1 /*
2 * Copyright (c) 1998, 2018, 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 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "jfr/jfrEvents.hpp"
28 #include "jfr/support/jfrThreadId.hpp"
29 #include "memory/allocation.inline.hpp"
30 #include "memory/resourceArea.hpp"
31 #include "oops/markOop.hpp"
32 #include "oops/oop.inline.hpp"
33 #include "runtime/atomic.hpp"
34 #include "runtime/handles.inline.hpp"
35 #include "runtime/interfaceSupport.inline.hpp"
36 #include "runtime/mutexLocker.hpp"
37 #include "runtime/objectMonitor.hpp"
38 #include "runtime/objectMonitor.inline.hpp"
39 #include "runtime/orderAccess.hpp"
40 #include "runtime/osThread.hpp"
41 #include "runtime/safepointMechanism.inline.hpp"
42 #include "runtime/sharedRuntime.hpp"
43 #include "runtime/stubRoutines.hpp"
44 #include "runtime/thread.inline.hpp"
45 #include "services/threadService.hpp"
46 #include "utilities/dtrace.hpp"
47 #include "utilities/macros.hpp"
48 #include "utilities/preserveException.hpp"
49 #if INCLUDE_JFR
50 #include "jfr/support/jfrFlush.hpp"
51 #endif
52
53 #ifdef DTRACE_ENABLED
54
55 // Only bother with this argument setup if dtrace is available
56 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
57
58
59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
60 char* bytes = NULL; \
61 int len = 0; \
62 jlong jtid = SharedRuntime::get_java_tid(thread); \
63 Symbol* klassname = ((oop)obj)->klass()->name(); \
64 if (klassname != NULL) { \
65 bytes = (char*)klassname->bytes(); \
66 len = klassname->utf8_length(); \
67 }
68
69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
70 { \
71 if (DTraceMonitorProbes) { \
72 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
73 HOTSPOT_MONITOR_WAIT(jtid, \
74 (monitor), bytes, len, (millis)); \
75 } \
76 }
77
78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
83
84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
85 { \
86 if (DTraceMonitorProbes) { \
87 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
88 HOTSPOT_MONITOR_##probe(jtid, \
89 (uintptr_t)(monitor), bytes, len); \
90 } \
91 }
92
93 #else // ndef DTRACE_ENABLED
94
95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
97
98 #endif // ndef DTRACE_ENABLED
99
100 // Tunables ...
101 // The knob* variables are effectively final. Once set they should
102 // never be modified hence. Consider using __read_mostly with GCC.
103
104 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
105
106 static int Knob_SuccEnabled = 1; // futile wake throttling
107 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
108 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
109 static int Knob_Bonus = 100; // spin success bonus
110 static int Knob_BonusB = 100; // spin success bonus
111 static int Knob_Penalty = 200; // spin failure penalty
112 static int Knob_Poverty = 1000;
113 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
114 static int Knob_FixedSpin = 0;
115 static int Knob_OState = 3; // Spinner checks thread state of _owner
116 static int Knob_UsePause = 1;
117 static int Knob_ExitPolicy = 0;
118 static int Knob_PreSpin = 10; // 20-100 likely better
119 static int Knob_ResetEvent = 0;
120
121 static int Knob_FastHSSEC = 0;
122 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
123 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
124 static volatile int InitDone = 0;
125
126 // -----------------------------------------------------------------------------
127 // Theory of operations -- Monitors lists, thread residency, etc:
128 //
129 // * A thread acquires ownership of a monitor by successfully
130 // CAS()ing the _owner field from null to non-null.
131 //
132 // * Invariant: A thread appears on at most one monitor list --
133 // cxq, EntryList or WaitSet -- at any one time.
134 //
135 // * Contending threads "push" themselves onto the cxq with CAS
136 // and then spin/park.
137 //
138 // * After a contending thread eventually acquires the lock it must
139 // dequeue itself from either the EntryList or the cxq.
140 //
141 // * The exiting thread identifies and unparks an "heir presumptive"
142 // tentative successor thread on the EntryList. Critically, the
143 // exiting thread doesn't unlink the successor thread from the EntryList.
144 // After having been unparked, the wakee will recontend for ownership of
145 // the monitor. The successor (wakee) will either acquire the lock or
146 // re-park itself.
147 //
148 // Succession is provided for by a policy of competitive handoff.
149 // The exiting thread does _not_ grant or pass ownership to the
150 // successor thread. (This is also referred to as "handoff" succession").
151 // Instead the exiting thread releases ownership and possibly wakes
152 // a successor, so the successor can (re)compete for ownership of the lock.
153 // If the EntryList is empty but the cxq is populated the exiting
154 // thread will drain the cxq into the EntryList. It does so by
155 // by detaching the cxq (installing null with CAS) and folding
156 // the threads from the cxq into the EntryList. The EntryList is
157 // doubly linked, while the cxq is singly linked because of the
158 // CAS-based "push" used to enqueue recently arrived threads (RATs).
159 //
160 // * Concurrency invariants:
161 //
162 // -- only the monitor owner may access or mutate the EntryList.
163 // The mutex property of the monitor itself protects the EntryList
164 // from concurrent interference.
165 // -- Only the monitor owner may detach the cxq.
166 //
167 // * The monitor entry list operations avoid locks, but strictly speaking
168 // they're not lock-free. Enter is lock-free, exit is not.
169 // For a description of 'Methods and apparatus providing non-blocking access
170 // to a resource,' see U.S. Pat. No. 7844973.
171 //
172 // * The cxq can have multiple concurrent "pushers" but only one concurrent
173 // detaching thread. This mechanism is immune from the ABA corruption.
174 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
175 //
176 // * Taken together, the cxq and the EntryList constitute or form a
177 // single logical queue of threads stalled trying to acquire the lock.
178 // We use two distinct lists to improve the odds of a constant-time
179 // dequeue operation after acquisition (in the ::enter() epilogue) and
180 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
181 // A key desideratum is to minimize queue & monitor metadata manipulation
182 // that occurs while holding the monitor lock -- that is, we want to
183 // minimize monitor lock holds times. Note that even a small amount of
184 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
185 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
186 // locks and monitor metadata.
187 //
188 // Cxq points to the set of Recently Arrived Threads attempting entry.
189 // Because we push threads onto _cxq with CAS, the RATs must take the form of
190 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
191 // the unlocking thread notices that EntryList is null but _cxq is != null.
192 //
193 // The EntryList is ordered by the prevailing queue discipline and
194 // can be organized in any convenient fashion, such as a doubly-linked list or
195 // a circular doubly-linked list. Critically, we want insert and delete operations
196 // to operate in constant-time. If we need a priority queue then something akin
197 // to Solaris' sleepq would work nicely. Viz.,
198 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
199 // Queue discipline is enforced at ::exit() time, when the unlocking thread
200 // drains the cxq into the EntryList, and orders or reorders the threads on the
201 // EntryList accordingly.
202 //
203 // Barring "lock barging", this mechanism provides fair cyclic ordering,
204 // somewhat similar to an elevator-scan.
205 //
206 // * The monitor synchronization subsystem avoids the use of native
207 // synchronization primitives except for the narrow platform-specific
208 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
209 // the semantics of park-unpark. Put another way, this monitor implementation
210 // depends only on atomic operations and park-unpark. The monitor subsystem
211 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
212 // underlying OS manages the READY<->RUN transitions.
213 //
214 // * Waiting threads reside on the WaitSet list -- wait() puts
215 // the caller onto the WaitSet.
216 //
217 // * notify() or notifyAll() simply transfers threads from the WaitSet to
218 // either the EntryList or cxq. Subsequent exit() operations will
219 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
220 // it's likely the notifyee would simply impale itself on the lock held
221 // by the notifier.
222 //
223 // * An interesting alternative is to encode cxq as (List,LockByte) where
224 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
225 // variable, like _recursions, in the scheme. The threads or Events that form
226 // the list would have to be aligned in 256-byte addresses. A thread would
227 // try to acquire the lock or enqueue itself with CAS, but exiting threads
228 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
229 // Note that is is *not* word-tearing, but it does presume that full-word
230 // CAS operations are coherent with intermix with STB operations. That's true
231 // on most common processors.
232 //
233 // * See also http://blogs.sun.com/dave
234
235
236 void* ObjectMonitor::operator new (size_t size) throw() {
237 return AllocateHeap(size, mtInternal);
238 }
239 void* ObjectMonitor::operator new[] (size_t size) throw() {
240 return operator new (size);
241 }
242 void ObjectMonitor::operator delete(void* p) {
243 FreeHeap(p);
244 }
245 void ObjectMonitor::operator delete[] (void *p) {
246 operator delete(p);
247 }
248
249 // -----------------------------------------------------------------------------
250 // Enter support
251
252 void ObjectMonitor::enter(TRAPS) {
253 // The following code is ordered to check the most common cases first
254 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
255 Thread * const Self = THREAD;
256
257 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
258 if (cur == NULL) {
259 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
260 assert(_recursions == 0, "invariant");
261 assert(_owner == Self, "invariant");
262 return;
263 }
264
265 if (cur == Self) {
266 // TODO-FIXME: check for integer overflow! BUGID 6557169.
267 _recursions++;
268 return;
269 }
270
271 if (Self->is_lock_owned ((address)cur)) {
272 assert(_recursions == 0, "internal state error");
273 _recursions = 1;
274 // Commute owner from a thread-specific on-stack BasicLockObject address to
275 // a full-fledged "Thread *".
276 _owner = Self;
277 return;
278 }
279
280 // We've encountered genuine contention.
281 assert(Self->_Stalled == 0, "invariant");
282 Self->_Stalled = intptr_t(this);
283
284 // Try one round of spinning *before* enqueueing Self
285 // and before going through the awkward and expensive state
286 // transitions. The following spin is strictly optional ...
