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_VerifyInUse = 0;
105 int ObjectMonitor::Knob_VerifyMatch = 0;
106 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
107
108 static int Knob_SpinBase = 0; // Floor AKA SpinMin
109 static int Knob_CASPenalty = -1; // Penalty for failed CAS
110 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
111 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
112 static int Knob_SpinEarly = 1;
113 static int Knob_SuccEnabled = 1; // futile wake throttling
114 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
115 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
116 static int Knob_Bonus = 100; // spin success bonus
117 static int Knob_BonusB = 100; // spin success bonus
118 static int Knob_Penalty = 200; // spin failure penalty
119 static int Knob_Poverty = 1000;
120 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
121 static int Knob_FixedSpin = 0;
122 static int Knob_OState = 3; // Spinner checks thread state of _owner
123 static int Knob_UsePause = 1;
124 static int Knob_ExitPolicy = 0;
125 static int Knob_PreSpin = 10; // 20-100 likely better
126 static int Knob_ResetEvent = 0;
127
128 static int Knob_FastHSSEC = 0;
129 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
130 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
131 static volatile int InitDone = 0;
132
133 // -----------------------------------------------------------------------------
134 // Theory of operations -- Monitors lists, thread residency, etc:
135 //
136 // * A thread acquires ownership of a monitor by successfully
137 // CAS()ing the _owner field from null to non-null.
138 //
139 // * Invariant: A thread appears on at most one monitor list --
140 // cxq, EntryList or WaitSet -- at any one time.
141 //
142 // * Contending threads "push" themselves onto the cxq with CAS
143 // and then spin/park.
144 //
145 // * After a contending thread eventually acquires the lock it must
146 // dequeue itself from either the EntryList or the cxq.
147 //
148 // * The exiting thread identifies and unparks an "heir presumptive"
149 // tentative successor thread on the EntryList. Critically, the
150 // exiting thread doesn't unlink the successor thread from the EntryList.
151 // After having been unparked, the wakee will recontend for ownership of
152 // the monitor. The successor (wakee) will either acquire the lock or
153 // re-park itself.
154 //
155 // Succession is provided for by a policy of competitive handoff.
156 // The exiting thread does _not_ grant or pass ownership to the
157 // successor thread. (This is also referred to as "handoff" succession").
158 // Instead the exiting thread releases ownership and possibly wakes
159 // a successor, so the successor can (re)compete for ownership of the lock.
160 // If the EntryList is empty but the cxq is populated the exiting
161 // thread will drain the cxq into the EntryList. It does so by
162 // by detaching the cxq (installing null with CAS) and folding
163 // the threads from the cxq into the EntryList. The EntryList is
164 // doubly linked, while the cxq is singly linked because of the
165 // CAS-based "push" used to enqueue recently arrived threads (RATs).
166 //
167 // * Concurrency invariants:
168 //
169 // -- only the monitor owner may access or mutate the EntryList.
170 // The mutex property of the monitor itself protects the EntryList
171 // from concurrent interference.
172 // -- Only the monitor owner may detach the cxq.
173 //
174 // * The monitor entry list operations avoid locks, but strictly speaking
175 // they're not lock-free. Enter is lock-free, exit is not.
176 // For a description of 'Methods and apparatus providing non-blocking access
177 // to a resource,' see U.S. Pat. No. 7844973.
178 //
179 // * The cxq can have multiple concurrent "pushers" but only one concurrent
180 // detaching thread. This mechanism is immune from the ABA corruption.
181 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
182 //
183 // * Taken together, the cxq and the EntryList constitute or form a
184 // single logical queue of threads stalled trying to acquire the lock.
185 // We use two distinct lists to improve the odds of a constant-time
186 // dequeue operation after acquisition (in the ::enter() epilogue) and
187 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
188 // A key desideratum is to minimize queue & monitor metadata manipulation
189 // that occurs while holding the monitor lock -- that is, we want to
190 // minimize monitor lock holds times. Note that even a small amount of
191 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
192 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
193 // locks and monitor metadata.
194 //
195 // Cxq points to the set of Recently Arrived Threads attempting entry.
196 // Because we push threads onto _cxq with CAS, the RATs must take the form of
197 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
198 // the unlocking thread notices that EntryList is null but _cxq is != null.
199 //
200 // The EntryList is ordered by the prevailing queue discipline and
201 // can be organized in any convenient fashion, such as a doubly-linked list or
202 // a circular doubly-linked list. Critically, we want insert and delete operations
203 // to operate in constant-time. If we need a priority queue then something akin
204 // to Solaris' sleepq would work nicely. Viz.,
205 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
206 // Queue discipline is enforced at ::exit() time, when the unlocking thread
207 // drains the cxq into the EntryList, and orders or reorders the threads on the
208 // EntryList accordingly.
209 //
210 // Barring "lock barging", this mechanism provides fair cyclic ordering,
211 // somewhat similar to an elevator-scan.
212 //
213 // * The monitor synchronization subsystem avoids the use of native
214 // synchronization primitives except for the narrow platform-specific
215 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
216 // the semantics of park-unpark. Put another way, this monitor implementation
217 // depends only on atomic operations and park-unpark. The monitor subsystem
218 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
219 // underlying OS manages the READY<->RUN transitions.
220 //
221 // * Waiting threads reside on the WaitSet list -- wait() puts
222 // the caller onto the WaitSet.
223 //
224 // * notify() or notifyAll() simply transfers threads from the WaitSet to
225 // either the EntryList or cxq. Subsequent exit() operations will
226 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
227 // it's likely the notifyee would simply impale itself on the lock held
228 // by the notifier.
229 //
230 // * An interesting alternative is to encode cxq as (List,LockByte) where
231 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
232 // variable, like _recursions, in the scheme. The threads or Events that form
233 // the list would have to be aligned in 256-byte addresses. A thread would
234 // try to acquire the lock or enqueue itself with CAS, but exiting threads
235 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
236 // Note that is is *not* word-tearing, but it does presume that full-word
237 // CAS operations are coherent with intermix with STB operations. That's true
238 // on most common processors.
239 //
240 // * See also http://blogs.sun.com/dave
241
242
243 void* ObjectMonitor::operator new (size_t size) throw() {
244 return AllocateHeap(size, mtInternal);
245 }
246 void* ObjectMonitor::operator new[] (size_t size) throw() {
247 return operator new (size);
248 }
249 void ObjectMonitor::operator delete(void* p) {
250 FreeHeap(p);
251 }
252 void ObjectMonitor::operator delete[] (void *p) {
253 operator delete(p);
254 }
255
256 // -----------------------------------------------------------------------------
257 // Enter support
258
259 void ObjectMonitor::enter(TRAPS) {
260 // The following code is ordered to check the most common cases first
261 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
262 Thread * const Self = THREAD;
263
264 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
265 if (cur == NULL) {
266 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
267 assert(_recursions == 0, "invariant");
268 assert(_owner == Self, "invariant");
269 return;
270 }
271
272 if (cur == Self) {
273 // TODO-FIXME: check for integer overflow! BUGID 6557169.
274 _recursions++;
275 return;
276 }
277
278 if (Self->is_lock_owned ((address)cur)) {
279 assert(_recursions == 0, "internal state error");
280 _recursions = 1;
281 // Commute owner from a thread-specific on-stack BasicLockObject address to
282 // a full-fledged "Thread *".
283 _owner = Self;
284 return;
285 }
286
287 // We've encountered genuine contention.
288 assert(Self->_Stalled == 0, "invariant");
289 Self->_Stalled = intptr_t(this);
290
291 // Try one round of spinning *before* enqueueing Self
292 // and before going through the awkward and expensive state
293 // transitions. The following spin is strictly optional ...
294 // Note that if we acquire the monitor from an initial spin
295 // we forgo posting JVMTI events and firing DTRACE probes.
296 if (Knob_SpinEarly && TrySpin (Self) > 0) {
297 assert(_owner == Self, "invariant");
298 assert(_recursions == 0, "invariant");
299 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
300 Self->_Stalled = 0;
301 return;
302 }
303
304 assert(_owner != Self, "invariant");
305 assert(_succ != Self, "invariant");
306 assert(Self->is_Java_thread(), "invariant");
307 JavaThread * jt = (JavaThread *) Self;
308 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
309 assert(jt->thread_state() != _thread_blocked, "invariant");
310 assert(this->object() != NULL, "invariant");
311 assert(_count >= 0, "invariant");
312
313 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
314 // Ensure the object-monitor relationship remains stable while there's contention.
315 Atomic::inc(&_count);
316
317 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
318 EventJavaMonitorEnter event;
319 if (event.should_commit()) {
320 event.set_monitorClass(((oop)this->object())->klass());
321 event.set_address((uintptr_t)(this->object_addr()));
322 }
323
324 { // Change java thread status to indicate blocked on monitor enter.
325 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
326
327 Self->set_current_pending_monitor(this);
328
329 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
330 if (JvmtiExport::should_post_monitor_contended_enter()) {
331 JvmtiExport::post_monitor_contended_enter(jt, this);
332
333 // The current thread does not yet own the monitor and does not
334 // yet appear on any queues that would get it made the successor.
335 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
336 // handler cannot accidentally consume an unpark() meant for the
337 // ParkEvent associated with this ObjectMonitor.
338 }
339
340 OSThreadContendState osts(Self->osthread());
341 ThreadBlockInVM tbivm(jt);
342
343 // TODO-FIXME: change the following for(;;) loop to straight-line code.
344 for (;;) {
345 jt->set_suspend_equivalent();
346 // cleared by handle_special_suspend_equivalent_condition()
347 // or java_suspend_self()
348
349 EnterI(THREAD);
350
351 if (!ExitSuspendEquivalent(jt)) break;
352
353 // We have acquired the contended monitor, but while we were
354 // waiting another thread suspended us. We don't want to enter
355 // the monitor while suspended because that would surprise the
356 // thread that suspended us.
