1 /*
2 * Copyright (c) 1998, 2017, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/atomic.hpp"
31 #include "runtime/handles.inline.hpp"
32 #include "runtime/interfaceSupport.hpp"
33 #include "runtime/mutexLocker.hpp"
34 #include "runtime/objectMonitor.hpp"
35 #include "runtime/objectMonitor.inline.hpp"
36 #include "runtime/orderAccess.inline.hpp"
37 #include "runtime/osThread.hpp"
38 #include "runtime/stubRoutines.hpp"
39 #include "runtime/thread.inline.hpp"
40 #include "services/threadService.hpp"
41 #include "trace/tracing.hpp"
42 #include "trace/traceMacros.hpp"
43 #include "utilities/dtrace.hpp"
44 #include "utilities/macros.hpp"
45 #include "utilities/preserveException.hpp"
46
47 #ifdef DTRACE_ENABLED
48
49 // Only bother with this argument setup if dtrace is available
50 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
51
52
53 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
54 char* bytes = NULL; \
55 int len = 0; \
56 jlong jtid = SharedRuntime::get_java_tid(thread); \
57 Symbol* klassname = ((oop)obj)->klass()->name(); \
58 if (klassname != NULL) { \
59 bytes = (char*)klassname->bytes(); \
60 len = klassname->utf8_length(); \
61 }
62
63 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
64 { \
65 if (DTraceMonitorProbes) { \
66 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
67 HOTSPOT_MONITOR_WAIT(jtid, \
68 (monitor), bytes, len, (millis)); \
69 } \
70 }
71
72 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
73 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
74 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
75 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
76 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
77
78 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
79 { \
80 if (DTraceMonitorProbes) { \
81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
82 HOTSPOT_MONITOR_##probe(jtid, \
83 (uintptr_t)(monitor), bytes, len); \
84 } \
85 }
86
87 #else // ndef DTRACE_ENABLED
88
89 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
90 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
91
92 #endif // ndef DTRACE_ENABLED
93
94 // Tunables ...
95 // The knob* variables are effectively final. Once set they should
96 // never be modified hence. Consider using __read_mostly with GCC.
97
98 int ObjectMonitor::Knob_ExitRelease = 0;
99 int ObjectMonitor::Knob_Verbose = 0;
100 int ObjectMonitor::Knob_VerifyInUse = 0;
101 int ObjectMonitor::Knob_VerifyMatch = 0;
102 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
103 static int Knob_LogSpins = 0; // enable jvmstat tally for spins
104 static int Knob_HandOff = 0;
105 static int Knob_ReportSettings = 0;
106
107 static int Knob_SpinBase = 0; // Floor AKA SpinMin
108 static int Knob_SpinBackOff = 0; // spin-loop backoff
109 static int Knob_CASPenalty = -1; // Penalty for failed CAS
110 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
111 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
112 static int Knob_SpinEarly = 1;
113 static int Knob_SuccEnabled = 1; // futile wake throttling
114 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
115 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
116 static int Knob_Bonus = 100; // spin success bonus
117 static int Knob_BonusB = 100; // spin success bonus
118 static int Knob_Penalty = 200; // spin failure penalty
119 static int Knob_Poverty = 1000;
120 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
121 static int Knob_FixedSpin = 0;
122 static int Knob_OState = 3; // Spinner checks thread state of _owner
123 static int Knob_UsePause = 1;
124 static int Knob_ExitPolicy = 0;
125 static int Knob_PreSpin = 10; // 20-100 likely better
126 static int Knob_ResetEvent = 0;
127 static int BackOffMask = 0;
128
129 static int Knob_FastHSSEC = 0;
130 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
131 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
132 static volatile int InitDone = 0;
133
134 // -----------------------------------------------------------------------------
135 // Theory of operations -- Monitors lists, thread residency, etc:
136 //
137 // * A thread acquires ownership of a monitor by successfully
138 // CAS()ing the _owner field from null to non-null.
139 //
140 // * Invariant: A thread appears on at most one monitor list --
141 // cxq, EntryList or WaitSet -- at any one time.
142 //
143 // * Contending threads "push" themselves onto the cxq with CAS
144 // and then spin/park.
145 //
146 // * After a contending thread eventually acquires the lock it must
147 // dequeue itself from either the EntryList or the cxq.
148 //
149 // * The exiting thread identifies and unparks an "heir presumptive"
150 // tentative successor thread on the EntryList. Critically, the
151 // exiting thread doesn't unlink the successor thread from the EntryList.
152 // After having been unparked, the wakee will recontend for ownership of
153 // the monitor. The successor (wakee) will either acquire the lock or
154 // re-park itself.
155 //
156 // Succession is provided for by a policy of competitive handoff.
157 // The exiting thread does _not_ grant or pass ownership to the
158 // successor thread. (This is also referred to as "handoff" succession").
159 // Instead the exiting thread releases ownership and possibly wakes
160 // a successor, so the successor can (re)compete for ownership of the lock.
161 // If the EntryList is empty but the cxq is populated the exiting
162 // thread will drain the cxq into the EntryList. It does so by
163 // by detaching the cxq (installing null with CAS) and folding
164 // the threads from the cxq into the EntryList. The EntryList is
165 // doubly linked, while the cxq is singly linked because of the
166 // CAS-based "push" used to enqueue recently arrived threads (RATs).
167 //
168 // * Concurrency invariants:
169 //
170 // -- only the monitor owner may access or mutate the EntryList.
171 // The mutex property of the monitor itself protects the EntryList
172 // from concurrent interference.
173 // -- Only the monitor owner may detach the cxq.
174 //
175 // * The monitor entry list operations avoid locks, but strictly speaking
176 // they're not lock-free. Enter is lock-free, exit is not.
177 // For a description of 'Methods and apparatus providing non-blocking access
178 // to a resource,' see U.S. Pat. No. 7844973.
179 //
180 // * The cxq can have multiple concurrent "pushers" but only one concurrent
181 // detaching thread. This mechanism is immune from the ABA corruption.
182 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
183 //
184 // * Taken together, the cxq and the EntryList constitute or form a
185 // single logical queue of threads stalled trying to acquire the lock.
186 // We use two distinct lists to improve the odds of a constant-time
187 // dequeue operation after acquisition (in the ::enter() epilogue) and
188 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
189 // A key desideratum is to minimize queue & monitor metadata manipulation
190 // that occurs while holding the monitor lock -- that is, we want to
191 // minimize monitor lock holds times. Note that even a small amount of
192 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
193 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
194 // locks and monitor metadata.
195 //
196 // Cxq points to the set of Recently Arrived Threads attempting entry.
197 // Because we push threads onto _cxq with CAS, the RATs must take the form of
198 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
199 // the unlocking thread notices that EntryList is null but _cxq is != null.
200 //
201 // The EntryList is ordered by the prevailing queue discipline and
202 // can be organized in any convenient fashion, such as a doubly-linked list or
203 // a circular doubly-linked list. Critically, we want insert and delete operations
204 // to operate in constant-time. If we need a priority queue then something akin
205 // to Solaris' sleepq would work nicely. Viz.,
206 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
207 // Queue discipline is enforced at ::exit() time, when the unlocking thread
208 // drains the cxq into the EntryList, and orders or reorders the threads on the
209 // EntryList accordingly.
210 //
211 // Barring "lock barging", this mechanism provides fair cyclic ordering,
212 // somewhat similar to an elevator-scan.
213 //
214 // * The monitor synchronization subsystem avoids the use of native
215 // synchronization primitives except for the narrow platform-specific
216 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
217 // the semantics of park-unpark. Put another way, this monitor implementation
218 // depends only on atomic operations and park-unpark. The monitor subsystem
219 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
220 // underlying OS manages the READY<->RUN transitions.
221 //
222 // * Waiting threads reside on the WaitSet list -- wait() puts
223 // the caller onto the WaitSet.
224 //
225 // * notify() or notifyAll() simply transfers threads from the WaitSet to
226 // either the EntryList or cxq. Subsequent exit() operations will
227 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
228 // it's likely the notifyee would simply impale itself on the lock held
229 // by the notifier.
230 //
231 // * An interesting alternative is to encode cxq as (List,LockByte) where
232 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
233 // variable, like _recursions, in the scheme. The threads or Events that form
234 // the list would have to be aligned in 256-byte addresses. A thread would
235 // try to acquire the lock or enqueue itself with CAS, but exiting threads
236 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
237 // Note that is is *not* word-tearing, but it does presume that full-word
238 // CAS operations are coherent with intermix with STB operations. That's true
239 // on most common processors.
240 //
241 // * See also http://blogs.sun.com/dave
242
243
244 // -----------------------------------------------------------------------------
245 // Enter support
246
247 void ObjectMonitor::enter(TRAPS) {
248 // The following code is ordered to check the most common cases first
249 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
250 Thread * const Self = THREAD;
251
252 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
253 if (cur == NULL) {
254 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
255 assert(_recursions == 0, "invariant");
256 assert(_owner == Self, "invariant");
257 return;
258 }
259
260 if (cur == Self) {
261 // TODO-FIXME: check for integer overflow! BUGID 6557169.
262 _recursions++;
263 return;
264 }
265
266 if (Self->is_lock_owned ((address)cur)) {
267 assert(_recursions == 0, "internal state error");
268 _recursions = 1;
269 // Commute owner from a thread-specific on-stack BasicLockObject address to
270 // a full-fledged "Thread *".
271 _owner = Self;
272 return;
273 }
274
275 // We've encountered genuine contention.
276 assert(Self->_Stalled == 0, "invariant");
277 Self->_Stalled = intptr_t(this);
278
279 // Try one round of spinning *before* enqueueing Self
280 // and before going through the awkward and expensive state
281 // transitions. The following spin is strictly optional ...
282 // Note that if we acquire the monitor from an initial spin
283 // we forgo posting JVMTI events and firing DTRACE probes.
284 if (Knob_SpinEarly && TrySpin (Self) > 0) {
285 assert(_owner == Self, "invariant");
286 assert(_recursions == 0, "invariant");
287 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
288 Self->_Stalled = 0;
289 return;
290 }
291
292 assert(_owner != Self, "invariant");
293 assert(_succ != Self, "invariant");
294 assert(Self->is_Java_thread(), "invariant");
295 JavaThread * jt = (JavaThread *) Self;
296 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
297 assert(jt->thread_state() != _thread_blocked, "invariant");
298 assert(this->object() != NULL, "invariant");
299 assert(_count >= 0, "invariant");
300
301 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
302 // Ensure the object-monitor relationship remains stable while there's contention.
303 Atomic::inc(&_count);
304
305 EventJavaMonitorEnter event;
306
307 { // Change java thread status to indicate blocked on monitor enter.
308 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
309
310 Self->set_current_pending_monitor(this);
311
312 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
313 if (JvmtiExport::should_post_monitor_contended_enter()) {
314 JvmtiExport::post_monitor_contended_enter(jt, this);
315
316 // The current thread does not yet own the monitor and does not
317 // yet appear on any queues that would get it made the successor.
318 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
319 // handler cannot accidentally consume an unpark() meant for the
320 // ParkEvent associated with this ObjectMonitor.