287 // Note that if we acquire the monitor from an initial spin
288 // we forgo posting JVMTI events and firing DTRACE probes.
289 if (TrySpin(Self) > 0) {
290 assert(_owner == Self, "invariant");
291 assert(_recursions == 0, "invariant");
292 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
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(this->object() != NULL, "invariant");
304 assert(_count >= 0, "invariant");
305
306 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
307 // Ensure the object-monitor relationship remains stable while there's contention.
308 Atomic::inc(&_count);
309
310 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
311 EventJavaMonitorEnter event;
312 if (event.should_commit()) {
313 event.set_monitorClass(((oop)this->object())->klass());
314 event.set_address((uintptr_t)(this->object_addr()));
315 }
316
317 { // Change java thread status to indicate blocked on monitor enter.
318 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
319
320 Self->set_current_pending_monitor(this);
321
322 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
323 if (JvmtiExport::should_post_monitor_contended_enter()) {
324 JvmtiExport::post_monitor_contended_enter(jt, this);
325
326 // The current thread does not yet own the monitor and does not
327 // yet appear on any queues that would get it made the successor.
328 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
329 // handler cannot accidentally consume an unpark() meant for the
330 // ParkEvent associated with this ObjectMonitor.
331 }
332
333 OSThreadContendState osts(Self->osthread());
334 ThreadBlockInVM tbivm(jt);
335
336 // TODO-FIXME: change the following for(;;) loop to straight-line code.
337 for (;;) {
338 jt->set_suspend_equivalent();
339 // cleared by handle_special_suspend_equivalent_condition()
340 // or java_suspend_self()
341
342 EnterI(THREAD);
343
344 if (!ExitSuspendEquivalent(jt)) break;
345
346 // We have acquired the contended monitor, but while we were
347 // waiting another thread suspended us. We don't want to enter
348 // the monitor while suspended because that would surprise the
349 // thread that suspended us.
350 //
351 _recursions = 0;
352 _succ = NULL;
353 exit(false, Self);
354
355 jt->java_suspend_self();
356 }
357 Self->set_current_pending_monitor(NULL);
358
359 // We cleared the pending monitor info since we've just gotten past
360 // the enter-check-for-suspend dance and we now own the monitor free
361 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
362 // destructor can go to a safepoint at the end of this block. If we
363 // do a thread dump during that safepoint, then this thread will show
364 // as having "-locked" the monitor, but the OS and java.lang.Thread
365 // states will still report that the thread is blocked trying to
366 // acquire it.
367 }
368
369 Atomic::dec(&_count);
370 assert(_count >= 0, "invariant");
371 Self->_Stalled = 0;
372
373 // Must either set _recursions = 0 or ASSERT _recursions == 0.
374 assert(_recursions == 0, "invariant");
375 assert(_owner == Self, "invariant");
376 assert(_succ != Self, "invariant");
377 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
378
379 // The thread -- now the owner -- is back in vm mode.
380 // Report the glorious news via TI,DTrace and jvmstat.
381 // The probe effect is non-trivial. All the reportage occurs
382 // while we hold the monitor, increasing the length of the critical
383 // section. Amdahl's parallel speedup law comes vividly into play.
384 //
385 // Another option might be to aggregate the events (thread local or
386 // per-monitor aggregation) and defer reporting until a more opportune
387 // time -- such as next time some thread encounters contention but has
388 // yet to acquire the lock. While spinning that thread could
389 // spinning we could increment JVMStat counters, etc.
390
391 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
392 if (JvmtiExport::should_post_monitor_contended_entered()) {
393 JvmtiExport::post_monitor_contended_entered(jt, this);
394
395 // The current thread already owns the monitor and is not going to
396 // call park() for the remainder of the monitor enter protocol. So
397 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
398 // event handler consumed an unpark() issued by the thread that
399 // just exited the monitor.
400 }
401 if (event.should_commit()) {
402 event.set_previousOwner((uintptr_t)_previous_owner_tid);
403 event.commit();
404 }
405 OM_PERFDATA_OP(ContendedLockAttempts, inc());
406 }
407
408 // Caveat: TryLock() is not necessarily serializing if it returns failure.
409 // Callers must compensate as needed.
410
411 int ObjectMonitor::TryLock(Thread * Self) {
412 void * own = _owner;
413 if (own != NULL) return 0;
414 if (Atomic::replace_if_null(Self, &_owner)) {
415 // Either guarantee _recursions == 0 or set _recursions = 0.
416 assert(_recursions == 0, "invariant");
417 assert(_owner == Self, "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 #define MAX_RECHECK_INTERVAL 1000
428
429 void ObjectMonitor::EnterI(TRAPS) {
430 Thread * const Self = THREAD;
431 assert(Self->is_Java_thread(), "invariant");
432 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
433
434 // Try the lock - TATAS
435 if (TryLock (Self) > 0) {
436 assert(_succ != Self, "invariant");
437 assert(_owner == Self, "invariant");
438 assert(_Responsible != Self, "invariant");
439 return;
440 }
441
442 DeferredInitialize();
443
444 // We try one round of spinning *before* enqueueing Self.
445 //
446 // If the _owner is ready but OFFPROC we could use a YieldTo()
447 // operation to donate the remainder of this thread's quantum
448 // to the owner. This has subtle but beneficial affinity
449 // effects.
450
451 if (TrySpin(Self) > 0) {
452 assert(_owner == Self, "invariant");
453 assert(_succ != Self, "invariant");
454 assert(_Responsible != Self, "invariant");
455 return;
456 }
457
458 // The Spin failed -- Enqueue and park the thread ...
459 assert(_succ != Self, "invariant");
460 assert(_owner != Self, "invariant");
461 assert(_Responsible != Self, "invariant");
462
463 // Enqueue "Self" on ObjectMonitor's _cxq.
464 //
465 // Node acts as a proxy for Self.
466 // As an aside, if were to ever rewrite the synchronization code mostly
467 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
468 // Java objects. This would avoid awkward lifecycle and liveness issues,
469 // as well as eliminate a subset of ABA issues.
470 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
471
472 ObjectWaiter node(Self);
473 Self->_ParkEvent->reset();
474 node._prev = (ObjectWaiter *) 0xBAD;
475 node.TState = ObjectWaiter::TS_CXQ;
476
477 // Push "Self" onto the front of the _cxq.
478 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
479 // Note that spinning tends to reduce the rate at which threads
480 // enqueue and dequeue on EntryList|cxq.
481 ObjectWaiter * nxt;
482 for (;;) {
483 node._next = nxt = _cxq;
484 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
485
486 // Interference - the CAS failed because _cxq changed. Just retry.
487 // As an optional optimization we retry the lock.
488 if (TryLock (Self) > 0) {
489 assert(_succ != Self, "invariant");
490 assert(_owner == Self, "invariant");
491 assert(_Responsible != Self, "invariant");
492 return;
493 }
494 }
495
496 // Check for cxq|EntryList edge transition to non-null. This indicates
497 // the onset of contention. While contention persists exiting threads
498 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
499 // operations revert to the faster 1-0 mode. This enter operation may interleave
500 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
501 // arrange for one of the contending thread to use a timed park() operations
502 // to detect and recover from the race. (Stranding is form of progress failure
503 // where the monitor is unlocked but all the contending threads remain parked).
504 // That is, at least one of the contended threads will periodically poll _owner.
505 // One of the contending threads will become the designated "Responsible" thread.
506 // The Responsible thread uses a timed park instead of a normal indefinite park
507 // operation -- it periodically wakes and checks for and recovers from potential
508 // strandings admitted by 1-0 exit operations. We need at most one Responsible
509 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
510 // be responsible for a monitor.
511 //
512 // Currently, one of the contended threads takes on the added role of "Responsible".
513 // A viable alternative would be to use a dedicated "stranding checker" thread
514 // that periodically iterated over all the threads (or active monitors) and unparked
515 // successors where there was risk of stranding. This would help eliminate the
516 // timer scalability issues we see on some platforms as we'd only have one thread
517 // -- the checker -- parked on a timer.
518
519 if (nxt == NULL && _EntryList == NULL) {
520 // Try to assume the role of responsible thread for the monitor.
521 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
522 Atomic::replace_if_null(Self, &_Responsible);
523 }
524
525 // The lock might have been released while this thread was occupied queueing
526 // itself onto _cxq. To close the race and avoid "stranding" and
527 // progress-liveness failure we must resample-retry _owner before parking.
528 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
529 // In this case the ST-MEMBAR is accomplished with CAS().
530 //
531 // TODO: Defer all thread state transitions until park-time.
532 // Since state transitions are heavy and inefficient we'd like
533 // to defer the state transitions until absolutely necessary,
534 // and in doing so avoid some transitions ...
535
536 int nWakeups = 0;
537 int recheckInterval = 1;
538
539 for (;;) {
540
541 if (TryLock(Self) > 0) break;
542 assert(_owner != Self, "invariant");
543
544 // park self
545 if (_Responsible == Self) {
546 Self->_ParkEvent->park((jlong) recheckInterval);
547 // Increase the recheckInterval, but clamp the value.
548 recheckInterval *= 8;
549 if (recheckInterval > MAX_RECHECK_INTERVAL) {
550 recheckInterval = MAX_RECHECK_INTERVAL;
551 }
552 } else {
553 Self->_ParkEvent->park();
554 }
555
556 if (TryLock(Self) > 0) break;
557
558 // The lock is still contested.
559 // Keep a tally of the # of futile wakeups.
560 // Note that the counter is not protected by a lock or updated by atomics.
561 // That is by design - we trade "lossy" counters which are exposed to
562 // races during updates for a lower probe effect.
563
564 // This PerfData object can be used in parallel with a safepoint.
565 // See the work around in PerfDataManager::destroy().
