1 /*
2 * Copyright (c) 1998, 2019, 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 DEBUG_ONLY(static volatile bool InitDone = false;)
114
115 // -----------------------------------------------------------------------------
116 // Theory of operations -- Monitors lists, thread residency, etc:
117 //
118 // * A thread acquires ownership of a monitor by successfully
119 // CAS()ing the _owner field from null to non-null.
120 //
121 // * Invariant: A thread appears on at most one monitor list --
122 // cxq, EntryList or WaitSet -- at any one time.
123 //
124 // * Contending threads "push" themselves onto the cxq with CAS
125 // and then spin/park.
126 //
127 // * After a contending thread eventually acquires the lock it must
128 // dequeue itself from either the EntryList or the cxq.
129 //
130 // * The exiting thread identifies and unparks an "heir presumptive"
131 // tentative successor thread on the EntryList. Critically, the
132 // exiting thread doesn't unlink the successor thread from the EntryList.
133 // After having been unparked, the wakee will recontend for ownership of
134 // the monitor. The successor (wakee) will either acquire the lock or
135 // re-park itself.
136 //
137 // Succession is provided for by a policy of competitive handoff.
138 // The exiting thread does _not_ grant or pass ownership to the
139 // successor thread. (This is also referred to as "handoff" succession").
140 // Instead the exiting thread releases ownership and possibly wakes
141 // a successor, so the successor can (re)compete for ownership of the lock.
142 // If the EntryList is empty but the cxq is populated the exiting
143 // thread will drain the cxq into the EntryList. It does so by
144 // by detaching the cxq (installing null with CAS) and folding
145 // the threads from the cxq into the EntryList. The EntryList is
146 // doubly linked, while the cxq is singly linked because of the
147 // CAS-based "push" used to enqueue recently arrived threads (RATs).
148 //
149 // * Concurrency invariants:
150 //
151 // -- only the monitor owner may access or mutate the EntryList.
152 // The mutex property of the monitor itself protects the EntryList
153 // from concurrent interference.
154 // -- Only the monitor owner may detach the cxq.
155 //
156 // * The monitor entry list operations avoid locks, but strictly speaking
157 // they're not lock-free. Enter is lock-free, exit is not.
158 // For a description of 'Methods and apparatus providing non-blocking access
159 // to a resource,' see U.S. Pat. No. 7844973.
160 //
161 // * The cxq can have multiple concurrent "pushers" but only one concurrent
162 // detaching thread. This mechanism is immune from the ABA corruption.
163 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
164 //
165 // * Taken together, the cxq and the EntryList constitute or form a
166 // single logical queue of threads stalled trying to acquire the lock.
167 // We use two distinct lists to improve the odds of a constant-time
168 // dequeue operation after acquisition (in the ::enter() epilogue) and
169 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
170 // A key desideratum is to minimize queue & monitor metadata manipulation
171 // that occurs while holding the monitor lock -- that is, we want to
172 // minimize monitor lock holds times. Note that even a small amount of
173 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
174 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
175 // locks and monitor metadata.
176 //
177 // Cxq points to the set of Recently Arrived Threads attempting entry.
178 // Because we push threads onto _cxq with CAS, the RATs must take the form of
179 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
180 // the unlocking thread notices that EntryList is null but _cxq is != null.
181 //
182 // The EntryList is ordered by the prevailing queue discipline and
183 // can be organized in any convenient fashion, such as a doubly-linked list or
184 // a circular doubly-linked list. Critically, we want insert and delete operations
185 // to operate in constant-time. If we need a priority queue then something akin
186 // to Solaris' sleepq would work nicely. Viz.,
187 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
188 // Queue discipline is enforced at ::exit() time, when the unlocking thread
189 // drains the cxq into the EntryList, and orders or reorders the threads on the
190 // EntryList accordingly.
191 //
192 // Barring "lock barging", this mechanism provides fair cyclic ordering,
193 // somewhat similar to an elevator-scan.
194 //
195 // * The monitor synchronization subsystem avoids the use of native
196 // synchronization primitives except for the narrow platform-specific
197 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
198 // the semantics of park-unpark. Put another way, this monitor implementation
199 // depends only on atomic operations and park-unpark. The monitor subsystem
200 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
201 // underlying OS manages the READY<->RUN transitions.
202 //
203 // * Waiting threads reside on the WaitSet list -- wait() puts
204 // the caller onto the WaitSet.
205 //
206 // * notify() or notifyAll() simply transfers threads from the WaitSet to
207 // either the EntryList or cxq. Subsequent exit() operations will
208 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
209 // it's likely the notifyee would simply impale itself on the lock held
210 // by the notifier.
211 //
212 // * An interesting alternative is to encode cxq as (List,LockByte) where
213 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
214 // variable, like _recursions, in the scheme. The threads or Events that form
215 // the list would have to be aligned in 256-byte addresses. A thread would
216 // try to acquire the lock or enqueue itself with CAS, but exiting threads
217 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
218 // Note that is is *not* word-tearing, but it does presume that full-word
219 // CAS operations are coherent with intermix with STB operations. That's true
220 // on most common processors.
221 //
222 // * See also http://blogs.sun.com/dave
223
224
225 void* ObjectMonitor::operator new (size_t size) throw() {
226 return AllocateHeap(size, mtInternal);
227 }
228 void* ObjectMonitor::operator new[] (size_t size) throw() {
229 return operator new (size);
230 }
231 void ObjectMonitor::operator delete(void* p) {
232 FreeHeap(p);
233 }
234 void ObjectMonitor::operator delete[] (void *p) {
235 operator delete(p);
236 }
237
238 // -----------------------------------------------------------------------------
239 // Enter support
240
241 void ObjectMonitor::enter(TRAPS) {
242 // The following code is ordered to check the most common cases first
243 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
244 Thread * const Self = THREAD;
245
246 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
247 if (cur == NULL) {
248 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
249 assert(_recursions == 0, "invariant");
250 assert(_owner == Self, "invariant");
251 return;
252 }
253
254 if (cur == Self) {
255 // TODO-FIXME: check for integer overflow! BUGID 6557169.
256 _recursions++;
257 return;
258 }
259
260 if (Self->is_lock_owned ((address)cur)) {
261 assert(_recursions == 0, "internal state error");
262 _recursions = 1;
263 // Commute owner from a thread-specific on-stack BasicLockObject address to
264 // a full-fledged "Thread *".
265 _owner = Self;
266 return;
267 }
268
269 // We've encountered genuine contention.
270 assert(Self->_Stalled == 0, "invariant");
271 Self->_Stalled = intptr_t(this);
272
273 // Try one round of spinning *before* enqueueing Self
274 // and before going through the awkward and expensive state
275 // transitions. The following spin is strictly optional ...
276 // Note that if we acquire the monitor from an initial spin
277 // we forgo posting JVMTI events and firing DTRACE probes.
278 if (TrySpin(Self) > 0) {
279 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner));
280 assert(_recursions == 0, "must be 0: recursions=" INTPTR_FORMAT,
281 _recursions);
282 assert(((oop)object())->mark() == markOopDesc::encode(this),
283 "object mark must match encoded this: mark=" INTPTR_FORMAT
284 ", encoded this=" INTPTR_FORMAT, p2i(((oop)object())->mark()),
285 p2i(markOopDesc::encode(this)));
286 Self->_Stalled = 0;
287 return;
288 }
289
290 assert(_owner != Self, "invariant");
291 assert(_succ != Self, "invariant");
292 assert(Self->is_Java_thread(), "invariant");
293 JavaThread * jt = (JavaThread *) Self;
294 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
295 assert(jt->thread_state() != _thread_blocked, "invariant");
296 assert(this->object() != NULL, "invariant");
297 assert(_contentions >= 0, "invariant");
298
299 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
300 // Ensure the object-monitor relationship remains stable while there's contention.
301 Atomic::inc(&_contentions);
302
303 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
304 EventJavaMonitorEnter event;
305 if (event.should_commit()) {
306 event.set_monitorClass(((oop)this->object())->klass());
307 event.set_address((uintptr_t)(this->object_addr()));
308 }
309
310 { // Change java thread status to indicate blocked on monitor enter.
311 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
312
313 Self->set_current_pending_monitor(this);
314
315 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
316 if (JvmtiExport::should_post_monitor_contended_enter()) {
317 JvmtiExport::post_monitor_contended_enter(jt, this);
318
319 // The current thread does not yet own the monitor and does not
320 // yet appear on any queues that would get it made the successor.
