1 /*
2 * Copyright (c) 1998, 2017, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/allocation.inline.hpp"
28 #include "memory/resourceArea.hpp"
29 #include "oops/markOop.hpp"
30 #include "oops/oop.inline.hpp"
31 #include "runtime/atomic.hpp"
32 #include "runtime/handles.inline.hpp"
33 #include "runtime/interfaceSupport.hpp"
34 #include "runtime/mutexLocker.hpp"
35 #include "runtime/objectMonitor.hpp"
36 #include "runtime/objectMonitor.inline.hpp"
37 #include "runtime/orderAccess.inline.hpp"
38 #include "runtime/osThread.hpp"
39 #include "runtime/safepointMechanism.inline.hpp"
40 #include "runtime/stubRoutines.hpp"
41 #include "runtime/thread.inline.hpp"
42 #include "services/threadService.hpp"
43 #include "trace/tracing.hpp"
44 #include "trace/traceMacros.hpp"
45 #include "utilities/dtrace.hpp"
46 #include "utilities/macros.hpp"
47 #include "utilities/preserveException.hpp"
48
49 #ifdef DTRACE_ENABLED
50
51 // Only bother with this argument setup if dtrace is available
52 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
53
54
55 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
56 char* bytes = NULL; \
57 int len = 0; \
58 jlong jtid = SharedRuntime::get_java_tid(thread); \
59 Symbol* klassname = ((oop)obj)->klass()->name(); \
60 if (klassname != NULL) { \
61 bytes = (char*)klassname->bytes(); \
62 len = klassname->utf8_length(); \
63 }
64
65 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
66 { \
67 if (DTraceMonitorProbes) { \
68 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
69 HOTSPOT_MONITOR_WAIT(jtid, \
70 (monitor), bytes, len, (millis)); \
71 } \
72 }
73
74 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
75 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
76 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
77 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
78 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
79
80 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
81 { \
82 if (DTraceMonitorProbes) { \
83 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
84 HOTSPOT_MONITOR_##probe(jtid, \
85 (uintptr_t)(monitor), bytes, len); \
86 } \
87 }
88
89 #else // ndef DTRACE_ENABLED
90
91 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
92 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
93
94 #endif // ndef DTRACE_ENABLED
95
96 // Tunables ...
97 // The knob* variables are effectively final. Once set they should
98 // never be modified hence. Consider using __read_mostly with GCC.
99
100 int ObjectMonitor::Knob_ExitRelease = 0;
101 int ObjectMonitor::Knob_Verbose = 0;
102 int ObjectMonitor::Knob_VerifyInUse = 0;
103 int ObjectMonitor::Knob_VerifyMatch = 0;
104 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
105 static int Knob_LogSpins = 0; // enable jvmstat tally for spins
106 static int Knob_HandOff = 0;
107 static int Knob_ReportSettings = 0;
108
109 static int Knob_SpinBase = 0; // Floor AKA SpinMin
110 static int Knob_SpinBackOff = 0; // spin-loop backoff
111 static int Knob_CASPenalty = -1; // Penalty for failed CAS
112 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
113 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
114 static int Knob_SpinEarly = 1;
115 static int Knob_SuccEnabled = 1; // futile wake throttling
116 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
117 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
118 static int Knob_Bonus = 100; // spin success bonus
119 static int Knob_BonusB = 100; // spin success bonus
120 static int Knob_Penalty = 200; // spin failure penalty
121 static int Knob_Poverty = 1000;
122 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
123 static int Knob_FixedSpin = 0;
124 static int Knob_OState = 3; // Spinner checks thread state of _owner
125 static int Knob_UsePause = 1;
126 static int Knob_ExitPolicy = 0;
127 static int Knob_PreSpin = 10; // 20-100 likely better
128 static int Knob_ResetEvent = 0;
129 static int BackOffMask = 0;
130
131 static int Knob_FastHSSEC = 0;
132 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
133 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
134 static volatile int InitDone = 0;
135
136 // -----------------------------------------------------------------------------
137 // Theory of operations -- Monitors lists, thread residency, etc:
138 //
139 // * A thread acquires ownership of a monitor by successfully
140 // CAS()ing the _owner field from null to non-null.
141 //
142 // * Invariant: A thread appears on at most one monitor list --
143 // cxq, EntryList or WaitSet -- at any one time.
144 //
145 // * Contending threads "push" themselves onto the cxq with CAS
146 // and then spin/park.
147 //
148 // * After a contending thread eventually acquires the lock it must
149 // dequeue itself from either the EntryList or the cxq.
150 //
151 // * The exiting thread identifies and unparks an "heir presumptive"
152 // tentative successor thread on the EntryList. Critically, the
153 // exiting thread doesn't unlink the successor thread from the EntryList.
154 // After having been unparked, the wakee will recontend for ownership of
155 // the monitor. The successor (wakee) will either acquire the lock or
156 // re-park itself.
157 //
158 // Succession is provided for by a policy of competitive handoff.
159 // The exiting thread does _not_ grant or pass ownership to the
160 // successor thread. (This is also referred to as "handoff" succession").
161 // Instead the exiting thread releases ownership and possibly wakes
162 // a successor, so the successor can (re)compete for ownership of the lock.
163 // If the EntryList is empty but the cxq is populated the exiting
164 // thread will drain the cxq into the EntryList. It does so by
165 // by detaching the cxq (installing null with CAS) and folding
166 // the threads from the cxq into the EntryList. The EntryList is
167 // doubly linked, while the cxq is singly linked because of the
168 // CAS-based "push" used to enqueue recently arrived threads (RATs).
169 //
170 // * Concurrency invariants:
171 //
172 // -- only the monitor owner may access or mutate the EntryList.
173 // The mutex property of the monitor itself protects the EntryList
174 // from concurrent interference.
175 // -- Only the monitor owner may detach the cxq.
176 //
177 // * The monitor entry list operations avoid locks, but strictly speaking
178 // they're not lock-free. Enter is lock-free, exit is not.
179 // For a description of 'Methods and apparatus providing non-blocking access
180 // to a resource,' see U.S. Pat. No. 7844973.
181 //
182 // * The cxq can have multiple concurrent "pushers" but only one concurrent
183 // detaching thread. This mechanism is immune from the ABA corruption.
184 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
185 //
186 // * Taken together, the cxq and the EntryList constitute or form a
187 // single logical queue of threads stalled trying to acquire the lock.
188 // We use two distinct lists to improve the odds of a constant-time
189 // dequeue operation after acquisition (in the ::enter() epilogue) and
190 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
191 // A key desideratum is to minimize queue & monitor metadata manipulation
192 // that occurs while holding the monitor lock -- that is, we want to
193 // minimize monitor lock holds times. Note that even a small amount of
194 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
195 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
196 // locks and monitor metadata.
197 //
198 // Cxq points to the set of Recently Arrived Threads attempting entry.
199 // Because we push threads onto _cxq with CAS, the RATs must take the form of
200 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
201 // the unlocking thread notices that EntryList is null but _cxq is != null.
202 //
203 // The EntryList is ordered by the prevailing queue discipline and
204 // can be organized in any convenient fashion, such as a doubly-linked list or
205 // a circular doubly-linked list. Critically, we want insert and delete operations
206 // to operate in constant-time. If we need a priority queue then something akin
207 // to Solaris' sleepq would work nicely. Viz.,
208 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
209 // Queue discipline is enforced at ::exit() time, when the unlocking thread
210 // drains the cxq into the EntryList, and orders or reorders the threads on the
211 // EntryList accordingly.
212 //
213 // Barring "lock barging", this mechanism provides fair cyclic ordering,
214 // somewhat similar to an elevator-scan.
215 //
216 // * The monitor synchronization subsystem avoids the use of native
217 // synchronization primitives except for the narrow platform-specific
218 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
219 // the semantics of park-unpark. Put another way, this monitor implementation
220 // depends only on atomic operations and park-unpark. The monitor subsystem
221 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
222 // underlying OS manages the READY<->RUN transitions.
223 //
224 // * Waiting threads reside on the WaitSet list -- wait() puts
225 // the caller onto the WaitSet.
226 //
227 // * notify() or notifyAll() simply transfers threads from the WaitSet to
228 // either the EntryList or cxq. Subsequent exit() operations will
229 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
230 // it's likely the notifyee would simply impale itself on the lock held
231 // by the notifier.
232 //
233 // * An interesting alternative is to encode cxq as (List,LockByte) where
234 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
235 // variable, like _recursions, in the scheme. The threads or Events that form
236 // the list would have to be aligned in 256-byte addresses. A thread would
237 // try to acquire the lock or enqueue itself with CAS, but exiting threads
238 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
239 // Note that is is *not* word-tearing, but it does presume that full-word
240 // CAS operations are coherent with intermix with STB operations. That's true
241 // on most common processors.
242 //
243 // * See also http://blogs.sun.com/dave
244
245
246 void* ObjectMonitor::operator new (size_t size) throw() {
247 return AllocateHeap(size, mtInternal);
248 }
249 void* ObjectMonitor::operator new[] (size_t size) throw() {
250 return operator new (size);
251 }
252 void ObjectMonitor::operator delete(void* p) {
253 FreeHeap(p);
254 }
255 void ObjectMonitor::operator delete[] (void *p) {
256 operator delete(p);
257 }
258
259 // -----------------------------------------------------------------------------
260 // Enter support
261
262 void ObjectMonitor::enter(TRAPS) {
263 // The following code is ordered to check the most common cases first
264 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
265 Thread * const Self = THREAD;
266
267 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
268 if (cur == NULL) {
269 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
270 assert(_recursions == 0, "invariant");
271 assert(_owner == Self, "invariant");
272 return;
273 }
274
275 if (cur == Self) {
276 // TODO-FIXME: check for integer overflow! BUGID 6557169.
277 _recursions++;
278 return;
279 }
280
281 if (Self->is_lock_owned ((address)cur)) {
282 assert(_recursions == 0, "internal state error");
283 _recursions = 1;
284 // Commute owner from a thread-specific on-stack BasicLockObject address to
285 // a full-fledged "Thread *".
286 _owner = Self;
287 return;
288 }
289
290 // We've encountered genuine contention.
291 assert(Self->_Stalled == 0, "invariant");
292 Self->_Stalled = intptr_t(this);
293
294 // Try one round of spinning *before* enqueueing Self
295 // and before going through the awkward and expensive state
296 // transitions. The following spin is strictly optional ...
297 // Note that if we acquire the monitor from an initial spin
298 // we forgo posting JVMTI events and firing DTRACE probes.
299 if (Knob_SpinEarly && TrySpin (Self) > 0) {
300 assert(_owner == Self, "invariant");
301 assert(_recursions == 0, "invariant");
302 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
303 Self->_Stalled = 0;
304 return;
305 }
306
307 assert(_owner != Self, "invariant");
308 assert(_succ != Self, "invariant");
309 assert(Self->is_Java_thread(), "invariant");
310 JavaThread * jt = (JavaThread *) Self;
311 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
312 assert(jt->thread_state() != _thread_blocked, "invariant");
313 assert(this->object() != NULL, "invariant");
314 assert(_count >= 0, "invariant");
315
316 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
317 // Ensure the object-monitor relationship remains stable while there's contention.
