1 /*
2 * Copyright (c) 1998, 2014, 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.inline.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 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
48 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
49 #define NOINLINE __attribute__((noinline))
50 #else
51 #define NOINLINE
52 #endif
53
54
55 #ifdef DTRACE_ENABLED
56
57 // Only bother with this argument setup if dtrace is available
58 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
59
60
61 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
62 char* bytes = NULL; \
63 int len = 0; \
64 jlong jtid = SharedRuntime::get_java_tid(thread); \
65 Symbol* klassname = ((oop)obj)->klass()->name(); \
66 if (klassname != NULL) { \
67 bytes = (char*)klassname->bytes(); \
68 len = klassname->utf8_length(); \
69 }
70
71 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
72 { \
73 if (DTraceMonitorProbes) { \
74 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
75 HOTSPOT_MONITOR_WAIT(jtid, \
76 (monitor), bytes, len, (millis)); \
77 } \
78 }
79
80 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
81 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
82 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
83 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
84 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
85
86 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
87 { \
88 if (DTraceMonitorProbes) { \
89 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
90 HOTSPOT_MONITOR_##probe(jtid, \
91 (uintptr_t)(monitor), bytes, len); \
92 } \
93 }
94
95 #else // ndef DTRACE_ENABLED
96
97 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
98 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
99
100 #endif // ndef DTRACE_ENABLED
101
102 // Tunables ...
103 // The knob* variables are effectively final. Once set they should
104 // never be modified hence. Consider using __read_mostly with GCC.
105
106 int ObjectMonitor::Knob_Verbose = 0;
107 int ObjectMonitor::Knob_VerifyInUse = 0;
108 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
109 static int Knob_LogSpins = 0; // enable jvmstat tally for spins
110 static int Knob_HandOff = 0;
111 static int Knob_ReportSettings = 0;
112
113 static int Knob_SpinBase = 0; // Floor AKA SpinMin
114 static int Knob_SpinBackOff = 0; // spin-loop backoff
115 static int Knob_CASPenalty = -1; // Penalty for failed CAS
116 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
117 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
118 static int Knob_SpinEarly = 1;
119 static int Knob_SuccEnabled = 1; // futile wake throttling
120 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
121 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
122 static int Knob_Bonus = 100; // spin success bonus
123 static int Knob_BonusB = 100; // spin success bonus
124 static int Knob_Penalty = 200; // spin failure penalty
125 static int Knob_Poverty = 1000;
126 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
127 static int Knob_FixedSpin = 0;
128 static int Knob_OState = 3; // Spinner checks thread state of _owner
129 static int Knob_UsePause = 1;
130 static int Knob_ExitPolicy = 0;
131 static int Knob_PreSpin = 10; // 20-100 likely better
132 static int Knob_ResetEvent = 0;
133 static int BackOffMask = 0;
134
135 static int Knob_FastHSSEC = 0;
136 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
137 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
138 static volatile int InitDone = 0;
139
140 #define TrySpin TrySpin_VaryDuration
141
142 // -----------------------------------------------------------------------------
143 // Theory of operations -- Monitors lists, thread residency, etc:
144 //
145 // * A thread acquires ownership of a monitor by successfully
146 // CAS()ing the _owner field from null to non-null.
147 //
148 // * Invariant: A thread appears on at most one monitor list --
149 // cxq, EntryList or WaitSet -- at any one time.
150 //
151 // * Contending threads "push" themselves onto the cxq with CAS
152 // and then spin/park.
153 //
154 // * After a contending thread eventually acquires the lock it must
155 // dequeue itself from either the EntryList or the cxq.
156 //
157 // * The exiting thread identifies and unparks an "heir presumptive"
158 // tentative successor thread on the EntryList. Critically, the
159 // exiting thread doesn't unlink the successor thread from the EntryList.
160 // After having been unparked, the wakee will recontend for ownership of
161 // the monitor. The successor (wakee) will either acquire the lock or
162 // re-park itself.
163 //
164 // Succession is provided for by a policy of competitive handoff.
165 // The exiting thread does _not_ grant or pass ownership to the
166 // successor thread. (This is also referred to as "handoff" succession").
167 // Instead the exiting thread releases ownership and possibly wakes
168 // a successor, so the successor can (re)compete for ownership of the lock.
169 // If the EntryList is empty but the cxq is populated the exiting
170 // thread will drain the cxq into the EntryList. It does so by
171 // by detaching the cxq (installing null with CAS) and folding
172 // the threads from the cxq into the EntryList. The EntryList is
173 // doubly linked, while the cxq is singly linked because of the
174 // CAS-based "push" used to enqueue recently arrived threads (RATs).
175 //
176 // * Concurrency invariants:
177 //
178 // -- only the monitor owner may access or mutate the EntryList.
179 // The mutex property of the monitor itself protects the EntryList
180 // from concurrent interference.
181 // -- Only the monitor owner may detach the cxq.
182 //
183 // * The monitor entry list operations avoid locks, but strictly speaking
184 // they're not lock-free. Enter is lock-free, exit is not.
185 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
186 //
187 // * The cxq can have multiple concurrent "pushers" but only one concurrent
188 // detaching thread. This mechanism is immune from the ABA corruption.
189 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
190 //
191 // * Taken together, the cxq and the EntryList constitute or form a
192 // single logical queue of threads stalled trying to acquire the lock.
193 // We use two distinct lists to improve the odds of a constant-time
194 // dequeue operation after acquisition (in the ::enter() epilogue) and
195 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
196 // A key desideratum is to minimize queue & monitor metadata manipulation
197 // that occurs while holding the monitor lock -- that is, we want to
198 // minimize monitor lock holds times. Note that even a small amount of
199 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
200 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
201 // locks and monitor metadata.
202 //
203 // Cxq points to the set of Recently Arrived Threads attempting entry.
204 // Because we push threads onto _cxq with CAS, the RATs must take the form of
205 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
206 // the unlocking thread notices that EntryList is null but _cxq is != null.
207 //
208 // The EntryList is ordered by the prevailing queue discipline and
209 // can be organized in any convenient fashion, such as a doubly-linked list or
210 // a circular doubly-linked list. Critically, we want insert and delete operations
211 // to operate in constant-time. If we need a priority queue then something akin
212 // to Solaris' sleepq would work nicely. Viz.,
213 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
214 // Queue discipline is enforced at ::exit() time, when the unlocking thread
215 // drains the cxq into the EntryList, and orders or reorders the threads on the
216 // EntryList accordingly.
217 //
218 // Barring "lock barging", this mechanism provides fair cyclic ordering,
219 // somewhat similar to an elevator-scan.
220 //
221 // * The monitor synchronization subsystem avoids the use of native
222 // synchronization primitives except for the narrow platform-specific
223 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
224 // the semantics of park-unpark. Put another way, this monitor implementation
225 // depends only on atomic operations and park-unpark. The monitor subsystem
226 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
227 // underlying OS manages the READY<->RUN transitions.
228 //
229 // * Waiting threads reside on the WaitSet list -- wait() puts
230 // the caller onto the WaitSet.
231 //
232 // * notify() or notifyAll() simply transfers threads from the WaitSet to
233 // either the EntryList or cxq. Subsequent exit() operations will
234 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
235 // it's likely the notifyee would simply impale itself on the lock held
236 // by the notifier.
237 //
238 // * An interesting alternative is to encode cxq as (List,LockByte) where
239 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
240 // variable, like _recursions, in the scheme. The threads or Events that form
241 // the list would have to be aligned in 256-byte addresses. A thread would
242 // try to acquire the lock or enqueue itself with CAS, but exiting threads
243 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
244 // Note that is is *not* word-tearing, but it does presume that full-word
245 // CAS operations are coherent with intermix with STB operations. That's true
246 // on most common processors.
247 //
248 // * See also http://blogs.sun.com/dave
249
250
251 // -----------------------------------------------------------------------------
252 // Enter support
253
254 bool ObjectMonitor::try_enter(Thread* THREAD) {
255 if (THREAD != _owner) {
256 if (THREAD->is_lock_owned ((address)_owner)) {
257 assert(_recursions == 0, "internal state error");
258 _owner = THREAD;
259 _recursions = 1;
260 return true;
261 }
262 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
263 return false;
264 }
265 return true;
266 } else {
267 _recursions++;
268 return true;
269 }
270 }
271
272 void NOINLINE ObjectMonitor::enter(TRAPS) {
273 // The following code is ordered to check the most common cases first
274 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
275 Thread * const Self = THREAD;
276
277 void * cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL);
278 if (cur == NULL) {
279 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
280 assert(_recursions == 0, "invariant");
281 assert(_owner == Self, "invariant");
282 return;
283 }
284
285 if (cur == Self) {
286 // TODO-FIXME: check for integer overflow! BUGID 6557169.
287 _recursions++;
288 return;
289 }
290
291 if (Self->is_lock_owned ((address)cur)) {
292 assert(_recursions == 0, "internal state error");
293 _recursions = 1;
294 // Commute owner from a thread-specific on-stack BasicLockObject address to
295 // a full-fledged "Thread *".
296 _owner = Self;
297 return;
298 }
299
300 // We've encountered genuine contention.
301 assert(Self->_Stalled == 0, "invariant");
302 Self->_Stalled = intptr_t(this);
303
304 // Try one round of spinning *before* enqueueing Self
305 // and before going through the awkward and expensive state
306 // transitions. The following spin is strictly optional ...
307 // Note that if we acquire the monitor from an initial spin
308 // we forgo posting JVMTI events and firing DTRACE probes.
309 if (Knob_SpinEarly && TrySpin (Self) > 0) {
310 assert(_owner == Self, "invariant");
311 assert(_recursions == 0, "invariant");
312 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
313 Self->_Stalled = 0;
314 return;
315 }
316
317 assert(_owner != Self, "invariant");
318 assert(_succ != Self, "invariant");
319 assert(Self->is_Java_thread(), "invariant");
320 JavaThread * jt = (JavaThread *) Self;
321 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
322 assert(jt->thread_state() != _thread_blocked, "invariant");
323 assert(this->object() != NULL, "invariant");
324 assert(_count >= 0, "invariant");
325
326 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
327 // Ensure the object-monitor relationship remains stable while there's contention.
328 Atomic::inc(&_count);
329
330 EventJavaMonitorEnter event;
331
332 { // Change java thread status to indicate blocked on monitor enter.
333 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
334
335 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
336 if (JvmtiExport::should_post_monitor_contended_enter()) {
337 JvmtiExport::post_monitor_contended_enter(jt, this);
338
339 // The current thread does not yet own the monitor and does not
340 // yet appear on any queues that would get it made the successor.
341 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
342 // handler cannot accidentally consume an unpark() meant for the
343 // ParkEvent associated with this ObjectMonitor.
344 }
345
346 OSThreadContendState osts(Self->osthread());
347 ThreadBlockInVM tbivm(jt);
348
349 Self->set_current_pending_monitor(this);
350
351 // TODO-FIXME: change the following for(;;) loop to straight-line code.
352 for (;;) {
353 jt->set_suspend_equivalent();
354 // cleared by handle_special_suspend_equivalent_condition()
355 // or java_suspend_self()
356
357 EnterI(THREAD);
358
359 if (!ExitSuspendEquivalent(jt)) break;
360
361 // We have acquired the contended monitor, but while we were
362 // waiting another thread suspended us. We don't want to enter
363 // the monitor while suspended because that would surprise the
364 // thread that suspended us.