321 }
322
323 OSThreadContendState osts(Self->osthread());
324 ThreadBlockInVM tbivm(jt);
325
326 // TODO-FIXME: change the following for(;;) loop to straight-line code.
327 for (;;) {
328 jt->set_suspend_equivalent();
329 // cleared by handle_special_suspend_equivalent_condition()
330 // or java_suspend_self()
331
332 EnterI(THREAD);
333
334 if (!ExitSuspendEquivalent(jt)) break;
335
336 // We have acquired the contended monitor, but while we were
337 // waiting another thread suspended us. We don't want to enter
338 // the monitor while suspended because that would surprise the
339 // thread that suspended us.
340 //
341 _recursions = 0;
342 _succ = NULL;
343 exit(false, Self);
344
345 jt->java_suspend_self();
346 }
347 Self->set_current_pending_monitor(NULL);
348
349 // We cleared the pending monitor info since we've just gotten past
350 // the enter-check-for-suspend dance and we now own the monitor free
351 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
352 // destructor can go to a safepoint at the end of this block. If we
353 // do a thread dump during that safepoint, then this thread will show
354 // as having "-locked" the monitor, but the OS and java.lang.Thread
355 // states will still report that the thread is blocked trying to
356 // acquire it.
357 }
358
359 Atomic::dec(&_count);
360 assert(_count >= 0, "invariant");
361 Self->_Stalled = 0;
362
363 // Must either set _recursions = 0 or ASSERT _recursions == 0.
364 assert(_recursions == 0, "invariant");
365 assert(_owner == Self, "invariant");
366 assert(_succ != Self, "invariant");
367 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
368
369 // The thread -- now the owner -- is back in vm mode.
370 // Report the glorious news via TI,DTrace and jvmstat.
371 // The probe effect is non-trivial. All the reportage occurs
372 // while we hold the monitor, increasing the length of the critical
373 // section. Amdahl's parallel speedup law comes vividly into play.
374 //
375 // Another option might be to aggregate the events (thread local or
376 // per-monitor aggregation) and defer reporting until a more opportune
377 // time -- such as next time some thread encounters contention but has
378 // yet to acquire the lock. While spinning that thread could
379 // spinning we could increment JVMStat counters, etc.
380
381 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
382 if (JvmtiExport::should_post_monitor_contended_entered()) {
383 JvmtiExport::post_monitor_contended_entered(jt, this);
384
385 // The current thread already owns the monitor and is not going to
386 // call park() for the remainder of the monitor enter protocol. So
387 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
388 // event handler consumed an unpark() issued by the thread that
389 // just exited the monitor.
390 }
391
392 if (event.should_commit()) {
393 event.set_monitorClass(((oop)this->object())->klass());
394 event.set_previousOwner((TYPE_THREAD)_previous_owner_tid);
395 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
396 event.commit();
397 }
398
399 OM_PERFDATA_OP(ContendedLockAttempts, inc());
400 }
401
402
403 // Caveat: TryLock() is not necessarily serializing if it returns failure.
404 // Callers must compensate as needed.
405
406 int ObjectMonitor::TryLock(Thread * Self) {
407 void * own = _owner;
408 if (own != NULL) return 0;
409 if (Atomic::cmpxchg(Self, &_owner, (void*)NULL) == NULL) {
410 // Either guarantee _recursions == 0 or set _recursions = 0.
411 assert(_recursions == 0, "invariant");
412 assert(_owner == Self, "invariant");
413 return 1;
414 }
415 // The lock had been free momentarily, but we lost the race to the lock.
416 // Interference -- the CAS failed.
417 // We can either return -1 or retry.
418 // Retry doesn't make as much sense because the lock was just acquired.
419 return -1;
420 }
421
422 #define MAX_RECHECK_INTERVAL 1000
423
424 void ObjectMonitor::EnterI(TRAPS) {
425 Thread * const Self = THREAD;
426 assert(Self->is_Java_thread(), "invariant");
427 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
428
429 // Try the lock - TATAS
430 if (TryLock (Self) > 0) {
431 assert(_succ != Self, "invariant");
432 assert(_owner == Self, "invariant");
433 assert(_Responsible != Self, "invariant");
434 return;
435 }
436
437 DeferredInitialize();
438
439 // We try one round of spinning *before* enqueueing Self.
440 //
441 // If the _owner is ready but OFFPROC we could use a YieldTo()
442 // operation to donate the remainder of this thread's quantum
443 // to the owner. This has subtle but beneficial affinity
444 // effects.
445
446 if (TrySpin (Self) > 0) {
447 assert(_owner == Self, "invariant");
448 assert(_succ != Self, "invariant");
449 assert(_Responsible != Self, "invariant");
450 return;
451 }
452
453 // The Spin failed -- Enqueue and park the thread ...
454 assert(_succ != Self, "invariant");
455 assert(_owner != Self, "invariant");
456 assert(_Responsible != Self, "invariant");
457
458 // Enqueue "Self" on ObjectMonitor's _cxq.
459 //
460 // Node acts as a proxy for Self.
461 // As an aside, if were to ever rewrite the synchronization code mostly
462 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
463 // Java objects. This would avoid awkward lifecycle and liveness issues,
464 // as well as eliminate a subset of ABA issues.
465 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
466
467 ObjectWaiter node(Self);
468 Self->_ParkEvent->reset();
469 node._prev = (ObjectWaiter *) 0xBAD;
470 node.TState = ObjectWaiter::TS_CXQ;
471
472 // Push "Self" onto the front of the _cxq.
473 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
474 // Note that spinning tends to reduce the rate at which threads
475 // enqueue and dequeue on EntryList|cxq.
476 ObjectWaiter * nxt;
477 for (;;) {
478 node._next = nxt = _cxq;
479 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
480
481 // Interference - the CAS failed because _cxq changed. Just retry.
482 // As an optional optimization we retry the lock.
483 if (TryLock (Self) > 0) {
484 assert(_succ != Self, "invariant");
485 assert(_owner == Self, "invariant");
486 assert(_Responsible != Self, "invariant");
487 return;
488 }
489 }
490
491 // Check for cxq|EntryList edge transition to non-null. This indicates
492 // the onset of contention. While contention persists exiting threads
493 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
494 // operations revert to the faster 1-0 mode. This enter operation may interleave
495 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
496 // arrange for one of the contending thread to use a timed park() operations
497 // to detect and recover from the race. (Stranding is form of progress failure
498 // where the monitor is unlocked but all the contending threads remain parked).
499 // That is, at least one of the contended threads will periodically poll _owner.
500 // One of the contending threads will become the designated "Responsible" thread.
501 // The Responsible thread uses a timed park instead of a normal indefinite park
502 // operation -- it periodically wakes and checks for and recovers from potential
503 // strandings admitted by 1-0 exit operations. We need at most one Responsible
504 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
505 // be responsible for a monitor.
506 //
507 // Currently, one of the contended threads takes on the added role of "Responsible".
508 // A viable alternative would be to use a dedicated "stranding checker" thread
509 // that periodically iterated over all the threads (or active monitors) and unparked
510 // successors where there was risk of stranding. This would help eliminate the
511 // timer scalability issues we see on some platforms as we'd only have one thread
512 // -- the checker -- parked on a timer.
513
514 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
515 // Try to assume the role of responsible thread for the monitor.
516 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
517 Atomic::cmpxchg(Self, &_Responsible, (Thread*)NULL);
518 }
519
520 // The lock might have been released while this thread was occupied queueing
521 // itself onto _cxq. To close the race and avoid "stranding" and
522 // progress-liveness failure we must resample-retry _owner before parking.
523 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
524 // In this case the ST-MEMBAR is accomplished with CAS().
525 //
526 // TODO: Defer all thread state transitions until park-time.
527 // Since state transitions are heavy and inefficient we'd like
528 // to defer the state transitions until absolutely necessary,
529 // and in doing so avoid some transitions ...
530
531 TEVENT(Inflated enter - Contention);
532 int nWakeups = 0;
533 int recheckInterval = 1;
534
535 for (;;) {
536
537 if (TryLock(Self) > 0) break;
538 assert(_owner != Self, "invariant");
539
540 if ((SyncFlags & 2) && _Responsible == NULL) {
541 Atomic::cmpxchg(Self, &_Responsible, (Thread*)NULL);
542 }
543
544 // park self
545 if (_Responsible == Self || (SyncFlags & 1)) {
546 TEVENT(Inflated enter - park TIMED);
547 Self->_ParkEvent->park((jlong) recheckInterval);
548 // Increase the recheckInterval, but clamp the value.
549 recheckInterval *= 8;
550 if (recheckInterval > MAX_RECHECK_INTERVAL) {
551 recheckInterval = MAX_RECHECK_INTERVAL;
552 }
553 } else {
554 TEVENT(Inflated enter - park UNTIMED);
555 Self->_ParkEvent->park();
556 }
557
558 if (TryLock(Self) > 0) break;
559
560 // The lock is still contested.
561 // Keep a tally of the # of futile wakeups.
562 // Note that the counter is not protected by a lock or updated by atomics.
563 // That is by design - we trade "lossy" counters which are exposed to
564 // races during updates for a lower probe effect.
565 TEVENT(Inflated enter - Futile wakeup);
566 // This PerfData object can be used in parallel with a safepoint.
567 // See the work around in PerfDataManager::destroy().
568 OM_PERFDATA_OP(FutileWakeups, inc());
569 ++nWakeups;
570
571 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
572 // We can defer clearing _succ until after the spin completes
573 // TrySpin() must tolerate being called with _succ == Self.
574 // Try yet another round of adaptive spinning.
575 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
576
577 // We can find that we were unpark()ed and redesignated _succ while
578 // we were spinning. That's harmless. If we iterate and call park(),
579 // park() will consume the event and return immediately and we'll
580 // just spin again. This pattern can repeat, leaving _succ to simply
581 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
582 // Alternately, we can sample fired() here, and if set, forgo spinning
583 // in the next iteration.
584
585 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
586 Self->_ParkEvent->reset();
587 OrderAccess::fence();
588 }
589 if (_succ == Self) _succ = NULL;
590
591 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
592 OrderAccess::fence();
593 }
594
595 // Egress :
596 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
597 // Normally we'll find Self on the EntryList .
598 // From the perspective of the lock owner (this thread), the
599 // EntryList is stable and cxq is prepend-only.
600 // The head of cxq is volatile but the interior is stable.
601 // In addition, Self.TState is stable.
602
603 assert(_owner == Self, "invariant");
604 assert(object() != NULL, "invariant");
605 // I'd like to write:
606 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
607 // but as we're at a safepoint that's not safe.
608
609 UnlinkAfterAcquire(Self, &node);
610 if (_succ == Self) _succ = NULL;
611
612 assert(_succ != Self, "invariant");
613 if (_Responsible == Self) {
614 _Responsible = NULL;
615 OrderAccess::fence(); // Dekker pivot-point
616
617 // We may leave threads on cxq|EntryList without a designated
618 // "Responsible" thread. This is benign. When this thread subsequently
619 // exits the monitor it can "see" such preexisting "old" threads --
620 // threads that arrived on the cxq|EntryList before the fence, above --
621 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
622 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
623 // non-null and elect a new "Responsible" timer thread.
