1 /*
2 * Copyright (c) 1998, 2009, 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 "incls/_precompiled.incl"
26 # include "incls/_objectMonitor.cpp.incl"
27
28 #if defined(__GNUC__) && !defined(IA64)
29 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
30 #define ATTR __attribute__((noinline))
31 #else
32 #define ATTR
33 #endif
34
35
36 #ifdef DTRACE_ENABLED
37
38 // Only bother with this argument setup if dtrace is available
39 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
40
41 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
42 jlong, uintptr_t, char*, int);
43 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
44 jlong, uintptr_t, char*, int);
45 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
46 jlong, uintptr_t, char*, int);
47 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
48 jlong, uintptr_t, char*, int);
49 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
50 jlong, uintptr_t, char*, int);
51
52 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \
53 char* bytes = NULL; \
54 int len = 0; \
55 jlong jtid = SharedRuntime::get_java_tid(thread); \
56 symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
57 if (klassname != NULL) { \
58 bytes = (char*)klassname->bytes(); \
59 len = klassname->utf8_length(); \
60 }
61
62 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
63 { \
64 if (DTraceMonitorProbes) { \
65 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
66 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
67 (monitor), bytes, len, (millis)); \
68 } \
69 }
70
71 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
72 { \
73 if (DTraceMonitorProbes) { \
74 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
75 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
76 (uintptr_t)(monitor), bytes, len); \
77 } \
78 }
79
80 #else // ndef DTRACE_ENABLED
81
82 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
83 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;}
84
85 #endif // ndef DTRACE_ENABLED
86
87 // Tunables ...
88 // The knob* variables are effectively final. Once set they should
89 // never be modified hence. Consider using __read_mostly with GCC.
90
91 int ObjectMonitor::Knob_Verbose = 0 ;
92 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
93 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
94 static int Knob_HandOff = 0 ;
95 static int Knob_ReportSettings = 0 ;
96
97 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
98 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
99 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
100 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
101 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
102 static int Knob_SpinEarly = 1 ;
103 static int Knob_SuccEnabled = 1 ; // futile wake throttling
104 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
105 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
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_SpinAfterFutile = 1 ; // Spin after returning from park()
111 static int Knob_FixedSpin = 0 ;
112 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
113 static int Knob_UsePause = 1 ;
114 static int Knob_ExitPolicy = 0 ;
115 static int Knob_PreSpin = 10 ; // 20-100 likely better
116 static int Knob_ResetEvent = 0 ;
117 static int BackOffMask = 0 ;
118
119 static int Knob_FastHSSEC = 0 ;
120 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
121 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
122 static volatile int InitDone = 0 ;
123
124 #define TrySpin TrySpin_VaryDuration
125
126 // -----------------------------------------------------------------------------
127 // Theory of operations -- Monitors lists, thread residency, etc:
128 //
129 // * A thread acquires ownership of a monitor by successfully
130 // CAS()ing the _owner field from null to non-null.
131 //
132 // * Invariant: A thread appears on at most one monitor list --
133 // cxq, EntryList or WaitSet -- at any one time.
134 //
135 // * Contending threads "push" themselves onto the cxq with CAS
136 // and then spin/park.
137 //
138 // * After a contending thread eventually acquires the lock it must
139 // dequeue itself from either the EntryList or the cxq.
140 //
141 // * The exiting thread identifies and unparks an "heir presumptive"
142 // tentative successor thread on the EntryList. Critically, the
143 // exiting thread doesn't unlink the successor thread from the EntryList.
144 // After having been unparked, the wakee will recontend for ownership of
145 // the monitor. The successor (wakee) will either acquire the lock or
146 // re-park itself.
147 //
148 // Succession is provided for by a policy of competitive handoff.
149 // The exiting thread does _not_ grant or pass ownership to the
150 // successor thread. (This is also referred to as "handoff" succession").
151 // Instead the exiting thread releases ownership and possibly wakes
152 // a successor, so the successor can (re)compete for ownership of the lock.
153 // If the EntryList is empty but the cxq is populated the exiting
154 // thread will drain the cxq into the EntryList. It does so by
155 // by detaching the cxq (installing null with CAS) and folding
156 // the threads from the cxq into the EntryList. The EntryList is
157 // doubly linked, while the cxq is singly linked because of the
158 // CAS-based "push" used to enqueue recently arrived threads (RATs).
159 //
160 // * Concurrency invariants:
161 //
162 // -- only the monitor owner may access or mutate the EntryList.
163 // The mutex property of the monitor itself protects the EntryList
164 // from concurrent interference.
165 // -- Only the monitor owner may detach the cxq.
166 //
167 // * The monitor entry list operations avoid locks, but strictly speaking
168 // they're not lock-free. Enter is lock-free, exit is not.
169 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
170 //
171 // * The cxq can have multiple concurrent "pushers" but only one concurrent
172 // detaching thread. This mechanism is immune from the ABA corruption.
173 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
174 //
175 // * Taken together, the cxq and the EntryList constitute or form a
176 // single logical queue of threads stalled trying to acquire the lock.
177 // We use two distinct lists to improve the odds of a constant-time
178 // dequeue operation after acquisition (in the ::enter() epilog) and
179 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
180 // A key desideratum is to minimize queue & monitor metadata manipulation
181 // that occurs while holding the monitor lock -- that is, we want to
182 // minimize monitor lock holds times. Note that even a small amount of
183 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
184 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
185 // locks and monitor metadata.
186 //
187 // Cxq points to the the set of Recently Arrived Threads attempting entry.
188 // Because we push threads onto _cxq with CAS, the RATs must take the form of
189 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
190 // the unlocking thread notices that EntryList is null but _cxq is != null.
191 //
192 // The EntryList is ordered by the prevailing queue discipline and
193 // can be organized in any convenient fashion, such as a doubly-linked list or
194 // a circular doubly-linked list. Critically, we want insert and delete operations
195 // to operate in constant-time. If we need a priority queue then something akin
196 // to Solaris' sleepq would work nicely. Viz.,
197 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
198 // Queue discipline is enforced at ::exit() time, when the unlocking thread
199 // drains the cxq into the EntryList, and orders or reorders the threads on the
200 // EntryList accordingly.
201 //
202 // Barring "lock barging", this mechanism provides fair cyclic ordering,
203 // somewhat similar to an elevator-scan.
204 //
205 // * The monitor synchronization subsystem avoids the use of native
206 // synchronization primitives except for the narrow platform-specific
207 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
208 // the semantics of park-unpark. Put another way, this monitor implementation
209 // depends only on atomic operations and park-unpark. The monitor subsystem
210 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
211 // underlying OS manages the READY<->RUN transitions.
212 //
213 // * Waiting threads reside on the WaitSet list -- wait() puts
214 // the caller onto the WaitSet.
215 //
216 // * notify() or notifyAll() simply transfers threads from the WaitSet to
217 // either the EntryList or cxq. Subsequent exit() operations will
218 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
219 // it's likely the notifyee would simply impale itself on the lock held
220 // by the notifier.
221 //
222 // * An interesting alternative is to encode cxq as (List,LockByte) where
223 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
224 // variable, like _recursions, in the scheme. The threads or Events that form
225 // the list would have to be aligned in 256-byte addresses. A thread would
226 // try to acquire the lock or enqueue itself with CAS, but exiting threads
227 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
228 // Note that is is *not* word-tearing, but it does presume that full-word
229 // CAS operations are coherent with intermix with STB operations. That's true
230 // on most common processors.
231 //
232 // * See also http://blogs.sun.com/dave
233
234
235 // -----------------------------------------------------------------------------
236 // Enter support
237
238 bool ObjectMonitor::try_enter(Thread* THREAD) {
239 if (THREAD != _owner) {
240 if (THREAD->is_lock_owned ((address)_owner)) {
241 assert(_recursions == 0, "internal state error");
242 _owner = THREAD ;
243 _recursions = 1 ;
244 OwnerIsThread = 1 ;
245 return true;
246 }
247 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
248 return false;
249 }
250 return true;
251 } else {
252 _recursions++;
253 return true;
254 }
255 }
256
257 void ATTR ObjectMonitor::enter(TRAPS) {
258 // The following code is ordered to check the most common cases first
259 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
260 Thread * const Self = THREAD ;
261 void * cur ;
262
263 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
264 if (cur == NULL) {
265 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
266 assert (_recursions == 0 , "invariant") ;
267 assert (_owner == Self, "invariant") ;
268 // CONSIDER: set or assert OwnerIsThread == 1
269 return ;
270 }
271
272 if (cur == Self) {
273 // TODO-FIXME: check for integer overflow! BUGID 6557169.
274 _recursions ++ ;
275 return ;
276 }
277
278 if (Self->is_lock_owned ((address)cur)) {
279 assert (_recursions == 0, "internal state error");
280 _recursions = 1 ;
281 // Commute owner from a thread-specific on-stack BasicLockObject address to
282 // a full-fledged "Thread *".
283 _owner = Self ;
284 OwnerIsThread = 1 ;
285 return ;
286 }
287
288 // We've encountered genuine contention.
289 assert (Self->_Stalled == 0, "invariant") ;
290 Self->_Stalled = intptr_t(this) ;
291
292 // Try one round of spinning *before* enqueueing Self
293 // and before going through the awkward and expensive state
294 // transitions. The following spin is strictly optional ...
295 // Note that if we acquire the monitor from an initial spin
296 // we forgo posting JVMTI events and firing DTRACE probes.
297 if (Knob_SpinEarly && TrySpin (Self) > 0) {
298 assert (_owner == Self , "invariant") ;
299 assert (_recursions == 0 , "invariant") ;
300 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
301 Self->_Stalled = 0 ;
302 return ;
303 }
304
305 assert (_owner != Self , "invariant") ;
306 assert (_succ != Self , "invariant") ;
307 assert (Self->is_Java_thread() , "invariant") ;
308 JavaThread * jt = (JavaThread *) Self ;
309 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
310 assert (jt->thread_state() != _thread_blocked , "invariant") ;
311 assert (this->object() != NULL , "invariant") ;
312 assert (_count >= 0, "invariant") ;
313
314 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
315 // Ensure the object-monitor relationship remains stable while there's contention.
316 Atomic::inc_ptr(&_count);
317
318 { // Change java thread status to indicate blocked on monitor enter.
319 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
320
321 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
322 if (JvmtiExport::should_post_monitor_contended_enter()) {
323 JvmtiExport::post_monitor_contended_enter(jt, this);
324 }
325
326 OSThreadContendState osts(Self->osthread());
327 ThreadBlockInVM tbivm(jt);
328
329 Self->set_current_pending_monitor(this);
330
331 // TODO-FIXME: change the following for(;;) loop to straight-line code.