365 //
366 _recursions = 0;
367 _succ = NULL;
368 exit(false, Self);
369
370 jt->java_suspend_self();
371 }
372 Self->set_current_pending_monitor(NULL);
373
374 // We cleared the pending monitor info since we've just gotten past
375 // the enter-check-for-suspend dance and we now own the monitor free
376 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
377 // destructor can go to a safepoint at the end of this block. If we
378 // do a thread dump during that safepoint, then this thread will show
379 // as having "-locked" the monitor, but the OS and java.lang.Thread
380 // states will still report that the thread is blocked trying to
381 // acquire it.
382 }
383
384 Atomic::dec(&_count);
385 assert(_count >= 0, "invariant");
386 Self->_Stalled = 0;
387
388 // Must either set _recursions = 0 or ASSERT _recursions == 0.
389 assert(_recursions == 0, "invariant");
390 assert(_owner == Self, "invariant");
391 assert(_succ != Self, "invariant");
392 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
393
394 // The thread -- now the owner -- is back in vm mode.
395 // Report the glorious news via TI,DTrace and jvmstat.
396 // The probe effect is non-trivial. All the reportage occurs
397 // while we hold the monitor, increasing the length of the critical
398 // section. Amdahl's parallel speedup law comes vividly into play.
399 //
400 // Another option might be to aggregate the events (thread local or
401 // per-monitor aggregation) and defer reporting until a more opportune
402 // time -- such as next time some thread encounters contention but has
403 // yet to acquire the lock. While spinning that thread could
404 // spinning we could increment JVMStat counters, etc.
405
406 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
407 if (JvmtiExport::should_post_monitor_contended_entered()) {
408 JvmtiExport::post_monitor_contended_entered(jt, this);
409
410 // The current thread already owns the monitor and is not going to
411 // call park() for the remainder of the monitor enter protocol. So
412 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
413 // event handler consumed an unpark() issued by the thread that
414 // just exited the monitor.
415 }
416
417 if (event.should_commit()) {
418 event.set_klass(((oop)this->object())->klass());
419 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
420 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
421 event.commit();
422 }
423
424 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
425 ObjectMonitor::_sync_ContendedLockAttempts->inc();
426 }
427 }
428
429
430 // Caveat: TryLock() is not necessarily serializing if it returns failure.
431 // Callers must compensate as needed.
432
433 int ObjectMonitor::TryLock(Thread * Self) {
434 void * own = _owner;
435 if (own != NULL) return 0;
436 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
437 // Either guarantee _recursions == 0 or set _recursions = 0.
438 assert(_recursions == 0, "invariant");
439 assert(_owner == Self, "invariant");
440 return 1;
441 }
442 // The lock had been free momentarily, but we lost the race to the lock.
443 // Interference -- the CAS failed.
444 // We can either return -1 or retry.
445 // Retry doesn't make as much sense because the lock was just acquired.
446 return -1;
447 }
448
449 void NOINLINE ObjectMonitor::EnterI(TRAPS) {
450 Thread * const Self = THREAD;
451 assert(Self->is_Java_thread(), "invariant");
452 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
453
454 // Try the lock - TATAS
455 if (TryLock (Self) > 0) {
456 assert(_succ != Self, "invariant");
457 assert(_owner == Self, "invariant");
458 assert(_Responsible != Self, "invariant");
459 return;
460 }
461
462 DeferredInitialize();
463
464 // We try one round of spinning *before* enqueueing Self.
465 //
466 // If the _owner is ready but OFFPROC we could use a YieldTo()
467 // operation to donate the remainder of this thread's quantum
468 // to the owner. This has subtle but beneficial affinity
469 // effects.
470
471 if (TrySpin (Self) > 0) {
472 assert(_owner == Self, "invariant");
473 assert(_succ != Self, "invariant");
474 assert(_Responsible != Self, "invariant");
475 return;
476 }
477
478 // The Spin failed -- Enqueue and park the thread ...
479 assert(_succ != Self, "invariant");
480 assert(_owner != Self, "invariant");
481 assert(_Responsible != Self, "invariant");
482
483 // Enqueue "Self" on ObjectMonitor's _cxq.
484 //
485 // Node acts as a proxy for Self.
486 // As an aside, if were to ever rewrite the synchronization code mostly
487 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
488 // Java objects. This would avoid awkward lifecycle and liveness issues,
489 // as well as eliminate a subset of ABA issues.
490 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
491
492 ObjectWaiter node(Self);
493 Self->_ParkEvent->reset();
494 node._prev = (ObjectWaiter *) 0xBAD;
495 node.TState = ObjectWaiter::TS_CXQ;
496
497 // Push "Self" onto the front of the _cxq.
498 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
499 // Note that spinning tends to reduce the rate at which threads
500 // enqueue and dequeue on EntryList|cxq.
501 ObjectWaiter * nxt;
502 for (;;) {
503 node._next = nxt = _cxq;
504 if (Atomic::cmpxchg_ptr(&node, &_cxq, nxt) == nxt) break;
505
506 // Interference - the CAS failed because _cxq changed. Just retry.
507 // As an optional optimization we retry the lock.
508 if (TryLock (Self) > 0) {
509 assert(_succ != Self, "invariant");
510 assert(_owner == Self, "invariant");
511 assert(_Responsible != Self, "invariant");
512 return;
513 }
514 }
515
516 // Check for cxq|EntryList edge transition to non-null. This indicates
517 // the onset of contention. While contention persists exiting threads
518 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
519 // operations revert to the faster 1-0 mode. This enter operation may interleave
520 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
521 // arrange for one of the contending thread to use a timed park() operations
522 // to detect and recover from the race. (Stranding is form of progress failure
523 // where the monitor is unlocked but all the contending threads remain parked).
524 // That is, at least one of the contended threads will periodically poll _owner.
525 // One of the contending threads will become the designated "Responsible" thread.
526 // The Responsible thread uses a timed park instead of a normal indefinite park
527 // operation -- it periodically wakes and checks for and recovers from potential
528 // strandings admitted by 1-0 exit operations. We need at most one Responsible
529 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
530 // be responsible for a monitor.
531 //
532 // Currently, one of the contended threads takes on the added role of "Responsible".
533 // A viable alternative would be to use a dedicated "stranding checker" thread
534 // that periodically iterated over all the threads (or active monitors) and unparked
535 // successors where there was risk of stranding. This would help eliminate the
536 // timer scalability issues we see on some platforms as we'd only have one thread
537 // -- the checker -- parked on a timer.
538
539 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
540 // Try to assume the role of responsible thread for the monitor.
541 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
542 Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
543 }
544
545 // The lock might have been released while this thread was occupied queueing
546 // itself onto _cxq. To close the race and avoid "stranding" and
547 // progress-liveness failure we must resample-retry _owner before parking.
548 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
549 // In this case the ST-MEMBAR is accomplished with CAS().
550 //
551 // TODO: Defer all thread state transitions until park-time.
552 // Since state transitions are heavy and inefficient we'd like
553 // to defer the state transitions until absolutely necessary,
554 // and in doing so avoid some transitions ...
555
556 TEVENT(Inflated enter - Contention);
557 int nWakeups = 0;
558 int RecheckInterval = 1;
559
560 for (;;) {
561
562 if (TryLock(Self) > 0) break;
563 assert(_owner != Self, "invariant");
564
565 if ((SyncFlags & 2) && _Responsible == NULL) {
566 Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
567 }
568
569 // park self
570 if (_Responsible == Self || (SyncFlags & 1)) {
571 TEVENT(Inflated enter - park TIMED);
572 Self->_ParkEvent->park((jlong) RecheckInterval);
573 // Increase the RecheckInterval, but clamp the value.
574 RecheckInterval *= 8;
575 if (RecheckInterval > 1000) RecheckInterval = 1000;
576 } else {
577 TEVENT(Inflated enter - park UNTIMED);
578 Self->_ParkEvent->park();
579 }
580
581 if (TryLock(Self) > 0) break;
582
583 // The lock is still contested.
584 // Keep a tally of the # of futile wakeups.
585 // Note that the counter is not protected by a lock or updated by atomics.
586 // That is by design - we trade "lossy" counters which are exposed to
587 // races during updates for a lower probe effect.
588 TEVENT(Inflated enter - Futile wakeup);
589 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
590 ObjectMonitor::_sync_FutileWakeups->inc();
591 }
592 ++nWakeups;
593
594 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
595 // We can defer clearing _succ until after the spin completes
596 // TrySpin() must tolerate being called with _succ == Self.
597 // Try yet another round of adaptive spinning.
598 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
599
600 // We can find that we were unpark()ed and redesignated _succ while
601 // we were spinning. That's harmless. If we iterate and call park(),
602 // park() will consume the event and return immediately and we'll
603 // just spin again. This pattern can repeat, leaving _succ to simply
604 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
605 // Alternately, we can sample fired() here, and if set, forgo spinning
606 // in the next iteration.
607
608 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
609 Self->_ParkEvent->reset();
610 OrderAccess::fence();
611 }
612 if (_succ == Self) _succ = NULL;
613
614 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
615 OrderAccess::fence();
616 }
617
618 // Egress :
619 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
620 // Normally we'll find Self on the EntryList .
621 // From the perspective of the lock owner (this thread), the
622 // EntryList is stable and cxq is prepend-only.
623 // The head of cxq is volatile but the interior is stable.
624 // In addition, Self.TState is stable.
625
626 assert(_owner == Self, "invariant");
627 assert(object() != NULL, "invariant");
628 // I'd like to write:
629 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
630 // but as we're at a safepoint that's not safe.
631
632 UnlinkAfterAcquire(Self, &node);
633 if (_succ == Self) _succ = NULL;
634
635 assert(_succ != Self, "invariant");
636 if (_Responsible == Self) {
637 _Responsible = NULL;
638 OrderAccess::fence(); // Dekker pivot-point
639
640 // We may leave threads on cxq|EntryList without a designated
641 // "Responsible" thread. This is benign. When this thread subsequently
642 // exits the monitor it can "see" such preexisting "old" threads --
643 // threads that arrived on the cxq|EntryList before the fence, above --
644 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
645 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
646 // non-null and elect a new "Responsible" timer thread.
647 //
648 // This thread executes:
649 // ST Responsible=null; MEMBAR (in enter epilogue - here)
650 // LD cxq|EntryList (in subsequent exit)
651 //
652 // Entering threads in the slow/contended path execute:
653 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
654 // The (ST cxq; MEMBAR) is accomplished with CAS().
655 //
656 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
657 // exit operation from floating above the ST Responsible=null.
658 }
659
660 // We've acquired ownership with CAS().
661 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
662 // But since the CAS() this thread may have also stored into _succ,
663 // EntryList, cxq or Responsible. These meta-data updates must be
664 // visible __before this thread subsequently drops the lock.
665 // Consider what could occur if we didn't enforce this constraint --
666 // STs to monitor meta-data and user-data could reorder with (become
667 // visible after) the ST in exit that drops ownership of the lock.
668 // Some other thread could then acquire the lock, but observe inconsistent
669 // or old monitor meta-data and heap data. That violates the JMM.
670 // To that end, the 1-0 exit() operation must have at least STST|LDST
671 // "release" barrier semantics. Specifically, there must be at least a
672 // STST|LDST barrier in exit() before the ST of null into _owner that drops
673 // the lock. The barrier ensures that changes to monitor meta-data and data
674 // protected by the lock will be visible before we release the lock, and
675 // therefore before some other thread (CPU) has a chance to acquire the lock.
