Wed, 17 Aug 2011 10:32:53 -0700
6791672: enable 1G and larger pages on solaris
Reviewed-by: ysr, iveresov, johnc
1 /*
2 * Copyright (c) 1998, 2011, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/osThread.hpp"
36 #include "runtime/stubRoutines.hpp"
37 #include "runtime/thread.hpp"
38 #include "services/threadService.hpp"
39 #include "utilities/dtrace.hpp"
40 #include "utilities/preserveException.hpp"
41 #ifdef TARGET_OS_FAMILY_linux
42 # include "os_linux.inline.hpp"
43 # include "thread_linux.inline.hpp"
44 #endif
45 #ifdef TARGET_OS_FAMILY_solaris
46 # include "os_solaris.inline.hpp"
47 # include "thread_solaris.inline.hpp"
48 #endif
49 #ifdef TARGET_OS_FAMILY_windows
50 # include "os_windows.inline.hpp"
51 # include "thread_windows.inline.hpp"
52 #endif
54 #if defined(__GNUC__) && !defined(IA64)
55 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
56 #define ATTR __attribute__((noinline))
57 #else
58 #define ATTR
59 #endif
62 #ifdef DTRACE_ENABLED
64 // Only bother with this argument setup if dtrace is available
65 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
67 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
68 jlong, uintptr_t, char*, int);
69 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
70 jlong, uintptr_t, char*, int);
71 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
72 jlong, uintptr_t, char*, int);
73 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
74 jlong, uintptr_t, char*, int);
75 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
76 jlong, uintptr_t, char*, int);
78 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \
79 char* bytes = NULL; \
80 int len = 0; \
81 jlong jtid = SharedRuntime::get_java_tid(thread); \
82 Symbol* klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
83 if (klassname != NULL) { \
84 bytes = (char*)klassname->bytes(); \
85 len = klassname->utf8_length(); \
86 }
88 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
89 { \
90 if (DTraceMonitorProbes) { \
91 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
92 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
93 (monitor), bytes, len, (millis)); \
94 } \
95 }
97 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
98 { \
99 if (DTraceMonitorProbes) { \
100 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
101 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
102 (uintptr_t)(monitor), bytes, len); \
103 } \
104 }
106 #else // ndef DTRACE_ENABLED
108 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
109 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;}
111 #endif // ndef DTRACE_ENABLED
113 // Tunables ...
114 // The knob* variables are effectively final. Once set they should
115 // never be modified hence. Consider using __read_mostly with GCC.
117 int ObjectMonitor::Knob_Verbose = 0 ;
118 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
119 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
120 static int Knob_HandOff = 0 ;
121 static int Knob_ReportSettings = 0 ;
123 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
124 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
125 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
126 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
127 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
128 static int Knob_SpinEarly = 1 ;
129 static int Knob_SuccEnabled = 1 ; // futile wake throttling
130 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
131 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
132 static int Knob_Bonus = 100 ; // spin success bonus
133 static int Knob_BonusB = 100 ; // spin success bonus
134 static int Knob_Penalty = 200 ; // spin failure penalty
135 static int Knob_Poverty = 1000 ;
136 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
137 static int Knob_FixedSpin = 0 ;
138 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
139 static int Knob_UsePause = 1 ;
140 static int Knob_ExitPolicy = 0 ;
141 static int Knob_PreSpin = 10 ; // 20-100 likely better
142 static int Knob_ResetEvent = 0 ;
143 static int BackOffMask = 0 ;
145 static int Knob_FastHSSEC = 0 ;
146 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
147 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
148 static volatile int InitDone = 0 ;
150 #define TrySpin TrySpin_VaryDuration
152 // -----------------------------------------------------------------------------
153 // Theory of operations -- Monitors lists, thread residency, etc:
154 //
155 // * A thread acquires ownership of a monitor by successfully
156 // CAS()ing the _owner field from null to non-null.
157 //
158 // * Invariant: A thread appears on at most one monitor list --
159 // cxq, EntryList or WaitSet -- at any one time.
160 //
161 // * Contending threads "push" themselves onto the cxq with CAS
162 // and then spin/park.
163 //
164 // * After a contending thread eventually acquires the lock it must
165 // dequeue itself from either the EntryList or the cxq.
166 //
167 // * The exiting thread identifies and unparks an "heir presumptive"
168 // tentative successor thread on the EntryList. Critically, the
169 // exiting thread doesn't unlink the successor thread from the EntryList.
170 // After having been unparked, the wakee will recontend for ownership of
171 // the monitor. The successor (wakee) will either acquire the lock or
172 // re-park itself.
173 //
174 // Succession is provided for by a policy of competitive handoff.
175 // The exiting thread does _not_ grant or pass ownership to the
176 // successor thread. (This is also referred to as "handoff" succession").
177 // Instead the exiting thread releases ownership and possibly wakes
178 // a successor, so the successor can (re)compete for ownership of the lock.
179 // If the EntryList is empty but the cxq is populated the exiting
180 // thread will drain the cxq into the EntryList. It does so by
181 // by detaching the cxq (installing null with CAS) and folding
182 // the threads from the cxq into the EntryList. The EntryList is
183 // doubly linked, while the cxq is singly linked because of the
184 // CAS-based "push" used to enqueue recently arrived threads (RATs).
185 //
186 // * Concurrency invariants:
187 //
188 // -- only the monitor owner may access or mutate the EntryList.
189 // The mutex property of the monitor itself protects the EntryList
190 // from concurrent interference.
191 // -- Only the monitor owner may detach the cxq.
192 //
193 // * The monitor entry list operations avoid locks, but strictly speaking
194 // they're not lock-free. Enter is lock-free, exit is not.
195 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
196 //
197 // * The cxq can have multiple concurrent "pushers" but only one concurrent
198 // detaching thread. This mechanism is immune from the ABA corruption.
199 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
200 //
201 // * Taken together, the cxq and the EntryList constitute or form a
202 // single logical queue of threads stalled trying to acquire the lock.
203 // We use two distinct lists to improve the odds of a constant-time
204 // dequeue operation after acquisition (in the ::enter() epilog) and
205 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
206 // A key desideratum is to minimize queue & monitor metadata manipulation
207 // that occurs while holding the monitor lock -- that is, we want to
208 // minimize monitor lock holds times. Note that even a small amount of
209 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
210 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
211 // locks and monitor metadata.
212 //
213 // Cxq points to the the set of Recently Arrived Threads attempting entry.
214 // Because we push threads onto _cxq with CAS, the RATs must take the form of
215 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
216 // the unlocking thread notices that EntryList is null but _cxq is != null.
217 //
218 // The EntryList is ordered by the prevailing queue discipline and
219 // can be organized in any convenient fashion, such as a doubly-linked list or
220 // a circular doubly-linked list. Critically, we want insert and delete operations
221 // to operate in constant-time. If we need a priority queue then something akin
222 // to Solaris' sleepq would work nicely. Viz.,
223 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
224 // Queue discipline is enforced at ::exit() time, when the unlocking thread
225 // drains the cxq into the EntryList, and orders or reorders the threads on the
226 // EntryList accordingly.
227 //
228 // Barring "lock barging", this mechanism provides fair cyclic ordering,
229 // somewhat similar to an elevator-scan.
230 //
231 // * The monitor synchronization subsystem avoids the use of native
232 // synchronization primitives except for the narrow platform-specific
233 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
234 // the semantics of park-unpark. Put another way, this monitor implementation
235 // depends only on atomic operations and park-unpark. The monitor subsystem
236 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
237 // underlying OS manages the READY<->RUN transitions.
238 //
239 // * Waiting threads reside on the WaitSet list -- wait() puts
240 // the caller onto the WaitSet.
241 //
242 // * notify() or notifyAll() simply transfers threads from the WaitSet to
243 // either the EntryList or cxq. Subsequent exit() operations will
244 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
245 // it's likely the notifyee would simply impale itself on the lock held
246 // by the notifier.
247 //
248 // * An interesting alternative is to encode cxq as (List,LockByte) where
249 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
250 // variable, like _recursions, in the scheme. The threads or Events that form
251 // the list would have to be aligned in 256-byte addresses. A thread would
252 // try to acquire the lock or enqueue itself with CAS, but exiting threads
253 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
254 // Note that is is *not* word-tearing, but it does presume that full-word
255 // CAS operations are coherent with intermix with STB operations. That's true
256 // on most common processors.
257 //
258 // * See also http://blogs.sun.com/dave
261 // -----------------------------------------------------------------------------
262 // Enter support
264 bool ObjectMonitor::try_enter(Thread* THREAD) {
265 if (THREAD != _owner) {
266 if (THREAD->is_lock_owned ((address)_owner)) {
267 assert(_recursions == 0, "internal state error");
268 _owner = THREAD ;
269 _recursions = 1 ;
270 OwnerIsThread = 1 ;
271 return true;
272 }
273 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
274 return false;
275 }
276 return true;
277 } else {
278 _recursions++;
279 return true;
280 }
281 }
283 void ATTR ObjectMonitor::enter(TRAPS) {
284 // The following code is ordered to check the most common cases first
285 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
286 Thread * const Self = THREAD ;
287 void * cur ;
289 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
290 if (cur == NULL) {
291 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
292 assert (_recursions == 0 , "invariant") ;
293 assert (_owner == Self, "invariant") ;
294 // CONSIDER: set or assert OwnerIsThread == 1
295 return ;
296 }
298 if (cur == Self) {
299 // TODO-FIXME: check for integer overflow! BUGID 6557169.
300 _recursions ++ ;
301 return ;
302 }
304 if (Self->is_lock_owned ((address)cur)) {
305 assert (_recursions == 0, "internal state error");
306 _recursions = 1 ;
307 // Commute owner from a thread-specific on-stack BasicLockObject address to
308 // a full-fledged "Thread *".
309 _owner = Self ;
310 OwnerIsThread = 1 ;
311 return ;
312 }
314 // We've encountered genuine contention.
315 assert (Self->_Stalled == 0, "invariant") ;
316 Self->_Stalled = intptr_t(this) ;
318 // Try one round of spinning *before* enqueueing Self
319 // and before going through the awkward and expensive state
320 // transitions. The following spin is strictly optional ...
321 // Note that if we acquire the monitor from an initial spin
322 // we forgo posting JVMTI events and firing DTRACE probes.
323 if (Knob_SpinEarly && TrySpin (Self) > 0) {
324 assert (_owner == Self , "invariant") ;
325 assert (_recursions == 0 , "invariant") ;
326 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
327 Self->_Stalled = 0 ;
328 return ;
329 }
331 assert (_owner != Self , "invariant") ;
332 assert (_succ != Self , "invariant") ;
333 assert (Self->is_Java_thread() , "invariant") ;
334 JavaThread * jt = (JavaThread *) Self ;
335 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
336 assert (jt->thread_state() != _thread_blocked , "invariant") ;
337 assert (this->object() != NULL , "invariant") ;
338 assert (_count >= 0, "invariant") ;
340 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
341 // Ensure the object-monitor relationship remains stable while there's contention.