332 for (;;) {
333 jt->set_suspend_equivalent();
334 // cleared by handle_special_suspend_equivalent_condition()
335 // or java_suspend_self()
336
337 EnterI (THREAD) ;
338
339 if (!ExitSuspendEquivalent(jt)) break ;
340
341 //
342 // We have acquired the contended monitor, but while we were
343 // waiting another thread suspended us. We don't want to enter
344 // the monitor while suspended because that would surprise the
345 // thread that suspended us.
346 //
347 _recursions = 0 ;
348 _succ = NULL ;
349 exit (Self) ;
350
351 jt->java_suspend_self();
352 }
353 Self->set_current_pending_monitor(NULL);
354 }
355
356 Atomic::dec_ptr(&_count);
357 assert (_count >= 0, "invariant") ;
358 Self->_Stalled = 0 ;
359
360 // Must either set _recursions = 0 or ASSERT _recursions == 0.
361 assert (_recursions == 0 , "invariant") ;
362 assert (_owner == Self , "invariant") ;
363 assert (_succ != Self , "invariant") ;
364 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
365
366 // The thread -- now the owner -- is back in vm mode.
367 // Report the glorious news via TI,DTrace and jvmstat.
368 // The probe effect is non-trivial. All the reportage occurs
369 // while we hold the monitor, increasing the length of the critical
370 // section. Amdahl's parallel speedup law comes vividly into play.
371 //
372 // Another option might be to aggregate the events (thread local or
373 // per-monitor aggregation) and defer reporting until a more opportune
374 // time -- such as next time some thread encounters contention but has
375 // yet to acquire the lock. While spinning that thread could
376 // spinning we could increment JVMStat counters, etc.
377
378 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
379 if (JvmtiExport::should_post_monitor_contended_entered()) {
380 JvmtiExport::post_monitor_contended_entered(jt, this);
381 }
382 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
383 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
384 }
385 }
386
387
388 // Caveat: TryLock() is not necessarily serializing if it returns failure.
389 // Callers must compensate as needed.
390
391 int ObjectMonitor::TryLock (Thread * Self) {
392 for (;;) {
393 void * own = _owner ;
394 if (own != NULL) return 0 ;
395 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
396 // Either guarantee _recursions == 0 or set _recursions = 0.
397 assert (_recursions == 0, "invariant") ;
398 assert (_owner == Self, "invariant") ;
399 // CONSIDER: set or assert that OwnerIsThread == 1
400 return 1 ;
401 }
402 // The lock had been free momentarily, but we lost the race to the lock.
403 // Interference -- the CAS failed.
404 // We can either return -1 or retry.
405 // Retry doesn't make as much sense because the lock was just acquired.
406 if (true) return -1 ;
407 }
408 }
409
410 void ATTR ObjectMonitor::EnterI (TRAPS) {
411 Thread * Self = THREAD ;
412 assert (Self->is_Java_thread(), "invariant") ;
413 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
414
415 // Try the lock - TATAS
416 if (TryLock (Self) > 0) {
417 assert (_succ != Self , "invariant") ;
418 assert (_owner == Self , "invariant") ;
419 assert (_Responsible != Self , "invariant") ;
420 return ;
421 }
422
423 DeferredInitialize () ;
424
425 // We try one round of spinning *before* enqueueing Self.
426 //
427 // If the _owner is ready but OFFPROC we could use a YieldTo()
428 // operation to donate the remainder of this thread's quantum
429 // to the owner. This has subtle but beneficial affinity
430 // effects.
431
432 if (TrySpin (Self) > 0) {
433 assert (_owner == Self , "invariant") ;
434 assert (_succ != Self , "invariant") ;
435 assert (_Responsible != Self , "invariant") ;
436 return ;
437 }
438
439 // The Spin failed -- Enqueue and park the thread ...
440 assert (_succ != Self , "invariant") ;
441 assert (_owner != Self , "invariant") ;
442 assert (_Responsible != Self , "invariant") ;
443
444 // Enqueue "Self" on ObjectMonitor's _cxq.
445 //
446 // Node acts as a proxy for Self.
447 // As an aside, if were to ever rewrite the synchronization code mostly
448 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
449 // Java objects. This would avoid awkward lifecycle and liveness issues,
450 // as well as eliminate a subset of ABA issues.
451 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
452 //
453
454 ObjectWaiter node(Self) ;
455 Self->_ParkEvent->reset() ;
456 node._prev = (ObjectWaiter *) 0xBAD ;
457 node.TState = ObjectWaiter::TS_CXQ ;
458
459 // Push "Self" onto the front of the _cxq.
460 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
461 // Note that spinning tends to reduce the rate at which threads
462 // enqueue and dequeue on EntryList|cxq.
463 ObjectWaiter * nxt ;
464 for (;;) {
465 node._next = nxt = _cxq ;
466 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
467
468 // Interference - the CAS failed because _cxq changed. Just retry.
469 // As an optional optimization we retry the lock.
470 if (TryLock (Self) > 0) {
471 assert (_succ != Self , "invariant") ;
472 assert (_owner == Self , "invariant") ;
473 assert (_Responsible != Self , "invariant") ;
474 return ;
475 }
476 }
477
478 // Check for cxq|EntryList edge transition to non-null. This indicates
479 // the onset of contention. While contention persists exiting threads
480 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
481 // operations revert to the faster 1-0 mode. This enter operation may interleave
482 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
483 // arrange for one of the contending thread to use a timed park() operations
484 // to detect and recover from the race. (Stranding is form of progress failure
485 // where the monitor is unlocked but all the contending threads remain parked).
486 // That is, at least one of the contended threads will periodically poll _owner.
487 // One of the contending threads will become the designated "Responsible" thread.
488 // The Responsible thread uses a timed park instead of a normal indefinite park
489 // operation -- it periodically wakes and checks for and recovers from potential
490 // strandings admitted by 1-0 exit operations. We need at most one Responsible
491 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
492 // be responsible for a monitor.
493 //
494 // Currently, one of the contended threads takes on the added role of "Responsible".
495 // A viable alternative would be to use a dedicated "stranding checker" thread
496 // that periodically iterated over all the threads (or active monitors) and unparked
497 // successors where there was risk of stranding. This would help eliminate the
498 // timer scalability issues we see on some platforms as we'd only have one thread
499 // -- the checker -- parked on a timer.
500
501 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
502 // Try to assume the role of responsible thread for the monitor.
503 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
504 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
505 }
506
507 // The lock have been released while this thread was occupied queueing
508 // itself onto _cxq. To close the race and avoid "stranding" and
509 // progress-liveness failure we must resample-retry _owner before parking.
510 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
511 // In this case the ST-MEMBAR is accomplished with CAS().
512 //
513 // TODO: Defer all thread state transitions until park-time.
514 // Since state transitions are heavy and inefficient we'd like
515 // to defer the state transitions until absolutely necessary,
516 // and in doing so avoid some transitions ...
517
518 TEVENT (Inflated enter - Contention) ;
519 int nWakeups = 0 ;
520 int RecheckInterval = 1 ;
521
522 for (;;) {
523
524 if (TryLock (Self) > 0) break ;
525 assert (_owner != Self, "invariant") ;
526
527 if ((SyncFlags & 2) && _Responsible == NULL) {
528 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
529 }
530
531 // park self
532 if (_Responsible == Self || (SyncFlags & 1)) {
533 TEVENT (Inflated enter - park TIMED) ;
534 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
535 // Increase the RecheckInterval, but clamp the value.
536 RecheckInterval *= 8 ;
537 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
538 } else {
539 TEVENT (Inflated enter - park UNTIMED) ;
540 Self->_ParkEvent->park() ;
541 }
542
543 if (TryLock(Self) > 0) break ;
544
545 // The lock is still contested.
546 // Keep a tally of the # of futile wakeups.
547 // Note that the counter is not protected by a lock or updated by atomics.
548 // That is by design - we trade "lossy" counters which are exposed to
549 // races during updates for a lower probe effect.
550 TEVENT (Inflated enter - Futile wakeup) ;
551 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
552 ObjectMonitor::_sync_FutileWakeups->inc() ;
553 }
554 ++ nWakeups ;
555
556 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
557 // We can defer clearing _succ until after the spin completes
558 // TrySpin() must tolerate being called with _succ == Self.
559 // Try yet another round of adaptive spinning.
560 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
561
562 // We can find that we were unpark()ed and redesignated _succ while
563 // we were spinning. That's harmless. If we iterate and call park(),
564 // park() will consume the event and return immediately and we'll
565 // just spin again. This pattern can repeat, leaving _succ to simply
566 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
567 // Alternately, we can sample fired() here, and if set, forgo spinning
568 // in the next iteration.
569
570 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
571 Self->_ParkEvent->reset() ;
572 OrderAccess::fence() ;
573 }
574 if (_succ == Self) _succ = NULL ;
575
576 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
577 OrderAccess::fence() ;
578 }
579
580 // Egress :
581 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
582 // Normally we'll find Self on the EntryList .
583 // From the perspective of the lock owner (this thread), the
584 // EntryList is stable and cxq is prepend-only.
585 // The head of cxq is volatile but the interior is stable.
586 // In addition, Self.TState is stable.
587
588 assert (_owner == Self , "invariant") ;
589 assert (object() != NULL , "invariant") ;
590 // I'd like to write:
591 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
592 // but as we're at a safepoint that's not safe.
593
594 UnlinkAfterAcquire (Self, &node) ;
595 if (_succ == Self) _succ = NULL ;
596
597 assert (_succ != Self, "invariant") ;
598 if (_Responsible == Self) {
599 _Responsible = NULL ;
600 // Dekker pivot-point.
601 // Consider OrderAccess::storeload() here
602
603 // We may leave threads on cxq|EntryList without a designated
604 // "Responsible" thread. This is benign. When this thread subsequently
605 // exits the monitor it can "see" such preexisting "old" threads --
606 // threads that arrived on the cxq|EntryList before the fence, above --
607 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
608 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
609 // non-null and elect a new "Responsible" timer thread.
610 //
611 // This thread executes:
612 // ST Responsible=null; MEMBAR (in enter epilog - here)
613 // LD cxq|EntryList (in subsequent exit)
614 //
615 // Entering threads in the slow/contended path execute:
616 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
617 // The (ST cxq; MEMBAR) is accomplished with CAS().
618 //
619 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
620 // exit operation from floating above the ST Responsible=null.
621 //
622 // In *practice* however, EnterI() is always followed by some atomic
623 // operation such as the decrement of _count in ::enter(). Those atomics
624 // obviate the need for the explicit MEMBAR, above.
625 }
626
627 // We've acquired ownership with CAS().
628 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
629 // But since the CAS() this thread may have also stored into _succ,
630 // EntryList, cxq or Responsible. These meta-data updates must be
631 // visible __before this thread subsequently drops the lock.
632 // Consider what could occur if we didn't enforce this constraint --
633 // STs to monitor meta-data and user-data could reorder with (become
634 // visible after) the ST in exit that drops ownership of the lock.