624 //
625 // This thread executes:
626 // ST Responsible=null; MEMBAR (in enter epilogue - here)
627 // LD cxq|EntryList (in subsequent exit)
628 //
629 // Entering threads in the slow/contended path execute:
630 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
631 // The (ST cxq; MEMBAR) is accomplished with CAS().
632 //
633 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
634 // exit operation from floating above the ST Responsible=null.
635 }
636
637 // We've acquired ownership with CAS().
638 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
639 // But since the CAS() this thread may have also stored into _succ,
640 // EntryList, cxq or Responsible. These meta-data updates must be
641 // visible __before this thread subsequently drops the lock.
642 // Consider what could occur if we didn't enforce this constraint --
643 // STs to monitor meta-data and user-data could reorder with (become
644 // visible after) the ST in exit that drops ownership of the lock.
645 // Some other thread could then acquire the lock, but observe inconsistent
646 // or old monitor meta-data and heap data. That violates the JMM.
647 // To that end, the 1-0 exit() operation must have at least STST|LDST
648 // "release" barrier semantics. Specifically, there must be at least a
649 // STST|LDST barrier in exit() before the ST of null into _owner that drops
650 // the lock. The barrier ensures that changes to monitor meta-data and data
651 // protected by the lock will be visible before we release the lock, and
652 // therefore before some other thread (CPU) has a chance to acquire the lock.
653 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
654 //
655 // Critically, any prior STs to _succ or EntryList must be visible before
656 // the ST of null into _owner in the *subsequent* (following) corresponding
657 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
658 // execute a serializing instruction.
659
660 if (SyncFlags & 8) {
661 OrderAccess::fence();
662 }
663 return;
664 }
665
666 // ReenterI() is a specialized inline form of the latter half of the
667 // contended slow-path from EnterI(). We use ReenterI() only for
668 // monitor reentry in wait().
669 //
670 // In the future we should reconcile EnterI() and ReenterI(), adding
671 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
672 // loop accordingly.
673
674 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
675 assert(Self != NULL, "invariant");
676 assert(SelfNode != NULL, "invariant");
677 assert(SelfNode->_thread == Self, "invariant");
678 assert(_waiters > 0, "invariant");
679 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
680 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
681 JavaThread * jt = (JavaThread *) Self;
682
683 int nWakeups = 0;
684 for (;;) {
685 ObjectWaiter::TStates v = SelfNode->TState;
686 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
687 assert(_owner != Self, "invariant");
688
689 if (TryLock(Self) > 0) break;
690 if (TrySpin(Self) > 0) break;
691
692 TEVENT(Wait Reentry - parking);
693
694 // State transition wrappers around park() ...
695 // ReenterI() wisely defers state transitions until
696 // it's clear we must park the thread.
697 {
698 OSThreadContendState osts(Self->osthread());
699 ThreadBlockInVM tbivm(jt);
700
701 // cleared by handle_special_suspend_equivalent_condition()
702 // or java_suspend_self()
703 jt->set_suspend_equivalent();
704 if (SyncFlags & 1) {
705 Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL);
706 } else {
707 Self->_ParkEvent->park();
708 }
709
710 // were we externally suspended while we were waiting?
711 for (;;) {
712 if (!ExitSuspendEquivalent(jt)) break;
713 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
714 jt->java_suspend_self();
715 jt->set_suspend_equivalent();
716 }
717 }
718
719 // Try again, but just so we distinguish between futile wakeups and
720 // successful wakeups. The following test isn't algorithmically
721 // necessary, but it helps us maintain sensible statistics.
722 if (TryLock(Self) > 0) break;
723
724 // The lock is still contested.
725 // Keep a tally of the # of futile wakeups.
726 // Note that the counter is not protected by a lock or updated by atomics.
727 // That is by design - we trade "lossy" counters which are exposed to
728 // races during updates for a lower probe effect.
729 TEVENT(Wait Reentry - futile wakeup);
730 ++nWakeups;
731
732 // Assuming this is not a spurious wakeup we'll normally
733 // find that _succ == Self.
734 if (_succ == Self) _succ = NULL;
735
736 // Invariant: after clearing _succ a contending thread
737 // *must* retry _owner before parking.
738 OrderAccess::fence();
739
740 // This PerfData object can be used in parallel with a safepoint.
741 // See the work around in PerfDataManager::destroy().
742 OM_PERFDATA_OP(FutileWakeups, inc());
743 }
744
745 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
746 // Normally we'll find Self on the EntryList.
747 // Unlinking from the EntryList is constant-time and atomic-free.
748 // From the perspective of the lock owner (this thread), the
749 // EntryList is stable and cxq is prepend-only.
750 // The head of cxq is volatile but the interior is stable.
751 // In addition, Self.TState is stable.
752
753 assert(_owner == Self, "invariant");
754 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
755 UnlinkAfterAcquire(Self, SelfNode);
756 if (_succ == Self) _succ = NULL;
757 assert(_succ != Self, "invariant");
758 SelfNode->TState = ObjectWaiter::TS_RUN;
759 OrderAccess::fence(); // see comments at the end of EnterI()
760 }
761
762 // By convention we unlink a contending thread from EntryList|cxq immediately
763 // after the thread acquires the lock in ::enter(). Equally, we could defer
764 // unlinking the thread until ::exit()-time.
765
766 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
767 assert(_owner == Self, "invariant");
768 assert(SelfNode->_thread == Self, "invariant");
769
770 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
771 // Normal case: remove Self from the DLL EntryList .
772 // This is a constant-time operation.
773 ObjectWaiter * nxt = SelfNode->_next;
774 ObjectWaiter * prv = SelfNode->_prev;
775 if (nxt != NULL) nxt->_prev = prv;
776 if (prv != NULL) prv->_next = nxt;
777 if (SelfNode == _EntryList) _EntryList = nxt;
778 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
779 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
780 TEVENT(Unlink from EntryList);
781 } else {
782 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
783 // Inopportune interleaving -- Self is still on the cxq.
784 // This usually means the enqueue of self raced an exiting thread.
785 // Normally we'll find Self near the front of the cxq, so
786 // dequeueing is typically fast. If needbe we can accelerate
787 // this with some MCS/CHL-like bidirectional list hints and advisory
788 // back-links so dequeueing from the interior will normally operate
789 // in constant-time.
790 // Dequeue Self from either the head (with CAS) or from the interior
791 // with a linear-time scan and normal non-atomic memory operations.
792 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
793 // and then unlink Self from EntryList. We have to drain eventually,
794 // so it might as well be now.
795
796 ObjectWaiter * v = _cxq;
797 assert(v != NULL, "invariant");
798 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
799 // The CAS above can fail from interference IFF a "RAT" arrived.
800 // In that case Self must be in the interior and can no longer be
801 // at the head of cxq.
802 if (v == SelfNode) {
803 assert(_cxq != v, "invariant");
804 v = _cxq; // CAS above failed - start scan at head of list
805 }
806 ObjectWaiter * p;
807 ObjectWaiter * q = NULL;
808 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
809 q = p;
810 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
811 }
812 assert(v != SelfNode, "invariant");
813 assert(p == SelfNode, "Node not found on cxq");
814 assert(p != _cxq, "invariant");
815 assert(q != NULL, "invariant");
816 assert(q->_next == p, "invariant");
817 q->_next = p->_next;
818 }
819 TEVENT(Unlink from cxq);
820 }
821
822 #ifdef ASSERT
823 // Diagnostic hygiene ...
824 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
825 SelfNode->_next = (ObjectWaiter *) 0xBAD;
826 SelfNode->TState = ObjectWaiter::TS_RUN;
827 #endif
828 }
829
830 // -----------------------------------------------------------------------------
831 // Exit support
832 //
833 // exit()
834 // ~~~~~~
835 // Note that the collector can't reclaim the objectMonitor or deflate
836 // the object out from underneath the thread calling ::exit() as the
837 // thread calling ::exit() never transitions to a stable state.
838 // This inhibits GC, which in turn inhibits asynchronous (and
839 // inopportune) reclamation of "this".
840 //
841 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
842 // There's one exception to the claim above, however. EnterI() can call
843 // exit() to drop a lock if the acquirer has been externally suspended.
844 // In that case exit() is called with _thread_state as _thread_blocked,
845 // but the monitor's _count field is > 0, which inhibits reclamation.
846 //
847 // 1-0 exit
848 // ~~~~~~~~
849 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
850 // the fast-path operators have been optimized so the common ::exit()
851 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
852 // The code emitted by fast_unlock() elides the usual MEMBAR. This
853 // greatly improves latency -- MEMBAR and CAS having considerable local
854 // latency on modern processors -- but at the cost of "stranding". Absent the
855 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
856 // ::enter() path, resulting in the entering thread being stranding
857 // and a progress-liveness failure. Stranding is extremely rare.
858 // We use timers (timed park operations) & periodic polling to detect
859 // and recover from stranding. Potentially stranded threads periodically
860 // wake up and poll the lock. See the usage of the _Responsible variable.
861 //
862 // The CAS() in enter provides for safety and exclusion, while the CAS or
863 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
864 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
865 // We detect and recover from stranding with timers.
866 //
867 // If a thread transiently strands it'll park until (a) another
868 // thread acquires the lock and then drops the lock, at which time the
869 // exiting thread will notice and unpark the stranded thread, or, (b)
870 // the timer expires. If the lock is high traffic then the stranding latency
871 // will be low due to (a). If the lock is low traffic then the odds of
872 // stranding are lower, although the worst-case stranding latency
873 // is longer. Critically, we don't want to put excessive load in the
874 // platform's timer subsystem. We want to minimize both the timer injection
875 // rate (timers created/sec) as well as the number of timers active at
876 // any one time. (more precisely, we want to minimize timer-seconds, which is
877 // the integral of the # of active timers at any instant over time).
878 // Both impinge on OS scalability. Given that, at most one thread parked on
879 // a monitor will use a timer.
880 //
881 // There is also the risk of a futile wake-up. If we drop the lock
882 // another thread can reacquire the lock immediately, and we can
883 // then wake a thread unnecessarily. This is benign, and we've
884 // structured the code so the windows are short and the frequency
885 // of such futile wakups is low.
886
887 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
888 Thread * const Self = THREAD;
889 if (THREAD != _owner) {
890 if (THREAD->is_lock_owned((address) _owner)) {
891 // Transmute _owner from a BasicLock pointer to a Thread address.
892 // We don't need to hold _mutex for this transition.
893 // Non-null to Non-null is safe as long as all readers can
894 // tolerate either flavor.
895 assert(_recursions == 0, "invariant");
896 _owner = THREAD;
897 _recursions = 0;
898 } else {
899 // Apparent unbalanced locking ...
900 // Naively we'd like to throw IllegalMonitorStateException.
901 // As a practical matter we can neither allocate nor throw an
902 // exception as ::exit() can be called from leaf routines.
903 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
904 // Upon deeper reflection, however, in a properly run JVM the only
905 // way we should encounter this situation is in the presence of
906 // unbalanced JNI locking. TODO: CheckJNICalls.