318 Atomic::inc(&_count);
319
320 EventJavaMonitorEnter event;
321
322 { // Change java thread status to indicate blocked on monitor enter.
323 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
324
325 Self->set_current_pending_monitor(this);
326
327 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
328 if (JvmtiExport::should_post_monitor_contended_enter()) {
329 JvmtiExport::post_monitor_contended_enter(jt, this);
330
331 // The current thread does not yet own the monitor and does not
332 // yet appear on any queues that would get it made the successor.
333 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
334 // handler cannot accidentally consume an unpark() meant for the
335 // ParkEvent associated with this ObjectMonitor.
336 }
337
338 OSThreadContendState osts(Self->osthread());
339 ThreadBlockInVM tbivm(jt);
340
341 // TODO-FIXME: change the following for(;;) loop to straight-line code.
342 for (;;) {
343 jt->set_suspend_equivalent();
344 // cleared by handle_special_suspend_equivalent_condition()
345 // or java_suspend_self()
346
347 EnterI(THREAD);
348
349 if (!ExitSuspendEquivalent(jt)) break;
350
351 // We have acquired the contended monitor, but while we were
352 // waiting another thread suspended us. We don't want to enter
353 // the monitor while suspended because that would surprise the
354 // thread that suspended us.
355 //
356 _recursions = 0;
357 _succ = NULL;
358 exit(false, Self);
359
360 jt->java_suspend_self();
361 }
362 Self->set_current_pending_monitor(NULL);
363
364 // We cleared the pending monitor info since we've just gotten past
365 // the enter-check-for-suspend dance and we now own the monitor free
366 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
367 // destructor can go to a safepoint at the end of this block. If we
368 // do a thread dump during that safepoint, then this thread will show
369 // as having "-locked" the monitor, but the OS and java.lang.Thread
370 // states will still report that the thread is blocked trying to
371 // acquire it.
372 }
373
374 Atomic::dec(&_count);
375 assert(_count >= 0, "invariant");
376 Self->_Stalled = 0;
377
378 // Must either set _recursions = 0 or ASSERT _recursions == 0.
379 assert(_recursions == 0, "invariant");
380 assert(_owner == Self, "invariant");
381 assert(_succ != Self, "invariant");
382 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
383
384 // The thread -- now the owner -- is back in vm mode.
385 // Report the glorious news via TI,DTrace and jvmstat.
386 // The probe effect is non-trivial. All the reportage occurs
387 // while we hold the monitor, increasing the length of the critical
388 // section. Amdahl's parallel speedup law comes vividly into play.
389 //
390 // Another option might be to aggregate the events (thread local or
391 // per-monitor aggregation) and defer reporting until a more opportune
392 // time -- such as next time some thread encounters contention but has
393 // yet to acquire the lock. While spinning that thread could
394 // spinning we could increment JVMStat counters, etc.
395
396 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
397 if (JvmtiExport::should_post_monitor_contended_entered()) {
398 JvmtiExport::post_monitor_contended_entered(jt, this);
399
400 // The current thread already owns the monitor and is not going to
401 // call park() for the remainder of the monitor enter protocol. So
402 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
403 // event handler consumed an unpark() issued by the thread that
404 // just exited the monitor.
405 }
406
407 if (event.should_commit()) {
408 event.set_monitorClass(((oop)this->object())->klass());
409 event.set_previousOwner((TYPE_THREAD)_previous_owner_tid);
410 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
411 event.commit();
412 }
413
414 OM_PERFDATA_OP(ContendedLockAttempts, inc());
415 }
416
417
418 // Caveat: TryLock() is not necessarily serializing if it returns failure.
419 // Callers must compensate as needed.
420
421 int ObjectMonitor::TryLock(Thread * Self) {
422 void * own = _owner;
423 if (own != NULL) return 0;
424 if (Atomic::cmpxchg(Self, &_owner, (void*)NULL) == NULL) {
425 // Either guarantee _recursions == 0 or set _recursions = 0.
426 assert(_recursions == 0, "invariant");
427 assert(_owner == Self, "invariant");
428 return 1;
429 }
430 // The lock had been free momentarily, but we lost the race to the lock.
431 // Interference -- the CAS failed.
432 // We can either return -1 or retry.
433 // Retry doesn't make as much sense because the lock was just acquired.
434 return -1;
435 }
436
437 #define MAX_RECHECK_INTERVAL 1000
438
439 void ObjectMonitor::EnterI(TRAPS) {
440 Thread * const Self = THREAD;
441 assert(Self->is_Java_thread(), "invariant");
442 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
443
444 // Try the lock - TATAS
445 if (TryLock (Self) > 0) {
446 assert(_succ != Self, "invariant");
447 assert(_owner == Self, "invariant");
448 assert(_Responsible != Self, "invariant");
449 return;
450 }
451
452 DeferredInitialize();
453
454 // We try one round of spinning *before* enqueueing Self.
455 //
456 // If the _owner is ready but OFFPROC we could use a YieldTo()
457 // operation to donate the remainder of this thread's quantum
458 // to the owner. This has subtle but beneficial affinity
459 // effects.
460
461 if (TrySpin (Self) > 0) {
462 assert(_owner == Self, "invariant");
463 assert(_succ != Self, "invariant");
464 assert(_Responsible != Self, "invariant");
465 return;
466 }
467
468 // The Spin failed -- Enqueue and park the thread ...
469 assert(_succ != Self, "invariant");
470 assert(_owner != Self, "invariant");
471 assert(_Responsible != Self, "invariant");
472
473 // Enqueue "Self" on ObjectMonitor's _cxq.
474 //
475 // Node acts as a proxy for Self.
476 // As an aside, if were to ever rewrite the synchronization code mostly
477 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
478 // Java objects. This would avoid awkward lifecycle and liveness issues,
479 // as well as eliminate a subset of ABA issues.
480 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
481
482 ObjectWaiter node(Self);
483 Self->_ParkEvent->reset();
484 node._prev = (ObjectWaiter *) 0xBAD;
485 node.TState = ObjectWaiter::TS_CXQ;
486
487 // Push "Self" onto the front of the _cxq.
488 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
489 // Note that spinning tends to reduce the rate at which threads
490 // enqueue and dequeue on EntryList|cxq.
491 ObjectWaiter * nxt;
492 for (;;) {
493 node._next = nxt = _cxq;
494 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
495
496 // Interference - the CAS failed because _cxq changed. Just retry.
497 // As an optional optimization we retry the lock.
498 if (TryLock (Self) > 0) {
499 assert(_succ != Self, "invariant");
500 assert(_owner == Self, "invariant");
501 assert(_Responsible != Self, "invariant");
502 return;
503 }
504 }
505
506 // Check for cxq|EntryList edge transition to non-null. This indicates
507 // the onset of contention. While contention persists exiting threads
508 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
509 // operations revert to the faster 1-0 mode. This enter operation may interleave
510 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
511 // arrange for one of the contending thread to use a timed park() operations
512 // to detect and recover from the race. (Stranding is form of progress failure
513 // where the monitor is unlocked but all the contending threads remain parked).
514 // That is, at least one of the contended threads will periodically poll _owner.
515 // One of the contending threads will become the designated "Responsible" thread.
516 // The Responsible thread uses a timed park instead of a normal indefinite park
517 // operation -- it periodically wakes and checks for and recovers from potential
518 // strandings admitted by 1-0 exit operations. We need at most one Responsible
519 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
520 // be responsible for a monitor.
521 //
522 // Currently, one of the contended threads takes on the added role of "Responsible".
523 // A viable alternative would be to use a dedicated "stranding checker" thread
524 // that periodically iterated over all the threads (or active monitors) and unparked
525 // successors where there was risk of stranding. This would help eliminate the
526 // timer scalability issues we see on some platforms as we'd only have one thread
527 // -- the checker -- parked on a timer.
528
529 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
530 // Try to assume the role of responsible thread for the monitor.
531 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
532 Atomic::cmpxchg(Self, &_Responsible, (Thread*)NULL);
533 }
534
535 // The lock might have been released while this thread was occupied queueing
536 // itself onto _cxq. To close the race and avoid "stranding" and
537 // progress-liveness failure we must resample-retry _owner before parking.
538 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
539 // In this case the ST-MEMBAR is accomplished with CAS().
540 //
541 // TODO: Defer all thread state transitions until park-time.
542 // Since state transitions are heavy and inefficient we'd like
543 // to defer the state transitions until absolutely necessary,
544 // and in doing so avoid some transitions ...
545
546 TEVENT(Inflated enter - Contention);
547 int nWakeups = 0;
548 int recheckInterval = 1;
549
550 for (;;) {
551
552 if (TryLock(Self) > 0) break;
553 assert(_owner != Self, "invariant");
554
555 if ((SyncFlags & 2) && _Responsible == NULL) {
556 Atomic::cmpxchg(Self, &_Responsible, (Thread*)NULL);
557 }
558
559 // park self
560 if (_Responsible == Self || (SyncFlags & 1)) {
561 TEVENT(Inflated enter - park TIMED);
562 Self->_ParkEvent->park((jlong) recheckInterval);
563 // Increase the recheckInterval, but clamp the value.
564 recheckInterval *= 8;
565 if (recheckInterval > MAX_RECHECK_INTERVAL) {
566 recheckInterval = MAX_RECHECK_INTERVAL;
567 }
568 } else {
569 TEVENT(Inflated enter - park UNTIMED);
570 Self->_ParkEvent->park();
571 }
572
573 if (TryLock(Self) > 0) break;
574
575 // The lock is still contested.
576 // Keep a tally of the # of futile wakeups.
577 // Note that the counter is not protected by a lock or updated by atomics.
578 // That is by design - we trade "lossy" counters which are exposed to
579 // races during updates for a lower probe effect.
580 TEVENT(Inflated enter - Futile wakeup);
581 // This PerfData object can be used in parallel with a safepoint.
582 // See the work around in PerfDataManager::destroy().
583 OM_PERFDATA_OP(FutileWakeups, inc());
584 ++nWakeups;
585
586 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
587 // We can defer clearing _succ until after the spin completes
588 // TrySpin() must tolerate being called with _succ == Self.
589 // Try yet another round of adaptive spinning.
590 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
591
592 // We can find that we were unpark()ed and redesignated _succ while
593 // we were spinning. That's harmless. If we iterate and call park(),
594 // park() will consume the event and return immediately and we'll
595 // just spin again. This pattern can repeat, leaving _succ to simply
596 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
597 // Alternately, we can sample fired() here, and if set, forgo spinning
598 // in the next iteration.
599
600 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
601 Self->_ParkEvent->reset();
602 OrderAccess::fence();
603 }
604 if (_succ == Self) _succ = NULL;
605
606 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
607 OrderAccess::fence();
608 }
609
610 // Egress :
611 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
612 // Normally we'll find Self on the EntryList .
613 // From the perspective of the lock owner (this thread), the
614 // EntryList is stable and cxq is prepend-only.
615 // The head of cxq is volatile but the interior is stable.
616 // In addition, Self.TState is stable.
617
618 assert(_owner == Self, "invariant");
619 assert(object() != NULL, "invariant");
620 // I'd like to write:
621 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
622 // but as we're at a safepoint that's not safe.