321 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
322 // handler cannot accidentally consume an unpark() meant for the
323 // ParkEvent associated with this ObjectMonitor.
324 }
325
326 OSThreadContendState osts(Self->osthread());
327 ThreadBlockInVM tbivm(jt);
328
329 // TODO-FIXME: change the following for(;;) loop to straight-line code.
330 for (;;) {
331 jt->set_suspend_equivalent();
332 // cleared by handle_special_suspend_equivalent_condition()
333 // or java_suspend_self()
334
335 EnterI(THREAD);
336
337 if (!ExitSuspendEquivalent(jt)) break;
338
339 // We have acquired the contended monitor, but while we were
340 // waiting another thread suspended us. We don't want to enter
341 // the monitor while suspended because that would surprise the
342 // thread that suspended us.
343 //
344 _recursions = 0;
345 _succ = NULL;
346 exit(false, Self);
347
348 jt->java_suspend_self();
349 }
350 Self->set_current_pending_monitor(NULL);
351
352 // We cleared the pending monitor info since we've just gotten past
353 // the enter-check-for-suspend dance and we now own the monitor free
354 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
355 // destructor can go to a safepoint at the end of this block. If we
356 // do a thread dump during that safepoint, then this thread will show
357 // as having "-locked" the monitor, but the OS and java.lang.Thread
358 // states will still report that the thread is blocked trying to
359 // acquire it.
360 }
361
362 Atomic::dec(&_contentions);
363 assert(_contentions >= 0, "invariant");
364 Self->_Stalled = 0;
365
366 // Must either set _recursions = 0 or ASSERT _recursions == 0.
367 assert(_recursions == 0, "invariant");
368 assert(_owner == Self, "invariant");
369 assert(_succ != Self, "invariant");
370 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
371
372 // The thread -- now the owner -- is back in vm mode.
373 // Report the glorious news via TI,DTrace and jvmstat.
374 // The probe effect is non-trivial. All the reportage occurs
375 // while we hold the monitor, increasing the length of the critical
376 // section. Amdahl's parallel speedup law comes vividly into play.
377 //
378 // Another option might be to aggregate the events (thread local or
379 // per-monitor aggregation) and defer reporting until a more opportune
380 // time -- such as next time some thread encounters contention but has
381 // yet to acquire the lock. While spinning that thread could
382 // spinning we could increment JVMStat counters, etc.
383
384 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
385 if (JvmtiExport::should_post_monitor_contended_entered()) {
386 JvmtiExport::post_monitor_contended_entered(jt, this);
387
388 // The current thread already owns the monitor and is not going to
389 // call park() for the remainder of the monitor enter protocol. So
390 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
391 // event handler consumed an unpark() issued by the thread that
392 // just exited the monitor.
393 }
394 if (event.should_commit()) {
395 event.set_previousOwner((uintptr_t)_previous_owner_tid);
396 event.commit();
397 }
398 OM_PERFDATA_OP(ContendedLockAttempts, inc());
399 }
400
401 // Caveat: TryLock() is not necessarily serializing if it returns failure.
402 // Callers must compensate as needed.
403
404 int ObjectMonitor::TryLock(Thread * Self) {
405 void * own = _owner;
406 if (own != NULL) return 0;
407 if (Atomic::replace_if_null(Self, &_owner)) {
408 // Either guarantee _recursions == 0 or set _recursions = 0.
409 assert(_recursions == 0, "invariant");
410 assert(_owner == Self, "invariant");
411 return 1;
412 }
413 // The lock had been free momentarily, but we lost the race to the lock.
414 // Interference -- the CAS failed.
415 // We can either return -1 or retry.
416 // Retry doesn't make as much sense because the lock was just acquired.
417 return -1;
418 }
419
420 #define MAX_RECHECK_INTERVAL 1000
421
422 void ObjectMonitor::EnterI(TRAPS) {
423 Thread * const Self = THREAD;
424 assert(Self->is_Java_thread(), "invariant");
425 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
426
427 // Try the lock - TATAS
428 if (TryLock (Self) > 0) {
429 assert(_succ != Self, "invariant");
430 assert(_owner == Self, "invariant");
431 assert(_Responsible != Self, "invariant");
432 return;
433 }
434
435 assert(InitDone, "Unexpectedly not initialized");
436
437 // We try one round of spinning *before* enqueueing Self.
438 //
439 // If the _owner is ready but OFFPROC we could use a YieldTo()
440 // operation to donate the remainder of this thread's quantum
441 // to the owner. This has subtle but beneficial affinity
442 // effects.
443
444 if (TrySpin(Self) > 0) {
445 assert(_owner == Self, "invariant");
446 assert(_succ != Self, "invariant");
447 assert(_Responsible != Self, "invariant");
448 return;
449 }
450
451 // The Spin failed -- Enqueue and park the thread ...
452 assert(_succ != Self, "invariant");
453 assert(_owner != Self, "invariant");
454 assert(_Responsible != Self, "invariant");
455
456 // Enqueue "Self" on ObjectMonitor's _cxq.
457 //
458 // Node acts as a proxy for Self.
459 // As an aside, if were to ever rewrite the synchronization code mostly
460 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
461 // Java objects. This would avoid awkward lifecycle and liveness issues,
462 // as well as eliminate a subset of ABA issues.
463 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
464
465 ObjectWaiter node(Self);
466 Self->_ParkEvent->reset();
467 node._prev = (ObjectWaiter *) 0xBAD;
468 node.TState = ObjectWaiter::TS_CXQ;
469
470 // Push "Self" onto the front of the _cxq.
471 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
472 // Note that spinning tends to reduce the rate at which threads
473 // enqueue and dequeue on EntryList|cxq.
474 ObjectWaiter * nxt;
475 for (;;) {
476 node._next = nxt = _cxq;
477 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
478
479 // Interference - the CAS failed because _cxq changed. Just retry.
480 // As an optional optimization we retry the lock.
481 if (TryLock (Self) > 0) {
482 assert(_succ != Self, "invariant");
483 assert(_owner == Self, "invariant");
484 assert(_Responsible != Self, "invariant");
485 return;
486 }
487 }
488
489 // Check for cxq|EntryList edge transition to non-null. This indicates
490 // the onset of contention. While contention persists exiting threads
491 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
492 // operations revert to the faster 1-0 mode. This enter operation may interleave
493 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
494 // arrange for one of the contending thread to use a timed park() operations
495 // to detect and recover from the race. (Stranding is form of progress failure
496 // where the monitor is unlocked but all the contending threads remain parked).
497 // That is, at least one of the contended threads will periodically poll _owner.
498 // One of the contending threads will become the designated "Responsible" thread.
499 // The Responsible thread uses a timed park instead of a normal indefinite park
500 // operation -- it periodically wakes and checks for and recovers from potential
501 // strandings admitted by 1-0 exit operations. We need at most one Responsible
502 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
503 // be responsible for a monitor.
504 //
505 // Currently, one of the contended threads takes on the added role of "Responsible".
506 // A viable alternative would be to use a dedicated "stranding checker" thread
507 // that periodically iterated over all the threads (or active monitors) and unparked
508 // successors where there was risk of stranding. This would help eliminate the
509 // timer scalability issues we see on some platforms as we'd only have one thread
510 // -- the checker -- parked on a timer.
511
512 if (nxt == NULL && _EntryList == NULL) {
513 // Try to assume the role of responsible thread for the monitor.
514 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
515 Atomic::replace_if_null(Self, &_Responsible);
516 }
517
518 // The lock might have been released while this thread was occupied queueing
519 // itself onto _cxq. To close the race and avoid "stranding" and
520 // progress-liveness failure we must resample-retry _owner before parking.
521 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
522 // In this case the ST-MEMBAR is accomplished with CAS().
523 //
524 // TODO: Defer all thread state transitions until park-time.
525 // Since state transitions are heavy and inefficient we'd like
526 // to defer the state transitions until absolutely necessary,
527 // and in doing so avoid some transitions ...
528
529 int nWakeups = 0;
530 int recheckInterval = 1;
531
532 for (;;) {
533
534 if (TryLock(Self) > 0) break;
535 assert(_owner != Self, "invariant");
536
537 // park self
538 if (_Responsible == Self) {
539 Self->_ParkEvent->park((jlong) recheckInterval);
540 // Increase the recheckInterval, but clamp the value.
541 recheckInterval *= 8;
542 if (recheckInterval > MAX_RECHECK_INTERVAL) {
543 recheckInterval = MAX_RECHECK_INTERVAL;
544 }
545 } else {
546 Self->_ParkEvent->park();
547 }
548
549 if (TryLock(Self) > 0) break;
550
551 // The lock is still contested.