907 // See also: CR4414101
908 TEVENT(Exit - Throw IMSX);
909 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
910 return;
911 }
912 }
913
914 if (_recursions != 0) {
915 _recursions--; // this is simple recursive enter
916 TEVENT(Inflated exit - recursive);
917 return;
918 }
919
920 // Invariant: after setting Responsible=null an thread must execute
921 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
922 if ((SyncFlags & 4) == 0) {
923 _Responsible = NULL;
924 }
925
926 #if INCLUDE_TRACE
927 // get the owner's thread id for the MonitorEnter event
928 // if it is enabled and the thread isn't suspended
929 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
930 _previous_owner_tid = THREAD_TRACE_ID(Self);
931 }
932 #endif
933
934 for (;;) {
935 assert(THREAD == _owner, "invariant");
936
937 if (Knob_ExitPolicy == 0) {
938 // release semantics: prior loads and stores from within the critical section
939 // must not float (reorder) past the following store that drops the lock.
940 // On SPARC that requires MEMBAR #loadstore|#storestore.
941 // But of course in TSO #loadstore|#storestore is not required.
942 // I'd like to write one of the following:
943 // A. OrderAccess::release() ; _owner = NULL
944 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
945 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
946 // store into a _dummy variable. That store is not needed, but can result
947 // in massive wasteful coherency traffic on classic SMP systems.
948 // Instead, I use release_store(), which is implemented as just a simple
949 // ST on x64, x86 and SPARC.
950 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
951 OrderAccess::storeload(); // See if we need to wake a successor
952 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
953 TEVENT(Inflated exit - simple egress);
954 return;
955 }
956 TEVENT(Inflated exit - complex egress);
957 // Other threads are blocked trying to acquire the lock.
958
959 // Normally the exiting thread is responsible for ensuring succession,
960 // but if other successors are ready or other entering threads are spinning
961 // then this thread can simply store NULL into _owner and exit without
962 // waking a successor. The existence of spinners or ready successors
963 // guarantees proper succession (liveness). Responsibility passes to the
964 // ready or running successors. The exiting thread delegates the duty.
965 // More precisely, if a successor already exists this thread is absolved
966 // of the responsibility of waking (unparking) one.
967 //
968 // The _succ variable is critical to reducing futile wakeup frequency.
969 // _succ identifies the "heir presumptive" thread that has been made
970 // ready (unparked) but that has not yet run. We need only one such
971 // successor thread to guarantee progress.
972 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
973 // section 3.3 "Futile Wakeup Throttling" for details.
974 //
975 // Note that spinners in Enter() also set _succ non-null.
976 // In the current implementation spinners opportunistically set
977 // _succ so that exiting threads might avoid waking a successor.
978 // Another less appealing alternative would be for the exiting thread
979 // to drop the lock and then spin briefly to see if a spinner managed
980 // to acquire the lock. If so, the exiting thread could exit
981 // immediately without waking a successor, otherwise the exiting
982 // thread would need to dequeue and wake a successor.
983 // (Note that we'd need to make the post-drop spin short, but no
984 // shorter than the worst-case round-trip cache-line migration time.
985 // The dropped lock needs to become visible to the spinner, and then
986 // the acquisition of the lock by the spinner must become visible to
987 // the exiting thread).
988
989 // It appears that an heir-presumptive (successor) must be made ready.
990 // Only the current lock owner can manipulate the EntryList or
991 // drain _cxq, so we need to reacquire the lock. If we fail
992 // to reacquire the lock the responsibility for ensuring succession
993 // falls to the new owner.
994 //
995 if (Atomic::cmpxchg(THREAD, &_owner, (void*)NULL) != NULL) {
996 return;
997 }
998 TEVENT(Exit - Reacquired);
999 } else {
1000 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1001 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
1002 OrderAccess::storeload();
1003 // Ratify the previously observed values.
1004 if (_cxq == NULL || _succ != NULL) {
1005 TEVENT(Inflated exit - simple egress);
1006 return;
1007 }
1008
1009 // inopportune interleaving -- the exiting thread (this thread)
1010 // in the fast-exit path raced an entering thread in the slow-enter
1011 // path.
1012 // We have two choices:
1013 // A. Try to reacquire the lock.
1014 // If the CAS() fails return immediately, otherwise
1015 // we either restart/rerun the exit operation, or simply
1016 // fall-through into the code below which wakes a successor.
1017 // B. If the elements forming the EntryList|cxq are TSM
1018 // we could simply unpark() the lead thread and return
1019 // without having set _succ.
1020 if (Atomic::cmpxchg(THREAD, &_owner, (void*)NULL) != NULL) {
1021 TEVENT(Inflated exit - reacquired succeeded);
1022 return;
1023 }
1024 TEVENT(Inflated exit - reacquired failed);
1025 } else {
1026 TEVENT(Inflated exit - complex egress);
1027 }
1028 }
1029
1030 guarantee(_owner == THREAD, "invariant");
1031
1032 ObjectWaiter * w = NULL;
1033 int QMode = Knob_QMode;
1034
1035 if (QMode == 2 && _cxq != NULL) {
1036 // QMode == 2 : cxq has precedence over EntryList.
1037 // Try to directly wake a successor from the cxq.
1038 // If successful, the successor will need to unlink itself from cxq.
1039 w = _cxq;
1040 assert(w != NULL, "invariant");
1041 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1042 ExitEpilog(Self, w);
1043 return;
1044 }
1045
1046 if (QMode == 3 && _cxq != NULL) {
1047 // Aggressively drain cxq into EntryList at the first opportunity.
1048 // This policy ensure that recently-run threads live at the head of EntryList.
1049 // Drain _cxq into EntryList - bulk transfer.
1050 // First, detach _cxq.
1051 // The following loop is tantamount to: w = swap(&cxq, NULL)
1052 w = _cxq;
1053 for (;;) {
1054 assert(w != NULL, "Invariant");
1055 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1056 if (u == w) break;
1057 w = u;
1058 }
1059 assert(w != NULL, "invariant");
1060
1061 ObjectWaiter * q = NULL;
1062 ObjectWaiter * p;
1063 for (p = w; p != NULL; p = p->_next) {
1064 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1065 p->TState = ObjectWaiter::TS_ENTER;
1066 p->_prev = q;
1067 q = p;
1068 }
1069
1070 // Append the RATs to the EntryList
1071 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1072 ObjectWaiter * Tail;
1073 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1074 Tail = Tail->_next)
1075 /* empty */;
1076 if (Tail == NULL) {
1077 _EntryList = w;
1078 } else {
1079 Tail->_next = w;
1080 w->_prev = Tail;
1081 }
1082
1083 // Fall thru into code that tries to wake a successor from EntryList
1084 }
1085
1086 if (QMode == 4 && _cxq != NULL) {
1087 // Aggressively drain cxq into EntryList at the first opportunity.
1088 // This policy ensure that recently-run threads live at the head of EntryList.
1089
1090 // Drain _cxq into EntryList - bulk transfer.
1091 // First, detach _cxq.
1092 // The following loop is tantamount to: w = swap(&cxq, NULL)
1093 w = _cxq;
1094 for (;;) {
1095 assert(w != NULL, "Invariant");
1096 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1097 if (u == w) break;
1098 w = u;
1099 }
1100 assert(w != NULL, "invariant");
1101
1102 ObjectWaiter * q = NULL;
1103 ObjectWaiter * p;
1104 for (p = w; p != NULL; p = p->_next) {
1105 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1106 p->TState = ObjectWaiter::TS_ENTER;
1107 p->_prev = q;
1108 q = p;
1109 }
1110
1111 // Prepend the RATs to the EntryList
1112 if (_EntryList != NULL) {
1113 q->_next = _EntryList;
1114 _EntryList->_prev = q;
1115 }
1116 _EntryList = w;
1117
1118 // Fall thru into code that tries to wake a successor from EntryList
1119 }
1120
1121 w = _EntryList;
1122 if (w != NULL) {
1123 // I'd like to write: guarantee (w->_thread != Self).
1124 // But in practice an exiting thread may find itself on the EntryList.
1125 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1126 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1127 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1128 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1129 // release the lock "O". T2 resumes immediately after the ST of null into
1130 // _owner, above. T2 notices that the EntryList is populated, so it
1131 // reacquires the lock and then finds itself on the EntryList.
1132 // Given all that, we have to tolerate the circumstance where "w" is
1133 // associated with Self.
1134 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1135 ExitEpilog(Self, w);
1136 return;
1137 }
1138
1139 // If we find that both _cxq and EntryList are null then just
1140 // re-run the exit protocol from the top.
1141 w = _cxq;
1142 if (w == NULL) continue;
1143
1144 // Drain _cxq into EntryList - bulk transfer.
1145 // First, detach _cxq.
1146 // The following loop is tantamount to: w = swap(&cxq, NULL)
1147 for (;;) {
1148 assert(w != NULL, "Invariant");
1149 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1150 if (u == w) break;
1151 w = u;
1152 }
1153 TEVENT(Inflated exit - drain cxq into EntryList);
1154
1155 assert(w != NULL, "invariant");
1156 assert(_EntryList == NULL, "invariant");
1157
1158 // Convert the LIFO SLL anchored by _cxq into a DLL.
1159 // The list reorganization step operates in O(LENGTH(w)) time.
1160 // It's critical that this step operate quickly as
1161 // "Self" still holds the outer-lock, restricting parallelism
1162 // and effectively lengthening the critical section.
1163 // Invariant: s chases t chases u.
1164 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1165 // we have faster access to the tail.
1166
1167 if (QMode == 1) {
1168 // QMode == 1 : drain cxq to EntryList, reversing order
1169 // We also reverse the order of the list.
1170 ObjectWaiter * s = NULL;
1171 ObjectWaiter * t = w;
1172 ObjectWaiter * u = NULL;
1173 while (t != NULL) {
1174 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1175 t->TState = ObjectWaiter::TS_ENTER;
1176 u = t->_next;
1177 t->_prev = u;
1178 t->_next = s;
1179 s = t;
1180 t = u;
1181 }
1182 _EntryList = s;
1183 assert(s != NULL, "invariant");
1184 } else {
1185 // QMode == 0 or QMode == 2
1186 _EntryList = w;
1187 ObjectWaiter * q = NULL;
1188 ObjectWaiter * p;
1189 for (p = w; p != NULL; p = p->_next) {
1190 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1191 p->TState = ObjectWaiter::TS_ENTER;
1192 p->_prev = q;
1193 q = p;
1194 }
1195 }
1196
1197 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1198 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1199
1200 // See if we can abdicate to a spinner instead of waking a thread.
1201 // A primary goal of the implementation is to reduce the
1202 // context-switch rate.
1203 if (_succ != NULL) continue;
1204
1205 w = _EntryList;
1206 if (w != NULL) {
1207 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1208 ExitEpilog(Self, w);
1209 return;
1210 }
1211 }
1212 }
1213
1214 // ExitSuspendEquivalent:
1215 // A faster alternate to handle_special_suspend_equivalent_condition()
1216 //
1217 // handle_special_suspend_equivalent_condition() unconditionally
1218 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1219 // operations have high latency. Note that in ::enter() we call HSSEC
1220 // while holding the monitor, so we effectively lengthen the critical sections.