623
624 UnlinkAfterAcquire(Self, &node);
625 if (_succ == Self) _succ = NULL;
626
627 assert(_succ != Self, "invariant");
628 if (_Responsible == Self) {
629 _Responsible = NULL;
630 OrderAccess::fence(); // Dekker pivot-point
631
632 // We may leave threads on cxq|EntryList without a designated
633 // "Responsible" thread. This is benign. When this thread subsequently
634 // exits the monitor it can "see" such preexisting "old" threads --
635 // threads that arrived on the cxq|EntryList before the fence, above --
636 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
637 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
638 // non-null and elect a new "Responsible" timer thread.
639 //
640 // This thread executes:
641 // ST Responsible=null; MEMBAR (in enter epilogue - here)
642 // LD cxq|EntryList (in subsequent exit)
643 //
644 // Entering threads in the slow/contended path execute:
645 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
646 // The (ST cxq; MEMBAR) is accomplished with CAS().
647 //
648 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
649 // exit operation from floating above the ST Responsible=null.
650 }
651
652 // We've acquired ownership with CAS().
653 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
654 // But since the CAS() this thread may have also stored into _succ,
655 // EntryList, cxq or Responsible. These meta-data updates must be
656 // visible __before this thread subsequently drops the lock.
657 // Consider what could occur if we didn't enforce this constraint --
658 // STs to monitor meta-data and user-data could reorder with (become
659 // visible after) the ST in exit that drops ownership of the lock.
660 // Some other thread could then acquire the lock, but observe inconsistent
661 // or old monitor meta-data and heap data. That violates the JMM.
662 // To that end, the 1-0 exit() operation must have at least STST|LDST
663 // "release" barrier semantics. Specifically, there must be at least a
664 // STST|LDST barrier in exit() before the ST of null into _owner that drops
665 // the lock. The barrier ensures that changes to monitor meta-data and data
666 // protected by the lock will be visible before we release the lock, and
667 // therefore before some other thread (CPU) has a chance to acquire the lock.
668 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
669 //
670 // Critically, any prior STs to _succ or EntryList must be visible before
671 // the ST of null into _owner in the *subsequent* (following) corresponding
672 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
673 // execute a serializing instruction.
674
675 if (SyncFlags & 8) {
676 OrderAccess::fence();
677 }
678 return;
679 }
680
681 // ReenterI() is a specialized inline form of the latter half of the
682 // contended slow-path from EnterI(). We use ReenterI() only for
683 // monitor reentry in wait().
684 //
685 // In the future we should reconcile EnterI() and ReenterI(), adding
686 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
687 // loop accordingly.
688
689 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
690 assert(Self != NULL, "invariant");
691 assert(SelfNode != NULL, "invariant");
692 assert(SelfNode->_thread == Self, "invariant");
693 assert(_waiters > 0, "invariant");
694 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
695 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
696 JavaThread * jt = (JavaThread *) Self;
697
698 int nWakeups = 0;
699 for (;;) {
700 ObjectWaiter::TStates v = SelfNode->TState;
701 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
702 assert(_owner != Self, "invariant");
703
704 if (TryLock(Self) > 0) break;
705 if (TrySpin(Self) > 0) break;
706
707 TEVENT(Wait Reentry - parking);
708
709 // State transition wrappers around park() ...
710 // ReenterI() wisely defers state transitions until
711 // it's clear we must park the thread.
712 {
713 OSThreadContendState osts(Self->osthread());
714 ThreadBlockInVM tbivm(jt);
715
716 // cleared by handle_special_suspend_equivalent_condition()
717 // or java_suspend_self()
718 jt->set_suspend_equivalent();
719 if (SyncFlags & 1) {
720 Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL);
721 } else {
722 Self->_ParkEvent->park();
723 }
724
725 // were we externally suspended while we were waiting?
726 for (;;) {
727 if (!ExitSuspendEquivalent(jt)) break;
728 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
729 jt->java_suspend_self();
730 jt->set_suspend_equivalent();
731 }
732 }
733
734 // Try again, but just so we distinguish between futile wakeups and
735 // successful wakeups. The following test isn't algorithmically
736 // necessary, but it helps us maintain sensible statistics.
737 if (TryLock(Self) > 0) break;
738
739 // The lock is still contested.
740 // Keep a tally of the # of futile wakeups.
741 // Note that the counter is not protected by a lock or updated by atomics.
742 // That is by design - we trade "lossy" counters which are exposed to
743 // races during updates for a lower probe effect.
744 TEVENT(Wait Reentry - futile wakeup);
745 ++nWakeups;
746
747 // Assuming this is not a spurious wakeup we'll normally
748 // find that _succ == Self.
749 if (_succ == Self) _succ = NULL;
750
751 // Invariant: after clearing _succ a contending thread
752 // *must* retry _owner before parking.
753 OrderAccess::fence();
754
755 // This PerfData object can be used in parallel with a safepoint.
756 // See the work around in PerfDataManager::destroy().
757 OM_PERFDATA_OP(FutileWakeups, inc());
758 }
759
760 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
761 // Normally we'll find Self on the EntryList.
762 // Unlinking from the EntryList is constant-time and atomic-free.
763 // From the perspective of the lock owner (this thread), the
764 // EntryList is stable and cxq is prepend-only.
765 // The head of cxq is volatile but the interior is stable.
766 // In addition, Self.TState is stable.
767
768 assert(_owner == Self, "invariant");
769 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
770 UnlinkAfterAcquire(Self, SelfNode);
771 if (_succ == Self) _succ = NULL;
772 assert(_succ != Self, "invariant");
773 SelfNode->TState = ObjectWaiter::TS_RUN;
774 OrderAccess::fence(); // see comments at the end of EnterI()
775 }
776
777 // By convention we unlink a contending thread from EntryList|cxq immediately
778 // after the thread acquires the lock in ::enter(). Equally, we could defer
779 // unlinking the thread until ::exit()-time.
780
781 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
782 assert(_owner == Self, "invariant");
783 assert(SelfNode->_thread == Self, "invariant");
784
785 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
786 // Normal case: remove Self from the DLL EntryList .
787 // This is a constant-time operation.
788 ObjectWaiter * nxt = SelfNode->_next;
789 ObjectWaiter * prv = SelfNode->_prev;
790 if (nxt != NULL) nxt->_prev = prv;
791 if (prv != NULL) prv->_next = nxt;
792 if (SelfNode == _EntryList) _EntryList = nxt;
793 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
794 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
795 TEVENT(Unlink from EntryList);
796 } else {
797 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
798 // Inopportune interleaving -- Self is still on the cxq.
799 // This usually means the enqueue of self raced an exiting thread.
800 // Normally we'll find Self near the front of the cxq, so
801 // dequeueing is typically fast. If needbe we can accelerate
802 // this with some MCS/CHL-like bidirectional list hints and advisory
803 // back-links so dequeueing from the interior will normally operate
804 // in constant-time.
805 // Dequeue Self from either the head (with CAS) or from the interior
806 // with a linear-time scan and normal non-atomic memory operations.
807 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
808 // and then unlink Self from EntryList. We have to drain eventually,
809 // so it might as well be now.
810
811 ObjectWaiter * v = _cxq;
812 assert(v != NULL, "invariant");
813 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
814 // The CAS above can fail from interference IFF a "RAT" arrived.
815 // In that case Self must be in the interior and can no longer be
816 // at the head of cxq.
817 if (v == SelfNode) {
818 assert(_cxq != v, "invariant");
819 v = _cxq; // CAS above failed - start scan at head of list
820 }
821 ObjectWaiter * p;
822 ObjectWaiter * q = NULL;
823 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
824 q = p;
825 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
826 }
827 assert(v != SelfNode, "invariant");
828 assert(p == SelfNode, "Node not found on cxq");
829 assert(p != _cxq, "invariant");
830 assert(q != NULL, "invariant");
831 assert(q->_next == p, "invariant");
832 q->_next = p->_next;
833 }
834 TEVENT(Unlink from cxq);
835 }
836
837 #ifdef ASSERT
838 // Diagnostic hygiene ...
839 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
840 SelfNode->_next = (ObjectWaiter *) 0xBAD;
841 SelfNode->TState = ObjectWaiter::TS_RUN;
842 #endif
843 }
844
845 // -----------------------------------------------------------------------------
846 // Exit support
847 //
848 // exit()
849 // ~~~~~~
850 // Note that the collector can't reclaim the objectMonitor or deflate
851 // the object out from underneath the thread calling ::exit() as the
852 // thread calling ::exit() never transitions to a stable state.
853 // This inhibits GC, which in turn inhibits asynchronous (and
854 // inopportune) reclamation of "this".
855 //
856 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
857 // There's one exception to the claim above, however. EnterI() can call
858 // exit() to drop a lock if the acquirer has been externally suspended.
859 // In that case exit() is called with _thread_state as _thread_blocked,
860 // but the monitor's _count field is > 0, which inhibits reclamation.
861 //
862 // 1-0 exit
863 // ~~~~~~~~
864 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
865 // the fast-path operators have been optimized so the common ::exit()
866 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
867 // The code emitted by fast_unlock() elides the usual MEMBAR. This
868 // greatly improves latency -- MEMBAR and CAS having considerable local
869 // latency on modern processors -- but at the cost of "stranding". Absent the
870 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
871 // ::enter() path, resulting in the entering thread being stranding
872 // and a progress-liveness failure. Stranding is extremely rare.
873 // We use timers (timed park operations) & periodic polling to detect
874 // and recover from stranding. Potentially stranded threads periodically
875 // wake up and poll the lock. See the usage of the _Responsible variable.
876 //
877 // The CAS() in enter provides for safety and exclusion, while the CAS or
878 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
879 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
880 // We detect and recover from stranding with timers.
881 //
882 // If a thread transiently strands it'll park until (a) another
883 // thread acquires the lock and then drops the lock, at which time the
884 // exiting thread will notice and unpark the stranded thread, or, (b)
885 // the timer expires. If the lock is high traffic then the stranding latency
886 // will be low due to (a). If the lock is low traffic then the odds of
887 // stranding are lower, although the worst-case stranding latency
888 // is longer. Critically, we don't want to put excessive load in the
889 // platform's timer subsystem. We want to minimize both the timer injection
890 // rate (timers created/sec) as well as the number of timers active at
891 // any one time. (more precisely, we want to minimize timer-seconds, which is
892 // the integral of the # of active timers at any instant over time).
893 // Both impinge on OS scalability. Given that, at most one thread parked on
894 // a monitor will use a timer.
895 //
896 // There is also the risk of a futile wake-up. If we drop the lock
897 // another thread can reacquire the lock immediately, and we can
898 // then wake a thread unnecessarily. This is benign, and we've
899 // structured the code so the windows are short and the frequency
900 // of such futile wakups is low.
901
902 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
903 Thread * const Self = THREAD;
904 if (THREAD != _owner) {
905 if (THREAD->is_lock_owned((address) _owner)) {
906 // Transmute _owner from a BasicLock pointer to a Thread address.
907 // We don't need to hold _mutex for this transition.
908 // Non-null to Non-null is safe as long as all readers can
909 // tolerate either flavor.
910 assert(_recursions == 0, "invariant");
911 _owner = THREAD;
912 _recursions = 0;
913 } else {
914 // Apparent unbalanced locking ...
