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