635 // Some other thread could then acquire the lock, but observe inconsistent
636 // or old monitor meta-data and heap data. That violates the JMM.
637 // To that end, the 1-0 exit() operation must have at least STST|LDST
638 // "release" barrier semantics. Specifically, there must be at least a
639 // STST|LDST barrier in exit() before the ST of null into _owner that drops
640 // the lock. The barrier ensures that changes to monitor meta-data and data
641 // protected by the lock will be visible before we release the lock, and
642 // therefore before some other thread (CPU) has a chance to acquire the lock.
643 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
644 //
645 // Critically, any prior STs to _succ or EntryList must be visible before
646 // the ST of null into _owner in the *subsequent* (following) corresponding
647 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
648 // execute a serializing instruction.
649
650 if (SyncFlags & 8) {
651 OrderAccess::fence() ;
652 }
653 return ;
654 }
655
656 // ReenterI() is a specialized inline form of the latter half of the
657 // contended slow-path from EnterI(). We use ReenterI() only for
658 // monitor reentry in wait().
659 //
660 // In the future we should reconcile EnterI() and ReenterI(), adding
661 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
662 // loop accordingly.
663
664 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
665 assert (Self != NULL , "invariant") ;
666 assert (SelfNode != NULL , "invariant") ;
667 assert (SelfNode->_thread == Self , "invariant") ;
668 assert (_waiters > 0 , "invariant") ;
669 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
670 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
671 JavaThread * jt = (JavaThread *) Self ;
672
673 int nWakeups = 0 ;
674 for (;;) {
675 ObjectWaiter::TStates v = SelfNode->TState ;
676 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
677 assert (_owner != Self, "invariant") ;
678
679 if (TryLock (Self) > 0) break ;
680 if (TrySpin (Self) > 0) break ;
681
682 TEVENT (Wait Reentry - parking) ;
683
684 // State transition wrappers around park() ...
685 // ReenterI() wisely defers state transitions until
686 // it's clear we must park the thread.
687 {
688 OSThreadContendState osts(Self->osthread());
689 ThreadBlockInVM tbivm(jt);
690
691 // cleared by handle_special_suspend_equivalent_condition()
692 // or java_suspend_self()
693 jt->set_suspend_equivalent();
694 if (SyncFlags & 1) {
695 Self->_ParkEvent->park ((jlong)1000) ;
696 } else {
697 Self->_ParkEvent->park () ;
698 }
699
700 // were we externally suspended while we were waiting?
701 for (;;) {
702 if (!ExitSuspendEquivalent (jt)) break ;
703 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
704 jt->java_suspend_self();
705 jt->set_suspend_equivalent();
706 }
707 }
708
709 // Try again, but just so we distinguish between futile wakeups and
710 // successful wakeups. The following test isn't algorithmically
711 // necessary, but it helps us maintain sensible statistics.
712 if (TryLock(Self) > 0) break ;
713
714 // The lock is still contested.
715 // Keep a tally of the # of futile wakeups.
716 // Note that the counter is not protected by a lock or updated by atomics.
717 // That is by design - we trade "lossy" counters which are exposed to
718 // races during updates for a lower probe effect.
719 TEVENT (Wait Reentry - futile wakeup) ;
720 ++ nWakeups ;
721
722 // Assuming this is not a spurious wakeup we'll normally
723 // find that _succ == Self.
724 if (_succ == Self) _succ = NULL ;
725
726 // Invariant: after clearing _succ a contending thread
727 // *must* retry _owner before parking.
728 OrderAccess::fence() ;
729
730 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
731 ObjectMonitor::_sync_FutileWakeups->inc() ;
732 }
733 }
734
735 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
736 // Normally we'll find Self on the EntryList.
737 // Unlinking from the EntryList is constant-time and atomic-free.
738 // From the perspective of the lock owner (this thread), the
739 // EntryList is stable and cxq is prepend-only.
740 // The head of cxq is volatile but the interior is stable.
741 // In addition, Self.TState is stable.
742
743 assert (_owner == Self, "invariant") ;
744 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
745 UnlinkAfterAcquire (Self, SelfNode) ;
746 if (_succ == Self) _succ = NULL ;
747 assert (_succ != Self, "invariant") ;
748 SelfNode->TState = ObjectWaiter::TS_RUN ;
749 OrderAccess::fence() ; // see comments at the end of EnterI()
750 }
751
752 // after the thread acquires the lock in ::enter(). Equally, we could defer
753 // unlinking the thread until ::exit()-time.
754
755 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
756 {
757 assert (_owner == Self, "invariant") ;
758 assert (SelfNode->_thread == Self, "invariant") ;
759
760 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
761 // Normal case: remove Self from the DLL EntryList .
762 // This is a constant-time operation.
763 ObjectWaiter * nxt = SelfNode->_next ;
764 ObjectWaiter * prv = SelfNode->_prev ;
765 if (nxt != NULL) nxt->_prev = prv ;
766 if (prv != NULL) prv->_next = nxt ;
767 if (SelfNode == _EntryList ) _EntryList = nxt ;
768 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
769 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
770 TEVENT (Unlink from EntryList) ;
771 } else {
772 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
773 // Inopportune interleaving -- Self is still on the cxq.
774 // This usually means the enqueue of self raced an exiting thread.
775 // Normally we'll find Self near the front of the cxq, so
776 // dequeueing is typically fast. If needbe we can accelerate
777 // this with some MCS/CHL-like bidirectional list hints and advisory
778 // back-links so dequeueing from the interior will normally operate
779 // in constant-time.
780 // Dequeue Self from either the head (with CAS) or from the interior
781 // with a linear-time scan and normal non-atomic memory operations.
782 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
783 // and then unlink Self from EntryList. We have to drain eventually,
784 // so it might as well be now.
785
786 ObjectWaiter * v = _cxq ;
787 assert (v != NULL, "invariant") ;
788 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
789 // The CAS above can fail from interference IFF a "RAT" arrived.
790 // In that case Self must be in the interior and can no longer be
791 // at the head of cxq.
792 if (v == SelfNode) {
793 assert (_cxq != v, "invariant") ;
794 v = _cxq ; // CAS above failed - start scan at head of list
795 }
796 ObjectWaiter * p ;
797 ObjectWaiter * q = NULL ;
798 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
799 q = p ;
800 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
801 }
802 assert (v != SelfNode, "invariant") ;
803 assert (p == SelfNode, "Node not found on cxq") ;
804 assert (p != _cxq, "invariant") ;
805 assert (q != NULL, "invariant") ;
806 assert (q->_next == p, "invariant") ;
807 q->_next = p->_next ;
808 }
809 TEVENT (Unlink from cxq) ;
810 }
811
812 // Diagnostic hygiene ...
813 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
814 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
815 SelfNode->TState = ObjectWaiter::TS_RUN ;
816 }
817
818 // -----------------------------------------------------------------------------
819 // Exit support
820 //
821 // exit()
822 // ~~~~~~
823 // Note that the collector can't reclaim the objectMonitor or deflate
824 // the object out from underneath the thread calling ::exit() as the
825 // thread calling ::exit() never transitions to a stable state.
826 // This inhibits GC, which in turn inhibits asynchronous (and
827 // inopportune) reclamation of "this".
828 //
829 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
830 // There's one exception to the claim above, however. EnterI() can call
831 // exit() to drop a lock if the acquirer has been externally suspended.
832 // In that case exit() is called with _thread_state as _thread_blocked,
833 // but the monitor's _count field is > 0, which inhibits reclamation.
834 //
835 // 1-0 exit
836 // ~~~~~~~~
837 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
838 // the fast-path operators have been optimized so the common ::exit()
839 // operation is 1-0. See i486.ad fast_unlock(), for instance.
840 // The code emitted by fast_unlock() elides the usual MEMBAR. This
841 // greatly improves latency -- MEMBAR and CAS having considerable local
842 // latency on modern processors -- but at the cost of "stranding". Absent the
843 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
844 // ::enter() path, resulting in the entering thread being stranding
845 // and a progress-liveness failure. Stranding is extremely rare.
846 // We use timers (timed park operations) & periodic polling to detect
847 // and recover from stranding. Potentially stranded threads periodically
848 // wake up and poll the lock. See the usage of the _Responsible variable.
849 //
850 // The CAS() in enter provides for safety and exclusion, while the CAS or
851 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
852 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
853 // We detect and recover from stranding with timers.
854 //
855 // If a thread transiently strands it'll park until (a) another
856 // thread acquires the lock and then drops the lock, at which time the
857 // exiting thread will notice and unpark the stranded thread, or, (b)
858 // the timer expires. If the lock is high traffic then the stranding latency
859 // will be low due to (a). If the lock is low traffic then the odds of
860 // stranding are lower, although the worst-case stranding latency
861 // is longer. Critically, we don't want to put excessive load in the
862 // platform's timer subsystem. We want to minimize both the timer injection
863 // rate (timers created/sec) as well as the number of timers active at
864 // any one time. (more precisely, we want to minimize timer-seconds, which is
865 // the integral of the # of active timers at any instant over time).
866 // Both impinge on OS scalability. Given that, at most one thread parked on
867 // a monitor will use a timer.
868
869 void ATTR ObjectMonitor::exit(TRAPS) {
870 Thread * Self = THREAD ;
871 if (THREAD != _owner) {
872 if (THREAD->is_lock_owned((address) _owner)) {
873 // Transmute _owner from a BasicLock pointer to a Thread address.
874 // We don't need to hold _mutex for this transition.
875 // Non-null to Non-null is safe as long as all readers can
876 // tolerate either flavor.
877 assert (_recursions == 0, "invariant") ;
878 _owner = THREAD ;
879 _recursions = 0 ;
880 OwnerIsThread = 1 ;
881 } else {
882 // NOTE: we need to handle unbalanced monitor enter/exit
883 // in native code by throwing an exception.
884 // TODO: Throw an IllegalMonitorStateException ?
885 TEVENT (Exit - Throw IMSX) ;
886 assert(false, "Non-balanced monitor enter/exit!");
887 if (false) {
888 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
889 }
890 return;
891 }
892 }
893
894 if (_recursions != 0) {
895 _recursions--; // this is simple recursive enter
896 TEVENT (Inflated exit - recursive) ;
897 return ;
898 }
899
900 // Invariant: after setting Responsible=null an thread must execute
901 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
902 if ((SyncFlags & 4) == 0) {
903 _Responsible = NULL ;
904 }
905
906 for (;;) {
907 assert (THREAD == _owner, "invariant") ;
908
909
910 if (Knob_ExitPolicy == 0) {
911 // release semantics: prior loads and stores from within the critical section
912 // must not float (reorder) past the following store that drops the lock.
913 // On SPARC that requires MEMBAR #loadstore|#storestore.
914 // But of course in TSO #loadstore|#storestore is not required.