915 // Naively we'd like to throw IllegalMonitorStateException.
916 // As a practical matter we can neither allocate nor throw an
917 // exception as ::exit() can be called from leaf routines.
918 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
919 // Upon deeper reflection, however, in a properly run JVM the only
920 // way we should encounter this situation is in the presence of
921 // unbalanced JNI locking. TODO: CheckJNICalls.
922 // See also: CR4414101
923 TEVENT(Exit - Throw IMSX);
924 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
925 return;
926 }
927 }
928
929 if (_recursions != 0) {
930 _recursions--; // this is simple recursive enter
931 TEVENT(Inflated exit - recursive);
932 return;
933 }
934
935 // Invariant: after setting Responsible=null an thread must execute
936 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
937 if ((SyncFlags & 4) == 0) {
938 _Responsible = NULL;
939 }
940
941 #if INCLUDE_TRACE
942 // get the owner's thread id for the MonitorEnter event
943 // if it is enabled and the thread isn't suspended
944 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
945 _previous_owner_tid = THREAD_TRACE_ID(Self);
946 }
947 #endif
948
949 for (;;) {
950 assert(THREAD == _owner, "invariant");
951
952 if (Knob_ExitPolicy == 0) {
953 // release semantics: prior loads and stores from within the critical section
954 // must not float (reorder) past the following store that drops the lock.
955 // On SPARC that requires MEMBAR #loadstore|#storestore.
956 // But of course in TSO #loadstore|#storestore is not required.
957 // I'd like to write one of the following:
958 // A. OrderAccess::release() ; _owner = NULL
959 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
960 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
961 // store into a _dummy variable. That store is not needed, but can result
962 // in massive wasteful coherency traffic on classic SMP systems.
963 // Instead, I use release_store(), which is implemented as just a simple
964 // ST on x64, x86 and SPARC.
965 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
966 OrderAccess::storeload(); // See if we need to wake a successor
967 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
968 TEVENT(Inflated exit - simple egress);
969 return;
970 }
971 TEVENT(Inflated exit - complex egress);
972 // Other threads are blocked trying to acquire the lock.
973
974 // Normally the exiting thread is responsible for ensuring succession,
975 // but if other successors are ready or other entering threads are spinning
976 // then this thread can simply store NULL into _owner and exit without
977 // waking a successor. The existence of spinners or ready successors
978 // guarantees proper succession (liveness). Responsibility passes to the
979 // ready or running successors. The exiting thread delegates the duty.
980 // More precisely, if a successor already exists this thread is absolved
981 // of the responsibility of waking (unparking) one.
982 //
983 // The _succ variable is critical to reducing futile wakeup frequency.
984 // _succ identifies the "heir presumptive" thread that has been made
985 // ready (unparked) but that has not yet run. We need only one such
986 // successor thread to guarantee progress.
987 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
988 // section 3.3 "Futile Wakeup Throttling" for details.
989 //
990 // Note that spinners in Enter() also set _succ non-null.
991 // In the current implementation spinners opportunistically set
992 // _succ so that exiting threads might avoid waking a successor.
993 // Another less appealing alternative would be for the exiting thread
994 // to drop the lock and then spin briefly to see if a spinner managed
995 // to acquire the lock. If so, the exiting thread could exit
996 // immediately without waking a successor, otherwise the exiting
997 // thread would need to dequeue and wake a successor.
998 // (Note that we'd need to make the post-drop spin short, but no
999 // shorter than the worst-case round-trip cache-line migration time.
1000 // The dropped lock needs to become visible to the spinner, and then
1001 // the acquisition of the lock by the spinner must become visible to
1002 // the exiting thread).
1003
1004 // It appears that an heir-presumptive (successor) must be made ready.
1005 // Only the current lock owner can manipulate the EntryList or
1006 // drain _cxq, so we need to reacquire the lock. If we fail
1007 // to reacquire the lock the responsibility for ensuring succession
1008 // falls to the new owner.
1009 //
1010 if (Atomic::cmpxchg(THREAD, &_owner, (void*)NULL) != NULL) {
1011 return;
1012 }
1013 TEVENT(Exit - Reacquired);
1014 } else {
1015 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1016 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
1017 OrderAccess::storeload();
1018 // Ratify the previously observed values.
1019 if (_cxq == NULL || _succ != NULL) {
1020 TEVENT(Inflated exit - simple egress);
1021 return;
1022 }
1023
1024 // inopportune interleaving -- the exiting thread (this thread)
1025 // in the fast-exit path raced an entering thread in the slow-enter
1026 // path.
1027 // We have two choices:
1028 // A. Try to reacquire the lock.
1029 // If the CAS() fails return immediately, otherwise
1030 // we either restart/rerun the exit operation, or simply
1031 // fall-through into the code below which wakes a successor.
1032 // B. If the elements forming the EntryList|cxq are TSM
1033 // we could simply unpark() the lead thread and return
1034 // without having set _succ.
1035 if (Atomic::cmpxchg(THREAD, &_owner, (void*)NULL) != NULL) {
1036 TEVENT(Inflated exit - reacquired succeeded);
1037 return;
1038 }
1039 TEVENT(Inflated exit - reacquired failed);
1040 } else {
1041 TEVENT(Inflated exit - complex egress);
1042 }
1043 }
1044
1045 guarantee(_owner == THREAD, "invariant");
1046
1047 ObjectWaiter * w = NULL;
1048 int QMode = Knob_QMode;
1049
1050 if (QMode == 2 && _cxq != NULL) {
1051 // QMode == 2 : cxq has precedence over EntryList.
1052 // Try to directly wake a successor from the cxq.
1053 // If successful, the successor will need to unlink itself from cxq.
1054 w = _cxq;
1055 assert(w != NULL, "invariant");
1056 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1057 ExitEpilog(Self, w);
1058 return;
1059 }
1060
1061 if (QMode == 3 && _cxq != NULL) {
1062 // Aggressively drain cxq into EntryList at the first opportunity.
1063 // This policy ensure that recently-run threads live at the head of EntryList.
1064 // Drain _cxq into EntryList - bulk transfer.
1065 // First, detach _cxq.
1066 // The following loop is tantamount to: w = swap(&cxq, NULL)
1067 w = _cxq;
1068 for (;;) {
1069 assert(w != NULL, "Invariant");
1070 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1071 if (u == w) break;
1072 w = u;
1073 }
1074 assert(w != NULL, "invariant");
1075
1076 ObjectWaiter * q = NULL;
1077 ObjectWaiter * p;
1078 for (p = w; p != NULL; p = p->_next) {
1079 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1080 p->TState = ObjectWaiter::TS_ENTER;
1081 p->_prev = q;
1082 q = p;
1083 }
1084
1085 // Append the RATs to the EntryList
1086 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1087 ObjectWaiter * Tail;
1088 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1089 Tail = Tail->_next)
1090 /* empty */;
1091 if (Tail == NULL) {
1092 _EntryList = w;
1093 } else {
1094 Tail->_next = w;
1095 w->_prev = Tail;
1096 }
1097
1098 // Fall thru into code that tries to wake a successor from EntryList
1099 }
1100
1101 if (QMode == 4 && _cxq != NULL) {
1102 // Aggressively drain cxq into EntryList at the first opportunity.
1103 // This policy ensure that recently-run threads live at the head of EntryList.
1104
1105 // Drain _cxq into EntryList - bulk transfer.
1106 // First, detach _cxq.
1107 // The following loop is tantamount to: w = swap(&cxq, NULL)
1108 w = _cxq;
1109 for (;;) {
1110 assert(w != NULL, "Invariant");
1111 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1112 if (u == w) break;
1113 w = u;
1114 }
1115 assert(w != NULL, "invariant");
1116
1117 ObjectWaiter * q = NULL;
1118 ObjectWaiter * p;
1119 for (p = w; p != NULL; p = p->_next) {
1120 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1121 p->TState = ObjectWaiter::TS_ENTER;
1122 p->_prev = q;
1123 q = p;
1124 }
1125
1126 // Prepend the RATs to the EntryList
1127 if (_EntryList != NULL) {
1128 q->_next = _EntryList;
1129 _EntryList->_prev = q;
1130 }
1131 _EntryList = w;
1132
1133 // Fall thru into code that tries to wake a successor from EntryList
1134 }
1135
1136 w = _EntryList;
1137 if (w != NULL) {
1138 // I'd like to write: guarantee (w->_thread != Self).
1139 // But in practice an exiting thread may find itself on the EntryList.
1140 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1141 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1142 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1143 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1144 // release the lock "O". T2 resumes immediately after the ST of null into
1145 // _owner, above. T2 notices that the EntryList is populated, so it
1146 // reacquires the lock and then finds itself on the EntryList.
1147 // Given all that, we have to tolerate the circumstance where "w" is
1148 // associated with Self.
1149 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1150 ExitEpilog(Self, w);
1151 return;
1152 }
1153
1154 // If we find that both _cxq and EntryList are null then just
1155 // re-run the exit protocol from the top.
1156 w = _cxq;
1157 if (w == NULL) continue;
1158
1159 // Drain _cxq into EntryList - bulk transfer.
1160 // First, detach _cxq.
1161 // The following loop is tantamount to: w = swap(&cxq, NULL)
1162 for (;;) {
1163 assert(w != NULL, "Invariant");
1164 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1165 if (u == w) break;
1166 w = u;
1167 }
1168 TEVENT(Inflated exit - drain cxq into EntryList);
1169
1170 assert(w != NULL, "invariant");
1171 assert(_EntryList == NULL, "invariant");
1172
1173 // Convert the LIFO SLL anchored by _cxq into a DLL.
1174 // The list reorganization step operates in O(LENGTH(w)) time.
1175 // It's critical that this step operate quickly as
1176 // "Self" still holds the outer-lock, restricting parallelism
1177 // and effectively lengthening the critical section.
1178 // Invariant: s chases t chases u.
1179 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1180 // we have faster access to the tail.
1181
1182 if (QMode == 1) {
1183 // QMode == 1 : drain cxq to EntryList, reversing order
1184 // We also reverse the order of the list.
1185 ObjectWaiter * s = NULL;
1186 ObjectWaiter * t = w;
1187 ObjectWaiter * u = NULL;
1188 while (t != NULL) {
1189 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1190 t->TState = ObjectWaiter::TS_ENTER;
1191 u = t->_next;
1192 t->_prev = u;
1193 t->_next = s;
1194 s = t;
1195 t = u;
1196 }
1197 _EntryList = s;
1198 assert(s != NULL, "invariant");
1199 } else {
1200 // QMode == 0 or QMode == 2
1201 _EntryList = w;
1202 ObjectWaiter * q = NULL;
1203 ObjectWaiter * p;
1204 for (p = w; p != NULL; p = p->_next) {
1205 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1206 p->TState = ObjectWaiter::TS_ENTER;
1207 p->_prev = q;
1208 q = p;
1209 }
1210 }
1211
1212 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1213 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1214
1215 // See if we can abdicate to a spinner instead of waking a thread.
1216 // A primary goal of the implementation is to reduce the
1217 // context-switch rate.