342 Atomic::inc_ptr(&_count);
344 { // Change java thread status to indicate blocked on monitor enter.
345 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
347 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
348 if (JvmtiExport::should_post_monitor_contended_enter()) {
349 JvmtiExport::post_monitor_contended_enter(jt, this);
350 }
352 OSThreadContendState osts(Self->osthread());
353 ThreadBlockInVM tbivm(jt);
355 Self->set_current_pending_monitor(this);
357 // TODO-FIXME: change the following for(;;) loop to straight-line code.
358 for (;;) {
359 jt->set_suspend_equivalent();
360 // cleared by handle_special_suspend_equivalent_condition()
361 // or java_suspend_self()
363 EnterI (THREAD) ;
365 if (!ExitSuspendEquivalent(jt)) break ;
367 //
368 // We have acquired the contended monitor, but while we were
369 // waiting another thread suspended us. We don't want to enter
370 // the monitor while suspended because that would surprise the
371 // thread that suspended us.
372 //
373 _recursions = 0 ;
374 _succ = NULL ;
375 exit (Self) ;
377 jt->java_suspend_self();
378 }
379 Self->set_current_pending_monitor(NULL);
380 }
382 Atomic::dec_ptr(&_count);
383 assert (_count >= 0, "invariant") ;
384 Self->_Stalled = 0 ;
386 // Must either set _recursions = 0 or ASSERT _recursions == 0.
387 assert (_recursions == 0 , "invariant") ;
388 assert (_owner == Self , "invariant") ;
389 assert (_succ != Self , "invariant") ;
390 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
392 // The thread -- now the owner -- is back in vm mode.
393 // Report the glorious news via TI,DTrace and jvmstat.
394 // The probe effect is non-trivial. All the reportage occurs
395 // while we hold the monitor, increasing the length of the critical
396 // section. Amdahl's parallel speedup law comes vividly into play.
397 //
398 // Another option might be to aggregate the events (thread local or
399 // per-monitor aggregation) and defer reporting until a more opportune
400 // time -- such as next time some thread encounters contention but has
401 // yet to acquire the lock. While spinning that thread could
402 // spinning we could increment JVMStat counters, etc.
404 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
405 if (JvmtiExport::should_post_monitor_contended_entered()) {
406 JvmtiExport::post_monitor_contended_entered(jt, this);
407 }
408 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
409 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
410 }
411 }
414 // Caveat: TryLock() is not necessarily serializing if it returns failure.
415 // Callers must compensate as needed.
417 int ObjectMonitor::TryLock (Thread * Self) {
418 for (;;) {
419 void * own = _owner ;
420 if (own != NULL) return 0 ;
421 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
422 // Either guarantee _recursions == 0 or set _recursions = 0.
423 assert (_recursions == 0, "invariant") ;
424 assert (_owner == Self, "invariant") ;
425 // CONSIDER: set or assert that OwnerIsThread == 1
426 return 1 ;
427 }
428 // The lock had been free momentarily, but we lost the race to the lock.
429 // Interference -- the CAS failed.
430 // We can either return -1 or retry.
431 // Retry doesn't make as much sense because the lock was just acquired.
432 if (true) return -1 ;
433 }
434 }
436 void ATTR ObjectMonitor::EnterI (TRAPS) {
437 Thread * Self = THREAD ;
438 assert (Self->is_Java_thread(), "invariant") ;
439 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
441 // Try the lock - TATAS
442 if (TryLock (Self) > 0) {
443 assert (_succ != Self , "invariant") ;
444 assert (_owner == Self , "invariant") ;
445 assert (_Responsible != Self , "invariant") ;
446 return ;
447 }
449 DeferredInitialize () ;
451 // We try one round of spinning *before* enqueueing Self.
452 //
453 // If the _owner is ready but OFFPROC we could use a YieldTo()
454 // operation to donate the remainder of this thread's quantum
455 // to the owner. This has subtle but beneficial affinity
456 // effects.
458 if (TrySpin (Self) > 0) {
459 assert (_owner == Self , "invariant") ;
460 assert (_succ != Self , "invariant") ;
461 assert (_Responsible != Self , "invariant") ;
462 return ;
463 }
465 // The Spin failed -- Enqueue and park the thread ...
466 assert (_succ != Self , "invariant") ;
467 assert (_owner != Self , "invariant") ;
468 assert (_Responsible != Self , "invariant") ;
470 // Enqueue "Self" on ObjectMonitor's _cxq.
471 //
472 // Node acts as a proxy for Self.
473 // As an aside, if were to ever rewrite the synchronization code mostly
474 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
475 // Java objects. This would avoid awkward lifecycle and liveness issues,
476 // as well as eliminate a subset of ABA issues.
477 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
478 //
480 ObjectWaiter node(Self) ;
481 Self->_ParkEvent->reset() ;
482 node._prev = (ObjectWaiter *) 0xBAD ;
483 node.TState = ObjectWaiter::TS_CXQ ;
485 // Push "Self" onto the front of the _cxq.
486 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
487 // Note that spinning tends to reduce the rate at which threads
488 // enqueue and dequeue on EntryList|cxq.
489 ObjectWaiter * nxt ;
490 for (;;) {
491 node._next = nxt = _cxq ;
492 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
494 // Interference - the CAS failed because _cxq changed. Just retry.
495 // As an optional optimization we retry the lock.
496 if (TryLock (Self) > 0) {
497 assert (_succ != Self , "invariant") ;
498 assert (_owner == Self , "invariant") ;
499 assert (_Responsible != Self , "invariant") ;
500 return ;
501 }
502 }
504 // Check for cxq|EntryList edge transition to non-null. This indicates
505 // the onset of contention. While contention persists exiting threads
506 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
507 // operations revert to the faster 1-0 mode. This enter operation may interleave
508 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
509 // arrange for one of the contending thread to use a timed park() operations
510 // to detect and recover from the race. (Stranding is form of progress failure
511 // where the monitor is unlocked but all the contending threads remain parked).
512 // That is, at least one of the contended threads will periodically poll _owner.
513 // One of the contending threads will become the designated "Responsible" thread.
514 // The Responsible thread uses a timed park instead of a normal indefinite park
515 // operation -- it periodically wakes and checks for and recovers from potential
516 // strandings admitted by 1-0 exit operations. We need at most one Responsible
517 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
518 // be responsible for a monitor.
519 //
520 // Currently, one of the contended threads takes on the added role of "Responsible".
521 // A viable alternative would be to use a dedicated "stranding checker" thread
522 // that periodically iterated over all the threads (or active monitors) and unparked
523 // successors where there was risk of stranding. This would help eliminate the
524 // timer scalability issues we see on some platforms as we'd only have one thread
525 // -- the checker -- parked on a timer.
527 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
528 // Try to assume the role of responsible thread for the monitor.
529 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
530 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
531 }
533 // The lock have been released while this thread was occupied queueing
534 // itself onto _cxq. To close the race and avoid "stranding" and
535 // progress-liveness failure we must resample-retry _owner before parking.
536 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
537 // In this case the ST-MEMBAR is accomplished with CAS().
538 //
539 // TODO: Defer all thread state transitions until park-time.
540 // Since state transitions are heavy and inefficient we'd like
541 // to defer the state transitions until absolutely necessary,
542 // and in doing so avoid some transitions ...
544 TEVENT (Inflated enter - Contention) ;
545 int nWakeups = 0 ;
546 int RecheckInterval = 1 ;
548 for (;;) {
550 if (TryLock (Self) > 0) break ;
551 assert (_owner != Self, "invariant") ;
553 if ((SyncFlags & 2) && _Responsible == NULL) {
554 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
555 }
557 // park self
558 if (_Responsible == Self || (SyncFlags & 1)) {
559 TEVENT (Inflated enter - park TIMED) ;
560 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
561 // Increase the RecheckInterval, but clamp the value.
562 RecheckInterval *= 8 ;
563 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
564 } else {
565 TEVENT (Inflated enter - park UNTIMED) ;
566 Self->_ParkEvent->park() ;
567 }
569 if (TryLock(Self) > 0) break ;
571 // The lock is still contested.
572 // Keep a tally of the # of futile wakeups.
573 // Note that the counter is not protected by a lock or updated by atomics.
574 // That is by design - we trade "lossy" counters which are exposed to
575 // races during updates for a lower probe effect.
576 TEVENT (Inflated enter - Futile wakeup) ;
577 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
578 ObjectMonitor::_sync_FutileWakeups->inc() ;
579 }
580 ++ nWakeups ;
582 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
583 // We can defer clearing _succ until after the spin completes
584 // TrySpin() must tolerate being called with _succ == Self.
585 // Try yet another round of adaptive spinning.
586 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
588 // We can find that we were unpark()ed and redesignated _succ while
589 // we were spinning. That's harmless. If we iterate and call park(),
590 // park() will consume the event and return immediately and we'll
591 // just spin again. This pattern can repeat, leaving _succ to simply
592 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
593 // Alternately, we can sample fired() here, and if set, forgo spinning
594 // in the next iteration.
596 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
597 Self->_ParkEvent->reset() ;
598 OrderAccess::fence() ;
599 }
600 if (_succ == Self) _succ = NULL ;
602 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
603 OrderAccess::fence() ;
604 }
606 // Egress :
607 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
608 // Normally we'll find Self on the EntryList .
609 // From the perspective of the lock owner (this thread), the
610 // EntryList is stable and cxq is prepend-only.
611 // The head of cxq is volatile but the interior is stable.
612 // In addition, Self.TState is stable.
614 assert (_owner == Self , "invariant") ;
615 assert (object() != NULL , "invariant") ;
616 // I'd like to write:
617 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
618 // but as we're at a safepoint that's not safe.
620 UnlinkAfterAcquire (Self, &node) ;
621 if (_succ == Self) _succ = NULL ;
623 assert (_succ != Self, "invariant") ;
624 if (_Responsible == Self) {
625 _Responsible = NULL ;
626 // Dekker pivot-point.
627 // Consider OrderAccess::storeload() here
629 // We may leave threads on cxq|EntryList without a designated
630 // "Responsible" thread. This is benign. When this thread subsequently
631 // exits the monitor it can "see" such preexisting "old" threads --
632 // threads that arrived on the cxq|EntryList before the fence, above --
633 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
634 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
635 // non-null and elect a new "Responsible" timer thread.