915 // I'd like to write one of the following:
916 // A. OrderAccess::release() ; _owner = NULL
917 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
918 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
919 // store into a _dummy variable. That store is not needed, but can result
920 // in massive wasteful coherency traffic on classic SMP systems.
921 // Instead, I use release_store(), which is implemented as just a simple
922 // ST on x64, x86 and SPARC.
923 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
924 OrderAccess::storeload() ; // See if we need to wake a successor
925 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
926 TEVENT (Inflated exit - simple egress) ;
927 return ;
928 }
929 TEVENT (Inflated exit - complex egress) ;
930
931 // Normally the exiting thread is responsible for ensuring succession,
932 // but if other successors are ready or other entering threads are spinning
933 // then this thread can simply store NULL into _owner and exit without
934 // waking a successor. The existence of spinners or ready successors
935 // guarantees proper succession (liveness). Responsibility passes to the
936 // ready or running successors. The exiting thread delegates the duty.
937 // More precisely, if a successor already exists this thread is absolved
938 // of the responsibility of waking (unparking) one.
939 //
940 // The _succ variable is critical to reducing futile wakeup frequency.
941 // _succ identifies the "heir presumptive" thread that has been made
942 // ready (unparked) but that has not yet run. We need only one such
943 // successor thread to guarantee progress.
944 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
945 // section 3.3 "Futile Wakeup Throttling" for details.
946 //
947 // Note that spinners in Enter() also set _succ non-null.
948 // In the current implementation spinners opportunistically set
949 // _succ so that exiting threads might avoid waking a successor.
950 // Another less appealing alternative would be for the exiting thread
951 // to drop the lock and then spin briefly to see if a spinner managed
952 // to acquire the lock. If so, the exiting thread could exit
953 // immediately without waking a successor, otherwise the exiting
954 // thread would need to dequeue and wake a successor.
955 // (Note that we'd need to make the post-drop spin short, but no
956 // shorter than the worst-case round-trip cache-line migration time.
957 // The dropped lock needs to become visible to the spinner, and then
958 // the acquisition of the lock by the spinner must become visible to
959 // the exiting thread).
960 //
961
962 // It appears that an heir-presumptive (successor) must be made ready.
963 // Only the current lock owner can manipulate the EntryList or
964 // drain _cxq, so we need to reacquire the lock. If we fail
965 // to reacquire the lock the responsibility for ensuring succession
966 // falls to the new owner.
967 //
968 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
969 return ;
970 }
971 TEVENT (Exit - Reacquired) ;
972 } else {
973 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
974 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
975 OrderAccess::storeload() ;
976 // Ratify the previously observed values.
977 if (_cxq == NULL || _succ != NULL) {
978 TEVENT (Inflated exit - simple egress) ;
979 return ;
980 }
981
982 // inopportune interleaving -- the exiting thread (this thread)
983 // in the fast-exit path raced an entering thread in the slow-enter
984 // path.
985 // We have two choices:
986 // A. Try to reacquire the lock.
987 // If the CAS() fails return immediately, otherwise
988 // we either restart/rerun the exit operation, or simply
989 // fall-through into the code below which wakes a successor.
990 // B. If the elements forming the EntryList|cxq are TSM
991 // we could simply unpark() the lead thread and return
992 // without having set _succ.
993 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
994 TEVENT (Inflated exit - reacquired succeeded) ;
995 return ;
996 }
997 TEVENT (Inflated exit - reacquired failed) ;
998 } else {
999 TEVENT (Inflated exit - complex egress) ;
1000 }
1001 }
1002
1003 guarantee (_owner == THREAD, "invariant") ;
1004
1005 ObjectWaiter * w = NULL ;
1006 int QMode = Knob_QMode ;
1007
1008 if (QMode == 2 && _cxq != NULL) {
1009 // QMode == 2 : cxq has precedence over EntryList.
1010 // Try to directly wake a successor from the cxq.
1011 // If successful, the successor will need to unlink itself from cxq.
1012 w = _cxq ;
1013 assert (w != NULL, "invariant") ;
1014 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1015 ExitEpilog (Self, w) ;
1016 return ;
1017 }
1018
1019 if (QMode == 3 && _cxq != NULL) {
1020 // Aggressively drain cxq into EntryList at the first opportunity.
1021 // This policy ensure that recently-run threads live at the head of EntryList.
1022 // Drain _cxq into EntryList - bulk transfer.
1023 // First, detach _cxq.
1024 // The following loop is tantamount to: w = swap (&cxq, NULL)
1025 w = _cxq ;
1026 for (;;) {
1027 assert (w != NULL, "Invariant") ;
1028 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1029 if (u == w) break ;
1030 w = u ;
1031 }
1032 assert (w != NULL , "invariant") ;
1033
1034 ObjectWaiter * q = NULL ;
1035 ObjectWaiter * p ;
1036 for (p = w ; p != NULL ; p = p->_next) {
1037 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1038 p->TState = ObjectWaiter::TS_ENTER ;
1039 p->_prev = q ;
1040 q = p ;
1041 }
1042
1043 // Append the RATs to the EntryList
1044 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1045 ObjectWaiter * Tail ;
1046 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1047 if (Tail == NULL) {
1048 _EntryList = w ;
1049 } else {
1050 Tail->_next = w ;
1051 w->_prev = Tail ;
1052 }
1053
1054 // Fall thru into code that tries to wake a successor from EntryList
1055 }
1056
1057 if (QMode == 4 && _cxq != NULL) {
1058 // Aggressively drain cxq into EntryList at the first opportunity.
1059 // This policy ensure that recently-run threads live at the head of EntryList.
1060
1061 // Drain _cxq into EntryList - bulk transfer.
1062 // First, detach _cxq.
1063 // The following loop is tantamount to: w = swap (&cxq, NULL)
1064 w = _cxq ;
1065 for (;;) {
1066 assert (w != NULL, "Invariant") ;
1067 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1068 if (u == w) break ;
1069 w = u ;
1070 }
1071 assert (w != NULL , "invariant") ;
1072
1073 ObjectWaiter * q = NULL ;
1074 ObjectWaiter * p ;
1075 for (p = w ; p != NULL ; p = p->_next) {
1076 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1077 p->TState = ObjectWaiter::TS_ENTER ;
1078 p->_prev = q ;
1079 q = p ;
1080 }
1081
1082 // Prepend the RATs to the EntryList
1083 if (_EntryList != NULL) {
1084 q->_next = _EntryList ;
1085 _EntryList->_prev = q ;
1086 }
1087 _EntryList = w ;
1088
1089 // Fall thru into code that tries to wake a successor from EntryList
1090 }
1091
1092 w = _EntryList ;
1093 if (w != NULL) {
1094 // I'd like to write: guarantee (w->_thread != Self).
1095 // But in practice an exiting thread may find itself on the EntryList.
1096 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1097 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1098 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1099 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1100 // release the lock "O". T2 resumes immediately after the ST of null into
1101 // _owner, above. T2 notices that the EntryList is populated, so it
1102 // reacquires the lock and then finds itself on the EntryList.
1103 // Given all that, we have to tolerate the circumstance where "w" is
1104 // associated with Self.
1105 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1106 ExitEpilog (Self, w) ;
1107 return ;
1108 }
1109
1110 // If we find that both _cxq and EntryList are null then just
1111 // re-run the exit protocol from the top.
1112 w = _cxq ;
1113 if (w == NULL) continue ;
1114
1115 // Drain _cxq into EntryList - bulk transfer.
1116 // First, detach _cxq.
1117 // The following loop is tantamount to: w = swap (&cxq, NULL)
1118 for (;;) {
1119 assert (w != NULL, "Invariant") ;
1120 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1121 if (u == w) break ;
1122 w = u ;
1123 }
1124 TEVENT (Inflated exit - drain cxq into EntryList) ;
1125
1126 assert (w != NULL , "invariant") ;
1127 assert (_EntryList == NULL , "invariant") ;
1128
1129 // Convert the LIFO SLL anchored by _cxq into a DLL.
1130 // The list reorganization step operates in O(LENGTH(w)) time.
1131 // It's critical that this step operate quickly as
1132 // "Self" still holds the outer-lock, restricting parallelism
1133 // and effectively lengthening the critical section.
1134 // Invariant: s chases t chases u.
1135 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1136 // we have faster access to the tail.
1137
1138 if (QMode == 1) {
1139 // QMode == 1 : drain cxq to EntryList, reversing order
1140 // We also reverse the order of the list.
1141 ObjectWaiter * s = NULL ;
1142 ObjectWaiter * t = w ;
1143 ObjectWaiter * u = NULL ;
1144 while (t != NULL) {
1145 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1146 t->TState = ObjectWaiter::TS_ENTER ;
1147 u = t->_next ;
1148 t->_prev = u ;
1149 t->_next = s ;
1150 s = t;
1151 t = u ;
1152 }
1153 _EntryList = s ;
1154 assert (s != NULL, "invariant") ;
1155 } else {
1156 // QMode == 0 or QMode == 2
1157 _EntryList = w ;
1158 ObjectWaiter * q = NULL ;
1159 ObjectWaiter * p ;
1160 for (p = w ; p != NULL ; p = p->_next) {
1161 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1162 p->TState = ObjectWaiter::TS_ENTER ;
1163 p->_prev = q ;
1164 q = p ;
1165 }
1166 }
1167
1168 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1169 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1170
1171 // See if we can abdicate to a spinner instead of waking a thread.
1172 // A primary goal of the implementation is to reduce the
1173 // context-switch rate.
1174 if (_succ != NULL) continue;
1175
1176 w = _EntryList ;
1177 if (w != NULL) {
1178 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1179 ExitEpilog (Self, w) ;
1180 return ;
1181 }
1182 }
1183 }
1184
1185 // ExitSuspendEquivalent:
1186 // A faster alternate to handle_special_suspend_equivalent_condition()
1187 //
1188 // handle_special_suspend_equivalent_condition() unconditionally
1189 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1190 // operations have high latency. Note that in ::enter() we call HSSEC
1191 // while holding the monitor, so we effectively lengthen the critical sections.
1192 //
1193 // There are a number of possible solutions:
1194 //
1195 // A. To ameliorate the problem we might also defer state transitions
1196 // to as late as possible -- just prior to parking.
1197 // Given that, we'd call HSSEC after having returned from park(),
1198 // but before attempting to acquire the monitor. This is only a
1199 // partial solution. It avoids calling HSSEC while holding the
1200 // monitor (good), but it still increases successor reacquisition latency --
1201 // the interval between unparking a successor and the time the successor
1202 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1203 // If we use this technique we can also avoid EnterI()-exit() loop
1204 // in ::enter() where we iteratively drop the lock and then attempt
1205 // to reacquire it after suspending.
1206 //
1207 // B. In the future we might fold all the suspend bits into a
1208 // composite per-thread suspend flag and then update it with CAS().