1218 if (_succ != NULL) continue;
1219
1220 w = _EntryList;
1221 if (w != NULL) {
1222 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1223 ExitEpilog(Self, w);
1224 return;
1225 }
1226 }
1227 }
1228
1229 // ExitSuspendEquivalent:
1230 // A faster alternate to handle_special_suspend_equivalent_condition()
1231 //
1232 // handle_special_suspend_equivalent_condition() unconditionally
1233 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1234 // operations have high latency. Note that in ::enter() we call HSSEC
1235 // while holding the monitor, so we effectively lengthen the critical sections.
1236 //
1237 // There are a number of possible solutions:
1238 //
1239 // A. To ameliorate the problem we might also defer state transitions
1240 // to as late as possible -- just prior to parking.
1241 // Given that, we'd call HSSEC after having returned from park(),
1242 // but before attempting to acquire the monitor. This is only a
1243 // partial solution. It avoids calling HSSEC while holding the
1244 // monitor (good), but it still increases successor reacquisition latency --
1245 // the interval between unparking a successor and the time the successor
1246 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1247 // If we use this technique we can also avoid EnterI()-exit() loop
1248 // in ::enter() where we iteratively drop the lock and then attempt
1249 // to reacquire it after suspending.
1250 //
1251 // B. In the future we might fold all the suspend bits into a
1252 // composite per-thread suspend flag and then update it with CAS().
1253 // Alternately, a Dekker-like mechanism with multiple variables
1254 // would suffice:
1255 // ST Self->_suspend_equivalent = false
1256 // MEMBAR
1257 // LD Self_>_suspend_flags
1258 //
1259 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1260 // with a more efficient implementation, the need to use "FastHSSEC" has
1261 // decreased. - Dave
1262
1263
1264 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1265 const int Mode = Knob_FastHSSEC;
1266 if (Mode && !jSelf->is_external_suspend()) {
1267 assert(jSelf->is_suspend_equivalent(), "invariant");
1268 jSelf->clear_suspend_equivalent();
1269 if (2 == Mode) OrderAccess::storeload();
1270 if (!jSelf->is_external_suspend()) return false;
1271 // We raced a suspension -- fall thru into the slow path
1272 TEVENT(ExitSuspendEquivalent - raced);
1273 jSelf->set_suspend_equivalent();
1274 }
1275 return jSelf->handle_special_suspend_equivalent_condition();
1276 }
1277
1278
1279 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1280 assert(_owner == Self, "invariant");
1281
1282 // Exit protocol:
1283 // 1. ST _succ = wakee
1284 // 2. membar #loadstore|#storestore;
1285 // 2. ST _owner = NULL
1286 // 3. unpark(wakee)
1287
1288 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1289 ParkEvent * Trigger = Wakee->_event;
1290
1291 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1292 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1293 // out-of-scope (non-extant).
1294 Wakee = NULL;
1295
1296 // Drop the lock
1297 OrderAccess::release_store(&_owner, (void*)NULL);
1298 OrderAccess::fence(); // ST _owner vs LD in unpark()
1299
1300 if (SafepointMechanism::poll(Self)) {
1301 TEVENT(unpark before SAFEPOINT);
1302 }
1303
1304 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1305 Trigger->unpark();
1306
1307 // Maintain stats and report events to JVMTI
1308 OM_PERFDATA_OP(Parks, inc());
1309 }
1310
1311
1312 // -----------------------------------------------------------------------------
1313 // Class Loader deadlock handling.
1314 //
1315 // complete_exit exits a lock returning recursion count
1316 // complete_exit/reenter operate as a wait without waiting
1317 // complete_exit requires an inflated monitor
1318 // The _owner field is not always the Thread addr even with an
1319 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1320 // thread due to contention.
1321 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1322 Thread * const Self = THREAD;
1323 assert(Self->is_Java_thread(), "Must be Java thread!");
1324 JavaThread *jt = (JavaThread *)THREAD;
1325
1326 DeferredInitialize();
1327
1328 if (THREAD != _owner) {
1329 if (THREAD->is_lock_owned ((address)_owner)) {
1330 assert(_recursions == 0, "internal state error");
1331 _owner = THREAD; // Convert from basiclock addr to Thread addr
1332 _recursions = 0;
1333 }
1334 }
1335
1336 guarantee(Self == _owner, "complete_exit not owner");
1337 intptr_t save = _recursions; // record the old recursion count
1338 _recursions = 0; // set the recursion level to be 0
1339 exit(true, Self); // exit the monitor
1340 guarantee(_owner != Self, "invariant");
1341 return save;
1342 }
1343
1344 // reenter() enters a lock and sets recursion count
1345 // complete_exit/reenter operate as a wait without waiting
1346 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1347 Thread * const Self = THREAD;
1348 assert(Self->is_Java_thread(), "Must be Java thread!");
1349 JavaThread *jt = (JavaThread *)THREAD;
1350
1351 guarantee(_owner != Self, "reenter already owner");
1352 enter(THREAD); // enter the monitor
1353 guarantee(_recursions == 0, "reenter recursion");
1354 _recursions = recursions;
1355 return;
1356 }
1357
1358
1359 // -----------------------------------------------------------------------------
1360 // A macro is used below because there may already be a pending
1361 // exception which should not abort the execution of the routines
1362 // which use this (which is why we don't put this into check_slow and
1363 // call it with a CHECK argument).
1364
1365 #define CHECK_OWNER() \
1366 do { \
1367 if (THREAD != _owner) { \
1368 if (THREAD->is_lock_owned((address) _owner)) { \
1369 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1370 _recursions = 0; \
1371 } else { \
1372 TEVENT(Throw IMSX); \
1373 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1374 } \
1375 } \
1376 } while (false)
1377
1378 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1379 // TODO-FIXME: remove check_slow() -- it's likely dead.
1380
1381 void ObjectMonitor::check_slow(TRAPS) {
1382 TEVENT(check_slow - throw IMSX);
1383 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1384 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1385 }
1386
1387 static int Adjust(volatile int * adr, int dx) {
1388 int v;
1389 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1390 return v;
1391 }
1392
1393 // helper method for posting a monitor wait event
1394 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1395 jlong notifier_tid,
1396 jlong timeout,
1397 bool timedout) {
1398 assert(event != NULL, "invariant");
1399 event->set_monitorClass(((oop)this->object())->klass());
1400 event->set_timeout(timeout);
1401 event->set_address((TYPE_ADDRESS)this->object_addr());
1402 event->set_notifier(notifier_tid);
1403 event->set_timedOut(timedout);
1404 event->commit();
1405 }
1406
1407 // -----------------------------------------------------------------------------
1408 // Wait/Notify/NotifyAll
1409 //
1410 // Note: a subset of changes to ObjectMonitor::wait()
1411 // will need to be replicated in complete_exit
1412 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1413 Thread * const Self = THREAD;
1414 assert(Self->is_Java_thread(), "Must be Java thread!");
1415 JavaThread *jt = (JavaThread *)THREAD;
1416
1417 DeferredInitialize();
1418
1419 // Throw IMSX or IEX.
1420 CHECK_OWNER();
1421
1422 EventJavaMonitorWait event;
1423
1424 // check for a pending interrupt
1425 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1426 // post monitor waited event. Note that this is past-tense, we are done waiting.
1427 if (JvmtiExport::should_post_monitor_waited()) {
1428 // Note: 'false' parameter is passed here because the
1429 // wait was not timed out due to thread interrupt.
1430 JvmtiExport::post_monitor_waited(jt, this, false);
1431
1432 // In this short circuit of the monitor wait protocol, the
1433 // current thread never drops ownership of the monitor and
1434 // never gets added to the wait queue so the current thread
1435 // cannot be made the successor. This means that the
1436 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1437 // consume an unpark() meant for the ParkEvent associated with
1438 // this ObjectMonitor.
1439 }
1440 if (event.should_commit()) {
1441 post_monitor_wait_event(&event, 0, millis, false);
1442 }
1443 TEVENT(Wait - Throw IEX);
1444 THROW(vmSymbols::java_lang_InterruptedException());
1445 return;
1446 }
1447
1448 TEVENT(Wait);
1449
1450 assert(Self->_Stalled == 0, "invariant");
1451 Self->_Stalled = intptr_t(this);
1452 jt->set_current_waiting_monitor(this);
1453
1454 // create a node to be put into the queue
1455 // Critically, after we reset() the event but prior to park(), we must check
1456 // for a pending interrupt.
1457 ObjectWaiter node(Self);
1458 node.TState = ObjectWaiter::TS_WAIT;
1459 Self->_ParkEvent->reset();
1460 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1461
1462 // Enter the waiting queue, which is a circular doubly linked list in this case
1463 // but it could be a priority queue or any data structure.
1464 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1465 // by the the owner of the monitor *except* in the case where park()
1466 // returns because of a timeout of interrupt. Contention is exceptionally rare
1467 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1468
1469 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1470 AddWaiter(&node);
1471 Thread::SpinRelease(&_WaitSetLock);
1472
1473 if ((SyncFlags & 4) == 0) {
1474 _Responsible = NULL;
1475 }
1476 intptr_t save = _recursions; // record the old recursion count
1477 _waiters++; // increment the number of waiters
1478 _recursions = 0; // set the recursion level to be 1
1479 exit(true, Self); // exit the monitor
1480 guarantee(_owner != Self, "invariant");
1481
1482 // The thread is on the WaitSet list - now park() it.
1483 // On MP systems it's conceivable that a brief spin before we park
1484 // could be profitable.
1485 //
1486 // TODO-FIXME: change the following logic to a loop of the form
1487 // while (!timeout && !interrupted && _notified == 0) park()
1488
1489 int ret = OS_OK;
1490 int WasNotified = 0;
1491 { // State transition wrappers
1492 OSThread* osthread = Self->osthread();
1493 OSThreadWaitState osts(osthread, true);
1494 {
1495 ThreadBlockInVM tbivm(jt);
1496 // Thread is in thread_blocked state and oop access is unsafe.
1497 jt->set_suspend_equivalent();
1498
1499 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1500 // Intentionally empty
1501 } else if (node._notified == 0) {
1502 if (millis <= 0) {
1503 Self->_ParkEvent->park();
1504 } else {
1505 ret = Self->_ParkEvent->park(millis);
1506 }
1507 }
1508
1509 // were we externally suspended while we were waiting?
1510 if (ExitSuspendEquivalent (jt)) {
1511 // TODO-FIXME: add -- if succ == Self then succ = null.
1512 jt->java_suspend_self();
1513 }
1514
1515 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1516
1517 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1518 // from the WaitSet to the EntryList.
1519 // See if we need to remove Node from the WaitSet.
1520 // We use double-checked locking to avoid grabbing _WaitSetLock
1521 // if the thread is not on the wait queue.
1522 //
1523 // Note that we don't need a fence before the fetch of TState.
1524 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1525 // written by the is thread. (perhaps the fetch might even be satisfied
1526 // by a look-aside into the processor's own store buffer, although given
1527 // the length of the code path between the prior ST and this load that's
1528 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1529 // then we'll acquire the lock and then re-fetch a fresh TState value.
1530 // That is, we fail toward safety.