357 //
358 _recursions = 0;
359 _succ = NULL;
360 exit(false, Self);
361
362 jt->java_suspend_self();
363 }
364 Self->set_current_pending_monitor(NULL);
365
366 // We cleared the pending monitor info since we've just gotten past
367 // the enter-check-for-suspend dance and we now own the monitor free
368 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
369 // destructor can go to a safepoint at the end of this block. If we
370 // do a thread dump during that safepoint, then this thread will show
371 // as having "-locked" the monitor, but the OS and java.lang.Thread
372 // states will still report that the thread is blocked trying to
373 // acquire it.
374 }
375
376 Atomic::dec(&_count);
377 assert(_count >= 0, "invariant");
378 Self->_Stalled = 0;
379
380 // Must either set _recursions = 0 or ASSERT _recursions == 0.
381 assert(_recursions == 0, "invariant");
382 assert(_owner == Self, "invariant");
383 assert(_succ != Self, "invariant");
384 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
385
386 // The thread -- now the owner -- is back in vm mode.
387 // Report the glorious news via TI,DTrace and jvmstat.
388 // The probe effect is non-trivial. All the reportage occurs
389 // while we hold the monitor, increasing the length of the critical
390 // section. Amdahl's parallel speedup law comes vividly into play.
391 //
392 // Another option might be to aggregate the events (thread local or
393 // per-monitor aggregation) and defer reporting until a more opportune
394 // time -- such as next time some thread encounters contention but has
395 // yet to acquire the lock. While spinning that thread could
396 // spinning we could increment JVMStat counters, etc.
397
398 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
399 if (JvmtiExport::should_post_monitor_contended_entered()) {
400 JvmtiExport::post_monitor_contended_entered(jt, this);
401
402 // The current thread already owns the monitor and is not going to
403 // call park() for the remainder of the monitor enter protocol. So
404 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
405 // event handler consumed an unpark() issued by the thread that
406 // just exited the monitor.
407 }
408 if (event.should_commit()) {
409 event.set_previousOwner((uintptr_t)_previous_owner_tid);
410 event.commit();
411 }
412 OM_PERFDATA_OP(ContendedLockAttempts, inc());
413 }
414
415 // Caveat: TryLock() is not necessarily serializing if it returns failure.
416 // Callers must compensate as needed.
417
418 int ObjectMonitor::TryLock(Thread * Self) {
419 void * own = _owner;
420 if (own != NULL) return 0;
421 if (Atomic::replace_if_null(Self, &_owner)) {
422 // Either guarantee _recursions == 0 or set _recursions = 0.
423 assert(_recursions == 0, "invariant");
424 assert(_owner == Self, "invariant");
425 return 1;
426 }
427 // The lock had been free momentarily, but we lost the race to the lock.
428 // Interference -- the CAS failed.
429 // We can either return -1 or retry.
430 // Retry doesn't make as much sense because the lock was just acquired.
431 return -1;
432 }
433
434 #define MAX_RECHECK_INTERVAL 1000
435
436 void ObjectMonitor::EnterI(TRAPS) {
437 Thread * const Self = THREAD;
438 assert(Self->is_Java_thread(), "invariant");
439 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
440
441 // Try the lock - TATAS
442 if (TryLock (Self) > 0) {
443 assert(_succ != Self, "invariant");
444 assert(_owner == Self, "invariant");
445 assert(_Responsible != Self, "invariant");
446 return;
447 }
448
449 DeferredInitialize();
450
451 // We try one round of spinning *before* enqueueing Self.
452 //
453 // If the _owner is ready but OFFPROC we could use a YieldTo()
454 // operation to donate the remainder of this thread's quantum
455 // to the owner. This has subtle but beneficial affinity
456 // effects.
457
458 if (TrySpin (Self) > 0) {
459 assert(_owner == Self, "invariant");
460 assert(_succ != Self, "invariant");
461 assert(_Responsible != Self, "invariant");
462 return;
463 }
464
465 // The Spin failed -- Enqueue and park the thread ...
466 assert(_succ != Self, "invariant");
467 assert(_owner != Self, "invariant");
468 assert(_Responsible != Self, "invariant");
469
470 // Enqueue "Self" on ObjectMonitor's _cxq.
471 //
472 // Node acts as a proxy for Self.
473 // As an aside, if were to ever rewrite the synchronization code mostly
474 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
475 // Java objects. This would avoid awkward lifecycle and liveness issues,
476 // as well as eliminate a subset of ABA issues.
477 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
478
479 ObjectWaiter node(Self);
480 Self->_ParkEvent->reset();
481 node._prev = (ObjectWaiter *) 0xBAD;
482 node.TState = ObjectWaiter::TS_CXQ;
483
484 // Push "Self" onto the front of the _cxq.
485 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
486 // Note that spinning tends to reduce the rate at which threads
487 // enqueue and dequeue on EntryList|cxq.
488 ObjectWaiter * nxt;
489 for (;;) {
490 node._next = nxt = _cxq;
491 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
492
493 // Interference - the CAS failed because _cxq changed. Just retry.
494 // As an optional optimization we retry the lock.
495 if (TryLock (Self) > 0) {
496 assert(_succ != Self, "invariant");
497 assert(_owner == Self, "invariant");
498 assert(_Responsible != Self, "invariant");
499 return;
500 }
501 }
502
503 // Check for cxq|EntryList edge transition to non-null. This indicates
504 // the onset of contention. While contention persists exiting threads
505 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
506 // operations revert to the faster 1-0 mode. This enter operation may interleave
507 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
508 // arrange for one of the contending thread to use a timed park() operations
509 // to detect and recover from the race. (Stranding is form of progress failure
510 // where the monitor is unlocked but all the contending threads remain parked).
511 // That is, at least one of the contended threads will periodically poll _owner.
512 // One of the contending threads will become the designated "Responsible" thread.
513 // The Responsible thread uses a timed park instead of a normal indefinite park
514 // operation -- it periodically wakes and checks for and recovers from potential
515 // strandings admitted by 1-0 exit operations. We need at most one Responsible
516 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
517 // be responsible for a monitor.
518 //
519 // Currently, one of the contended threads takes on the added role of "Responsible".
520 // A viable alternative would be to use a dedicated "stranding checker" thread
521 // that periodically iterated over all the threads (or active monitors) and unparked
522 // successors where there was risk of stranding. This would help eliminate the
523 // timer scalability issues we see on some platforms as we'd only have one thread
524 // -- the checker -- parked on a timer.
525
526 if (nxt == NULL && _EntryList == NULL) {
527 // Try to assume the role of responsible thread for the monitor.
528 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
529 Atomic::replace_if_null(Self, &_Responsible);
530 }
531
532 // The lock might have been released while this thread was occupied queueing
533 // itself onto _cxq. To close the race and avoid "stranding" and
534 // progress-liveness failure we must resample-retry _owner before parking.
535 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
536 // In this case the ST-MEMBAR is accomplished with CAS().
537 //
538 // TODO: Defer all thread state transitions until park-time.
539 // Since state transitions are heavy and inefficient we'd like
540 // to defer the state transitions until absolutely necessary,
541 // and in doing so avoid some transitions ...
542
543 int nWakeups = 0;
544 int recheckInterval = 1;
545
546 for (;;) {
547
548 if (TryLock(Self) > 0) break;
549 assert(_owner != Self, "invariant");
550
551 // park self
552 if (_Responsible == Self) {
553 Self->_ParkEvent->park((jlong) recheckInterval);
554 // Increase the recheckInterval, but clamp the value.
555 recheckInterval *= 8;
556 if (recheckInterval > MAX_RECHECK_INTERVAL) {
557 recheckInterval = MAX_RECHECK_INTERVAL;
558 }
559 } else {
560 Self->_ParkEvent->park();
561 }
562
563 if (TryLock(Self) > 0) break;
564
565 // The lock is still contested.
566 // Keep a tally of the # of futile wakeups.
567 // Note that the counter is not protected by a lock or updated by atomics.
568 // That is by design - we trade "lossy" counters which are exposed to
569 // races during updates for a lower probe effect.
570
571 // This PerfData object can be used in parallel with a safepoint.
572 // See the work around in PerfDataManager::destroy().
573 OM_PERFDATA_OP(FutileWakeups, inc());
574 ++nWakeups;
575
576 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
577 // We can defer clearing _succ until after the spin completes
578 // TrySpin() must tolerate being called with _succ == Self.
579 // Try yet another round of adaptive spinning.
580 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
581
582 // We can find that we were unpark()ed and redesignated _succ while
583 // we were spinning. That's harmless. If we iterate and call park(),
584 // park() will consume the event and return immediately and we'll
585 // just spin again. This pattern can repeat, leaving _succ to simply
586 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
587 // Alternately, we can sample fired() here, and if set, forgo spinning
588 // in the next iteration.
589
590 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
591 Self->_ParkEvent->reset();
592 OrderAccess::fence();
593 }
594 if (_succ == Self) _succ = NULL;
595
596 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
597 OrderAccess::fence();
598 }
599
600 // Egress :
601 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
602 // Normally we'll find Self on the EntryList .
603 // From the perspective of the lock owner (this thread), the
604 // EntryList is stable and cxq is prepend-only.
605 // The head of cxq is volatile but the interior is stable.
606 // In addition, Self.TState is stable.
607
608 assert(_owner == Self, "invariant");
609 assert(object() != NULL, "invariant");
610 // I'd like to write:
611 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
612 // but as we're at a safepoint that's not safe.