676 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
677 //
678 // Critically, any prior STs to _succ or EntryList must be visible before
679 // the ST of null into _owner in the *subsequent* (following) corresponding
680 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
681 // execute a serializing instruction.
682
683 if (SyncFlags & 8) {
684 OrderAccess::fence();
685 }
686 return;
687 }
688
689 // ReenterI() is a specialized inline form of the latter half of the
690 // contended slow-path from EnterI(). We use ReenterI() only for
691 // monitor reentry in wait().
692 //
693 // In the future we should reconcile EnterI() and ReenterI(), adding
694 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
695 // loop accordingly.
696
697 void NOINLINE ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
698 assert(Self != NULL, "invariant");
699 assert(SelfNode != NULL, "invariant");
700 assert(SelfNode->_thread == Self, "invariant");
701 assert(_waiters > 0, "invariant");
702 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
703 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
704 JavaThread * jt = (JavaThread *) Self;
705
706 int nWakeups = 0;
707 for (;;) {
708 ObjectWaiter::TStates v = SelfNode->TState;
709 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
710 assert(_owner != Self, "invariant");
711
712 if (TryLock(Self) > 0) break;
713 if (TrySpin(Self) > 0) break;
714
715 TEVENT(Wait Reentry - parking);
716
717 // State transition wrappers around park() ...
718 // ReenterI() wisely defers state transitions until
719 // it's clear we must park the thread.
720 {
721 OSThreadContendState osts(Self->osthread());
722 ThreadBlockInVM tbivm(jt);
723
724 // cleared by handle_special_suspend_equivalent_condition()
725 // or java_suspend_self()
726 jt->set_suspend_equivalent();
727 if (SyncFlags & 1) {
728 Self->_ParkEvent->park((jlong)1000);
729 } else {
730 Self->_ParkEvent->park();
731 }
732
733 // were we externally suspended while we were waiting?
734 for (;;) {
735 if (!ExitSuspendEquivalent(jt)) break;
736 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
737 jt->java_suspend_self();
738 jt->set_suspend_equivalent();
739 }
740 }
741
742 // Try again, but just so we distinguish between futile wakeups and
743 // successful wakeups. The following test isn't algorithmically
744 // necessary, but it helps us maintain sensible statistics.
745 if (TryLock(Self) > 0) break;
746
747 // The lock is still contested.
748 // Keep a tally of the # of futile wakeups.
749 // Note that the counter is not protected by a lock or updated by atomics.
750 // That is by design - we trade "lossy" counters which are exposed to
751 // races during updates for a lower probe effect.
752 TEVENT(Wait Reentry - futile wakeup);
753 ++nWakeups;
754
755 // Assuming this is not a spurious wakeup we'll normally
756 // find that _succ == Self.
757 if (_succ == Self) _succ = NULL;
758
759 // Invariant: after clearing _succ a contending thread
760 // *must* retry _owner before parking.
761 OrderAccess::fence();
762
763 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
764 ObjectMonitor::_sync_FutileWakeups->inc();
765 }
766 }
767
768 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
769 // Normally we'll find Self on the EntryList.
770 // Unlinking from the EntryList is constant-time and atomic-free.
771 // From the perspective of the lock owner (this thread), the
772 // EntryList is stable and cxq is prepend-only.
773 // The head of cxq is volatile but the interior is stable.
774 // In addition, Self.TState is stable.
775
776 assert(_owner == Self, "invariant");
777 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
778 UnlinkAfterAcquire(Self, SelfNode);
779 if (_succ == Self) _succ = NULL;
780 assert(_succ != Self, "invariant");
781 SelfNode->TState = ObjectWaiter::TS_RUN;
782 OrderAccess::fence(); // see comments at the end of EnterI()
783 }
784
785 // By convention we unlink a contending thread from EntryList|cxq immediately
786 // after the thread acquires the lock in ::enter(). Equally, we could defer
787 // unlinking the thread until ::exit()-time.
788
789 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
790 assert(_owner == Self, "invariant");
791 assert(SelfNode->_thread == Self, "invariant");
792
793 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
794 // Normal case: remove Self from the DLL EntryList .
795 // This is a constant-time operation.
796 ObjectWaiter * nxt = SelfNode->_next;
797 ObjectWaiter * prv = SelfNode->_prev;
798 if (nxt != NULL) nxt->_prev = prv;
799 if (prv != NULL) prv->_next = nxt;
800 if (SelfNode == _EntryList) _EntryList = nxt;
801 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
802 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
803 TEVENT(Unlink from EntryList);
804 } else {
805 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
806 // Inopportune interleaving -- Self is still on the cxq.
807 // This usually means the enqueue of self raced an exiting thread.
808 // Normally we'll find Self near the front of the cxq, so
809 // dequeueing is typically fast. If needbe we can accelerate
810 // this with some MCS/CHL-like bidirectional list hints and advisory
811 // back-links so dequeueing from the interior will normally operate
812 // in constant-time.
813 // Dequeue Self from either the head (with CAS) or from the interior
814 // with a linear-time scan and normal non-atomic memory operations.
815 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
816 // and then unlink Self from EntryList. We have to drain eventually,
817 // so it might as well be now.
818
819 ObjectWaiter * v = _cxq;
820 assert(v != NULL, "invariant");
821 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
822 // The CAS above can fail from interference IFF a "RAT" arrived.
823 // In that case Self must be in the interior and can no longer be
824 // at the head of cxq.
825 if (v == SelfNode) {
826 assert(_cxq != v, "invariant");
827 v = _cxq; // CAS above failed - start scan at head of list
828 }
829 ObjectWaiter * p;
830 ObjectWaiter * q = NULL;
831 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
832 q = p;
833 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
834 }
835 assert(v != SelfNode, "invariant");
836 assert(p == SelfNode, "Node not found on cxq");
837 assert(p != _cxq, "invariant");
838 assert(q != NULL, "invariant");
839 assert(q->_next == p, "invariant");
840 q->_next = p->_next;
841 }
842 TEVENT(Unlink from cxq);
843 }
844
845 #ifdef ASSERT
846 // Diagnostic hygiene ...
847 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
848 SelfNode->_next = (ObjectWaiter *) 0xBAD;
849 SelfNode->TState = ObjectWaiter::TS_RUN;
850 #endif
851 }
852
853 // -----------------------------------------------------------------------------
854 // Exit support
855 //
856 // exit()
857 // ~~~~~~
858 // Note that the collector can't reclaim the objectMonitor or deflate
859 // the object out from underneath the thread calling ::exit() as the
860 // thread calling ::exit() never transitions to a stable state.
861 // This inhibits GC, which in turn inhibits asynchronous (and
862 // inopportune) reclamation of "this".
863 //
864 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
865 // There's one exception to the claim above, however. EnterI() can call
866 // exit() to drop a lock if the acquirer has been externally suspended.
867 // In that case exit() is called with _thread_state as _thread_blocked,
868 // but the monitor's _count field is > 0, which inhibits reclamation.
869 //
870 // 1-0 exit
871 // ~~~~~~~~
872 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
873 // the fast-path operators have been optimized so the common ::exit()
874 // operation is 1-0. See i486.ad fast_unlock(), for instance.
875 // The code emitted by fast_unlock() elides the usual MEMBAR. This
876 // greatly improves latency -- MEMBAR and CAS having considerable local
877 // latency on modern processors -- but at the cost of "stranding". Absent the
878 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
879 // ::enter() path, resulting in the entering thread being stranding
880 // and a progress-liveness failure. Stranding is extremely rare.
881 // We use timers (timed park operations) & periodic polling to detect
882 // and recover from stranding. Potentially stranded threads periodically
883 // wake up and poll the lock. See the usage of the _Responsible variable.
884 //
885 // The CAS() in enter provides for safety and exclusion, while the CAS or
886 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
887 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
888 // We detect and recover from stranding with timers.
889 //
890 // If a thread transiently strands it'll park until (a) another
891 // thread acquires the lock and then drops the lock, at which time the
892 // exiting thread will notice and unpark the stranded thread, or, (b)
893 // the timer expires. If the lock is high traffic then the stranding latency
894 // will be low due to (a). If the lock is low traffic then the odds of
895 // stranding are lower, although the worst-case stranding latency
896 // is longer. Critically, we don't want to put excessive load in the
897 // platform's timer subsystem. We want to minimize both the timer injection
898 // rate (timers created/sec) as well as the number of timers active at
899 // any one time. (more precisely, we want to minimize timer-seconds, which is
900 // the integral of the # of active timers at any instant over time).
901 // Both impinge on OS scalability. Given that, at most one thread parked on
902 // a monitor will use a timer.
903 //
904 // There is also the risk of a futile wake-up. If we drop the lock
905 // another thread can reacquire the lock immediately, and we can
906 // then wake a thread unnecessarily. This is benign, and we've
907 // structured the code so the windows are short and the frequency
908 // of such futile wakups is low.
909
910 void NOINLINE ObjectMonitor::exit(bool not_suspended, TRAPS) {
911 Thread * const Self = THREAD;
912 if (THREAD != _owner) {
913 if (THREAD->is_lock_owned((address) _owner)) {
914 // Transmute _owner from a BasicLock pointer to a Thread address.
915 // We don't need to hold _mutex for this transition.
916 // Non-null to Non-null is safe as long as all readers can
917 // tolerate either flavor.
918 assert(_recursions == 0, "invariant");
919 _owner = THREAD;
920 _recursions = 0;
921 } else {
922 // Apparent unbalanced locking ...
923 // Naively we'd like to throw IllegalMonitorStateException.
924 // As a practical matter we can neither allocate nor throw an
925 // exception as ::exit() can be called from leaf routines.
926 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
927 // Upon deeper reflection, however, in a properly run JVM the only
928 // way we should encounter this situation is in the presence of
929 // unbalanced JNI locking. TODO: CheckJNICalls.
930 // See also: CR4414101
931 TEVENT(Exit - Throw IMSX);
932 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
933 return;
934 }
935 }
936
937 if (_recursions != 0) {
938 _recursions--; // this is simple recursive enter
939 TEVENT(Inflated exit - recursive);
940 return;
941 }
942
943 // Invariant: after setting Responsible=null an thread must execute
944 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
945 if ((SyncFlags & 4) == 0) {
946 _Responsible = NULL;
947 }
948
949 #if INCLUDE_TRACE
950 // get the owner's thread id for the MonitorEnter event
951 // if it is enabled and the thread isn't suspended
952 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
953 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
954 }
955 #endif
956
957 for (;;) {
958 assert(THREAD == _owner, "invariant");
959
960
961 if (Knob_ExitPolicy == 0) {
962 // release semantics: prior loads and stores from within the critical section
963 // must not float (reorder) past the following store that drops the lock.
964 // On SPARC that requires MEMBAR #loadstore|#storestore.
965 // But of course in TSO #loadstore|#storestore is not required.
966 // I'd like to write one of the following:
967 // A. OrderAccess::release() ; _owner = NULL
968 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
969 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
970 // store into a _dummy variable. That store is not needed, but can result
971 // in massive wasteful coherency traffic on classic SMP systems.
972 // Instead, I use release_store(), which is implemented as just a simple
973 // ST on x64, x86 and SPARC.
974 OrderAccess::release_store_ptr(&_owner, NULL); // drop the lock
975 OrderAccess::storeload(); // See if we need to wake a successor
976 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
977 TEVENT(Inflated exit - simple egress);
978 return;
979 }
980 TEVENT(Inflated exit - complex egress);
981 // Other threads are blocked trying to acquire the lock.