636 //
637 // This thread executes:
638 // ST Responsible=null; MEMBAR (in enter epilog - here)
639 // LD cxq|EntryList (in subsequent exit)
640 //
641 // Entering threads in the slow/contended path execute:
642 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
643 // The (ST cxq; MEMBAR) is accomplished with CAS().
644 //
645 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
646 // exit operation from floating above the ST Responsible=null.
647 //
648 // In *practice* however, EnterI() is always followed by some atomic
649 // operation such as the decrement of _count in ::enter(). Those atomics
650 // obviate the need for the explicit MEMBAR, above.
651 }
653 // We've acquired ownership with CAS().
654 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
655 // But since the CAS() this thread may have also stored into _succ,
656 // EntryList, cxq or Responsible. These meta-data updates must be
657 // visible __before this thread subsequently drops the lock.
658 // Consider what could occur if we didn't enforce this constraint --
659 // STs to monitor meta-data and user-data could reorder with (become
660 // visible after) the ST in exit that drops ownership of the lock.
661 // Some other thread could then acquire the lock, but observe inconsistent
662 // or old monitor meta-data and heap data. That violates the JMM.
663 // To that end, the 1-0 exit() operation must have at least STST|LDST
664 // "release" barrier semantics. Specifically, there must be at least a
665 // STST|LDST barrier in exit() before the ST of null into _owner that drops
666 // the lock. The barrier ensures that changes to monitor meta-data and data
667 // protected by the lock will be visible before we release the lock, and
668 // therefore before some other thread (CPU) has a chance to acquire the lock.
669 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
670 //
671 // Critically, any prior STs to _succ or EntryList must be visible before
672 // the ST of null into _owner in the *subsequent* (following) corresponding
673 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
674 // execute a serializing instruction.
676 if (SyncFlags & 8) {
677 OrderAccess::fence() ;
678 }
679 return ;
680 }
682 // ReenterI() is a specialized inline form of the latter half of the
683 // contended slow-path from EnterI(). We use ReenterI() only for
684 // monitor reentry in wait().
685 //
686 // In the future we should reconcile EnterI() and ReenterI(), adding
687 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
688 // loop accordingly.
690 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
691 assert (Self != NULL , "invariant") ;
692 assert (SelfNode != NULL , "invariant") ;
693 assert (SelfNode->_thread == Self , "invariant") ;
694 assert (_waiters > 0 , "invariant") ;
695 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
696 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
697 JavaThread * jt = (JavaThread *) Self ;
699 int nWakeups = 0 ;
700 for (;;) {
701 ObjectWaiter::TStates v = SelfNode->TState ;
702 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
703 assert (_owner != Self, "invariant") ;
705 if (TryLock (Self) > 0) break ;
706 if (TrySpin (Self) > 0) break ;
708 TEVENT (Wait Reentry - parking) ;
710 // State transition wrappers around park() ...
711 // ReenterI() wisely defers state transitions until
712 // it's clear we must park the thread.
713 {
714 OSThreadContendState osts(Self->osthread());
715 ThreadBlockInVM tbivm(jt);
717 // cleared by handle_special_suspend_equivalent_condition()
718 // or java_suspend_self()
719 jt->set_suspend_equivalent();
720 if (SyncFlags & 1) {
721 Self->_ParkEvent->park ((jlong)1000) ;
722 } else {
723 Self->_ParkEvent->park () ;
724 }
726 // were we externally suspended while we were waiting?
727 for (;;) {
728 if (!ExitSuspendEquivalent (jt)) break ;
729 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
730 jt->java_suspend_self();
731 jt->set_suspend_equivalent();
732 }
733 }
735 // Try again, but just so we distinguish between futile wakeups and
736 // successful wakeups. The following test isn't algorithmically
737 // necessary, but it helps us maintain sensible statistics.
738 if (TryLock(Self) > 0) break ;
740 // The lock is still contested.
741 // Keep a tally of the # of futile wakeups.
742 // Note that the counter is not protected by a lock or updated by atomics.
743 // That is by design - we trade "lossy" counters which are exposed to
744 // races during updates for a lower probe effect.
745 TEVENT (Wait Reentry - futile wakeup) ;
746 ++ nWakeups ;
748 // Assuming this is not a spurious wakeup we'll normally
749 // find that _succ == Self.
750 if (_succ == Self) _succ = NULL ;
752 // Invariant: after clearing _succ a contending thread
753 // *must* retry _owner before parking.
754 OrderAccess::fence() ;
756 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
757 ObjectMonitor::_sync_FutileWakeups->inc() ;
758 }
759 }
761 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
762 // Normally we'll find Self on the EntryList.
763 // Unlinking from the EntryList is constant-time and atomic-free.
764 // From the perspective of the lock owner (this thread), the
765 // EntryList is stable and cxq is prepend-only.
766 // The head of cxq is volatile but the interior is stable.
767 // In addition, Self.TState is stable.
769 assert (_owner == Self, "invariant") ;
770 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
771 UnlinkAfterAcquire (Self, SelfNode) ;
772 if (_succ == Self) _succ = NULL ;
773 assert (_succ != Self, "invariant") ;
774 SelfNode->TState = ObjectWaiter::TS_RUN ;
775 OrderAccess::fence() ; // see comments at the end of EnterI()
776 }
778 // after the thread acquires the lock in ::enter(). Equally, we could defer
779 // unlinking the thread until ::exit()-time.
781 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
782 {
783 assert (_owner == Self, "invariant") ;
784 assert (SelfNode->_thread == Self, "invariant") ;
786 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
787 // Normal case: remove Self from the DLL EntryList .
788 // This is a constant-time operation.
789 ObjectWaiter * nxt = SelfNode->_next ;
790 ObjectWaiter * prv = SelfNode->_prev ;
791 if (nxt != NULL) nxt->_prev = prv ;
792 if (prv != NULL) prv->_next = nxt ;
793 if (SelfNode == _EntryList ) _EntryList = nxt ;
794 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
795 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
796 TEVENT (Unlink from EntryList) ;
797 } else {
798 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
799 // Inopportune interleaving -- Self is still on the cxq.
800 // This usually means the enqueue of self raced an exiting thread.
801 // Normally we'll find Self near the front of the cxq, so
802 // dequeueing is typically fast. If needbe we can accelerate
803 // this with some MCS/CHL-like bidirectional list hints and advisory
804 // back-links so dequeueing from the interior will normally operate
805 // in constant-time.
806 // Dequeue Self from either the head (with CAS) or from the interior
807 // with a linear-time scan and normal non-atomic memory operations.
808 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
809 // and then unlink Self from EntryList. We have to drain eventually,
810 // so it might as well be now.
812 ObjectWaiter * v = _cxq ;
813 assert (v != NULL, "invariant") ;
814 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
815 // The CAS above can fail from interference IFF a "RAT" arrived.
816 // In that case Self must be in the interior and can no longer be
817 // at the head of cxq.
818 if (v == SelfNode) {
819 assert (_cxq != v, "invariant") ;
820 v = _cxq ; // CAS above failed - start scan at head of list
821 }
822 ObjectWaiter * p ;
823 ObjectWaiter * q = NULL ;
824 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
825 q = p ;
826 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
827 }
828 assert (v != SelfNode, "invariant") ;
829 assert (p == SelfNode, "Node not found on cxq") ;
830 assert (p != _cxq, "invariant") ;
831 assert (q != NULL, "invariant") ;
832 assert (q->_next == p, "invariant") ;
833 q->_next = p->_next ;
834 }
835 TEVENT (Unlink from cxq) ;
836 }
838 // Diagnostic hygiene ...
839 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
840 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
841 SelfNode->TState = ObjectWaiter::TS_RUN ;
842 }
844 // -----------------------------------------------------------------------------
845 // Exit support
846 //
847 // exit()
848 // ~~~~~~
849 // Note that the collector can't reclaim the objectMonitor or deflate
850 // the object out from underneath the thread calling ::exit() as the
851 // thread calling ::exit() never transitions to a stable state.
852 // This inhibits GC, which in turn inhibits asynchronous (and
853 // inopportune) reclamation of "this".
854 //
855 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
856 // There's one exception to the claim above, however. EnterI() can call
857 // exit() to drop a lock if the acquirer has been externally suspended.
858 // In that case exit() is called with _thread_state as _thread_blocked,
859 // but the monitor's _count field is > 0, which inhibits reclamation.
860 //
861 // 1-0 exit
862 // ~~~~~~~~
863 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
864 // the fast-path operators have been optimized so the common ::exit()
865 // operation is 1-0. See i486.ad fast_unlock(), for instance.
866 // The code emitted by fast_unlock() elides the usual MEMBAR. This
867 // greatly improves latency -- MEMBAR and CAS having considerable local
868 // latency on modern processors -- but at the cost of "stranding". Absent the
869 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
870 // ::enter() path, resulting in the entering thread being stranding
871 // and a progress-liveness failure. Stranding is extremely rare.
872 // We use timers (timed park operations) & periodic polling to detect
873 // and recover from stranding. Potentially stranded threads periodically
874 // wake up and poll the lock. See the usage of the _Responsible variable.
875 //
876 // The CAS() in enter provides for safety and exclusion, while the CAS or
877 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
878 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
879 // We detect and recover from stranding with timers.
880 //
881 // If a thread transiently strands it'll park until (a) another
882 // thread acquires the lock and then drops the lock, at which time the
883 // exiting thread will notice and unpark the stranded thread, or, (b)
884 // the timer expires. If the lock is high traffic then the stranding latency
885 // will be low due to (a). If the lock is low traffic then the odds of
886 // stranding are lower, although the worst-case stranding latency
887 // is longer. Critically, we don't want to put excessive load in the
888 // platform's timer subsystem. We want to minimize both the timer injection
889 // rate (timers created/sec) as well as the number of timers active at
890 // any one time. (more precisely, we want to minimize timer-seconds, which is
891 // the integral of the # of active timers at any instant over time).
892 // Both impinge on OS scalability. Given that, at most one thread parked on
893 // a monitor will use a timer.
895 void ATTR ObjectMonitor::exit(TRAPS) {
896 Thread * Self = THREAD ;
897 if (THREAD != _owner) {
898 if (THREAD->is_lock_owned((address) _owner)) {
899 // Transmute _owner from a BasicLock pointer to a Thread address.
900 // We don't need to hold _mutex for this transition.
901 // Non-null to Non-null is safe as long as all readers can
902 // tolerate either flavor.
903 assert (_recursions == 0, "invariant") ;
904 _owner = THREAD ;
905 _recursions = 0 ;
906 OwnerIsThread = 1 ;
907 } else {
908 // NOTE: we need to handle unbalanced monitor enter/exit
909 // in native code by throwing an exception.
910 // TODO: Throw an IllegalMonitorStateException ?
