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