982
983 // Normally the exiting thread is responsible for ensuring succession,
984 // but if other successors are ready or other entering threads are spinning
985 // then this thread can simply store NULL into _owner and exit without
986 // waking a successor. The existence of spinners or ready successors
987 // guarantees proper succession (liveness). Responsibility passes to the
988 // ready or running successors. The exiting thread delegates the duty.
989 // More precisely, if a successor already exists this thread is absolved
990 // of the responsibility of waking (unparking) one.
991 //
992 // The _succ variable is critical to reducing futile wakeup frequency.
993 // _succ identifies the "heir presumptive" thread that has been made
994 // ready (unparked) but that has not yet run. We need only one such
995 // successor thread to guarantee progress.
996 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
997 // section 3.3 "Futile Wakeup Throttling" for details.
998 //
999 // Note that spinners in Enter() also set _succ non-null.
1000 // In the current implementation spinners opportunistically set
1001 // _succ so that exiting threads might avoid waking a successor.
1002 // Another less appealing alternative would be for the exiting thread
1003 // to drop the lock and then spin briefly to see if a spinner managed
1004 // to acquire the lock. If so, the exiting thread could exit
1005 // immediately without waking a successor, otherwise the exiting
1006 // thread would need to dequeue and wake a successor.
1007 // (Note that we'd need to make the post-drop spin short, but no
1008 // shorter than the worst-case round-trip cache-line migration time.
1009 // The dropped lock needs to become visible to the spinner, and then
1010 // the acquisition of the lock by the spinner must become visible to
1011 // the exiting thread).
1012
1013 // It appears that an heir-presumptive (successor) must be made ready.
1014 // Only the current lock owner can manipulate the EntryList or
1015 // drain _cxq, so we need to reacquire the lock. If we fail
1016 // to reacquire the lock the responsibility for ensuring succession
1017 // falls to the new owner.
1018 //
1019 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1020 return;
1021 }
1022 TEVENT(Exit - Reacquired);
1023 } else {
1024 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1025 OrderAccess::release_store_ptr(&_owner, NULL); // drop the lock
1026 OrderAccess::storeload();
1027 // Ratify the previously observed values.
1028 if (_cxq == NULL || _succ != NULL) {
1029 TEVENT(Inflated exit - simple egress);
1030 return;
1031 }
1032
1033 // inopportune interleaving -- the exiting thread (this thread)
1034 // in the fast-exit path raced an entering thread in the slow-enter
1035 // path.
1036 // We have two choices:
1037 // A. Try to reacquire the lock.
1038 // If the CAS() fails return immediately, otherwise
1039 // we either restart/rerun the exit operation, or simply
1040 // fall-through into the code below which wakes a successor.
1041 // B. If the elements forming the EntryList|cxq are TSM
1042 // we could simply unpark() the lead thread and return
1043 // without having set _succ.
1044 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1045 TEVENT(Inflated exit - reacquired succeeded);
1046 return;
1047 }
1048 TEVENT(Inflated exit - reacquired failed);
1049 } else {
1050 TEVENT(Inflated exit - complex egress);
1051 }
1052 }
1053
1054 guarantee(_owner == THREAD, "invariant");
1055
1056 ObjectWaiter * w = NULL;
1057 int QMode = Knob_QMode;
1058
1059 if (QMode == 2 && _cxq != NULL) {
1060 // QMode == 2 : cxq has precedence over EntryList.
1061 // Try to directly wake a successor from the cxq.
1062 // If successful, the successor will need to unlink itself from cxq.
1063 w = _cxq;
1064 assert(w != NULL, "invariant");
1065 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1066 ExitEpilog(Self, w);
1067 return;
1068 }
1069
1070 if (QMode == 3 && _cxq != NULL) {
1071 // Aggressively drain cxq into EntryList at the first opportunity.
1072 // This policy ensure that recently-run threads live at the head of EntryList.
1073 // Drain _cxq into EntryList - bulk transfer.
1074 // First, detach _cxq.
1075 // The following loop is tantamount to: w = swap(&cxq, NULL)
1076 w = _cxq;
1077 for (;;) {
1078 assert(w != NULL, "Invariant");
1079 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1080 if (u == w) break;
1081 w = u;
1082 }
1083 assert(w != NULL, "invariant");
1084
1085 ObjectWaiter * q = NULL;
1086 ObjectWaiter * p;
1087 for (p = w; p != NULL; p = p->_next) {
1088 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1089 p->TState = ObjectWaiter::TS_ENTER;
1090 p->_prev = q;
1091 q = p;
1092 }
1093
1094 // Append the RATs to the EntryList
1095 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1096 ObjectWaiter * Tail;
1097 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1098 Tail = Tail->_next)
1099 /* empty */;
1100 if (Tail == NULL) {
1101 _EntryList = w;
1102 } else {
1103 Tail->_next = w;
1104 w->_prev = Tail;
1105 }
1106
1107 // Fall thru into code that tries to wake a successor from EntryList
1108 }
1109
1110 if (QMode == 4 && _cxq != NULL) {
1111 // Aggressively drain cxq into EntryList at the first opportunity.
1112 // This policy ensure that recently-run threads live at the head of EntryList.
1113
1114 // Drain _cxq into EntryList - bulk transfer.
1115 // First, detach _cxq.
1116 // The following loop is tantamount to: w = swap(&cxq, NULL)
1117 w = _cxq;
1118 for (;;) {
1119 assert(w != NULL, "Invariant");
1120 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1121 if (u == w) break;
1122 w = u;
1123 }
1124 assert(w != NULL, "invariant");
1125
1126 ObjectWaiter * q = NULL;
1127 ObjectWaiter * p;
1128 for (p = w; p != NULL; p = p->_next) {
1129 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1130 p->TState = ObjectWaiter::TS_ENTER;
1131 p->_prev = q;
1132 q = p;
1133 }
1134
1135 // Prepend the RATs to the EntryList
1136 if (_EntryList != NULL) {
1137 q->_next = _EntryList;
1138 _EntryList->_prev = q;
1139 }
1140 _EntryList = w;
1141
1142 // Fall thru into code that tries to wake a successor from EntryList
1143 }
1144
1145 w = _EntryList;
1146 if (w != NULL) {
1147 // I'd like to write: guarantee (w->_thread != Self).
1148 // But in practice an exiting thread may find itself on the EntryList.
1149 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1150 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1151 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1152 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1153 // release the lock "O". T2 resumes immediately after the ST of null into
1154 // _owner, above. T2 notices that the EntryList is populated, so it
1155 // reacquires the lock and then finds itself on the EntryList.
1156 // Given all that, we have to tolerate the circumstance where "w" is
1157 // associated with Self.
1158 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1159 ExitEpilog(Self, w);
1160 return;
1161 }
1162
1163 // If we find that both _cxq and EntryList are null then just
1164 // re-run the exit protocol from the top.
1165 w = _cxq;
1166 if (w == NULL) continue;
1167
1168 // Drain _cxq into EntryList - bulk transfer.
1169 // First, detach _cxq.
1170 // The following loop is tantamount to: w = swap(&cxq, NULL)
1171 for (;;) {
1172 assert(w != NULL, "Invariant");
1173 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1174 if (u == w) break;
1175 w = u;
1176 }
1177 TEVENT(Inflated exit - drain cxq into EntryList);
1178
1179 assert(w != NULL, "invariant");
1180 assert(_EntryList == NULL, "invariant");
1181
1182 // Convert the LIFO SLL anchored by _cxq into a DLL.
1183 // The list reorganization step operates in O(LENGTH(w)) time.
1184 // It's critical that this step operate quickly as
1185 // "Self" still holds the outer-lock, restricting parallelism
1186 // and effectively lengthening the critical section.
1187 // Invariant: s chases t chases u.
1188 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1189 // we have faster access to the tail.
1190
1191 if (QMode == 1) {
1192 // QMode == 1 : drain cxq to EntryList, reversing order
1193 // We also reverse the order of the list.
1194 ObjectWaiter * s = NULL;
1195 ObjectWaiter * t = w;
1196 ObjectWaiter * u = NULL;
1197 while (t != NULL) {
1198 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1199 t->TState = ObjectWaiter::TS_ENTER;
1200 u = t->_next;
1201 t->_prev = u;
1202 t->_next = s;
1203 s = t;
1204 t = u;
1205 }
1206 _EntryList = s;
1207 assert(s != NULL, "invariant");
1208 } else {
1209 // QMode == 0 or QMode == 2
1210 _EntryList = w;
1211 ObjectWaiter * q = NULL;
1212 ObjectWaiter * p;
1213 for (p = w; p != NULL; p = p->_next) {
1214 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1215 p->TState = ObjectWaiter::TS_ENTER;
1216 p->_prev = q;
1217 q = p;
1218 }
1219 }
1220
1221 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1222 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1223
1224 // See if we can abdicate to a spinner instead of waking a thread.
1225 // A primary goal of the implementation is to reduce the
1226 // context-switch rate.
1227 if (_succ != NULL) continue;
1228
1229 w = _EntryList;
1230 if (w != NULL) {
1231 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1232 ExitEpilog(Self, w);
1233 return;
1234 }
1235 }
1236 }
1237
1238 // ExitSuspendEquivalent:
1239 // A faster alternate to handle_special_suspend_equivalent_condition()
1240 //
1241 // handle_special_suspend_equivalent_condition() unconditionally
1242 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1243 // operations have high latency. Note that in ::enter() we call HSSEC
1244 // while holding the monitor, so we effectively lengthen the critical sections.
1245 //
1246 // There are a number of possible solutions:
1247 //
1248 // A. To ameliorate the problem we might also defer state transitions
1249 // to as late as possible -- just prior to parking.
1250 // Given that, we'd call HSSEC after having returned from park(),
1251 // but before attempting to acquire the monitor. This is only a
1252 // partial solution. It avoids calling HSSEC while holding the
1253 // monitor (good), but it still increases successor reacquisition latency --
1254 // the interval between unparking a successor and the time the successor
1255 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1256 // If we use this technique we can also avoid EnterI()-exit() loop
1257 // in ::enter() where we iteratively drop the lock and then attempt
1258 // to reacquire it after suspending.
1259 //
1260 // B. In the future we might fold all the suspend bits into a
1261 // composite per-thread suspend flag and then update it with CAS().
1262 // Alternately, a Dekker-like mechanism with multiple variables
1263 // would suffice:
1264 // ST Self->_suspend_equivalent = false
1265 // MEMBAR
1266 // LD Self_>_suspend_flags
1267 //
1268 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1269 // with a more efficient implementation, the need to use "FastHSSEC" has
1270 // decreased. - Dave
1271
1272
1273 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1274 const int Mode = Knob_FastHSSEC;
1275 if (Mode && !jSelf->is_external_suspend()) {
1276 assert(jSelf->is_suspend_equivalent(), "invariant");
1277 jSelf->clear_suspend_equivalent();
1278 if (2 == Mode) OrderAccess::storeload();
1279 if (!jSelf->is_external_suspend()) return false;
1280 // We raced a suspension -- fall thru into the slow path
1281 TEVENT(ExitSuspendEquivalent - raced);
1282 jSelf->set_suspend_equivalent();
1283 }
1284 return jSelf->handle_special_suspend_equivalent_condition();
1285 }
1286
1287
1288 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1289 assert(_owner == Self, "invariant");
1290
1291 // Exit protocol:
1292 // 1. ST _succ = wakee
1293 // 2. membar #loadstore|#storestore;
1294 // 2. ST _owner = NULL
1295 // 3. unpark(wakee)
1296
1297 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1298 ParkEvent * Trigger = Wakee->_event;
1299
1300 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1301 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1302 // out-of-scope (non-extant).