566 OM_PERFDATA_OP(FutileWakeups, inc());
567 ++nWakeups;
568
569 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
570 // We can defer clearing _succ until after the spin completes
571 // TrySpin() must tolerate being called with _succ == Self.
572 // Try yet another round of adaptive spinning.
573 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
574
575 // We can find that we were unpark()ed and redesignated _succ while
576 // we were spinning. That's harmless. If we iterate and call park(),
577 // park() will consume the event and return immediately and we'll
578 // just spin again. This pattern can repeat, leaving _succ to simply
579 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
580 // Alternately, we can sample fired() here, and if set, forgo spinning
581 // in the next iteration.
582
583 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
584 Self->_ParkEvent->reset();
585 OrderAccess::fence();
586 }
587 if (_succ == Self) _succ = NULL;
588
589 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
590 OrderAccess::fence();
591 }
592
593 // Egress :
594 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
595 // Normally we'll find Self on the EntryList .
596 // From the perspective of the lock owner (this thread), the
597 // EntryList is stable and cxq is prepend-only.
598 // The head of cxq is volatile but the interior is stable.
599 // In addition, Self.TState is stable.
600
601 assert(_owner == Self, "invariant");
602 assert(object() != NULL, "invariant");
603 // I'd like to write:
604 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
605 // but as we're at a safepoint that's not safe.
606
607 UnlinkAfterAcquire(Self, &node);
608 if (_succ == Self) _succ = NULL;
609
610 assert(_succ != Self, "invariant");
611 if (_Responsible == Self) {
612 _Responsible = NULL;
613 OrderAccess::fence(); // Dekker pivot-point
614
615 // We may leave threads on cxq|EntryList without a designated
616 // "Responsible" thread. This is benign. When this thread subsequently
617 // exits the monitor it can "see" such preexisting "old" threads --
618 // threads that arrived on the cxq|EntryList before the fence, above --
619 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
620 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
621 // non-null and elect a new "Responsible" timer thread.
622 //
623 // This thread executes:
624 // ST Responsible=null; MEMBAR (in enter epilogue - here)
625 // LD cxq|EntryList (in subsequent exit)
626 //
627 // Entering threads in the slow/contended path execute:
628 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
629 // The (ST cxq; MEMBAR) is accomplished with CAS().
630 //
631 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
632 // exit operation from floating above the ST Responsible=null.
633 }
634
635 // We've acquired ownership with CAS().
636 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
637 // But since the CAS() this thread may have also stored into _succ,
638 // EntryList, cxq or Responsible. These meta-data updates must be
639 // visible __before this thread subsequently drops the lock.
640 // Consider what could occur if we didn't enforce this constraint --
641 // STs to monitor meta-data and user-data could reorder with (become
642 // visible after) the ST in exit that drops ownership of the lock.
643 // Some other thread could then acquire the lock, but observe inconsistent
644 // or old monitor meta-data and heap data. That violates the JMM.
645 // To that end, the 1-0 exit() operation must have at least STST|LDST
646 // "release" barrier semantics. Specifically, there must be at least a
647 // STST|LDST barrier in exit() before the ST of null into _owner that drops
648 // the lock. The barrier ensures that changes to monitor meta-data and data
649 // protected by the lock will be visible before we release the lock, and
650 // therefore before some other thread (CPU) has a chance to acquire the lock.
651 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
652 //
653 // Critically, any prior STs to _succ or EntryList must be visible before
654 // the ST of null into _owner in the *subsequent* (following) corresponding
655 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
656 // execute a serializing instruction.
657
658 return;
659 }
660
661 // ReenterI() is a specialized inline form of the latter half of the
662 // contended slow-path from EnterI(). We use ReenterI() only for
663 // monitor reentry in wait().
664 //
665 // In the future we should reconcile EnterI() and ReenterI(), adding
666 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
667 // loop accordingly.
668
669 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
670 assert(Self != NULL, "invariant");
671 assert(SelfNode != NULL, "invariant");
672 assert(SelfNode->_thread == Self, "invariant");
673 assert(_waiters > 0, "invariant");
674 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
675 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
676 JavaThread * jt = (JavaThread *) Self;
677
678 int nWakeups = 0;
679 for (;;) {
680 ObjectWaiter::TStates v = SelfNode->TState;
681 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
682 assert(_owner != Self, "invariant");
683
684 if (TryLock(Self) > 0) break;
685 if (TrySpin(Self) > 0) break;
686
687 // State transition wrappers around park() ...
688 // ReenterI() wisely defers state transitions until
689 // it's clear we must park the thread.
690 {
691 OSThreadContendState osts(Self->osthread());
692 ThreadBlockInVM tbivm(jt);
693
694 // cleared by handle_special_suspend_equivalent_condition()
695 // or java_suspend_self()
696 jt->set_suspend_equivalent();
697 Self->_ParkEvent->park();
698
699 // were we externally suspended while we were waiting?
700 for (;;) {
701 if (!ExitSuspendEquivalent(jt)) break;
702 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
703 jt->java_suspend_self();
704 jt->set_suspend_equivalent();
705 }
706 }
707
708 // Try again, but just so we distinguish between futile wakeups and
709 // successful wakeups. The following test isn't algorithmically
710 // necessary, but it helps us maintain sensible statistics.
711 if (TryLock(Self) > 0) break;
712
713 // The lock is still contested.
714 // Keep a tally of the # of futile wakeups.
715 // Note that the counter is not protected by a lock or updated by atomics.
716 // That is by design - we trade "lossy" counters which are exposed to
717 // races during updates for a lower probe effect.
718 ++nWakeups;
719
720 // Assuming this is not a spurious wakeup we'll normally
721 // find that _succ == Self.
722 if (_succ == Self) _succ = NULL;
723
724 // Invariant: after clearing _succ a contending thread
725 // *must* retry _owner before parking.
726 OrderAccess::fence();
727
728 // This PerfData object can be used in parallel with a safepoint.
729 // See the work around in PerfDataManager::destroy().
730 OM_PERFDATA_OP(FutileWakeups, inc());
731 }
732
733 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
734 // Normally we'll find Self on the EntryList.
735 // Unlinking from the EntryList is constant-time and atomic-free.
736 // From the perspective of the lock owner (this thread), the
737 // EntryList is stable and cxq is prepend-only.
738 // The head of cxq is volatile but the interior is stable.
739 // In addition, Self.TState is stable.
740
741 assert(_owner == Self, "invariant");
742 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
743 UnlinkAfterAcquire(Self, SelfNode);
744 if (_succ == Self) _succ = NULL;
745 assert(_succ != Self, "invariant");
746 SelfNode->TState = ObjectWaiter::TS_RUN;
747 OrderAccess::fence(); // see comments at the end of EnterI()
748 }
749
750 // By convention we unlink a contending thread from EntryList|cxq immediately
751 // after the thread acquires the lock in ::enter(). Equally, we could defer
752 // unlinking the thread until ::exit()-time.
753
754 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
755 assert(_owner == Self, "invariant");
756 assert(SelfNode->_thread == Self, "invariant");
757
758 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
759 // Normal case: remove Self from the DLL EntryList .
760 // This is a constant-time operation.
761 ObjectWaiter * nxt = SelfNode->_next;
762 ObjectWaiter * prv = SelfNode->_prev;
763 if (nxt != NULL) nxt->_prev = prv;
764 if (prv != NULL) prv->_next = nxt;
765 if (SelfNode == _EntryList) _EntryList = nxt;
766 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
767 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
768 } else {
769 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
770 // Inopportune interleaving -- Self is still on the cxq.
771 // This usually means the enqueue of self raced an exiting thread.
772 // Normally we'll find Self near the front of the cxq, so
773 // dequeueing is typically fast. If needbe we can accelerate
774 // this with some MCS/CHL-like bidirectional list hints and advisory
775 // back-links so dequeueing from the interior will normally operate
776 // in constant-time.
777 // Dequeue Self from either the head (with CAS) or from the interior
778 // with a linear-time scan and normal non-atomic memory operations.
779 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
780 // and then unlink Self from EntryList. We have to drain eventually,
781 // so it might as well be now.
782
783 ObjectWaiter * v = _cxq;
784 assert(v != NULL, "invariant");
785 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
786 // The CAS above can fail from interference IFF a "RAT" arrived.
787 // In that case Self must be in the interior and can no longer be
788 // at the head of cxq.
789 if (v == SelfNode) {
790 assert(_cxq != v, "invariant");
791 v = _cxq; // CAS above failed - start scan at head of list
792 }
793 ObjectWaiter * p;
794 ObjectWaiter * q = NULL;
795 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
796 q = p;
797 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
798 }
799 assert(v != SelfNode, "invariant");
800 assert(p == SelfNode, "Node not found on cxq");
801 assert(p != _cxq, "invariant");
802 assert(q != NULL, "invariant");
803 assert(q->_next == p, "invariant");
804 q->_next = p->_next;
805 }
806 }
807
808 #ifdef ASSERT
809 // Diagnostic hygiene ...
810 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
811 SelfNode->_next = (ObjectWaiter *) 0xBAD;
812 SelfNode->TState = ObjectWaiter::TS_RUN;
813 #endif
814 }
815
816 // -----------------------------------------------------------------------------
817 // Exit support
818 //
819 // exit()
820 // ~~~~~~
821 // Note that the collector can't reclaim the objectMonitor or deflate
822 // the object out from underneath the thread calling ::exit() as the
823 // thread calling ::exit() never transitions to a stable state.
824 // This inhibits GC, which in turn inhibits asynchronous (and
825 // inopportune) reclamation of "this".
826 //
827 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
828 // There's one exception to the claim above, however. EnterI() can call
829 // exit() to drop a lock if the acquirer has been externally suspended.
830 // In that case exit() is called with _thread_state as _thread_blocked,
831 // but the monitor's _count field is > 0, which inhibits reclamation.