1209 // Alternately, a Dekker-like mechanism with multiple variables
1210 // would suffice:
1211 // ST Self->_suspend_equivalent = false
1212 // MEMBAR
1213 // LD Self_>_suspend_flags
1214 //
1215
1216
1217 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1218 int Mode = Knob_FastHSSEC ;
1219 if (Mode && !jSelf->is_external_suspend()) {
1220 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1221 jSelf->clear_suspend_equivalent() ;
1222 if (2 == Mode) OrderAccess::storeload() ;
1223 if (!jSelf->is_external_suspend()) return false ;
1224 // We raced a suspension -- fall thru into the slow path
1225 TEVENT (ExitSuspendEquivalent - raced) ;
1226 jSelf->set_suspend_equivalent() ;
1227 }
1228 return jSelf->handle_special_suspend_equivalent_condition() ;
1229 }
1230
1231
1232 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1233 assert (_owner == Self, "invariant") ;
1234
1235 // Exit protocol:
1236 // 1. ST _succ = wakee
1237 // 2. membar #loadstore|#storestore;
1238 // 2. ST _owner = NULL
1239 // 3. unpark(wakee)
1240
1241 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1242 ParkEvent * Trigger = Wakee->_event ;
1243
1244 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1245 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1246 // out-of-scope (non-extant).
1247 Wakee = NULL ;
1248
1249 // Drop the lock
1250 OrderAccess::release_store_ptr (&_owner, NULL) ;
1251 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1252
1253 if (SafepointSynchronize::do_call_back()) {
1254 TEVENT (unpark before SAFEPOINT) ;
1255 }
1256
1257 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1258 Trigger->unpark() ;
1259
1260 // Maintain stats and report events to JVMTI
1261 if (ObjectMonitor::_sync_Parks != NULL) {
1262 ObjectMonitor::_sync_Parks->inc() ;
1263 }
1264 }
1265
1266
1267 // -----------------------------------------------------------------------------
1268 // Class Loader deadlock handling.
1269 //
1270 // complete_exit exits a lock returning recursion count
1271 // complete_exit/reenter operate as a wait without waiting
1272 // complete_exit requires an inflated monitor
1273 // The _owner field is not always the Thread addr even with an
1274 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1275 // thread due to contention.
1276 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1277 Thread * const Self = THREAD;
1278 assert(Self->is_Java_thread(), "Must be Java thread!");
1279 JavaThread *jt = (JavaThread *)THREAD;
1280
1281 DeferredInitialize();
1282
1283 if (THREAD != _owner) {
1284 if (THREAD->is_lock_owned ((address)_owner)) {
1285 assert(_recursions == 0, "internal state error");
1286 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1287 _recursions = 0 ;
1288 OwnerIsThread = 1 ;
1289 }
1290 }
1291
1292 guarantee(Self == _owner, "complete_exit not owner");
1293 intptr_t save = _recursions; // record the old recursion count
1294 _recursions = 0; // set the recursion level to be 0
1295 exit (Self) ; // exit the monitor
1296 guarantee (_owner != Self, "invariant");
1297 return save;
1298 }
1299
1300 // reenter() enters a lock and sets recursion count
1301 // complete_exit/reenter operate as a wait without waiting
1302 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1303 Thread * const Self = THREAD;
1304 assert(Self->is_Java_thread(), "Must be Java thread!");
1305 JavaThread *jt = (JavaThread *)THREAD;
1306
1307 guarantee(_owner != Self, "reenter already owner");
1308 enter (THREAD); // enter the monitor
1309 guarantee (_recursions == 0, "reenter recursion");
1310 _recursions = recursions;
1311 return;
1312 }
1313
1314
1315 // -----------------------------------------------------------------------------
1316 // A macro is used below because there may already be a pending
1317 // exception which should not abort the execution of the routines
1318 // which use this (which is why we don't put this into check_slow and
1319 // call it with a CHECK argument).
1320
1321 #define CHECK_OWNER() \
1322 do { \
1323 if (THREAD != _owner) { \
1324 if (THREAD->is_lock_owned((address) _owner)) { \
1325 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1326 _recursions = 0; \
1327 OwnerIsThread = 1 ; \
1328 } else { \
1329 TEVENT (Throw IMSX) ; \
1330 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1331 } \
1332 } \
1333 } while (false)
1334
1335 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1336 // TODO-FIXME: remove check_slow() -- it's likely dead.
1337
1338 void ObjectMonitor::check_slow(TRAPS) {
1339 TEVENT (check_slow - throw IMSX) ;
1340 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1341 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1342 }
1343
1344 static int Adjust (volatile int * adr, int dx) {
1345 int v ;
1346 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1347 return v ;
1348 }
1349 // -----------------------------------------------------------------------------
1350 // Wait/Notify/NotifyAll
1351 //
1352 // Note: a subset of changes to ObjectMonitor::wait()
1353 // will need to be replicated in complete_exit above
1354 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1355 Thread * const Self = THREAD ;
1356 assert(Self->is_Java_thread(), "Must be Java thread!");
1357 JavaThread *jt = (JavaThread *)THREAD;
1358
1359 DeferredInitialize () ;
1360
1361 // Throw IMSX or IEX.
1362 CHECK_OWNER();
1363
1364 // check for a pending interrupt
1365 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1366 // post monitor waited event. Note that this is past-tense, we are done waiting.
1367 if (JvmtiExport::should_post_monitor_waited()) {
1368 // Note: 'false' parameter is passed here because the
1369 // wait was not timed out due to thread interrupt.
1370 JvmtiExport::post_monitor_waited(jt, this, false);
1371 }
1372 TEVENT (Wait - Throw IEX) ;
1373 THROW(vmSymbols::java_lang_InterruptedException());
1374 return ;
1375 }
1376 TEVENT (Wait) ;
1377
1378 assert (Self->_Stalled == 0, "invariant") ;
1379 Self->_Stalled = intptr_t(this) ;
1380 jt->set_current_waiting_monitor(this);
1381
1382 // create a node to be put into the queue
1383 // Critically, after we reset() the event but prior to park(), we must check
1384 // for a pending interrupt.
1385 ObjectWaiter node(Self);
1386 node.TState = ObjectWaiter::TS_WAIT ;
1387 Self->_ParkEvent->reset() ;
1388 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1389
1390 // Enter the waiting queue, which is a circular doubly linked list in this case
1391 // but it could be a priority queue or any data structure.
1392 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1393 // by the the owner of the monitor *except* in the case where park()
1394 // returns because of a timeout of interrupt. Contention is exceptionally rare
1395 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1396
1397 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1398 AddWaiter (&node) ;
1399 Thread::SpinRelease (&_WaitSetLock) ;
1400
1401 if ((SyncFlags & 4) == 0) {
1402 _Responsible = NULL ;
1403 }
1404 intptr_t save = _recursions; // record the old recursion count
1405 _waiters++; // increment the number of waiters
1406 _recursions = 0; // set the recursion level to be 1
1407 exit (Self) ; // exit the monitor
1408 guarantee (_owner != Self, "invariant") ;
1409
1410 // As soon as the ObjectMonitor's ownership is dropped in the exit()
1411 // call above, another thread can enter() the ObjectMonitor, do the
1412 // notify(), and exit() the ObjectMonitor. If the other thread's
1413 // exit() call chooses this thread as the successor and the unpark()
1414 // call happens to occur while this thread is posting a
1415 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
1416 // handler using RawMonitors and consuming the unpark().
1417 //
1418 // To avoid the problem, we re-post the event. This does no harm
1419 // even if the original unpark() was not consumed because we are the
1420 // chosen successor for this monitor.
1421 if (node._notified != 0 && _succ == Self) {
1422 node._event->unpark();
1423 }
1424
1425 // The thread is on the WaitSet list - now park() it.
1426 // On MP systems it's conceivable that a brief spin before we park
1427 // could be profitable.
1428 //
1429 // TODO-FIXME: change the following logic to a loop of the form
1430 // while (!timeout && !interrupted && _notified == 0) park()
1431
1432 int ret = OS_OK ;
1433 int WasNotified = 0 ;
1434 { // State transition wrappers
1435 OSThread* osthread = Self->osthread();
1436 OSThreadWaitState osts(osthread, true);
1437 {
1438 ThreadBlockInVM tbivm(jt);
1439 // Thread is in thread_blocked state and oop access is unsafe.
1440 jt->set_suspend_equivalent();
1441
1442 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1443 // Intentionally empty
1444 } else
1445 if (node._notified == 0) {
1446 if (millis <= 0) {
1447 Self->_ParkEvent->park () ;
1448 } else {
1449 ret = Self->_ParkEvent->park (millis) ;
1450 }
1451 }
1452
1453 // were we externally suspended while we were waiting?
1454 if (ExitSuspendEquivalent (jt)) {
1455 // TODO-FIXME: add -- if succ == Self then succ = null.
1456 jt->java_suspend_self();
1457 }
1458
1459 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1460
1461
1462 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1463 // from the WaitSet to the EntryList.
1464 // See if we need to remove Node from the WaitSet.
1465 // We use double-checked locking to avoid grabbing _WaitSetLock
1466 // if the thread is not on the wait queue.
1467 //
1468 // Note that we don't need a fence before the fetch of TState.
1469 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1470 // written by the is thread. (perhaps the fetch might even be satisfied
1471 // by a look-aside into the processor's own store buffer, although given
1472 // the length of the code path between the prior ST and this load that's
1473 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1474 // then we'll acquire the lock and then re-fetch a fresh TState value.
1475 // That is, we fail toward safety.
1476
1477 if (node.TState == ObjectWaiter::TS_WAIT) {
1478 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1479 if (node.TState == ObjectWaiter::TS_WAIT) {
1480 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1481 assert(node._notified == 0, "invariant");
1482 node.TState = ObjectWaiter::TS_RUN ;
1483 }
1484 Thread::SpinRelease (&_WaitSetLock) ;
1485 }
1486
1487 // The thread is now either on off-list (TS_RUN),
1488 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1489 // The Node's TState variable is stable from the perspective of this thread.
1490 // No other threads will asynchronously modify TState.
1491 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1492 OrderAccess::loadload() ;
1493 if (_succ == Self) _succ = NULL ;
1494 WasNotified = node._notified ;
1495
1496 // Reentry phase -- reacquire the monitor.
1497 // re-enter contended monitor after object.wait().
1498 // retain OBJECT_WAIT state until re-enter successfully completes
1499 // Thread state is thread_in_vm and oop access is again safe,
1500 // although the raw address of the object may have changed.
1501 // (Don't cache naked oops over safepoints, of course).
1502
1503 // post monitor waited event. Note that this is past-tense, we are done waiting.