1531
1532 if (node.TState == ObjectWaiter::TS_WAIT) {
1533 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1534 if (node.TState == ObjectWaiter::TS_WAIT) {
1535 DequeueSpecificWaiter(&node); // unlink from WaitSet
1536 assert(node._notified == 0, "invariant");
1537 node.TState = ObjectWaiter::TS_RUN;
1538 }
1539 Thread::SpinRelease(&_WaitSetLock);
1540 }
1541
1542 // The thread is now either on off-list (TS_RUN),
1543 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1544 // The Node's TState variable is stable from the perspective of this thread.
1545 // No other threads will asynchronously modify TState.
1546 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1547 OrderAccess::loadload();
1548 if (_succ == Self) _succ = NULL;
1549 WasNotified = node._notified;
1550
1551 // Reentry phase -- reacquire the monitor.
1552 // re-enter contended monitor after object.wait().
1553 // retain OBJECT_WAIT state until re-enter successfully completes
1554 // Thread state is thread_in_vm and oop access is again safe,
1555 // although the raw address of the object may have changed.
1556 // (Don't cache naked oops over safepoints, of course).
1557
1558 // post monitor waited event. Note that this is past-tense, we are done waiting.
1559 if (JvmtiExport::should_post_monitor_waited()) {
1560 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1561
1562 if (node._notified != 0 && _succ == Self) {
1563 // In this part of the monitor wait-notify-reenter protocol it
1564 // is possible (and normal) for another thread to do a fastpath
1565 // monitor enter-exit while this thread is still trying to get
1566 // to the reenter portion of the protocol.
1567 //
1568 // The ObjectMonitor was notified and the current thread is
1569 // the successor which also means that an unpark() has already
1570 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1571 // consume the unpark() that was done when the successor was
1572 // set because the same ParkEvent is shared between Java
1573 // monitors and JVM/TI RawMonitors (for now).
1574 //
1575 // We redo the unpark() to ensure forward progress, i.e., we
1576 // don't want all pending threads hanging (parked) with none
1577 // entering the unlocked monitor.
1578 node._event->unpark();
1579 }
1580 }
1581
1582 if (event.should_commit()) {
1583 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1584 }
1585
1586 OrderAccess::fence();
1587
1588 assert(Self->_Stalled != 0, "invariant");
1589 Self->_Stalled = 0;
1590
1591 assert(_owner != Self, "invariant");
1592 ObjectWaiter::TStates v = node.TState;
1593 if (v == ObjectWaiter::TS_RUN) {
1594 enter(Self);
1595 } else {
1596 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1597 ReenterI(Self, &node);
1598 node.wait_reenter_end(this);
1599 }
1600
1601 // Self has reacquired the lock.
1602 // Lifecycle - the node representing Self must not appear on any queues.
1603 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1604 // want residual elements associated with this thread left on any lists.
1605 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1606 assert(_owner == Self, "invariant");
1607 assert(_succ != Self, "invariant");
1608 } // OSThreadWaitState()
1609
1610 jt->set_current_waiting_monitor(NULL);
1611
1612 guarantee(_recursions == 0, "invariant");
1613 _recursions = save; // restore the old recursion count
1614 _waiters--; // decrement the number of waiters
1615
1616 // Verify a few postconditions
1617 assert(_owner == Self, "invariant");
1618 assert(_succ != Self, "invariant");
1619 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1620
1621 if (SyncFlags & 32) {
1622 OrderAccess::fence();
1623 }
1624
1625 // check if the notification happened
1626 if (!WasNotified) {
1627 // no, it could be timeout or Thread.interrupt() or both
1628 // check for interrupt event, otherwise it is timeout
1629 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1630 TEVENT(Wait - throw IEX from epilog);
1631 THROW(vmSymbols::java_lang_InterruptedException());
1632 }
1633 }
1634
1635 // NOTE: Spurious wake up will be consider as timeout.
1636 // Monitor notify has precedence over thread interrupt.
1637 }
1638
1639
1640 // Consider:
1641 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1642 // then instead of transferring a thread from the WaitSet to the EntryList
1643 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1644
1645 void ObjectMonitor::INotify(Thread * Self) {
1646 const int policy = Knob_MoveNotifyee;
1647
1648 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1649 ObjectWaiter * iterator = DequeueWaiter();
1650 if (iterator != NULL) {
1651 TEVENT(Notify1 - Transfer);
1652 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1653 guarantee(iterator->_notified == 0, "invariant");
1654 // Disposition - what might we do with iterator ?
1655 // a. add it directly to the EntryList - either tail (policy == 1)
1656 // or head (policy == 0).
1657 // b. push it onto the front of the _cxq (policy == 2).
1658 // For now we use (b).
1659 if (policy != 4) {
1660 iterator->TState = ObjectWaiter::TS_ENTER;
1661 }
1662 iterator->_notified = 1;
1663 iterator->_notifier_tid = THREAD_TRACE_ID(Self);
1664
1665 ObjectWaiter * list = _EntryList;
1666 if (list != NULL) {
1667 assert(list->_prev == NULL, "invariant");
1668 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1669 assert(list != iterator, "invariant");
1670 }
1671
1672 if (policy == 0) { // prepend to EntryList
1673 if (list == NULL) {
1674 iterator->_next = iterator->_prev = NULL;
1675 _EntryList = iterator;
1676 } else {
1677 list->_prev = iterator;
1678 iterator->_next = list;
1679 iterator->_prev = NULL;
1680 _EntryList = iterator;
1681 }
1682 } else if (policy == 1) { // append to EntryList
1683 if (list == NULL) {
1684 iterator->_next = iterator->_prev = NULL;
1685 _EntryList = iterator;
1686 } else {
1687 // CONSIDER: finding the tail currently requires a linear-time walk of
1688 // the EntryList. We can make tail access constant-time by converting to
1689 // a CDLL instead of using our current DLL.
1690 ObjectWaiter * tail;
1691 for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1692 assert(tail != NULL && tail->_next == NULL, "invariant");
1693 tail->_next = iterator;
1694 iterator->_prev = tail;
1695 iterator->_next = NULL;
1696 }
1697 } else if (policy == 2) { // prepend to cxq
1698 if (list == NULL) {
1699 iterator->_next = iterator->_prev = NULL;
1700 _EntryList = iterator;
1701 } else {
1702 iterator->TState = ObjectWaiter::TS_CXQ;
1703 for (;;) {
1704 ObjectWaiter * front = _cxq;
1705 iterator->_next = front;
1706 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1707 break;
1708 }
1709 }
1710 }
1711 } else if (policy == 3) { // append to cxq
1712 iterator->TState = ObjectWaiter::TS_CXQ;
1713 for (;;) {
1714 ObjectWaiter * tail = _cxq;
1715 if (tail == NULL) {
1716 iterator->_next = NULL;
1717 if (Atomic::cmpxchg(iterator, &_cxq, (ObjectWaiter*)NULL) == NULL) {
1718 break;
1719 }
1720 } else {
1721 while (tail->_next != NULL) tail = tail->_next;
1722 tail->_next = iterator;
1723 iterator->_prev = tail;
1724 iterator->_next = NULL;
1725 break;
1726 }
1727 }
1728 } else {
1729 ParkEvent * ev = iterator->_event;
1730 iterator->TState = ObjectWaiter::TS_RUN;
1731 OrderAccess::fence();
1732 ev->unpark();
1733 }
1734
1735 // _WaitSetLock protects the wait queue, not the EntryList. We could
1736 // move the add-to-EntryList operation, above, outside the critical section
1737 // protected by _WaitSetLock. In practice that's not useful. With the
1738 // exception of wait() timeouts and interrupts the monitor owner
1739 // is the only thread that grabs _WaitSetLock. There's almost no contention
1740 // on _WaitSetLock so it's not profitable to reduce the length of the
1741 // critical section.
1742
1743 if (policy < 4) {
1744 iterator->wait_reenter_begin(this);
1745 }
1746 }
1747 Thread::SpinRelease(&_WaitSetLock);
1748 }
1749
1750 // Consider: a not-uncommon synchronization bug is to use notify() when
1751 // notifyAll() is more appropriate, potentially resulting in stranded
1752 // threads; this is one example of a lost wakeup. A useful diagnostic
1753 // option is to force all notify() operations to behave as notifyAll().
1754 //
1755 // Note: We can also detect many such problems with a "minimum wait".
1756 // When the "minimum wait" is set to a small non-zero timeout value
1757 // and the program does not hang whereas it did absent "minimum wait",
1758 // that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses
1759 // a "minimum wait" for all park() operations; see the recheckInterval
1760 // variable and MAX_RECHECK_INTERVAL.
1761
1762 void ObjectMonitor::notify(TRAPS) {
1763 CHECK_OWNER();
1764 if (_WaitSet == NULL) {
1765 TEVENT(Empty-Notify);
1766 return;
1767 }
1768 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1769 INotify(THREAD);
1770 OM_PERFDATA_OP(Notifications, inc(1));
1771 }
1772
1773
1774 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1775 // from the waitset to the EntryList. This could be done more efficiently with a
1776 // single bulk transfer but in practice it's not time-critical. Beware too,
1777 // that in prepend-mode we invert the order of the waiters. Let's say that the
1778 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1779 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1780
1781 void ObjectMonitor::notifyAll(TRAPS) {
1782 CHECK_OWNER();
1783 if (_WaitSet == NULL) {
1784 TEVENT(Empty-NotifyAll);
1785 return;
1786 }
1787
1788 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1789 int tally = 0;
1790 while (_WaitSet != NULL) {
1791 tally++;
1792 INotify(THREAD);
1793 }
1794
1795 OM_PERFDATA_OP(Notifications, inc(tally));
1796 }
1797
1798 // -----------------------------------------------------------------------------
1799 // Adaptive Spinning Support
1800 //
1801 // Adaptive spin-then-block - rational spinning
1802 //
1803 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1804 // algorithm. On high order SMP systems it would be better to start with
1805 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1806 // a contending thread could enqueue itself on the cxq and then spin locally
1807 // on a thread-specific variable such as its ParkEvent._Event flag.
1808 // That's left as an exercise for the reader. Note that global spinning is
1809 // not problematic on Niagara, as the L2 cache serves the interconnect and
1810 // has both low latency and massive bandwidth.
1811 //
1812 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1813 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1814 // (duration) or we can fix the count at approximately the duration of
1815 // a context switch and vary the frequency. Of course we could also
1816 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1817 // For a description of 'Adaptive spin-then-block mutual exclusion in
1818 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1819 //
1820 // This implementation varies the duration "D", where D varies with
1821 // the success rate of recent spin attempts. (D is capped at approximately
1822 // length of a round-trip context switch). The success rate for recent
1823 // spin attempts is a good predictor of the success rate of future spin
1824 // attempts. The mechanism adapts automatically to varying critical
1825 // section length (lock modality), system load and degree of parallelism.
1826 // D is maintained per-monitor in _SpinDuration and is initialized
1827 // optimistically. Spin frequency is fixed at 100%.
1828 //
1829 // Note that _SpinDuration is volatile, but we update it without locks
1830 // or atomics. The code is designed so that _SpinDuration stays within
1831 // a reasonable range even in the presence of races. The arithmetic
1832 // operations on _SpinDuration are closed over the domain of legal values,
1833 // so at worst a race will install and older but still legal value.
1834 // At the very worst this introduces some apparent non-determinism.
1835 // We might spin when we shouldn't or vice-versa, but since the spin
1836 // count are relatively short, even in the worst case, the effect is harmless.