832 //
833 // 1-0 exit
834 // ~~~~~~~~
835 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
836 // the fast-path operators have been optimized so the common ::exit()
837 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
838 // The code emitted by fast_unlock() elides the usual MEMBAR. This
839 // greatly improves latency -- MEMBAR and CAS having considerable local
840 // latency on modern processors -- but at the cost of "stranding". Absent the
841 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
842 // ::enter() path, resulting in the entering thread being stranding
843 // and a progress-liveness failure. Stranding is extremely rare.
844 // We use timers (timed park operations) & periodic polling to detect
845 // and recover from stranding. Potentially stranded threads periodically
846 // wake up and poll the lock. See the usage of the _Responsible variable.
847 //
848 // The CAS() in enter provides for safety and exclusion, while the CAS or
849 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
850 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
851 // We detect and recover from stranding with timers.
852 //
853 // If a thread transiently strands it'll park until (a) another
854 // thread acquires the lock and then drops the lock, at which time the
855 // exiting thread will notice and unpark the stranded thread, or, (b)
856 // the timer expires. If the lock is high traffic then the stranding latency
857 // will be low due to (a). If the lock is low traffic then the odds of
858 // stranding are lower, although the worst-case stranding latency
859 // is longer. Critically, we don't want to put excessive load in the
860 // platform's timer subsystem. We want to minimize both the timer injection
861 // rate (timers created/sec) as well as the number of timers active at
862 // any one time. (more precisely, we want to minimize timer-seconds, which is
863 // the integral of the # of active timers at any instant over time).
864 // Both impinge on OS scalability. Given that, at most one thread parked on
865 // a monitor will use a timer.
866 //
867 // There is also the risk of a futile wake-up. If we drop the lock
868 // another thread can reacquire the lock immediately, and we can
869 // then wake a thread unnecessarily. This is benign, and we've
870 // structured the code so the windows are short and the frequency
871 // of such futile wakups is low.
872
873 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
874 Thread * const Self = THREAD;
875 if (THREAD != _owner) {
876 if (THREAD->is_lock_owned((address) _owner)) {
877 // Transmute _owner from a BasicLock pointer to a Thread address.
878 // We don't need to hold _mutex for this transition.
879 // Non-null to Non-null is safe as long as all readers can
880 // tolerate either flavor.
881 assert(_recursions == 0, "invariant");
882 _owner = THREAD;
883 _recursions = 0;
884 } else {
885 // Apparent unbalanced locking ...
886 // Naively we'd like to throw IllegalMonitorStateException.
887 // As a practical matter we can neither allocate nor throw an
888 // exception as ::exit() can be called from leaf routines.
889 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
890 // Upon deeper reflection, however, in a properly run JVM the only
891 // way we should encounter this situation is in the presence of
892 // unbalanced JNI locking. TODO: CheckJNICalls.
893 // See also: CR4414101
894 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
895 return;
896 }
897 }
898
899 if (_recursions != 0) {
900 _recursions--; // this is simple recursive enter
901 return;
902 }
903
904 // Invariant: after setting Responsible=null an thread must execute
905 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
906 _Responsible = NULL;
907
908 #if INCLUDE_JFR
909 // get the owner's thread id for the MonitorEnter event
910 // if it is enabled and the thread isn't suspended
911 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
912 _previous_owner_tid = JFR_THREAD_ID(Self);
913 }
914 #endif
915
916 for (;;) {
917 assert(THREAD == _owner, "invariant");
918
919 if (Knob_ExitPolicy == 0) {
920 // release semantics: prior loads and stores from within the critical section
921 // must not float (reorder) past the following store that drops the lock.
922 // On SPARC that requires MEMBAR #loadstore|#storestore.
923 // But of course in TSO #loadstore|#storestore is not required.
924 // I'd like to write one of the following:
925 // A. OrderAccess::release() ; _owner = NULL
926 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
927 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
928 // store into a _dummy variable. That store is not needed, but can result
929 // in massive wasteful coherency traffic on classic SMP systems.
930 // Instead, I use release_store(), which is implemented as just a simple
931 // ST on x64, x86 and SPARC.
932 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
933 OrderAccess::storeload(); // See if we need to wake a successor
934 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
935 return;
936 }
937 // Other threads are blocked trying to acquire the lock.
938
939 // Normally the exiting thread is responsible for ensuring succession,
940 // but if other successors are ready or other entering threads are spinning
941 // then this thread can simply store NULL into _owner and exit without
942 // waking a successor. The existence of spinners or ready successors
943 // guarantees proper succession (liveness). Responsibility passes to the
944 // ready or running successors. The exiting thread delegates the duty.
945 // More precisely, if a successor already exists this thread is absolved
946 // of the responsibility of waking (unparking) one.
947 //
948 // The _succ variable is critical to reducing futile wakeup frequency.
949 // _succ identifies the "heir presumptive" thread that has been made
950 // ready (unparked) but that has not yet run. We need only one such
951 // successor thread to guarantee progress.
952 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
953 // section 3.3 "Futile Wakeup Throttling" for details.
954 //
955 // Note that spinners in Enter() also set _succ non-null.
956 // In the current implementation spinners opportunistically set
957 // _succ so that exiting threads might avoid waking a successor.
958 // Another less appealing alternative would be for the exiting thread
959 // to drop the lock and then spin briefly to see if a spinner managed
960 // to acquire the lock. If so, the exiting thread could exit
961 // immediately without waking a successor, otherwise the exiting
962 // thread would need to dequeue and wake a successor.
963 // (Note that we'd need to make the post-drop spin short, but no
964 // shorter than the worst-case round-trip cache-line migration time.
965 // The dropped lock needs to become visible to the spinner, and then
966 // the acquisition of the lock by the spinner must become visible to
967 // the exiting thread).
968
969 // It appears that an heir-presumptive (successor) must be made ready.
970 // Only the current lock owner can manipulate the EntryList or
971 // drain _cxq, so we need to reacquire the lock. If we fail
972 // to reacquire the lock the responsibility for ensuring succession
973 // falls to the new owner.
974 //
975 if (!Atomic::replace_if_null(THREAD, &_owner)) {
976 return;
977 }
978 } else {
979 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
980 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
981 OrderAccess::storeload();
982 // Ratify the previously observed values.
983 if (_cxq == NULL || _succ != NULL) {
984 return;
985 }
986
987 // inopportune interleaving -- the exiting thread (this thread)
988 // in the fast-exit path raced an entering thread in the slow-enter
989 // path.
990 // We have two choices:
991 // A. Try to reacquire the lock.
992 // If the CAS() fails return immediately, otherwise
993 // we either restart/rerun the exit operation, or simply
994 // fall-through into the code below which wakes a successor.
995 // B. If the elements forming the EntryList|cxq are TSM
996 // we could simply unpark() the lead thread and return
997 // without having set _succ.
998 if (!Atomic::replace_if_null(THREAD, &_owner)) {
999 return;
1000 }
1001 }
1002 }
1003
1004 guarantee(_owner == THREAD, "invariant");
1005
1006 ObjectWaiter * w = NULL;
1007 int QMode = Knob_QMode;
1008
1009 if (QMode == 2 && _cxq != NULL) {
1010 // QMode == 2 : cxq has precedence over EntryList.
1011 // Try to directly wake a successor from the cxq.
1012 // If successful, the successor will need to unlink itself from cxq.
1013 w = _cxq;
1014 assert(w != NULL, "invariant");
1015 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1016 ExitEpilog(Self, w);
1017 return;
1018 }
1019
1020 if (QMode == 3 && _cxq != NULL) {
1021 // Aggressively drain cxq into EntryList at the first opportunity.
1022 // This policy ensure that recently-run threads live at the head of EntryList.
1023 // Drain _cxq into EntryList - bulk transfer.
1024 // First, detach _cxq.
1025 // The following loop is tantamount to: w = swap(&cxq, NULL)
1026 w = _cxq;
1027 for (;;) {
1028 assert(w != NULL, "Invariant");
1029 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1030 if (u == w) break;
1031 w = u;
1032 }
1033 assert(w != NULL, "invariant");
1034
1035 ObjectWaiter * q = NULL;
1036 ObjectWaiter * p;
1037 for (p = w; p != NULL; p = p->_next) {
1038 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1039 p->TState = ObjectWaiter::TS_ENTER;
1040 p->_prev = q;
1041 q = p;
1042 }
1043
1044 // Append the RATs to the EntryList
1045 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1046 ObjectWaiter * Tail;
1047 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1048 Tail = Tail->_next)
1049 /* empty */;
1050 if (Tail == NULL) {
1051 _EntryList = w;
1052 } else {
1053 Tail->_next = w;
1054 w->_prev = Tail;
1055 }
1056
1057 // Fall thru into code that tries to wake a successor from EntryList
1058 }
1059
1060 if (QMode == 4 && _cxq != NULL) {
1061 // Aggressively drain cxq into EntryList at the first opportunity.
1062 // This policy ensure that recently-run threads live at the head of EntryList.
1063
1064 // Drain _cxq into EntryList - bulk transfer.
1065 // First, detach _cxq.
1066 // The following loop is tantamount to: w = swap(&cxq, NULL)
1067 w = _cxq;
1068 for (;;) {
1069 assert(w != NULL, "Invariant");
1070 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1071 if (u == w) break;
1072 w = u;
1073 }
1074 assert(w != NULL, "invariant");
1075
1076 ObjectWaiter * q = NULL;
1077 ObjectWaiter * p;
1078 for (p = w; p != NULL; p = p->_next) {
1079 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1080 p->TState = ObjectWaiter::TS_ENTER;
1081 p->_prev = q;
1082 q = p;
1083 }
1084
1085 // Prepend the RATs to the EntryList
1086 if (_EntryList != NULL) {
1087 q->_next = _EntryList;
1088 _EntryList->_prev = q;
1089 }
1090 _EntryList = w;
1091
1092 // Fall thru into code that tries to wake a successor from EntryList
1093 }
1094
1095 w = _EntryList;
1096 if (w != NULL) {
1097 // I'd like to write: guarantee (w->_thread != Self).
