965 // TODO-FIXME: add -- if succ == Self then succ = null.
3966 jt->java_suspend_self();
3967 }
3969 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
3972 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
3973 // from the WaitSet to the EntryList.
3974 // See if we need to remove Node from the WaitSet.
3975 // We use double-checked locking to avoid grabbing _WaitSetLock
3976 // if the thread is not on the wait queue.
3977 //
3978 // Note that we don't need a fence before the fetch of TState.
3979 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
3980 // written by the is thread. (perhaps the fetch might even be satisfied
3981 // by a look-aside into the processor's own store buffer, although given
3982 // the length of the code path between the prior ST and this load that's
3983 // highly unlikely). If the following LD fetches a stale TS_WAIT value
3984 // then we'll acquire the lock and then re-fetch a fresh TState value.
3985 // That is, we fail toward safety.
3987 if (node.TState == ObjectWaiter::TS_WAIT) {
3988 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
3989 if (node.TState == ObjectWaiter::TS_WAIT) {
3990 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
3991 assert(node._notified == 0, "invariant");
3992 node.TState = ObjectWaiter::TS_RUN ;
3993 }
3994 Thread::SpinRelease (&_WaitSetLock) ;
3995 }
3997 // The thread is now either on off-list (TS_RUN),
3998 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
3999 // The Node's TState variable is stable from the perspective of this thread.
4000 // No other threads will asynchronously modify TState.
4001 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
4002 OrderAccess::loadload() ;
4003 if (_succ == Self) _succ = NULL ;
4004 WasNotified = node._notified ;
4006 // Reentry phase -- reacquire the monitor.
4007 // re-enter contended monitor after object.wait().
4008 // retain OBJECT_WAIT state until re-enter successfully completes
4009 // Thread state is thread_in_vm and oop access is again safe,
4010 // although the raw address of the object may have changed.
4011 // (Don't cache naked oops over safepoints, of course).
4013 // post monitor waited event. Note that this is past-tense, we are done waiting.
4014 if (JvmtiExport::should_post_monitor_waited()) {
4015 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
4016 }
4017 OrderAccess::fence() ;
4019 assert (Self->_Stalled != 0, "invariant") ;
4020 Self->_Stalled = 0 ;
4022 assert (_owner != Self, "invariant") ;
4023 ObjectWaiter::TStates v = node.TState ;
4024 if (v == ObjectWaiter::TS_RUN) {
4025 enter (Self) ;
4026 } else {
4027 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
4028 ReenterI (Self, &node) ;
4029 node.wait_reenter_end(this);
4030 }
4032 // Self has reacquired the lock.
4033 // Lifecycle - the node representing Self must not appear on any queues.
4034 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
4035 // want residual elements associated with this thread left on any lists.
4036 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
4037 assert (_owner == Self, "invariant") ;
4038 assert (_succ != Self , "invariant") ;
4039 } // OSThreadWaitState()
4041 jt->set_current_waiting_monitor(NULL);
4043 guarantee (_recursions == 0, "invariant") ;
4044 _recursions = save; // restore the old recursion count
4045 _waiters--; // decrement the number of waiters
4047 // Verify a few postconditions
4048 assert (_owner == Self , "invariant") ;
4049 assert (_succ != Self , "invariant") ;
4050 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
4052 if (SyncFlags & 32) {
4053 OrderAccess::fence() ;
4054 }
4056 // check if the notification happened
4057 if (!WasNotified) {
4058 // no, it could be timeout or Thread.interrupt() or both
4059 // check for interrupt event, otherwise it is timeout
4060 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
4061 TEVENT (Wait - throw IEX from epilog) ;
4062 THROW(vmSymbols::java_lang_InterruptedException());
4063 }
4064 }
4066 // NOTE: Spurious wake up will be consider as timeout.
4067 // Monitor notify has precedence over thread interrupt.
4068 }
4071 // Consider:
4072 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
4073 // then instead of transferring a thread from the WaitSet to the EntryList
4074 // we might just dequeue a thread from the WaitSet and directly unpark() it.