552 // Keep a tally of the # of futile wakeups.
553 // Note that the counter is not protected by a lock or updated by atomics.
554 // That is by design - we trade "lossy" counters which are exposed to
555 // races during updates for a lower probe effect.
556
557 // This PerfData object can be used in parallel with a safepoint.
558 // See the work around in PerfDataManager::destroy().
559 OM_PERFDATA_OP(FutileWakeups, inc());
560 ++nWakeups;
561
562 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
563 // We can defer clearing _succ until after the spin completes
564 // TrySpin() must tolerate being called with _succ == Self.
565 // Try yet another round of adaptive spinning.
566 if (TrySpin(Self) > 0) break;
567
568 // We can find that we were unpark()ed and redesignated _succ while
569 // we were spinning. That's harmless. If we iterate and call park(),
570 // park() will consume the event and return immediately and we'll
571 // just spin again. This pattern can repeat, leaving _succ to simply
572 // spin on a CPU.
573
574 if (_succ == Self) _succ = NULL;
575
576 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
577 OrderAccess::fence();
578 }
579
580 // Egress :
581 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
582 // Normally we'll find Self on the EntryList .
583 // From the perspective of the lock owner (this thread), the
584 // EntryList is stable and cxq is prepend-only.
585 // The head of cxq is volatile but the interior is stable.
586 // In addition, Self.TState is stable.
587
588 assert(_owner == Self, "invariant");
589 assert(object() != NULL, "invariant");
590 // I'd like to write:
591 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
592 // but as we're at a safepoint that's not safe.
593
594 UnlinkAfterAcquire(Self, &node);
595 if (_succ == Self) _succ = NULL;
596
597 assert(_succ != Self, "invariant");
598 if (_Responsible == Self) {
599 _Responsible = NULL;
600 OrderAccess::fence(); // Dekker pivot-point
601
602 // We may leave threads on cxq|EntryList without a designated
603 // "Responsible" thread. This is benign. When this thread subsequently
604 // exits the monitor it can "see" such preexisting "old" threads --
605 // threads that arrived on the cxq|EntryList before the fence, above --
606 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
607 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
608 // non-null and elect a new "Responsible" timer thread.
609 //
610 // This thread executes:
611 // ST Responsible=null; MEMBAR (in enter epilogue - here)
612 // LD cxq|EntryList (in subsequent exit)
613 //
614 // Entering threads in the slow/contended path execute:
615 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
616 // The (ST cxq; MEMBAR) is accomplished with CAS().
617 //
618 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
619 // exit operation from floating above the ST Responsible=null.
620 }
621
622 // We've acquired ownership with CAS().
623 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
624 // But since the CAS() this thread may have also stored into _succ,
625 // EntryList, cxq or Responsible. These meta-data updates must be
626 // visible __before this thread subsequently drops the lock.
627 // Consider what could occur if we didn't enforce this constraint --
628 // STs to monitor meta-data and user-data could reorder with (become
629 // visible after) the ST in exit that drops ownership of the lock.
630 // Some other thread could then acquire the lock, but observe inconsistent
631 // or old monitor meta-data and heap data. That violates the JMM.
632 // To that end, the 1-0 exit() operation must have at least STST|LDST
633 // "release" barrier semantics. Specifically, there must be at least a
634 // STST|LDST barrier in exit() before the ST of null into _owner that drops
635 // the lock. The barrier ensures that changes to monitor meta-data and data
636 // protected by the lock will be visible before we release the lock, and
637 // therefore before some other thread (CPU) has a chance to acquire the lock.
638 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
639 //
640 // Critically, any prior STs to _succ or EntryList must be visible before
641 // the ST of null into _owner in the *subsequent* (following) corresponding
642 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
643 // execute a serializing instruction.
644
645 return;
646 }
647
648 // ReenterI() is a specialized inline form of the latter half of the
649 // contended slow-path from EnterI(). We use ReenterI() only for
650 // monitor reentry in wait().
651 //
652 // In the future we should reconcile EnterI() and ReenterI().
653
654 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
655 assert(Self != NULL, "invariant");
656 assert(SelfNode != NULL, "invariant");
657 assert(SelfNode->_thread == Self, "invariant");
658 assert(_waiters > 0, "invariant");
659 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
660 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
661 JavaThread * jt = (JavaThread *) Self;
662
663 int nWakeups = 0;
664 for (;;) {
665 ObjectWaiter::TStates v = SelfNode->TState;
666 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
667 assert(_owner != Self, "invariant");
668
669 if (TryLock(Self) > 0) break;
670 if (TrySpin(Self) > 0) break;
671
672 // State transition wrappers around park() ...
673 // ReenterI() wisely defers state transitions until
674 // it's clear we must park the thread.
675 {
676 OSThreadContendState osts(Self->osthread());
677 ThreadBlockInVM tbivm(jt);
678
679 // cleared by handle_special_suspend_equivalent_condition()
680 // or java_suspend_self()
681 jt->set_suspend_equivalent();
682 Self->_ParkEvent->park();
683
684 // were we externally suspended while we were waiting?
685 for (;;) {
686 if (!ExitSuspendEquivalent(jt)) break;
687 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
688 jt->java_suspend_self();
689 jt->set_suspend_equivalent();
690 }
691 }
692
693 // Try again, but just so we distinguish between futile wakeups and
694 // successful wakeups. The following test isn't algorithmically
695 // necessary, but it helps us maintain sensible statistics.
696 if (TryLock(Self) > 0) break;
697
698 // The lock is still contested.
699 // Keep a tally of the # of futile wakeups.
700 // Note that the counter is not protected by a lock or updated by atomics.
701 // That is by design - we trade "lossy" counters which are exposed to
702 // races during updates for a lower probe effect.
703 ++nWakeups;
704
705 // Assuming this is not a spurious wakeup we'll normally
706 // find that _succ == Self.
707 if (_succ == Self) _succ = NULL;
708
709 // Invariant: after clearing _succ a contending thread
710 // *must* retry _owner before parking.
711 OrderAccess::fence();
712
713 // This PerfData object can be used in parallel with a safepoint.
714 // See the work around in PerfDataManager::destroy().
715 OM_PERFDATA_OP(FutileWakeups, inc());
716 }
717
718 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
719 // Normally we'll find Self on the EntryList.
720 // Unlinking from the EntryList is constant-time and atomic-free.
721 // From the perspective of the lock owner (this thread), the
722 // EntryList is stable and cxq is prepend-only.
723 // The head of cxq is volatile but the interior is stable.
724 // In addition, Self.TState is stable.
725
726 assert(_owner == Self, "invariant");
727 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
728 UnlinkAfterAcquire(Self, SelfNode);
729 if (_succ == Self) _succ = NULL;
730 assert(_succ != Self, "invariant");
731 SelfNode->TState = ObjectWaiter::TS_RUN;
732 OrderAccess::fence(); // see comments at the end of EnterI()
733 }
734
735 // By convention we unlink a contending thread from EntryList|cxq immediately
736 // after the thread acquires the lock in ::enter(). Equally, we could defer
737 // unlinking the thread until ::exit()-time.
738
739 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
740 assert(_owner == Self, "invariant");
741 assert(SelfNode->_thread == Self, "invariant");
742
743 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
744 // Normal case: remove Self from the DLL EntryList .
745 // This is a constant-time operation.
746 ObjectWaiter * nxt = SelfNode->_next;
747 ObjectWaiter * prv = SelfNode->_prev;
748 if (nxt != NULL) nxt->_prev = prv;
749 if (prv != NULL) prv->_next = nxt;
750 if (SelfNode == _EntryList) _EntryList = nxt;
751 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
752 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
753 } else {
754 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
755 // Inopportune interleaving -- Self is still on the cxq.
756 // This usually means the enqueue of self raced an exiting thread.
757 // Normally we'll find Self near the front of the cxq, so
758 // dequeueing is typically fast. If needbe we can accelerate
759 // this with some MCS/CHL-like bidirectional list hints and advisory
760 // back-links so dequeueing from the interior will normally operate
761 // in constant-time.
762 // Dequeue Self from either the head (with CAS) or from the interior
763 // with a linear-time scan and normal non-atomic memory operations.
764 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
765 // and then unlink Self from EntryList. We have to drain eventually,
766 // so it might as well be now.
767
768 ObjectWaiter * v = _cxq;
769 assert(v != NULL, "invariant");
770 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
771 // The CAS above can fail from interference IFF a "RAT" arrived.