1303 Wakee = NULL;
1304
1305 // Drop the lock
1306 OrderAccess::release_store_ptr(&_owner, NULL);
1307 OrderAccess::fence(); // ST _owner vs LD in unpark()
1308
1309 if (SafepointSynchronize::do_call_back()) {
1310 TEVENT(unpark before SAFEPOINT);
1311 }
1312
1313 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1314 Trigger->unpark();
1315
1316 // Maintain stats and report events to JVMTI
1317 if (ObjectMonitor::_sync_Parks != NULL) {
1318 ObjectMonitor::_sync_Parks->inc();
1319 }
1320 }
1321
1322
1323 // -----------------------------------------------------------------------------
1324 // Class Loader deadlock handling.
1325 //
1326 // complete_exit exits a lock returning recursion count
1327 // complete_exit/reenter operate as a wait without waiting
1328 // complete_exit requires an inflated monitor
1329 // The _owner field is not always the Thread addr even with an
1330 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1331 // thread due to contention.
1332 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1333 Thread * const Self = THREAD;
1334 assert(Self->is_Java_thread(), "Must be Java thread!");
1335 JavaThread *jt = (JavaThread *)THREAD;
1336
1337 DeferredInitialize();
1338
1339 if (THREAD != _owner) {
1340 if (THREAD->is_lock_owned ((address)_owner)) {
1341 assert(_recursions == 0, "internal state error");
1342 _owner = THREAD; // Convert from basiclock addr to Thread addr
1343 _recursions = 0;
1344 }
1345 }
1346
1347 guarantee(Self == _owner, "complete_exit not owner");
1348 intptr_t save = _recursions; // record the old recursion count
1349 _recursions = 0; // set the recursion level to be 0
1350 exit(true, Self); // exit the monitor
1351 guarantee(_owner != Self, "invariant");
1352 return save;
1353 }
1354
1355 // reenter() enters a lock and sets recursion count
1356 // complete_exit/reenter operate as a wait without waiting
1357 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1358 Thread * const Self = THREAD;
1359 assert(Self->is_Java_thread(), "Must be Java thread!");
1360 JavaThread *jt = (JavaThread *)THREAD;
1361
1362 guarantee(_owner != Self, "reenter already owner");
1363 enter(THREAD); // enter the monitor
1364 guarantee(_recursions == 0, "reenter recursion");
1365 _recursions = recursions;
1366 return;
1367 }
1368
1369
1370 // -----------------------------------------------------------------------------
1371 // A macro is used below because there may already be a pending
1372 // exception which should not abort the execution of the routines
1373 // which use this (which is why we don't put this into check_slow and
1374 // call it with a CHECK argument).
1375
1376 #define CHECK_OWNER() \
1377 do { \
1378 if (THREAD != _owner) { \
1379 if (THREAD->is_lock_owned((address) _owner)) { \
1380 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1381 _recursions = 0; \
1382 } else { \
1383 TEVENT(Throw IMSX); \
1384 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1385 } \
1386 } \
1387 } while (false)
1388
1389 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1390 // TODO-FIXME: remove check_slow() -- it's likely dead.
1391
1392 void ObjectMonitor::check_slow(TRAPS) {
1393 TEVENT(check_slow - throw IMSX);
1394 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1395 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1396 }
1397
1398 static int Adjust(volatile int * adr, int dx) {
1399 int v;
1400 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1401 return v;
1402 }
1403
1404 // helper method for posting a monitor wait event
1405 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1406 jlong notifier_tid,
1407 jlong timeout,
1408 bool timedout) {
1409 event->set_klass(((oop)this->object())->klass());
1410 event->set_timeout((TYPE_ULONG)timeout);
1411 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1412 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1413 event->set_timedOut((TYPE_BOOLEAN)timedout);
1414 event->commit();
1415 }
1416
1417 // -----------------------------------------------------------------------------
1418 // Wait/Notify/NotifyAll
1419 //
1420 // Note: a subset of changes to ObjectMonitor::wait()
1421 // will need to be replicated in complete_exit
1422 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1423 Thread * const Self = THREAD;
1424 assert(Self->is_Java_thread(), "Must be Java thread!");
1425 JavaThread *jt = (JavaThread *)THREAD;
1426
1427 DeferredInitialize();
1428
1429 // Throw IMSX or IEX.
1430 CHECK_OWNER();
1431
1432 EventJavaMonitorWait event;
1433
1434 // check for a pending interrupt
1435 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1436 // post monitor waited event. Note that this is past-tense, we are done waiting.
1437 if (JvmtiExport::should_post_monitor_waited()) {
1438 // Note: 'false' parameter is passed here because the
1439 // wait was not timed out due to thread interrupt.
1440 JvmtiExport::post_monitor_waited(jt, this, false);
1441
1442 // In this short circuit of the monitor wait protocol, the
1443 // current thread never drops ownership of the monitor and
1444 // never gets added to the wait queue so the current thread
1445 // cannot be made the successor. This means that the
1446 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1447 // consume an unpark() meant for the ParkEvent associated with
1448 // this ObjectMonitor.
1449 }
1450 if (event.should_commit()) {
1451 post_monitor_wait_event(&event, 0, millis, false);
1452 }
1453 TEVENT(Wait - Throw IEX);
1454 THROW(vmSymbols::java_lang_InterruptedException());
1455 return;
1456 }
1457
1458 TEVENT(Wait);
1459
1460 assert(Self->_Stalled == 0, "invariant");
1461 Self->_Stalled = intptr_t(this);
1462 jt->set_current_waiting_monitor(this);
1463
1464 // create a node to be put into the queue
1465 // Critically, after we reset() the event but prior to park(), we must check
1466 // for a pending interrupt.
1467 ObjectWaiter node(Self);
1468 node.TState = ObjectWaiter::TS_WAIT;
1469 Self->_ParkEvent->reset();
1470 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1471
1472 // Enter the waiting queue, which is a circular doubly linked list in this case
1473 // but it could be a priority queue or any data structure.
1474 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1475 // by the the owner of the monitor *except* in the case where park()
1476 // returns because of a timeout of interrupt. Contention is exceptionally rare
1477 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1478
1479 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1480 AddWaiter(&node);
1481 Thread::SpinRelease(&_WaitSetLock);
1482
1483 if ((SyncFlags & 4) == 0) {
1484 _Responsible = NULL;
1485 }
1486 intptr_t save = _recursions; // record the old recursion count
1487 _waiters++; // increment the number of waiters
1488 _recursions = 0; // set the recursion level to be 1
1489 exit(true, Self); // exit the monitor
1490 guarantee(_owner != Self, "invariant");
1491
1492 // The thread is on the WaitSet list - now park() it.
1493 // On MP systems it's conceivable that a brief spin before we park
1494 // could be profitable.
1495 //
1496 // TODO-FIXME: change the following logic to a loop of the form
1497 // while (!timeout && !interrupted && _notified == 0) park()
1498
1499 int ret = OS_OK;
1500 int WasNotified = 0;
1501 { // State transition wrappers
1502 OSThread* osthread = Self->osthread();
1503 OSThreadWaitState osts(osthread, true);
1504 {
1505 ThreadBlockInVM tbivm(jt);
1506 // Thread is in thread_blocked state and oop access is unsafe.
1507 jt->set_suspend_equivalent();
1508
1509 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1510 // Intentionally empty
1511 } else if (node._notified == 0) {
1512 if (millis <= 0) {
1513 Self->_ParkEvent->park();
1514 } else {
1515 ret = Self->_ParkEvent->park(millis);
1516 }
1517 }
1518
1519 // were we externally suspended while we were waiting?
1520 if (ExitSuspendEquivalent (jt)) {
1521 // TODO-FIXME: add -- if succ == Self then succ = null.
1522 jt->java_suspend_self();
1523 }
1524
1525 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1526
1527 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1528 // from the WaitSet to the EntryList.
1529 // See if we need to remove Node from the WaitSet.
1530 // We use double-checked locking to avoid grabbing _WaitSetLock
1531 // if the thread is not on the wait queue.
1532 //
1533 // Note that we don't need a fence before the fetch of TState.
1534 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1535 // written by the is thread. (perhaps the fetch might even be satisfied
1536 // by a look-aside into the processor's own store buffer, although given
1537 // the length of the code path between the prior ST and this load that's
1538 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1539 // then we'll acquire the lock and then re-fetch a fresh TState value.
1540 // That is, we fail toward safety.
1541
1542 if (node.TState == ObjectWaiter::TS_WAIT) {
1543 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1544 if (node.TState == ObjectWaiter::TS_WAIT) {
1545 DequeueSpecificWaiter(&node); // unlink from WaitSet
1546 assert(node._notified == 0, "invariant");
1547 node.TState = ObjectWaiter::TS_RUN;
1548 }
1549 Thread::SpinRelease(&_WaitSetLock);
1550 }
1551
1552 // The thread is now either on off-list (TS_RUN),
1553 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1554 // The Node's TState variable is stable from the perspective of this thread.
1555 // No other threads will asynchronously modify TState.
1556 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1557 OrderAccess::loadload();
1558 if (_succ == Self) _succ = NULL;
1559 WasNotified = node._notified;
1560
1561 // Reentry phase -- reacquire the monitor.
1562 // re-enter contended monitor after object.wait().
1563 // retain OBJECT_WAIT state until re-enter successfully completes
1564 // Thread state is thread_in_vm and oop access is again safe,
1565 // although the raw address of the object may have changed.
1566 // (Don't cache naked oops over safepoints, of course).
1567
1568 // post monitor waited event. Note that this is past-tense, we are done waiting.
1569 if (JvmtiExport::should_post_monitor_waited()) {
1570 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1571
1572 if (node._notified != 0 && _succ == Self) {
1573 // In this part of the monitor wait-notify-reenter protocol it
1574 // is possible (and normal) for another thread to do a fastpath
1575 // monitor enter-exit while this thread is still trying to get
1576 // to the reenter portion of the protocol.
1577 //
1578 // The ObjectMonitor was notified and the current thread is
1579 // the successor which also means that an unpark() has already
1580 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1581 // consume the unpark() that was done when the successor was
1582 // set because the same ParkEvent is shared between Java
1583 // monitors and JVM/TI RawMonitors (for now).
1584 //
1585 // We redo the unpark() to ensure forward progress, i.e., we
1586 // don't want all pending threads hanging (parked) with none
1587 // entering the unlocked monitor.
1588 node._event->unpark();
1589 }
1590 }
1591
1592 if (event.should_commit()) {
1593 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1594 }
1595
1596 OrderAccess::fence();
1597
1598 assert(Self->_Stalled != 0, "invariant");
1599 Self->_Stalled = 0;
1600
1601 assert(_owner != Self, "invariant");
1602 ObjectWaiter::TStates v = node.TState;
1603 if (v == ObjectWaiter::TS_RUN) {
1604 enter(Self);
1605 } else {
1606 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1607 ReenterI(Self, &node);
1608 node.wait_reenter_end(this);
1609 }
1610
1611 // Self has reacquired the lock.
1612 // Lifecycle - the node representing Self must not appear on any queues.
1613 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1614 // want residual elements associated with this thread left on any lists.