1504 if (JvmtiExport::should_post_monitor_waited()) {
1505 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1506 }
1507 OrderAccess::fence() ;
1508
1509 assert (Self->_Stalled != 0, "invariant") ;
1510 Self->_Stalled = 0 ;
1511
1512 assert (_owner != Self, "invariant") ;
1513 ObjectWaiter::TStates v = node.TState ;
1514 if (v == ObjectWaiter::TS_RUN) {
1515 enter (Self) ;
1516 } else {
1517 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1518 ReenterI (Self, &node) ;
1519 node.wait_reenter_end(this);
1520 }
1521
1522 // Self has reacquired the lock.
1523 // Lifecycle - the node representing Self must not appear on any queues.
1524 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1525 // want residual elements associated with this thread left on any lists.
1526 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1527 assert (_owner == Self, "invariant") ;
1528 assert (_succ != Self , "invariant") ;
1529 } // OSThreadWaitState()
1530
1531 jt->set_current_waiting_monitor(NULL);
1532
1533 guarantee (_recursions == 0, "invariant") ;
1534 _recursions = save; // restore the old recursion count
1535 _waiters--; // decrement the number of waiters
1536
1537 // Verify a few postconditions
1538 assert (_owner == Self , "invariant") ;
1539 assert (_succ != Self , "invariant") ;
1540 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1541
1542 if (SyncFlags & 32) {
1543 OrderAccess::fence() ;
1544 }
1545
1546 // check if the notification happened
1547 if (!WasNotified) {
1548 // no, it could be timeout or Thread.interrupt() or both
1549 // check for interrupt event, otherwise it is timeout
1550 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1551 TEVENT (Wait - throw IEX from epilog) ;
1552 THROW(vmSymbols::java_lang_InterruptedException());
1553 }
1554 }
1555
1556 // NOTE: Spurious wake up will be consider as timeout.
1557 // Monitor notify has precedence over thread interrupt.
1558 }
1559
1560
1561 // Consider:
1562 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1563 // then instead of transferring a thread from the WaitSet to the EntryList
1564 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1565
1566 void ObjectMonitor::notify(TRAPS) {
1567 CHECK_OWNER();
1568 if (_WaitSet == NULL) {
1569 TEVENT (Empty-Notify) ;
1570 return ;
1571 }
1572 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1573
1574 int Policy = Knob_MoveNotifyee ;
1575
1576 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1577 ObjectWaiter * iterator = DequeueWaiter() ;
1578 if (iterator != NULL) {
1579 TEVENT (Notify1 - Transfer) ;
1580 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1581 guarantee (iterator->_notified == 0, "invariant") ;
1582 if (Policy != 4) {
1583 iterator->TState = ObjectWaiter::TS_ENTER ;
1584 }
1585 iterator->_notified = 1 ;
1586
1587 ObjectWaiter * List = _EntryList ;
1588 if (List != NULL) {
1589 assert (List->_prev == NULL, "invariant") ;
1590 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1591 assert (List != iterator, "invariant") ;
1592 }
1593
1594 if (Policy == 0) { // prepend to EntryList
1595 if (List == NULL) {
1596 iterator->_next = iterator->_prev = NULL ;
1597 _EntryList = iterator ;
1598 } else {
1599 List->_prev = iterator ;
1600 iterator->_next = List ;
1601 iterator->_prev = NULL ;
1602 _EntryList = iterator ;
1603 }
1604 } else
1605 if (Policy == 1) { // append to EntryList
1606 if (List == NULL) {
1607 iterator->_next = iterator->_prev = NULL ;
1608 _EntryList = iterator ;
1609 } else {
1610 // CONSIDER: finding the tail currently requires a linear-time walk of
1611 // the EntryList. We can make tail access constant-time by converting to
1612 // a CDLL instead of using our current DLL.
1613 ObjectWaiter * Tail ;
1614 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1615 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1616 Tail->_next = iterator ;
1617 iterator->_prev = Tail ;
1618 iterator->_next = NULL ;
1619 }
1620 } else
1621 if (Policy == 2) { // prepend to cxq
1622 // prepend to cxq
1623 if (List == NULL) {
1624 iterator->_next = iterator->_prev = NULL ;
1625 _EntryList = iterator ;
1626 } else {
1627 iterator->TState = ObjectWaiter::TS_CXQ ;
1628 for (;;) {
1629 ObjectWaiter * Front = _cxq ;
1630 iterator->_next = Front ;
1631 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1632 break ;
1633 }
1634 }
1635 }
1636 } else
1637 if (Policy == 3) { // append to cxq
1638 iterator->TState = ObjectWaiter::TS_CXQ ;
1639 for (;;) {
1640 ObjectWaiter * Tail ;
1641 Tail = _cxq ;
1642 if (Tail == NULL) {
1643 iterator->_next = NULL ;
1644 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1645 break ;
1646 }
1647 } else {
1648 while (Tail->_next != NULL) Tail = Tail->_next ;
1649 Tail->_next = iterator ;
1650 iterator->_prev = Tail ;
1651 iterator->_next = NULL ;
1652 break ;
1653 }
1654 }
1655 } else {
1656 ParkEvent * ev = iterator->_event ;
1657 iterator->TState = ObjectWaiter::TS_RUN ;
1658 OrderAccess::fence() ;
1659 ev->unpark() ;
1660 }
1661
1662 if (Policy < 4) {
1663 iterator->wait_reenter_begin(this);
1664 }
1665
1666 // _WaitSetLock protects the wait queue, not the EntryList. We could
1667 // move the add-to-EntryList operation, above, outside the critical section
1668 // protected by _WaitSetLock. In practice that's not useful. With the
1669 // exception of wait() timeouts and interrupts the monitor owner
1670 // is the only thread that grabs _WaitSetLock. There's almost no contention
1671 // on _WaitSetLock so it's not profitable to reduce the length of the
1672 // critical section.
1673 }
1674
1675 Thread::SpinRelease (&_WaitSetLock) ;
1676
1677 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1678 ObjectMonitor::_sync_Notifications->inc() ;
1679 }
1680 }
1681
1682
1683 void ObjectMonitor::notifyAll(TRAPS) {
1684 CHECK_OWNER();
1685 ObjectWaiter* iterator;
1686 if (_WaitSet == NULL) {
1687 TEVENT (Empty-NotifyAll) ;
1688 return ;
1689 }
1690 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1691
1692 int Policy = Knob_MoveNotifyee ;
1693 int Tally = 0 ;
1694 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1695
1696 for (;;) {
1697 iterator = DequeueWaiter () ;
1698 if (iterator == NULL) break ;
1699 TEVENT (NotifyAll - Transfer1) ;
1700 ++Tally ;
1701
1702 // Disposition - what might we do with iterator ?
1703 // a. add it directly to the EntryList - either tail or head.
1704 // b. push it onto the front of the _cxq.
1705 // For now we use (a).
1706
1707 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1708 guarantee (iterator->_notified == 0, "invariant") ;
1709 iterator->_notified = 1 ;
1710 if (Policy != 4) {
1711 iterator->TState = ObjectWaiter::TS_ENTER ;
1712 }
1713
1714 ObjectWaiter * List = _EntryList ;
1715 if (List != NULL) {
1716 assert (List->_prev == NULL, "invariant") ;
1717 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1718 assert (List != iterator, "invariant") ;
1719 }
1720
1721 if (Policy == 0) { // prepend to EntryList
1722 if (List == NULL) {
1723 iterator->_next = iterator->_prev = NULL ;
1724 _EntryList = iterator ;
1725 } else {
1726 List->_prev = iterator ;
1727 iterator->_next = List ;
1728 iterator->_prev = NULL ;
1729 _EntryList = iterator ;
1730 }
1731 } else
1732 if (Policy == 1) { // append to EntryList
1733 if (List == NULL) {
1734 iterator->_next = iterator->_prev = NULL ;
1735 _EntryList = iterator ;
1736 } else {
1737 // CONSIDER: finding the tail currently requires a linear-time walk of
1738 // the EntryList. We can make tail access constant-time by converting to
1739 // a CDLL instead of using our current DLL.
1740 ObjectWaiter * Tail ;
1741 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1742 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1743 Tail->_next = iterator ;
1744 iterator->_prev = Tail ;
1745 iterator->_next = NULL ;
1746 }
1747 } else
1748 if (Policy == 2) { // prepend to cxq
1749 // prepend to cxq
1750 iterator->TState = ObjectWaiter::TS_CXQ ;
1751 for (;;) {
1752 ObjectWaiter * Front = _cxq ;
1753 iterator->_next = Front ;
1754 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1755 break ;
1756 }
1757 }
1758 } else
1759 if (Policy == 3) { // append to cxq
1760 iterator->TState = ObjectWaiter::TS_CXQ ;
1761 for (;;) {
1762 ObjectWaiter * Tail ;
1763 Tail = _cxq ;
1764 if (Tail == NULL) {
1765 iterator->_next = NULL ;
1766 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1767 break ;
1768 }
1769 } else {
1770 while (Tail->_next != NULL) Tail = Tail->_next ;
1771 Tail->_next = iterator ;
1772 iterator->_prev = Tail ;
1773 iterator->_next = NULL ;
1774 break ;
1775 }
1776 }
1777 } else {
1778 ParkEvent * ev = iterator->_event ;
1779 iterator->TState = ObjectWaiter::TS_RUN ;
1780 OrderAccess::fence() ;
1781 ev->unpark() ;
1782 }
1783
1784 if (Policy < 4) {
1785 iterator->wait_reenter_begin(this);
1786 }
1787
1788 // _WaitSetLock protects the wait queue, not the EntryList. We could
1789 // move the add-to-EntryList operation, above, outside the critical section
1790 // protected by _WaitSetLock. In practice that's not useful. With the
1791 // exception of wait() timeouts and interrupts the monitor owner
1792 // is the only thread that grabs _WaitSetLock. There's almost no contention
1793 // on _WaitSetLock so it's not profitable to reduce the length of the
1794 // critical section.
1795 }
1796
1797 Thread::SpinRelease (&_WaitSetLock) ;
1798
1799 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1800 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1801 }
1802 }
1803
1804 // -----------------------------------------------------------------------------
1805 // Adaptive Spinning Support
1806 //
1807 // Adaptive spin-then-block - rational spinning
1808 //
1809 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1810 // algorithm. On high order SMP systems it would be better to start with
1811 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1812 // a contending thread could enqueue itself on the cxq and then spin locally
1813 // on a thread-specific variable such as its ParkEvent._Event flag.
1814 // That's left as an exercise for the reader. Note that global spinning is
1815 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1816 // low latency and massive bandwidth.
1817 //
1818 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1819 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1820 // (duration) or we can fix the count at approximately the duration of
1821 // a context switch and vary the frequency. Of course we could also
1822 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1823 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1824 //
1825 // This implementation varies the duration "D", where D varies with
1826 // the success rate of recent spin attempts. (D is capped at approximately
1827 // length of a round-trip context switch). The success rate for recent
1828 // spin attempts is a good predictor of the success rate of future spin
1829 // attempts. The mechanism adapts automatically to varying critical
1830 // section length (lock modality), system load and degree of parallelism.