1221 //
1222 // There are a number of possible solutions:
1223 //
1224 // A. To ameliorate the problem we might also defer state transitions
1225 // to as late as possible -- just prior to parking.
1226 // Given that, we'd call HSSEC after having returned from park(),
1227 // but before attempting to acquire the monitor. This is only a
1228 // partial solution. It avoids calling HSSEC while holding the
1229 // monitor (good), but it still increases successor reacquisition latency --
1230 // the interval between unparking a successor and the time the successor
1231 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1232 // If we use this technique we can also avoid EnterI()-exit() loop
1233 // in ::enter() where we iteratively drop the lock and then attempt
1234 // to reacquire it after suspending.
1235 //
1236 // B. In the future we might fold all the suspend bits into a
1237 // composite per-thread suspend flag and then update it with CAS().
1238 // Alternately, a Dekker-like mechanism with multiple variables
1239 // would suffice:
1240 // ST Self->_suspend_equivalent = false
1241 // MEMBAR
1242 // LD Self_>_suspend_flags
1243 //
1244 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1245 // with a more efficient implementation, the need to use "FastHSSEC" has
1246 // decreased. - Dave
1247
1248
1249 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1250 const int Mode = Knob_FastHSSEC;
1251 if (Mode && !jSelf->is_external_suspend()) {
1252 assert(jSelf->is_suspend_equivalent(), "invariant");
1253 jSelf->clear_suspend_equivalent();
1254 if (2 == Mode) OrderAccess::storeload();
1255 if (!jSelf->is_external_suspend()) return false;
1256 // We raced a suspension -- fall thru into the slow path
1257 TEVENT(ExitSuspendEquivalent - raced);
1258 jSelf->set_suspend_equivalent();
1259 }
1260 return jSelf->handle_special_suspend_equivalent_condition();
1261 }
1262
1263
1264 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1265 assert(_owner == Self, "invariant");
1266
1267 // Exit protocol:
1268 // 1. ST _succ = wakee
1269 // 2. membar #loadstore|#storestore;
1270 // 2. ST _owner = NULL
1271 // 3. unpark(wakee)
1272
1273 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1274 ParkEvent * Trigger = Wakee->_event;
1275
1276 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1277 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1278 // out-of-scope (non-extant).
1279 Wakee = NULL;
1280
1281 // Drop the lock
1282 OrderAccess::release_store(&_owner, (void*)NULL);
1283 OrderAccess::fence(); // ST _owner vs LD in unpark()
1284
1285 if (SafepointSynchronize::do_call_back()) {
1286 TEVENT(unpark before SAFEPOINT);
1287 }
1288
1289 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1290 Trigger->unpark();
1291
1292 // Maintain stats and report events to JVMTI
1293 OM_PERFDATA_OP(Parks, inc());
1294 }
1295
1296
1297 // -----------------------------------------------------------------------------
1298 // Class Loader deadlock handling.
1299 //
1300 // complete_exit exits a lock returning recursion count
1301 // complete_exit/reenter operate as a wait without waiting
1302 // complete_exit requires an inflated monitor
1303 // The _owner field is not always the Thread addr even with an
1304 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1305 // thread due to contention.
1306 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1307 Thread * const Self = THREAD;
1308 assert(Self->is_Java_thread(), "Must be Java thread!");
1309 JavaThread *jt = (JavaThread *)THREAD;
1310
1311 DeferredInitialize();
1312
1313 if (THREAD != _owner) {
1314 if (THREAD->is_lock_owned ((address)_owner)) {
1315 assert(_recursions == 0, "internal state error");
1316 _owner = THREAD; // Convert from basiclock addr to Thread addr
1317 _recursions = 0;
1318 }
1319 }
1320
1321 guarantee(Self == _owner, "complete_exit not owner");
1322 intptr_t save = _recursions; // record the old recursion count
1323 _recursions = 0; // set the recursion level to be 0
1324 exit(true, Self); // exit the monitor
1325 guarantee(_owner != Self, "invariant");
1326 return save;
1327 }
1328
1329 // reenter() enters a lock and sets recursion count
1330 // complete_exit/reenter operate as a wait without waiting
1331 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1332 Thread * const Self = THREAD;
1333 assert(Self->is_Java_thread(), "Must be Java thread!");
1334 JavaThread *jt = (JavaThread *)THREAD;
1335
1336 guarantee(_owner != Self, "reenter already owner");
1337 enter(THREAD); // enter the monitor
1338 guarantee(_recursions == 0, "reenter recursion");
1339 _recursions = recursions;
1340 return;
1341 }
1342
1343
1344 // -----------------------------------------------------------------------------
1345 // A macro is used below because there may already be a pending
1346 // exception which should not abort the execution of the routines
1347 // which use this (which is why we don't put this into check_slow and
1348 // call it with a CHECK argument).
1349
1350 #define CHECK_OWNER() \
1351 do { \
1352 if (THREAD != _owner) { \
1353 if (THREAD->is_lock_owned((address) _owner)) { \
1354 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1355 _recursions = 0; \
1356 } else { \
1357 TEVENT(Throw IMSX); \
1358 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1359 } \
1360 } \
1361 } while (false)
1362
1363 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1364 // TODO-FIXME: remove check_slow() -- it's likely dead.
1365
1366 void ObjectMonitor::check_slow(TRAPS) {
1367 TEVENT(check_slow - throw IMSX);
1368 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1369 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1370 }
1371
1372 static int Adjust(volatile int * adr, int dx) {
1373 int v;
1374 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1375 return v;
1376 }
1377
1378 // helper method for posting a monitor wait event
1379 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1380 jlong notifier_tid,
1381 jlong timeout,
1382 bool timedout) {
1383 assert(event != NULL, "invariant");
1384 event->set_monitorClass(((oop)this->object())->klass());
1385 event->set_timeout(timeout);
1386 event->set_address((TYPE_ADDRESS)this->object_addr());
1387 event->set_notifier(notifier_tid);
1388 event->set_timedOut(timedout);
1389 event->commit();
1390 }
1391
1392 // -----------------------------------------------------------------------------
1393 // Wait/Notify/NotifyAll
1394 //
1395 // Note: a subset of changes to ObjectMonitor::wait()
1396 // will need to be replicated in complete_exit
1397 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1398 Thread * const Self = THREAD;
1399 assert(Self->is_Java_thread(), "Must be Java thread!");
1400 JavaThread *jt = (JavaThread *)THREAD;
1401
1402 DeferredInitialize();
1403
1404 // Throw IMSX or IEX.
1405 CHECK_OWNER();
1406
1407 EventJavaMonitorWait event;
1408
1409 // check for a pending interrupt
1410 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1411 // post monitor waited event. Note that this is past-tense, we are done waiting.
1412 if (JvmtiExport::should_post_monitor_waited()) {
1413 // Note: 'false' parameter is passed here because the
1414 // wait was not timed out due to thread interrupt.
1415 JvmtiExport::post_monitor_waited(jt, this, false);
1416
1417 // In this short circuit of the monitor wait protocol, the
1418 // current thread never drops ownership of the monitor and
1419 // never gets added to the wait queue so the current thread
1420 // cannot be made the successor. This means that the
1421 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1422 // consume an unpark() meant for the ParkEvent associated with
1423 // this ObjectMonitor.
1424 }
1425 if (event.should_commit()) {
1426 post_monitor_wait_event(&event, 0, millis, false);
1427 }
1428 TEVENT(Wait - Throw IEX);
1429 THROW(vmSymbols::java_lang_InterruptedException());
1430 return;
1431 }
1432
1433 TEVENT(Wait);
1434
1435 assert(Self->_Stalled == 0, "invariant");
1436 Self->_Stalled = intptr_t(this);
1437 jt->set_current_waiting_monitor(this);
1438
1439 // create a node to be put into the queue
1440 // Critically, after we reset() the event but prior to park(), we must check
1441 // for a pending interrupt.
1442 ObjectWaiter node(Self);
1443 node.TState = ObjectWaiter::TS_WAIT;
1444 Self->_ParkEvent->reset();
1445 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1446
1447 // Enter the waiting queue, which is a circular doubly linked list in this case
1448 // but it could be a priority queue or any data structure.
1449 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1450 // by the the owner of the monitor *except* in the case where park()
1451 // returns because of a timeout of interrupt. Contention is exceptionally rare
1452 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1453
1454 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1455 AddWaiter(&node);
1456 Thread::SpinRelease(&_WaitSetLock);
1457
1458 if ((SyncFlags & 4) == 0) {
1459 _Responsible = NULL;
1460 }
1461 intptr_t save = _recursions; // record the old recursion count
1462 _waiters++; // increment the number of waiters
1463 _recursions = 0; // set the recursion level to be 1
1464 exit(true, Self); // exit the monitor
1465 guarantee(_owner != Self, "invariant");
1466
1467 // The thread is on the WaitSet list - now park() it.
1468 // On MP systems it's conceivable that a brief spin before we park
1469 // could be profitable.
1470 //
1471 // TODO-FIXME: change the following logic to a loop of the form
1472 // while (!timeout && !interrupted && _notified == 0) park()
1473
1474 int ret = OS_OK;
1475 int WasNotified = 0;
1476 { // State transition wrappers
1477 OSThread* osthread = Self->osthread();
1478 OSThreadWaitState osts(osthread, true);
1479 {
1480 ThreadBlockInVM tbivm(jt);
1481 // Thread is in thread_blocked state and oop access is unsafe.
1482 jt->set_suspend_equivalent();
1483
1484 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1485 // Intentionally empty
1486 } else if (node._notified == 0) {
1487 if (millis <= 0) {
1488 Self->_ParkEvent->park();
1489 } else {
1490 ret = Self->_ParkEvent->park(millis);
1491 }
1492 }
1493
1494 // were we externally suspended while we were waiting?
1495 if (ExitSuspendEquivalent (jt)) {
1496 // TODO-FIXME: add -- if succ == Self then succ = null.
1497 jt->java_suspend_self();
1498 }
1499
1500 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1501
1502 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1503 // from the WaitSet to the EntryList.
1504 // See if we need to remove Node from the WaitSet.
1505 // We use double-checked locking to avoid grabbing _WaitSetLock
1506 // if the thread is not on the wait queue.
1507 //
1508 // Note that we don't need a fence before the fetch of TState.
1509 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1510 // written by the is thread. (perhaps the fetch might even be satisfied
1511 // by a look-aside into the processor's own store buffer, although given
1512 // the length of the code path between the prior ST and this load that's
1513 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1514 // then we'll acquire the lock and then re-fetch a fresh TState value.
1515 // That is, we fail toward safety.
1516
1517 if (node.TState == ObjectWaiter::TS_WAIT) {
1518 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1519 if (node.TState == ObjectWaiter::TS_WAIT) {
1520 DequeueSpecificWaiter(&node); // unlink from WaitSet
1521 assert(node._notified == 0, "invariant");
1522 node.TState = ObjectWaiter::TS_RUN;
1523 }
1524 Thread::SpinRelease(&_WaitSetLock);
1525 }
1526
1527 // The thread is now either on off-list (TS_RUN),
1528 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1529 // The Node's TState variable is stable from the perspective of this thread.