772 // In that case Self must be in the interior and can no longer be
773 // at the head of cxq.
774 if (v == SelfNode) {
775 assert(_cxq != v, "invariant");
776 v = _cxq; // CAS above failed - start scan at head of list
777 }
778 ObjectWaiter * p;
779 ObjectWaiter * q = NULL;
780 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
781 q = p;
782 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
783 }
784 assert(v != SelfNode, "invariant");
785 assert(p == SelfNode, "Node not found on cxq");
786 assert(p != _cxq, "invariant");
787 assert(q != NULL, "invariant");
788 assert(q->_next == p, "invariant");
789 q->_next = p->_next;
790 }
791 }
792
793 #ifdef ASSERT
794 // Diagnostic hygiene ...
795 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
796 SelfNode->_next = (ObjectWaiter *) 0xBAD;
797 SelfNode->TState = ObjectWaiter::TS_RUN;
798 #endif
799 }
800
801 // -----------------------------------------------------------------------------
802 // Exit support
803 //
804 // exit()
805 // ~~~~~~
806 // Note that the collector can't reclaim the objectMonitor or deflate
807 // the object out from underneath the thread calling ::exit() as the
808 // thread calling ::exit() never transitions to a stable state.
809 // This inhibits GC, which in turn inhibits asynchronous (and
810 // inopportune) reclamation of "this".
811 //
812 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
813 // There's one exception to the claim above, however. EnterI() can call
814 // exit() to drop a lock if the acquirer has been externally suspended.
815 // In that case exit() is called with _thread_state as _thread_blocked,
816 // but the monitor's _contentions field is > 0, which inhibits reclamation.
817 //
818 // 1-0 exit
819 // ~~~~~~~~
820 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
821 // the fast-path operators have been optimized so the common ::exit()
822 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
823 // The code emitted by fast_unlock() elides the usual MEMBAR. This
824 // greatly improves latency -- MEMBAR and CAS having considerable local
825 // latency on modern processors -- but at the cost of "stranding". Absent the
826 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
827 // ::enter() path, resulting in the entering thread being stranding
828 // and a progress-liveness failure. Stranding is extremely rare.
829 // We use timers (timed park operations) & periodic polling to detect
830 // and recover from stranding. Potentially stranded threads periodically
831 // wake up and poll the lock. See the usage of the _Responsible variable.
832 //
833 // The CAS() in enter provides for safety and exclusion, while the CAS or
834 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
835 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
836 // We detect and recover from stranding with timers.
837 //
838 // If a thread transiently strands it'll park until (a) another
839 // thread acquires the lock and then drops the lock, at which time the
840 // exiting thread will notice and unpark the stranded thread, or, (b)
841 // the timer expires. If the lock is high traffic then the stranding latency
842 // will be low due to (a). If the lock is low traffic then the odds of
843 // stranding are lower, although the worst-case stranding latency
844 // is longer. Critically, we don't want to put excessive load in the
845 // platform's timer subsystem. We want to minimize both the timer injection
846 // rate (timers created/sec) as well as the number of timers active at
847 // any one time. (more precisely, we want to minimize timer-seconds, which is
848 // the integral of the # of active timers at any instant over time).
849 // Both impinge on OS scalability. Given that, at most one thread parked on
850 // a monitor will use a timer.
851 //
852 // There is also the risk of a futile wake-up. If we drop the lock
853 // another thread can reacquire the lock immediately, and we can
854 // then wake a thread unnecessarily. This is benign, and we've
855 // structured the code so the windows are short and the frequency
856 // of such futile wakups is low.
857
858 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
859 Thread * const Self = THREAD;
860 if (THREAD != _owner) {
861 if (THREAD->is_lock_owned((address) _owner)) {
862 // Transmute _owner from a BasicLock pointer to a Thread address.
863 // We don't need to hold _mutex for this transition.
864 // Non-null to Non-null is safe as long as all readers can
865 // tolerate either flavor.
866 assert(_recursions == 0, "invariant");
867 _owner = THREAD;
868 _recursions = 0;
869 } else {
870 // Apparent unbalanced locking ...
871 // Naively we'd like to throw IllegalMonitorStateException.
872 // As a practical matter we can neither allocate nor throw an
873 // exception as ::exit() can be called from leaf routines.
874 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
875 // Upon deeper reflection, however, in a properly run JVM the only
876 // way we should encounter this situation is in the presence of
877 // unbalanced JNI locking. TODO: CheckJNICalls.
878 // See also: CR4414101
879 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
880 return;
881 }
882 }
883
884 if (_recursions != 0) {
885 _recursions--; // this is simple recursive enter
886 return;
887 }
888
889 // Invariant: after setting Responsible=null an thread must execute
890 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
891 _Responsible = NULL;
892
893 #if INCLUDE_JFR
894 // get the owner's thread id for the MonitorEnter event
895 // if it is enabled and the thread isn't suspended
896 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
897 _previous_owner_tid = JFR_THREAD_ID(Self);
898 }
899 #endif
900
901 for (;;) {
902 assert(THREAD == _owner, "invariant");
903
904 // release semantics: prior loads and stores from within the critical section
905 // must not float (reorder) past the following store that drops the lock.
906 // On SPARC that requires MEMBAR #loadstore|#storestore.
907 // But of course in TSO #loadstore|#storestore is not required.
908 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
909 OrderAccess::storeload(); // See if we need to wake a successor
910 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
911 return;
912 }
913 // Other threads are blocked trying to acquire the lock.
914
915 // Normally the exiting thread is responsible for ensuring succession,
916 // but if other successors are ready or other entering threads are spinning
917 // then this thread can simply store NULL into _owner and exit without
918 // waking a successor. The existence of spinners or ready successors
919 // guarantees proper succession (liveness). Responsibility passes to the
920 // ready or running successors. The exiting thread delegates the duty.
921 // More precisely, if a successor already exists this thread is absolved
922 // of the responsibility of waking (unparking) one.
923 //
924 // The _succ variable is critical to reducing futile wakeup frequency.
925 // _succ identifies the "heir presumptive" thread that has been made
926 // ready (unparked) but that has not yet run. We need only one such
927 // successor thread to guarantee progress.
928 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
929 // section 3.3 "Futile Wakeup Throttling" for details.
930 //
931 // Note that spinners in Enter() also set _succ non-null.
932 // In the current implementation spinners opportunistically set
933 // _succ so that exiting threads might avoid waking a successor.
934 // Another less appealing alternative would be for the exiting thread
935 // to drop the lock and then spin briefly to see if a spinner managed
936 // to acquire the lock. If so, the exiting thread could exit
937 // immediately without waking a successor, otherwise the exiting
938 // thread would need to dequeue and wake a successor.
939 // (Note that we'd need to make the post-drop spin short, but no
940 // shorter than the worst-case round-trip cache-line migration time.
941 // The dropped lock needs to become visible to the spinner, and then
942 // the acquisition of the lock by the spinner must become visible to
943 // the exiting thread).
944
945 // It appears that an heir-presumptive (successor) must be made ready.
946 // Only the current lock owner can manipulate the EntryList or
947 // drain _cxq, so we need to reacquire the lock. If we fail
948 // to reacquire the lock the responsibility for ensuring succession
949 // falls to the new owner.
950 //
951 if (!Atomic::replace_if_null(THREAD, &_owner)) {
952 return;
953 }
954
955 guarantee(_owner == THREAD, "invariant");
956
957 ObjectWaiter * w = NULL;
958
959 w = _EntryList;
960 if (w != NULL) {
961 // I'd like to write: guarantee (w->_thread != Self).
962 // But in practice an exiting thread may find itself on the EntryList.
963 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
964 // then calls exit(). Exit release the lock by setting O._owner to NULL.
965 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
966 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
967 // release the lock "O". T2 resumes immediately after the ST of null into
968 // _owner, above. T2 notices that the EntryList is populated, so it
969 // reacquires the lock and then finds itself on the EntryList.
970 // Given all that, we have to tolerate the circumstance where "w" is
971 // associated with Self.
972 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
973 ExitEpilog(Self, w);
974 return;
975 }
976
977 // If we find that both _cxq and EntryList are null then just
978 // re-run the exit protocol from the top.
979 w = _cxq;
980 if (w == NULL) continue;
981
982 // Drain _cxq into EntryList - bulk transfer.
983 // First, detach _cxq.