4076 void ObjectMonitor::notify(TRAPS) {
4077 CHECK_OWNER();
4078 if (_WaitSet == NULL) {
4079 TEVENT (Empty-Notify) ;
4080 return ;
4081 }
4082 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
4084 int Policy = Knob_MoveNotifyee ;
4086 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
4087 ObjectWaiter * iterator = DequeueWaiter() ;
4088 if (iterator != NULL) {
4089 TEVENT (Notify1 - Transfer) ;
4090 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
4091 guarantee (iterator->_notified == 0, "invariant") ;
4092 // Disposition - what might we do with iterator ?
4093 // a. add it directly to the EntryList - either tail or head.
4094 // b. push it onto the front of the _cxq.
4095 // For now we use (a).
4096 if (Policy != 4) {
4097 iterator->TState = ObjectWaiter::TS_ENTER ;
4098 }
4099 iterator->_notified = 1 ;
4101 ObjectWaiter * List = _EntryList ;
4102 if (List != NULL) {
4103 assert (List->_prev == NULL, "invariant") ;
4104 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4105 assert (List != iterator, "invariant") ;
4106 }
4108 if (Policy == 0) { // prepend to EntryList
4109 if (List == NULL) {
4110 iterator->_next = iterator->_prev = NULL ;
4111 _EntryList = iterator ;
4112 } else {
4113 List->_prev = iterator ;
4114 iterator->_next = List ;
4115 iterator->_prev = NULL ;
4116 _EntryList = iterator ;
4117 }
4118 } else
4119 if (Policy == 1) { // append to EntryList
4120 if (List == NULL) {
4121 iterator->_next = iterator->_prev = NULL ;
4122 _EntryList = iterator ;
4123 } else {
4124 // CONSIDER: finding the tail currently requires a linear-time walk of
4125 // the EntryList. We can make tail access constant-time by converting to
4126 // a CDLL instead of using our current DLL.
4127 ObjectWaiter * Tail ;
4128 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
4129 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
4130 Tail->_next = iterator ;
4131 iterator->_prev = Tail ;
4132 iterator->_next = NULL ;
4133 }
4134 } else
4135 if (Policy == 2) { // prepend to cxq
4136 // prepend to cxq
4137 if (List == NULL) {
4138 iterator->_next = iterator->_prev = NULL ;
4139 _EntryList = iterator ;
4140 } else {
4141 iterator->TState = ObjectWaiter::TS_CXQ ;
4142 for (;;) {
4143 ObjectWaiter * Front = _cxq ;
4144 iterator->_next = Front ;
4145 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
4146 break ;
4147 }
4148 }
4149 }
4150 } else
4151 if (Policy == 3) { // append to cxq
4152 iterator->TState = ObjectWaiter::TS_CXQ ;
4153 for (;;) {
4154 ObjectWaiter * Tail ;
4155 Tail = _cxq ;
4156 if (Tail == NULL) {
4157 iterator->_next = NULL ;
4158 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
4159 break ;
4160 }
4161 } else {
4162 while (Tail->_next != NULL) Tail = Tail->_next ;
4163 Tail->_next = iterator ;
4164 iterator->_prev = Tail ;
4165 iterator->_next = NULL ;
4166 break ;
4167 }
4168 }
4169 } else {
4170 ParkEvent * ev = iterator->_event ;
4171 iterator->TState = ObjectWaiter::TS_RUN ;
4172 OrderAccess::fence() ;
4173 ev->unpark() ;
4174 }
4176 if (Policy < 4) {
4177 iterator->wait_reenter_begin(this);
4178 }
4180 // _WaitSetLock protects the wait queue, not the EntryList. We could
4181 // move the add-to-EntryList operation, above, outside the critical section
4182 // protected by _WaitSetLock. In practice that's not useful. With the
4183 // exception of wait() timeouts and interrupts the monitor owner
4184 // is the only thread that grabs _WaitSetLock. There's almost no contention
4185 // on _WaitSetLock so it's not profitable to reduce the length of the
4186 // critical section.
