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 "logging/log.hpp"
30 #include "logging/logStream.hpp"
31 #include "memory/allocation.inline.hpp"
32 #include "memory/resourceArea.hpp"
33 #include "oops/markWord.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/atomic.hpp"
36 #include "runtime/handles.inline.hpp"
37 #include "runtime/interfaceSupport.inline.hpp"
38 #include "runtime/mutexLocker.hpp"
39 #include "runtime/objectMonitor.hpp"
40 #include "runtime/objectMonitor.inline.hpp"
41 #include "runtime/orderAccess.hpp"
42 #include "runtime/osThread.hpp"
43 #include "runtime/safepointMechanism.inline.hpp"
44 #include "runtime/sharedRuntime.hpp"
45 #include "runtime/stubRoutines.hpp"
46 #include "runtime/thread.inline.hpp"
47 #include "services/threadService.hpp"
48 #include "utilities/dtrace.hpp"
49 #include "utilities/macros.hpp"
50 #include "utilities/preserveException.hpp"
51 #if INCLUDE_JFR
52 #include "jfr/support/jfrFlush.hpp"
53 #endif
54
55 #ifdef DTRACE_ENABLED
56
57 // Only bother with this argument setup if dtrace is available
58 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
59
60
61 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
62 char* bytes = NULL; \
63 int len = 0; \
64 jlong jtid = SharedRuntime::get_java_tid(thread); \
65 Symbol* klassname = ((oop)obj)->klass()->name(); \
66 if (klassname != NULL) { \
67 bytes = (char*)klassname->bytes(); \
68 len = klassname->utf8_length(); \
69 }
70
71 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
72 { \
73 if (DTraceMonitorProbes) { \
74 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
75 HOTSPOT_MONITOR_WAIT(jtid, \
76 (monitor), bytes, len, (millis)); \
77 } \
78 }
79
80 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
81 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
82 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
83 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
84 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
85
86 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
87 { \
88 if (DTraceMonitorProbes) { \
89 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
90 HOTSPOT_MONITOR_##probe(jtid, \
91 (uintptr_t)(monitor), bytes, len); \
92 } \
93 }
94
95 #else // ndef DTRACE_ENABLED
96
97 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
98 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
99
100 #endif // ndef DTRACE_ENABLED
101
102 // Tunables ...
103 // The knob* variables are effectively final. Once set they should
104 // never be modified hence. Consider using __read_mostly with GCC.
105
106 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
107
108 static int Knob_Bonus = 100; // spin success bonus
109 static int Knob_BonusB = 100; // spin success bonus
110 static int Knob_Penalty = 200; // spin failure penalty
111 static int Knob_Poverty = 1000;
112 static int Knob_FixedSpin = 0;
113 static int Knob_PreSpin = 10; // 20-100 likely better
114
115 DEBUG_ONLY(static volatile bool InitDone = false;)
116
117 // -----------------------------------------------------------------------------
118 // Theory of operations -- Monitors lists, thread residency, etc:
119 //
120 // * A thread acquires ownership of a monitor by successfully
121 // CAS()ing the _owner field from null to non-null.
122 //
123 // * Invariant: A thread appears on at most one monitor list --
124 // cxq, EntryList or WaitSet -- at any one time.
125 //
126 // * Contending threads "push" themselves onto the cxq with CAS
127 // and then spin/park.
128 //
129 // * After a contending thread eventually acquires the lock it must
130 // dequeue itself from either the EntryList or the cxq.
131 //
132 // * The exiting thread identifies and unparks an "heir presumptive"
133 // tentative successor thread on the EntryList. Critically, the
134 // exiting thread doesn't unlink the successor thread from the EntryList.
135 // After having been unparked, the wakee will recontend for ownership of
136 // the monitor. The successor (wakee) will either acquire the lock or
137 // re-park itself.
138 //
139 // Succession is provided for by a policy of competitive handoff.