984 // The following loop is tantamount to: w = swap(&cxq, NULL)
985 for (;;) {
986 assert(w != NULL, "Invariant");
987 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
988 if (u == w) break;
989 w = u;
990 }
991
992 assert(w != NULL, "invariant");
993 assert(_EntryList == NULL, "invariant");
994
995 // Convert the LIFO SLL anchored by _cxq into a DLL.
996 // The list reorganization step operates in O(LENGTH(w)) time.
997 // It's critical that this step operate quickly as
998 // "Self" still holds the outer-lock, restricting parallelism
999 // and effectively lengthening the critical section.
1000 // Invariant: s chases t chases u.
1001 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1002 // we have faster access to the tail.
1003
1004 _EntryList = w;
1005 ObjectWaiter * q = NULL;
1006 ObjectWaiter * p;
1007 for (p = w; p != NULL; p = p->_next) {
1008 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1009 p->TState = ObjectWaiter::TS_ENTER;
1010 p->_prev = q;
1011 q = p;
1012 }
1013
1014 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1015 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1016
1017 // See if we can abdicate to a spinner instead of waking a thread.
1018 // A primary goal of the implementation is to reduce the
1019 // context-switch rate.
1020 if (_succ != NULL) continue;
1021
1022 w = _EntryList;
1023 if (w != NULL) {
1024 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1025 ExitEpilog(Self, w);
1026 return;
1027 }
1028 }
1029 }
1030
1031 // ExitSuspendEquivalent:
1032 // A faster alternate to handle_special_suspend_equivalent_condition()
1033 //
1034 // handle_special_suspend_equivalent_condition() unconditionally
1035 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1036 // operations have high latency. Note that in ::enter() we call HSSEC
1037 // while holding the monitor, so we effectively lengthen the critical sections.
1038 //
1039 // There are a number of possible solutions:
1040 //
1041 // A. To ameliorate the problem we might also defer state transitions
1042 // to as late as possible -- just prior to parking.
1043 // Given that, we'd call HSSEC after having returned from park(),
1044 // but before attempting to acquire the monitor. This is only a
1045 // partial solution. It avoids calling HSSEC while holding the
1046 // monitor (good), but it still increases successor reacquisition latency --
1047 // the interval between unparking a successor and the time the successor
1048 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1049 // If we use this technique we can also avoid EnterI()-exit() loop
1050 // in ::enter() where we iteratively drop the lock and then attempt
1051 // to reacquire it after suspending.
1052 //
1053 // B. In the future we might fold all the suspend bits into a
1054 // composite per-thread suspend flag and then update it with CAS().
1055 // Alternately, a Dekker-like mechanism with multiple variables
1056 // would suffice:
1057 // ST Self->_suspend_equivalent = false
1058 // MEMBAR
1059 // LD Self_>_suspend_flags
1060
1061 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1062 return jSelf->handle_special_suspend_equivalent_condition();
1063 }
1064
1065
1066 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1067 assert(_owner == Self, "invariant");
1068
1069 // Exit protocol:
1070 // 1. ST _succ = wakee
1071 // 2. membar #loadstore|#storestore;
1072 // 2. ST _owner = NULL
1073 // 3. unpark(wakee)
1074
1075 _succ = Wakee->_thread;
1076 ParkEvent * Trigger = Wakee->_event;
1077
1078 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1079 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1080 // out-of-scope (non-extant).
1081 Wakee = NULL;
1082
1083 // Drop the lock
1084 OrderAccess::release_store(&_owner, (void*)NULL);
1085 OrderAccess::fence(); // ST _owner vs LD in unpark()
1086
1087 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1088 Trigger->unpark();
1089
1090 // Maintain stats and report events to JVMTI
1091 OM_PERFDATA_OP(Parks, inc());
1092 }
1093
1094
1095 // -----------------------------------------------------------------------------
1096 // Class Loader deadlock handling.
1097 //
1098 // complete_exit exits a lock returning recursion count
1099 // complete_exit/reenter operate as a wait without waiting
1100 // complete_exit requires an inflated monitor
1101 // The _owner field is not always the Thread addr even with an
1102 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1103 // thread due to contention.
1104 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1105 Thread * const Self = THREAD;
1106 assert(Self->is_Java_thread(), "Must be Java thread!");
1107 JavaThread *jt = (JavaThread *)THREAD;
1108
1109 assert(InitDone, "Unexpectedly not initialized");
1110
1111 if (THREAD != _owner) {
1112 if (THREAD->is_lock_owned ((address)_owner)) {
1113 assert(_recursions == 0, "internal state error");
1114 _owner = THREAD; // Convert from basiclock addr to Thread addr
1115 _recursions = 0;
1116 }
1117 }
1118
1119 guarantee(Self == _owner, "complete_exit not owner");
1120 intptr_t save = _recursions; // record the old recursion count
1121 _recursions = 0; // set the recursion level to be 0
1122 exit(true, Self); // exit the monitor
1123 guarantee(_owner != Self, "invariant");
1124 return save;
1125 }
1126
1127 // reenter() enters a lock and sets recursion count
1128 // complete_exit/reenter operate as a wait without waiting
1129 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1130 Thread * const Self = THREAD;
1131 assert(Self->is_Java_thread(), "Must be Java thread!");
1132 JavaThread *jt = (JavaThread *)THREAD;
1133
1134 guarantee(_owner != Self, "reenter already owner");
1135 enter(THREAD); // enter the monitor
1136 guarantee(_recursions == 0, "reenter recursion");
1137 _recursions = recursions;
1138 return;
1139 }
1140
1141
1142 // -----------------------------------------------------------------------------
1143 // A macro is used below because there may already be a pending
1144 // exception which should not abort the execution of the routines
1145 // which use this (which is why we don't put this into check_slow and
1146 // call it with a CHECK argument).
1147
1148 #define CHECK_OWNER() \
1149 do { \
1150 if (THREAD != _owner) { \
1151 if (THREAD->is_lock_owned((address) _owner)) { \
1152 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1153 _recursions = 0; \
1154 } else { \
1155 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1156 } \
1157 } \
1158 } while (false)
1159
1160 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1161 // TODO-FIXME: remove check_slow() -- it's likely dead.
1162
1163 void ObjectMonitor::check_slow(TRAPS) {
1164 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1165 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1166 }
1167
1168 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1169 ObjectMonitor* monitor,
1170 jlong notifier_tid,
1171 jlong timeout,
1172 bool timedout) {
1173 assert(event != NULL, "invariant");
1174 assert(monitor != NULL, "invariant");
1175 event->set_monitorClass(((oop)monitor->object())->klass());
1176 event->set_timeout(timeout);
1177 event->set_address((uintptr_t)monitor->object_addr());
1178 event->set_notifier(notifier_tid);
1179 event->set_timedOut(timedout);
1180 event->commit();
1181 }
1182
1183 // -----------------------------------------------------------------------------
1184 // Wait/Notify/NotifyAll
1185 //
1186 // Note: a subset of changes to ObjectMonitor::wait()
1187 // will need to be replicated in complete_exit
1188 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1189 Thread * const Self = THREAD;
1190 assert(Self->is_Java_thread(), "Must be Java thread!");
1191 JavaThread *jt = (JavaThread *)THREAD;
1192
1193 assert(InitDone, "Unexpectedly not initialized");
1194
1195 // Throw IMSX or IEX.
1196 CHECK_OWNER();
1197
1198 EventJavaMonitorWait event;
1199
1200 // check for a pending interrupt
1201 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1202 // post monitor waited event. Note that this is past-tense, we are done waiting.
1203 if (JvmtiExport::should_post_monitor_waited()) {
1204 // Note: 'false' parameter is passed here because the
1205 // wait was not timed out due to thread interrupt.
1206 JvmtiExport::post_monitor_waited(jt, this, false);
1207
1208 // In this short circuit of the monitor wait protocol, the
1209 // current thread never drops ownership of the monitor and
1210 // never gets added to the wait queue so the current thread
1211 // cannot be made the successor. This means that the
1212 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1213 // consume an unpark() meant for the ParkEvent associated with
1214 // this ObjectMonitor.
1215 }
1216 if (event.should_commit()) {
1217 post_monitor_wait_event(&event, this, 0, millis, false);
1218 }
1219 THROW(vmSymbols::java_lang_InterruptedException());
1220 return;
1221 }
1222
1223 assert(Self->_Stalled == 0, "invariant");
1224 Self->_Stalled = intptr_t(this);
1225 jt->set_current_waiting_monitor(this);
1226
1227 // create a node to be put into the queue
1228 // Critically, after we reset() the event but prior to park(), we must check
1229 // for a pending interrupt.