1615 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1616 assert(_owner == Self, "invariant");
1617 assert(_succ != Self, "invariant");
1618 } // OSThreadWaitState()
1619
1620 jt->set_current_waiting_monitor(NULL);
1621
1622 guarantee(_recursions == 0, "invariant");
1623 _recursions = save; // restore the old recursion count
1624 _waiters--; // decrement the number of waiters
1625
1626 // Verify a few postconditions
1627 assert(_owner == Self, "invariant");
1628 assert(_succ != Self, "invariant");
1629 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1630
1631 if (SyncFlags & 32) {
1632 OrderAccess::fence();
1633 }
1634
1635 // check if the notification happened
1636 if (!WasNotified) {
1637 // no, it could be timeout or Thread.interrupt() or both
1638 // check for interrupt event, otherwise it is timeout
1639 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1640 TEVENT(Wait - throw IEX from epilog);
1641 THROW(vmSymbols::java_lang_InterruptedException());
1642 }
1643 }
1644
1645 // NOTE: Spurious wake up will be consider as timeout.
1646 // Monitor notify has precedence over thread interrupt.
1647 }
1648
1649
1650 // Consider:
1651 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1652 // then instead of transferring a thread from the WaitSet to the EntryList
1653 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1654
1655 void ObjectMonitor::notify(TRAPS) {
1656 CHECK_OWNER();
1657 if (_WaitSet == NULL) {
1658 TEVENT(Empty-Notify);
1659 return;
1660 }
1661 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1662
1663 int Policy = Knob_MoveNotifyee;
1664
1665 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1666 ObjectWaiter * iterator = DequeueWaiter();
1667 if (iterator != NULL) {
1668 TEVENT(Notify1 - Transfer);
1669 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1670 guarantee(iterator->_notified == 0, "invariant");
1671 if (Policy != 4) {
1672 iterator->TState = ObjectWaiter::TS_ENTER;
1673 }
1674 iterator->_notified = 1;
1675 Thread * Self = THREAD;
1676 iterator->_notifier_tid = Self->osthread()->thread_id();
1677
1678 ObjectWaiter * List = _EntryList;
1679 if (List != NULL) {
1680 assert(List->_prev == NULL, "invariant");
1681 assert(List->TState == ObjectWaiter::TS_ENTER, "invariant");
1682 assert(List != iterator, "invariant");
1683 }
1684
1685 if (Policy == 0) { // prepend to EntryList
1686 if (List == NULL) {
1687 iterator->_next = iterator->_prev = NULL;
1688 _EntryList = iterator;
1689 } else {
1690 List->_prev = iterator;
1691 iterator->_next = List;
1692 iterator->_prev = NULL;
1693 _EntryList = iterator;
1694 }
1695 } else if (Policy == 1) { // append to EntryList
1696 if (List == NULL) {
1697 iterator->_next = iterator->_prev = NULL;
1698 _EntryList = iterator;
1699 } else {
1700 // CONSIDER: finding the tail currently requires a linear-time walk of
1701 // the EntryList. We can make tail access constant-time by converting to
1702 // a CDLL instead of using our current DLL.
1703 ObjectWaiter * Tail;
1704 for (Tail = List; Tail->_next != NULL; Tail = Tail->_next) /* empty */;
1705 assert(Tail != NULL && Tail->_next == NULL, "invariant");
1706 Tail->_next = iterator;
1707 iterator->_prev = Tail;
1708 iterator->_next = NULL;
1709 }
1710 } else if (Policy == 2) { // prepend to cxq
1711 // prepend to cxq
1712 if (List == NULL) {
1713 iterator->_next = iterator->_prev = NULL;
1714 _EntryList = iterator;
1715 } else {
1716 iterator->TState = ObjectWaiter::TS_CXQ;
1717 for (;;) {
1718 ObjectWaiter * Front = _cxq;
1719 iterator->_next = Front;
1720 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1721 break;
1722 }
1723 }
1724 }
1725 } else if (Policy == 3) { // append to cxq
1726 iterator->TState = ObjectWaiter::TS_CXQ;
1727 for (;;) {
1728 ObjectWaiter * Tail;
1729 Tail = _cxq;
1730 if (Tail == NULL) {
1731 iterator->_next = NULL;
1732 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1733 break;
1734 }
1735 } else {
1736 while (Tail->_next != NULL) Tail = Tail->_next;
1737 Tail->_next = iterator;
1738 iterator->_prev = Tail;
1739 iterator->_next = NULL;
1740 break;
1741 }
1742 }
1743 } else {
1744 ParkEvent * ev = iterator->_event;
1745 iterator->TState = ObjectWaiter::TS_RUN;
1746 OrderAccess::fence();
1747 ev->unpark();
1748 }
1749
1750 if (Policy < 4) {
1751 iterator->wait_reenter_begin(this);
1752 }
1753
1754 // _WaitSetLock protects the wait queue, not the EntryList. We could
1755 // move the add-to-EntryList operation, above, outside the critical section
1756 // protected by _WaitSetLock. In practice that's not useful. With the
1757 // exception of wait() timeouts and interrupts the monitor owner
1758 // is the only thread that grabs _WaitSetLock. There's almost no contention
1759 // on _WaitSetLock so it's not profitable to reduce the length of the
1760 // critical section.
1761 }
1762
1763 Thread::SpinRelease(&_WaitSetLock);
1764
1765 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1766 ObjectMonitor::_sync_Notifications->inc();
1767 }
1768 }
1769
1770
1771 void ObjectMonitor::notifyAll(TRAPS) {
1772 CHECK_OWNER();
1773 ObjectWaiter* iterator;
1774 if (_WaitSet == NULL) {
1775 TEVENT(Empty-NotifyAll);
1776 return;
1777 }
1778 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1779
1780 int Policy = Knob_MoveNotifyee;
1781 int Tally = 0;
1782 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notifyall");
1783
1784 for (;;) {
1785 iterator = DequeueWaiter();
1786 if (iterator == NULL) break;
1787 TEVENT(NotifyAll - Transfer1);
1788 ++Tally;
1789
1790 // Disposition - what might we do with iterator ?
1791 // a. add it directly to the EntryList - either tail or head.
1792 // b. push it onto the front of the _cxq.
1793 // For now we use (a).
1794
1795 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1796 guarantee(iterator->_notified == 0, "invariant");
1797 iterator->_notified = 1;
1798 Thread * Self = THREAD;
1799 iterator->_notifier_tid = Self->osthread()->thread_id();
1800 if (Policy != 4) {
1801 iterator->TState = ObjectWaiter::TS_ENTER;
1802 }
1803
1804 ObjectWaiter * List = _EntryList;
1805 if (List != NULL) {
1806 assert(List->_prev == NULL, "invariant");
1807 assert(List->TState == ObjectWaiter::TS_ENTER, "invariant");
1808 assert(List != iterator, "invariant");
1809 }
1810
1811 if (Policy == 0) { // prepend to EntryList
1812 if (List == NULL) {
1813 iterator->_next = iterator->_prev = NULL;
1814 _EntryList = iterator;
1815 } else {
1816 List->_prev = iterator;
1817 iterator->_next = List;
1818 iterator->_prev = NULL;
1819 _EntryList = iterator;
1820 }
1821 } else if (Policy == 1) { // append to EntryList
1822 if (List == NULL) {
1823 iterator->_next = iterator->_prev = NULL;
1824 _EntryList = iterator;
1825 } else {
1826 // CONSIDER: finding the tail currently requires a linear-time walk of
1827 // the EntryList. We can make tail access constant-time by converting to
1828 // a CDLL instead of using our current DLL.
1829 ObjectWaiter * Tail;
1830 for (Tail = List; Tail->_next != NULL; Tail = Tail->_next) /* empty */;
1831 assert(Tail != NULL && Tail->_next == NULL, "invariant");
1832 Tail->_next = iterator;
1833 iterator->_prev = Tail;
1834 iterator->_next = NULL;
1835 }
1836 } else if (Policy == 2) { // prepend to cxq
1837 // prepend to cxq
1838 iterator->TState = ObjectWaiter::TS_CXQ;
1839 for (;;) {
1840 ObjectWaiter * Front = _cxq;
1841 iterator->_next = Front;
1842 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1843 break;
1844 }
1845 }
1846 } else if (Policy == 3) { // append to cxq
1847 iterator->TState = ObjectWaiter::TS_CXQ;
1848 for (;;) {
1849 ObjectWaiter * Tail;
1850 Tail = _cxq;
1851 if (Tail == NULL) {
1852 iterator->_next = NULL;
1853 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1854 break;
1855 }
1856 } else {
1857 while (Tail->_next != NULL) Tail = Tail->_next;
1858 Tail->_next = iterator;
1859 iterator->_prev = Tail;
1860 iterator->_next = NULL;
1861 break;
1862 }
1863 }
1864 } else {
1865 ParkEvent * ev = iterator->_event;
1866 iterator->TState = ObjectWaiter::TS_RUN;
1867 OrderAccess::fence();
1868 ev->unpark();
1869 }
1870
1871 if (Policy < 4) {
1872 iterator->wait_reenter_begin(this);
1873 }
1874
1875 // _WaitSetLock protects the wait queue, not the EntryList. We could
1876 // move the add-to-EntryList operation, above, outside the critical section
1877 // protected by _WaitSetLock. In practice that's not useful. With the
1878 // exception of wait() timeouts and interrupts the monitor owner
1879 // is the only thread that grabs _WaitSetLock. There's almost no contention
1880 // on _WaitSetLock so it's not profitable to reduce the length of the
1881 // critical section.
1882 }
1883
1884 Thread::SpinRelease(&_WaitSetLock);
1885
1886 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1887 ObjectMonitor::_sync_Notifications->inc(Tally);
1888 }
1889 }
1890
1891 // -----------------------------------------------------------------------------
1892 // Adaptive Spinning Support
1893 //
1894 // Adaptive spin-then-block - rational spinning
1895 //
1896 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1897 // algorithm. On high order SMP systems it would be better to start with
1898 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1899 // a contending thread could enqueue itself on the cxq and then spin locally
1900 // on a thread-specific variable such as its ParkEvent._Event flag.
1901 // That's left as an exercise for the reader. Note that global spinning is
1902 // not problematic on Niagara, as the L2 cache serves the interconnect and
1903 // has both low latency and massive bandwidth.
1904 //
1905 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1906 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1907 // (duration) or we can fix the count at approximately the duration of
1908 // a context switch and vary the frequency. Of course we could also
1909 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1910 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1911 //
1912 // This implementation varies the duration "D", where D varies with
1913 // the success rate of recent spin attempts. (D is capped at approximately
1914 // length of a round-trip context switch). The success rate for recent
1915 // spin attempts is a good predictor of the success rate of future spin
1916 // attempts. The mechanism adapts automatically to varying critical
1917 // section length (lock modality), system load and degree of parallelism.
1918 // D is maintained per-monitor in _SpinDuration and is initialized
1919 // optimistically. Spin frequency is fixed at 100%.
1920 //
1921 // Note that _SpinDuration is volatile, but we update it without locks
1922 // or atomics. The code is designed so that _SpinDuration stays within
1923 // a reasonable range even in the presence of races. The arithmetic
1924 // operations on _SpinDuration are closed over the domain of legal values,
1925 // so at worst a race will install and older but still legal value.
1926 // At the very worst this introduces some apparent non-determinism.
1927 // We might spin when we shouldn't or vice-versa, but since the spin
1928 // count are relatively short, even in the worst case, the effect is harmless.
