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