1230 ObjectWaiter node(Self);
1231 node.TState = ObjectWaiter::TS_WAIT;
1232 Self->_ParkEvent->reset();
1233 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1234
1235 // Enter the waiting queue, which is a circular doubly linked list in this case
1236 // but it could be a priority queue or any data structure.
1237 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1238 // by the the owner of the monitor *except* in the case where park()
1239 // returns because of a timeout of interrupt. Contention is exceptionally rare
1240 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1241
1242 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1243 AddWaiter(&node);
1244 Thread::SpinRelease(&_WaitSetLock);
1245
1246 _Responsible = NULL;
1247
1248 intptr_t save = _recursions; // record the old recursion count
1249 _waiters++; // increment the number of waiters
1250 _recursions = 0; // set the recursion level to be 1
1251 exit(true, Self); // exit the monitor
1252 guarantee(_owner != Self, "invariant");
1253
1254 // The thread is on the WaitSet list - now park() it.
1255 // On MP systems it's conceivable that a brief spin before we park
1256 // could be profitable.
1257 //
1258 // TODO-FIXME: change the following logic to a loop of the form
1259 // while (!timeout && !interrupted && _notified == 0) park()
1260
1261 int ret = OS_OK;
1262 int WasNotified = 0;
1263 { // State transition wrappers
1264 OSThread* osthread = Self->osthread();
1265 OSThreadWaitState osts(osthread, true);
1266 {
1267 ThreadBlockInVM tbivm(jt);
1268 // Thread is in thread_blocked state and oop access is unsafe.
1269 jt->set_suspend_equivalent();
1270
1271 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1272 // Intentionally empty
1273 } else if (node._notified == 0) {
1274 if (millis <= 0) {
1275 Self->_ParkEvent->park();
1276 } else {
1277 ret = Self->_ParkEvent->park(millis);
1278 }
1279 }
1280
1281 // were we externally suspended while we were waiting?
1282 if (ExitSuspendEquivalent (jt)) {
1283 // TODO-FIXME: add -- if succ == Self then succ = null.
1284 jt->java_suspend_self();
1285 }
1286
1287 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1288
1289 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1290 // from the WaitSet to the EntryList.
1291 // See if we need to remove Node from the WaitSet.
1292 // We use double-checked locking to avoid grabbing _WaitSetLock
1293 // if the thread is not on the wait queue.
1294 //
1295 // Note that we don't need a fence before the fetch of TState.
1296 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1297 // written by the is thread. (perhaps the fetch might even be satisfied
1298 // by a look-aside into the processor's own store buffer, although given
1299 // the length of the code path between the prior ST and this load that's
1300 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1301 // then we'll acquire the lock and then re-fetch a fresh TState value.
1302 // That is, we fail toward safety.
1303
1304 if (node.TState == ObjectWaiter::TS_WAIT) {
1305 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1306 if (node.TState == ObjectWaiter::TS_WAIT) {
1307 DequeueSpecificWaiter(&node); // unlink from WaitSet
1308 assert(node._notified == 0, "invariant");
1309 node.TState = ObjectWaiter::TS_RUN;
1310 }
1311 Thread::SpinRelease(&_WaitSetLock);
1312 }
1313
1314 // The thread is now either on off-list (TS_RUN),
1315 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1316 // The Node's TState variable is stable from the perspective of this thread.
1317 // No other threads will asynchronously modify TState.
1318 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1319 OrderAccess::loadload();
1320 if (_succ == Self) _succ = NULL;
1321 WasNotified = node._notified;
1322
1323 // Reentry phase -- reacquire the monitor.
1324 // re-enter contended monitor after object.wait().
1325 // retain OBJECT_WAIT state until re-enter successfully completes
1326 // Thread state is thread_in_vm and oop access is again safe,
1327 // although the raw address of the object may have changed.
1328 // (Don't cache naked oops over safepoints, of course).
1329
1330 // post monitor waited event. Note that this is past-tense, we are done waiting.
1331 if (JvmtiExport::should_post_monitor_waited()) {
1332 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1333
1334 if (node._notified != 0 && _succ == Self) {
1335 // In this part of the monitor wait-notify-reenter protocol it
1336 // is possible (and normal) for another thread to do a fastpath
1337 // monitor enter-exit while this thread is still trying to get
1338 // to the reenter portion of the protocol.
1339 //
1340 // The ObjectMonitor was notified and the current thread is
1341 // the successor which also means that an unpark() has already
1342 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1343 // consume the unpark() that was done when the successor was
1344 // set because the same ParkEvent is shared between Java
1345 // monitors and JVM/TI RawMonitors (for now).
1346 //
1347 // We redo the unpark() to ensure forward progress, i.e., we
1348 // don't want all pending threads hanging (parked) with none
1349 // entering the unlocked monitor.
1350 node._event->unpark();
1351 }
1352 }
1353
1354 if (event.should_commit()) {
1355 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1356 }
1357
1358 OrderAccess::fence();
1359
1360 assert(Self->_Stalled != 0, "invariant");
1361 Self->_Stalled = 0;
1362
1363 assert(_owner != Self, "invariant");
1364 ObjectWaiter::TStates v = node.TState;
1365 if (v == ObjectWaiter::TS_RUN) {
1366 enter(Self);
1367 } else {
1368 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1369 ReenterI(Self, &node);
1370 node.wait_reenter_end(this);
1371 }
1372
1373 // Self has reacquired the lock.
1374 // Lifecycle - the node representing Self must not appear on any queues.
1375 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1376 // want residual elements associated with this thread left on any lists.
1377 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1378 assert(_owner == Self, "invariant");
1379 assert(_succ != Self, "invariant");
1380 } // OSThreadWaitState()
1381
1382 jt->set_current_waiting_monitor(NULL);
1383
1384 guarantee(_recursions == 0, "invariant");
1385 _recursions = save; // restore the old recursion count
1386 _waiters--; // decrement the number of waiters
1387
1388 // Verify a few postconditions
1389 assert(_owner == Self, "invariant");
1390 assert(_succ != Self, "invariant");
1391 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1392
1393 // check if the notification happened
1394 if (!WasNotified) {
1395 // no, it could be timeout or Thread.interrupt() or both
1396 // check for interrupt event, otherwise it is timeout
1397 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1398 THROW(vmSymbols::java_lang_InterruptedException());
1399 }
1400 }
1401
1402 // NOTE: Spurious wake up will be consider as timeout.
1403 // Monitor notify has precedence over thread interrupt.
1404 }
1405
1406
1407 // Consider:
1408 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1409 // then instead of transferring a thread from the WaitSet to the EntryList
1410 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1411
1412 void ObjectMonitor::INotify(Thread * Self) {
1413 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1414 ObjectWaiter * iterator = DequeueWaiter();
1415 if (iterator != NULL) {
1416 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1417 guarantee(iterator->_notified == 0, "invariant");
1418 // Disposition - what might we do with iterator ?
1419 // a. add it directly to the EntryList - either tail (policy == 1)
1420 // or head (policy == 0).
1421 // b. push it onto the front of the _cxq (policy == 2).
1422 // For now we use (b).
1423
1424 iterator->TState = ObjectWaiter::TS_ENTER;
1425
1426 iterator->_notified = 1;
1427 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1428
1429 ObjectWaiter * list = _EntryList;
1430 if (list != NULL) {
1431 assert(list->_prev == NULL, "invariant");
1432 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1433 assert(list != iterator, "invariant");
1434 }
1435
1436 // prepend to cxq
1437 if (list == NULL) {
1438 iterator->_next = iterator->_prev = NULL;
1439 _EntryList = iterator;
1440 } else {
1441 iterator->TState = ObjectWaiter::TS_CXQ;
1442 for (;;) {
1443 ObjectWaiter * front = _cxq;
1444 iterator->_next = front;
1445 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1446 break;
1447 }
1448 }
1449 }
1450
1451 // _WaitSetLock protects the wait queue, not the EntryList. We could
1452 // move the add-to-EntryList operation, above, outside the critical section
1453 // protected by _WaitSetLock. In practice that's not useful. With the
1454 // exception of wait() timeouts and interrupts the monitor owner
1455 // is the only thread that grabs _WaitSetLock. There's almost no contention
1456 // on _WaitSetLock so it's not profitable to reduce the length of the
1457 // critical section.