1837 //
1838 // Care must be taken that a low "D" value does not become an
1839 // an absorbing state. Transient spinning failures -- when spinning
1840 // is overall profitable -- should not cause the system to converge
1841 // on low "D" values. We want spinning to be stable and predictable
1842 // and fairly responsive to change and at the same time we don't want
1843 // it to oscillate, become metastable, be "too" non-deterministic,
1844 // or converge on or enter undesirable stable absorbing states.
1845 //
1846 // We implement a feedback-based control system -- using past behavior
1847 // to predict future behavior. We face two issues: (a) if the
1848 // input signal is random then the spin predictor won't provide optimal
1849 // results, and (b) if the signal frequency is too high then the control
1850 // system, which has some natural response lag, will "chase" the signal.
1851 // (b) can arise from multimodal lock hold times. Transient preemption
1852 // can also result in apparent bimodal lock hold times.
1853 // Although sub-optimal, neither condition is particularly harmful, as
1854 // in the worst-case we'll spin when we shouldn't or vice-versa.
1855 // The maximum spin duration is rather short so the failure modes aren't bad.
1856 // To be conservative, I've tuned the gain in system to bias toward
1857 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1858 // "rings" or oscillates between spinning and not spinning. This happens
1859 // when spinning is just on the cusp of profitability, however, so the
1860 // situation is not dire. The state is benign -- there's no need to add
1861 // hysteresis control to damp the transition rate between spinning and
1862 // not spinning.
1863
1864 // Spinning: Fixed frequency (100%), vary duration
1865 int ObjectMonitor::TrySpin(Thread * Self) {
1866 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1867 int ctr = Knob_FixedSpin;
1868 if (ctr != 0) {
1869 while (--ctr >= 0) {
1870 if (TryLock(Self) > 0) return 1;
1871 SpinPause();
1872 }
1873 return 0;
1874 }
1875
1876 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1877 if (TryLock(Self) > 0) {
1878 // Increase _SpinDuration ...
1879 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1880 // Raising _SpurDuration to the poverty line is key.
1881 int x = _SpinDuration;
1882 if (x < Knob_SpinLimit) {
1883 if (x < Knob_Poverty) x = Knob_Poverty;
1884 _SpinDuration = x + Knob_BonusB;
1885 }
1886 return 1;
1887 }
1888 SpinPause();
1889 }
1890
1891 // Admission control - verify preconditions for spinning
1892 //
1893 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1894 // becoming an absorbing state. Put another way, we spin briefly to
1895 // sample, just in case the system load, parallelism, contention, or lock
1896 // modality changed.
1897 //
1898 // Consider the following alternative:
1899 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1900 // spin attempt. "Periodically" might mean after a tally of
1901 // the # of failed spin attempts (or iterations) reaches some threshold.
1902 // This takes us into the realm of 1-out-of-N spinning, where we
1903 // hold the duration constant but vary the frequency.
1904
1905 ctr = _SpinDuration;
1906 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1907 if (ctr <= 0) return 0;
1908
1909 if (Knob_SuccRestrict && _succ != NULL) return 0;
1910 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1911 TEVENT(Spin abort - notrunnable [TOP]);
1912 return 0;
1913 }
1914
1915 int MaxSpin = Knob_MaxSpinners;
1916 if (MaxSpin >= 0) {
1917 if (_Spinner > MaxSpin) {
1918 TEVENT(Spin abort -- too many spinners);
1919 return 0;
1920 }
1921 // Slightly racy, but benign ...
1922 Adjust(&_Spinner, 1);
1923 }
1924
1925 // We're good to spin ... spin ingress.
1926 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1927 // when preparing to LD...CAS _owner, etc and the CAS is likely
1928 // to succeed.
1929 int hits = 0;
1930 int msk = 0;
1931 int caspty = Knob_CASPenalty;
1932 int oxpty = Knob_OXPenalty;
1933 int sss = Knob_SpinSetSucc;
1934 if (sss && _succ == NULL) _succ = Self;
1935 Thread * prv = NULL;
1936
1937 // There are three ways to exit the following loop:
1938 // 1. A successful spin where this thread has acquired the lock.
1939 // 2. Spin failure with prejudice
1940 // 3. Spin failure without prejudice
1941
1942 while (--ctr >= 0) {
1943
1944 // Periodic polling -- Check for pending GC
1945 // Threads may spin while they're unsafe.
1946 // We don't want spinning threads to delay the JVM from reaching
1947 // a stop-the-world safepoint or to steal cycles from GC.
1948 // If we detect a pending safepoint we abort in order that
1949 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1950 // this thread, if safe, doesn't steal cycles from GC.
1951 // This is in keeping with the "no loitering in runtime" rule.
1952 // We periodically check to see if there's a safepoint pending.
1953 if ((ctr & 0xFF) == 0) {
1954 if (SafepointMechanism::poll(Self)) {
1955 TEVENT(Spin: safepoint);
1956 goto Abort; // abrupt spin egress
1957 }
1958 if (Knob_UsePause & 1) SpinPause();
1959 }
1960
1961 if (Knob_UsePause & 2) SpinPause();
1962
1963 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
1964 // This is useful on classic SMP systems, but is of less utility on
1965 // N1-style CMT platforms.
1966 //
1967 // Trade-off: lock acquisition latency vs coherency bandwidth.
1968 // Lock hold times are typically short. A histogram
1969 // of successful spin attempts shows that we usually acquire
1970 // the lock early in the spin. That suggests we want to
1971 // sample _owner frequently in the early phase of the spin,
1972 // but then back-off and sample less frequently as the spin
1973 // progresses. The back-off makes a good citizen on SMP big
1974 // SMP systems. Oversampling _owner can consume excessive
1975 // coherency bandwidth. Relatedly, if we _oversample _owner we
1976 // can inadvertently interfere with the the ST m->owner=null.
1977 // executed by the lock owner.
1978 if (ctr & msk) continue;
1979 ++hits;
1980 if ((hits & 0xF) == 0) {
1981 // The 0xF, above, corresponds to the exponent.
1982 // Consider: (msk+1)|msk
1983 msk = ((msk << 2)|3) & BackOffMask;
1984 }
1985
1986 // Probe _owner with TATAS
1987 // If this thread observes the monitor transition or flicker
1988 // from locked to unlocked to locked, then the odds that this
1989 // thread will acquire the lock in this spin attempt go down
1990 // considerably. The same argument applies if the CAS fails
1991 // or if we observe _owner change from one non-null value to
1992 // another non-null value. In such cases we might abort
1993 // the spin without prejudice or apply a "penalty" to the
1994 // spin count-down variable "ctr", reducing it by 100, say.
1995
1996 Thread * ox = (Thread *) _owner;
1997 if (ox == NULL) {
1998 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1999 if (ox == NULL) {
2000 // The CAS succeeded -- this thread acquired ownership
2001 // Take care of some bookkeeping to exit spin state.
2002 if (sss && _succ == Self) {
2003 _succ = NULL;
2004 }
2005 if (MaxSpin > 0) Adjust(&_Spinner, -1);
2006
2007 // Increase _SpinDuration :
2008 // The spin was successful (profitable) so we tend toward
2009 // longer spin attempts in the future.
2010 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2011 // If we acquired the lock early in the spin cycle it
2012 // makes sense to increase _SpinDuration proportionally.
2013 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2014 int x = _SpinDuration;
2015 if (x < Knob_SpinLimit) {
2016 if (x < Knob_Poverty) x = Knob_Poverty;
2017 _SpinDuration = x + Knob_Bonus;
2018 }
2019 return 1;
2020 }
2021
2022 // The CAS failed ... we can take any of the following actions:
2023 // * penalize: ctr -= Knob_CASPenalty
2024 // * exit spin with prejudice -- goto Abort;
2025 // * exit spin without prejudice.
2026 // * Since CAS is high-latency, retry again immediately.
2027 prv = ox;
2028 TEVENT(Spin: cas failed);
2029 if (caspty == -2) break;
2030 if (caspty == -1) goto Abort;
2031 ctr -= caspty;
2032 continue;
2033 }
2034
2035 // Did lock ownership change hands ?
2036 if (ox != prv && prv != NULL) {
2037 TEVENT(spin: Owner changed)
2038 if (oxpty == -2) break;
2039 if (oxpty == -1) goto Abort;
2040 ctr -= oxpty;
2041 }
2042 prv = ox;
2043
2044 // Abort the spin if the owner is not executing.
2045 // The owner must be executing in order to drop the lock.
2046 // Spinning while the owner is OFFPROC is idiocy.
2047 // Consider: ctr -= RunnablePenalty ;
2048 if (Knob_OState && NotRunnable (Self, ox)) {
2049 TEVENT(Spin abort - notrunnable);
2050 goto Abort;
2051 }
2052 if (sss && _succ == NULL) _succ = Self;
2053 }
2054
2055 // Spin failed with prejudice -- reduce _SpinDuration.
2056 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2057 // AIMD is globally stable.
2058 TEVENT(Spin failure);
2059 {
2060 int x = _SpinDuration;
2061 if (x > 0) {
2062 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2063 // This is globally sample and tends to damp the response.
2064 x -= Knob_Penalty;
2065 if (x < 0) x = 0;
2066 _SpinDuration = x;
2067 }
2068 }
2069
2070 Abort:
2071 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2072 if (sss && _succ == Self) {
2073 _succ = NULL;
2074 // Invariant: after setting succ=null a contending thread
2075 // must recheck-retry _owner before parking. This usually happens
2076 // in the normal usage of TrySpin(), but it's safest
2077 // to make TrySpin() as foolproof as possible.
2078 OrderAccess::fence();
2079 if (TryLock(Self) > 0) return 1;
2080 }
2081 return 0;
2082 }
2083
2084 // NotRunnable() -- informed spinning
2085 //
2086 // Don't bother spinning if the owner is not eligible to drop the lock.
2087 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2088 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2089 // The thread must be runnable in order to drop the lock in timely fashion.
2090 // If the _owner is not runnable then spinning will not likely be
2091 // successful (profitable).
2092 //
2093 // Beware -- the thread referenced by _owner could have died
2094 // so a simply fetch from _owner->_thread_state might trap.
2095 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2096 // Because of the lifecycle issues the schedctl and _thread_state values
2097 // observed by NotRunnable() might be garbage. NotRunnable must
2098 // tolerate this and consider the observed _thread_state value
2099 // as advisory.
2100 //
2101 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2102 // a thread pointer.
2103 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2104 // with the LSB of _owner. Another option would be to probablistically probe
2105 // the putative _owner->TypeTag value.
2106 //
2107 // Checking _thread_state isn't perfect. Even if the thread is
2108 // in_java it might be blocked on a page-fault or have been preempted
2109 // and sitting on a ready/dispatch queue. _thread state in conjunction
2110 // with schedctl.sc_state gives us a good picture of what the
2111 // thread is doing, however.
2112 //
2113 // TODO: check schedctl.sc_state.
2114 // We'll need to use SafeFetch32() to read from the schedctl block.
2115 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2116 //
2117 // The return value from NotRunnable() is *advisory* -- the
2118 // result is based on sampling and is not necessarily coherent.
2119 // The caller must tolerate false-negative and false-positive errors.
2120 // Spinning, in general, is probabilistic anyway.