4187 }
4189 Thread::SpinRelease (&_WaitSetLock) ;
4191 if (iterator != NULL && ObjectSynchronizer::_sync_Notifications != NULL) {
4192 ObjectSynchronizer::_sync_Notifications->inc() ;
4193 }
4194 }
4197 void ObjectMonitor::notifyAll(TRAPS) {
4198 CHECK_OWNER();
4199 ObjectWaiter* iterator;
4200 if (_WaitSet == NULL) {
4201 TEVENT (Empty-NotifyAll) ;
4202 return ;
4203 }
4204 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
4206 int Policy = Knob_MoveNotifyee ;
4207 int Tally = 0 ;
4208 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
4210 for (;;) {
4211 iterator = DequeueWaiter () ;
4212 if (iterator == NULL) break ;
4213 TEVENT (NotifyAll - Transfer1) ;
4214 ++Tally ;
4216 // Disposition - what might we do with iterator ?
4217 // a. add it directly to the EntryList - either tail or head.
4218 // b. push it onto the front of the _cxq.
4219 // For now we use (a).
4220 //
4221 // TODO-FIXME: currently notifyAll() transfers the waiters one-at-a-time from the waitset
4222 // to the EntryList. This could be done more efficiently with a single bulk transfer,
4223 // but in practice it's not time-critical. Beware too, that in prepend-mode we invert the
4224 // order of the waiters. Lets say that the waitset is "ABCD" and the EntryList is "XYZ".
4225 // After a notifyAll() in prepend mode the waitset will be empty and the EntryList will
4226 // be "DCBAXYZ".
4228 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
4229 guarantee (iterator->_notified == 0, "invariant") ;
4230 iterator->_notified = 1 ;
4231 if (Policy != 4) {
4232 iterator->TState = ObjectWaiter::TS_ENTER ;
4233 }
4235 ObjectWaiter * List = _EntryList ;
4236 if (List != NULL) {
4237 assert (List->_prev == NULL, "invariant") ;
4238 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4239 assert (List != iterator, "invariant") ;
4240 }
4242 if (Policy == 0) { // prepend to EntryList
4243 if (List == NULL) {
4244 iterator->_next = iterator->_prev = NULL ;
4245 _EntryList = iterator ;
4246 } else {
4247 List->_prev = iterator ;
4248 iterator->_next = List ;
4249 iterator->_prev = NULL ;
4250 _EntryList = iterator ;
4251 }
4252 } else
4253 if (Policy == 1) { // append to EntryList
4254 if (List == NULL) {
4255 iterator->_next = iterator->_prev = NULL ;
4256 _EntryList = iterator ;
4257 } else {
4258 // CONSIDER: finding the tail currently requires a linear-time walk of
4259 // the EntryList. We can make tail access constant-time by converting to
4260 // a CDLL instead of using our current DLL.
4261 ObjectWaiter * Tail ;
4262 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
4263 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
4264 Tail->_next = iterator ;
4265 iterator->_prev = Tail ;
4266 iterator->_next = NULL ;
4267 }
4268 } else
4269 if (Policy == 2) { // prepend to cxq
4270 // prepend to cxq
4271 iterator->TState = ObjectWaiter::TS_CXQ ;
4272 for (;;) {
4273 ObjectWaiter * Front = _cxq ;
4274 iterator->_next = Front ;
4275 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
4276 break ;
4277 }
4278 }
4279 } else
4280 if (Policy == 3) { // append to cxq
4281 iterator->TState = ObjectWaiter::TS_CXQ ;
4282 for (;;) {
4283 ObjectWaiter * Tail ;
4284 Tail = _cxq ;
4285 if (Tail == NULL) {
4286 iterator->_next = NULL ;
4287 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
4288 break ;
4289 }
4290 } else {
4291 while (Tail->_next != NULL) Tail = Tail->_next ;
4292 Tail->_next = iterator ;
4293 iterator->_prev = Tail ;
4294 iterator->_next = NULL ;
4295 break ;
4296 }
4297 }
4298 } else {
4299 ParkEvent * ev = iterator->_event ;
4300 iterator->TState = ObjectWaiter::TS_RUN ;
4301 OrderAccess::fence() ;
4302 ev->unpark() ;
4303 }
4305 if (Policy < 4) {
4306 iterator->wait_reenter_begin(this);
4307 }
4309 // _WaitSetLock protects the wait queue, not the EntryList. We could
4310 // move the add-to-EntryList operation, above, outside the critical section
4311 // protected by _WaitSetLock. In practice that's not useful. With the
4312 // exception of wait() timeouts and interrupts the monitor owner
4313 // is the only thread that grabs _WaitSetLock. There's almost no contention
4314 // on _WaitSetLock so it's not profitable to reduce the length of the
4315 // critical section.