1929 //
1930 // Care must be taken that a low "D" value does not become an
1931 // an absorbing state. Transient spinning failures -- when spinning
1932 // is overall profitable -- should not cause the system to converge
1933 // on low "D" values. We want spinning to be stable and predictable
1934 // and fairly responsive to change and at the same time we don't want
1935 // it to oscillate, become metastable, be "too" non-deterministic,
1936 // or converge on or enter undesirable stable absorbing states.
1937 //
1938 // We implement a feedback-based control system -- using past behavior
1939 // to predict future behavior. We face two issues: (a) if the
1940 // input signal is random then the spin predictor won't provide optimal
1941 // results, and (b) if the signal frequency is too high then the control
1942 // system, which has some natural response lag, will "chase" the signal.
1943 // (b) can arise from multimodal lock hold times. Transient preemption
1944 // can also result in apparent bimodal lock hold times.
1945 // Although sub-optimal, neither condition is particularly harmful, as
1946 // in the worst-case we'll spin when we shouldn't or vice-versa.
1947 // The maximum spin duration is rather short so the failure modes aren't bad.
1948 // To be conservative, I've tuned the gain in system to bias toward
1949 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1950 // "rings" or oscillates between spinning and not spinning. This happens
1951 // when spinning is just on the cusp of profitability, however, so the
1952 // situation is not dire. The state is benign -- there's no need to add
1953 // hysteresis control to damp the transition rate between spinning and
1954 // not spinning.
1955
1956 intptr_t ObjectMonitor::SpinCallbackArgument = 0;
1957 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL;
1958
1959 // Spinning: Fixed frequency (100%), vary duration
1960
1961
1962 int ObjectMonitor::TrySpin_VaryDuration(Thread * Self) {
1963 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1964 int ctr = Knob_FixedSpin;
1965 if (ctr != 0) {
1966 while (--ctr >= 0) {
1967 if (TryLock(Self) > 0) return 1;
1968 SpinPause();
1969 }
1970 return 0;
1971 }
1972
1973 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1974 if (TryLock(Self) > 0) {
1975 // Increase _SpinDuration ...
1976 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1977 // Raising _SpurDuration to the poverty line is key.
1978 int x = _SpinDuration;
1979 if (x < Knob_SpinLimit) {
1980 if (x < Knob_Poverty) x = Knob_Poverty;
1981 _SpinDuration = x + Knob_BonusB;
1982 }
1983 return 1;
1984 }
1985 SpinPause();
1986 }
1987
1988 // Admission control - verify preconditions for spinning
1989 //
1990 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1991 // becoming an absorbing state. Put another way, we spin briefly to
1992 // sample, just in case the system load, parallelism, contention, or lock
1993 // modality changed.
1994 //
1995 // Consider the following alternative:
1996 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1997 // spin attempt. "Periodically" might mean after a tally of
1998 // the # of failed spin attempts (or iterations) reaches some threshold.
1999 // This takes us into the realm of 1-out-of-N spinning, where we
2000 // hold the duration constant but vary the frequency.
2001
2002 ctr = _SpinDuration;
2003 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
2004 if (ctr <= 0) return 0;
2005
2006 if (Knob_SuccRestrict && _succ != NULL) return 0;
2007 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2008 TEVENT(Spin abort - notrunnable [TOP]);
2009 return 0;
2010 }
2011
2012 int MaxSpin = Knob_MaxSpinners;
2013 if (MaxSpin >= 0) {
2014 if (_Spinner > MaxSpin) {
2015 TEVENT(Spin abort -- too many spinners);
2016 return 0;
2017 }
2018 // Slightly racy, but benign ...
2019 Adjust(&_Spinner, 1);
2020 }
2021
2022 // We're good to spin ... spin ingress.
2023 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2024 // when preparing to LD...CAS _owner, etc and the CAS is likely
2025 // to succeed.
2026 int hits = 0;
2027 int msk = 0;
2028 int caspty = Knob_CASPenalty;
2029 int oxpty = Knob_OXPenalty;
2030 int sss = Knob_SpinSetSucc;
2031 if (sss && _succ == NULL) _succ = Self;
2032 Thread * prv = NULL;
2033
2034 // There are three ways to exit the following loop:
2035 // 1. A successful spin where this thread has acquired the lock.
2036 // 2. Spin failure with prejudice
2037 // 3. Spin failure without prejudice
2038
2039 while (--ctr >= 0) {
2040
2041 // Periodic polling -- Check for pending GC
2042 // Threads may spin while they're unsafe.
2043 // We don't want spinning threads to delay the JVM from reaching
2044 // a stop-the-world safepoint or to steal cycles from GC.
2045 // If we detect a pending safepoint we abort in order that
2046 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2047 // this thread, if safe, doesn't steal cycles from GC.
2048 // This is in keeping with the "no loitering in runtime" rule.
2049 // We periodically check to see if there's a safepoint pending.
2050 if ((ctr & 0xFF) == 0) {
2051 if (SafepointSynchronize::do_call_back()) {
2052 TEVENT(Spin: safepoint);
2053 goto Abort; // abrupt spin egress
2054 }
2055 if (Knob_UsePause & 1) SpinPause();
2056
2057 int (*scb)(intptr_t,int) = SpinCallbackFunction;
2058 if (hits > 50 && scb != NULL) {
2059 int abend = (*scb)(SpinCallbackArgument, 0);
2060 }
2061 }
2062
2063 if (Knob_UsePause & 2) SpinPause();
2064
2065 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2066 // This is useful on classic SMP systems, but is of less utility on
2067 // N1-style CMT platforms.
2068 //
2069 // Trade-off: lock acquisition latency vs coherency bandwidth.
2070 // Lock hold times are typically short. A histogram
2071 // of successful spin attempts shows that we usually acquire
2072 // the lock early in the spin. That suggests we want to
2073 // sample _owner frequently in the early phase of the spin,
2074 // but then back-off and sample less frequently as the spin
2075 // progresses. The back-off makes a good citizen on SMP big
2076 // SMP systems. Oversampling _owner can consume excessive
2077 // coherency bandwidth. Relatedly, if we _oversample _owner we
2078 // can inadvertently interfere with the the ST m->owner=null.
2079 // executed by the lock owner.
2080 if (ctr & msk) continue;
2081 ++hits;
2082 if ((hits & 0xF) == 0) {
2083 // The 0xF, above, corresponds to the exponent.
2084 // Consider: (msk+1)|msk
2085 msk = ((msk << 2)|3) & BackOffMask;
2086 }
2087
2088 // Probe _owner with TATAS
2089 // If this thread observes the monitor transition or flicker
2090 // from locked to unlocked to locked, then the odds that this
2091 // thread will acquire the lock in this spin attempt go down
2092 // considerably. The same argument applies if the CAS fails
2093 // or if we observe _owner change from one non-null value to
2094 // another non-null value. In such cases we might abort
2095 // the spin without prejudice or apply a "penalty" to the
2096 // spin count-down variable "ctr", reducing it by 100, say.
2097
2098 Thread * ox = (Thread *) _owner;
2099 if (ox == NULL) {
2100 ox = (Thread *) Atomic::cmpxchg_ptr(Self, &_owner, NULL);
2101 if (ox == NULL) {
2102 // The CAS succeeded -- this thread acquired ownership
2103 // Take care of some bookkeeping to exit spin state.
2104 if (sss && _succ == Self) {
2105 _succ = NULL;
2106 }
2107 if (MaxSpin > 0) Adjust(&_Spinner, -1);
2108
2109 // Increase _SpinDuration :
2110 // The spin was successful (profitable) so we tend toward
2111 // longer spin attempts in the future.
2112 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2113 // If we acquired the lock early in the spin cycle it
2114 // makes sense to increase _SpinDuration proportionally.
2115 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2116 int x = _SpinDuration;
2117 if (x < Knob_SpinLimit) {
2118 if (x < Knob_Poverty) x = Knob_Poverty;
2119 _SpinDuration = x + Knob_Bonus;
2120 }
2121 return 1;
2122 }
2123
2124 // The CAS failed ... we can take any of the following actions:
2125 // * penalize: ctr -= Knob_CASPenalty
2126 // * exit spin with prejudice -- goto Abort;
2127 // * exit spin without prejudice.
2128 // * Since CAS is high-latency, retry again immediately.
2129 prv = ox;
2130 TEVENT(Spin: cas failed);
2131 if (caspty == -2) break;
2132 if (caspty == -1) goto Abort;
2133 ctr -= caspty;
2134 continue;
2135 }
2136
2137 // Did lock ownership change hands ?
2138 if (ox != prv && prv != NULL) {
2139 TEVENT(spin: Owner changed)
2140 if (oxpty == -2) break;
2141 if (oxpty == -1) goto Abort;
2142 ctr -= oxpty;
2143 }
2144 prv = ox;
2145
2146 // Abort the spin if the owner is not executing.
2147 // The owner must be executing in order to drop the lock.
2148 // Spinning while the owner is OFFPROC is idiocy.
2149 // Consider: ctr -= RunnablePenalty ;
2150 if (Knob_OState && NotRunnable (Self, ox)) {
2151 TEVENT(Spin abort - notrunnable);
2152 goto Abort;
2153 }
2154 if (sss && _succ == NULL) _succ = Self;
2155 }
2156
2157 // Spin failed with prejudice -- reduce _SpinDuration.
2158 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2159 // AIMD is globally stable.
2160 TEVENT(Spin failure);
2161 {
2162 int x = _SpinDuration;
2163 if (x > 0) {
2164 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2165 // This is globally sample and tends to damp the response.
2166 x -= Knob_Penalty;
2167 if (x < 0) x = 0;
2168 _SpinDuration = x;
2169 }
2170 }
2171
2172 Abort:
2173 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2174 if (sss && _succ == Self) {
2175 _succ = NULL;
2176 // Invariant: after setting succ=null a contending thread
2177 // must recheck-retry _owner before parking. This usually happens
2178 // in the normal usage of TrySpin(), but it's safest
2179 // to make TrySpin() as foolproof as possible.
2180 OrderAccess::fence();
2181 if (TryLock(Self) > 0) return 1;
2182 }
2183 return 0;
2184 }
2185
2186 // NotRunnable() -- informed spinning
2187 //
2188 // Don't bother spinning if the owner is not eligible to drop the lock.
2189 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2190 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2191 // The thread must be runnable in order to drop the lock in timely fashion.
2192 // If the _owner is not runnable then spinning will not likely be
2193 // successful (profitable).
2194 //
2195 // Beware -- the thread referenced by _owner could have died
2196 // so a simply fetch from _owner->_thread_state might trap.
2197 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2198 // Because of the lifecycle issues the schedctl and _thread_state values
2199 // observed by NotRunnable() might be garbage. NotRunnable must
2200 // tolerate this and consider the observed _thread_state value
2201 // as advisory.
2202 //
2203 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2204 // a thread pointer.
2205 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2206 // with the LSB of _owner. Another option would be to probablistically probe
2207 // the putative _owner->TypeTag value.
2208 //
2209 // Checking _thread_state isn't perfect. Even if the thread is
2210 // in_java it might be blocked on a page-fault or have been preempted
2211 // and sitting on a ready/dispatch queue. _thread state in conjunction
2212 // with schedctl.sc_state gives us a good picture of what the
2213 // thread is doing, however.
2214 //
2215 // TODO: check schedctl.sc_state.