2121
2122
2123 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2124 // Check ox->TypeTag == 2BAD.
2125 if (ox == NULL) return 0;
2126
2127 // Avoid transitive spinning ...
2128 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2129 // Immediately after T1 acquires L it's possible that T2, also
2130 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2131 // This occurs transiently after T1 acquired L but before
2132 // T1 managed to clear T1.Stalled. T2 does not need to abort
2133 // its spin in this circumstance.
2134 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2135
2136 if (BlockedOn == 1) return 1;
2137 if (BlockedOn != 0) {
2138 return BlockedOn != intptr_t(this) && _owner == ox;
2139 }
2140
2141 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2142 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2143 // consider also: jst != _thread_in_Java -- but that's overspecific.
2144 return jst == _thread_blocked || jst == _thread_in_native;
2145 }
2146
2147
2148 // -----------------------------------------------------------------------------
2149 // WaitSet management ...
2150
2151 ObjectWaiter::ObjectWaiter(Thread* thread) {
2152 _next = NULL;
2153 _prev = NULL;
2154 _notified = 0;
2155 _notifier_tid = 0;
2156 TState = TS_RUN;
2157 _thread = thread;
2158 _event = thread->_ParkEvent;
2159 _active = false;
2160 assert(_event != NULL, "invariant");
2161 }
2162
2163 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2164 JavaThread *jt = (JavaThread *)this->_thread;
2165 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2166 }
2167
2168 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2169 JavaThread *jt = (JavaThread *)this->_thread;
2170 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2171 }
2172
2173 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2174 assert(node != NULL, "should not add NULL node");
2175 assert(node->_prev == NULL, "node already in list");
2176 assert(node->_next == NULL, "node already in list");
2177 // put node at end of queue (circular doubly linked list)
2178 if (_WaitSet == NULL) {
2179 _WaitSet = node;
2180 node->_prev = node;
2181 node->_next = node;
2182 } else {
2183 ObjectWaiter* head = _WaitSet;
2184 ObjectWaiter* tail = head->_prev;
2185 assert(tail->_next == head, "invariant check");
2186 tail->_next = node;
2187 head->_prev = node;
2188 node->_next = head;
2189 node->_prev = tail;
2190 }
2191 }
2192
2193 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2194 // dequeue the very first waiter
2195 ObjectWaiter* waiter = _WaitSet;
2196 if (waiter) {
2197 DequeueSpecificWaiter(waiter);
2198 }
2199 return waiter;
2200 }
2201
2202 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2203 assert(node != NULL, "should not dequeue NULL node");
2204 assert(node->_prev != NULL, "node already removed from list");
2205 assert(node->_next != NULL, "node already removed from list");
2206 // when the waiter has woken up because of interrupt,
2207 // timeout or other spurious wake-up, dequeue the
2208 // waiter from waiting list
2209 ObjectWaiter* next = node->_next;
2210 if (next == node) {
2211 assert(node->_prev == node, "invariant check");
2212 _WaitSet = NULL;
2213 } else {
2214 ObjectWaiter* prev = node->_prev;
2215 assert(prev->_next == node, "invariant check");
2216 assert(next->_prev == node, "invariant check");
2217 next->_prev = prev;
2218 prev->_next = next;
2219 if (_WaitSet == node) {
2220 _WaitSet = next;
2221 }
2222 }
2223 node->_next = NULL;
2224 node->_prev = NULL;
2225 }
2226
2227 // -----------------------------------------------------------------------------
2228 // PerfData support
2229 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2230 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2231 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2232 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL;
2233 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2234 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL;
2235 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL;
2236 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL;
2237 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL;
2238 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL;
2239 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL;
2240 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL;
2241 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL;
2242 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL;
2243 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL;
2244 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2245 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2246 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2247
2248 // One-shot global initialization for the sync subsystem.
2249 // We could also defer initialization and initialize on-demand
2250 // the first time we call inflate(). Initialization would
2251 // be protected - like so many things - by the MonitorCache_lock.
2252
2253 void ObjectMonitor::Initialize() {
2254 static int InitializationCompleted = 0;
2255 assert(InitializationCompleted == 0, "invariant");
2256 InitializationCompleted = 1;
2257 if (UsePerfData) {
2258 EXCEPTION_MARK;
2259 #define NEWPERFCOUNTER(n) \
2260 { \
2261 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2262 CHECK); \
2263 }
2264 #define NEWPERFVARIABLE(n) \
2265 { \
2266 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2267 CHECK); \
2268 }
2269 NEWPERFCOUNTER(_sync_Inflations);
2270 NEWPERFCOUNTER(_sync_Deflations);
2271 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2272 NEWPERFCOUNTER(_sync_FutileWakeups);
2273 NEWPERFCOUNTER(_sync_Parks);
2274 NEWPERFCOUNTER(_sync_EmptyNotifications);
2275 NEWPERFCOUNTER(_sync_Notifications);
2276 NEWPERFCOUNTER(_sync_SlowEnter);
2277 NEWPERFCOUNTER(_sync_SlowExit);
2278 NEWPERFCOUNTER(_sync_SlowNotify);
2279 NEWPERFCOUNTER(_sync_SlowNotifyAll);
2280 NEWPERFCOUNTER(_sync_FailedSpins);
2281 NEWPERFCOUNTER(_sync_SuccessfulSpins);
2282 NEWPERFCOUNTER(_sync_PrivateA);
2283 NEWPERFCOUNTER(_sync_PrivateB);
2284 NEWPERFCOUNTER(_sync_MonInCirculation);
2285 NEWPERFCOUNTER(_sync_MonScavenged);
2286 NEWPERFVARIABLE(_sync_MonExtant);
2287 #undef NEWPERFCOUNTER
2288 #undef NEWPERFVARIABLE
2289 }
2290 }
2291
2292 static char * kvGet(char * kvList, const char * Key) {
2293 if (kvList == NULL) return NULL;
2294 size_t n = strlen(Key);
2295 char * Search;
2296 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2297 if (strncmp (Search, Key, n) == 0) {
2298 if (Search[n] == '=') return Search + n + 1;
2299 if (Search[n] == 0) return(char *) "1";
2300 }
2301 }
2302 return NULL;
2303 }
2304
2305 static int kvGetInt(char * kvList, const char * Key, int Default) {
2306 char * v = kvGet(kvList, Key);
2307 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2308 if (Knob_ReportSettings && v != NULL) {
2309 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ;
2310 tty->flush();
2311 }
2312 return rslt;
2313 }
2314
2315 void ObjectMonitor::DeferredInitialize() {
2316 if (InitDone > 0) return;
2317 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2318 while (InitDone != 1) /* empty */;
2319 return;
2320 }
2321
2322 // One-shot global initialization ...
2323 // The initialization is idempotent, so we don't need locks.
2324 // In the future consider doing this via os::init_2().
2325 // SyncKnobs consist of <Key>=<Value> pairs in the style
2326 // of environment variables. Start by converting ':' to NUL.
2327
2328 if (SyncKnobs == NULL) SyncKnobs = "";
2329
2330 size_t sz = strlen(SyncKnobs);
2331 char * knobs = (char *) malloc(sz + 2);
2332 if (knobs == NULL) {
2333 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2334 guarantee(0, "invariant");
2335 }
2336 strcpy(knobs, SyncKnobs);
2337 knobs[sz+1] = 0;
2338 for (char * p = knobs; *p; p++) {
2339 if (*p == ':') *p = 0;
2340 }
2341
2342 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2343 SETKNOB(ReportSettings);
2344 SETKNOB(ExitRelease);
2345 SETKNOB(Verbose);
2346 SETKNOB(VerifyInUse);
2347 SETKNOB(VerifyMatch);
2348 SETKNOB(FixedSpin);
2349 SETKNOB(SpinLimit);
2350 SETKNOB(SpinBase);
2351 SETKNOB(SpinBackOff);
2352 SETKNOB(CASPenalty);
2353 SETKNOB(OXPenalty);
2354 SETKNOB(LogSpins);
2355 SETKNOB(SpinSetSucc);
2356 SETKNOB(SuccEnabled);
2357 SETKNOB(SuccRestrict);
2358 SETKNOB(Penalty);
2359 SETKNOB(Bonus);
2360 SETKNOB(BonusB);
2361 SETKNOB(Poverty);
2362 SETKNOB(SpinAfterFutile);
2363 SETKNOB(UsePause);
2364 SETKNOB(SpinEarly);
2365 SETKNOB(OState);
2366 SETKNOB(MaxSpinners);
2367 SETKNOB(PreSpin);
2368 SETKNOB(ExitPolicy);
2369 SETKNOB(QMode);
2370 SETKNOB(ResetEvent);
2371 SETKNOB(MoveNotifyee);
2372 SETKNOB(FastHSSEC);
2373 #undef SETKNOB
2374
2375 if (Knob_Verbose) {
2376 sanity_checks();
2377 }
2378
2379 if (os::is_MP()) {
2380 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2381 if (Knob_ReportSettings) {
2382 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask);
2383 }
2384 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2385 } else {
2386 Knob_SpinLimit = 0;
2387 Knob_SpinBase = 0;
2388 Knob_PreSpin = 0;
2389 Knob_FixedSpin = -1;
2390 }
2391
2392 if (Knob_LogSpins == 0) {
2393 ObjectMonitor::_sync_FailedSpins = NULL;
2394 }
2395
2396 free(knobs);
2397 OrderAccess::fence();
2398 InitDone = 1;
2399 }
2400
2401 void ObjectMonitor::sanity_checks() {
2402 int error_cnt = 0;
2403 int warning_cnt = 0;
2404 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2405
2406 if (verbose) {
2407 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2408 sizeof(ObjectMonitor));
2409 tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT,
2410 sizeof(PaddedEnd<ObjectMonitor>));
2411 }
2412
2413 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2414 if (verbose) {
2415 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2416 }
2417
2418 ObjectMonitor dummy;
2419 u_char *addr_begin = (u_char*)&dummy;
2420 u_char *addr_header = (u_char*)&dummy._header;
2421 u_char *addr_owner = (u_char*)&dummy._owner;
2422
2423 uint offset_header = (uint)(addr_header - addr_begin);
2424 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2425
2426 uint offset_owner = (uint)(addr_owner - addr_begin);
2427 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2428
2429 if ((uint)(addr_header - addr_begin) != 0) {
2430 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2431 error_cnt++;
2432 }
2433
2434 if (cache_line_size != 0) {
2435 // We were able to determine the L1 data cache line size so
2436 // do some cache line specific sanity checks
2437
2438 if ((offset_owner - offset_header) < cache_line_size) {
2439 tty->print_cr("WARNING: the _header and _owner fields are closer "
2440 "than a cache line which permits false sharing.");
2441 warning_cnt++;
2442 }
2443
2444 if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) {
2445 tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a "
2446 "multiple of a cache line which permits false sharing.");
2447 warning_cnt++;
2448 }
2449 }
2450
2451 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2452 &warning_cnt);
2453
2454 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2455 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2456 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2457 }
2458
2459 guarantee(error_cnt == 0,
2460 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2461 }
2462
2463 #ifndef PRODUCT
2464 void ObjectMonitor_test() {
2465 ObjectMonitor::sanity_checks();
2466 }
2467 #endif