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