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