4316 }
4318 Thread::SpinRelease (&_WaitSetLock) ;
4320 if (Tally != 0 && ObjectSynchronizer::_sync_Notifications != NULL) {
4321 ObjectSynchronizer::_sync_Notifications->inc(Tally) ;
4322 }
4323 }
4325 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
4326 // TODO-FIXME: remove check_slow() -- it's likely dead.
4328 void ObjectMonitor::check_slow(TRAPS) {
4329 TEVENT (check_slow - throw IMSX) ;
4330 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
4331 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
4332 }
4335 // -------------------------------------------------------------------------
4336 // The raw monitor subsystem is entirely distinct from normal
4337 // java-synchronization or jni-synchronization. raw monitors are not
4338 // associated with objects. They can be implemented in any manner
4339 // that makes sense. The original implementors decided to piggy-back
4340 // the raw-monitor implementation on the existing Java objectMonitor mechanism.
4341 // This flaw needs to fixed. We should reimplement raw monitors as sui-generis.
4342 // Specifically, we should not implement raw monitors via java monitors.
4343 // Time permitting, we should disentangle and deconvolve the two implementations
4344 // and move the resulting raw monitor implementation over to the JVMTI directories.
4345 // Ideally, the raw monitor implementation would be built on top of
4346 // park-unpark and nothing else.
4347 //
4348 // raw monitors are used mainly by JVMTI
4349 // The raw monitor implementation borrows the ObjectMonitor structure,
4350 // but the operators are degenerate and extremely simple.
4351 //
4352 // Mixed use of a single objectMonitor instance -- as both a raw monitor
4353 // and a normal java monitor -- is not permissible.
4354 //
4355 // Note that we use the single RawMonitor_lock to protect queue operations for
4356 // _all_ raw monitors. This is a scalability impediment, but since raw monitor usage
4357 // is deprecated and rare, this is not of concern. The RawMonitor_lock can not
4358 // be held indefinitely. The critical sections must be short and bounded.
4359 //
4360 // -------------------------------------------------------------------------
4362 int ObjectMonitor::SimpleEnter (Thread * Self) {
4363 for (;;) {
4364 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
4365 return OS_OK ;
4366 }
4368 ObjectWaiter Node (Self) ;
4369 Self->_ParkEvent->reset() ; // strictly optional
4370 Node.TState = ObjectWaiter::TS_ENTER ;
4372 RawMonitor_lock->lock_without_safepoint_check() ;
4373 Node._next = _EntryList ;
4374 _EntryList = &Node ;
4375 OrderAccess::fence() ;
4376 if (_owner == NULL && Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
4377 _EntryList = Node._next ;
4378 RawMonitor_lock->unlock() ;
4379 return OS_OK ;
4380 }
4381 RawMonitor_lock->unlock() ;
4382 while (Node.TState == ObjectWaiter::TS_ENTER) {
4383 Self->_ParkEvent->park() ;