613
614 UnlinkAfterAcquire(Self, &node);
615 if (_succ == Self) _succ = NULL;
616
617 assert(_succ != Self, "invariant");
618 if (_Responsible == Self) {
619 _Responsible = NULL;
620 OrderAccess::fence(); // Dekker pivot-point
621
622 // We may leave threads on cxq|EntryList without a designated
623 // "Responsible" thread. This is benign. When this thread subsequently
624 // exits the monitor it can "see" such preexisting "old" threads --
625 // threads that arrived on the cxq|EntryList before the fence, above --
626 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
627 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
628 // non-null and elect a new "Responsible" timer thread.
629 //
630 // This thread executes:
631 // ST Responsible=null; MEMBAR (in enter epilogue - here)
632 // LD cxq|EntryList (in subsequent exit)
633 //
634 // Entering threads in the slow/contended path execute:
635 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
636 // The (ST cxq; MEMBAR) is accomplished with CAS().
637 //
638 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
639 // exit operation from floating above the ST Responsible=null.
640 }
641
642 // We've acquired ownership with CAS().
643 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
644 // But since the CAS() this thread may have also stored into _succ,
645 // EntryList, cxq or Responsible. These meta-data updates must be
646 // visible __before this thread subsequently drops the lock.
647 // Consider what could occur if we didn't enforce this constraint --
648 // STs to monitor meta-data and user-data could reorder with (become
649 // visible after) the ST in exit that drops ownership of the lock.
650 // Some other thread could then acquire the lock, but observe inconsistent
651 // or old monitor meta-data and heap data. That violates the JMM.
652 // To that end, the 1-0 exit() operation must have at least STST|LDST
653 // "release" barrier semantics. Specifically, there must be at least a
654 // STST|LDST barrier in exit() before the ST of null into _owner that drops
655 // the lock. The barrier ensures that changes to monitor meta-data and data
656 // protected by the lock will be visible before we release the lock, and
657 // therefore before some other thread (CPU) has a chance to acquire the lock.
658 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
659 //
660 // Critically, any prior STs to _succ or EntryList must be visible before
661 // the ST of null into _owner in the *subsequent* (following) corresponding
662 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
663 // execute a serializing instruction.
664
665 return;
666 }
667
668 // ReenterI() is a specialized inline form of the latter half of the
669 // contended slow-path from EnterI(). We use ReenterI() only for
670 // monitor reentry in wait().
671 //
672 // In the future we should reconcile EnterI() and ReenterI(), adding
673 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
674 // loop accordingly.
675
676 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
677 assert(Self != NULL, "invariant");
678 assert(SelfNode != NULL, "invariant");
679 assert(SelfNode->_thread == Self, "invariant");
680 assert(_waiters > 0, "invariant");
681 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
682 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
683 JavaThread * jt = (JavaThread *) Self;
684
685 int nWakeups = 0;
686 for (;;) {
687 ObjectWaiter::TStates v = SelfNode->TState;
688 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
689 assert(_owner != Self, "invariant");
690
691 if (TryLock(Self) > 0) break;
692 if (TrySpin(Self) > 0) break;
693
694 // State transition wrappers around park() ...
695 // ReenterI() wisely defers state transitions until
696 // it's clear we must park the thread.
697 {
698 OSThreadContendState osts(Self->osthread());
699 ThreadBlockInVM tbivm(jt);
700
701 // cleared by handle_special_suspend_equivalent_condition()
702 // or java_suspend_self()
703 jt->set_suspend_equivalent();
704 Self->_ParkEvent->park();
705
706 // were we externally suspended while we were waiting?
707 for (;;) {
708 if (!ExitSuspendEquivalent(jt)) break;
709 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
710 jt->java_suspend_self();
711 jt->set_suspend_equivalent();
712 }
713 }
714
715 // Try again, but just so we distinguish between futile wakeups and
716 // successful wakeups. The following test isn't algorithmically
717 // necessary, but it helps us maintain sensible statistics.
718 if (TryLock(Self) > 0) break;
719
720 // The lock is still contested.
721 // Keep a tally of the # of futile wakeups.
722 // Note that the counter is not protected by a lock or updated by atomics.
723 // That is by design - we trade "lossy" counters which are exposed to
724 // races during updates for a lower probe effect.
725 ++nWakeups;
726
727 // Assuming this is not a spurious wakeup we'll normally
728 // find that _succ == Self.
729 if (_succ == Self) _succ = NULL;
730
731 // Invariant: after clearing _succ a contending thread
732 // *must* retry _owner before parking.
733 OrderAccess::fence();
734
735 // This PerfData object can be used in parallel with a safepoint.
736 // See the work around in PerfDataManager::destroy().
737 OM_PERFDATA_OP(FutileWakeups, inc());
738 }
739
740 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
741 // Normally we'll find Self on the EntryList.
742 // Unlinking from the EntryList is constant-time and atomic-free.
743 // From the perspective of the lock owner (this thread), the
744 // EntryList is stable and cxq is prepend-only.
745 // The head of cxq is volatile but the interior is stable.
746 // In addition, Self.TState is stable.
747
748 assert(_owner == Self, "invariant");
749 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
750 UnlinkAfterAcquire(Self, SelfNode);
751 if (_succ == Self) _succ = NULL;
752 assert(_succ != Self, "invariant");
753 SelfNode->TState = ObjectWaiter::TS_RUN;
754 OrderAccess::fence(); // see comments at the end of EnterI()
755 }
756
757 // By convention we unlink a contending thread from EntryList|cxq immediately
758 // after the thread acquires the lock in ::enter(). Equally, we could defer
759 // unlinking the thread until ::exit()-time.
760
761 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
762 assert(_owner == Self, "invariant");
763 assert(SelfNode->_thread == Self, "invariant");
764
765 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
766 // Normal case: remove Self from the DLL EntryList .
767 // This is a constant-time operation.
768 ObjectWaiter * nxt = SelfNode->_next;
769 ObjectWaiter * prv = SelfNode->_prev;
770 if (nxt != NULL) nxt->_prev = prv;
771 if (prv != NULL) prv->_next = nxt;
772 if (SelfNode == _EntryList) _EntryList = nxt;
773 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
774 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
775 } else {
776 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
777 // Inopportune interleaving -- Self is still on the cxq.
778 // This usually means the enqueue of self raced an exiting thread.
779 // Normally we'll find Self near the front of the cxq, so
780 // dequeueing is typically fast. If needbe we can accelerate
781 // this with some MCS/CHL-like bidirectional list hints and advisory
782 // back-links so dequeueing from the interior will normally operate
783 // in constant-time.
784 // Dequeue Self from either the head (with CAS) or from the interior
785 // with a linear-time scan and normal non-atomic memory operations.
786 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
787 // and then unlink Self from EntryList. We have to drain eventually,
788 // so it might as well be now.
789
790 ObjectWaiter * v = _cxq;
791 assert(v != NULL, "invariant");
792 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
793 // The CAS above can fail from interference IFF a "RAT" arrived.
794 // In that case Self must be in the interior and can no longer be
795 // at the head of cxq.
796 if (v == SelfNode) {
797 assert(_cxq != v, "invariant");
798 v = _cxq; // CAS above failed - start scan at head of list
799 }
800 ObjectWaiter * p;
801 ObjectWaiter * q = NULL;
802 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
803 q = p;
804 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
805 }
806 assert(v != SelfNode, "invariant");
807 assert(p == SelfNode, "Node not found on cxq");
808 assert(p != _cxq, "invariant");
809 assert(q != NULL, "invariant");
810 assert(q->_next == p, "invariant");
811 q->_next = p->_next;
812 }
813 }
814
815 #ifdef ASSERT
816 // Diagnostic hygiene ...
817 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
818 SelfNode->_next = (ObjectWaiter *) 0xBAD;
819 SelfNode->TState = ObjectWaiter::TS_RUN;
820 #endif
821 }
822
823 // -----------------------------------------------------------------------------
824 // Exit support
825 //
826 // exit()
827 // ~~~~~~
828 // Note that the collector can't reclaim the objectMonitor or deflate
829 // the object out from underneath the thread calling ::exit() as the
830 // thread calling ::exit() never transitions to a stable state.
831 // This inhibits GC, which in turn inhibits asynchronous (and
832 // inopportune) reclamation of "this".
833 //
834 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
835 // There's one exception to the claim above, however. EnterI() can call
836 // exit() to drop a lock if the acquirer has been externally suspended.
837 // In that case exit() is called with _thread_state as _thread_blocked,
838 // but the monitor's _count field is > 0, which inhibits reclamation.
839 //
840 // 1-0 exit
841 // ~~~~~~~~
842 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
843 // the fast-path operators have been optimized so the common ::exit()
844 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
845 // The code emitted by fast_unlock() elides the usual MEMBAR. This
846 // greatly improves latency -- MEMBAR and CAS having considerable local
847 // latency on modern processors -- but at the cost of "stranding". Absent the
848 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
849 // ::enter() path, resulting in the entering thread being stranding
850 // and a progress-liveness failure. Stranding is extremely rare.
851 // We use timers (timed park operations) & periodic polling to detect
852 // and recover from stranding. Potentially stranded threads periodically
853 // wake up and poll the lock. See the usage of the _Responsible variable.
854 //
855 // The CAS() in enter provides for safety and exclusion, while the CAS or
856 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
857 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
858 // We detect and recover from stranding with timers.
