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