222 // * See also http://blogs.sun.com/dave
223
224
225 void* ObjectMonitor::operator new (size_t size) throw() {
226 return AllocateHeap(size, mtInternal);
227 }
228 void* ObjectMonitor::operator new[] (size_t size) throw() {
229 return operator new (size);
230 }
231 void ObjectMonitor::operator delete(void* p) {
232 FreeHeap(p);
233 }
234 void ObjectMonitor::operator delete[] (void *p) {
235 operator delete(p);
236 }
237
238 // -----------------------------------------------------------------------------
239 // Enter support
240
241 void ObjectMonitor::enter(TRAPS) {
242 // The following code is ordered to check the most common cases first
243 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
244 Thread * const Self = THREAD;
245
246 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
247 if (cur == NULL) {
248 assert(_recursions == 0, "invariant");
249 return;
250 }
251
252 if (cur == Self) {
253 // TODO-FIXME: check for integer overflow! BUGID 6557169.
254 _recursions++;
255 return;
256 }
257
258 if (Self->is_lock_owned ((address)cur)) {
259 assert(_recursions == 0, "internal state error");
260 _recursions = 1;
261 // Commute owner from a thread-specific on-stack BasicLockObject address to
262 // a full-fledged "Thread *".
263 _owner = Self;
264 return;
265 }
266
267 // We've encountered genuine contention.
268 assert(Self->_Stalled == 0, "invariant");
269 Self->_Stalled = intptr_t(this);
270
271 // Try one round of spinning *before* enqueueing Self
272 // and before going through the awkward and expensive state
273 // transitions. The following spin is strictly optional ...
274 // Note that if we acquire the monitor from an initial spin
275 // we forgo posting JVMTI events and firing DTRACE probes.
276 if (TrySpin(Self) > 0) {
277 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner));
278 assert(_recursions == 0, "must be 0: recursions=" INTPTR_FORMAT,
279 _recursions);
280 assert(((oop)object())->mark() == markWord::encode(this),
281 "object mark must match encoded this: mark=" INTPTR_FORMAT
282 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(),
283 markWord::encode(this).value());
284 Self->_Stalled = 0;
285 return;
286 }
287
288 assert(_owner != Self, "invariant");
289 assert(_succ != Self, "invariant");
290 assert(Self->is_Java_thread(), "invariant");
291 JavaThread * jt = (JavaThread *) Self;
292 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
293 assert(jt->thread_state() != _thread_blocked, "invariant");
294 assert(this->object() != NULL, "invariant");
295 assert(_contentions >= 0, "invariant");
296
297 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
298 // Ensure the object-monitor relationship remains stable while there's contention.
299 Atomic::inc(&_contentions);
300
301 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
302 EventJavaMonitorEnter event;
303 if (event.should_commit()) {
304 event.set_monitorClass(((oop)this->object())->klass());
305 event.set_address((uintptr_t)(this->object_addr()));
306 }
307
308 { // Change java thread status to indicate blocked on monitor enter.
309 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
310
311 Self->set_current_pending_monitor(this);
312
313 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
314 if (JvmtiExport::should_post_monitor_contended_enter()) {
315 JvmtiExport::post_monitor_contended_enter(jt, this);
316
317 // The current thread does not yet own the monitor and does not
318 // yet appear on any queues that would get it made the successor.
319 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
341 //
342 _recursions = 0;
343 _succ = NULL;
344 exit(false, Self);
345
346 jt->java_suspend_self();
347 }
348 Self->set_current_pending_monitor(NULL);
349
350 // We cleared the pending monitor info since we've just gotten past
351 // the enter-check-for-suspend dance and we now own the monitor free
352 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
353 // destructor can go to a safepoint at the end of this block. If we
354 // do a thread dump during that safepoint, then this thread will show
355 // as having "-locked" the monitor, but the OS and java.lang.Thread
356 // states will still report that the thread is blocked trying to
357 // acquire it.
358 }
359
360 Atomic::dec(&_contentions);
361 assert(_contentions >= 0, "invariant");
362 Self->_Stalled = 0;
363
364 // Must either set _recursions = 0 or ASSERT _recursions == 0.
365 assert(_recursions == 0, "invariant");
366 assert(_owner == Self, "invariant");
367 assert(_succ != Self, "invariant");
368 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
369
370 // The thread -- now the owner -- is back in vm mode.
371 // Report the glorious news via TI,DTrace and jvmstat.
372 // The probe effect is non-trivial. All the reportage occurs
373 // while we hold the monitor, increasing the length of the critical
374 // section. Amdahl's parallel speedup law comes vividly into play.
375 //
376 // Another option might be to aggregate the events (thread local or
377 // per-monitor aggregation) and defer reporting until a more opportune
378 // time -- such as next time some thread encounters contention but has
379 // yet to acquire the lock. While spinning that thread could
380 // spinning we could increment JVMStat counters, etc.
381
385
386 // The current thread already owns the monitor and is not going to
387 // call park() for the remainder of the monitor enter protocol. So
388 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
389 // event handler consumed an unpark() issued by the thread that
390 // just exited the monitor.
391 }
392 if (event.should_commit()) {
393 event.set_previousOwner((uintptr_t)_previous_owner_tid);
394 event.commit();
395 }
396 OM_PERFDATA_OP(ContendedLockAttempts, inc());
397 }
398
399 // Caveat: TryLock() is not necessarily serializing if it returns failure.
400 // Callers must compensate as needed.
401
402 int ObjectMonitor::TryLock(Thread * Self) {
403 void * own = _owner;
404 if (own != NULL) return 0;
405 if (Atomic::replace_if_null(Self, &_owner)) {
406 assert(_recursions == 0, "invariant");
407 return 1;
408 }
409 // The lock had been free momentarily, but we lost the race to the lock.
410 // Interference -- the CAS failed.
411 // We can either return -1 or retry.
412 // Retry doesn't make as much sense because the lock was just acquired.
413 return -1;
414 }
415
416 // Convert the fields used by is_busy() to a string that can be
417 // used for diagnostic output.
418 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
419 ss->print("is_busy: contentions=%d, waiters=%d, owner=" INTPTR_FORMAT
420 ", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, _contentions,
421 _waiters, p2i(_owner), p2i(_cxq), p2i(_EntryList));
422 return ss->base();
423 }
424
425 #define MAX_RECHECK_INTERVAL 1000
426
427 void ObjectMonitor::EnterI(TRAPS) {
428 Thread * const Self = THREAD;
429 assert(Self->is_Java_thread(), "invariant");
430 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
431
432 // Try the lock - TATAS
433 if (TryLock (Self) > 0) {
434 assert(_succ != Self, "invariant");
435 assert(_owner == Self, "invariant");
436 assert(_Responsible != Self, "invariant");
437 return;
438 }
439
440 assert(InitDone, "Unexpectedly not initialized");
441
442 // We try one round of spinning *before* enqueueing Self.
443 //
444 // If the _owner is ready but OFFPROC we could use a YieldTo()
445 // operation to donate the remainder of this thread's quantum
446 // to the owner. This has subtle but beneficial affinity
447 // effects.
448
449 if (TrySpin(Self) > 0) {
450 assert(_owner == Self, "invariant");
451 assert(_succ != Self, "invariant");
452 assert(_Responsible != Self, "invariant");
453 return;
454 }
455
456 // The Spin failed -- Enqueue and park the thread ...