859 //
860 // If a thread transiently strands it'll park until (a) another
861 // thread acquires the lock and then drops the lock, at which time the
862 // exiting thread will notice and unpark the stranded thread, or, (b)
863 // the timer expires. If the lock is high traffic then the stranding latency
864 // will be low due to (a). If the lock is low traffic then the odds of
865 // stranding are lower, although the worst-case stranding latency
866 // is longer. Critically, we don't want to put excessive load in the
867 // platform's timer subsystem. We want to minimize both the timer injection
868 // rate (timers created/sec) as well as the number of timers active at
869 // any one time. (more precisely, we want to minimize timer-seconds, which is
870 // the integral of the # of active timers at any instant over time).
871 // Both impinge on OS scalability. Given that, at most one thread parked on
872 // a monitor will use a timer.
873 //
874 // There is also the risk of a futile wake-up. If we drop the lock
875 // another thread can reacquire the lock immediately, and we can
876 // then wake a thread unnecessarily. This is benign, and we've
877 // structured the code so the windows are short and the frequency
878 // of such futile wakups is low.
879
880 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
881 Thread * const Self = THREAD;
882 if (THREAD != _owner) {
883 if (THREAD->is_lock_owned((address) _owner)) {
884 // Transmute _owner from a BasicLock pointer to a Thread address.
885 // We don't need to hold _mutex for this transition.
886 // Non-null to Non-null is safe as long as all readers can
887 // tolerate either flavor.
888 assert(_recursions == 0, "invariant");
889 _owner = THREAD;
890 _recursions = 0;
891 } else {
892 // Apparent unbalanced locking ...
893 // Naively we'd like to throw IllegalMonitorStateException.
894 // As a practical matter we can neither allocate nor throw an
895 // exception as ::exit() can be called from leaf routines.
896 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
897 // Upon deeper reflection, however, in a properly run JVM the only
898 // way we should encounter this situation is in the presence of
899 // unbalanced JNI locking. TODO: CheckJNICalls.
900 // See also: CR4414101
901 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
902 return;
903 }
904 }
905
906 if (_recursions != 0) {
907 _recursions--; // this is simple recursive enter
908 return;
909 }
910
911 // Invariant: after setting Responsible=null an thread must execute
912 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
913 _Responsible = NULL;
914
915 #if INCLUDE_JFR
916 // get the owner's thread id for the MonitorEnter event
917 // if it is enabled and the thread isn't suspended
918 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
919 _previous_owner_tid = JFR_THREAD_ID(Self);
920 }
921 #endif
922
923 for (;;) {
924 assert(THREAD == _owner, "invariant");
925
926 if (Knob_ExitPolicy == 0) {
927 // release semantics: prior loads and stores from within the critical section
928 // must not float (reorder) past the following store that drops the lock.
929 // On SPARC that requires MEMBAR #loadstore|#storestore.
930 // But of course in TSO #loadstore|#storestore is not required.
931 // I'd like to write one of the following:
932 // A. OrderAccess::release() ; _owner = NULL
933 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
934 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
935 // store into a _dummy variable. That store is not needed, but can result
936 // in massive wasteful coherency traffic on classic SMP systems.
937 // Instead, I use release_store(), which is implemented as just a simple
938 // ST on x64, x86 and SPARC.
939 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
940 OrderAccess::storeload(); // See if we need to wake a successor
941 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
942 return;
943 }
944 // Other threads are blocked trying to acquire the lock.
945
946 // Normally the exiting thread is responsible for ensuring succession,
947 // but if other successors are ready or other entering threads are spinning
948 // then this thread can simply store NULL into _owner and exit without
949 // waking a successor. The existence of spinners or ready successors
950 // guarantees proper succession (liveness). Responsibility passes to the
951 // ready or running successors. The exiting thread delegates the duty.
952 // More precisely, if a successor already exists this thread is absolved
953 // of the responsibility of waking (unparking) one.
954 //
955 // The _succ variable is critical to reducing futile wakeup frequency.
956 // _succ identifies the "heir presumptive" thread that has been made
957 // ready (unparked) but that has not yet run. We need only one such
958 // successor thread to guarantee progress.
959 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
960 // section 3.3 "Futile Wakeup Throttling" for details.
961 //
962 // Note that spinners in Enter() also set _succ non-null.
963 // In the current implementation spinners opportunistically set
964 // _succ so that exiting threads might avoid waking a successor.
965 // Another less appealing alternative would be for the exiting thread
966 // to drop the lock and then spin briefly to see if a spinner managed
967 // to acquire the lock. If so, the exiting thread could exit
968 // immediately without waking a successor, otherwise the exiting
969 // thread would need to dequeue and wake a successor.
970 // (Note that we'd need to make the post-drop spin short, but no
971 // shorter than the worst-case round-trip cache-line migration time.
972 // The dropped lock needs to become visible to the spinner, and then
973 // the acquisition of the lock by the spinner must become visible to
974 // the exiting thread).
975
976 // It appears that an heir-presumptive (successor) must be made ready.
977 // Only the current lock owner can manipulate the EntryList or
978 // drain _cxq, so we need to reacquire the lock. If we fail
979 // to reacquire the lock the responsibility for ensuring succession
980 // falls to the new owner.
981 //
982 if (!Atomic::replace_if_null(THREAD, &_owner)) {
983 return;
984 }
985 } else {
986 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
987 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
988 OrderAccess::storeload();
989 // Ratify the previously observed values.
990 if (_cxq == NULL || _succ != NULL) {
991 return;
992 }
993
994 // inopportune interleaving -- the exiting thread (this thread)
995 // in the fast-exit path raced an entering thread in the slow-enter
996 // path.
997 // We have two choices:
998 // A. Try to reacquire the lock.
999 // If the CAS() fails return immediately, otherwise
1000 // we either restart/rerun the exit operation, or simply
1001 // fall-through into the code below which wakes a successor.
1002 // B. If the elements forming the EntryList|cxq are TSM
1003 // we could simply unpark() the lead thread and return
1004 // without having set _succ.
1005 if (!Atomic::replace_if_null(THREAD, &_owner)) {
1006 return;
1007 }
1008 }
1009 }
1010
1011 guarantee(_owner == THREAD, "invariant");
1012
1013 ObjectWaiter * w = NULL;
1014 int QMode = Knob_QMode;
1015
1016 if (QMode == 2 && _cxq != NULL) {
1017 // QMode == 2 : cxq has precedence over EntryList.
1018 // Try to directly wake a successor from the cxq.
1019 // If successful, the successor will need to unlink itself from cxq.
1020 w = _cxq;
1021 assert(w != NULL, "invariant");
1022 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1023 ExitEpilog(Self, w);
1024 return;
1025 }
1026
1027 if (QMode == 3 && _cxq != NULL) {
1028 // Aggressively drain cxq into EntryList at the first opportunity.
1029 // This policy ensure that recently-run threads live at the head of EntryList.
1030 // Drain _cxq into EntryList - bulk transfer.
1031 // First, detach _cxq.
1032 // The following loop is tantamount to: w = swap(&cxq, NULL)
1033 w = _cxq;
1034 for (;;) {
1035 assert(w != NULL, "Invariant");
1036 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1037 if (u == w) break;
1038 w = u;
1039 }
1040 assert(w != NULL, "invariant");
1041
1042 ObjectWaiter * q = NULL;
1043 ObjectWaiter * p;
1044 for (p = w; p != NULL; p = p->_next) {
1045 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1046 p->TState = ObjectWaiter::TS_ENTER;
1047 p->_prev = q;
1048 q = p;
1049 }
1050
1051 // Append the RATs to the EntryList
1052 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1053 ObjectWaiter * Tail;
1054 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1055 Tail = Tail->_next)
1056 /* empty */;
1057 if (Tail == NULL) {
1058 _EntryList = w;
1059 } else {
1060 Tail->_next = w;
1061 w->_prev = Tail;
1062 }
1063
1064 // Fall thru into code that tries to wake a successor from EntryList
1065 }
1066
1067 if (QMode == 4 && _cxq != NULL) {
1068 // Aggressively drain cxq into EntryList at the first opportunity.
1069 // This policy ensure that recently-run threads live at the head of EntryList.
1070
1071 // Drain _cxq into EntryList - bulk transfer.
1072 // First, detach _cxq.
1073 // The following loop is tantamount to: w = swap(&cxq, NULL)
1074 w = _cxq;
1075 for (;;) {
1076 assert(w != NULL, "Invariant");
1077 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1078 if (u == w) break;
1079 w = u;
1080 }
1081 assert(w != NULL, "invariant");
1082
1083 ObjectWaiter * q = NULL;
1084 ObjectWaiter * p;
1085 for (p = w; p != NULL; p = p->_next) {
1086 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1087 p->TState = ObjectWaiter::TS_ENTER;
1088 p->_prev = q;
1089 q = p;
1090 }
1091
1092 // Prepend the RATs to the EntryList
1093 if (_EntryList != NULL) {
1094 q->_next = _EntryList;
1095 _EntryList->_prev = q;
1096 }
1097 _EntryList = w;
1098
1099 // Fall thru into code that tries to wake a successor from EntryList
1100 }
1101
1102 w = _EntryList;
1103 if (w != NULL) {
1104 // I'd like to write: guarantee (w->_thread != Self).
1105 // But in practice an exiting thread may find itself on the EntryList.
1106 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1107 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1108 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1109 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1110 // release the lock "O". T2 resumes immediately after the ST of null into
1111 // _owner, above. T2 notices that the EntryList is populated, so it
1112 // reacquires the lock and then finds itself on the EntryList.
1113 // Given all that, we have to tolerate the circumstance where "w" is
1114 // associated with Self.
1115 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1116 ExitEpilog(Self, w);
1117 return;
1118 }
1119
1120 // If we find that both _cxq and EntryList are null then just
1121 // re-run the exit protocol from the top.
1122 w = _cxq;
1123 if (w == NULL) continue;
1124
1125 // Drain _cxq into EntryList - bulk transfer.
