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