911 TEVENT (Exit - Throw IMSX) ;
912 assert(false, "Non-balanced monitor enter/exit!");
913 if (false) {
914 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
915 }
916 return;
917 }
918 }
920 if (_recursions != 0) {
921 _recursions--; // this is simple recursive enter
922 TEVENT (Inflated exit - recursive) ;
923 return ;
924 }
926 // Invariant: after setting Responsible=null an thread must execute
927 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
928 if ((SyncFlags & 4) == 0) {
929 _Responsible = NULL ;
930 }
932 for (;;) {
933 assert (THREAD == _owner, "invariant") ;
936 if (Knob_ExitPolicy == 0) {
937 // release semantics: prior loads and stores from within the critical section
938 // must not float (reorder) past the following store that drops the lock.
939 // On SPARC that requires MEMBAR #loadstore|#storestore.
940 // But of course in TSO #loadstore|#storestore is not required.
941 // I'd like to write one of the following:
942 // A. OrderAccess::release() ; _owner = NULL
943 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
944 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
945 // store into a _dummy variable. That store is not needed, but can result
946 // in massive wasteful coherency traffic on classic SMP systems.
947 // Instead, I use release_store(), which is implemented as just a simple
948 // ST on x64, x86 and SPARC.
949 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
950 OrderAccess::storeload() ; // See if we need to wake a successor
951 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
952 TEVENT (Inflated exit - simple egress) ;
953 return ;
954 }
955 TEVENT (Inflated exit - complex egress) ;
957 // Normally the exiting thread is responsible for ensuring succession,
958 // but if other successors are ready or other entering threads are spinning
959 // then this thread can simply store NULL into _owner and exit without
960 // waking a successor. The existence of spinners or ready successors
961 // guarantees proper succession (liveness). Responsibility passes to the
962 // ready or running successors. The exiting thread delegates the duty.
963 // More precisely, if a successor already exists this thread is absolved
964 // of the responsibility of waking (unparking) one.
965 //
966 // The _succ variable is critical to reducing futile wakeup frequency.
967 // _succ identifies the "heir presumptive" thread that has been made
968 // ready (unparked) but that has not yet run. We need only one such
969 // successor thread to guarantee progress.
970 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
971 // section 3.3 "Futile Wakeup Throttling" for details.
972 //
973 // Note that spinners in Enter() also set _succ non-null.
974 // In the current implementation spinners opportunistically set
975 // _succ so that exiting threads might avoid waking a successor.
976 // Another less appealing alternative would be for the exiting thread
977 // to drop the lock and then spin briefly to see if a spinner managed
978 // to acquire the lock. If so, the exiting thread could exit
979 // immediately without waking a successor, otherwise the exiting
980 // thread would need to dequeue and wake a successor.
981 // (Note that we'd need to make the post-drop spin short, but no
982 // shorter than the worst-case round-trip cache-line migration time.
983 // The dropped lock needs to become visible to the spinner, and then
984 // the acquisition of the lock by the spinner must become visible to
985 // the exiting thread).
986 //
988 // It appears that an heir-presumptive (successor) must be made ready.
989 // Only the current lock owner can manipulate the EntryList or
990 // drain _cxq, so we need to reacquire the lock. If we fail
991 // to reacquire the lock the responsibility for ensuring succession
992 // falls to the new owner.
993 //
994 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
995 return ;
996 }
997 TEVENT (Exit - Reacquired) ;
998 } else {
999 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1000 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1001 OrderAccess::storeload() ;
1002 // Ratify the previously observed values.
1003 if (_cxq == NULL || _succ != NULL) {
1004 TEVENT (Inflated exit - simple egress) ;
1005 return ;
1006 }
1008 // inopportune interleaving -- the exiting thread (this thread)
1009 // in the fast-exit path raced an entering thread in the slow-enter
1010 // path.
1011 // We have two choices:
1012 // A. Try to reacquire the lock.
1013 // If the CAS() fails return immediately, otherwise
1014 // we either restart/rerun the exit operation, or simply
1015 // fall-through into the code below which wakes a successor.
1016 // B. If the elements forming the EntryList|cxq are TSM
1017 // we could simply unpark() the lead thread and return
1018 // without having set _succ.
1019 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1020 TEVENT (Inflated exit - reacquired succeeded) ;
1021 return ;
1022 }
1023 TEVENT (Inflated exit - reacquired failed) ;
1024 } else {
1025 TEVENT (Inflated exit - complex egress) ;
1026 }
1027 }
1029 guarantee (_owner == THREAD, "invariant") ;
1031 ObjectWaiter * w = NULL ;
1032 int QMode = Knob_QMode ;
1034 if (QMode == 2 && _cxq != NULL) {
1035 // QMode == 2 : cxq has precedence over EntryList.
1036 // Try to directly wake a successor from the cxq.
1037 // If successful, the successor will need to unlink itself from cxq.
1038 w = _cxq ;
1039 assert (w != NULL, "invariant") ;
1040 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1041 ExitEpilog (Self, w) ;
1042 return ;
1043 }
1045 if (QMode == 3 && _cxq != NULL) {
1046 // Aggressively drain cxq into EntryList at the first opportunity.
1047 // This policy ensure that recently-run threads live at the head of EntryList.
1048 // Drain _cxq into EntryList - bulk transfer.
1049 // First, detach _cxq.
1050 // The following loop is tantamount to: w = swap (&cxq, NULL)
1051 w = _cxq ;
1052 for (;;) {
1053 assert (w != NULL, "Invariant") ;
1054 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1055 if (u == w) break ;
1056 w = u ;
1057 }
1058 assert (w != NULL , "invariant") ;
1060 ObjectWaiter * q = NULL ;
1061 ObjectWaiter * p ;
1062 for (p = w ; p != NULL ; p = p->_next) {
1063 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1064 p->TState = ObjectWaiter::TS_ENTER ;
1065 p->_prev = q ;
1066 q = p ;
1067 }
1069 // Append the RATs to the EntryList
1070 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1071 ObjectWaiter * Tail ;
1072 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1073 if (Tail == NULL) {
1074 _EntryList = w ;
1075 } else {
1076 Tail->_next = w ;
1077 w->_prev = Tail ;
1078 }
1080 // Fall thru into code that tries to wake a successor from EntryList
1081 }
1083 if (QMode == 4 && _cxq != NULL) {
1084 // Aggressively drain cxq into EntryList at the first opportunity.
1085 // This policy ensure that recently-run threads live at the head of EntryList.
1087 // Drain _cxq into EntryList - bulk transfer.
1088 // First, detach _cxq.
1089 // The following loop is tantamount to: w = swap (&cxq, NULL)
1090 w = _cxq ;
1091 for (;;) {
1092 assert (w != NULL, "Invariant") ;
1093 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1094 if (u == w) break ;
1095 w = u ;
1096 }
1097 assert (w != NULL , "invariant") ;
1099 ObjectWaiter * q = NULL ;
1100 ObjectWaiter * p ;
1101 for (p = w ; p != NULL ; p = p->_next) {
1102 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1103 p->TState = ObjectWaiter::TS_ENTER ;
1104 p->_prev = q ;
1105 q = p ;
1106 }
1108 // Prepend the RATs to the EntryList
1109 if (_EntryList != NULL) {
1110 q->_next = _EntryList ;
1111 _EntryList->_prev = q ;
1112 }
1113 _EntryList = w ;
1115 // Fall thru into code that tries to wake a successor from EntryList
1116 }
1118 w = _EntryList ;
1119 if (w != NULL) {
1120 // I'd like to write: guarantee (w->_thread != Self).
1121 // But in practice an exiting thread may find itself on the EntryList.
1122 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1123 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1124 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1125 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1126 // release the lock "O". T2 resumes immediately after the ST of null into
1127 // _owner, above. T2 notices that the EntryList is populated, so it
1128 // reacquires the lock and then finds itself on the EntryList.
1129 // Given all that, we have to tolerate the circumstance where "w" is
1130 // associated with Self.
1131 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1132 ExitEpilog (Self, w) ;
1133 return ;
1134 }
1136 // If we find that both _cxq and EntryList are null then just
1137 // re-run the exit protocol from the top.
1138 w = _cxq ;
1139 if (w == NULL) continue ;
1141 // Drain _cxq into EntryList - bulk transfer.
1142 // First, detach _cxq.
1143 // The following loop is tantamount to: w = swap (&cxq, NULL)
1144 for (;;) {
1145 assert (w != NULL, "Invariant") ;
1146 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1147 if (u == w) break ;
1148 w = u ;
1149 }
1150 TEVENT (Inflated exit - drain cxq into EntryList) ;
1152 assert (w != NULL , "invariant") ;
1153 assert (_EntryList == NULL , "invariant") ;
1155 // Convert the LIFO SLL anchored by _cxq into a DLL.
1156 // The list reorganization step operates in O(LENGTH(w)) time.
1157 // It's critical that this step operate quickly as
1158 // "Self" still holds the outer-lock, restricting parallelism
1159 // and effectively lengthening the critical section.
1160 // Invariant: s chases t chases u.
1161 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1162 // we have faster access to the tail.
1164 if (QMode == 1) {
1165 // QMode == 1 : drain cxq to EntryList, reversing order
1166 // We also reverse the order of the list.
1167 ObjectWaiter * s = NULL ;
1168 ObjectWaiter * t = w ;
1169 ObjectWaiter * u = NULL ;
1170 while (t != NULL) {
1171 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1172 t->TState = ObjectWaiter::TS_ENTER ;
1173 u = t->_next ;
1174 t->_prev = u ;
1175 t->_next = s ;
1176 s = t;
1177 t = u ;
1178 }
1179 _EntryList = s ;
1180 assert (s != NULL, "invariant") ;
1181 } else {
1182 // QMode == 0 or QMode == 2
1183 _EntryList = w ;
1184 ObjectWaiter * q = NULL ;
1185 ObjectWaiter * p ;
1186 for (p = w ; p != NULL ; p = p->_next) {
1187 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1188 p->TState = ObjectWaiter::TS_ENTER ;
1189 p->_prev = q ;
1190 q = p ;
1191 }
1192 }
1194 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1195 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1197 // See if we can abdicate to a spinner instead of waking a thread.
1198 // A primary goal of the implementation is to reduce the
1199 // context-switch rate.
1200 if (_succ != NULL) continue;
1202 w = _EntryList ;
1203 if (w != NULL) {
1204 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1205 ExitEpilog (Self, w) ;
1206 return ;
1207 }
1208 }
1209 }
1211 // ExitSuspendEquivalent:
1212 // A faster alternate to handle_special_suspend_equivalent_condition()
1213 //
1214 // handle_special_suspend_equivalent_condition() unconditionally
1215 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1216 // operations have high latency. Note that in ::enter() we call HSSEC
1217 // while holding the monitor, so we effectively lengthen the critical sections.