4384 }
4385 }
4386 }
4388 int ObjectMonitor::SimpleExit (Thread * Self) {
4389 guarantee (_owner == Self, "invariant") ;
4390 OrderAccess::release_store_ptr (&_owner, NULL) ;
4391 OrderAccess::fence() ;
4392 if (_EntryList == NULL) return OS_OK ;
4393 ObjectWaiter * w ;
4395 RawMonitor_lock->lock_without_safepoint_check() ;
4396 w = _EntryList ;
4397 if (w != NULL) {
4398 _EntryList = w->_next ;
4399 }
4400 RawMonitor_lock->unlock() ;
4401 if (w != NULL) {
4402 guarantee (w ->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4403 ParkEvent * ev = w->_event ;
4404 w->TState = ObjectWaiter::TS_RUN ;
4405 OrderAccess::fence() ;
4406 ev->unpark() ;
4407 }
4408 return OS_OK ;
4409 }
4411 int ObjectMonitor::SimpleWait (Thread * Self, jlong millis) {
4412 guarantee (_owner == Self , "invariant") ;
4413 guarantee (_recursions == 0, "invariant") ;
4415 ObjectWaiter Node (Self) ;
4416 Node._notified = 0 ;
4417 Node.TState = ObjectWaiter::TS_WAIT ;
4419 RawMonitor_lock->lock_without_safepoint_check() ;
4420 Node._next = _WaitSet ;
4421 _WaitSet = &Node ;
4422 RawMonitor_lock->unlock() ;
4424 SimpleExit (Self) ;
4425 guarantee (_owner != Self, "invariant") ;
4427 int ret = OS_OK ;
4428 if (millis <= 0) {
4429 Self->_ParkEvent->park();
4430 } else {
4431 ret = Self->_ParkEvent->park(millis);
4432 }
4434 // If thread still resides on the waitset then unlink it.
4435 // Double-checked locking -- the usage is safe in this context
4436 // as we TState is volatile and the lock-unlock operators are
4437 // serializing (barrier-equivalent).
4439 if (Node.TState == ObjectWaiter::TS_WAIT) {
4440 RawMonitor_lock->lock_without_safepoint_check() ;
4441 if (Node.TState == ObjectWaiter::TS_WAIT) {
4442 // Simple O(n) unlink, but performance isn't critical here.
4443 ObjectWaiter * p ;
4444 ObjectWaiter * q = NULL ;
4445 for (p = _WaitSet ; p != &Node; p = p->_next) {
4446 q = p ;
4447 }
4448 guarantee (p == &Node, "invariant") ;
4449 if (q == NULL) {
4450 guarantee (p == _WaitSet, "invariant") ;
4451 _WaitSet = p->_next ;
4452 } else {
4453 guarantee (p == q->_next, "invariant") ;
4454 q->_next = p->_next ;
4455 }
4456 Node.TState = ObjectWaiter::TS_RUN ;
4457 }
4458 RawMonitor_lock->unlock() ;
4459 }
4461 guarantee (Node.TState == ObjectWaiter::TS_RUN, "invariant") ;
4462 SimpleEnter (Self) ;
4464 guarantee (_owner == Self, "invariant") ;
4465 guarantee (_recursions == 0, "invariant") ;
4466 return ret ;
4467 }
4469 int ObjectMonitor::SimpleNotify (Thread * Self, bool All) {
4470 guarantee (_owner == Self, "invariant") ;
4471 if (_WaitSet == NULL) return OS_OK ;
4473 // We have two options:
4474 // A. Transfer the threads from the WaitSet to the EntryList
4475 // B. Remove the thread from the WaitSet and unpark() it.
4476 //
4477 // We use (B), which is crude and results in lots of futile
4478 // context switching. In particular (B) induces lots of contention.