1530 // No other threads will asynchronously modify TState.
1531 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1532 OrderAccess::loadload();
1533 if (_succ == Self) _succ = NULL;
1534 WasNotified = node._notified;
1535
1536 // Reentry phase -- reacquire the monitor.
1537 // re-enter contended monitor after object.wait().
1538 // retain OBJECT_WAIT state until re-enter successfully completes
1539 // Thread state is thread_in_vm and oop access is again safe,
1540 // although the raw address of the object may have changed.
1541 // (Don't cache naked oops over safepoints, of course).
1542
1543 // post monitor waited event. Note that this is past-tense, we are done waiting.
1544 if (JvmtiExport::should_post_monitor_waited()) {
1545 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1546
1547 if (node._notified != 0 && _succ == Self) {
1548 // In this part of the monitor wait-notify-reenter protocol it
1549 // is possible (and normal) for another thread to do a fastpath
1550 // monitor enter-exit while this thread is still trying to get
1551 // to the reenter portion of the protocol.
1552 //
1553 // The ObjectMonitor was notified and the current thread is
1554 // the successor which also means that an unpark() has already
1555 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1556 // consume the unpark() that was done when the successor was
1557 // set because the same ParkEvent is shared between Java
1558 // monitors and JVM/TI RawMonitors (for now).
1559 //
1560 // We redo the unpark() to ensure forward progress, i.e., we
1561 // don't want all pending threads hanging (parked) with none
1562 // entering the unlocked monitor.
1563 node._event->unpark();
1564 }
1565 }
1566
1567 if (event.should_commit()) {
1568 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1569 }
1570
1571 OrderAccess::fence();
1572
1573 assert(Self->_Stalled != 0, "invariant");
1574 Self->_Stalled = 0;
1575
1576 assert(_owner != Self, "invariant");
1577 ObjectWaiter::TStates v = node.TState;
1578 if (v == ObjectWaiter::TS_RUN) {
1579 enter(Self);
1580 } else {
1581 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1582 ReenterI(Self, &node);
1583 node.wait_reenter_end(this);
1584 }
1585
1586 // Self has reacquired the lock.
1587 // Lifecycle - the node representing Self must not appear on any queues.
1588 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1589 // want residual elements associated with this thread left on any lists.
1590 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1591 assert(_owner == Self, "invariant");
1592 assert(_succ != Self, "invariant");
1593 } // OSThreadWaitState()
1594
1595 jt->set_current_waiting_monitor(NULL);
1596
1597 guarantee(_recursions == 0, "invariant");
1598 _recursions = save; // restore the old recursion count
1599 _waiters--; // decrement the number of waiters
1600
1601 // Verify a few postconditions
1602 assert(_owner == Self, "invariant");
1603 assert(_succ != Self, "invariant");
1604 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1605
1606 if (SyncFlags & 32) {
1607 OrderAccess::fence();
1608 }
1609
1610 // check if the notification happened
1611 if (!WasNotified) {
1612 // no, it could be timeout or Thread.interrupt() or both
1613 // check for interrupt event, otherwise it is timeout
1614 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1615 TEVENT(Wait - throw IEX from epilog);
1616 THROW(vmSymbols::java_lang_InterruptedException());
1617 }
1618 }
1619
1620 // NOTE: Spurious wake up will be consider as timeout.
1621 // Monitor notify has precedence over thread interrupt.
1622 }
1623
1624
1625 // Consider:
1626 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1627 // then instead of transferring a thread from the WaitSet to the EntryList
1628 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1629
1630 void ObjectMonitor::INotify(Thread * Self) {
1631 const int policy = Knob_MoveNotifyee;
1632
1633 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1634 ObjectWaiter * iterator = DequeueWaiter();
1635 if (iterator != NULL) {
1636 TEVENT(Notify1 - Transfer);
1637 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1638 guarantee(iterator->_notified == 0, "invariant");
1639 // Disposition - what might we do with iterator ?
1640 // a. add it directly to the EntryList - either tail (policy == 1)
1641 // or head (policy == 0).
1642 // b. push it onto the front of the _cxq (policy == 2).
1643 // For now we use (b).
1644 if (policy != 4) {
1645 iterator->TState = ObjectWaiter::TS_ENTER;
1646 }
1647 iterator->_notified = 1;
1648 iterator->_notifier_tid = THREAD_TRACE_ID(Self);
1649
1650 ObjectWaiter * list = _EntryList;
1651 if (list != NULL) {
1652 assert(list->_prev == NULL, "invariant");
1653 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1654 assert(list != iterator, "invariant");
1655 }
1656
1657 if (policy == 0) { // prepend to EntryList
1658 if (list == NULL) {
1659 iterator->_next = iterator->_prev = NULL;
1660 _EntryList = iterator;
1661 } else {
1662 list->_prev = iterator;
1663 iterator->_next = list;
1664 iterator->_prev = NULL;
1665 _EntryList = iterator;
1666 }
1667 } else if (policy == 1) { // append to EntryList
1668 if (list == NULL) {
1669 iterator->_next = iterator->_prev = NULL;
1670 _EntryList = iterator;
1671 } else {
1672 // CONSIDER: finding the tail currently requires a linear-time walk of
1673 // the EntryList. We can make tail access constant-time by converting to
1674 // a CDLL instead of using our current DLL.
1675 ObjectWaiter * tail;
1676 for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1677 assert(tail != NULL && tail->_next == NULL, "invariant");
1678 tail->_next = iterator;
1679 iterator->_prev = tail;
1680 iterator->_next = NULL;
1681 }
1682 } else if (policy == 2) { // prepend to cxq
1683 if (list == NULL) {
1684 iterator->_next = iterator->_prev = NULL;
1685 _EntryList = iterator;
1686 } else {
1687 iterator->TState = ObjectWaiter::TS_CXQ;
1688 for (;;) {
1689 ObjectWaiter * front = _cxq;
1690 iterator->_next = front;
1691 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1692 break;
1693 }
1694 }
1695 }
1696 } else if (policy == 3) { // append to cxq
1697 iterator->TState = ObjectWaiter::TS_CXQ;
1698 for (;;) {
1699 ObjectWaiter * tail = _cxq;
1700 if (tail == NULL) {
1701 iterator->_next = NULL;
1702 if (Atomic::cmpxchg(iterator, &_cxq, (ObjectWaiter*)NULL) == NULL) {
1703 break;
1704 }
1705 } else {
1706 while (tail->_next != NULL) tail = tail->_next;
1707 tail->_next = iterator;
1708 iterator->_prev = tail;
1709 iterator->_next = NULL;
1710 break;
1711 }
1712 }
1713 } else {
1714 ParkEvent * ev = iterator->_event;
1715 iterator->TState = ObjectWaiter::TS_RUN;
1716 OrderAccess::fence();
1717 ev->unpark();
1718 }
1719
1720 // _WaitSetLock protects the wait queue, not the EntryList. We could
1721 // move the add-to-EntryList operation, above, outside the critical section
1722 // protected by _WaitSetLock. In practice that's not useful. With the
1723 // exception of wait() timeouts and interrupts the monitor owner
1724 // is the only thread that grabs _WaitSetLock. There's almost no contention
1725 // on _WaitSetLock so it's not profitable to reduce the length of the
1726 // critical section.
1727
1728 if (policy < 4) {
1729 iterator->wait_reenter_begin(this);
1730 }
1731 }
1732 Thread::SpinRelease(&_WaitSetLock);
1733 }
1734
1735 // Consider: a not-uncommon synchronization bug is to use notify() when
1736 // notifyAll() is more appropriate, potentially resulting in stranded
1737 // threads; this is one example of a lost wakeup. A useful diagnostic
1738 // option is to force all notify() operations to behave as notifyAll().
1739 //
1740 // Note: We can also detect many such problems with a "minimum wait".
1741 // When the "minimum wait" is set to a small non-zero timeout value
1742 // and the program does not hang whereas it did absent "minimum wait",
1743 // that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses
1744 // a "minimum wait" for all park() operations; see the recheckInterval
1745 // variable and MAX_RECHECK_INTERVAL.
1746
1747 void ObjectMonitor::notify(TRAPS) {
1748 CHECK_OWNER();
1749 if (_WaitSet == NULL) {
1750 TEVENT(Empty-Notify);
1751 return;
1752 }
1753 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1754 INotify(THREAD);
1755 OM_PERFDATA_OP(Notifications, inc(1));
1756 }
1757
1758
1759 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1760 // from the waitset to the EntryList. This could be done more efficiently with a
1761 // single bulk transfer but in practice it's not time-critical. Beware too,
1762 // that in prepend-mode we invert the order of the waiters. Let's say that the
1763 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1764 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1765
1766 void ObjectMonitor::notifyAll(TRAPS) {
1767 CHECK_OWNER();
1768 if (_WaitSet == NULL) {
1769 TEVENT(Empty-NotifyAll);
1770 return;
1771 }
1772
1773 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1774 int tally = 0;
1775 while (_WaitSet != NULL) {
1776 tally++;
1777 INotify(THREAD);
1778 }
1779
1780 OM_PERFDATA_OP(Notifications, inc(tally));
1781 }
1782
1783 // -----------------------------------------------------------------------------
1784 // Adaptive Spinning Support
1785 //
1786 // Adaptive spin-then-block - rational spinning
1787 //
1788 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1789 // algorithm. On high order SMP systems it would be better to start with
1790 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1791 // a contending thread could enqueue itself on the cxq and then spin locally
1792 // on a thread-specific variable such as its ParkEvent._Event flag.
1793 // That's left as an exercise for the reader. Note that global spinning is
1794 // not problematic on Niagara, as the L2 cache serves the interconnect and
1795 // has both low latency and massive bandwidth.
1796 //
1797 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1798 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1799 // (duration) or we can fix the count at approximately the duration of
1800 // a context switch and vary the frequency. Of course we could also
1801 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1802 // For a description of 'Adaptive spin-then-block mutual exclusion in
1803 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1804 //
1805 // This implementation varies the duration "D", where D varies with
1806 // the success rate of recent spin attempts. (D is capped at approximately
1807 // length of a round-trip context switch). The success rate for recent
1808 // spin attempts is a good predictor of the success rate of future spin
1809 // attempts. The mechanism adapts automatically to varying critical
1810 // section length (lock modality), system load and degree of parallelism.
1811 // D is maintained per-monitor in _SpinDuration and is initialized
1812 // optimistically. Spin frequency is fixed at 100%.
1813 //
1814 // Note that _SpinDuration is volatile, but we update it without locks
1815 // or atomics. The code is designed so that _SpinDuration stays within
1816 // a reasonable range even in the presence of races. The arithmetic
1817 // operations on _SpinDuration are closed over the domain of legal values,
1818 // so at worst a race will install and older but still legal value.
1819 // At the very worst this introduces some apparent non-determinism.
1820 // We might spin when we shouldn't or vice-versa, but since the spin
1821 // count are relatively short, even in the worst case, the effect is harmless.