457 assert(_succ != Self, "invariant");
458 assert(_owner != Self, "invariant");
459 assert(_Responsible != Self, "invariant");
536
537 for (;;) {
538
539 if (TryLock(Self) > 0) break;
540 assert(_owner != Self, "invariant");
541
542 // park self
543 if (_Responsible == Self) {
544 Self->_ParkEvent->park((jlong) recheckInterval);
545 // Increase the recheckInterval, but clamp the value.
546 recheckInterval *= 8;
547 if (recheckInterval > MAX_RECHECK_INTERVAL) {
548 recheckInterval = MAX_RECHECK_INTERVAL;
549 }
550 } else {
551 Self->_ParkEvent->park();
552 }
553
554 if (TryLock(Self) > 0) break;
555
556 // The lock is still contested.
557 // Keep a tally of the # of futile wakeups.
558 // Note that the counter is not protected by a lock or updated by atomics.
559 // That is by design - we trade "lossy" counters which are exposed to
560 // races during updates for a lower probe effect.
561
562 // This PerfData object can be used in parallel with a safepoint.
563 // See the work around in PerfDataManager::destroy().
564 OM_PERFDATA_OP(FutileWakeups, inc());
565 ++nWakeups;
566
567 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
568 // We can defer clearing _succ until after the spin completes
569 // TrySpin() must tolerate being called with _succ == Self.
570 // Try yet another round of adaptive spinning.
571 if (TrySpin(Self) > 0) break;
572
573 // We can find that we were unpark()ed and redesignated _succ while
574 // we were spinning. That's harmless. If we iterate and call park(),
575 // park() will consume the event and return immediately and we'll
640 // the lock. The barrier ensures that changes to monitor meta-data and data
641 // protected by the lock will be visible before we release the lock, and
642 // therefore before some other thread (CPU) has a chance to acquire the lock.
643 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
644 //
645 // Critically, any prior STs to _succ or EntryList must be visible before
646 // the ST of null into _owner in the *subsequent* (following) corresponding
647 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
648 // execute a serializing instruction.
649
650 return;
651 }
652
653 // ReenterI() is a specialized inline form of the latter half of the
654 // contended slow-path from EnterI(). We use ReenterI() only for
655 // monitor reentry in wait().
656 //
657 // In the future we should reconcile EnterI() and ReenterI().
658
659 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
660 assert(Self != NULL, "invariant");
661 assert(SelfNode != NULL, "invariant");
662 assert(SelfNode->_thread == Self, "invariant");
663 assert(_waiters > 0, "invariant");
664 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
665 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
666 JavaThread * jt = (JavaThread *) Self;
667
668 int nWakeups = 0;
669 for (;;) {
670 ObjectWaiter::TStates v = SelfNode->TState;
671 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
672 assert(_owner != Self, "invariant");
673
674 if (TryLock(Self) > 0) break;
675 if (TrySpin(Self) > 0) break;
676
677 // State transition wrappers around park() ...
678 // ReenterI() wisely defers state transitions until
679 // it's clear we must park the thread.
680 {
681 OSThreadContendState osts(Self->osthread());
682 ThreadBlockInVM tbivm(jt);
683
684 // cleared by handle_special_suspend_equivalent_condition()
685 // or java_suspend_self()
686 jt->set_suspend_equivalent();
687 Self->_ParkEvent->park();
688
689 // were we externally suspended while we were waiting?
690 for (;;) {
691 if (!ExitSuspendEquivalent(jt)) break;
692 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
693 jt->java_suspend_self();
694 jt->set_suspend_equivalent();
695 }
696 }
846 // the timer expires. If the lock is high traffic then the stranding latency
847 // will be low due to (a). If the lock is low traffic then the odds of
848 // stranding are lower, although the worst-case stranding latency
849 // is longer. Critically, we don't want to put excessive load in the
850 // platform's timer subsystem. We want to minimize both the timer injection
851 // rate (timers created/sec) as well as the number of timers active at
852 // any one time. (more precisely, we want to minimize timer-seconds, which is
853 // the integral of the # of active timers at any instant over time).
854 // Both impinge on OS scalability. Given that, at most one thread parked on
855 // a monitor will use a timer.
856 //
857 // There is also the risk of a futile wake-up. If we drop the lock
858 // another thread can reacquire the lock immediately, and we can
859 // then wake a thread unnecessarily. This is benign, and we've
860 // structured the code so the windows are short and the frequency
861 // of such futile wakups is low.
862
863 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
864 Thread * const Self = THREAD;
865 if (THREAD != _owner) {
866 if (THREAD->is_lock_owned((address) _owner)) {
867 // Transmute _owner from a BasicLock pointer to a Thread address.
868 // We don't need to hold _mutex for this transition.
869 // Non-null to Non-null is safe as long as all readers can
870 // tolerate either flavor.
871 assert(_recursions == 0, "invariant");
872 _owner = THREAD;
873 _recursions = 0;
874 } else {
875 // Apparent unbalanced locking ...
876 // Naively we'd like to throw IllegalMonitorStateException.
877 // As a practical matter we can neither allocate nor throw an
878 // exception as ::exit() can be called from leaf routines.
879 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
880 // Upon deeper reflection, however, in a properly run JVM the only
881 // way we should encounter this situation is in the presence of
882 // unbalanced JNI locking. TODO: CheckJNICalls.
883 // See also: CR4414101
884 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
885 return;
886 }
887 }
888
889 if (_recursions != 0) {
890 _recursions--; // this is simple recursive enter
891 return;
892 }
893
894 // Invariant: after setting Responsible=null an thread must execute
895 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
896 _Responsible = NULL;
897
898 #if INCLUDE_JFR
899 // get the owner's thread id for the MonitorEnter event
900 // if it is enabled and the thread isn't suspended
901 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
902 _previous_owner_tid = JFR_THREAD_ID(Self);
903 }
904 #endif
905
906 for (;;) {
907 assert(THREAD == _owner, "invariant");
908
909 // release semantics: prior loads and stores from within the critical section
910 // must not float (reorder) past the following store that drops the lock.
911 // On SPARC that requires MEMBAR #loadstore|#storestore.
912 // But of course in TSO #loadstore|#storestore is not required.
913 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
914 OrderAccess::storeload(); // See if we need to wake a successor
915 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
916 return;
917 }
918 // Other threads are blocked trying to acquire the lock.
919
920 // Normally the exiting thread is responsible for ensuring succession,
921 // but if other successors are ready or other entering threads are spinning
922 // then this thread can simply store NULL into _owner and exit without
923 // waking a successor. The existence of spinners or ready successors
924 // guarantees proper succession (liveness). Responsibility passes to the
925 // ready or running successors. The exiting thread delegates the duty.
926 // More precisely, if a successor already exists this thread is absolved
927 // of the responsibility of waking (unparking) one.
928 //
929 // The _succ variable is critical to reducing futile wakeup frequency.
930 // _succ identifies the "heir presumptive" thread that has been made
931 // ready (unparked) but that has not yet run. We need only one such
932 // successor thread to guarantee progress.
933 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
934 // section 3.3 "Futile Wakeup Throttling" for details.
936 // Note that spinners in Enter() also set _succ non-null.