1126 // First, detach _cxq.
1127 // The following loop is tantamount to: w = swap(&cxq, NULL)
1128 for (;;) {
1129 assert(w != NULL, "Invariant");
1130 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1131 if (u == w) break;
1132 w = u;
1133 }
1134
1135 assert(w != NULL, "invariant");
1136 assert(_EntryList == NULL, "invariant");
1137
1138 // Convert the LIFO SLL anchored by _cxq into a DLL.
1139 // The list reorganization step operates in O(LENGTH(w)) time.
1140 // It's critical that this step operate quickly as
1141 // "Self" still holds the outer-lock, restricting parallelism
1142 // and effectively lengthening the critical section.
1143 // Invariant: s chases t chases u.
1144 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1145 // we have faster access to the tail.
1146
1147 if (QMode == 1) {
1148 // QMode == 1 : drain cxq to EntryList, reversing order
1149 // We also reverse the order of the list.
1150 ObjectWaiter * s = NULL;
1151 ObjectWaiter * t = w;
1152 ObjectWaiter * u = NULL;
1153 while (t != NULL) {
1154 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1155 t->TState = ObjectWaiter::TS_ENTER;
1156 u = t->_next;
1157 t->_prev = u;
1158 t->_next = s;
1159 s = t;
1160 t = u;
1161 }
1162 _EntryList = s;
1163 assert(s != NULL, "invariant");
1164 } else {
1165 // QMode == 0 or QMode == 2
1166 _EntryList = w;
1167 ObjectWaiter * q = NULL;
1168 ObjectWaiter * p;
1169 for (p = w; p != NULL; p = p->_next) {
1170 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1171 p->TState = ObjectWaiter::TS_ENTER;
1172 p->_prev = q;
1173 q = p;
1174 }
1175 }
1176
1177 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1178 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1179
1180 // See if we can abdicate to a spinner instead of waking a thread.
1181 // A primary goal of the implementation is to reduce the
1182 // context-switch rate.
1183 if (_succ != NULL) continue;
1184
1185 w = _EntryList;
1186 if (w != NULL) {
1187 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1188 ExitEpilog(Self, w);
1189 return;
1190 }
1191 }
1192 }
1193
1194 // ExitSuspendEquivalent:
1195 // A faster alternate to handle_special_suspend_equivalent_condition()
1196 //
1197 // handle_special_suspend_equivalent_condition() unconditionally
1198 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1199 // operations have high latency. Note that in ::enter() we call HSSEC
1200 // while holding the monitor, so we effectively lengthen the critical sections.
1201 //
1202 // There are a number of possible solutions:
1203 //
1204 // A. To ameliorate the problem we might also defer state transitions
1205 // to as late as possible -- just prior to parking.
1206 // Given that, we'd call HSSEC after having returned from park(),
1207 // but before attempting to acquire the monitor. This is only a
1208 // partial solution. It avoids calling HSSEC while holding the
1209 // monitor (good), but it still increases successor reacquisition latency --
1210 // the interval between unparking a successor and the time the successor
1211 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1212 // If we use this technique we can also avoid EnterI()-exit() loop
1213 // in ::enter() where we iteratively drop the lock and then attempt
1214 // to reacquire it after suspending.
1215 //
1216 // B. In the future we might fold all the suspend bits into a
1217 // composite per-thread suspend flag and then update it with CAS().
1218 // Alternately, a Dekker-like mechanism with multiple variables
1219 // would suffice:
1220 // ST Self->_suspend_equivalent = false
1221 // MEMBAR
1222 // LD Self_>_suspend_flags
1223 //
1224 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1225 // with a more efficient implementation, the need to use "FastHSSEC" has
1226 // decreased. - Dave
1227
1228
1229 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1230 const int Mode = Knob_FastHSSEC;
1231 if (Mode && !jSelf->is_external_suspend()) {
1232 assert(jSelf->is_suspend_equivalent(), "invariant");
1233 jSelf->clear_suspend_equivalent();
1234 if (2 == Mode) OrderAccess::storeload();
1235 if (!jSelf->is_external_suspend()) return false;
1236 // We raced a suspension -- fall thru into the slow path
1237 jSelf->set_suspend_equivalent();
1238 }
1239 return jSelf->handle_special_suspend_equivalent_condition();
1240 }
1241
1242
1243 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1244 assert(_owner == Self, "invariant");
1245
1246 // Exit protocol:
1247 // 1. ST _succ = wakee
1248 // 2. membar #loadstore|#storestore;
1249 // 2. ST _owner = NULL
1250 // 3. unpark(wakee)
1251
1252 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1253 ParkEvent * Trigger = Wakee->_event;
1254
1255 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1256 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1257 // out-of-scope (non-extant).
1258 Wakee = NULL;
1259
1260 // Drop the lock
1261 OrderAccess::release_store(&_owner, (void*)NULL);
1262 OrderAccess::fence(); // ST _owner vs LD in unpark()
1263
1264 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1265 Trigger->unpark();
1266
1267 // Maintain stats and report events to JVMTI
1268 OM_PERFDATA_OP(Parks, inc());
1269 }
1270
1271
1272 // -----------------------------------------------------------------------------
1273 // Class Loader deadlock handling.
1274 //
1275 // complete_exit exits a lock returning recursion count
1276 // complete_exit/reenter operate as a wait without waiting
1277 // complete_exit requires an inflated monitor
1278 // The _owner field is not always the Thread addr even with an
1279 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1280 // thread due to contention.
1281 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1282 Thread * const Self = THREAD;
1283 assert(Self->is_Java_thread(), "Must be Java thread!");
1284 JavaThread *jt = (JavaThread *)THREAD;
1285
1286 DeferredInitialize();
1287
1288 if (THREAD != _owner) {
1289 if (THREAD->is_lock_owned ((address)_owner)) {
1290 assert(_recursions == 0, "internal state error");
1291 _owner = THREAD; // Convert from basiclock addr to Thread addr
1292 _recursions = 0;
1293 }
1294 }
1295
1296 guarantee(Self == _owner, "complete_exit not owner");
1297 intptr_t save = _recursions; // record the old recursion count
1298 _recursions = 0; // set the recursion level to be 0
1299 exit(true, Self); // exit the monitor
1300 guarantee(_owner != Self, "invariant");
1301 return save;
1302 }
1303
1304 // reenter() enters a lock and sets recursion count
1305 // complete_exit/reenter operate as a wait without waiting
1306 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1307 Thread * const Self = THREAD;
1308 assert(Self->is_Java_thread(), "Must be Java thread!");
1309 JavaThread *jt = (JavaThread *)THREAD;
1310
1311 guarantee(_owner != Self, "reenter already owner");
1312 enter(THREAD); // enter the monitor
1313 guarantee(_recursions == 0, "reenter recursion");
1314 _recursions = recursions;
1315 return;
1316 }
1317
1318
1319 // -----------------------------------------------------------------------------
1320 // A macro is used below because there may already be a pending
1321 // exception which should not abort the execution of the routines
1322 // which use this (which is why we don't put this into check_slow and
1323 // call it with a CHECK argument).
1324
1325 #define CHECK_OWNER() \
1326 do { \
1327 if (THREAD != _owner) { \
1328 if (THREAD->is_lock_owned((address) _owner)) { \
1329 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1330 _recursions = 0; \
1331 } else { \
1332 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1333 } \
1334 } \
1335 } while (false)
1336
1337 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1338 // TODO-FIXME: remove check_slow() -- it's likely dead.
1339
1340 void ObjectMonitor::check_slow(TRAPS) {
1341 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1342 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1343 }
1344
1345 static int Adjust(volatile int * adr, int dx) {
1346 int v;
1347 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1348 return v;
1349 }
1350
1351 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1352 ObjectMonitor* monitor,
1353 jlong notifier_tid,
1354 jlong timeout,
1355 bool timedout) {
1356 assert(event != NULL, "invariant");
1357 assert(monitor != NULL, "invariant");
1358 event->set_monitorClass(((oop)monitor->object())->klass());
1359 event->set_timeout(timeout);
1360 event->set_address((uintptr_t)monitor->object_addr());
1361 event->set_notifier(notifier_tid);
1362 event->set_timedOut(timedout);
1363 event->commit();
1364 }
1365
1366 // -----------------------------------------------------------------------------
1367 // Wait/Notify/NotifyAll
1368 //
1369 // Note: a subset of changes to ObjectMonitor::wait()
1370 // will need to be replicated in complete_exit
1371 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1372 Thread * const Self = THREAD;
1373 assert(Self->is_Java_thread(), "Must be Java thread!");
1374 JavaThread *jt = (JavaThread *)THREAD;
1375
1376 DeferredInitialize();
1377
1378 // Throw IMSX or IEX.
1379 CHECK_OWNER();
1380
1381 EventJavaMonitorWait event;
1382
1383 // check for a pending interrupt
1384 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1385 // post monitor waited event. Note that this is past-tense, we are done waiting.
1386 if (JvmtiExport::should_post_monitor_waited()) {
1387 // Note: 'false' parameter is passed here because the
1388 // wait was not timed out due to thread interrupt.
1389 JvmtiExport::post_monitor_waited(jt, this, false);
1390
1391 // In this short circuit of the monitor wait protocol, the
1392 // current thread never drops ownership of the monitor and
1393 // never gets added to the wait queue so the current thread
1394 // cannot be made the successor. This means that the
1395 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1396 // consume an unpark() meant for the ParkEvent associated with
1397 // this ObjectMonitor.