1822 //
1823 // Care must be taken that a low "D" value does not become an
1824 // an absorbing state. Transient spinning failures -- when spinning
1825 // is overall profitable -- should not cause the system to converge
1826 // on low "D" values. We want spinning to be stable and predictable
1827 // and fairly responsive to change and at the same time we don't want
1828 // it to oscillate, become metastable, be "too" non-deterministic,
1829 // or converge on or enter undesirable stable absorbing states.
1830 //
1831 // We implement a feedback-based control system -- using past behavior
1832 // to predict future behavior. We face two issues: (a) if the
1833 // input signal is random then the spin predictor won't provide optimal
1834 // results, and (b) if the signal frequency is too high then the control
1835 // system, which has some natural response lag, will "chase" the signal.
1836 // (b) can arise from multimodal lock hold times. Transient preemption
1837 // can also result in apparent bimodal lock hold times.
1838 // Although sub-optimal, neither condition is particularly harmful, as
1839 // in the worst-case we'll spin when we shouldn't or vice-versa.
1840 // The maximum spin duration is rather short so the failure modes aren't bad.
1841 // To be conservative, I've tuned the gain in system to bias toward
1842 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1843 // "rings" or oscillates between spinning and not spinning. This happens
1844 // when spinning is just on the cusp of profitability, however, so the
1845 // situation is not dire. The state is benign -- there's no need to add
1846 // hysteresis control to damp the transition rate between spinning and
1847 // not spinning.
1848
1849 // Spinning: Fixed frequency (100%), vary duration
1850 int ObjectMonitor::TrySpin(Thread * Self) {
1851 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1852 int ctr = Knob_FixedSpin;
1853 if (ctr != 0) {
1854 while (--ctr >= 0) {
1855 if (TryLock(Self) > 0) return 1;
1856 SpinPause();
1857 }
1858 return 0;
1859 }
1860
1861 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1862 if (TryLock(Self) > 0) {
1863 // Increase _SpinDuration ...
1864 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1865 // Raising _SpurDuration to the poverty line is key.
1866 int x = _SpinDuration;
1867 if (x < Knob_SpinLimit) {
1868 if (x < Knob_Poverty) x = Knob_Poverty;
1869 _SpinDuration = x + Knob_BonusB;
1870 }
1871 return 1;
1872 }
1873 SpinPause();
1874 }
1875
1876 // Admission control - verify preconditions for spinning
1877 //
1878 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1879 // becoming an absorbing state. Put another way, we spin briefly to
1880 // sample, just in case the system load, parallelism, contention, or lock
1881 // modality changed.
1882 //
1883 // Consider the following alternative:
1884 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1885 // spin attempt. "Periodically" might mean after a tally of
1886 // the # of failed spin attempts (or iterations) reaches some threshold.
1887 // This takes us into the realm of 1-out-of-N spinning, where we
1888 // hold the duration constant but vary the frequency.
1889
1890 ctr = _SpinDuration;
1891 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1892 if (ctr <= 0) return 0;
1893
1894 if (Knob_SuccRestrict && _succ != NULL) return 0;
1895 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1896 TEVENT(Spin abort - notrunnable [TOP]);
1897 return 0;
1898 }
1899
1900 int MaxSpin = Knob_MaxSpinners;
1901 if (MaxSpin >= 0) {
1902 if (_Spinner > MaxSpin) {
1903 TEVENT(Spin abort -- too many spinners);
1904 return 0;
1905 }
1906 // Slightly racy, but benign ...
1907 Adjust(&_Spinner, 1);
1908 }
1909
1910 // We're good to spin ... spin ingress.
1911 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1912 // when preparing to LD...CAS _owner, etc and the CAS is likely
1913 // to succeed.
1914 int hits = 0;
1915 int msk = 0;
1916 int caspty = Knob_CASPenalty;
1917 int oxpty = Knob_OXPenalty;
1918 int sss = Knob_SpinSetSucc;
1919 if (sss && _succ == NULL) _succ = Self;
1920 Thread * prv = NULL;
1921
1922 // There are three ways to exit the following loop:
1923 // 1. A successful spin where this thread has acquired the lock.
1924 // 2. Spin failure with prejudice
1925 // 3. Spin failure without prejudice
1926
1927 while (--ctr >= 0) {
1928
1929 // Periodic polling -- Check for pending GC
1930 // Threads may spin while they're unsafe.
1931 // We don't want spinning threads to delay the JVM from reaching
1932 // a stop-the-world safepoint or to steal cycles from GC.
1933 // If we detect a pending safepoint we abort in order that
1934 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1935 // this thread, if safe, doesn't steal cycles from GC.
1936 // This is in keeping with the "no loitering in runtime" rule.
1937 // We periodically check to see if there's a safepoint pending.
1938 if ((ctr & 0xFF) == 0) {
1939 if (SafepointSynchronize::do_call_back()) {
1940 TEVENT(Spin: safepoint);
1941 goto Abort; // abrupt spin egress
1942 }
1943 if (Knob_UsePause & 1) SpinPause();
1944 }
1945
1946 if (Knob_UsePause & 2) SpinPause();
1947
1948 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
1949 // This is useful on classic SMP systems, but is of less utility on
1950 // N1-style CMT platforms.
1951 //
1952 // Trade-off: lock acquisition latency vs coherency bandwidth.
1953 // Lock hold times are typically short. A histogram
1954 // of successful spin attempts shows that we usually acquire
1955 // the lock early in the spin. That suggests we want to
1956 // sample _owner frequently in the early phase of the spin,
1957 // but then back-off and sample less frequently as the spin
1958 // progresses. The back-off makes a good citizen on SMP big
1959 // SMP systems. Oversampling _owner can consume excessive
1960 // coherency bandwidth. Relatedly, if we _oversample _owner we
1961 // can inadvertently interfere with the the ST m->owner=null.
1962 // executed by the lock owner.
1963 if (ctr & msk) continue;
1964 ++hits;
1965 if ((hits & 0xF) == 0) {
1966 // The 0xF, above, corresponds to the exponent.
1967 // Consider: (msk+1)|msk
1968 msk = ((msk << 2)|3) & BackOffMask;
1969 }
1970
1971 // Probe _owner with TATAS
1972 // If this thread observes the monitor transition or flicker
1973 // from locked to unlocked to locked, then the odds that this
1974 // thread will acquire the lock in this spin attempt go down
1975 // considerably. The same argument applies if the CAS fails
1976 // or if we observe _owner change from one non-null value to
1977 // another non-null value. In such cases we might abort
1978 // the spin without prejudice or apply a "penalty" to the
1979 // spin count-down variable "ctr", reducing it by 100, say.
1980
1981 Thread * ox = (Thread *) _owner;
1982 if (ox == NULL) {
1983 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1984 if (ox == NULL) {
1985 // The CAS succeeded -- this thread acquired ownership
1986 // Take care of some bookkeeping to exit spin state.
1987 if (sss && _succ == Self) {
1988 _succ = NULL;
1989 }
1990 if (MaxSpin > 0) Adjust(&_Spinner, -1);
1991
1992 // Increase _SpinDuration :
1993 // The spin was successful (profitable) so we tend toward
1994 // longer spin attempts in the future.
1995 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1996 // If we acquired the lock early in the spin cycle it
1997 // makes sense to increase _SpinDuration proportionally.
1998 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1999 int x = _SpinDuration;
2000 if (x < Knob_SpinLimit) {
2001 if (x < Knob_Poverty) x = Knob_Poverty;
2002 _SpinDuration = x + Knob_Bonus;
2003 }
2004 return 1;
2005 }
2006
2007 // The CAS failed ... we can take any of the following actions:
2008 // * penalize: ctr -= Knob_CASPenalty
2009 // * exit spin with prejudice -- goto Abort;
2010 // * exit spin without prejudice.
2011 // * Since CAS is high-latency, retry again immediately.
2012 prv = ox;
2013 TEVENT(Spin: cas failed);
2014 if (caspty == -2) break;
2015 if (caspty == -1) goto Abort;
2016 ctr -= caspty;
2017 continue;
2018 }
2019
2020 // Did lock ownership change hands ?
2021 if (ox != prv && prv != NULL) {
2022 TEVENT(spin: Owner changed)
2023 if (oxpty == -2) break;
2024 if (oxpty == -1) goto Abort;
2025 ctr -= oxpty;
2026 }
2027 prv = ox;
2028
2029 // Abort the spin if the owner is not executing.
2030 // The owner must be executing in order to drop the lock.
2031 // Spinning while the owner is OFFPROC is idiocy.
2032 // Consider: ctr -= RunnablePenalty ;
2033 if (Knob_OState && NotRunnable (Self, ox)) {
2034 TEVENT(Spin abort - notrunnable);
2035 goto Abort;
2036 }
2037 if (sss && _succ == NULL) _succ = Self;
2038 }
2039
2040 // Spin failed with prejudice -- reduce _SpinDuration.
2041 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2042 // AIMD is globally stable.
2043 TEVENT(Spin failure);
2044 {
2045 int x = _SpinDuration;
2046 if (x > 0) {
2047 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2048 // This is globally sample and tends to damp the response.
2049 x -= Knob_Penalty;
2050 if (x < 0) x = 0;
2051 _SpinDuration = x;
2052 }
2053 }
2054
2055 Abort:
2056 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2057 if (sss && _succ == Self) {
2058 _succ = NULL;
2059 // Invariant: after setting succ=null a contending thread
2060 // must recheck-retry _owner before parking. This usually happens
2061 // in the normal usage of TrySpin(), but it's safest
2062 // to make TrySpin() as foolproof as possible.
2063 OrderAccess::fence();
2064 if (TryLock(Self) > 0) return 1;
2065 }
2066 return 0;
2067 }
2068
2069 // NotRunnable() -- informed spinning
2070 //
2071 // Don't bother spinning if the owner is not eligible to drop the lock.
2072 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2073 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2074 // The thread must be runnable in order to drop the lock in timely fashion.
2075 // If the _owner is not runnable then spinning will not likely be
2076 // successful (profitable).
2077 //
2078 // Beware -- the thread referenced by _owner could have died
2079 // so a simply fetch from _owner->_thread_state might trap.
2080 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2081 // Because of the lifecycle issues the schedctl and _thread_state values
2082 // observed by NotRunnable() might be garbage. NotRunnable must
2083 // tolerate this and consider the observed _thread_state value
2084 // as advisory.
2085 //
2086 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2087 // a thread pointer.
2088 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2089 // with the LSB of _owner. Another option would be to probablistically probe
2090 // the putative _owner->TypeTag value.
2091 //
2092 // Checking _thread_state isn't perfect. Even if the thread is
2093 // in_java it might be blocked on a page-fault or have been preempted
2094 // and sitting on a ready/dispatch queue. _thread state in conjunction
2095 // with schedctl.sc_state gives us a good picture of what the
2096 // thread is doing, however.
2097 //
2098 // TODO: check schedctl.sc_state.
2099 // We'll need to use SafeFetch32() to read from the schedctl block.
2100 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2101 //
2102 // The return value from NotRunnable() is *advisory* -- the
2103 // result is based on sampling and is not necessarily coherent.
2104 // The caller must tolerate false-negative and false-positive errors.
2105 // Spinning, in general, is probabilistic anyway.
2106
2107
2108 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2109 // Check ox->TypeTag == 2BAD.