1458
1459 iterator->wait_reenter_begin(this);
1460 }
1461 Thread::SpinRelease(&_WaitSetLock);
1462 }
1463
1464 // Consider: a not-uncommon synchronization bug is to use notify() when
1465 // notifyAll() is more appropriate, potentially resulting in stranded
1466 // threads; this is one example of a lost wakeup. A useful diagnostic
1467 // option is to force all notify() operations to behave as notifyAll().
1468 //
1469 // Note: We can also detect many such problems with a "minimum wait".
1470 // When the "minimum wait" is set to a small non-zero timeout value
1471 // and the program does not hang whereas it did absent "minimum wait",
1472 // that suggests a lost wakeup bug.
1473
1474 void ObjectMonitor::notify(TRAPS) {
1475 CHECK_OWNER();
1476 if (_WaitSet == NULL) {
1477 return;
1478 }
1479 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1480 INotify(THREAD);
1481 OM_PERFDATA_OP(Notifications, inc(1));
1482 }
1483
1484
1485 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1486 // from the waitset to the EntryList. This could be done more efficiently with a
1487 // single bulk transfer but in practice it's not time-critical. Beware too,
1488 // that in prepend-mode we invert the order of the waiters. Let's say that the
1489 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1490 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1491
1492 void ObjectMonitor::notifyAll(TRAPS) {
1493 CHECK_OWNER();
1494 if (_WaitSet == NULL) {
1495 return;
1496 }
1497
1498 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1499 int tally = 0;
1500 while (_WaitSet != NULL) {
1501 tally++;
1502 INotify(THREAD);
1503 }
1504
1505 OM_PERFDATA_OP(Notifications, inc(tally));
1506 }
1507
1508 // -----------------------------------------------------------------------------
1509 // Adaptive Spinning Support
1510 //
1511 // Adaptive spin-then-block - rational spinning
1512 //
1513 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1514 // algorithm. On high order SMP systems it would be better to start with
1515 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1516 // a contending thread could enqueue itself on the cxq and then spin locally
1517 // on a thread-specific variable such as its ParkEvent._Event flag.
1518 // That's left as an exercise for the reader. Note that global spinning is
1519 // not problematic on Niagara, as the L2 cache serves the interconnect and
1520 // has both low latency and massive bandwidth.
1521 //
1522 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1523 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1524 // (duration) or we can fix the count at approximately the duration of
1525 // a context switch and vary the frequency. Of course we could also
1526 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1527 // For a description of 'Adaptive spin-then-block mutual exclusion in
1528 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1529 //
1530 // This implementation varies the duration "D", where D varies with
1531 // the success rate of recent spin attempts. (D is capped at approximately
1532 // length of a round-trip context switch). The success rate for recent
1533 // spin attempts is a good predictor of the success rate of future spin
1534 // attempts. The mechanism adapts automatically to varying critical
1535 // section length (lock modality), system load and degree of parallelism.
1536 // D is maintained per-monitor in _SpinDuration and is initialized
1537 // optimistically. Spin frequency is fixed at 100%.
1538 //
1539 // Note that _SpinDuration is volatile, but we update it without locks
1540 // or atomics. The code is designed so that _SpinDuration stays within
1541 // a reasonable range even in the presence of races. The arithmetic
1542 // operations on _SpinDuration are closed over the domain of legal values,
1543 // so at worst a race will install and older but still legal value.
1544 // At the very worst this introduces some apparent non-determinism.
1545 // We might spin when we shouldn't or vice-versa, but since the spin
1546 // count are relatively short, even in the worst case, the effect is harmless.
1547 //
1548 // Care must be taken that a low "D" value does not become an
1549 // an absorbing state. Transient spinning failures -- when spinning
1550 // is overall profitable -- should not cause the system to converge
1551 // on low "D" values. We want spinning to be stable and predictable
1552 // and fairly responsive to change and at the same time we don't want
1553 // it to oscillate, become metastable, be "too" non-deterministic,
1554 // or converge on or enter undesirable stable absorbing states.
1555 //
1556 // We implement a feedback-based control system -- using past behavior
1557 // to predict future behavior. We face two issues: (a) if the
1558 // input signal is random then the spin predictor won't provide optimal
1559 // results, and (b) if the signal frequency is too high then the control
1560 // system, which has some natural response lag, will "chase" the signal.
1561 // (b) can arise from multimodal lock hold times. Transient preemption
1562 // can also result in apparent bimodal lock hold times.
1563 // Although sub-optimal, neither condition is particularly harmful, as
1564 // in the worst-case we'll spin when we shouldn't or vice-versa.
1565 // The maximum spin duration is rather short so the failure modes aren't bad.
1566 // To be conservative, I've tuned the gain in system to bias toward
1567 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1568 // "rings" or oscillates between spinning and not spinning. This happens
1569 // when spinning is just on the cusp of profitability, however, so the
1570 // situation is not dire. The state is benign -- there's no need to add
1571 // hysteresis control to damp the transition rate between spinning and
1572 // not spinning.
1573
1574 // Spinning: Fixed frequency (100%), vary duration
1575 int ObjectMonitor::TrySpin(Thread * Self) {
1576 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1577 int ctr = Knob_FixedSpin;
1578 if (ctr != 0) {
1579 while (--ctr >= 0) {
1580 if (TryLock(Self) > 0) return 1;
1581 SpinPause();
1582 }
1583 return 0;
1584 }
1585
1586 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1587 if (TryLock(Self) > 0) {
1588 // Increase _SpinDuration ...
1589 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1590 // Raising _SpurDuration to the poverty line is key.
1591 int x = _SpinDuration;
1592 if (x < Knob_SpinLimit) {
1593 if (x < Knob_Poverty) x = Knob_Poverty;
1594 _SpinDuration = x + Knob_BonusB;
1595 }
1596 return 1;
1597 }
1598 SpinPause();
1599 }
1600
1601 // Admission control - verify preconditions for spinning
1602 //
1603 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1604 // becoming an absorbing state. Put another way, we spin briefly to
1605 // sample, just in case the system load, parallelism, contention, or lock
1606 // modality changed.
1607 //
1608 // Consider the following alternative:
1609 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1610 // spin attempt. "Periodically" might mean after a tally of
1611 // the # of failed spin attempts (or iterations) reaches some threshold.
1612 // This takes us into the realm of 1-out-of-N spinning, where we
1613 // hold the duration constant but vary the frequency.
1614
1615 ctr = _SpinDuration;
1616 if (ctr <= 0) return 0;
1617
1618 if (NotRunnable(Self, (Thread *) _owner)) {
1619 return 0;
1620 }
1621
1622 // We're good to spin ... spin ingress.
1623 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1624 // when preparing to LD...CAS _owner, etc and the CAS is likely
1625 // to succeed.
1626 if (_succ == NULL) {
1627 _succ = Self;
1628 }
1629 Thread * prv = NULL;
1630
1631 // There are three ways to exit the following loop:
1632 // 1. A successful spin where this thread has acquired the lock.
1633 // 2. Spin failure with prejudice
1634 // 3. Spin failure without prejudice
1635
1636 while (--ctr >= 0) {
1637
1638 // Periodic polling -- Check for pending GC
1639 // Threads may spin while they're unsafe.
1640 // We don't want spinning threads to delay the JVM from reaching
1641 // a stop-the-world safepoint or to steal cycles from GC.
1642 // If we detect a pending safepoint we abort in order that
1643 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1644 // this thread, if safe, doesn't steal cycles from GC.
1645 // This is in keeping with the "no loitering in runtime" rule.
1646 // We periodically check to see if there's a safepoint pending.
1647 if ((ctr & 0xFF) == 0) {
1648 if (SafepointMechanism::should_block(Self)) {
1649 goto Abort; // abrupt spin egress
1650 }
1651 SpinPause();
1652 }
1653
1654 // Probe _owner with TATAS
1655 // If this thread observes the monitor transition or flicker
1656 // from locked to unlocked to locked, then the odds that this
1657 // thread will acquire the lock in this spin attempt go down
1658 // considerably. The same argument applies if the CAS fails
1659 // or if we observe _owner change from one non-null value to
1660 // another non-null value. In such cases we might abort
1661 // the spin without prejudice or apply a "penalty" to the
1662 // spin count-down variable "ctr", reducing it by 100, say.
1663
1664 Thread * ox = (Thread *) _owner;
1665 if (ox == NULL) {
1666 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1667 if (ox == NULL) {
1668 // The CAS succeeded -- this thread acquired ownership
1669 // Take care of some bookkeeping to exit spin state.
1670 if (_succ == Self) {
1671 _succ = NULL;
1672 }
1673
1674 // Increase _SpinDuration :
1675 // The spin was successful (profitable) so we tend toward
1676 // longer spin attempts in the future.