1831 // D is maintained per-monitor in _SpinDuration and is initialized
1832 // optimistically. Spin frequency is fixed at 100%.
1833 //
1834 // Note that _SpinDuration is volatile, but we update it without locks
1835 // or atomics. The code is designed so that _SpinDuration stays within
1836 // a reasonable range even in the presence of races. The arithmetic
1837 // operations on _SpinDuration are closed over the domain of legal values,
1838 // so at worst a race will install and older but still legal value.
1839 // At the very worst this introduces some apparent non-determinism.
1840 // We might spin when we shouldn't or vice-versa, but since the spin
1841 // count are relatively short, even in the worst case, the effect is harmless.
1842 //
1843 // Care must be taken that a low "D" value does not become an
1844 // an absorbing state. Transient spinning failures -- when spinning
1845 // is overall profitable -- should not cause the system to converge
1846 // on low "D" values. We want spinning to be stable and predictable
1847 // and fairly responsive to change and at the same time we don't want
1848 // it to oscillate, become metastable, be "too" non-deterministic,
1849 // or converge on or enter undesirable stable absorbing states.
1850 //
1851 // We implement a feedback-based control system -- using past behavior
1852 // to predict future behavior. We face two issues: (a) if the
1853 // input signal is random then the spin predictor won't provide optimal
1854 // results, and (b) if the signal frequency is too high then the control
1855 // system, which has some natural response lag, will "chase" the signal.
1856 // (b) can arise from multimodal lock hold times. Transient preemption
1857 // can also result in apparent bimodal lock hold times.
1858 // Although sub-optimal, neither condition is particularly harmful, as
1859 // in the worst-case we'll spin when we shouldn't or vice-versa.
1860 // The maximum spin duration is rather short so the failure modes aren't bad.
1861 // To be conservative, I've tuned the gain in system to bias toward
1862 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1863 // "rings" or oscillates between spinning and not spinning. This happens
1864 // when spinning is just on the cusp of profitability, however, so the
1865 // situation is not dire. The state is benign -- there's no need to add
1866 // hysteresis control to damp the transition rate between spinning and
1867 // not spinning.
1868 //
1869
1870 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1871 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1872
1873 // Spinning: Fixed frequency (100%), vary duration
1874
1875
1876 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1877
1878 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1879 int ctr = Knob_FixedSpin ;
1880 if (ctr != 0) {
1881 while (--ctr >= 0) {
1882 if (TryLock (Self) > 0) return 1 ;
1883 SpinPause () ;
1884 }
1885 return 0 ;
1886 }
1887
1888 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1889 if (TryLock(Self) > 0) {
1890 // Increase _SpinDuration ...
1891 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1892 // Raising _SpurDuration to the poverty line is key.
1893 int x = _SpinDuration ;
1894 if (x < Knob_SpinLimit) {
1895 if (x < Knob_Poverty) x = Knob_Poverty ;
1896 _SpinDuration = x + Knob_BonusB ;
1897 }
1898 return 1 ;
1899 }
1900 SpinPause () ;
1901 }
1902
1903 // Admission control - verify preconditions for spinning
1904 //
1905 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1906 // becoming an absorbing state. Put another way, we spin briefly to
1907 // sample, just in case the system load, parallelism, contention, or lock
1908 // modality changed.
1909 //
1910 // Consider the following alternative:
1911 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1912 // spin attempt. "Periodically" might mean after a tally of
1913 // the # of failed spin attempts (or iterations) reaches some threshold.
1914 // This takes us into the realm of 1-out-of-N spinning, where we
1915 // hold the duration constant but vary the frequency.
1916
1917 ctr = _SpinDuration ;
1918 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
1919 if (ctr <= 0) return 0 ;
1920
1921 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
1922 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1923 TEVENT (Spin abort - notrunnable [TOP]);
1924 return 0 ;
1925 }
1926
1927 int MaxSpin = Knob_MaxSpinners ;
1928 if (MaxSpin >= 0) {
1929 if (_Spinner > MaxSpin) {
1930 TEVENT (Spin abort -- too many spinners) ;
1931 return 0 ;
1932 }
1933 // Slighty racy, but benign ...
1934 Adjust (&_Spinner, 1) ;
1935 }
1936
1937 // We're good to spin ... spin ingress.
1938 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1939 // when preparing to LD...CAS _owner, etc and the CAS is likely
1940 // to succeed.
1941 int hits = 0 ;
1942 int msk = 0 ;
1943 int caspty = Knob_CASPenalty ;
1944 int oxpty = Knob_OXPenalty ;
1945 int sss = Knob_SpinSetSucc ;
1946 if (sss && _succ == NULL ) _succ = Self ;
1947 Thread * prv = NULL ;
1948
1949 // There are three ways to exit the following loop:
1950 // 1. A successful spin where this thread has acquired the lock.
1951 // 2. Spin failure with prejudice
1952 // 3. Spin failure without prejudice
1953
1954 while (--ctr >= 0) {
1955
1956 // Periodic polling -- Check for pending GC
1957 // Threads may spin while they're unsafe.
1958 // We don't want spinning threads to delay the JVM from reaching
1959 // a stop-the-world safepoint or to steal cycles from GC.
1960 // If we detect a pending safepoint we abort in order that
1961 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1962 // this thread, if safe, doesn't steal cycles from GC.
1963 // This is in keeping with the "no loitering in runtime" rule.
1964 // We periodically check to see if there's a safepoint pending.
1965 if ((ctr & 0xFF) == 0) {
1966 if (SafepointSynchronize::do_call_back()) {
1967 TEVENT (Spin: safepoint) ;
1968 goto Abort ; // abrupt spin egress
1969 }
1970 if (Knob_UsePause & 1) SpinPause () ;
1971
1972 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
1973 if (hits > 50 && scb != NULL) {
1974 int abend = (*scb)(SpinCallbackArgument, 0) ;
1975 }
1976 }
1977
1978 if (Knob_UsePause & 2) SpinPause() ;
1979
1980 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
1981 // This is useful on classic SMP systems, but is of less utility on
1982 // N1-style CMT platforms.
1983 //
1984 // Trade-off: lock acquisition latency vs coherency bandwidth.
1985 // Lock hold times are typically short. A histogram
1986 // of successful spin attempts shows that we usually acquire
1987 // the lock early in the spin. That suggests we want to
1988 // sample _owner frequently in the early phase of the spin,
1989 // but then back-off and sample less frequently as the spin
1990 // progresses. The back-off makes a good citizen on SMP big
1991 // SMP systems. Oversampling _owner can consume excessive
1992 // coherency bandwidth. Relatedly, if we _oversample _owner we
1993 // can inadvertently interfere with the the ST m->owner=null.
1994 // executed by the lock owner.
1995 if (ctr & msk) continue ;
1996 ++hits ;
1997 if ((hits & 0xF) == 0) {
1998 // The 0xF, above, corresponds to the exponent.
1999 // Consider: (msk+1)|msk
2000 msk = ((msk << 2)|3) & BackOffMask ;
2001 }
2002
2003 // Probe _owner with TATAS
2004 // If this thread observes the monitor transition or flicker
2005 // from locked to unlocked to locked, then the odds that this
2006 // thread will acquire the lock in this spin attempt go down
2007 // considerably. The same argument applies if the CAS fails
2008 // or if we observe _owner change from one non-null value to
2009 // another non-null value. In such cases we might abort
2010 // the spin without prejudice or apply a "penalty" to the
2011 // spin count-down variable "ctr", reducing it by 100, say.
2012
2013 Thread * ox = (Thread *) _owner ;
2014 if (ox == NULL) {
2015 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2016 if (ox == NULL) {
2017 // The CAS succeeded -- this thread acquired ownership
2018 // Take care of some bookkeeping to exit spin state.
2019 if (sss && _succ == Self) {
2020 _succ = NULL ;
2021 }
2022 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2023
2024 // Increase _SpinDuration :
2025 // The spin was successful (profitable) so we tend toward
2026 // longer spin attempts in the future.
2027 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2028 // If we acquired the lock early in the spin cycle it
2029 // makes sense to increase _SpinDuration proportionally.
2030 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2031 int x = _SpinDuration ;
2032 if (x < Knob_SpinLimit) {
2033 if (x < Knob_Poverty) x = Knob_Poverty ;
2034 _SpinDuration = x + Knob_Bonus ;
2035 }
2036 return 1 ;
2037 }
2038
2039 // The CAS failed ... we can take any of the following actions:
2040 // * penalize: ctr -= Knob_CASPenalty
2041 // * exit spin with prejudice -- goto Abort;
2042 // * exit spin without prejudice.
2043 // * Since CAS is high-latency, retry again immediately.
2044 prv = ox ;
2045 TEVENT (Spin: cas failed) ;
2046 if (caspty == -2) break ;
2047 if (caspty == -1) goto Abort ;
2048 ctr -= caspty ;
2049 continue ;
2050 }
2051
2052 // Did lock ownership change hands ?
2053 if (ox != prv && prv != NULL ) {
2054 TEVENT (spin: Owner changed)
2055 if (oxpty == -2) break ;
2056 if (oxpty == -1) goto Abort ;
2057 ctr -= oxpty ;
2058 }
2059 prv = ox ;
2060
2061 // Abort the spin if the owner is not executing.
2062 // The owner must be executing in order to drop the lock.
2063 // Spinning while the owner is OFFPROC is idiocy.
2064 // Consider: ctr -= RunnablePenalty ;
2065 if (Knob_OState && NotRunnable (Self, ox)) {
2066 TEVENT (Spin abort - notrunnable);
2067 goto Abort ;
2068 }
2069 if (sss && _succ == NULL ) _succ = Self ;
2070 }
2071
2072 // Spin failed with prejudice -- reduce _SpinDuration.
2073 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2074 // AIMD is globally stable.
2075 TEVENT (Spin failure) ;
2076 {
2077 int x = _SpinDuration ;
2078 if (x > 0) {
2079 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2080 // This is globally sample and tends to damp the response.
2081 x -= Knob_Penalty ;
2082 if (x < 0) x = 0 ;
2083 _SpinDuration = x ;
2084 }
2085 }
2086
2087 Abort:
2088 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2089 if (sss && _succ == Self) {
2090 _succ = NULL ;
2091 // Invariant: after setting succ=null a contending thread
2092 // must recheck-retry _owner before parking. This usually happens
2093 // in the normal usage of TrySpin(), but it's safest
2094 // to make TrySpin() as foolproof as possible.
2095 OrderAccess::fence() ;
2096 if (TryLock(Self) > 0) return 1 ;
2097 }
2098 return 0 ;
2099 }
2100
2101 // NotRunnable() -- informed spinning
2102 //
2103 // Don't bother spinning if the owner is not eligible to drop the lock.