140 // The exiting thread does _not_ grant or pass ownership to the
141 // successor thread. (This is also referred to as "handoff" succession").
142 // Instead the exiting thread releases ownership and possibly wakes
143 // a successor, so the successor can (re)compete for ownership of the lock.
144 // If the EntryList is empty but the cxq is populated the exiting
145 // thread will drain the cxq into the EntryList. It does so by
146 // by detaching the cxq (installing null with CAS) and folding
147 // the threads from the cxq into the EntryList. The EntryList is
148 // doubly linked, while the cxq is singly linked because of the
149 // CAS-based "push" used to enqueue recently arrived threads (RATs).
150 //
151 // * Concurrency invariants:
152 //
153 // -- only the monitor owner may access or mutate the EntryList.
154 // The mutex property of the monitor itself protects the EntryList
155 // from concurrent interference.
156 // -- Only the monitor owner may detach the cxq.
157 //
158 // * The monitor entry list operations avoid locks, but strictly speaking
159 // they're not lock-free. Enter is lock-free, exit is not.
160 // For a description of 'Methods and apparatus providing non-blocking access
161 // to a resource,' see U.S. Pat. No. 7844973.
162 //
163 // * The cxq can have multiple concurrent "pushers" but only one concurrent
164 // detaching thread. This mechanism is immune from the ABA corruption.
165 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
166 //
167 // * Taken together, the cxq and the EntryList constitute or form a
168 // single logical queue of threads stalled trying to acquire the lock.
169 // We use two distinct lists to improve the odds of a constant-time
170 // dequeue operation after acquisition (in the ::enter() epilogue) and
171 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
172 // A key desideratum is to minimize queue & monitor metadata manipulation
173 // that occurs while holding the monitor lock -- that is, we want to
174 // minimize monitor lock holds times. Note that even a small amount of
175 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
176 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
177 // locks and monitor metadata.
178 //
179 // Cxq points to the set of Recently Arrived Threads attempting entry.
180 // Because we push threads onto _cxq with CAS, the RATs must take the form of
181 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
182 // the unlocking thread notices that EntryList is null but _cxq is != null.
183 //
184 // The EntryList is ordered by the prevailing queue discipline and
185 // can be organized in any convenient fashion, such as a doubly-linked list or
186 // a circular doubly-linked list. Critically, we want insert and delete operations
187 // to operate in constant-time. If we need a priority queue then something akin
188 // to Solaris' sleepq would work nicely. Viz.,
189 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
190 // Queue discipline is enforced at ::exit() time, when the unlocking thread
191 // drains the cxq into the EntryList, and orders or reorders the threads on the
192 // EntryList accordingly.
193 //
194 // Barring "lock barging", this mechanism provides fair cyclic ordering,
195 // somewhat similar to an elevator-scan.
196 //
197 // * The monitor synchronization subsystem avoids the use of native
198 // synchronization primitives except for the narrow platform-specific
199 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
200 // the semantics of park-unpark. Put another way, this monitor implementation
201 // depends only on atomic operations and park-unpark. The monitor subsystem
202 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
203 // underlying OS manages the READY<->RUN transitions.
204 //
205 // * Waiting threads reside on the WaitSet list -- wait() puts
206 // the caller onto the WaitSet.
207 //
208 // * notify() or notifyAll() simply transfers threads from the WaitSet to
209 // either the EntryList or cxq. Subsequent exit() operations will
210 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
211 // it's likely the notifyee would simply impale itself on the lock held
212 // by the notifier.
213 //
214 // * An interesting alternative is to encode cxq as (List,LockByte) where
215 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
216 // variable, like _recursions, in the scheme. The threads or Events that form
217 // the list would have to be aligned in 256-byte addresses. A thread would
218 // try to acquire the lock or enqueue itself with CAS, but exiting threads
219 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
220 // Note that is is *not* word-tearing, but it does presume that full-word
221 // CAS operations are coherent with intermix with STB operations. That's true
222 // on most common processors.