1218 //
1219 // There are a number of possible solutions:
1220 //
1221 // A. To ameliorate the problem we might also defer state transitions
1222 // to as late as possible -- just prior to parking.
1223 // Given that, we'd call HSSEC after having returned from park(),
1224 // but before attempting to acquire the monitor. This is only a
1225 // partial solution. It avoids calling HSSEC while holding the
1226 // monitor (good), but it still increases successor reacquisition latency --
1227 // the interval between unparking a successor and the time the successor
1228 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1229 // If we use this technique we can also avoid EnterI()-exit() loop
1230 // in ::enter() where we iteratively drop the lock and then attempt
1231 // to reacquire it after suspending.
1232 //
1233 // B. In the future we might fold all the suspend bits into a
1234 // composite per-thread suspend flag and then update it with CAS().
1235 // Alternately, a Dekker-like mechanism with multiple variables
1236 // would suffice:
1237 // ST Self->_suspend_equivalent = false
1238 // MEMBAR
1239 // LD Self_>_suspend_flags
1240 //
1243 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1244 int Mode = Knob_FastHSSEC ;
1245 if (Mode && !jSelf->is_external_suspend()) {
1246 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1247 jSelf->clear_suspend_equivalent() ;
1248 if (2 == Mode) OrderAccess::storeload() ;
1249 if (!jSelf->is_external_suspend()) return false ;
1250 // We raced a suspension -- fall thru into the slow path
1251 TEVENT (ExitSuspendEquivalent - raced) ;
1252 jSelf->set_suspend_equivalent() ;
1253 }
1254 return jSelf->handle_special_suspend_equivalent_condition() ;
1255 }
1258 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1259 assert (_owner == Self, "invariant") ;
1261 // Exit protocol:
1262 // 1. ST _succ = wakee
1263 // 2. membar #loadstore|#storestore;
1264 // 2. ST _owner = NULL
1265 // 3. unpark(wakee)
1267 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1268 ParkEvent * Trigger = Wakee->_event ;
1270 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1271 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1272 // out-of-scope (non-extant).
1273 Wakee = NULL ;
1275 // Drop the lock
1276 OrderAccess::release_store_ptr (&_owner, NULL) ;
1277 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1279 if (SafepointSynchronize::do_call_back()) {
1280 TEVENT (unpark before SAFEPOINT) ;
1281 }
1283 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1284 Trigger->unpark() ;
1286 // Maintain stats and report events to JVMTI
1287 if (ObjectMonitor::_sync_Parks != NULL) {
1288 ObjectMonitor::_sync_Parks->inc() ;
1289 }
1290 }
1293 // -----------------------------------------------------------------------------
1294 // Class Loader deadlock handling.
1295 //
1296 // complete_exit exits a lock returning recursion count
1297 // complete_exit/reenter operate as a wait without waiting
1298 // complete_exit requires an inflated monitor
1299 // The _owner field is not always the Thread addr even with an
1300 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1301 // thread due to contention.
1302 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1303 Thread * const Self = THREAD;
1304 assert(Self->is_Java_thread(), "Must be Java thread!");
1305 JavaThread *jt = (JavaThread *)THREAD;
1307 DeferredInitialize();
1309 if (THREAD != _owner) {
1310 if (THREAD->is_lock_owned ((address)_owner)) {
1311 assert(_recursions == 0, "internal state error");
1312 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1313 _recursions = 0 ;
1314 OwnerIsThread = 1 ;
1315 }
1316 }
1318 guarantee(Self == _owner, "complete_exit not owner");
1319 intptr_t save = _recursions; // record the old recursion count
1320 _recursions = 0; // set the recursion level to be 0
1321 exit (Self) ; // exit the monitor
1322 guarantee (_owner != Self, "invariant");
1323 return save;
1324 }
1326 // reenter() enters a lock and sets recursion count
1327 // complete_exit/reenter operate as a wait without waiting
1328 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1329 Thread * const Self = THREAD;
1330 assert(Self->is_Java_thread(), "Must be Java thread!");
1331 JavaThread *jt = (JavaThread *)THREAD;
1333 guarantee(_owner != Self, "reenter already owner");
1334 enter (THREAD); // enter the monitor
1335 guarantee (_recursions == 0, "reenter recursion");
1336 _recursions = recursions;
1337 return;
1338 }
1341 // -----------------------------------------------------------------------------
1342 // A macro is used below because there may already be a pending
1343 // exception which should not abort the execution of the routines
1344 // which use this (which is why we don't put this into check_slow and
1345 // call it with a CHECK argument).
1347 #define CHECK_OWNER() \
1348 do { \
1349 if (THREAD != _owner) { \
1350 if (THREAD->is_lock_owned((address) _owner)) { \
1351 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1352 _recursions = 0; \
1353 OwnerIsThread = 1 ; \
1354 } else { \
1355 TEVENT (Throw IMSX) ; \
1356 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1357 } \
1358 } \
1359 } while (false)
1361 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1362 // TODO-FIXME: remove check_slow() -- it's likely dead.
1364 void ObjectMonitor::check_slow(TRAPS) {
1365 TEVENT (check_slow - throw IMSX) ;
1366 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1367 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1368 }
1370 static int Adjust (volatile int * adr, int dx) {
1371 int v ;
1372 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1373 return v ;
1374 }
1375 // -----------------------------------------------------------------------------
1376 // Wait/Notify/NotifyAll
1377 //
1378 // Note: a subset of changes to ObjectMonitor::wait()
1379 // will need to be replicated in complete_exit above
1380 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1381 Thread * const Self = THREAD ;
1382 assert(Self->is_Java_thread(), "Must be Java thread!");
1383 JavaThread *jt = (JavaThread *)THREAD;
1385 DeferredInitialize () ;
1387 // Throw IMSX or IEX.
1388 CHECK_OWNER();
1390 // check for a pending interrupt
1391 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1392 // post monitor waited event. Note that this is past-tense, we are done waiting.
1393 if (JvmtiExport::should_post_monitor_waited()) {
1394 // Note: 'false' parameter is passed here because the
1395 // wait was not timed out due to thread interrupt.
1396 JvmtiExport::post_monitor_waited(jt, this, false);
1397 }
1398 TEVENT (Wait - Throw IEX) ;
1399 THROW(vmSymbols::java_lang_InterruptedException());
1400 return ;
1401 }
1402 TEVENT (Wait) ;
1404 assert (Self->_Stalled == 0, "invariant") ;
1405 Self->_Stalled = intptr_t(this) ;
1406 jt->set_current_waiting_monitor(this);
1408 // create a node to be put into the queue
1409 // Critically, after we reset() the event but prior to park(), we must check
1410 // for a pending interrupt.
1411 ObjectWaiter node(Self);
1412 node.TState = ObjectWaiter::TS_WAIT ;
1413 Self->_ParkEvent->reset() ;
1414 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1416 // Enter the waiting queue, which is a circular doubly linked list in this case
1417 // but it could be a priority queue or any data structure.
1418 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1419 // by the the owner of the monitor *except* in the case where park()
1420 // returns because of a timeout of interrupt. Contention is exceptionally rare
1421 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1423 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1424 AddWaiter (&node) ;
1425 Thread::SpinRelease (&_WaitSetLock) ;
1427 if ((SyncFlags & 4) == 0) {
1428 _Responsible = NULL ;
1429 }
1430 intptr_t save = _recursions; // record the old recursion count
1431 _waiters++; // increment the number of waiters
1432 _recursions = 0; // set the recursion level to be 1
1433 exit (Self) ; // exit the monitor
1434 guarantee (_owner != Self, "invariant") ;
1436 // As soon as the ObjectMonitor's ownership is dropped in the exit()
1437 // call above, another thread can enter() the ObjectMonitor, do the
1438 // notify(), and exit() the ObjectMonitor. If the other thread's
1439 // exit() call chooses this thread as the successor and the unpark()
1440 // call happens to occur while this thread is posting a
1441 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
1442 // handler using RawMonitors and consuming the unpark().
1443 //
1444 // To avoid the problem, we re-post the event. This does no harm
1445 // even if the original unpark() was not consumed because we are the
1446 // chosen successor for this monitor.
1447 if (node._notified != 0 && _succ == Self) {
1448 node._event->unpark();
1449 }
1451 // The thread is on the WaitSet list - now park() it.
1452 // On MP systems it's conceivable that a brief spin before we park
1453 // could be profitable.
1454 //
1455 // TODO-FIXME: change the following logic to a loop of the form
1456 // while (!timeout && !interrupted && _notified == 0) park()
1458 int ret = OS_OK ;
1459 int WasNotified = 0 ;
1460 { // State transition wrappers
1461 OSThread* osthread = Self->osthread();
1462 OSThreadWaitState osts(osthread, true);
1463 {
1464 ThreadBlockInVM tbivm(jt);
1465 // Thread is in thread_blocked state and oop access is unsafe.
1466 jt->set_suspend_equivalent();
1468 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1469 // Intentionally empty
1470 } else
1471 if (node._notified == 0) {
1472 if (millis <= 0) {
1473 Self->_ParkEvent->park () ;
1474 } else {
1475 ret = Self->_ParkEvent->park (millis) ;
1476 }
1477 }
1479 // were we externally suspended while we were waiting?
1480 if (ExitSuspendEquivalent (jt)) {
1481 // TODO-FIXME: add -- if succ == Self then succ = null.
1482 jt->java_suspend_self();
1483 }
1485 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1488 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1489 // from the WaitSet to the EntryList.
1490 // See if we need to remove Node from the WaitSet.
1491 // We use double-checked locking to avoid grabbing _WaitSetLock
1492 // if the thread is not on the wait queue.
1493 //
1494 // Note that we don't need a fence before the fetch of TState.
1495 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1496 // written by the is thread. (perhaps the fetch might even be satisfied
1497 // by a look-aside into the processor's own store buffer, although given
1498 // the length of the code path between the prior ST and this load that's
1499 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1500 // then we'll acquire the lock and then re-fetch a fresh TState value.
1501 // That is, we fail toward safety.
1503 if (node.TState == ObjectWaiter::TS_WAIT) {
1504 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1505 if (node.TState == ObjectWaiter::TS_WAIT) {
1506 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1507 assert(node._notified == 0, "invariant");
1508 node.TState = ObjectWaiter::TS_RUN ;
1509 }
1510 Thread::SpinRelease (&_WaitSetLock) ;
1511 }
1513 // The thread is now either on off-list (TS_RUN),
1514 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1515 // The Node's TState variable is stable from the perspective of this thread.
1516 // No other threads will asynchronously modify TState.
1517 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1518 OrderAccess::loadload() ;
1519 if (_succ == Self) _succ = NULL ;
1520 WasNotified = node._notified ;
1522 // Reentry phase -- reacquire the monitor.
1523 // re-enter contended monitor after object.wait().