937 // In the current implementation spinners opportunistically set
938 // _succ so that exiting threads might avoid waking a successor.
939 // Another less appealing alternative would be for the exiting thread
940 // to drop the lock and then spin briefly to see if a spinner managed
941 // to acquire the lock. If so, the exiting thread could exit
942 // immediately without waking a successor, otherwise the exiting
943 // thread would need to dequeue and wake a successor.
944 // (Note that we'd need to make the post-drop spin short, but no
945 // shorter than the worst-case round-trip cache-line migration time.
946 // The dropped lock needs to become visible to the spinner, and then
947 // the acquisition of the lock by the spinner must become visible to
948 // the exiting thread).
949
950 // It appears that an heir-presumptive (successor) must be made ready.
951 // Only the current lock owner can manipulate the EntryList or
952 // drain _cxq, so we need to reacquire the lock. If we fail
953 // to reacquire the lock the responsibility for ensuring succession
954 // falls to the new owner.
955 //
956 if (!Atomic::replace_if_null(THREAD, &_owner)) {
957 return;
958 }
959
960 guarantee(_owner == THREAD, "invariant");
961
962 ObjectWaiter * w = NULL;
963
964 w = _EntryList;
965 if (w != NULL) {
966 // I'd like to write: guarantee (w->_thread != Self).
967 // But in practice an exiting thread may find itself on the EntryList.
968 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
969 // then calls exit(). Exit release the lock by setting O._owner to NULL.
970 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
971 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
972 // release the lock "O". T2 resumes immediately after the ST of null into
973 // _owner, above. T2 notices that the EntryList is populated, so it
974 // reacquires the lock and then finds itself on the EntryList.
975 // Given all that, we have to tolerate the circumstance where "w" is
976 // associated with Self.
1069
1070
1071 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1072 assert(_owner == Self, "invariant");
1073
1074 // Exit protocol:
1075 // 1. ST _succ = wakee
1076 // 2. membar #loadstore|#storestore;
1077 // 2. ST _owner = NULL
1078 // 3. unpark(wakee)
1079
1080 _succ = Wakee->_thread;
1081 ParkEvent * Trigger = Wakee->_event;
1082
1083 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1084 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1085 // out-of-scope (non-extant).
1086 Wakee = NULL;
1087
1088 // Drop the lock
1089 OrderAccess::release_store(&_owner, (void*)NULL);
1090 OrderAccess::fence(); // ST _owner vs LD in unpark()
1091
1092 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1093 Trigger->unpark();
1094
1095 // Maintain stats and report events to JVMTI
1096 OM_PERFDATA_OP(Parks, inc());
1097 }
1098
1099
1100 // -----------------------------------------------------------------------------
1101 // Class Loader deadlock handling.
1102 //
1103 // complete_exit exits a lock returning recursion count
1104 // complete_exit/reenter operate as a wait without waiting
1105 // complete_exit requires an inflated monitor
1106 // The _owner field is not always the Thread addr even with an
1107 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1108 // thread due to contention.
1109 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1110 Thread * const Self = THREAD;
1111 assert(Self->is_Java_thread(), "Must be Java thread!");
1112 JavaThread *jt = (JavaThread *)THREAD;
1113
1114 assert(InitDone, "Unexpectedly not initialized");
1115
1116 if (THREAD != _owner) {
1117 if (THREAD->is_lock_owned ((address)_owner)) {
1118 assert(_recursions == 0, "internal state error");
1119 _owner = THREAD; // Convert from basiclock addr to Thread addr
1120 _recursions = 0;
1121 }
1122 }
1123
1124 guarantee(Self == _owner, "complete_exit not owner");
1125 intptr_t save = _recursions; // record the old recursion count
1126 _recursions = 0; // set the recursion level to be 0
1127 exit(true, Self); // exit the monitor
1128 guarantee(_owner != Self, "invariant");
1129 return save;
1130 }
1131
1132 // reenter() enters a lock and sets recursion count
1133 // complete_exit/reenter operate as a wait without waiting
1134 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1135 Thread * const Self = THREAD;
1136 assert(Self->is_Java_thread(), "Must be Java thread!");
1137 JavaThread *jt = (JavaThread *)THREAD;
1138
1139 guarantee(_owner != Self, "reenter already owner");
1140 enter(THREAD); // enter the monitor
1141 guarantee(_recursions == 0, "reenter recursion");
1142 _recursions = recursions;
1143 return;
1144 }
1145
1146 // Checks that the current THREAD owns this monitor and causes an
1147 // immediate return if it doesn't. We don't use the CHECK macro
1148 // because we want the IMSE to be the only exception that is thrown
1149 // from the call site when false is returned. Any other pending
1150 // exception is ignored.
1151 #define CHECK_OWNER() \
1152 do { \
1153 if (!check_owner(THREAD)) { \
1154 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1155 return; \
1156 } \
1157 } while (false)
1158
1159 // Returns true if the specified thread owns the ObjectMonitor.
1160 // Otherwise returns false and throws IllegalMonitorStateException
1161 // (IMSE). If there is a pending exception and the specified thread
1162 // is not the owner, that exception will be replaced by the IMSE.
1163 bool ObjectMonitor::check_owner(Thread* THREAD) {
1164 if (_owner == THREAD) {
1165 return true;
1166 }
1167 if (THREAD->is_lock_owned((address)_owner)) {
1168 _owner = THREAD; // convert from BasicLock addr to Thread addr
1169 _recursions = 0;
1170 return true;
1171 }
1172 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1173 "current thread is not owner", false);
1174 }
1175
1176 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1177 ObjectMonitor* monitor,
1178 jlong notifier_tid,
1179 jlong timeout,
1180 bool timedout) {
1181 assert(event != NULL, "invariant");
1182 assert(monitor != NULL, "invariant");
1183 event->set_monitorClass(((oop)monitor->object())->klass());
1184 event->set_timeout(timeout);
1185 event->set_address((uintptr_t)monitor->object_addr());
1186 event->set_notifier(notifier_tid);
1187 event->set_timedOut(timedout);
1188 event->commit();
1653 // We periodically check to see if there's a safepoint pending.
1654 if ((ctr & 0xFF) == 0) {
1655 if (SafepointMechanism::should_block(Self)) {
1656 goto Abort; // abrupt spin egress
1657 }
1658 SpinPause();
1659 }
1660
1661 // Probe _owner with TATAS
1662 // If this thread observes the monitor transition or flicker
1663 // from locked to unlocked to locked, then the odds that this
1664 // thread will acquire the lock in this spin attempt go down
1665 // considerably. The same argument applies if the CAS fails
1666 // or if we observe _owner change from one non-null value to
1667 // another non-null value. In such cases we might abort
1668 // the spin without prejudice or apply a "penalty" to the
1669 // spin count-down variable "ctr", reducing it by 100, say.
1670
1671 Thread * ox = (Thread *) _owner;
1672 if (ox == NULL) {
1673 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1674 if (ox == NULL) {
1675 // The CAS succeeded -- this thread acquired ownership
1676 // Take care of some bookkeeping to exit spin state.
1677 if (_succ == Self) {
1678 _succ = NULL;
1679 }
1680
1681 // Increase _SpinDuration :
1682 // The spin was successful (profitable) so we tend toward
1683 // longer spin attempts in the future.
