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