1098 // But in practice an exiting thread may find itself on the EntryList.
1099 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1100 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1101 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1102 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1103 // release the lock "O". T2 resumes immediately after the ST of null into
1104 // _owner, above. T2 notices that the EntryList is populated, so it
1105 // reacquires the lock and then finds itself on the EntryList.
1106 // Given all that, we have to tolerate the circumstance where "w" is
1107 // associated with Self.
1108 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1109 ExitEpilog(Self, w);
1110 return;
1111 }
1112
1113 // If we find that both _cxq and EntryList are null then just
1114 // re-run the exit protocol from the top.
1115 w = _cxq;
1116 if (w == NULL) continue;
1117
1118 // Drain _cxq into EntryList - bulk transfer.
1119 // First, detach _cxq.
1120 // The following loop is tantamount to: w = swap(&cxq, NULL)
1121 for (;;) {
1122 assert(w != NULL, "Invariant");
1123 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1124 if (u == w) break;
1125 w = u;
1126 }
1127
1128 assert(w != NULL, "invariant");
1129 assert(_EntryList == NULL, "invariant");
1130
1131 // Convert the LIFO SLL anchored by _cxq into a DLL.
1132 // The list reorganization step operates in O(LENGTH(w)) time.
1133 // It's critical that this step operate quickly as
1134 // "Self" still holds the outer-lock, restricting parallelism
1135 // and effectively lengthening the critical section.
1136 // Invariant: s chases t chases u.
1137 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1138 // we have faster access to the tail.
1139
1140 if (QMode == 1) {
1141 // QMode == 1 : drain cxq to EntryList, reversing order
1142 // We also reverse the order of the list.
1143 ObjectWaiter * s = NULL;
1144 ObjectWaiter * t = w;
1145 ObjectWaiter * u = NULL;
1146 while (t != NULL) {
1147 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1148 t->TState = ObjectWaiter::TS_ENTER;
1149 u = t->_next;
1150 t->_prev = u;
1151 t->_next = s;
1152 s = t;
1153 t = u;
1154 }
1155 _EntryList = s;
1156 assert(s != NULL, "invariant");
1157 } else {
1158 // QMode == 0 or QMode == 2
1159 _EntryList = w;
1160 ObjectWaiter * q = NULL;
1161 ObjectWaiter * p;
1162 for (p = w; p != NULL; p = p->_next) {
1163 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1164 p->TState = ObjectWaiter::TS_ENTER;
1165 p->_prev = q;
1166 q = p;
1167 }
1168 }
1169
1170 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1171 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1172
1173 // See if we can abdicate to a spinner instead of waking a thread.
1174 // A primary goal of the implementation is to reduce the
1175 // context-switch rate.
1176 if (_succ != NULL) continue;
1177
1178 w = _EntryList;
1179 if (w != NULL) {
1180 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1181 ExitEpilog(Self, w);
1182 return;
1183 }
1184 }
1185 }
1186
1187 // ExitSuspendEquivalent:
1188 // A faster alternate to handle_special_suspend_equivalent_condition()
1189 //
1190 // handle_special_suspend_equivalent_condition() unconditionally
1191 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1192 // operations have high latency. Note that in ::enter() we call HSSEC
1193 // while holding the monitor, so we effectively lengthen the critical sections.
1194 //
1195 // There are a number of possible solutions:
1196 //
1197 // A. To ameliorate the problem we might also defer state transitions
1198 // to as late as possible -- just prior to parking.
1199 // Given that, we'd call HSSEC after having returned from park(),
1200 // but before attempting to acquire the monitor. This is only a
1201 // partial solution. It avoids calling HSSEC while holding the
1202 // monitor (good), but it still increases successor reacquisition latency --
1203 // the interval between unparking a successor and the time the successor
1204 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1205 // If we use this technique we can also avoid EnterI()-exit() loop
1206 // in ::enter() where we iteratively drop the lock and then attempt
1207 // to reacquire it after suspending.
1208 //
1209 // B. In the future we might fold all the suspend bits into a
1210 // composite per-thread suspend flag and then update it with CAS().
1211 // Alternately, a Dekker-like mechanism with multiple variables
1212 // would suffice:
1213 // ST Self->_suspend_equivalent = false
1214 // MEMBAR
1215 // LD Self_>_suspend_flags
1216 //
1217 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1218 // with a more efficient implementation, the need to use "FastHSSEC" has
1219 // decreased. - Dave
1220
1221
1222 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1223 const int Mode = Knob_FastHSSEC;
1224 if (Mode && !jSelf->is_external_suspend()) {
1225 assert(jSelf->is_suspend_equivalent(), "invariant");
1226 jSelf->clear_suspend_equivalent();
1227 if (2 == Mode) OrderAccess::storeload();
1228 if (!jSelf->is_external_suspend()) return false;
1229 // We raced a suspension -- fall thru into the slow path
1230 jSelf->set_suspend_equivalent();
1231 }
1232 return jSelf->handle_special_suspend_equivalent_condition();
1233 }
1234
1235
1236 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1237 assert(_owner == Self, "invariant");
1238
1239 // Exit protocol:
1240 // 1. ST _succ = wakee
1241 // 2. membar #loadstore|#storestore;
1242 // 2. ST _owner = NULL
1243 // 3. unpark(wakee)
1244
1245 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1246 ParkEvent * Trigger = Wakee->_event;
1247
1248 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1249 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1250 // out-of-scope (non-extant).
1251 Wakee = NULL;
1252
1253 // Drop the lock
1254 OrderAccess::release_store(&_owner, (void*)NULL);
1255 OrderAccess::fence(); // ST _owner vs LD in unpark()
1256
1257 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1258 Trigger->unpark();
1259
1260 // Maintain stats and report events to JVMTI
1261 OM_PERFDATA_OP(Parks, inc());
1262 }
1263
1264
1265 // -----------------------------------------------------------------------------
1266 // Class Loader deadlock handling.
1267 //
1268 // complete_exit exits a lock returning recursion count
1269 // complete_exit/reenter operate as a wait without waiting
1270 // complete_exit requires an inflated monitor
1271 // The _owner field is not always the Thread addr even with an
1272 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1273 // thread due to contention.
1274 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1275 Thread * const Self = THREAD;
1276 assert(Self->is_Java_thread(), "Must be Java thread!");
1277 JavaThread *jt = (JavaThread *)THREAD;
1278
1279 DeferredInitialize();
1280
1281 if (THREAD != _owner) {
1282 if (THREAD->is_lock_owned ((address)_owner)) {
1283 assert(_recursions == 0, "internal state error");
1284 _owner = THREAD; // Convert from basiclock addr to Thread addr
1285 _recursions = 0;
1286 }
1287 }
1288
1289 guarantee(Self == _owner, "complete_exit not owner");
1290 intptr_t save = _recursions; // record the old recursion count
1291 _recursions = 0; // set the recursion level to be 0
1292 exit(true, Self); // exit the monitor
1293 guarantee(_owner != Self, "invariant");
1294 return save;
1295 }
1296
1297 // reenter() enters a lock and sets recursion count
1298 // complete_exit/reenter operate as a wait without waiting
1299 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1300 Thread * const Self = THREAD;
1301 assert(Self->is_Java_thread(), "Must be Java thread!");
1302 JavaThread *jt = (JavaThread *)THREAD;
1303
1304 guarantee(_owner != Self, "reenter already owner");
1305 enter(THREAD); // enter the monitor
1306 guarantee(_recursions == 0, "reenter recursion");
1307 _recursions = recursions;
1308 return;
1309 }
1310
1311
1312 // -----------------------------------------------------------------------------
1313 // A macro is used below because there may already be a pending
1314 // exception which should not abort the execution of the routines
1315 // which use this (which is why we don't put this into check_slow and
1316 // call it with a CHECK argument).
1317
1318 #define CHECK_OWNER() \
1319 do { \
1320 if (THREAD != _owner) { \
1321 if (THREAD->is_lock_owned((address) _owner)) { \
1322 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1323 _recursions = 0; \
1324 } else { \
1325 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1326 } \
1327 } \
1328 } while (false)
1329
1330 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1331 // TODO-FIXME: remove check_slow() -- it's likely dead.
1332
1333 void ObjectMonitor::check_slow(TRAPS) {
1334 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1335 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1336 }
1337
1338 static int Adjust(volatile int * adr, int dx) {
1339 int v;
1340 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1341 return v;
1342 }
1343
1344 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1345 ObjectMonitor* monitor,
1346 jlong notifier_tid,
1347 jlong timeout,
1348 bool timedout) {
1349 assert(event != NULL, "invariant");
1350 assert(monitor != NULL, "invariant");
1351 event->set_monitorClass(((oop)monitor->object())->klass());
1352 event->set_timeout(timeout);
1353 event->set_address((uintptr_t)monitor->object_addr());
1354 event->set_notifier(notifier_tid);
1355 event->set_timedOut(timedout);
1356 event->commit();
1357 }
1358
1359 // -----------------------------------------------------------------------------
1360 // Wait/Notify/NotifyAll
1361 //
1362 // Note: a subset of changes to ObjectMonitor::wait()
1363 // will need to be replicated in complete_exit
1364 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1365 Thread * const Self = THREAD;
1366 assert(Self->is_Java_thread(), "Must be Java thread!");
1367 JavaThread *jt = (JavaThread *)THREAD;
1368
1369 DeferredInitialize();
1370
1371 // Throw IMSX or IEX.
1372 CHECK_OWNER();
1373
1374 EventJavaMonitorWait event;
1375
1376 // check for a pending interrupt
1377 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1378 // post monitor waited event. Note that this is past-tense, we are done waiting.