1684 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1685 // If we acquired the lock early in the spin cycle it
1686 // makes sense to increase _SpinDuration proportionally.
1687 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1688 int x = _SpinDuration;
1689 if (x < Knob_SpinLimit) {
1690 if (x < Knob_Poverty) x = Knob_Poverty;
1691 _SpinDuration = x + Knob_Bonus;
1692 }
1693 return 1;
1917 }
1918 #define NEWPERFVARIABLE(n) \
1919 { \
1920 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
1921 CHECK); \
1922 }
1923 NEWPERFCOUNTER(_sync_Inflations);
1924 NEWPERFCOUNTER(_sync_Deflations);
1925 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
1926 NEWPERFCOUNTER(_sync_FutileWakeups);
1927 NEWPERFCOUNTER(_sync_Parks);
1928 NEWPERFCOUNTER(_sync_Notifications);
1929 NEWPERFVARIABLE(_sync_MonExtant);
1930 #undef NEWPERFCOUNTER
1931 #undef NEWPERFVARIABLE
1932 }
1933
1934 DEBUG_ONLY(InitDone = true;)
1935 }
1936
1937 void ObjectMonitor::print_on(outputStream* st) const {
1938 // The minimal things to print for markWord printing, more can be added for debugging and logging.
1939 st->print("{contentions=0x%08x,waiters=0x%08x"
1940 ",recursions=" INTPTR_FORMAT ",owner=" INTPTR_FORMAT "}",
1941 contentions(), waiters(), recursions(),
1942 p2i(owner()));
1943 }
1944 void ObjectMonitor::print() const { print_on(tty); }
|
222 // * See also http://blogs.sun.com/dave
223
224
225 void* ObjectMonitor::operator new (size_t size) throw() {
226 return AllocateHeap(size, mtInternal);
227 }
228 void* ObjectMonitor::operator new[] (size_t size) throw() {
229 return operator new (size);
230 }
231 void ObjectMonitor::operator delete(void* p) {
232 FreeHeap(p);
233 }
234 void ObjectMonitor::operator delete[] (void *p) {
235 operator delete(p);
236 }
237
238 // -----------------------------------------------------------------------------
239 // Enter support
240
241 void ObjectMonitor::enter(TRAPS) {
242 ADIM_guarantee(ref_count() > 0, "must be positive: ref_count=%d", ref_count());
243
244 // The following code is ordered to check the most common cases first
245 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
246 Thread * const Self = THREAD;
247
248 void* cur = try_set_owner_from(Self, NULL);
249 if (cur == NULL) {
250 assert(_recursions == 0, "invariant");
251 return;
252 }
253
254 if (cur == Self) {
255 // TODO-FIXME: check for integer overflow! BUGID 6557169.
256 _recursions++;
257 return;
258 }
259
260 if (Self->is_lock_owned ((address)cur)) {
261 assert(_recursions == 0, "internal state error");
262 _recursions = 1;
263 set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*.
264 return;
265 }
266
267 if (AsyncDeflateIdleMonitors &&
268 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) {
269 // The deflation protocol finished the first part (setting owner),
270 // but it failed the second part (making ref_count negative) and
271 // bailed. Or the ObjectMonitor was async deflated and reused.
272 // Acquired the monitor.
273 assert(_recursions == 0, "invariant");
274 return;
275 }
276
277 // We've encountered genuine contention.
278 assert(Self->_Stalled == 0, "invariant");
279 Self->_Stalled = intptr_t(this);
280
281 // Try one round of spinning *before* enqueueing Self
282 // and before going through the awkward and expensive state
283 // transitions. The following spin is strictly optional ...
284 // Note that if we acquire the monitor from an initial spin
285 // we forgo posting JVMTI events and firing DTRACE probes.
286 if (TrySpin(Self) > 0) {
287 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner));
288 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
289 assert(((oop)object())->mark() == markWord::encode(this),
290 "object mark must match encoded this: mark=" INTPTR_FORMAT
291 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(),
292 markWord::encode(this).value());
293 Self->_Stalled = 0;
294 return;
295 }
296
297 assert(_owner != Self, "invariant");
298 assert(_succ != Self, "invariant");
299 assert(Self->is_Java_thread(), "invariant");
300 JavaThread * jt = (JavaThread *) Self;
301 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
302 assert(jt->thread_state() != _thread_blocked, "invariant");
303 assert(AsyncDeflateIdleMonitors || this->object() != NULL, "invariant");
304 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
305
306 // Prevent deflation. See ObjectSynchronizer::deflate_monitor(),
307 // ObjectSynchronizer::deflate_monitor_using_JT() and is_busy().
308 // Ensure the object <-> monitor relationship remains stable while
309 // there's contention.
310 Atomic::add(1, &_contentions);
311
312 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
313 EventJavaMonitorEnter event;
314 if (event.should_commit()) {
315 event.set_monitorClass(((oop)this->object())->klass());
316 event.set_address((uintptr_t)(this->object_addr()));
317 }
318
319 { // Change java thread status to indicate blocked on monitor enter.
320 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
321
322 Self->set_current_pending_monitor(this);
323
324 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
325 if (JvmtiExport::should_post_monitor_contended_enter()) {
326 JvmtiExport::post_monitor_contended_enter(jt, this);
327
328 // The current thread does not yet own the monitor and does not
329 // yet appear on any queues that would get it made the successor.
330 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
352 //
353 _recursions = 0;
354 _succ = NULL;
355 exit(false, Self);
356
357 jt->java_suspend_self();
358 }
359 Self->set_current_pending_monitor(NULL);
360
361 // We cleared the pending monitor info since we've just gotten past
362 // the enter-check-for-suspend dance and we now own the monitor free
363 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
364 // destructor can go to a safepoint at the end of this block. If we
365 // do a thread dump during that safepoint, then this thread will show
366 // as having "-locked" the monitor, but the OS and java.lang.Thread
367 // states will still report that the thread is blocked trying to
368 // acquire it.
369 }
370
371 Atomic::dec(&_contentions);
372 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
373 Self->_Stalled = 0;
374
375 // Must either set _recursions = 0 or ASSERT _recursions == 0.
376 assert(_recursions == 0, "invariant");
377 assert(_owner == Self, "invariant");
378 assert(_succ != Self, "invariant");
379 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
380
381 // The thread -- now the owner -- is back in vm mode.
382 // Report the glorious news via TI,DTrace and jvmstat.
383 // The probe effect is non-trivial. All the reportage occurs
384 // while we hold the monitor, increasing the length of the critical
385 // section. Amdahl's parallel speedup law comes vividly into play.
386 //
387 // Another option might be to aggregate the events (thread local or
388 // per-monitor aggregation) and defer reporting until a more opportune
389 // time -- such as next time some thread encounters contention but has
390 // yet to acquire the lock. While spinning that thread could
391 // spinning we could increment JVMStat counters, etc.
392
396
397 // The current thread already owns the monitor and is not going to
398 // call park() for the remainder of the monitor enter protocol. So
399 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
400 // event handler consumed an unpark() issued by the thread that
401 // just exited the monitor.
402 }
403 if (event.should_commit()) {
404 event.set_previousOwner((uintptr_t)_previous_owner_tid);
405 event.commit();
406 }
407 OM_PERFDATA_OP(ContendedLockAttempts, inc());
408 }
409
410 // Caveat: TryLock() is not necessarily serializing if it returns failure.