2110 if (ox == NULL) return 0;
2111
2112 // Avoid transitive spinning ...
2113 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2114 // Immediately after T1 acquires L it's possible that T2, also
2115 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2116 // This occurs transiently after T1 acquired L but before
2117 // T1 managed to clear T1.Stalled. T2 does not need to abort
2118 // its spin in this circumstance.
2119 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2120
2121 if (BlockedOn == 1) return 1;
2122 if (BlockedOn != 0) {
2123 return BlockedOn != intptr_t(this) && _owner == ox;
2124 }
2125
2126 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2127 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2128 // consider also: jst != _thread_in_Java -- but that's overspecific.
2129 return jst == _thread_blocked || jst == _thread_in_native;
2130 }
2131
2132
2133 // -----------------------------------------------------------------------------
2134 // WaitSet management ...
2135
2136 ObjectWaiter::ObjectWaiter(Thread* thread) {
2137 _next = NULL;
2138 _prev = NULL;
2139 _notified = 0;
2140 TState = TS_RUN;
2141 _thread = thread;
2142 _event = thread->_ParkEvent;
2143 _active = false;
2144 assert(_event != NULL, "invariant");
2145 }
2146
2147 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2148 JavaThread *jt = (JavaThread *)this->_thread;
2149 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2150 }
2151
2152 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2153 JavaThread *jt = (JavaThread *)this->_thread;
2154 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2155 }
2156
2157 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2158 assert(node != NULL, "should not add NULL node");
2159 assert(node->_prev == NULL, "node already in list");
2160 assert(node->_next == NULL, "node already in list");
2161 // put node at end of queue (circular doubly linked list)
2162 if (_WaitSet == NULL) {
2163 _WaitSet = node;
2164 node->_prev = node;
2165 node->_next = node;
2166 } else {
2167 ObjectWaiter* head = _WaitSet;
2168 ObjectWaiter* tail = head->_prev;
2169 assert(tail->_next == head, "invariant check");
2170 tail->_next = node;
2171 head->_prev = node;
2172 node->_next = head;
2173 node->_prev = tail;
2174 }
2175 }
2176
2177 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2178 // dequeue the very first waiter
2179 ObjectWaiter* waiter = _WaitSet;
2180 if (waiter) {
2181 DequeueSpecificWaiter(waiter);
2182 }
2183 return waiter;
2184 }
2185
2186 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2187 assert(node != NULL, "should not dequeue NULL node");
2188 assert(node->_prev != NULL, "node already removed from list");
2189 assert(node->_next != NULL, "node already removed from list");
2190 // when the waiter has woken up because of interrupt,
2191 // timeout or other spurious wake-up, dequeue the
2192 // waiter from waiting list
2193 ObjectWaiter* next = node->_next;
2194 if (next == node) {
2195 assert(node->_prev == node, "invariant check");
2196 _WaitSet = NULL;
2197 } else {
2198 ObjectWaiter* prev = node->_prev;
2199 assert(prev->_next == node, "invariant check");
2200 assert(next->_prev == node, "invariant check");
2201 next->_prev = prev;
2202 prev->_next = next;
2203 if (_WaitSet == node) {
2204 _WaitSet = next;
2205 }
2206 }
2207 node->_next = NULL;
2208 node->_prev = NULL;
2209 }
2210
2211 // -----------------------------------------------------------------------------
2212 // PerfData support
2213 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2214 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2215 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2216 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL;
2217 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2218 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL;
2219 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL;
2220 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL;
2221 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL;
2222 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL;
2223 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL;
2224 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL;
2225 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL;
2226 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL;
2227 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL;
2228 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2229 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2230 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2231
2232 // One-shot global initialization for the sync subsystem.
2233 // We could also defer initialization and initialize on-demand
2234 // the first time we call inflate(). Initialization would
2235 // be protected - like so many things - by the MonitorCache_lock.
2236
2237 void ObjectMonitor::Initialize() {
2238 static int InitializationCompleted = 0;
2239 assert(InitializationCompleted == 0, "invariant");
2240 InitializationCompleted = 1;
2241 if (UsePerfData) {
2242 EXCEPTION_MARK;
2243 #define NEWPERFCOUNTER(n) \
2244 { \
2245 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2246 CHECK); \
2247 }
2248 #define NEWPERFVARIABLE(n) \
2249 { \
2250 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2251 CHECK); \
2252 }
2253 NEWPERFCOUNTER(_sync_Inflations);
2254 NEWPERFCOUNTER(_sync_Deflations);
2255 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2256 NEWPERFCOUNTER(_sync_FutileWakeups);
2257 NEWPERFCOUNTER(_sync_Parks);
2258 NEWPERFCOUNTER(_sync_EmptyNotifications);
2259 NEWPERFCOUNTER(_sync_Notifications);
2260 NEWPERFCOUNTER(_sync_SlowEnter);
2261 NEWPERFCOUNTER(_sync_SlowExit);
2262 NEWPERFCOUNTER(_sync_SlowNotify);
2263 NEWPERFCOUNTER(_sync_SlowNotifyAll);
2264 NEWPERFCOUNTER(_sync_FailedSpins);
2265 NEWPERFCOUNTER(_sync_SuccessfulSpins);
2266 NEWPERFCOUNTER(_sync_PrivateA);
2267 NEWPERFCOUNTER(_sync_PrivateB);
2268 NEWPERFCOUNTER(_sync_MonInCirculation);
2269 NEWPERFCOUNTER(_sync_MonScavenged);
2270 NEWPERFVARIABLE(_sync_MonExtant);
2271 #undef NEWPERFCOUNTER
2272 #undef NEWPERFVARIABLE
2273 }
2274 }
2275
2276 static char * kvGet(char * kvList, const char * Key) {
2277 if (kvList == NULL) return NULL;
2278 size_t n = strlen(Key);
2279 char * Search;
2280 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2281 if (strncmp (Search, Key, n) == 0) {
2282 if (Search[n] == '=') return Search + n + 1;
2283 if (Search[n] == 0) return(char *) "1";
2284 }
2285 }
2286 return NULL;
2287 }
2288
2289 static int kvGetInt(char * kvList, const char * Key, int Default) {
2290 char * v = kvGet(kvList, Key);
2291 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2292 if (Knob_ReportSettings && v != NULL) {
2293 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ;
2294 tty->flush();
2295 }
2296 return rslt;
2297 }
2298
2299 void ObjectMonitor::DeferredInitialize() {
2300 if (InitDone > 0) return;
2301 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2302 while (InitDone != 1) /* empty */;
2303 return;
2304 }
2305
2306 // One-shot global initialization ...
2307 // The initialization is idempotent, so we don't need locks.
2308 // In the future consider doing this via os::init_2().
2309 // SyncKnobs consist of <Key>=<Value> pairs in the style
2310 // of environment variables. Start by converting ':' to NUL.
2311
2312 if (SyncKnobs == NULL) SyncKnobs = "";
2313
2314 size_t sz = strlen(SyncKnobs);
2315 char * knobs = (char *) malloc(sz + 2);
2316 if (knobs == NULL) {
2317 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2318 guarantee(0, "invariant");
2319 }
2320 strcpy(knobs, SyncKnobs);
2321 knobs[sz+1] = 0;
2322 for (char * p = knobs; *p; p++) {
2323 if (*p == ':') *p = 0;
2324 }
2325
2326 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2327 SETKNOB(ReportSettings);
2328 SETKNOB(ExitRelease);
2329 SETKNOB(Verbose);
2330 SETKNOB(VerifyInUse);
2331 SETKNOB(VerifyMatch);
2332 SETKNOB(FixedSpin);
2333 SETKNOB(SpinLimit);
2334 SETKNOB(SpinBase);
2335 SETKNOB(SpinBackOff);
2336 SETKNOB(CASPenalty);
2337 SETKNOB(OXPenalty);
2338 SETKNOB(LogSpins);
2339 SETKNOB(SpinSetSucc);
2340 SETKNOB(SuccEnabled);
2341 SETKNOB(SuccRestrict);
2342 SETKNOB(Penalty);
2343 SETKNOB(Bonus);
2344 SETKNOB(BonusB);
2345 SETKNOB(Poverty);
2346 SETKNOB(SpinAfterFutile);
2347 SETKNOB(UsePause);
2348 SETKNOB(SpinEarly);
2349 SETKNOB(OState);
2350 SETKNOB(MaxSpinners);
2351 SETKNOB(PreSpin);
2352 SETKNOB(ExitPolicy);
2353 SETKNOB(QMode);
2354 SETKNOB(ResetEvent);
2355 SETKNOB(MoveNotifyee);
2356 SETKNOB(FastHSSEC);
2357 #undef SETKNOB
2358
2359 if (Knob_Verbose) {
2360 sanity_checks();
2361 }
2362
2363 if (os::is_MP()) {
2364 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2365 if (Knob_ReportSettings) {
2366 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask);
2367 }
2368 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2369 } else {
2370 Knob_SpinLimit = 0;
2371 Knob_SpinBase = 0;
2372 Knob_PreSpin = 0;
2373 Knob_FixedSpin = -1;
2374 }
2375
2376 if (Knob_LogSpins == 0) {
2377 ObjectMonitor::_sync_FailedSpins = NULL;
2378 }
2379
2380 free(knobs);
2381 OrderAccess::fence();
2382 InitDone = 1;
2383 }
2384
2385 void ObjectMonitor::sanity_checks() {
2386 int error_cnt = 0;
2387 int warning_cnt = 0;
2388 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2389
2390 if (verbose) {
2391 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2392 sizeof(ObjectMonitor));
2393 tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT,
2394 sizeof(PaddedEnd<ObjectMonitor>));
2395 }
2396
2397 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2398 if (verbose) {
2399 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2400 }
2401
2402 ObjectMonitor dummy;
2403 u_char *addr_begin = (u_char*)&dummy;
2404 u_char *addr_header = (u_char*)&dummy._header;
2405 u_char *addr_owner = (u_char*)&dummy._owner;
2406
2407 uint offset_header = (uint)(addr_header - addr_begin);
2408 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2409
2410 uint offset_owner = (uint)(addr_owner - addr_begin);
2411 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2412
2413 if ((uint)(addr_header - addr_begin) != 0) {
2414 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2415 error_cnt++;
2416 }
2417
2418 if (cache_line_size != 0) {
2419 // We were able to determine the L1 data cache line size so
2420 // do some cache line specific sanity checks
2421
2422 if ((offset_owner - offset_header) < cache_line_size) {
2423 tty->print_cr("WARNING: the _header and _owner fields are closer "
2424 "than a cache line which permits false sharing.");
2425 warning_cnt++;
2426 }
2427
2428 if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) {
2429 tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a "
2430 "multiple of a cache line which permits false sharing.");
2431 warning_cnt++;
2432 }
2433 }
2434
2435 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2436 &warning_cnt);
2437
2438 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2439 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2440 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2441 }
2442
2443 guarantee(error_cnt == 0,
2444 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2445 }
2446
2447 #ifndef PRODUCT
2448 void ObjectMonitor_test() {
2449 ObjectMonitor::sanity_checks();
2450 }
2451 #endif