1379 if (JvmtiExport::should_post_monitor_waited()) {
1380 // Note: 'false' parameter is passed here because the
1381 // wait was not timed out due to thread interrupt.
1382 JvmtiExport::post_monitor_waited(jt, this, false);
1383
1384 // In this short circuit of the monitor wait protocol, the
1385 // current thread never drops ownership of the monitor and
1386 // never gets added to the wait queue so the current thread
1387 // cannot be made the successor. This means that the
1388 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1389 // consume an unpark() meant for the ParkEvent associated with
1390 // this ObjectMonitor.
1391 }
1392 if (event.should_commit()) {
1393 post_monitor_wait_event(&event, this, 0, millis, false);
1394 }
1395 THROW(vmSymbols::java_lang_InterruptedException());
1396 return;
1397 }
1398
1399 assert(Self->_Stalled == 0, "invariant");
1400 Self->_Stalled = intptr_t(this);
1401 jt->set_current_waiting_monitor(this);
1402
1403 // create a node to be put into the queue
1404 // Critically, after we reset() the event but prior to park(), we must check
1405 // for a pending interrupt.
1406 ObjectWaiter node(Self);
1407 node.TState = ObjectWaiter::TS_WAIT;
1408 Self->_ParkEvent->reset();
1409 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1410
1411 // Enter the waiting queue, which is a circular doubly linked list in this case
1412 // but it could be a priority queue or any data structure.
1413 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1414 // by the the owner of the monitor *except* in the case where park()
1415 // returns because of a timeout of interrupt. Contention is exceptionally rare
1416 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1417
1418 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1419 AddWaiter(&node);
1420 Thread::SpinRelease(&_WaitSetLock);
1421
1422 _Responsible = NULL;
1423
1424 intptr_t save = _recursions; // record the old recursion count
1425 _waiters++; // increment the number of waiters
1426 _recursions = 0; // set the recursion level to be 1
1427 exit(true, Self); // exit the monitor
1428 guarantee(_owner != Self, "invariant");
1429
1430 // The thread is on the WaitSet list - now park() it.
1431 // On MP systems it's conceivable that a brief spin before we park
1432 // could be profitable.
1433 //
1434 // TODO-FIXME: change the following logic to a loop of the form
1435 // while (!timeout && !interrupted && _notified == 0) park()
1436
1437 int ret = OS_OK;
1438 int WasNotified = 0;
1439 { // State transition wrappers
1440 OSThread* osthread = Self->osthread();
1441 OSThreadWaitState osts(osthread, true);
1442 {
1443 ThreadBlockInVM tbivm(jt);
1444 // Thread is in thread_blocked state and oop access is unsafe.
1445 jt->set_suspend_equivalent();
1446
1447 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1448 // Intentionally empty
1449 } else if (node._notified == 0) {
1450 if (millis <= 0) {
1451 Self->_ParkEvent->park();
1452 } else {
1453 ret = Self->_ParkEvent->park(millis);
1454 }
1455 }
1456
1457 // were we externally suspended while we were waiting?
1458 if (ExitSuspendEquivalent (jt)) {
1459 // TODO-FIXME: add -- if succ == Self then succ = null.
1460 jt->java_suspend_self();
1461 }
1462
1463 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1464
1465 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1466 // from the WaitSet to the EntryList.
1467 // See if we need to remove Node from the WaitSet.
1468 // We use double-checked locking to avoid grabbing _WaitSetLock
1469 // if the thread is not on the wait queue.
1470 //
1471 // Note that we don't need a fence before the fetch of TState.
1472 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1473 // written by the is thread. (perhaps the fetch might even be satisfied
1474 // by a look-aside into the processor's own store buffer, although given
1475 // the length of the code path between the prior ST and this load that's
1476 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1477 // then we'll acquire the lock and then re-fetch a fresh TState value.
1478 // That is, we fail toward safety.
1479
1480 if (node.TState == ObjectWaiter::TS_WAIT) {
1481 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1482 if (node.TState == ObjectWaiter::TS_WAIT) {
1483 DequeueSpecificWaiter(&node); // unlink from WaitSet
1484 assert(node._notified == 0, "invariant");
1485 node.TState = ObjectWaiter::TS_RUN;
1486 }
1487 Thread::SpinRelease(&_WaitSetLock);
1488 }
1489
1490 // The thread is now either on off-list (TS_RUN),
1491 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1492 // The Node's TState variable is stable from the perspective of this thread.
1493 // No other threads will asynchronously modify TState.
1494 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1495 OrderAccess::loadload();
1496 if (_succ == Self) _succ = NULL;
1497 WasNotified = node._notified;
1498
1499 // Reentry phase -- reacquire the monitor.
1500 // re-enter contended monitor after object.wait().
1501 // retain OBJECT_WAIT state until re-enter successfully completes
1502 // Thread state is thread_in_vm and oop access is again safe,
1503 // although the raw address of the object may have changed.
1504 // (Don't cache naked oops over safepoints, of course).
1505
1506 // post monitor waited event. Note that this is past-tense, we are done waiting.
1507 if (JvmtiExport::should_post_monitor_waited()) {
1508 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1509
1510 if (node._notified != 0 && _succ == Self) {
1511 // In this part of the monitor wait-notify-reenter protocol it
1512 // is possible (and normal) for another thread to do a fastpath
1513 // monitor enter-exit while this thread is still trying to get
1514 // to the reenter portion of the protocol.
1515 //
1516 // The ObjectMonitor was notified and the current thread is
1517 // the successor which also means that an unpark() has already
1518 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1519 // consume the unpark() that was done when the successor was
1520 // set because the same ParkEvent is shared between Java
1521 // monitors and JVM/TI RawMonitors (for now).
1522 //
1523 // We redo the unpark() to ensure forward progress, i.e., we
1524 // don't want all pending threads hanging (parked) with none
1525 // entering the unlocked monitor.
1526 node._event->unpark();
1527 }
1528 }
1529
1530 if (event.should_commit()) {
1531 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1532 }
1533
1534 OrderAccess::fence();
1535
1536 assert(Self->_Stalled != 0, "invariant");
1537 Self->_Stalled = 0;
1538
1539 assert(_owner != Self, "invariant");
1540 ObjectWaiter::TStates v = node.TState;
1541 if (v == ObjectWaiter::TS_RUN) {
1542 enter(Self);
1543 } else {
1544 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1545 ReenterI(Self, &node);
1546 node.wait_reenter_end(this);
1547 }
1548
1549 // Self has reacquired the lock.
1550 // Lifecycle - the node representing Self must not appear on any queues.
1551 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1552 // want residual elements associated with this thread left on any lists.
1553 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1554 assert(_owner == Self, "invariant");
1555 assert(_succ != Self, "invariant");
1556 } // OSThreadWaitState()
1557
1558 jt->set_current_waiting_monitor(NULL);
1559
1560 guarantee(_recursions == 0, "invariant");
1561 _recursions = save; // restore the old recursion count
1562 _waiters--; // decrement the number of waiters
1563
1564 // Verify a few postconditions
1565 assert(_owner == Self, "invariant");
1566 assert(_succ != Self, "invariant");
1567 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1568
1569 // check if the notification happened
1570 if (!WasNotified) {
1571 // no, it could be timeout or Thread.interrupt() or both
1572 // check for interrupt event, otherwise it is timeout
1573 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1574 THROW(vmSymbols::java_lang_InterruptedException());
1575 }
1576 }
1577
1578 // NOTE: Spurious wake up will be consider as timeout.
1579 // Monitor notify has precedence over thread interrupt.
1580 }
1581
1582
1583 // Consider:
1584 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1585 // then instead of transferring a thread from the WaitSet to the EntryList
1586 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1587
1588 void ObjectMonitor::INotify(Thread * Self) {
1589 const int policy = Knob_MoveNotifyee;
1590
1591 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1592 ObjectWaiter * iterator = DequeueWaiter();
1593 if (iterator != NULL) {
1594 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1595 guarantee(iterator->_notified == 0, "invariant");
1596 // Disposition - what might we do with iterator ?
1597 // a. add it directly to the EntryList - either tail (policy == 1)
1598 // or head (policy == 0).
1599 // b. push it onto the front of the _cxq (policy == 2).
1600 // For now we use (b).
1601 if (policy != 4) {
1602 iterator->TState = ObjectWaiter::TS_ENTER;
1603 }
1604 iterator->_notified = 1;
1605 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1606
1607 ObjectWaiter * list = _EntryList;
1608 if (list != NULL) {
1609 assert(list->_prev == NULL, "invariant");
1610 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1611 assert(list != iterator, "invariant");
1612 }
1613
1614 if (policy == 0) { // prepend to EntryList
1615 if (list == NULL) {
1616 iterator->_next = iterator->_prev = NULL;
1617 _EntryList = iterator;
1618 } else {
1619 list->_prev = iterator;
1620 iterator->_next = list;
1621 iterator->_prev = NULL;
1622 _EntryList = iterator;
1623 }
1624 } else if (policy == 1) { // append to EntryList
1625 if (list == NULL) {
1626 iterator->_next = iterator->_prev = NULL;
1627 _EntryList = iterator;
1628 } else {
1629 // CONSIDER: finding the tail currently requires a linear-time walk of
1630 // the EntryList. We can make tail access constant-time by converting to
1631 // a CDLL instead of using our current DLL.