411 // Callers must compensate as needed.
412
413 int ObjectMonitor::TryLock(Thread * Self) {
414 void * own = _owner;
415 if (own != NULL) return 0;
416 if (try_set_owner_from(Self, NULL) == NULL) {
417 assert(_recursions == 0, "invariant");
418 return 1;
419 }
420 // The lock had been free momentarily, but we lost the race to the lock.
421 // Interference -- the CAS failed.
422 // We can either return -1 or retry.
423 // Retry doesn't make as much sense because the lock was just acquired.
424 return -1;
425 }
426
427 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
428 // into the header of the object associated with the monitor. This
429 // idempotent method is called by a thread that is deflating a
430 // monitor and by other threads that have detected a race with the
431 // deflation process.
432 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
433 // This function must only be called when (owner == DEFLATER_MARKER
434 // && ref_count <= 0), but we can't guarantee that here because
435 // those values could change when the ObjectMonitor gets moved from
436 // the global free list to a per-thread free list.
437
438 guarantee(obj != NULL, "must be non-NULL");
439 if (object() != obj) {
440 // ObjectMonitor's object ref no longer refers to the target object
441 // so the object's header has already been restored.
442 return;
443 }
444
445 markWord dmw = header();
446 if (dmw.value() == 0) {
447 // ObjectMonitor's header/dmw has been cleared so the ObjectMonitor
448 // has been deflated and taken off the global free list.
449 return;
450 }
451
452 // A non-NULL dmw has to be either neutral (not locked and not marked)
453 // or is already participating in this restoration protocol.
454 assert(dmw.is_neutral() || (dmw.is_marked() && dmw.hash() == 0),
455 "failed precondition: dmw=" INTPTR_FORMAT, dmw.value());
456
457 markWord marked_dmw = markWord::zero();
458 if (!dmw.is_marked() && dmw.hash() == 0) {
459 // This dmw has not yet started the restoration protocol so we
460 // mark a copy of the dmw to begin the protocol.
461 // Note: A dmw with a hashcode does not take this code path.
462 marked_dmw = dmw.set_marked();
463
464 // All of the callers to this function can be racing with each
465 // other trying to update the _header field.
466 dmw = (markWord) Atomic::cmpxchg(marked_dmw, &_header, dmw);
467 if (dmw.value() == 0) {
468 // ObjectMonitor's header/dmw has been cleared so the object's
469 // header has already been restored.
470 return;
471 }
472 // The _header field is now marked. The winner's 'dmw' variable
473 // contains the original, unmarked header/dmw value and any
474 // losers have a marked header/dmw value that will be cleaned
475 // up below.
476 }
477
478 if (dmw.is_marked()) {
479 // Clear the mark from the header/dmw copy in preparation for
480 // possible restoration from this thread.
481 assert(dmw.hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT,
482 dmw.value());
483 dmw = dmw.set_unmarked();
484 }
485 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
486
487 // Install displaced mark word if the object's header still points
488 // to this ObjectMonitor. All racing callers to this function will
489 // reach this point, but only one can win.
490 obj->cas_set_mark(dmw, markWord::encode(this));
491
492 // Note: It does not matter which thread restored the header/dmw
493 // into the object's header. The thread deflating the monitor just
494 // wanted the object's header restored and it is. The threads that
495 // detected a race with the deflation process also wanted the
496 // object's header restored before they retry their operation and
497 // because it is restored they will only retry once.
498 }
499
500 // Convert the fields used by is_busy() to a string that can be
501 // used for diagnostic output.
502 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
503 ss->print("is_busy: contentions=%d, waiters=%d, ", _contentions, _waiters);
504 if (!AsyncDeflateIdleMonitors) {
505 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
506 } else if (_owner != DEFLATER_MARKER) {
507 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
508 } else {
509 ss->print("owner=" INTPTR_FORMAT, NULL);
510 }
511 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
512 p2i(_EntryList));
513 return ss->base();
514 }
515
516 #define MAX_RECHECK_INTERVAL 1000
517
518 void ObjectMonitor::EnterI(TRAPS) {
519 ADIM_guarantee(ref_count() > 0, "must be positive: ref_count=%d", ref_count());
520
521 Thread * const Self = THREAD;
522 assert(Self->is_Java_thread(), "invariant");
523 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
524
525 // Try the lock - TATAS
526 if (TryLock (Self) > 0) {
527 assert(_succ != Self, "invariant");
528 assert(_owner == Self, "invariant");
529 assert(_Responsible != Self, "invariant");
530 return;
531 }
532
533 if (AsyncDeflateIdleMonitors &&
534 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) {
535 // The deflation protocol finished the first part (setting owner),
536 // but it failed the second part (making ref_count negative) and
537 // bailed. Or the ObjectMonitor was async deflated and reused.
538 // Acquired the monitor.
539 assert(_succ != Self, "invariant");
540 assert(_Responsible != Self, "invariant");
541 return;
542 }
543
544 assert(InitDone, "Unexpectedly not initialized");
545
546 // We try one round of spinning *before* enqueueing Self.
547 //
548 // If the _owner is ready but OFFPROC we could use a YieldTo()
549 // operation to donate the remainder of this thread's quantum
550 // to the owner. This has subtle but beneficial affinity
551 // effects.
552
553 if (TrySpin(Self) > 0) {
554 assert(_owner == Self, "invariant");
555 assert(_succ != Self, "invariant");
556 assert(_Responsible != Self, "invariant");
557 return;
558 }
559
560 // The Spin failed -- Enqueue and park the thread ...
561 assert(_succ != Self, "invariant");
562 assert(_owner != Self, "invariant");
563 assert(_Responsible != Self, "invariant");
640
641 for (;;) {
642
643 if (TryLock(Self) > 0) break;
644 assert(_owner != Self, "invariant");
645
646 // park self
647 if (_Responsible == Self) {
648 Self->_ParkEvent->park((jlong) recheckInterval);
649 // Increase the recheckInterval, but clamp the value.
650 recheckInterval *= 8;
651 if (recheckInterval > MAX_RECHECK_INTERVAL) {
652 recheckInterval = MAX_RECHECK_INTERVAL;
653 }
654 } else {
655 Self->_ParkEvent->park();
656 }
657
658 if (TryLock(Self) > 0) break;
659
660 if (AsyncDeflateIdleMonitors &&
661 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) {
662 // The deflation protocol finished the first part (setting owner),
663 // but it failed the second part (making ref_count negative) and
664 // bailed. Or the ObjectMonitor was async deflated and reused.
665 // Acquired the monitor.
666 break;
667 }
668
669 // The lock is still contested.
670 // Keep a tally of the # of futile wakeups.
671 // Note that the counter is not protected by a lock or updated by atomics.
672 // That is by design - we trade "lossy" counters which are exposed to
673 // races during updates for a lower probe effect.
674
675 // This PerfData object can be used in parallel with a safepoint.
676 // See the work around in PerfDataManager::destroy().
677 OM_PERFDATA_OP(FutileWakeups, inc());
678 ++nWakeups;
679
680 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
681 // We can defer clearing _succ until after the spin completes
682 // TrySpin() must tolerate being called with _succ == Self.
683 // Try yet another round of adaptive spinning.
