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