1632 ObjectWaiter * tail;
1633 for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1634 assert(tail != NULL && tail->_next == NULL, "invariant");
1635 tail->_next = iterator;
1636 iterator->_prev = tail;
1637 iterator->_next = NULL;
1638 }
1639 } else if (policy == 2) { // prepend to cxq
1640 if (list == NULL) {
1641 iterator->_next = iterator->_prev = NULL;
1642 _EntryList = iterator;
1643 } else {
1644 iterator->TState = ObjectWaiter::TS_CXQ;
1645 for (;;) {
1646 ObjectWaiter * front = _cxq;
1647 iterator->_next = front;
1648 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1649 break;
1650 }
1651 }
1652 }
1653 } else if (policy == 3) { // append to cxq
1654 iterator->TState = ObjectWaiter::TS_CXQ;
1655 for (;;) {
1656 ObjectWaiter * tail = _cxq;
1657 if (tail == NULL) {
1658 iterator->_next = NULL;
1659 if (Atomic::replace_if_null(iterator, &_cxq)) {
1660 break;
1661 }
1662 } else {
1663 while (tail->_next != NULL) tail = tail->_next;
1664 tail->_next = iterator;
1665 iterator->_prev = tail;
1666 iterator->_next = NULL;
1667 break;
1668 }
1669 }
1670 } else {
1671 ParkEvent * ev = iterator->_event;
1672 iterator->TState = ObjectWaiter::TS_RUN;
1673 OrderAccess::fence();
1674 ev->unpark();
1675 }
1676
1677 // _WaitSetLock protects the wait queue, not the EntryList. We could
1678 // move the add-to-EntryList operation, above, outside the critical section
1679 // protected by _WaitSetLock. In practice that's not useful. With the
1680 // exception of wait() timeouts and interrupts the monitor owner
1681 // is the only thread that grabs _WaitSetLock. There's almost no contention
1682 // on _WaitSetLock so it's not profitable to reduce the length of the
1683 // critical section.
1684
1685 if (policy < 4) {
1686 iterator->wait_reenter_begin(this);
1687 }
1688 }
1689 Thread::SpinRelease(&_WaitSetLock);
1690 }
1691
1692 // Consider: a not-uncommon synchronization bug is to use notify() when
1693 // notifyAll() is more appropriate, potentially resulting in stranded
1694 // threads; this is one example of a lost wakeup. A useful diagnostic
1695 // option is to force all notify() operations to behave as notifyAll().
1696 //
1697 // Note: We can also detect many such problems with a "minimum wait".
1698 // When the "minimum wait" is set to a small non-zero timeout value
1699 // and the program does not hang whereas it did absent "minimum wait",
1700 // that suggests a lost wakeup bug.
1701
1702 void ObjectMonitor::notify(TRAPS) {
1703 CHECK_OWNER();
1704 if (_WaitSet == NULL) {
1705 return;
1706 }
1707 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1708 INotify(THREAD);
1709 OM_PERFDATA_OP(Notifications, inc(1));
1710 }
1711
1712
1713 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1714 // from the waitset to the EntryList. This could be done more efficiently with a
1715 // single bulk transfer but in practice it's not time-critical. Beware too,
1716 // that in prepend-mode we invert the order of the waiters. Let's say that the
1717 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1718 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1719
1720 void ObjectMonitor::notifyAll(TRAPS) {
1721 CHECK_OWNER();
1722 if (_WaitSet == NULL) {
1723 return;
1724 }
1725
1726 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1727 int tally = 0;
1728 while (_WaitSet != NULL) {
1729 tally++;
1730 INotify(THREAD);
1731 }
1732
1733 OM_PERFDATA_OP(Notifications, inc(tally));
1734 }
1735
1736 // -----------------------------------------------------------------------------
1737 // Adaptive Spinning Support
1738 //
1739 // Adaptive spin-then-block - rational spinning
1740 //
1741 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1742 // algorithm. On high order SMP systems it would be better to start with
1743 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1744 // a contending thread could enqueue itself on the cxq and then spin locally
1745 // on a thread-specific variable such as its ParkEvent._Event flag.
1746 // That's left as an exercise for the reader. Note that global spinning is
1747 // not problematic on Niagara, as the L2 cache serves the interconnect and
1748 // has both low latency and massive bandwidth.
1749 //
1750 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1751 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1752 // (duration) or we can fix the count at approximately the duration of
1753 // a context switch and vary the frequency. Of course we could also
1754 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1755 // For a description of 'Adaptive spin-then-block mutual exclusion in
1756 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1757 //
1758 // This implementation varies the duration "D", where D varies with
1759 // the success rate of recent spin attempts. (D is capped at approximately
1760 // length of a round-trip context switch). The success rate for recent
1761 // spin attempts is a good predictor of the success rate of future spin
1762 // attempts. The mechanism adapts automatically to varying critical
1763 // section length (lock modality), system load and degree of parallelism.
1764 // D is maintained per-monitor in _SpinDuration and is initialized
1765 // optimistically. Spin frequency is fixed at 100%.
1766 //
1767 // Note that _SpinDuration is volatile, but we update it without locks
1768 // or atomics. The code is designed so that _SpinDuration stays within
1769 // a reasonable range even in the presence of races. The arithmetic
1770 // operations on _SpinDuration are closed over the domain of legal values,
1771 // so at worst a race will install and older but still legal value.
1772 // At the very worst this introduces some apparent non-determinism.
1773 // We might spin when we shouldn't or vice-versa, but since the spin
1774 // count are relatively short, even in the worst case, the effect is harmless.
1775 //
1776 // Care must be taken that a low "D" value does not become an
1777 // an absorbing state. Transient spinning failures -- when spinning
1778 // is overall profitable -- should not cause the system to converge
1779 // on low "D" values. We want spinning to be stable and predictable
1780 // and fairly responsive to change and at the same time we don't want
1781 // it to oscillate, become metastable, be "too" non-deterministic,
1782 // or converge on or enter undesirable stable absorbing states.
1783 //
1784 // We implement a feedback-based control system -- using past behavior
1785 // to predict future behavior. We face two issues: (a) if the
1786 // input signal is random then the spin predictor won't provide optimal
1787 // results, and (b) if the signal frequency is too high then the control
1788 // system, which has some natural response lag, will "chase" the signal.
1789 // (b) can arise from multimodal lock hold times. Transient preemption
1790 // can also result in apparent bimodal lock hold times.
1791 // Although sub-optimal, neither condition is particularly harmful, as
1792 // in the worst-case we'll spin when we shouldn't or vice-versa.
1793 // The maximum spin duration is rather short so the failure modes aren't bad.
1794 // To be conservative, I've tuned the gain in system to bias toward
1795 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1796 // "rings" or oscillates between spinning and not spinning. This happens
1797 // when spinning is just on the cusp of profitability, however, so the
1798 // situation is not dire. The state is benign -- there's no need to add
1799 // hysteresis control to damp the transition rate between spinning and
1800 // not spinning.
1801
1802 // Spinning: Fixed frequency (100%), vary duration
1803 int ObjectMonitor::TrySpin(Thread * Self) {
1804 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1805 int ctr = Knob_FixedSpin;
1806 if (ctr != 0) {
1807 while (--ctr >= 0) {
1808 if (TryLock(Self) > 0) return 1;
1809 SpinPause();
1810 }
1811 return 0;
1812 }
1813
1814 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1815 if (TryLock(Self) > 0) {
1816 // Increase _SpinDuration ...
1817 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1818 // Raising _SpurDuration to the poverty line is key.
1819 int x = _SpinDuration;
1820 if (x < Knob_SpinLimit) {
1821 if (x < Knob_Poverty) x = Knob_Poverty;
1822 _SpinDuration = x + Knob_BonusB;
1823 }
1824 return 1;
1825 }
1826 SpinPause();
1827 }
1828
1829 // Admission control - verify preconditions for spinning
1830 //
1831 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1832 // becoming an absorbing state. Put another way, we spin briefly to
1833 // sample, just in case the system load, parallelism, contention, or lock
1834 // modality changed.
1835 //
1836 // Consider the following alternative:
1837 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1838 // spin attempt. "Periodically" might mean after a tally of
1839 // the # of failed spin attempts (or iterations) reaches some threshold.
1840 // This takes us into the realm of 1-out-of-N spinning, where we
1841 // hold the duration constant but vary the frequency.
1842
1843 ctr = _SpinDuration;
1844 if (ctr <= 0) return 0;
1845
1846 if (Knob_SuccRestrict && _succ != NULL) return 0;
1847 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1848 return 0;
1849 }
1850
1851 int MaxSpin = Knob_MaxSpinners;
1852 if (MaxSpin >= 0) {
1853 if (_Spinner > MaxSpin) {
1854 return 0;
1855 }
1856 // Slightly racy, but benign ...
1857 Adjust(&_Spinner, 1);
1858 }
1859
1860 // We're good to spin ... spin ingress.
1861 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1862 // when preparing to LD...CAS _owner, etc and the CAS is likely
1863 // to succeed.
1864 if (_succ == NULL) {
1865 _succ = Self;
1866 }
1867 Thread * prv = NULL;
1868
1869 // There are three ways to exit the following loop:
1870 // 1. A successful spin where this thread has acquired the lock.
1871 // 2. Spin failure with prejudice
1872 // 3. Spin failure without prejudice
1873
1874 while (--ctr >= 0) {
1875
1876 // Periodic polling -- Check for pending GC
1877 // Threads may spin while they're unsafe.
1878 // We don't want spinning threads to delay the JVM from reaching
1879 // a stop-the-world safepoint or to steal cycles from GC.
1880 // If we detect a pending safepoint we abort in order that
1881 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1882 // this thread, if safe, doesn't steal cycles from GC.
1883 // This is in keeping with the "no loitering in runtime" rule.
1884 // We periodically check to see if there's a safepoint pending.