684 if (TrySpin(Self) > 0) break;
685
686 // We can find that we were unpark()ed and redesignated _succ while
687 // we were spinning. That's harmless. If we iterate and call park(),
688 // park() will consume the event and return immediately and we'll
753 // the lock. The barrier ensures that changes to monitor meta-data and data
754 // protected by the lock will be visible before we release the lock, and
755 // therefore before some other thread (CPU) has a chance to acquire the lock.
756 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
757 //
758 // Critically, any prior STs to _succ or EntryList must be visible before
759 // the ST of null into _owner in the *subsequent* (following) corresponding
760 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
761 // execute a serializing instruction.
762
763 return;
764 }
765
766 // ReenterI() is a specialized inline form of the latter half of the
767 // contended slow-path from EnterI(). We use ReenterI() only for
768 // monitor reentry in wait().
769 //
770 // In the future we should reconcile EnterI() and ReenterI().
771
772 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
773 ADIM_guarantee(ref_count() > 0, "must be positive: ref_count=%d", ref_count());
774
775 assert(Self != NULL, "invariant");
776 assert(SelfNode != NULL, "invariant");
777 assert(SelfNode->_thread == Self, "invariant");
778 assert(_waiters > 0, "invariant");
779 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
780 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
781 JavaThread * jt = (JavaThread *) Self;
782
783 int nWakeups = 0;
784 for (;;) {
785 ObjectWaiter::TStates v = SelfNode->TState;
786 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
787 assert(_owner != Self, "invariant");
788
789 if (TryLock(Self) > 0) break;
790 if (TrySpin(Self) > 0) break;
791
792 if (AsyncDeflateIdleMonitors &&
793 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) {
794 // The deflation protocol finished the first part (setting owner),
795 // but it failed the second part (making ref_count negative) and
796 // bailed. Or the ObjectMonitor was async deflated and reused.
797 // Acquired the monitor.
798 break;
799 }
800
801 // State transition wrappers around park() ...
802 // ReenterI() wisely defers state transitions until
803 // it's clear we must park the thread.
804 {
805 OSThreadContendState osts(Self->osthread());
806 ThreadBlockInVM tbivm(jt);
807
808 // cleared by handle_special_suspend_equivalent_condition()
809 // or java_suspend_self()
810 jt->set_suspend_equivalent();
811 Self->_ParkEvent->park();
812
813 // were we externally suspended while we were waiting?
814 for (;;) {
815 if (!ExitSuspendEquivalent(jt)) break;
816 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
817 jt->java_suspend_self();
818 jt->set_suspend_equivalent();
819 }
820 }
970 // the timer expires. If the lock is high traffic then the stranding latency
971 // will be low due to (a). If the lock is low traffic then the odds of
972 // stranding are lower, although the worst-case stranding latency
973 // is longer. Critically, we don't want to put excessive load in the
974 // platform's timer subsystem. We want to minimize both the timer injection
975 // rate (timers created/sec) as well as the number of timers active at
976 // any one time. (more precisely, we want to minimize timer-seconds, which is
977 // the integral of the # of active timers at any instant over time).
978 // Both impinge on OS scalability. Given that, at most one thread parked on
979 // a monitor will use a timer.
980 //
981 // There is also the risk of a futile wake-up. If we drop the lock
982 // another thread can reacquire the lock immediately, and we can
983 // then wake a thread unnecessarily. This is benign, and we've
984 // structured the code so the windows are short and the frequency
985 // of such futile wakups is low.
986
987 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
988 Thread * const Self = THREAD;
989 if (THREAD != _owner) {
990 void* cur = _owner;
991 if (THREAD->is_lock_owned((address)cur)) {
992 assert(_recursions == 0, "invariant");
993 set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*.
994 _recursions = 0;
995 } else {
996 // Apparent unbalanced locking ...
997 // Naively we'd like to throw IllegalMonitorStateException.
998 // As a practical matter we can neither allocate nor throw an
999 // exception as ::exit() can be called from leaf routines.
1000 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1001 // Upon deeper reflection, however, in a properly run JVM the only
1002 // way we should encounter this situation is in the presence of
1003 // unbalanced JNI locking. TODO: CheckJNICalls.
1004 // See also: CR4414101
1005 tty->print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1006 " is exiting an ObjectMonitor it does not own.",
1007 p2i(THREAD));
1008 tty->print_cr("The imbalance is possibly caused by JNI locking.");
1009 print_debug_style_on(tty);
1010 // Changing this from an assert() to ADIM_guarantee() may run
1011 // afoul of any test that is inducing non-balanced JNI locking.
1012 ADIM_guarantee(false, "Non-balanced monitor enter/exit!");
1013 return;
1014 }
1015 }
1016
1017 if (_recursions != 0) {
1018 _recursions--; // this is simple recursive enter
1019 return;
1020 }
1021
1022 // Invariant: after setting Responsible=null an thread must execute
1023 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1024 _Responsible = NULL;
1025
1026 #if INCLUDE_JFR
1027 // get the owner's thread id for the MonitorEnter event
1028 // if it is enabled and the thread isn't suspended
1029 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1030 _previous_owner_tid = JFR_THREAD_ID(Self);
1031 }
1032 #endif
1033
1034 for (;;) {
1035 assert(THREAD == _owner, "invariant");
1036
1037 // release semantics: prior loads and stores from within the critical section
1038 // must not float (reorder) past the following store that drops the lock.
1039 // On SPARC that requires MEMBAR #loadstore|#storestore.
1040 // But of course in TSO #loadstore|#storestore is not required.
1041 if (AsyncDeflateIdleMonitors) {
1042 set_owner_from(NULL, Self);
1043 } else {
1044 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
1045 OrderAccess::storeload(); // See if we need to wake a successor
1046 }
1047 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1048 return;
1049 }
1050 // Other threads are blocked trying to acquire the lock.
1051
1052 // Normally the exiting thread is responsible for ensuring succession,
1053 // but if other successors are ready or other entering threads are spinning
1054 // then this thread can simply store NULL into _owner and exit without
1055 // waking a successor. The existence of spinners or ready successors
1056 // guarantees proper succession (liveness). Responsibility passes to the
1057 // ready or running successors. The exiting thread delegates the duty.
1058 // More precisely, if a successor already exists this thread is absolved
1059 // of the responsibility of waking (unparking) one.
1060 //
1061 // The _succ variable is critical to reducing futile wakeup frequency.
1062 // _succ identifies the "heir presumptive" thread that has been made
1063 // ready (unparked) but that has not yet run. We need only one such
1064 // successor thread to guarantee progress.
1065 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1066 // section 3.3 "Futile Wakeup Throttling" for details.
1068 // Note that spinners in Enter() also set _succ non-null.
1069 // In the current implementation spinners opportunistically set
1070 // _succ so that exiting threads might avoid waking a successor.
1071 // Another less appealing alternative would be for the exiting thread
1072 // to drop the lock and then spin briefly to see if a spinner managed
1073 // to acquire the lock. If so, the exiting thread could exit
1074 // immediately without waking a successor, otherwise the exiting
1075 // thread would need to dequeue and wake a successor.
1076 // (Note that we'd need to make the post-drop spin short, but no
1077 // shorter than the worst-case round-trip cache-line migration time.
1078 // The dropped lock needs to become visible to the spinner, and then
1079 // the acquisition of the lock by the spinner must become visible to
1080 // the exiting thread).