1398 }
1399 if (event.should_commit()) {
1400 post_monitor_wait_event(&event, this, 0, millis, false);
1401 }
1402 THROW(vmSymbols::java_lang_InterruptedException());
1403 return;
1404 }
1405
1406 assert(Self->_Stalled == 0, "invariant");
1407 Self->_Stalled = intptr_t(this);
1408 jt->set_current_waiting_monitor(this);
1409
1410 // create a node to be put into the queue
1411 // Critically, after we reset() the event but prior to park(), we must check
1412 // for a pending interrupt.
1413 ObjectWaiter node(Self);
1414 node.TState = ObjectWaiter::TS_WAIT;
1415 Self->_ParkEvent->reset();
1416 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1417
1418 // Enter the waiting queue, which is a circular doubly linked list in this case
1419 // but it could be a priority queue or any data structure.
1420 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1421 // by the the owner of the monitor *except* in the case where park()
1422 // returns because of a timeout of interrupt. Contention is exceptionally rare
1423 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1424
1425 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1426 AddWaiter(&node);
1427 Thread::SpinRelease(&_WaitSetLock);
1428
1429 _Responsible = NULL;
1430
1431 intptr_t save = _recursions; // record the old recursion count
1432 _waiters++; // increment the number of waiters
1433 _recursions = 0; // set the recursion level to be 1
1434 exit(true, Self); // exit the monitor
1435 guarantee(_owner != Self, "invariant");
1436
1437 // The thread is on the WaitSet list - now park() it.
1438 // On MP systems it's conceivable that a brief spin before we park
1439 // could be profitable.
1440 //
1441 // TODO-FIXME: change the following logic to a loop of the form
1442 // while (!timeout && !interrupted && _notified == 0) park()
1443
1444 int ret = OS_OK;
1445 int WasNotified = 0;
1446 { // State transition wrappers
1447 OSThread* osthread = Self->osthread();
1448 OSThreadWaitState osts(osthread, true);
1449 {
1450 ThreadBlockInVM tbivm(jt);
1451 // Thread is in thread_blocked state and oop access is unsafe.
1452 jt->set_suspend_equivalent();
1453
1454 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1455 // Intentionally empty
1456 } else if (node._notified == 0) {
1457 if (millis <= 0) {
1458 Self->_ParkEvent->park();
1459 } else {
1460 ret = Self->_ParkEvent->park(millis);
1461 }
1462 }
1463
1464 // were we externally suspended while we were waiting?
1465 if (ExitSuspendEquivalent (jt)) {
1466 // TODO-FIXME: add -- if succ == Self then succ = null.
1467 jt->java_suspend_self();
1468 }
1469
1470 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1471
1472 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1473 // from the WaitSet to the EntryList.
1474 // See if we need to remove Node from the WaitSet.
1475 // We use double-checked locking to avoid grabbing _WaitSetLock
1476 // if the thread is not on the wait queue.
1477 //
1478 // Note that we don't need a fence before the fetch of TState.
1479 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1480 // written by the is thread. (perhaps the fetch might even be satisfied
1481 // by a look-aside into the processor's own store buffer, although given
1482 // the length of the code path between the prior ST and this load that's
1483 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1484 // then we'll acquire the lock and then re-fetch a fresh TState value.
1485 // That is, we fail toward safety.
1486
1487 if (node.TState == ObjectWaiter::TS_WAIT) {
1488 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1489 if (node.TState == ObjectWaiter::TS_WAIT) {
1490 DequeueSpecificWaiter(&node); // unlink from WaitSet
1491 assert(node._notified == 0, "invariant");
1492 node.TState = ObjectWaiter::TS_RUN;
1493 }
1494 Thread::SpinRelease(&_WaitSetLock);
1495 }
1496
1497 // The thread is now either on off-list (TS_RUN),
1498 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1499 // The Node's TState variable is stable from the perspective of this thread.
1500 // No other threads will asynchronously modify TState.
1501 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1502 OrderAccess::loadload();
1503 if (_succ == Self) _succ = NULL;
1504 WasNotified = node._notified;
1505
1506 // Reentry phase -- reacquire the monitor.
1507 // re-enter contended monitor after object.wait().
1508 // retain OBJECT_WAIT state until re-enter successfully completes
1509 // Thread state is thread_in_vm and oop access is again safe,
1510 // although the raw address of the object may have changed.
1511 // (Don't cache naked oops over safepoints, of course).
1512
1513 // post monitor waited event. Note that this is past-tense, we are done waiting.
1514 if (JvmtiExport::should_post_monitor_waited()) {
1515 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1516
1517 if (node._notified != 0 && _succ == Self) {
1518 // In this part of the monitor wait-notify-reenter protocol it
1519 // is possible (and normal) for another thread to do a fastpath
1520 // monitor enter-exit while this thread is still trying to get
1521 // to the reenter portion of the protocol.
1522 //
1523 // The ObjectMonitor was notified and the current thread is
1524 // the successor which also means that an unpark() has already
1525 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1526 // consume the unpark() that was done when the successor was
1527 // set because the same ParkEvent is shared between Java
1528 // monitors and JVM/TI RawMonitors (for now).
1529 //
1530 // We redo the unpark() to ensure forward progress, i.e., we
1531 // don't want all pending threads hanging (parked) with none
1532 // entering the unlocked monitor.
1533 node._event->unpark();
1534 }
1535 }
1536
1537 if (event.should_commit()) {
1538 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1539 }
1540
1541 OrderAccess::fence();
1542
1543 assert(Self->_Stalled != 0, "invariant");
1544 Self->_Stalled = 0;
1545
1546 assert(_owner != Self, "invariant");
1547 ObjectWaiter::TStates v = node.TState;
1548 if (v == ObjectWaiter::TS_RUN) {
1549 enter(Self);
1550 } else {
1551 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1552 ReenterI(Self, &node);
1553 node.wait_reenter_end(this);
1554 }
1555
1556 // Self has reacquired the lock.
1557 // Lifecycle - the node representing Self must not appear on any queues.
1558 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1559 // want residual elements associated with this thread left on any lists.
1560 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1561 assert(_owner == Self, "invariant");
1562 assert(_succ != Self, "invariant");
1563 } // OSThreadWaitState()
1564
1565 jt->set_current_waiting_monitor(NULL);
1566
1567 guarantee(_recursions == 0, "invariant");
1568 _recursions = save; // restore the old recursion count
1569 _waiters--; // decrement the number of waiters
1570
1571 // Verify a few postconditions
1572 assert(_owner == Self, "invariant");
1573 assert(_succ != Self, "invariant");
1574 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1575
1576 // check if the notification happened
1577 if (!WasNotified) {
1578 // no, it could be timeout or Thread.interrupt() or both
1579 // check for interrupt event, otherwise it is timeout
1580 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1581 THROW(vmSymbols::java_lang_InterruptedException());
1582 }
1583 }
1584
1585 // NOTE: Spurious wake up will be consider as timeout.
1586 // Monitor notify has precedence over thread interrupt.
1587 }
1588
1589
1590 // Consider:
1591 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1592 // then instead of transferring a thread from the WaitSet to the EntryList
1593 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1594
1595 void ObjectMonitor::INotify(Thread * Self) {
1596 const int policy = Knob_MoveNotifyee;
1597
1598 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1599 ObjectWaiter * iterator = DequeueWaiter();
1600 if (iterator != NULL) {
1601 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1602 guarantee(iterator->_notified == 0, "invariant");
1603 // Disposition - what might we do with iterator ?
1604 // a. add it directly to the EntryList - either tail (policy == 1)
1605 // or head (policy == 0).
1606 // b. push it onto the front of the _cxq (policy == 2).
1607 // For now we use (b).
1608 if (policy != 4) {
1609 iterator->TState = ObjectWaiter::TS_ENTER;
1610 }
1611 iterator->_notified = 1;
1612 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1613
1614 ObjectWaiter * list = _EntryList;
1615 if (list != NULL) {
1616 assert(list->_prev == NULL, "invariant");
1617 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1618 assert(list != iterator, "invariant");
1619 }
1620
1621 if (policy == 0) { // prepend to EntryList
1622 if (list == NULL) {
1623 iterator->_next = iterator->_prev = NULL;
1624 _EntryList = iterator;
1625 } else {
1626 list->_prev = iterator;
1627 iterator->_next = list;
1628 iterator->_prev = NULL;
1629 _EntryList = iterator;
1630 }
1631 } else if (policy == 1) { // append to EntryList
1632 if (list == NULL) {
1633 iterator->_next = iterator->_prev = NULL;
1634 _EntryList = iterator;
1635 } else {
1636 // CONSIDER: finding the tail currently requires a linear-time walk of
1637 // the EntryList. We can make tail access constant-time by converting to
1638 // a CDLL instead of using our current DLL.
1639 ObjectWaiter * tail;
1640 for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1641 assert(tail != NULL && tail->_next == NULL, "invariant");
1642 tail->_next = iterator;
1643 iterator->_prev = tail;
1644 iterator->_next = NULL;
1645 }
1646 } else if (policy == 2) { // prepend to cxq
1647 if (list == NULL) {
1648 iterator->_next = iterator->_prev = NULL;
1649 _EntryList = iterator;
1650 } else {
1651 iterator->TState = ObjectWaiter::TS_CXQ;
1652 for (;;) {
1653 ObjectWaiter * front = _cxq;
1654 iterator->_next = front;
1655 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1656 break;
1657 }
1658 }
1659 }
1660 } else if (policy == 3) { // append to cxq
1661 iterator->TState = ObjectWaiter::TS_CXQ;
1662 for (;;) {
1663 ObjectWaiter * tail = _cxq;
1664 if (tail == NULL) {
1665 iterator->_next = NULL;
1666 if (Atomic::replace_if_null(iterator, &_cxq)) {
1667 break;
1668 }
1669 } else {
1670 while (tail->_next != NULL) tail = tail->_next;
1671 tail->_next = iterator;
1672 iterator->_prev = tail;
1673 iterator->_next = NULL;
1674 break;
1675 }
1676 }
1677 } else {
1678 ParkEvent * ev = iterator->_event;
1679 iterator->TState = ObjectWaiter::TS_RUN;
1680 OrderAccess::fence();
1681 ev->unpark();
1682 }
1683
1684 // _WaitSetLock protects the wait queue, not the EntryList. We could
1685 // move the add-to-EntryList operation, above, outside the critical section
1686 // protected by _WaitSetLock. In practice that's not useful. With the
1687 // exception of wait() timeouts and interrupts the monitor owner
1688 // is the only thread that grabs _WaitSetLock. There's almost no contention
1689 // on _WaitSetLock so it's not profitable to reduce the length of the
1690 // critical section.