1524 // retain OBJECT_WAIT state until re-enter successfully completes
1525 // Thread state is thread_in_vm and oop access is again safe,
1526 // although the raw address of the object may have changed.
1527 // (Don't cache naked oops over safepoints, of course).
1529 // post monitor waited event. Note that this is past-tense, we are done waiting.
1530 if (JvmtiExport::should_post_monitor_waited()) {
1531 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1532 }
1533 OrderAccess::fence() ;
1535 assert (Self->_Stalled != 0, "invariant") ;
1536 Self->_Stalled = 0 ;
1538 assert (_owner != Self, "invariant") ;
1539 ObjectWaiter::TStates v = node.TState ;
1540 if (v == ObjectWaiter::TS_RUN) {
1541 enter (Self) ;
1542 } else {
1543 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1544 ReenterI (Self, &node) ;
1545 node.wait_reenter_end(this);
1546 }
1548 // Self has reacquired the lock.
1549 // Lifecycle - the node representing Self must not appear on any queues.
1550 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1551 // want residual elements associated with this thread left on any lists.
1552 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1553 assert (_owner == Self, "invariant") ;
1554 assert (_succ != Self , "invariant") ;
1555 } // OSThreadWaitState()
1557 jt->set_current_waiting_monitor(NULL);
1559 guarantee (_recursions == 0, "invariant") ;
1560 _recursions = save; // restore the old recursion count
1561 _waiters--; // decrement the number of waiters
1563 // Verify a few postconditions
1564 assert (_owner == Self , "invariant") ;
1565 assert (_succ != Self , "invariant") ;
1566 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1568 if (SyncFlags & 32) {
1569 OrderAccess::fence() ;
1570 }
1572 // check if the notification happened
1573 if (!WasNotified) {
1574 // no, it could be timeout or Thread.interrupt() or both
1575 // check for interrupt event, otherwise it is timeout
1576 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1577 TEVENT (Wait - throw IEX from epilog) ;
1578 THROW(vmSymbols::java_lang_InterruptedException());
1579 }
1580 }
1582 // NOTE: Spurious wake up will be consider as timeout.
1583 // Monitor notify has precedence over thread interrupt.
1584 }
1587 // Consider:
1588 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1589 // then instead of transferring a thread from the WaitSet to the EntryList
1590 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1592 void ObjectMonitor::notify(TRAPS) {
1593 CHECK_OWNER();
1594 if (_WaitSet == NULL) {
1595 TEVENT (Empty-Notify) ;
1596 return ;
1597 }
1598 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1600 int Policy = Knob_MoveNotifyee ;
1602 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1603 ObjectWaiter * iterator = DequeueWaiter() ;
1604 if (iterator != NULL) {
1605 TEVENT (Notify1 - Transfer) ;
1606 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1607 guarantee (iterator->_notified == 0, "invariant") ;
1608 if (Policy != 4) {
1609 iterator->TState = ObjectWaiter::TS_ENTER ;
1610 }
1611 iterator->_notified = 1 ;
1613 ObjectWaiter * List = _EntryList ;
1614 if (List != NULL) {
1615 assert (List->_prev == NULL, "invariant") ;
1616 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1617 assert (List != iterator, "invariant") ;
1618 }
1620 if (Policy == 0) { // prepend to EntryList
1621 if (List == NULL) {
1622 iterator->_next = iterator->_prev = NULL ;
1623 _EntryList = iterator ;
1624 } else {
1625 List->_prev = iterator ;
1626 iterator->_next = List ;
1627 iterator->_prev = NULL ;
1628 _EntryList = iterator ;
1629 }
1630 } else
1631 if (Policy == 1) { // append to EntryList
1632 if (List == NULL) {
1633 iterator->_next = iterator->_prev = NULL ;
1634 _EntryList = iterator ;
1635 } else {
1636 // CONSIDER: finding the tail currently requires a linear-time walk of
1637 // the EntryList. We can make tail access constant-time by converting to
1638 // a CDLL instead of using our current DLL.
1639 ObjectWaiter * Tail ;
1640 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1641 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1642 Tail->_next = iterator ;
1643 iterator->_prev = Tail ;
1644 iterator->_next = NULL ;
1645 }
1646 } else
1647 if (Policy == 2) { // prepend to cxq
1648 // prepend to cxq
1649 if (List == NULL) {
1650 iterator->_next = iterator->_prev = NULL ;
1651 _EntryList = iterator ;
1652 } else {
1653 iterator->TState = ObjectWaiter::TS_CXQ ;
1654 for (;;) {
1655 ObjectWaiter * Front = _cxq ;
1656 iterator->_next = Front ;
1657 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1658 break ;
1659 }
1660 }
1661 }
1662 } else
1663 if (Policy == 3) { // append to cxq
1664 iterator->TState = ObjectWaiter::TS_CXQ ;
1665 for (;;) {
1666 ObjectWaiter * Tail ;
1667 Tail = _cxq ;
1668 if (Tail == NULL) {
1669 iterator->_next = NULL ;
1670 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1671 break ;
1672 }
1673 } else {
1674 while (Tail->_next != NULL) Tail = Tail->_next ;
1675 Tail->_next = iterator ;
1676 iterator->_prev = Tail ;
1677 iterator->_next = NULL ;
1678 break ;
1679 }
1680 }
1681 } else {
1682 ParkEvent * ev = iterator->_event ;
1683 iterator->TState = ObjectWaiter::TS_RUN ;
1684 OrderAccess::fence() ;
1685 ev->unpark() ;
1686 }
1688 if (Policy < 4) {
1689 iterator->wait_reenter_begin(this);
1690 }
1692 // _WaitSetLock protects the wait queue, not the EntryList. We could
1693 // move the add-to-EntryList operation, above, outside the critical section
1694 // protected by _WaitSetLock. In practice that's not useful. With the
1695 // exception of wait() timeouts and interrupts the monitor owner
1696 // is the only thread that grabs _WaitSetLock. There's almost no contention
1697 // on _WaitSetLock so it's not profitable to reduce the length of the
1698 // critical section.
1699 }
1701 Thread::SpinRelease (&_WaitSetLock) ;
1703 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1704 ObjectMonitor::_sync_Notifications->inc() ;
1705 }
1706 }
1709 void ObjectMonitor::notifyAll(TRAPS) {
1710 CHECK_OWNER();
1711 ObjectWaiter* iterator;
1712 if (_WaitSet == NULL) {
1713 TEVENT (Empty-NotifyAll) ;
1714 return ;
1715 }
1716 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1718 int Policy = Knob_MoveNotifyee ;
1719 int Tally = 0 ;
1720 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1722 for (;;) {
1723 iterator = DequeueWaiter () ;
1724 if (iterator == NULL) break ;
1725 TEVENT (NotifyAll - Transfer1) ;
1726 ++Tally ;
1728 // Disposition - what might we do with iterator ?
1729 // a. add it directly to the EntryList - either tail or head.
1730 // b. push it onto the front of the _cxq.
1731 // For now we use (a).
1733 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1734 guarantee (iterator->_notified == 0, "invariant") ;
1735 iterator->_notified = 1 ;
1736 if (Policy != 4) {
1737 iterator->TState = ObjectWaiter::TS_ENTER ;
1738 }
1740 ObjectWaiter * List = _EntryList ;
1741 if (List != NULL) {
1742 assert (List->_prev == NULL, "invariant") ;
1743 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1744 assert (List != iterator, "invariant") ;
1745 }
1747 if (Policy == 0) { // prepend to EntryList
1748 if (List == NULL) {
1749 iterator->_next = iterator->_prev = NULL ;
1750 _EntryList = iterator ;
1751 } else {
1752 List->_prev = iterator ;
1753 iterator->_next = List ;
1754 iterator->_prev = NULL ;
1755 _EntryList = iterator ;
1756 }
1757 } else
1758 if (Policy == 1) { // append to EntryList
1759 if (List == NULL) {
1760 iterator->_next = iterator->_prev = NULL ;
1761 _EntryList = iterator ;
1762 } else {
1763 // CONSIDER: finding the tail currently requires a linear-time walk of
1764 // the EntryList. We can make tail access constant-time by converting to
1765 // a CDLL instead of using our current DLL.
1766 ObjectWaiter * Tail ;
1767 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1768 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1769 Tail->_next = iterator ;
1770 iterator->_prev = Tail ;
1771 iterator->_next = NULL ;
1772 }
1773 } else
1774 if (Policy == 2) { // prepend to cxq
1775 // prepend to cxq
1776 iterator->TState = ObjectWaiter::TS_CXQ ;
1777 for (;;) {
1778 ObjectWaiter * Front = _cxq ;
1779 iterator->_next = Front ;
1780 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1781 break ;
1782 }
1783 }
1784 } else
1785 if (Policy == 3) { // append to cxq
1786 iterator->TState = ObjectWaiter::TS_CXQ ;
1787 for (;;) {
1788 ObjectWaiter * Tail ;
1789 Tail = _cxq ;
1790 if (Tail == NULL) {
1791 iterator->_next = NULL ;
1792 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1793 break ;
1794 }
1795 } else {
1796 while (Tail->_next != NULL) Tail = Tail->_next ;
1797 Tail->_next = iterator ;
1798 iterator->_prev = Tail ;
1799 iterator->_next = NULL ;
1800 break ;
1801 }
1802 }
1803 } else {
1804 ParkEvent * ev = iterator->_event ;
1805 iterator->TState = ObjectWaiter::TS_RUN ;
1806 OrderAccess::fence() ;
1807 ev->unpark() ;
1808 }
1810 if (Policy < 4) {
1811 iterator->wait_reenter_begin(this);
1812 }
1814 // _WaitSetLock protects the wait queue, not the EntryList. We could
1815 // move the add-to-EntryList operation, above, outside the critical section
1816 // protected by _WaitSetLock. In practice that's not useful. With the
1817 // exception of wait() timeouts and interrupts the monitor owner
1818 // is the only thread that grabs _WaitSetLock. There's almost no contention
1819 // on _WaitSetLock so it's not profitable to reduce the length of the
1820 // critical section.
1821 }
1823 Thread::SpinRelease (&_WaitSetLock) ;
1825 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1826 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1827 }
1828 }
1830 // -----------------------------------------------------------------------------
1831 // Adaptive Spinning Support
1832 //
1833 // Adaptive spin-then-block - rational spinning
1834 //
1835 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1836 // algorithm. On high order SMP systems it would be better to start with
1837 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1838 // a contending thread could enqueue itself on the cxq and then spin locally
1839 // on a thread-specific variable such as its ParkEvent._Event flag.
1840 // That's left as an exercise for the reader. Note that global spinning is
1841 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1842 // low latency and massive bandwidth.