1081
1082 // It appears that an heir-presumptive (successor) must be made ready.
1083 // Only the current lock owner can manipulate the EntryList or
1084 // drain _cxq, so we need to reacquire the lock. If we fail
1085 // to reacquire the lock the responsibility for ensuring succession
1086 // falls to the new owner.
1087 //
1088 if (try_set_owner_from(Self, NULL) != NULL) {
1089 return;
1090 }
1091
1092 guarantee(_owner == THREAD, "invariant");
1093
1094 ObjectWaiter * w = NULL;
1095
1096 w = _EntryList;
1097 if (w != NULL) {
1098 // I'd like to write: guarantee (w->_thread != Self).
1099 // But in practice an exiting thread may find itself on the EntryList.
1100 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1101 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1102 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1103 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1104 // release the lock "O". T2 resumes immediately after the ST of null into
1105 // _owner, above. T2 notices that the EntryList is populated, so it
1106 // reacquires the lock and then finds itself on the EntryList.
1107 // Given all that, we have to tolerate the circumstance where "w" is
1108 // associated with Self.
1201
1202
1203 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1204 assert(_owner == Self, "invariant");
1205
1206 // Exit protocol:
1207 // 1. ST _succ = wakee
1208 // 2. membar #loadstore|#storestore;
1209 // 2. ST _owner = NULL
1210 // 3. unpark(wakee)
1211
1212 _succ = Wakee->_thread;
1213 ParkEvent * Trigger = Wakee->_event;
1214
1215 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1216 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1217 // out-of-scope (non-extant).
1218 Wakee = NULL;
1219
1220 // Drop the lock
1221 if (AsyncDeflateIdleMonitors) {
1222 set_owner_from(NULL, Self);
1223 } else {
1224 OrderAccess::release_store(&_owner, (void*)NULL);
1225 OrderAccess::fence(); // ST _owner vs LD in unpark()
1226 }
1227
1228 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1229 Trigger->unpark();
1230
1231 // Maintain stats and report events to JVMTI
1232 OM_PERFDATA_OP(Parks, inc());
1233 }
1234
1235
1236 // -----------------------------------------------------------------------------
1237 // Class Loader deadlock handling.
1238 //
1239 // complete_exit exits a lock returning recursion count
1240 // complete_exit/reenter operate as a wait without waiting
1241 // complete_exit requires an inflated monitor
1242 // The _owner field is not always the Thread addr even with an
1243 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1244 // thread due to contention.
1245 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1246 Thread * const Self = THREAD;
1247 assert(Self->is_Java_thread(), "Must be Java thread!");
1248 JavaThread *jt = (JavaThread *)THREAD;
1249
1250 assert(InitDone, "Unexpectedly not initialized");
1251
1252 if (THREAD != _owner) {
1253 void* cur = _owner;
1254 if (THREAD->is_lock_owned((address)cur)) {
1255 assert(_recursions == 0, "internal state error");
1256 set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*.
1257 _recursions = 0;
1258 }
1259 }
1260
1261 guarantee(Self == _owner, "complete_exit not owner");
1262 intptr_t save = _recursions; // record the old recursion count
1263 _recursions = 0; // set the recursion level to be 0
1264 exit(true, Self); // exit the monitor
1265 guarantee(_owner != Self, "invariant");
1266 return save;
1267 }
1268
1269 // reenter() enters a lock and sets recursion count
1270 // complete_exit/reenter operate as a wait without waiting
1271 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1272 Thread * const Self = THREAD;
1273 assert(Self->is_Java_thread(), "Must be Java thread!");
1274 JavaThread *jt = (JavaThread *)THREAD;
1275
1276 guarantee(_owner != Self, "reenter already owner");
1277 enter(THREAD);
1278 // Entered the monitor.
1279 guarantee(_recursions == 0, "reenter recursion");
1280 _recursions = recursions;
1281 }
1282
1283 // Checks that the current THREAD owns this monitor and causes an
1284 // immediate return if it doesn't. We don't use the CHECK macro
1285 // because we want the IMSE to be the only exception that is thrown
1286 // from the call site when false is returned. Any other pending
1287 // exception is ignored.
1288 #define CHECK_OWNER() \
1289 do { \
1290 if (!check_owner(THREAD)) { \
1291 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1292 return; \
1293 } \
1294 } while (false)
1295
1296 // Returns true if the specified thread owns the ObjectMonitor.
1297 // Otherwise returns false and throws IllegalMonitorStateException
1298 // (IMSE). If there is a pending exception and the specified thread
1299 // is not the owner, that exception will be replaced by the IMSE.
1300 bool ObjectMonitor::check_owner(Thread* THREAD) {
1301 if (_owner == THREAD) {
1302 return true;
1303 }
1304 void* cur = _owner;
1305 if (THREAD->is_lock_owned((address)cur)) {
1306 set_owner_from_BasicLock(THREAD, cur); // Convert from BasicLock* to Thread*.
1307 _recursions = 0;
1308 return true;
1309 }
1310 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1311 "current thread is not owner", false);
1312 }
1313
1314 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1315 ObjectMonitor* monitor,
1316 jlong notifier_tid,
1317 jlong timeout,
1318 bool timedout) {
1319 assert(event != NULL, "invariant");
1320 assert(monitor != NULL, "invariant");
1321 event->set_monitorClass(((oop)monitor->object())->klass());
1322 event->set_timeout(timeout);
1323 event->set_address((uintptr_t)monitor->object_addr());
1324 event->set_notifier(notifier_tid);
1325 event->set_timedOut(timedout);
1326 event->commit();
1791 // We periodically check to see if there's a safepoint pending.
1792 if ((ctr & 0xFF) == 0) {
1793 if (SafepointMechanism::should_block(Self)) {
1794 goto Abort; // abrupt spin egress
1795 }
1796 SpinPause();
1797 }
1798
1799 // Probe _owner with TATAS
1800 // If this thread observes the monitor transition or flicker
1801 // from locked to unlocked to locked, then the odds that this
1802 // thread will acquire the lock in this spin attempt go down
1803 // considerably. The same argument applies if the CAS fails
1804 // or if we observe _owner change from one non-null value to
1805 // another non-null value. In such cases we might abort
1806 // the spin without prejudice or apply a "penalty" to the
1807 // spin count-down variable "ctr", reducing it by 100, say.
1808
1809 Thread * ox = (Thread *) _owner;
1810 if (ox == NULL) {
1811 ox = (Thread*)try_set_owner_from(Self, NULL);
1812 if (ox == NULL) {
1813 // The CAS succeeded -- this thread acquired ownership
1814 // Take care of some bookkeeping to exit spin state.
1815 if (_succ == Self) {
1816 _succ = NULL;
1817 }
1818
1819 // Increase _SpinDuration :
1820 // The spin was successful (profitable) so we tend toward
1821 // longer spin attempts in the future.
1822 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1823 // If we acquired the lock early in the spin cycle it
1824 // makes sense to increase _SpinDuration proportionally.