2216 // We'll need to use SafeFetch32() to read from the schedctl block.
2217 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2218 //
2219 // The return value from NotRunnable() is *advisory* -- the
2220 // result is based on sampling and is not necessarily coherent.
2221 // The caller must tolerate false-negative and false-positive errors.
2222 // Spinning, in general, is probabilistic anyway.
2223
2224
2225 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2226 // Check ox->TypeTag == 2BAD.
2227 if (ox == NULL) return 0;
2228
2229 // Avoid transitive spinning ...
2230 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2231 // Immediately after T1 acquires L it's possible that T2, also
2232 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2233 // This occurs transiently after T1 acquired L but before
2234 // T1 managed to clear T1.Stalled. T2 does not need to abort
2235 // its spin in this circumstance.
2236 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2237
2238 if (BlockedOn == 1) return 1;
2239 if (BlockedOn != 0) {
2240 return BlockedOn != intptr_t(this) && _owner == ox;
2241 }
2242
2243 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2244 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2245 // consider also: jst != _thread_in_Java -- but that's overspecific.
2246 return jst == _thread_blocked || jst == _thread_in_native;
2247 }
2248
2249
2250 // -----------------------------------------------------------------------------
2251 // WaitSet management ...
2252
2253 ObjectWaiter::ObjectWaiter(Thread* thread) {
2254 _next = NULL;
2255 _prev = NULL;
2256 _notified = 0;
2257 TState = TS_RUN;
2258 _thread = thread;
2259 _event = thread->_ParkEvent;
2260 _active = false;
2261 assert(_event != NULL, "invariant");
2262 }
2263
2264 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2265 JavaThread *jt = (JavaThread *)this->_thread;
2266 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2267 }
2268
2269 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2270 JavaThread *jt = (JavaThread *)this->_thread;
2271 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2272 }
2273
2274 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2275 assert(node != NULL, "should not dequeue NULL node");
2276 assert(node->_prev == NULL, "node already in list");
2277 assert(node->_next == NULL, "node already in list");
2278 // put node at end of queue (circular doubly linked list)
2279 if (_WaitSet == NULL) {
2280 _WaitSet = node;
2281 node->_prev = node;
2282 node->_next = node;
2283 } else {
2284 ObjectWaiter* head = _WaitSet;
2285 ObjectWaiter* tail = head->_prev;
2286 assert(tail->_next == head, "invariant check");
2287 tail->_next = node;
2288 head->_prev = node;
2289 node->_next = head;
2290 node->_prev = tail;
2291 }
2292 }
2293
2294 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2295 // dequeue the very first waiter
2296 ObjectWaiter* waiter = _WaitSet;
2297 if (waiter) {
2298 DequeueSpecificWaiter(waiter);
2299 }
2300 return waiter;
2301 }
2302
2303 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2304 assert(node != NULL, "should not dequeue NULL node");
2305 assert(node->_prev != NULL, "node already removed from list");
2306 assert(node->_next != NULL, "node already removed from list");
2307 // when the waiter has woken up because of interrupt,
2308 // timeout or other spurious wake-up, dequeue the
2309 // waiter from waiting list
2310 ObjectWaiter* next = node->_next;
2311 if (next == node) {
2312 assert(node->_prev == node, "invariant check");
2313 _WaitSet = NULL;
2314 } else {
2315 ObjectWaiter* prev = node->_prev;
2316 assert(prev->_next == node, "invariant check");
2317 assert(next->_prev == node, "invariant check");
2318 next->_prev = prev;
2319 prev->_next = next;
2320 if (_WaitSet == node) {
2321 _WaitSet = next;
2322 }
2323 }
2324 node->_next = NULL;
2325 node->_prev = NULL;
2326 }
2327
2328 // -----------------------------------------------------------------------------
2329 // PerfData support
2330 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2331 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2332 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2333 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL;
2334 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2335 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL;
2336 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL;
2337 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL;
2338 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL;
2339 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL;
2340 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL;
2341 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL;
2342 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL;
2343 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL;
2344 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL;
2345 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2346 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2347 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2348
2349 // One-shot global initialization for the sync subsystem.
2350 // We could also defer initialization and initialize on-demand
2351 // the first time we call inflate(). Initialization would
2352 // be protected - like so many things - by the MonitorCache_lock.
2353
2354 void ObjectMonitor::Initialize() {
2355 static int InitializationCompleted = 0;
2356 assert(InitializationCompleted == 0, "invariant");
2357 InitializationCompleted = 1;
2358 if (UsePerfData) {
2359 EXCEPTION_MARK;
2360 #define NEWPERFCOUNTER(n) \
2361 { \
2362 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2363 CHECK); \
2364 }
2365 #define NEWPERFVARIABLE(n) \
2366 { \
2367 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2368 CHECK); \
2369 }
2370 NEWPERFCOUNTER(_sync_Inflations);
2371 NEWPERFCOUNTER(_sync_Deflations);
2372 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2373 NEWPERFCOUNTER(_sync_FutileWakeups);
2374 NEWPERFCOUNTER(_sync_Parks);
2375 NEWPERFCOUNTER(_sync_EmptyNotifications);
2376 NEWPERFCOUNTER(_sync_Notifications);
2377 NEWPERFCOUNTER(_sync_SlowEnter);
2378 NEWPERFCOUNTER(_sync_SlowExit);
2379 NEWPERFCOUNTER(_sync_SlowNotify);
2380 NEWPERFCOUNTER(_sync_SlowNotifyAll);
2381 NEWPERFCOUNTER(_sync_FailedSpins);
2382 NEWPERFCOUNTER(_sync_SuccessfulSpins);
2383 NEWPERFCOUNTER(_sync_PrivateA);
2384 NEWPERFCOUNTER(_sync_PrivateB);
2385 NEWPERFCOUNTER(_sync_MonInCirculation);
2386 NEWPERFCOUNTER(_sync_MonScavenged);
2387 NEWPERFVARIABLE(_sync_MonExtant);
2388 #undef NEWPERFCOUNTER
2389 #undef NEWPERFVARIABLE
2390 }
2391 }
2392
2393 static char * kvGet(char * kvList, const char * Key) {
2394 if (kvList == NULL) return NULL;
2395 size_t n = strlen(Key);
2396 char * Search;
2397 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2398 if (strncmp (Search, Key, n) == 0) {
2399 if (Search[n] == '=') return Search + n + 1;
2400 if (Search[n] == 0) return(char *) "1";
2401 }
2402 }
2403 return NULL;
2404 }
2405
2406 static int kvGetInt(char * kvList, const char * Key, int Default) {
2407 char * v = kvGet(kvList, Key);
2408 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2409 if (Knob_ReportSettings && v != NULL) {
2410 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2411 ::fflush(stdout);
2412 }
2413 return rslt;
2414 }
2415
2416 void ObjectMonitor::DeferredInitialize() {
2417 if (InitDone > 0) return;
2418 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2419 while (InitDone != 1) /* empty */;
2420 return;
2421 }
2422
2423 // One-shot global initialization ...
2424 // The initialization is idempotent, so we don't need locks.
2425 // In the future consider doing this via os::init_2().
2426 // SyncKnobs consist of <Key>=<Value> pairs in the style
2427 // of environment variables. Start by converting ':' to NUL.
2428
2429 if (SyncKnobs == NULL) SyncKnobs = "";
2430
2431 size_t sz = strlen(SyncKnobs);
2432 char * knobs = (char *) malloc(sz + 2);
2433 if (knobs == NULL) {
2434 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2435 guarantee(0, "invariant");
2436 }
2437 strcpy(knobs, SyncKnobs);
2438 knobs[sz+1] = 0;
2439 for (char * p = knobs; *p; p++) {
2440 if (*p == ':') *p = 0;
2441 }
2442
2443 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2444 SETKNOB(ReportSettings);
2445 SETKNOB(Verbose);
2446 SETKNOB(VerifyInUse);
2447 SETKNOB(FixedSpin);
2448 SETKNOB(SpinLimit);
2449 SETKNOB(SpinBase);
2450 SETKNOB(SpinBackOff);
2451 SETKNOB(CASPenalty);
2452 SETKNOB(OXPenalty);
2453 SETKNOB(LogSpins);
2454 SETKNOB(SpinSetSucc);
2455 SETKNOB(SuccEnabled);
2456 SETKNOB(SuccRestrict);
2457 SETKNOB(Penalty);
2458 SETKNOB(Bonus);
2459 SETKNOB(BonusB);
2460 SETKNOB(Poverty);
2461 SETKNOB(SpinAfterFutile);
2462 SETKNOB(UsePause);
2463 SETKNOB(SpinEarly);
2464 SETKNOB(OState);
2465 SETKNOB(MaxSpinners);
2466 SETKNOB(PreSpin);
2467 SETKNOB(ExitPolicy);
2468 SETKNOB(QMode);
2469 SETKNOB(ResetEvent);
2470 SETKNOB(MoveNotifyee);
2471 SETKNOB(FastHSSEC);
2472 #undef SETKNOB
2473
2474 if (Knob_Verbose) {
2475 sanity_checks();
2476 }
2477
2478 if (os::is_MP()) {
2479 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2480 if (Knob_ReportSettings) ::printf("BackOffMask=%X\n", BackOffMask);
2481 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2482 } else {
2483 Knob_SpinLimit = 0;
2484 Knob_SpinBase = 0;
2485 Knob_PreSpin = 0;
2486 Knob_FixedSpin = -1;
2487 }
2488
2489 if (Knob_LogSpins == 0) {
2490 ObjectMonitor::_sync_FailedSpins = NULL;
2491 }
2492
2493 free(knobs);
2494 OrderAccess::fence();
2495 InitDone = 1;
2496 }
2497
2498 void ObjectMonitor::sanity_checks() {
2499 int error_cnt = 0;
2500 int warning_cnt = 0;
2501 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2502
2503 if (verbose) {
2504 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2505 sizeof(ObjectMonitor));
2506 tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT,
2507 sizeof(PaddedEnd<ObjectMonitor>));
2508 }
2509
2510 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2511 if (verbose) {
2512 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2513 }
2514
2515 ObjectMonitor dummy;
2516 u_char *addr_begin = (u_char*)&dummy;
2517 u_char *addr_header = (u_char*)&dummy._header;
2518 u_char *addr_owner = (u_char*)&dummy._owner;
2519
2520 uint offset_header = (uint)(addr_header - addr_begin);
2521 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2522
2523 uint offset_owner = (uint)(addr_owner - addr_begin);
2524 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2525
2526 if ((uint)(addr_header - addr_begin) != 0) {
2527 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2528 error_cnt++;
2529 }
2530
2531 if (cache_line_size != 0) {
2532 // We were able to determine the L1 data cache line size so
2533 // do some cache line specific sanity checks
2534
2535 if ((offset_owner - offset_header) < cache_line_size) {
2536 tty->print_cr("WARNING: the _header and _owner fields are closer "
2537 "than a cache line which permits false sharing.");
2538 warning_cnt++;
2539 }
2540
2541 if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) {
2542 tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a "
2543 "multiple of a cache line which permits false sharing.");
2544 warning_cnt++;
2545 }
2546 }
2547
2548 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2549 &warning_cnt);
2550
2551 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2552 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2553 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2554 }
2555
2556 guarantee(error_cnt == 0,
2557 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2558 }
2559
2560 #ifndef PRODUCT
2561 void ObjectMonitor::verify() {
2562 }
2563
2564 void ObjectMonitor::print() {
2565 }
2566 #endif