223 //
224 // * See also http://blogs.sun.com/dave
225
226
227 void* ObjectMonitor::operator new (size_t size) throw() {
228 return AllocateHeap(size, mtInternal);
229 }
230 void* ObjectMonitor::operator new[] (size_t size) throw() {
231 return operator new (size);
232 }
233 void ObjectMonitor::operator delete(void* p) {
234 FreeHeap(p);
235 }
236 void ObjectMonitor::operator delete[] (void *p) {
237 operator delete(p);
238 }
239
240 // -----------------------------------------------------------------------------
241 // Enter support
242
243 void ObjectMonitor::enter(TRAPS) {
244 // The following code is ordered to check the most common cases first
245 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
246 Thread * const Self = THREAD;
247
248 void * cur = Atomic::cmpxchg(&_owner, (void*)NULL, Self);
249 if (cur == NULL) {
250 assert(_recursions == 0, "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=" INTX_FORMAT, _recursions);
281 assert(((oop)object())->mark() == markWord::encode(this),
282 "object mark must match encoded this: mark=" INTPTR_FORMAT
283 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(),
284 markWord::encode(this).value());
285 Self->_Stalled = 0;
286 return;
287 }
288
289 assert(_owner != Self, "invariant");
290 assert(_succ != Self, "invariant");
291 assert(Self->is_Java_thread(), "invariant");
292 JavaThread * jt = (JavaThread *) Self;
293 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
294 assert(jt->thread_state() != _thread_blocked, "invariant");
295 assert(this->object() != NULL, "invariant");
296 assert(_contentions >= 0, "invariant");
297
298 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
299 // Ensure the object-monitor relationship remains stable while there's contention.
300 Atomic::inc(&_contentions);
301
302 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
303 EventJavaMonitorEnter event;
304 if (event.should_commit()) {
305 event.set_monitorClass(((oop)this->object())->klass());
306 event.set_address((uintptr_t)(this->object_addr()));
307 }
308
309 { // Change java thread status to indicate blocked on monitor enter.
310 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
311
312 Self->set_current_pending_monitor(this);
313
314 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
315 if (JvmtiExport::should_post_monitor_contended_enter()) {
316 JvmtiExport::post_monitor_contended_enter(jt, this);
317
318 // The current thread does not yet own the monitor and does not
319 // yet appear on any queues that would get it made the successor.
320 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
321 // handler cannot accidentally consume an unpark() meant for the
322 // ParkEvent associated with this ObjectMonitor.
323 }
324
325 OSThreadContendState osts(Self->osthread());
326 ThreadBlockInVM tbivm(jt);
327
328 // TODO-FIXME: change the following for(;;) loop to straight-line code.
329 for (;;) {
330 jt->set_suspend_equivalent();
331 // cleared by handle_special_suspend_equivalent_condition()
332 // or java_suspend_self()
333
334 EnterI(THREAD);
335
336 if (!ExitSuspendEquivalent(jt)) break;
337
338 // We have acquired the contended monitor, but while we were
339 // waiting another thread suspended us. We don't want to enter
340 // the monitor while suspended because that would surprise the
341 // thread that suspended us.
342 //
343 _recursions = 0;
344 _succ = NULL;
345 exit(false, Self);
346
347 jt->java_suspend_self();
348 }
349 Self->set_current_pending_monitor(NULL);
350
351 // We cleared the pending monitor info since we've just gotten past
352 // the enter-check-for-suspend dance and we now own the monitor free
353 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
354 // destructor can go to a safepoint at the end of this block. If we
355 // do a thread dump during that safepoint, then this thread will show
356 // as having "-locked" the monitor, but the OS and java.lang.Thread
357 // states will still report that the thread is blocked trying to
358 // acquire it.
359 }
360
361 Atomic::dec(&_contentions);
362 assert(_contentions >= 0, "invariant");
363 Self->_Stalled = 0;
364
365 // Must either set _recursions = 0 or ASSERT _recursions == 0.