1825 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1826 int x = _SpinDuration;
1827 if (x < Knob_SpinLimit) {
1828 if (x < Knob_Poverty) x = Knob_Poverty;
1829 _SpinDuration = x + Knob_Bonus;
1830 }
1831 return 1;
2055 }
2056 #define NEWPERFVARIABLE(n) \
2057 { \
2058 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2059 CHECK); \
2060 }
2061 NEWPERFCOUNTER(_sync_Inflations);
2062 NEWPERFCOUNTER(_sync_Deflations);
2063 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2064 NEWPERFCOUNTER(_sync_FutileWakeups);
2065 NEWPERFCOUNTER(_sync_Parks);
2066 NEWPERFCOUNTER(_sync_Notifications);
2067 NEWPERFVARIABLE(_sync_MonExtant);
2068 #undef NEWPERFCOUNTER
2069 #undef NEWPERFVARIABLE
2070 }
2071
2072 DEBUG_ONLY(InitDone = true;)
2073 }
2074
2075 // For internal use by ObjectSynchronizer::monitors_iterate().
2076 ObjectMonitorHandle::ObjectMonitorHandle(ObjectMonitor * om_ptr) {
2077 om_ptr->inc_ref_count();
2078 _om_ptr = om_ptr;
2079 }
2080
2081 ObjectMonitorHandle::~ObjectMonitorHandle() {
2082 if (_om_ptr != NULL) {
2083 _om_ptr->dec_ref_count();
2084 _om_ptr = NULL;
2085 }
2086 }
2087
2088 // Save the ObjectMonitor* associated with the specified markWord and
2089 // increment the ref_count. This function should only be called if
2090 // the caller has verified mark.has_monitor() == true. The object
2091 // parameter is needed to verify that ObjectMonitor* has not been
2092 // deflated and reused for another object.
2093 //
2094 // This function returns true if the ObjectMonitor* has been safely
2095 // saved. This function returns false if we have lost a race with
2096 // async deflation; the caller should retry as appropriate.
2097 //
2098 bool ObjectMonitorHandle::save_om_ptr(oop object, markWord mark) {
2099 guarantee(mark.has_monitor(), "sanity check: mark=" INTPTR_FORMAT,
2100 mark.value());
2101
2102 ObjectMonitor * om_ptr = mark.monitor();
2103 om_ptr->inc_ref_count();
2104
2105 if (AsyncDeflateIdleMonitors) {
2106 // Race here if monitor is not owned! The above ref_count bump
2107 // will cause subsequent async deflation to skip it. However,
2108 // previous or concurrent async deflation is a race.
2109 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) {
2110 // Async deflation is in progress and our ref_count increment
2111 // above lost the race to async deflation. Attempt to restore
2112 // the header/dmw to the object's header so that we only retry
2113 // once if the deflater thread happens to be slow.
2114 om_ptr->install_displaced_markword_in_object(object);
2115 om_ptr->dec_ref_count();
2116 return false;
2117 }
2118 if (om_ptr->ref_count() <= 0) {
2119 // Async deflation is in the process of bailing out, but has not
2120 // yet restored the ref_count field so we return false to force
2121 // a retry. We want a positive ref_count value for a true return.
2122 om_ptr->dec_ref_count();
2123 return false;
2124 }
2125 // The ObjectMonitor could have been deflated and reused for
2126 // another object before we bumped the ref_count so make sure
2127 // our object still refers to this ObjectMonitor.
2128 const markWord tmp = object->mark();
2129 if (!tmp.has_monitor() || tmp.monitor() != om_ptr) {
2130 // Async deflation and reuse won the race so we have to retry.
2131 // Skip object header restoration since that's already done.
2132 om_ptr->dec_ref_count();
2133 return false;
2134 }
2135 }
2136
2137 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
2138 p2i(_om_ptr));
2139 _om_ptr = om_ptr;
2140 return true;
2141 }
2142
2143 // For internal use by ObjectSynchronizer::inflate().
2144 void ObjectMonitorHandle::set_om_ptr(ObjectMonitor * om_ptr) {
2145 if (_om_ptr == NULL) {
2146 ADIM_guarantee(om_ptr != NULL, "cannot clear an unset om_ptr");
2147 om_ptr->inc_ref_count();
2148 _om_ptr = om_ptr;
2149 } else {
2150 ADIM_guarantee(om_ptr == NULL, "can only clear a set om_ptr");
2151 _om_ptr->dec_ref_count();
2152 _om_ptr = NULL;
2153 }
2154 }
2155
2156 void ObjectMonitor::print_on(outputStream* st) const {
2157 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2158 st->print("{contentions=0x%08x,waiters=0x%08x"
2159 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2160 contentions(), waiters(), recursions(),
2161 p2i(owner()));
2162 }
2163 void ObjectMonitor::print() const { print_on(tty); }
2164
2165 // Print the ObjectMonitor like a debugger would:
2166 //
2167 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2168 // _header = (_value = 1)
2169 // _object = 0x000000070ff45fd0
2170 // _allocation_state = Old
2171 // _pad_buf0 = {
2172 // [0] = '\0'
2173 // ...
2174 // [43] = '\0'
2175 // }
2176 // _owner = 0x0000000000000000
2177 // _previous_owner_tid = 0
2178 // _pad_buf1 = {
2179 // [0] = '\0'
2180 // ...
2181 // [47] = '\0'
2182 // }
2183 // _ref_count = 1
2184 // _pad_buf2 = {
2185 // [0] = '\0'
2186 // ...
2187 // [59] = '\0'
2188 // }
2189 // _next_om = 0x0000000000000000
2190 // _recursions = 0
2191 // _EntryList = 0x0000000000000000
2192 // _cxq = 0x0000000000000000
2193 // _succ = 0x0000000000000000
2194 // _Responsible = 0x0000000000000000
2195 // _Spinner = 0
2196 // _SpinDuration = 5000
2197 // _contentions = 0
2198 // _WaitSet = 0x0000700009756248
2199 // _waiters = 1
2200 // _WaitSetLock = 0
2201 // }
2202 //
2203 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2204 st->print_cr("(ObjectMonitor *) " INTPTR_FORMAT " = {", p2i(this));
2205 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2206 st->print_cr(" _object = " INTPTR_FORMAT, p2i(_object));
2207 st->print(" _allocation_state = ");
2208 if (is_free()) {
2209 st->print("Free");
2210 } else if (is_old()) {
2211 st->print("Old");
2212 } else if (is_new()) {
2213 st->print("New");
2214 } else {
2215 st->print("unknown=%d", _allocation_state);
2216 }
2217 st->cr();
2218 st->print_cr(" _pad_buf0 = {");
2219 st->print_cr(" [0] = '\\0'");
2220 st->print_cr(" ...");
2221 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2222 st->print_cr(" }");
2223 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(_owner));
2224 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid);
2225 st->print_cr(" _pad_buf1 = {");
2226 st->print_cr(" [0] = '\\0'");
2227 st->print_cr(" ...");
2228 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2229 st->print_cr(" }");
2230 st->print_cr(" _ref_count = %d", ref_count());
2231 st->print_cr(" _pad_buf2 = {");
2232 st->print_cr(" [0] = '\\0'");
2233 st->print_cr(" ...");
2234 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2235 st->print_cr(" }");
2236 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(_next_om));
2237 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2238 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2239 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2240 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2241 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2242 st->print_cr(" _Spinner = %d", _Spinner);
2243 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2244 st->print_cr(" _contentions = %d", _contentions);
2245 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2246 st->print_cr(" _waiters = %d", _waiters);
2247 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2248 st->print_cr("}");
2249 }
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