1885 if ((ctr & 0xFF) == 0) {
1886 if (SafepointMechanism::poll(Self)) {
1887 goto Abort; // abrupt spin egress
1888 }
1889 if (Knob_UsePause & 1) SpinPause();
1890 }
1891
1892 if (Knob_UsePause & 2) SpinPause();
1893
1894 // Probe _owner with TATAS
1895 // If this thread observes the monitor transition or flicker
1896 // from locked to unlocked to locked, then the odds that this
1897 // thread will acquire the lock in this spin attempt go down
1898 // considerably. The same argument applies if the CAS fails
1899 // or if we observe _owner change from one non-null value to
1900 // another non-null value. In such cases we might abort
1901 // the spin without prejudice or apply a "penalty" to the
1902 // spin count-down variable "ctr", reducing it by 100, say.
1903
1904 Thread * ox = (Thread *) _owner;
1905 if (ox == NULL) {
1906 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1907 if (ox == NULL) {
1908 // The CAS succeeded -- this thread acquired ownership
1909 // Take care of some bookkeeping to exit spin state.
1910 if (_succ == Self) {
1911 _succ = NULL;
1912 }
1913 if (MaxSpin > 0) Adjust(&_Spinner, -1);
1914
1915 // Increase _SpinDuration :
1916 // The spin was successful (profitable) so we tend toward
1917 // longer spin attempts in the future.
1918 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1919 // If we acquired the lock early in the spin cycle it
1920 // makes sense to increase _SpinDuration proportionally.
1921 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1922 int x = _SpinDuration;
1923 if (x < Knob_SpinLimit) {
1924 if (x < Knob_Poverty) x = Knob_Poverty;
1925 _SpinDuration = x + Knob_Bonus;
1926 }
1927 return 1;
1928 }
1929
1930 // The CAS failed ... we can take any of the following actions:
1931 // * penalize: ctr -= CASPenalty
1932 // * exit spin with prejudice -- goto Abort;
1933 // * exit spin without prejudice.
1934 // * Since CAS is high-latency, retry again immediately.
1935 prv = ox;
1936 goto Abort;
1937 }
1938
1939 // Did lock ownership change hands ?
1940 if (ox != prv && prv != NULL) {
1941 goto Abort;
1942 }
1943 prv = ox;
1944
1945 // Abort the spin if the owner is not executing.
1946 // The owner must be executing in order to drop the lock.
1947 // Spinning while the owner is OFFPROC is idiocy.
1948 // Consider: ctr -= RunnablePenalty ;
1949 if (Knob_OState && NotRunnable (Self, ox)) {
1950 goto Abort;
1951 }
1952 if (_succ == NULL) {
1953 _succ = Self;
1954 }
1955 }
1956
1957 // Spin failed with prejudice -- reduce _SpinDuration.
1958 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1959 // AIMD is globally stable.
1960 {
1961 int x = _SpinDuration;
1962 if (x > 0) {
1963 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1964 // This is globally sample and tends to damp the response.
1965 x -= Knob_Penalty;
1966 if (x < 0) x = 0;
1967 _SpinDuration = x;
1968 }
1969 }
1970
1971 Abort:
1972 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
1973 if (_succ == Self) {
1974 _succ = NULL;
1975 // Invariant: after setting succ=null a contending thread
1976 // must recheck-retry _owner before parking. This usually happens
1977 // in the normal usage of TrySpin(), but it's safest
1978 // to make TrySpin() as foolproof as possible.
1979 OrderAccess::fence();
1980 if (TryLock(Self) > 0) return 1;
1981 }
1982 return 0;
1983 }
1984
1985 // NotRunnable() -- informed spinning
1986 //
1987 // Don't bother spinning if the owner is not eligible to drop the lock.
1988 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1989 // The thread must be runnable in order to drop the lock in timely fashion.
1990 // If the _owner is not runnable then spinning will not likely be
1991 // successful (profitable).
1992 //
1993 // Beware -- the thread referenced by _owner could have died
1994 // so a simply fetch from _owner->_thread_state might trap.
1995 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1996 // Because of the lifecycle issues, the _thread_state values
1997 // observed by NotRunnable() might be garbage. NotRunnable must
1998 // tolerate this and consider the observed _thread_state value
1999 // as advisory.
2000 //
2001 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2002 // a thread pointer.
2003 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2004 // with the LSB of _owner. Another option would be to probabilistically probe
2005 // the putative _owner->TypeTag value.
2006 //
2007 // Checking _thread_state isn't perfect. Even if the thread is
2008 // in_java it might be blocked on a page-fault or have been preempted
2009 // and sitting on a ready/dispatch queue.
2010 //
2011 // The return value from NotRunnable() is *advisory* -- the
2012 // result is based on sampling and is not necessarily coherent.
2013 // The caller must tolerate false-negative and false-positive errors.
2014 // Spinning, in general, is probabilistic anyway.
2015
2016
2017 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2018 // Check ox->TypeTag == 2BAD.
2019 if (ox == NULL) return 0;
2020
2021 // Avoid transitive spinning ...
2022 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2023 // Immediately after T1 acquires L it's possible that T2, also
2024 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2025 // This occurs transiently after T1 acquired L but before
2026 // T1 managed to clear T1.Stalled. T2 does not need to abort
2027 // its spin in this circumstance.
2028 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2029
2030 if (BlockedOn == 1) return 1;
2031 if (BlockedOn != 0) {
2032 return BlockedOn != intptr_t(this) && _owner == ox;
2033 }
2034
2035 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2036 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2037 // consider also: jst != _thread_in_Java -- but that's overspecific.
2038 return jst == _thread_blocked || jst == _thread_in_native;
2039 }
2040
2041
2042 // -----------------------------------------------------------------------------
2043 // WaitSet management ...
2044
2045 ObjectWaiter::ObjectWaiter(Thread* thread) {
2046 _next = NULL;
2047 _prev = NULL;
2048 _notified = 0;
2049 _notifier_tid = 0;
2050 TState = TS_RUN;
2051 _thread = thread;
2052 _event = thread->_ParkEvent;
2053 _active = false;
2054 assert(_event != NULL, "invariant");
2055 }
2056
2057 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2058 JavaThread *jt = (JavaThread *)this->_thread;
2059 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2060 }
2061
2062 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2063 JavaThread *jt = (JavaThread *)this->_thread;
2064 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2065 }
2066
2067 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2068 assert(node != NULL, "should not add NULL node");
2069 assert(node->_prev == NULL, "node already in list");
2070 assert(node->_next == NULL, "node already in list");
2071 // put node at end of queue (circular doubly linked list)
2072 if (_WaitSet == NULL) {
2073 _WaitSet = node;
2074 node->_prev = node;
2075 node->_next = node;
2076 } else {
2077 ObjectWaiter* head = _WaitSet;
2078 ObjectWaiter* tail = head->_prev;
2079 assert(tail->_next == head, "invariant check");
2080 tail->_next = node;
2081 head->_prev = node;
2082 node->_next = head;
2083 node->_prev = tail;
2084 }
2085 }
2086
2087 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2088 // dequeue the very first waiter
2089 ObjectWaiter* waiter = _WaitSet;
2090 if (waiter) {
2091 DequeueSpecificWaiter(waiter);
2092 }
2093 return waiter;
2094 }
2095
2096 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2097 assert(node != NULL, "should not dequeue NULL node");
2098 assert(node->_prev != NULL, "node already removed from list");
2099 assert(node->_next != NULL, "node already removed from list");
2100 // when the waiter has woken up because of interrupt,
2101 // timeout or other spurious wake-up, dequeue the
2102 // waiter from waiting list
2103 ObjectWaiter* next = node->_next;
2104 if (next == node) {
2105 assert(node->_prev == node, "invariant check");
2106 _WaitSet = NULL;
2107 } else {
2108 ObjectWaiter* prev = node->_prev;
2109 assert(prev->_next == node, "invariant check");
2110 assert(next->_prev == node, "invariant check");
2111 next->_prev = prev;
2112 prev->_next = next;
2113 if (_WaitSet == node) {
2114 _WaitSet = next;
2115 }
2116 }
2117 node->_next = NULL;
2118 node->_prev = NULL;
2119 }
2120
2121 // -----------------------------------------------------------------------------
2122 // PerfData support
2123 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2124 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2125 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2126 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2127 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2128 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2129 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2130
2131 // One-shot global initialization for the sync subsystem.
2132 // We could also defer initialization and initialize on-demand
2133 // the first time we call inflate(). Initialization would
2134 // be protected - like so many things - by the MonitorCache_lock.
2135
2136 void ObjectMonitor::Initialize() {
2137 static int InitializationCompleted = 0;
2138 assert(InitializationCompleted == 0, "invariant");
2139 InitializationCompleted = 1;
2140 if (UsePerfData) {
2141 EXCEPTION_MARK;
2142 #define NEWPERFCOUNTER(n) \
2143 { \
2144 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2145 CHECK); \
2146 }
2147 #define NEWPERFVARIABLE(n) \
2148 { \
2149 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2150 CHECK); \
2151 }
2152 NEWPERFCOUNTER(_sync_Inflations);
2153 NEWPERFCOUNTER(_sync_Deflations);
2154 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2155 NEWPERFCOUNTER(_sync_FutileWakeups);
2156 NEWPERFCOUNTER(_sync_Parks);
2157 NEWPERFCOUNTER(_sync_Notifications);
2158 NEWPERFVARIABLE(_sync_MonExtant);
2159 #undef NEWPERFCOUNTER
2160 #undef NEWPERFVARIABLE
2161 }
2162 }
2163
2164 void ObjectMonitor::DeferredInitialize() {
2165 if (InitDone > 0) return;
2166 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2167 while (InitDone != 1) /* empty */;
2168 return;
2169 }
2170
2171 // One-shot global initialization ...
2172 // The initialization is idempotent, so we don't need locks.
2173 // In the future consider doing this via os::init_2().
2174
2175 if (!os::is_MP()) {
2176 Knob_SpinLimit = 0;
2177 Knob_PreSpin = 0;
2178 Knob_FixedSpin = -1;
2179 }
2180
2181 OrderAccess::fence();
2182 InitDone = 1;
2183 }
2184