1677 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1678 // If we acquired the lock early in the spin cycle it
1679 // makes sense to increase _SpinDuration proportionally.
1680 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1681 int x = _SpinDuration;
1682 if (x < Knob_SpinLimit) {
1683 if (x < Knob_Poverty) x = Knob_Poverty;
1684 _SpinDuration = x + Knob_Bonus;
1685 }
1686 return 1;
1687 }
1688
1689 // The CAS failed ... we can take any of the following actions:
1690 // * penalize: ctr -= CASPenalty
1691 // * exit spin with prejudice -- goto Abort;
1692 // * exit spin without prejudice.
1693 // * Since CAS is high-latency, retry again immediately.
1694 prv = ox;
1695 goto Abort;
1696 }
1697
1698 // Did lock ownership change hands ?
1699 if (ox != prv && prv != NULL) {
1700 goto Abort;
1701 }
1702 prv = ox;
1703
1704 // Abort the spin if the owner is not executing.
1705 // The owner must be executing in order to drop the lock.
1706 // Spinning while the owner is OFFPROC is idiocy.
1707 // Consider: ctr -= RunnablePenalty ;
1708 if (NotRunnable(Self, ox)) {
1709 goto Abort;
1710 }
1711 if (_succ == NULL) {
1712 _succ = Self;
1713 }
1714 }
1715
1716 // Spin failed with prejudice -- reduce _SpinDuration.
1717 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1718 // AIMD is globally stable.
1719 {
1720 int x = _SpinDuration;
1721 if (x > 0) {
1722 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1723 // This is globally sample and tends to damp the response.
1724 x -= Knob_Penalty;
1725 if (x < 0) x = 0;
1726 _SpinDuration = x;
1727 }
1728 }
1729
1730 Abort:
1731 if (_succ == Self) {
1732 _succ = NULL;
1733 // Invariant: after setting succ=null a contending thread
1734 // must recheck-retry _owner before parking. This usually happens
1735 // in the normal usage of TrySpin(), but it's safest
1736 // to make TrySpin() as foolproof as possible.
1737 OrderAccess::fence();
1738 if (TryLock(Self) > 0) return 1;
1739 }
1740 return 0;
1741 }
1742
1743 // NotRunnable() -- informed spinning
1744 //
1745 // Don't bother spinning if the owner is not eligible to drop the lock.
1746 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1747 // The thread must be runnable in order to drop the lock in timely fashion.
1748 // If the _owner is not runnable then spinning will not likely be
1749 // successful (profitable).
1750 //
1751 // Beware -- the thread referenced by _owner could have died
1752 // so a simply fetch from _owner->_thread_state might trap.
1753 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1754 // Because of the lifecycle issues, the _thread_state values
1755 // observed by NotRunnable() might be garbage. NotRunnable must
1756 // tolerate this and consider the observed _thread_state value
1757 // as advisory.
1758 //
1759 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1760 // a thread pointer.
1761 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1762 // with the LSB of _owner. Another option would be to probabilistically probe
1763 // the putative _owner->TypeTag value.
1764 //
1765 // Checking _thread_state isn't perfect. Even if the thread is
1766 // in_java it might be blocked on a page-fault or have been preempted
1767 // and sitting on a ready/dispatch queue.
1768 //
1769 // The return value from NotRunnable() is *advisory* -- the
1770 // result is based on sampling and is not necessarily coherent.
1771 // The caller must tolerate false-negative and false-positive errors.
1772 // Spinning, in general, is probabilistic anyway.
1773
1774
1775 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1776 // Check ox->TypeTag == 2BAD.
1777 if (ox == NULL) return 0;
1778
1779 // Avoid transitive spinning ...
1780 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
1781 // Immediately after T1 acquires L it's possible that T2, also
1782 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1783 // This occurs transiently after T1 acquired L but before
1784 // T1 managed to clear T1.Stalled. T2 does not need to abort
1785 // its spin in this circumstance.
1786 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1787
1788 if (BlockedOn == 1) return 1;
1789 if (BlockedOn != 0) {
1790 return BlockedOn != intptr_t(this) && _owner == ox;
1791 }
1792
1793 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1794 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1795 // consider also: jst != _thread_in_Java -- but that's overspecific.
1796 return jst == _thread_blocked || jst == _thread_in_native;
1797 }
1798
1799
1800 // -----------------------------------------------------------------------------
1801 // WaitSet management ...
1802
1803 ObjectWaiter::ObjectWaiter(Thread* thread) {
1804 _next = NULL;
1805 _prev = NULL;
1806 _notified = 0;
1807 _notifier_tid = 0;
1808 TState = TS_RUN;
1809 _thread = thread;
1810 _event = thread->_ParkEvent;
1811 _active = false;
1812 assert(_event != NULL, "invariant");
1813 }
1814
1815 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1816 JavaThread *jt = (JavaThread *)this->_thread;
1817 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1818 }
1819
1820 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1821 JavaThread *jt = (JavaThread *)this->_thread;
1822 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1823 }
1824
1825 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1826 assert(node != NULL, "should not add NULL node");
1827 assert(node->_prev == NULL, "node already in list");
1828 assert(node->_next == NULL, "node already in list");
1829 // put node at end of queue (circular doubly linked list)
1830 if (_WaitSet == NULL) {
1831 _WaitSet = node;
1832 node->_prev = node;
1833 node->_next = node;
1834 } else {
1835 ObjectWaiter* head = _WaitSet;
1836 ObjectWaiter* tail = head->_prev;
1837 assert(tail->_next == head, "invariant check");
1838 tail->_next = node;
1839 head->_prev = node;
1840 node->_next = head;
1841 node->_prev = tail;
1842 }
1843 }
1844
1845 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1846 // dequeue the very first waiter
1847 ObjectWaiter* waiter = _WaitSet;
1848 if (waiter) {
1849 DequeueSpecificWaiter(waiter);
1850 }
1851 return waiter;
1852 }
1853
1854 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1855 assert(node != NULL, "should not dequeue NULL node");
1856 assert(node->_prev != NULL, "node already removed from list");
1857 assert(node->_next != NULL, "node already removed from list");
1858 // when the waiter has woken up because of interrupt,
1859 // timeout or other spurious wake-up, dequeue the
1860 // waiter from waiting list
1861 ObjectWaiter* next = node->_next;
1862 if (next == node) {
1863 assert(node->_prev == node, "invariant check");
1864 _WaitSet = NULL;
1865 } else {
1866 ObjectWaiter* prev = node->_prev;
1867 assert(prev->_next == node, "invariant check");
1868 assert(next->_prev == node, "invariant check");
1869 next->_prev = prev;
1870 prev->_next = next;
1871 if (_WaitSet == node) {
1872 _WaitSet = next;
1873 }
1874 }
1875 node->_next = NULL;
1876 node->_prev = NULL;
1877 }
1878
1879 // -----------------------------------------------------------------------------
1880 // PerfData support
1881 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
1882 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
1883 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
1884 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
1885 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
1886 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
1887 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
1888
1889 // One-shot global initialization for the sync subsystem.
1890 // We could also defer initialization and initialize on-demand
1891 // the first time we call ObjectSynchronizer::inflate().
1892 // Initialization would be protected - like so many things - by
1893 // the MonitorCache_lock.
1894
1895 void ObjectMonitor::Initialize() {
1896 assert(!InitDone, "invariant");
1897
1898 if (!os::is_MP()) {
1899 Knob_SpinLimit = 0;
1900 Knob_PreSpin = 0;
1901 Knob_FixedSpin = -1;
1902 }
1903
1904 if (UsePerfData) {
1905 EXCEPTION_MARK;
1906 #define NEWPERFCOUNTER(n) \
1907 { \
1908 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
1909 CHECK); \
1910 }
1911 #define NEWPERFVARIABLE(n) \
1912 { \
1913 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
1914 CHECK); \
1915 }
1916 NEWPERFCOUNTER(_sync_Inflations);
1917 NEWPERFCOUNTER(_sync_Deflations);
1918 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
1919 NEWPERFCOUNTER(_sync_FutileWakeups);
1920 NEWPERFCOUNTER(_sync_Parks);
1921 NEWPERFCOUNTER(_sync_Notifications);
1922 NEWPERFVARIABLE(_sync_MonExtant);
1923 #undef NEWPERFCOUNTER
1924 #undef NEWPERFVARIABLE
1925 }
1926
1927 DEBUG_ONLY(InitDone = true;)
1928 }