1843 //
1844 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1845 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1846 // (duration) or we can fix the count at approximately the duration of
1847 // a context switch and vary the frequency. Of course we could also
1848 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1849 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1850 //
1851 // This implementation varies the duration "D", where D varies with
1852 // the success rate of recent spin attempts. (D is capped at approximately
1853 // length of a round-trip context switch). The success rate for recent
1854 // spin attempts is a good predictor of the success rate of future spin
1855 // attempts. The mechanism adapts automatically to varying critical
1856 // section length (lock modality), system load and degree of parallelism.
1857 // D is maintained per-monitor in _SpinDuration and is initialized
1858 // optimistically. Spin frequency is fixed at 100%.
1859 //
1860 // Note that _SpinDuration is volatile, but we update it without locks
1861 // or atomics. The code is designed so that _SpinDuration stays within
1862 // a reasonable range even in the presence of races. The arithmetic
1863 // operations on _SpinDuration are closed over the domain of legal values,
1864 // so at worst a race will install and older but still legal value.
1865 // At the very worst this introduces some apparent non-determinism.
1866 // We might spin when we shouldn't or vice-versa, but since the spin
1867 // count are relatively short, even in the worst case, the effect is harmless.
1868 //
1869 // Care must be taken that a low "D" value does not become an
1870 // an absorbing state. Transient spinning failures -- when spinning
1871 // is overall profitable -- should not cause the system to converge
1872 // on low "D" values. We want spinning to be stable and predictable
1873 // and fairly responsive to change and at the same time we don't want
1874 // it to oscillate, become metastable, be "too" non-deterministic,
1875 // or converge on or enter undesirable stable absorbing states.
1876 //
1877 // We implement a feedback-based control system -- using past behavior
1878 // to predict future behavior. We face two issues: (a) if the
1879 // input signal is random then the spin predictor won't provide optimal
1880 // results, and (b) if the signal frequency is too high then the control
1881 // system, which has some natural response lag, will "chase" the signal.
1882 // (b) can arise from multimodal lock hold times. Transient preemption
1883 // can also result in apparent bimodal lock hold times.
1884 // Although sub-optimal, neither condition is particularly harmful, as
1885 // in the worst-case we'll spin when we shouldn't or vice-versa.
1886 // The maximum spin duration is rather short so the failure modes aren't bad.
1887 // To be conservative, I've tuned the gain in system to bias toward
1888 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1889 // "rings" or oscillates between spinning and not spinning. This happens
1890 // when spinning is just on the cusp of profitability, however, so the
1891 // situation is not dire. The state is benign -- there's no need to add
1892 // hysteresis control to damp the transition rate between spinning and
1893 // not spinning.
1894 //
1896 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1897 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1899 // Spinning: Fixed frequency (100%), vary duration
1902 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1904 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1905 int ctr = Knob_FixedSpin ;
1906 if (ctr != 0) {
1907 while (--ctr >= 0) {
1908 if (TryLock (Self) > 0) return 1 ;
1909 SpinPause () ;
1910 }
1911 return 0 ;
1912 }
1914 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1915 if (TryLock(Self) > 0) {
1916 // Increase _SpinDuration ...
1917 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1918 // Raising _SpurDuration to the poverty line is key.
1919 int x = _SpinDuration ;
1920 if (x < Knob_SpinLimit) {
1921 if (x < Knob_Poverty) x = Knob_Poverty ;
1922 _SpinDuration = x + Knob_BonusB ;
1923 }
1924 return 1 ;
1925 }
1926 SpinPause () ;
1927 }
1929 // Admission control - verify preconditions for spinning
1930 //
1931 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1932 // becoming an absorbing state. Put another way, we spin briefly to
1933 // sample, just in case the system load, parallelism, contention, or lock
1934 // modality changed.
1935 //
1936 // Consider the following alternative:
1937 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1938 // spin attempt. "Periodically" might mean after a tally of
1939 // the # of failed spin attempts (or iterations) reaches some threshold.
1940 // This takes us into the realm of 1-out-of-N spinning, where we
1941 // hold the duration constant but vary the frequency.
1943 ctr = _SpinDuration ;
1944 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
1945 if (ctr <= 0) return 0 ;
1947 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
1948 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1949 TEVENT (Spin abort - notrunnable [TOP]);
1950 return 0 ;
1951 }
1953 int MaxSpin = Knob_MaxSpinners ;
1954 if (MaxSpin >= 0) {
1955 if (_Spinner > MaxSpin) {
1956 TEVENT (Spin abort -- too many spinners) ;
1957 return 0 ;
1958 }
1959 // Slighty racy, but benign ...
1960 Adjust (&_Spinner, 1) ;
1961 }
1963 // We're good to spin ... spin ingress.
1964 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1965 // when preparing to LD...CAS _owner, etc and the CAS is likely
1966 // to succeed.
1967 int hits = 0 ;
1968 int msk = 0 ;
1969 int caspty = Knob_CASPenalty ;
1970 int oxpty = Knob_OXPenalty ;
1971 int sss = Knob_SpinSetSucc ;
1972 if (sss && _succ == NULL ) _succ = Self ;
1973 Thread * prv = NULL ;
1975 // There are three ways to exit the following loop:
1976 // 1. A successful spin where this thread has acquired the lock.
1977 // 2. Spin failure with prejudice
1978 // 3. Spin failure without prejudice
1980 while (--ctr >= 0) {
1982 // Periodic polling -- Check for pending GC
1983 // Threads may spin while they're unsafe.
1984 // We don't want spinning threads to delay the JVM from reaching
1985 // a stop-the-world safepoint or to steal cycles from GC.
1986 // If we detect a pending safepoint we abort in order that
1987 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1988 // this thread, if safe, doesn't steal cycles from GC.
1989 // This is in keeping with the "no loitering in runtime" rule.
1990 // We periodically check to see if there's a safepoint pending.
1991 if ((ctr & 0xFF) == 0) {
1992 if (SafepointSynchronize::do_call_back()) {
1993 TEVENT (Spin: safepoint) ;
1994 goto Abort ; // abrupt spin egress
1995 }
1996 if (Knob_UsePause & 1) SpinPause () ;
1998 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
1999 if (hits > 50 && scb != NULL) {
2000 int abend = (*scb)(SpinCallbackArgument, 0) ;
2001 }
2002 }
2004 if (Knob_UsePause & 2) SpinPause() ;
2006 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2007 // This is useful on classic SMP systems, but is of less utility on
2008 // N1-style CMT platforms.
2009 //
2010 // Trade-off: lock acquisition latency vs coherency bandwidth.
2011 // Lock hold times are typically short. A histogram
2012 // of successful spin attempts shows that we usually acquire
2013 // the lock early in the spin. That suggests we want to
2014 // sample _owner frequently in the early phase of the spin,
2015 // but then back-off and sample less frequently as the spin
2016 // progresses. The back-off makes a good citizen on SMP big
2017 // SMP systems. Oversampling _owner can consume excessive
2018 // coherency bandwidth. Relatedly, if we _oversample _owner we
2019 // can inadvertently interfere with the the ST m->owner=null.
2020 // executed by the lock owner.
2021 if (ctr & msk) continue ;
2022 ++hits ;
2023 if ((hits & 0xF) == 0) {
2024 // The 0xF, above, corresponds to the exponent.
2025 // Consider: (msk+1)|msk
2026 msk = ((msk << 2)|3) & BackOffMask ;
2027 }
2029 // Probe _owner with TATAS
2030 // If this thread observes the monitor transition or flicker
2031 // from locked to unlocked to locked, then the odds that this
2032 // thread will acquire the lock in this spin attempt go down
2033 // considerably. The same argument applies if the CAS fails
2034 // or if we observe _owner change from one non-null value to
2035 // another non-null value. In such cases we might abort
2036 // the spin without prejudice or apply a "penalty" to the
2037 // spin count-down variable "ctr", reducing it by 100, say.
2039 Thread * ox = (Thread *) _owner ;
2040 if (ox == NULL) {
2041 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2042 if (ox == NULL) {
2043 // The CAS succeeded -- this thread acquired ownership
2044 // Take care of some bookkeeping to exit spin state.
2045 if (sss && _succ == Self) {
2046 _succ = NULL ;
2047 }
2048 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2050 // Increase _SpinDuration :
2051 // The spin was successful (profitable) so we tend toward
2052 // longer spin attempts in the future.
2053 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2054 // If we acquired the lock early in the spin cycle it
2055 // makes sense to increase _SpinDuration proportionally.
2056 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2057 int x = _SpinDuration ;
2058 if (x < Knob_SpinLimit) {
2059 if (x < Knob_Poverty) x = Knob_Poverty ;
2060 _SpinDuration = x + Knob_Bonus ;
2061 }
2062 return 1 ;
2063 }
2065 // The CAS failed ... we can take any of the following actions:
2066 // * penalize: ctr -= Knob_CASPenalty
2067 // * exit spin with prejudice -- goto Abort;
2068 // * exit spin without prejudice.
2069 // * Since CAS is high-latency, retry again immediately.
2070 prv = ox ;
2071 TEVENT (Spin: cas failed) ;
2072 if (caspty == -2) break ;
2073 if (caspty == -1) goto Abort ;
2074 ctr -= caspty ;
2075 continue ;
2076 }
2078 // Did lock ownership change hands ?
2079 if (ox != prv && prv != NULL ) {
2080 TEVENT (spin: Owner changed)
2081 if (oxpty == -2) break ;
2082 if (oxpty == -1) goto Abort ;
2083 ctr -= oxpty ;
2084 }
2085 prv = ox ;
2087 // Abort the spin if the owner is not executing.
2088 // The owner must be executing in order to drop the lock.
2089 // Spinning while the owner is OFFPROC is idiocy.
2090 // Consider: ctr -= RunnablePenalty ;
2091 if (Knob_OState && NotRunnable (Self, ox)) {
2092 TEVENT (Spin abort - notrunnable);
2093 goto Abort ;
2094 }
2095 if (sss && _succ == NULL ) _succ = Self ;
2096 }
2098 // Spin failed with prejudice -- reduce _SpinDuration.
2099 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2100 // AIMD is globally stable.
2101 TEVENT (Spin failure) ;
2102 {
2103 int x = _SpinDuration ;
2104 if (x > 0) {
2105 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2106 // This is globally sample and tends to damp the response.
2107 x -= Knob_Penalty ;
2108 if (x < 0) x = 0 ;
2109 _SpinDuration = x ;
2110 }
2111 }
2113 Abort:
2114 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2115 if (sss && _succ == Self) {
2116 _succ = NULL ;
2117 // Invariant: after setting succ=null a contending thread
2118 // must recheck-retry _owner before parking. This usually happens
2119 // in the normal usage of TrySpin(), but it's safest
2120 // to make TrySpin() as foolproof as possible.
