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