1691
1692 if (policy < 4) {
1693 iterator->wait_reenter_begin(this);
1694 }
1695 }
1696 Thread::SpinRelease(&_WaitSetLock);
1697 }
1698
1699 // Consider: a not-uncommon synchronization bug is to use notify() when
1700 // notifyAll() is more appropriate, potentially resulting in stranded
1701 // threads; this is one example of a lost wakeup. A useful diagnostic
1702 // option is to force all notify() operations to behave as notifyAll().
1703 //
1704 // Note: We can also detect many such problems with a "minimum wait".
1705 // When the "minimum wait" is set to a small non-zero timeout value
1706 // and the program does not hang whereas it did absent "minimum wait",
1707 // that suggests a lost wakeup bug.
1708
1709 void ObjectMonitor::notify(TRAPS) {
1710 CHECK_OWNER();
1711 if (_WaitSet == NULL) {
1712 return;
1713 }
1714 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1715 INotify(THREAD);
1716 OM_PERFDATA_OP(Notifications, inc(1));
1717 }
1718
1719
1720 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1721 // from the waitset to the EntryList. This could be done more efficiently with a
1722 // single bulk transfer but in practice it's not time-critical. Beware too,
1723 // that in prepend-mode we invert the order of the waiters. Let's say that the
1724 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1725 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1726
1727 void ObjectMonitor::notifyAll(TRAPS) {
1728 CHECK_OWNER();
1729 if (_WaitSet == NULL) {
1730 return;
1731 }
1732
1733 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1734 int tally = 0;
1735 while (_WaitSet != NULL) {
1736 tally++;
1737 INotify(THREAD);
1738 }
1739
1740 OM_PERFDATA_OP(Notifications, inc(tally));
1741 }
1742
1743 // -----------------------------------------------------------------------------
1744 // Adaptive Spinning Support
1745 //
1746 // Adaptive spin-then-block - rational spinning
1747 //
1748 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1749 // algorithm. On high order SMP systems it would be better to start with
1750 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1751 // a contending thread could enqueue itself on the cxq and then spin locally
1752 // on a thread-specific variable such as its ParkEvent._Event flag.
1753 // That's left as an exercise for the reader. Note that global spinning is
1754 // not problematic on Niagara, as the L2 cache serves the interconnect and
1755 // has both low latency and massive bandwidth.
1756 //
1757 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1758 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1759 // (duration) or we can fix the count at approximately the duration of
1760 // a context switch and vary the frequency. Of course we could also
1761 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1762 // For a description of 'Adaptive spin-then-block mutual exclusion in
1763 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1764 //
1765 // This implementation varies the duration "D", where D varies with
1766 // the success rate of recent spin attempts. (D is capped at approximately
1767 // length of a round-trip context switch). The success rate for recent
1768 // spin attempts is a good predictor of the success rate of future spin
1769 // attempts. The mechanism adapts automatically to varying critical
1770 // section length (lock modality), system load and degree of parallelism.
1771 // D is maintained per-monitor in _SpinDuration and is initialized
1772 // optimistically. Spin frequency is fixed at 100%.
1773 //
1774 // Note that _SpinDuration is volatile, but we update it without locks
1775 // or atomics. The code is designed so that _SpinDuration stays within
1776 // a reasonable range even in the presence of races. The arithmetic
1777 // operations on _SpinDuration are closed over the domain of legal values,
1778 // so at worst a race will install and older but still legal value.
1779 // At the very worst this introduces some apparent non-determinism.
1780 // We might spin when we shouldn't or vice-versa, but since the spin
1781 // count are relatively short, even in the worst case, the effect is harmless.
1782 //
1783 // Care must be taken that a low "D" value does not become an
1784 // an absorbing state. Transient spinning failures -- when spinning
1785 // is overall profitable -- should not cause the system to converge
1786 // on low "D" values. We want spinning to be stable and predictable
1787 // and fairly responsive to change and at the same time we don't want
1788 // it to oscillate, become metastable, be "too" non-deterministic,
1789 // or converge on or enter undesirable stable absorbing states.
1790 //
1791 // We implement a feedback-based control system -- using past behavior
1792 // to predict future behavior. We face two issues: (a) if the
1793 // input signal is random then the spin predictor won't provide optimal
1794 // results, and (b) if the signal frequency is too high then the control
1795 // system, which has some natural response lag, will "chase" the signal.
1796 // (b) can arise from multimodal lock hold times. Transient preemption
1797 // can also result in apparent bimodal lock hold times.
1798 // Although sub-optimal, neither condition is particularly harmful, as
1799 // in the worst-case we'll spin when we shouldn't or vice-versa.
1800 // The maximum spin duration is rather short so the failure modes aren't bad.
1801 // To be conservative, I've tuned the gain in system to bias toward
1802 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1803 // "rings" or oscillates between spinning and not spinning. This happens
1804 // when spinning is just on the cusp of profitability, however, so the
1805 // situation is not dire. The state is benign -- there's no need to add
1806 // hysteresis control to damp the transition rate between spinning and
1807 // not spinning.
1808
1809 // Spinning: Fixed frequency (100%), vary duration
1810 int ObjectMonitor::TrySpin(Thread * Self) {
1811 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1812 int ctr = Knob_FixedSpin;
1813 if (ctr != 0) {
1814 while (--ctr >= 0) {
1815 if (TryLock(Self) > 0) return 1;
1816 SpinPause();
1817 }
1818 return 0;
1819 }
1820
1821 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1822 if (TryLock(Self) > 0) {
1823 // Increase _SpinDuration ...
1824 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1825 // Raising _SpurDuration to the poverty line is key.
1826 int x = _SpinDuration;
1827 if (x < Knob_SpinLimit) {
1828 if (x < Knob_Poverty) x = Knob_Poverty;
1829 _SpinDuration = x + Knob_BonusB;
1830 }
1831 return 1;
1832 }
1833 SpinPause();
1834 }
1835
1836 // Admission control - verify preconditions for spinning
1837 //
1838 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1839 // becoming an absorbing state. Put another way, we spin briefly to
1840 // sample, just in case the system load, parallelism, contention, or lock
1841 // modality changed.
1842 //
1843 // Consider the following alternative:
1844 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1845 // spin attempt. "Periodically" might mean after a tally of
1846 // the # of failed spin attempts (or iterations) reaches some threshold.
1847 // This takes us into the realm of 1-out-of-N spinning, where we
1848 // hold the duration constant but vary the frequency.
1849
1850 ctr = _SpinDuration;
1851 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1852 if (ctr <= 0) return 0;
1853
1854 if (Knob_SuccRestrict && _succ != NULL) return 0;
1855 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1856 return 0;
1857 }
1858
1859 int MaxSpin = Knob_MaxSpinners;
1860 if (MaxSpin >= 0) {
1861 if (_Spinner > MaxSpin) {
1862 return 0;
1863 }
1864 // Slightly racy, but benign ...
1865 Adjust(&_Spinner, 1);
1866 }
1867
1868 // We're good to spin ... spin ingress.
1869 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1870 // when preparing to LD...CAS _owner, etc and the CAS is likely
1871 // to succeed.
1872 int caspty = Knob_CASPenalty;
1873 int oxpty = Knob_OXPenalty;
1874 int sss = Knob_SpinSetSucc;
1875 if (sss && _succ == NULL) _succ = Self;
1876 Thread * prv = NULL;
1877
1878 // There are three ways to exit the following loop:
1879 // 1. A successful spin where this thread has acquired the lock.
1880 // 2. Spin failure with prejudice
1881 // 3. Spin failure without prejudice
1882
1883 while (--ctr >= 0) {
1884
1885 // Periodic polling -- Check for pending GC
1886 // Threads may spin while they're unsafe.
1887 // We don't want spinning threads to delay the JVM from reaching
1888 // a stop-the-world safepoint or to steal cycles from GC.
1889 // If we detect a pending safepoint we abort in order that
1890 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1891 // this thread, if safe, doesn't steal cycles from GC.
1892 // This is in keeping with the "no loitering in runtime" rule.
1893 // We periodically check to see if there's a safepoint pending.
1894 if ((ctr & 0xFF) == 0) {
1895 if (SafepointMechanism::poll(Self)) {
1896 goto Abort; // abrupt spin egress
1897 }
1898 if (Knob_UsePause & 1) SpinPause();
1899 }
1900
1901 if (Knob_UsePause & 2) SpinPause();
1902
1903 // Probe _owner with TATAS
1904 // If this thread observes the monitor transition or flicker
1905 // from locked to unlocked to locked, then the odds that this
1906 // thread will acquire the lock in this spin attempt go down
1907 // considerably. The same argument applies if the CAS fails
1908 // or if we observe _owner change from one non-null value to
1909 // another non-null value. In such cases we might abort
1910 // the spin without prejudice or apply a "penalty" to the
1911 // spin count-down variable "ctr", reducing it by 100, say.