4480 ParkEvent * ev = NULL ; // consider using a small auto array ...
4481 RawMonitor_lock->lock_without_safepoint_check() ;
4482 for (;;) {
4483 ObjectWaiter * w = _WaitSet ;
4484 if (w == NULL) break ;
4485 _WaitSet = w->_next ;
4486 if (ev != NULL) { ev->unpark(); ev = NULL; }
4487 ev = w->_event ;
4488 OrderAccess::loadstore() ;
4489 w->TState = ObjectWaiter::TS_RUN ;
4490 OrderAccess::storeload();
4491 if (!All) break ;
4492 }
4493 RawMonitor_lock->unlock() ;
4494 if (ev != NULL) ev->unpark();
4495 return OS_OK ;
4496 }
4498 // Any JavaThread will enter here with state _thread_blocked
4499 int ObjectMonitor::raw_enter(TRAPS) {
4500 TEVENT (raw_enter) ;
4501 void * Contended ;
4503 // don't enter raw monitor if thread is being externally suspended, it will
4504 // surprise the suspender if a "suspended" thread can still enter monitor
4505 JavaThread * jt = (JavaThread *)THREAD;
4506 if (THREAD->is_Java_thread()) {
4507 jt->SR_lock()->lock_without_safepoint_check();
4508 while (jt->is_external_suspend()) {
4509 jt->SR_lock()->unlock();
4510 jt->java_suspend_self();
4511 jt->SR_lock()->lock_without_safepoint_check();
4512 }
4513 // guarded by SR_lock to avoid racing with new external suspend requests.
4514 Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
4515 jt->SR_lock()->unlock();
4516 } else {
4517 Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
4518 }
4520 if (Contended == THREAD) {
4521 _recursions ++ ;
4522 return OM_OK ;
4523 }
4525 if (Contended == NULL) {
4526 guarantee (_owner == THREAD, "invariant") ;
4527 guarantee (_recursions == 0, "invariant") ;
4528 return OM_OK ;
4529 }
4531 THREAD->set_current_pending_monitor(this);
4533 if (!THREAD->is_Java_thread()) {
4534 // No other non-Java threads besides VM thread would acquire
4535 // a raw monitor.
4536 assert(THREAD->is_VM_thread(), "must be VM thread");
4537 SimpleEnter (THREAD) ;
4538 } else {
4539 guarantee (jt->thread_state() == _thread_blocked, "invariant") ;
4540 for (;;) {
4541 jt->set_suspend_equivalent();
4542 // cleared by handle_special_suspend_equivalent_condition() or
4543 // java_suspend_self()
4544 SimpleEnter (THREAD) ;
4546 // were we externally suspended while we were waiting?
4547 if (!jt->handle_special_suspend_equivalent_condition()) break ;
4549 // This thread was externally suspended
4550 //
4551 // This logic isn't needed for JVMTI raw monitors,
4552 // but doesn't hurt just in case the suspend rules change. This
4553 // logic is needed for the ObjectMonitor.wait() reentry phase.
4554 // We have reentered the contended monitor, but while we were
4555 // waiting another thread suspended us. We don't want to reenter
4556 // the monitor while suspended because that would surprise the
4557 // thread that suspended us.
4558 //
4559 // Drop the lock -
4560 SimpleExit (THREAD) ;
4562 jt->java_suspend_self();
4563 }
4565 assert(_owner == THREAD, "Fatal error with monitor owner!");
4566 assert(_recursions == 0, "Fatal error with monitor recursions!");
4567 }
4569 THREAD->set_current_pending_monitor(NULL);
4570 guarantee (_recursions == 0, "invariant") ;
4571 return OM_OK;
4572 }
4574 // Used mainly for JVMTI raw monitor implementation
4575 // Also used for ObjectMonitor::wait().
4576 int ObjectMonitor::raw_exit(TRAPS) {
4577 TEVENT (raw_exit) ;
4578 if (THREAD != _owner) {
4579 return OM_ILLEGAL_MONITOR_STATE;
4580 }
4581 if (_recursions > 0) {
4582 --_recursions ;
4583 return OM_OK ;
4584 }
4586 void * List = _EntryList ;
4587 SimpleExit (THREAD) ;
4589 return OM_OK;
4590 }
4592 // Used for JVMTI raw monitor implementation.
4593 // All JavaThreads will enter here with state _thread_blocked
4595 int ObjectMonitor::raw_wait(jlong millis, bool interruptible, TRAPS) {
4596 TEVENT (raw_wait) ;
4597 if (THREAD != _owner) {
4598 return OM_ILLEGAL_MONITOR_STATE;
4599 }
4601 // To avoid spurious wakeups we reset the parkevent -- This is strictly optional.