2104 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2105 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2106 // The thread must be runnable in order to drop the lock in timely fashion.
2107 // If the _owner is not runnable then spinning will not likely be
2108 // successful (profitable).
2109 //
2110 // Beware -- the thread referenced by _owner could have died
2111 // so a simply fetch from _owner->_thread_state might trap.
2112 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2113 // Because of the lifecycle issues the schedctl and _thread_state values
2114 // observed by NotRunnable() might be garbage. NotRunnable must
2115 // tolerate this and consider the observed _thread_state value
2116 // as advisory.
2117 //
2118 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2119 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2120 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2121 // with the LSB of _owner. Another option would be to probablistically probe
2122 // the putative _owner->TypeTag value.
2123 //
2124 // Checking _thread_state isn't perfect. Even if the thread is
2125 // in_java it might be blocked on a page-fault or have been preempted
2126 // and sitting on a ready/dispatch queue. _thread state in conjunction
2127 // with schedctl.sc_state gives us a good picture of what the
2128 // thread is doing, however.
2129 //
2130 // TODO: check schedctl.sc_state.
2131 // We'll need to use SafeFetch32() to read from the schedctl block.
2132 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2133 //
2134 // The return value from NotRunnable() is *advisory* -- the
2135 // result is based on sampling and is not necessarily coherent.
2136 // The caller must tolerate false-negative and false-positive errors.
2137 // Spinning, in general, is probabilistic anyway.
2138
2139
2140 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2141 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2142 if (!OwnerIsThread) return 0 ;
2143
2144 if (ox == NULL) return 0 ;
2145
2146 // Avoid transitive spinning ...
2147 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2148 // Immediately after T1 acquires L it's possible that T2, also
2149 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2150 // This occurs transiently after T1 acquired L but before
2151 // T1 managed to clear T1.Stalled. T2 does not need to abort
2152 // its spin in this circumstance.
2153 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2154
2155 if (BlockedOn == 1) return 1 ;
2156 if (BlockedOn != 0) {
2157 return BlockedOn != intptr_t(this) && _owner == ox ;
2158 }
2159
2160 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2161 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2162 // consider also: jst != _thread_in_Java -- but that's overspecific.
2163 return jst == _thread_blocked || jst == _thread_in_native ;
2164 }
2165
2166
2167 // -----------------------------------------------------------------------------
2168 // WaitSet management ...
2169
2170 ObjectWaiter::ObjectWaiter(Thread* thread) {
2171 _next = NULL;
2172 _prev = NULL;
2173 _notified = 0;
2174 TState = TS_RUN ;
2175 _thread = thread;
2176 _event = thread->_ParkEvent ;
2177 _active = false;
2178 assert (_event != NULL, "invariant") ;
2179 }
2180
2181 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2182 JavaThread *jt = (JavaThread *)this->_thread;
2183 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2184 }
2185
2186 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2187 JavaThread *jt = (JavaThread *)this->_thread;
2188 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2189 }
2190
2191 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2192 assert(node != NULL, "should not dequeue NULL node");
2193 assert(node->_prev == NULL, "node already in list");
2194 assert(node->_next == NULL, "node already in list");
2195 // put node at end of queue (circular doubly linked list)
2196 if (_WaitSet == NULL) {
2197 _WaitSet = node;
2198 node->_prev = node;
2199 node->_next = node;
2200 } else {
2201 ObjectWaiter* head = _WaitSet ;
2202 ObjectWaiter* tail = head->_prev;
2203 assert(tail->_next == head, "invariant check");
2204 tail->_next = node;
2205 head->_prev = node;
2206 node->_next = head;
2207 node->_prev = tail;
2208 }
2209 }
2210
2211 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2212 // dequeue the very first waiter
2213 ObjectWaiter* waiter = _WaitSet;
2214 if (waiter) {
2215 DequeueSpecificWaiter(waiter);
2216 }
2217 return waiter;
2218 }
2219
2220 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2221 assert(node != NULL, "should not dequeue NULL node");
2222 assert(node->_prev != NULL, "node already removed from list");
2223 assert(node->_next != NULL, "node already removed from list");
2224 // when the waiter has woken up because of interrupt,
2225 // timeout or other spurious wake-up, dequeue the
2226 // waiter from waiting list
2227 ObjectWaiter* next = node->_next;
2228 if (next == node) {
2229 assert(node->_prev == node, "invariant check");
2230 _WaitSet = NULL;
2231 } else {
2232 ObjectWaiter* prev = node->_prev;
2233 assert(prev->_next == node, "invariant check");
2234 assert(next->_prev == node, "invariant check");
2235 next->_prev = prev;
2236 prev->_next = next;
2237 if (_WaitSet == node) {
2238 _WaitSet = next;
2239 }
2240 }
2241 node->_next = NULL;
2242 node->_prev = NULL;
2243 }
2244
2245 // -----------------------------------------------------------------------------
2246 // PerfData support
2247 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2248 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2249 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2250 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2251 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2252 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2253 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2254 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2255 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2256 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2257 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2258 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2259 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2260 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2261 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2262 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2263 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2264 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2265
2266 // One-shot global initialization for the sync subsystem.
2267 // We could also defer initialization and initialize on-demand
2268 // the first time we call inflate(). Initialization would
2269 // be protected - like so many things - by the MonitorCache_lock.
2270
2271 void ObjectMonitor::Initialize () {
2272 static int InitializationCompleted = 0 ;
2273 assert (InitializationCompleted == 0, "invariant") ;
2274 InitializationCompleted = 1 ;
2275 if (UsePerfData) {
2276 EXCEPTION_MARK ;
2277 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2278 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2279 NEWPERFCOUNTER(_sync_Inflations) ;
2280 NEWPERFCOUNTER(_sync_Deflations) ;
2281 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2282 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2283 NEWPERFCOUNTER(_sync_Parks) ;
2284 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2285 NEWPERFCOUNTER(_sync_Notifications) ;
2286 NEWPERFCOUNTER(_sync_SlowEnter) ;
2287 NEWPERFCOUNTER(_sync_SlowExit) ;
2288 NEWPERFCOUNTER(_sync_SlowNotify) ;
2289 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2290 NEWPERFCOUNTER(_sync_FailedSpins) ;
2291 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2292 NEWPERFCOUNTER(_sync_PrivateA) ;
2293 NEWPERFCOUNTER(_sync_PrivateB) ;
2294 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2295 NEWPERFCOUNTER(_sync_MonScavenged) ;
2296 NEWPERFVARIABLE(_sync_MonExtant) ;
2297 #undef NEWPERFCOUNTER
2298 }
2299 }
2300
2301
2302 // Compile-time asserts
2303 // When possible, it's better to catch errors deterministically at
2304 // compile-time than at runtime. The down-side to using compile-time
2305 // asserts is that error message -- often something about negative array
2306 // indices -- is opaque.
2307
2308 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2309
2310 void ObjectMonitor::ctAsserts() {
2311 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2312 }
2313
2314
2315 static char * kvGet (char * kvList, const char * Key) {
2316 if (kvList == NULL) return NULL ;
2317 size_t n = strlen (Key) ;
2318 char * Search ;
2319 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2320 if (strncmp (Search, Key, n) == 0) {
2321 if (Search[n] == '=') return Search + n + 1 ;
2322 if (Search[n] == 0) return (char *) "1" ;
2323 }
2324 }
2325 return NULL ;
2326 }
2327
2328 static int kvGetInt (char * kvList, const char * Key, int Default) {
2329 char * v = kvGet (kvList, Key) ;
2330 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2331 if (Knob_ReportSettings && v != NULL) {
2332 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2333 ::fflush (stdout) ;
2334 }
2335 return rslt ;
2336 }
2337
2338 void ObjectMonitor::DeferredInitialize () {
2339 if (InitDone > 0) return ;
2340 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2341 while (InitDone != 1) ;
2342 return ;
2343 }
2344
2345 // One-shot global initialization ...
2346 // The initialization is idempotent, so we don't need locks.
2347 // In the future consider doing this via os::init_2().
2348 // SyncKnobs consist of <Key>=<Value> pairs in the style
2349 // of environment variables. Start by converting ':' to NUL.
2350
2351 if (SyncKnobs == NULL) SyncKnobs = "" ;
2352
2353 size_t sz = strlen (SyncKnobs) ;
2354 char * knobs = (char *) malloc (sz + 2) ;
2355 if (knobs == NULL) {
2356 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2357 guarantee (0, "invariant") ;
2358 }
2359 strcpy (knobs, SyncKnobs) ;
2360 knobs[sz+1] = 0 ;
2361 for (char * p = knobs ; *p ; p++) {
2362 if (*p == ':') *p = 0 ;
2363 }
2364
2365 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2366 SETKNOB(ReportSettings) ;
2367 SETKNOB(Verbose) ;
2368 SETKNOB(FixedSpin) ;
2369 SETKNOB(SpinLimit) ;
2370 SETKNOB(SpinBase) ;
2371 SETKNOB(SpinBackOff);
2372 SETKNOB(CASPenalty) ;
2373 SETKNOB(OXPenalty) ;
2374 SETKNOB(LogSpins) ;
2375 SETKNOB(SpinSetSucc) ;
2376 SETKNOB(SuccEnabled) ;
2377 SETKNOB(SuccRestrict) ;
2378 SETKNOB(Penalty) ;
2379 SETKNOB(Bonus) ;
2380 SETKNOB(BonusB) ;
2381 SETKNOB(Poverty) ;
2382 SETKNOB(SpinAfterFutile) ;
2383 SETKNOB(UsePause) ;
2384 SETKNOB(SpinEarly) ;
2385 SETKNOB(OState) ;
2386 SETKNOB(MaxSpinners) ;
2387 SETKNOB(PreSpin) ;
2388 SETKNOB(ExitPolicy) ;
2389 SETKNOB(QMode);
2390 SETKNOB(ResetEvent) ;
2391 SETKNOB(MoveNotifyee) ;
2392 SETKNOB(FastHSSEC) ;
2393 #undef SETKNOB
2394
2395 if (os::is_MP()) {
2396 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2397 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2398 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2399 } else {
2400 Knob_SpinLimit = 0 ;
2401 Knob_SpinBase = 0 ;
2402 Knob_PreSpin = 0 ;
2403 Knob_FixedSpin = -1 ;
2404 }
2405
2406 if (Knob_LogSpins == 0) {
2407 ObjectMonitor::_sync_FailedSpins = NULL ;
2408 }
2409
2410 free (knobs) ;
2411 OrderAccess::fence() ;
2412 InitDone = 1 ;
2413 }
2414
2415 #ifndef PRODUCT
2416 void ObjectMonitor::verify() {
2417 }
2418
2419 void ObjectMonitor::print() {
2420 }
2421 #endif