1912
1913 Thread * ox = (Thread *) _owner;
1914 if (ox == NULL) {
1915 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1916 if (ox == NULL) {
1917 // The CAS succeeded -- this thread acquired ownership
1918 // Take care of some bookkeeping to exit spin state.
1919 if (sss && _succ == Self) {
1920 _succ = NULL;
1921 }
1922 if (MaxSpin > 0) Adjust(&_Spinner, -1);
1923
1924 // Increase _SpinDuration :
1925 // The spin was successful (profitable) so we tend toward
1926 // longer spin attempts in the future.
1927 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1928 // If we acquired the lock early in the spin cycle it
1929 // makes sense to increase _SpinDuration proportionally.
1930 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1931 int x = _SpinDuration;
1932 if (x < Knob_SpinLimit) {
1933 if (x < Knob_Poverty) x = Knob_Poverty;
1934 _SpinDuration = x + Knob_Bonus;
1935 }
1936 return 1;
1937 }
1938
1939 // The CAS failed ... we can take any of the following actions:
1940 // * penalize: ctr -= Knob_CASPenalty
1941 // * exit spin with prejudice -- goto Abort;
1942 // * exit spin without prejudice.
1943 // * Since CAS is high-latency, retry again immediately.
1944 prv = ox;
1945 if (caspty == -2) break;
1946 if (caspty == -1) goto Abort;
1947 ctr -= caspty;
1948 continue;
1949 }
1950
1951 // Did lock ownership change hands ?
1952 if (ox != prv && prv != NULL) {
1953 if (oxpty == -2) break;
1954 if (oxpty == -1) goto Abort;
1955 ctr -= oxpty;
1956 }
1957 prv = ox;
1958
1959 // Abort the spin if the owner is not executing.
1960 // The owner must be executing in order to drop the lock.
1961 // Spinning while the owner is OFFPROC is idiocy.
1962 // Consider: ctr -= RunnablePenalty ;
1963 if (Knob_OState && NotRunnable (Self, ox)) {
1964 goto Abort;
1965 }
1966 if (sss && _succ == NULL) _succ = Self;
1967 }
1968
1969 // Spin failed with prejudice -- reduce _SpinDuration.
1970 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1971 // AIMD is globally stable.
1972 {
1973 int x = _SpinDuration;
1974 if (x > 0) {
1975 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1976 // This is globally sample and tends to damp the response.
1977 x -= Knob_Penalty;
1978 if (x < 0) x = 0;
1979 _SpinDuration = x;
1980 }
1981 }
1982
1983 Abort:
1984 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
1985 if (sss && _succ == Self) {
1986 _succ = NULL;
1987 // Invariant: after setting succ=null a contending thread
1988 // must recheck-retry _owner before parking. This usually happens
1989 // in the normal usage of TrySpin(), but it's safest
1990 // to make TrySpin() as foolproof as possible.
1991 OrderAccess::fence();
1992 if (TryLock(Self) > 0) return 1;
1993 }
1994 return 0;
1995 }
1996
1997 // NotRunnable() -- informed spinning
1998 //
1999 // Don't bother spinning if the owner is not eligible to drop the lock.
2000 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2001 // The thread must be runnable in order to drop the lock in timely fashion.
2002 // If the _owner is not runnable then spinning will not likely be
2003 // successful (profitable).
2004 //
2005 // Beware -- the thread referenced by _owner could have died
2006 // so a simply fetch from _owner->_thread_state might trap.
2007 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2008 // Because of the lifecycle issues, the _thread_state values
2009 // observed by NotRunnable() might be garbage. NotRunnable must
2010 // tolerate this and consider the observed _thread_state value
2011 // as advisory.
2012 //
2013 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2014 // a thread pointer.
2015 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2016 // with the LSB of _owner. Another option would be to probabilistically probe
2017 // the putative _owner->TypeTag value.
2018 //
2019 // Checking _thread_state isn't perfect. Even if the thread is
2020 // in_java it might be blocked on a page-fault or have been preempted
2021 // and sitting on a ready/dispatch queue.
2022 //
2023 // The return value from NotRunnable() is *advisory* -- the
2024 // result is based on sampling and is not necessarily coherent.
2025 // The caller must tolerate false-negative and false-positive errors.
2026 // Spinning, in general, is probabilistic anyway.
2027
2028
2029 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2030 // Check ox->TypeTag == 2BAD.
2031 if (ox == NULL) return 0;
2032
2033 // Avoid transitive spinning ...
2034 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2035 // Immediately after T1 acquires L it's possible that T2, also
2036 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2037 // This occurs transiently after T1 acquired L but before
2038 // T1 managed to clear T1.Stalled. T2 does not need to abort
2039 // its spin in this circumstance.
2040 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2041
2042 if (BlockedOn == 1) return 1;
2043 if (BlockedOn != 0) {
2044 return BlockedOn != intptr_t(this) && _owner == ox;
2045 }
2046
2047 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2048 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2049 // consider also: jst != _thread_in_Java -- but that's overspecific.
2050 return jst == _thread_blocked || jst == _thread_in_native;
2051 }
2052
2053
2054 // -----------------------------------------------------------------------------
2055 // WaitSet management ...
2056
2057 ObjectWaiter::ObjectWaiter(Thread* thread) {
2058 _next = NULL;
2059 _prev = NULL;
2060 _notified = 0;
2061 _notifier_tid = 0;
2062 TState = TS_RUN;
2063 _thread = thread;
2064 _event = thread->_ParkEvent;
2065 _active = false;
2066 assert(_event != NULL, "invariant");
2067 }
2068
2069 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2070 JavaThread *jt = (JavaThread *)this->_thread;
2071 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2072 }
2073
2074 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2075 JavaThread *jt = (JavaThread *)this->_thread;
2076 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2077 }
2078
2079 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2080 assert(node != NULL, "should not add NULL node");
2081 assert(node->_prev == NULL, "node already in list");
2082 assert(node->_next == NULL, "node already in list");
2083 // put node at end of queue (circular doubly linked list)
2084 if (_WaitSet == NULL) {
2085 _WaitSet = node;
2086 node->_prev = node;
2087 node->_next = node;
2088 } else {
2089 ObjectWaiter* head = _WaitSet;
2090 ObjectWaiter* tail = head->_prev;
2091 assert(tail->_next == head, "invariant check");
2092 tail->_next = node;
2093 head->_prev = node;
2094 node->_next = head;
2095 node->_prev = tail;
2096 }
2097 }
2098
2099 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2100 // dequeue the very first waiter
2101 ObjectWaiter* waiter = _WaitSet;
2102 if (waiter) {
2103 DequeueSpecificWaiter(waiter);
2104 }
2105 return waiter;
2106 }
2107
2108 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2109 assert(node != NULL, "should not dequeue NULL node");
2110 assert(node->_prev != NULL, "node already removed from list");
2111 assert(node->_next != NULL, "node already removed from list");
2112 // when the waiter has woken up because of interrupt,
2113 // timeout or other spurious wake-up, dequeue the
2114 // waiter from waiting list
2115 ObjectWaiter* next = node->_next;
2116 if (next == node) {
2117 assert(node->_prev == node, "invariant check");
2118 _WaitSet = NULL;
2119 } else {
2120 ObjectWaiter* prev = node->_prev;
2121 assert(prev->_next == node, "invariant check");
2122 assert(next->_prev == node, "invariant check");
2123 next->_prev = prev;
2124 prev->_next = next;
2125 if (_WaitSet == node) {
2126 _WaitSet = next;
2127 }
2128 }
2129 node->_next = NULL;
2130 node->_prev = NULL;
2131 }
2132
2133 // -----------------------------------------------------------------------------
2134 // PerfData support
2135 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2136 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2137 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2138 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2139 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2140 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2141 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2142
2143 // One-shot global initialization for the sync subsystem.
2144 // We could also defer initialization and initialize on-demand
2145 // the first time we call inflate(). Initialization would
2146 // be protected - like so many things - by the MonitorCache_lock.
2147
2148 void ObjectMonitor::Initialize() {
2149 static int InitializationCompleted = 0;
2150 assert(InitializationCompleted == 0, "invariant");
2151 InitializationCompleted = 1;
2152 if (UsePerfData) {
2153 EXCEPTION_MARK;
2154 #define NEWPERFCOUNTER(n) \
2155 { \
2156 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2157 CHECK); \
2158 }
2159 #define NEWPERFVARIABLE(n) \
2160 { \
2161 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2162 CHECK); \
2163 }
2164 NEWPERFCOUNTER(_sync_Inflations);
2165 NEWPERFCOUNTER(_sync_Deflations);
2166 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2167 NEWPERFCOUNTER(_sync_FutileWakeups);
2168 NEWPERFCOUNTER(_sync_Parks);
2169 NEWPERFCOUNTER(_sync_Notifications);
2170 NEWPERFVARIABLE(_sync_MonExtant);
2171 #undef NEWPERFCOUNTER
2172 #undef NEWPERFVARIABLE
2173 }
2174 }
2175
2176 void ObjectMonitor::DeferredInitialize() {
2177 if (InitDone > 0) return;
2178 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2179 while (InitDone != 1) /* empty */;
2180 return;
2181 }
2182
2183 // One-shot global initialization ...
2184 // The initialization is idempotent, so we don't need locks.
2185 // In the future consider doing this via os::init_2().
2186
2187 if (!os::is_MP()) {
2188 Knob_SpinLimit = 0;
2189 Knob_SpinBase = 0;
2190 Knob_PreSpin = 0;
2191 Knob_FixedSpin = -1;
2192 }
2193
2194 OrderAccess::fence();
2195 InitDone = 1;
2196 }
2197