4602 // The caller must be able to tolerate spurious returns from raw_wait().
4603 THREAD->_ParkEvent->reset() ;
4604 OrderAccess::fence() ;
4606 // check interrupt event
4607 if (interruptible && Thread::is_interrupted(THREAD, true)) {
4608 return OM_INTERRUPTED;
4609 }
4611 intptr_t save = _recursions ;
4612 _recursions = 0 ;
4613 _waiters ++ ;
4614 if (THREAD->is_Java_thread()) {
4615 guarantee (((JavaThread *) THREAD)->thread_state() == _thread_blocked, "invariant") ;
4616 ((JavaThread *)THREAD)->set_suspend_equivalent();
4617 }
4618 int rv = SimpleWait (THREAD, millis) ;
4619 _recursions = save ;
4620 _waiters -- ;
4622 guarantee (THREAD == _owner, "invariant") ;
4623 if (THREAD->is_Java_thread()) {
4624 JavaThread * jSelf = (JavaThread *) THREAD ;
4625 for (;;) {
4626 if (!jSelf->handle_special_suspend_equivalent_condition()) break ;
4627 SimpleExit (THREAD) ;
4628 jSelf->java_suspend_self();
4629 SimpleEnter (THREAD) ;
4630 jSelf->set_suspend_equivalent() ;
4631 }
4632 }
4633 guarantee (THREAD == _owner, "invariant") ;
4635 if (interruptible && Thread::is_interrupted(THREAD, true)) {
4636 return OM_INTERRUPTED;
4637 }
4638 return OM_OK ;
4639 }
4641 int ObjectMonitor::raw_notify(TRAPS) {
4642 TEVENT (raw_notify) ;
4643 if (THREAD != _owner) {
4644 return OM_ILLEGAL_MONITOR_STATE;
4645 }
4646 SimpleNotify (THREAD, false) ;
4647 return OM_OK;
4648 }
4650 int ObjectMonitor::raw_notifyAll(TRAPS) {
4651 TEVENT (raw_notifyAll) ;
4652 if (THREAD != _owner) {
4653 return OM_ILLEGAL_MONITOR_STATE;
4654 }
4655 SimpleNotify (THREAD, true) ;
4656 return OM_OK;
4657 }
4659 #ifndef PRODUCT
4660 void ObjectMonitor::verify() {
4661 }
4663 void ObjectMonitor::print() {
4664 }
4665 #endif
4667 //------------------------------------------------------------------------------
4668 // Non-product code
4670 #ifndef PRODUCT
4672 void ObjectSynchronizer::trace_locking(Handle locking_obj, bool is_compiled,
4673 bool is_method, bool is_locking) {
4674 // Don't know what to do here
4675 }
4677 // Verify all monitors in the monitor cache, the verification is weak.
4678 void ObjectSynchronizer::verify() {
4679 ObjectMonitor* block = gBlockList;
4680 ObjectMonitor* mid;
4681 while (block) {
4682 assert(block->object() == CHAINMARKER, "must be a block header");
4683 for (int i = 1; i < _BLOCKSIZE; i++) {
4684 mid = block + i;
4685 oop object = (oop) mid->object();
4686 if (object != NULL) {
4687 mid->verify();
4688 }
4689 }
4690 block = (ObjectMonitor*) block->FreeNext;
4691 }
4692 }
4694 // Check if monitor belongs to the monitor cache
4695 // The list is grow-only so it's *relatively* safe to traverse
4696 // the list of extant blocks without taking a lock.
4698 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
4699 ObjectMonitor* block = gBlockList;
4701 while (block) {
4702 assert(block->object() == CHAINMARKER, "must be a block header");
4703 if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
4704 address mon = (address) monitor;
4705 address blk = (address) block;
4706 size_t diff = mon - blk;
4707 assert((diff % sizeof(ObjectMonitor)) == 0, "check");
4708 return 1;
4709 }
4710 block = (ObjectMonitor*) block->FreeNext;
4711 }
4712 return 0;
4713 }
4715 #endif