2121 OrderAccess::fence() ;
2122 if (TryLock(Self) > 0) return 1 ;
2123 }
2124 return 0 ;
2125 }
2127 // NotRunnable() -- informed spinning
2128 //
2129 // Don't bother spinning if the owner is not eligible to drop the lock.
2130 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2131 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2132 // The thread must be runnable in order to drop the lock in timely fashion.
2133 // If the _owner is not runnable then spinning will not likely be
2134 // successful (profitable).
2135 //
2136 // Beware -- the thread referenced by _owner could have died
2137 // so a simply fetch from _owner->_thread_state might trap.
2138 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2139 // Because of the lifecycle issues the schedctl and _thread_state values
2140 // observed by NotRunnable() might be garbage. NotRunnable must
2141 // tolerate this and consider the observed _thread_state value
2142 // as advisory.
2143 //
2144 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2145 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2146 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2147 // with the LSB of _owner. Another option would be to probablistically probe
2148 // the putative _owner->TypeTag value.
2149 //
2150 // Checking _thread_state isn't perfect. Even if the thread is
2151 // in_java it might be blocked on a page-fault or have been preempted
2152 // and sitting on a ready/dispatch queue. _thread state in conjunction
2153 // with schedctl.sc_state gives us a good picture of what the
2154 // thread is doing, however.
2155 //
2156 // TODO: check schedctl.sc_state.
2157 // We'll need to use SafeFetch32() to read from the schedctl block.
2158 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2159 //
2160 // The return value from NotRunnable() is *advisory* -- the
2161 // result is based on sampling and is not necessarily coherent.
2162 // The caller must tolerate false-negative and false-positive errors.
2163 // Spinning, in general, is probabilistic anyway.
2166 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2167 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2168 if (!OwnerIsThread) return 0 ;
2170 if (ox == NULL) return 0 ;
2172 // Avoid transitive spinning ...
2173 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2174 // Immediately after T1 acquires L it's possible that T2, also
2175 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2176 // This occurs transiently after T1 acquired L but before
2177 // T1 managed to clear T1.Stalled. T2 does not need to abort
2178 // its spin in this circumstance.
2179 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2181 if (BlockedOn == 1) return 1 ;
2182 if (BlockedOn != 0) {
2183 return BlockedOn != intptr_t(this) && _owner == ox ;
2184 }
2186 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2187 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2188 // consider also: jst != _thread_in_Java -- but that's overspecific.
2189 return jst == _thread_blocked || jst == _thread_in_native ;
2190 }
2193 // -----------------------------------------------------------------------------
2194 // WaitSet management ...
2196 ObjectWaiter::ObjectWaiter(Thread* thread) {
2197 _next = NULL;
2198 _prev = NULL;
2199 _notified = 0;
2200 TState = TS_RUN ;
2201 _thread = thread;
2202 _event = thread->_ParkEvent ;
2203 _active = false;
2204 assert (_event != NULL, "invariant") ;
2205 }
2207 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2208 JavaThread *jt = (JavaThread *)this->_thread;
2209 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2210 }
2212 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2213 JavaThread *jt = (JavaThread *)this->_thread;
2214 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2215 }
2217 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2218 assert(node != NULL, "should not dequeue NULL node");
2219 assert(node->_prev == NULL, "node already in list");
2220 assert(node->_next == NULL, "node already in list");
2221 // put node at end of queue (circular doubly linked list)
2222 if (_WaitSet == NULL) {
2223 _WaitSet = node;
2224 node->_prev = node;
2225 node->_next = node;
2226 } else {
2227 ObjectWaiter* head = _WaitSet ;
2228 ObjectWaiter* tail = head->_prev;
2229 assert(tail->_next == head, "invariant check");
2230 tail->_next = node;
2231 head->_prev = node;
2232 node->_next = head;
2233 node->_prev = tail;
2234 }
2235 }
2237 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2238 // dequeue the very first waiter
2239 ObjectWaiter* waiter = _WaitSet;
2240 if (waiter) {
2241 DequeueSpecificWaiter(waiter);
2242 }
2243 return waiter;
2244 }
2246 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2247 assert(node != NULL, "should not dequeue NULL node");
2248 assert(node->_prev != NULL, "node already removed from list");
2249 assert(node->_next != NULL, "node already removed from list");
2250 // when the waiter has woken up because of interrupt,
2251 // timeout or other spurious wake-up, dequeue the
2252 // waiter from waiting list
2253 ObjectWaiter* next = node->_next;
2254 if (next == node) {
2255 assert(node->_prev == node, "invariant check");
2256 _WaitSet = NULL;
2257 } else {
2258 ObjectWaiter* prev = node->_prev;
2259 assert(prev->_next == node, "invariant check");
2260 assert(next->_prev == node, "invariant check");
2261 next->_prev = prev;
2262 prev->_next = next;
2263 if (_WaitSet == node) {
2264 _WaitSet = next;
2265 }
2266 }
2267 node->_next = NULL;
2268 node->_prev = NULL;
2269 }
2271 // -----------------------------------------------------------------------------
2272 // PerfData support
2273 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2274 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2275 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2276 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2277 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2278 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2279 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2280 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2281 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2282 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2283 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2284 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2285 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2286 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2287 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2288 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2289 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2290 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2292 // One-shot global initialization for the sync subsystem.
2293 // We could also defer initialization and initialize on-demand
2294 // the first time we call inflate(). Initialization would
2295 // be protected - like so many things - by the MonitorCache_lock.
2297 void ObjectMonitor::Initialize () {
2298 static int InitializationCompleted = 0 ;
2299 assert (InitializationCompleted == 0, "invariant") ;
2300 InitializationCompleted = 1 ;
2301 if (UsePerfData) {
2302 EXCEPTION_MARK ;
2303 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2304 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2305 NEWPERFCOUNTER(_sync_Inflations) ;
2306 NEWPERFCOUNTER(_sync_Deflations) ;
2307 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2308 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2309 NEWPERFCOUNTER(_sync_Parks) ;
2310 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2311 NEWPERFCOUNTER(_sync_Notifications) ;
2312 NEWPERFCOUNTER(_sync_SlowEnter) ;
2313 NEWPERFCOUNTER(_sync_SlowExit) ;
2314 NEWPERFCOUNTER(_sync_SlowNotify) ;
2315 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2316 NEWPERFCOUNTER(_sync_FailedSpins) ;
2317 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2318 NEWPERFCOUNTER(_sync_PrivateA) ;
2319 NEWPERFCOUNTER(_sync_PrivateB) ;
2320 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2321 NEWPERFCOUNTER(_sync_MonScavenged) ;
2322 NEWPERFVARIABLE(_sync_MonExtant) ;
2323 #undef NEWPERFCOUNTER
2324 }
2325 }
2328 // Compile-time asserts
2329 // When possible, it's better to catch errors deterministically at
2330 // compile-time than at runtime. The down-side to using compile-time
2331 // asserts is that error message -- often something about negative array
2332 // indices -- is opaque.
2334 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2336 void ObjectMonitor::ctAsserts() {
2337 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2338 }
2341 static char * kvGet (char * kvList, const char * Key) {
2342 if (kvList == NULL) return NULL ;
2343 size_t n = strlen (Key) ;
2344 char * Search ;
2345 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2346 if (strncmp (Search, Key, n) == 0) {
2347 if (Search[n] == '=') return Search + n + 1 ;
2348 if (Search[n] == 0) return (char *) "1" ;
2349 }
2350 }
2351 return NULL ;
2352 }
2354 static int kvGetInt (char * kvList, const char * Key, int Default) {
2355 char * v = kvGet (kvList, Key) ;
2356 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2357 if (Knob_ReportSettings && v != NULL) {
2358 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2359 ::fflush (stdout) ;
2360 }
2361 return rslt ;
2362 }
2364 void ObjectMonitor::DeferredInitialize () {
2365 if (InitDone > 0) return ;
2366 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2367 while (InitDone != 1) ;
2368 return ;
2369 }
2371 // One-shot global initialization ...
2372 // The initialization is idempotent, so we don't need locks.
2373 // In the future consider doing this via os::init_2().
2374 // SyncKnobs consist of <Key>=<Value> pairs in the style
2375 // of environment variables. Start by converting ':' to NUL.
2377 if (SyncKnobs == NULL) SyncKnobs = "" ;
2379 size_t sz = strlen (SyncKnobs) ;
2380 char * knobs = (char *) malloc (sz + 2) ;
2381 if (knobs == NULL) {
2382 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2383 guarantee (0, "invariant") ;
2384 }
2385 strcpy (knobs, SyncKnobs) ;
2386 knobs[sz+1] = 0 ;
2387 for (char * p = knobs ; *p ; p++) {
2388 if (*p == ':') *p = 0 ;
2389 }
2391 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2392 SETKNOB(ReportSettings) ;
2393 SETKNOB(Verbose) ;
2394 SETKNOB(FixedSpin) ;
2395 SETKNOB(SpinLimit) ;
2396 SETKNOB(SpinBase) ;
2397 SETKNOB(SpinBackOff);
2398 SETKNOB(CASPenalty) ;
2399 SETKNOB(OXPenalty) ;
2400 SETKNOB(LogSpins) ;
2401 SETKNOB(SpinSetSucc) ;
2402 SETKNOB(SuccEnabled) ;
2403 SETKNOB(SuccRestrict) ;
2404 SETKNOB(Penalty) ;
2405 SETKNOB(Bonus) ;
2406 SETKNOB(BonusB) ;
2407 SETKNOB(Poverty) ;
2408 SETKNOB(SpinAfterFutile) ;
2409 SETKNOB(UsePause) ;
2410 SETKNOB(SpinEarly) ;
2411 SETKNOB(OState) ;
2412 SETKNOB(MaxSpinners) ;
2413 SETKNOB(PreSpin) ;
2414 SETKNOB(ExitPolicy) ;
2415 SETKNOB(QMode);
2416 SETKNOB(ResetEvent) ;
2417 SETKNOB(MoveNotifyee) ;
2418 SETKNOB(FastHSSEC) ;
2419 #undef SETKNOB
2421 if (os::is_MP()) {
2422 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2423 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2424 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2425 } else {
2426 Knob_SpinLimit = 0 ;
2427 Knob_SpinBase = 0 ;
2428 Knob_PreSpin = 0 ;
2429 Knob_FixedSpin = -1 ;
2430 }
2432 if (Knob_LogSpins == 0) {
2433 ObjectMonitor::_sync_FailedSpins = NULL ;
2434 }
2436 free (knobs) ;
2437 OrderAccess::fence() ;
2438 InitDone = 1 ;
2439 }
2441 #ifndef PRODUCT
2442 void ObjectMonitor::verify() {
2443 }
2445 void ObjectMonitor::print() {
2446 }
2447 #endif