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