366 assert(_recursions == 0, "invariant");
367 assert(_owner == Self, "invariant");
368 assert(_succ != Self, "invariant");
369 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
370
371 // The thread -- now the owner -- is back in vm mode.
372 // Report the glorious news via TI,DTrace and jvmstat.
373 // The probe effect is non-trivial. All the reportage occurs
374 // while we hold the monitor, increasing the length of the critical
375 // section. Amdahl's parallel speedup law comes vividly into play.
376 //
377 // Another option might be to aggregate the events (thread local or
378 // per-monitor aggregation) and defer reporting until a more opportune
379 // time -- such as next time some thread encounters contention but has
380 // yet to acquire the lock. While spinning that thread could
381 // spinning we could increment JVMStat counters, etc.
382
383 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
384 if (JvmtiExport::should_post_monitor_contended_entered()) {
385 JvmtiExport::post_monitor_contended_entered(jt, this);
386
387 // The current thread already owns the monitor and is not going to
388 // call park() for the remainder of the monitor enter protocol. So
389 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
390 // event handler consumed an unpark() issued by the thread that
391 // just exited the monitor.
392 }
393 if (event.should_commit()) {
394 event.set_previousOwner((uintptr_t)_previous_owner_tid);
395 event.commit();
396 }
397 OM_PERFDATA_OP(ContendedLockAttempts, inc());
398 }
399
400 // Caveat: TryLock() is not necessarily serializing if it returns failure.
401 // Callers must compensate as needed.
402
403 int ObjectMonitor::TryLock(Thread * Self) {
404 void * own = _owner;
405 if (own != NULL) return 0;
406 if (Atomic::replace_if_null(&_owner, Self)) {
407 assert(_recursions == 0, "invariant");
408 return 1;
409 }
410 // The lock had been free momentarily, but we lost the race to the lock.
411 // Interference -- the CAS failed.
412 // We can either return -1 or retry.
413 // Retry doesn't make as much sense because the lock was just acquired.
414 return -1;
415 }
416
417 // Convert the fields used by is_busy() to a string that can be
418 // used for diagnostic output.
419 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
420 ss->print("is_busy: contentions=%d, waiters=%d, owner=" INTPTR_FORMAT
421 ", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, _contentions,
422 _waiters, p2i(_owner), p2i(_cxq), p2i(_EntryList));
423 return ss->base();
424 }
425
426 #define MAX_RECHECK_INTERVAL 1000
427
428 void ObjectMonitor::EnterI(TRAPS) {
429 Thread * const Self = THREAD;
430 assert(Self->is_Java_thread(), "invariant");
431 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
432
433 // Try the lock - TATAS
434 if (TryLock (Self) > 0) {
435 assert(_succ != Self, "invariant");
436 assert(_owner == Self, "invariant");
437 assert(_Responsible != Self, "invariant");
438 return;
439 }
440
441 assert(InitDone, "Unexpectedly not initialized");
442
443 // We try one round of spinning *before* enqueueing Self.
444 //
445 // If the _owner is ready but OFFPROC we could use a YieldTo()
446 // operation to donate the remainder of this thread's quantum
447 // to the owner. This has subtle but beneficial affinity
448 // effects.
449
450 if (TrySpin(Self) > 0) {
451 assert(_owner == Self, "invariant");
452 assert(_succ != Self, "invariant");
453 assert(_Responsible != Self, "invariant");
454 return;
455 }
456
457 // The Spin failed -- Enqueue and park the thread ...
458 assert(_succ != Self, "invariant");
459 assert(_owner != Self, "invariant");
460 assert(_Responsible != Self, "invariant");
461
462 // Enqueue "Self" on ObjectMonitor's _cxq.
463 //
464 // Node acts as a proxy for Self.
465 // As an aside, if were to ever rewrite the synchronization code mostly
466 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
467 // Java objects. This would avoid awkward lifecycle and liveness issues,
468 // as well as eliminate a subset of ABA issues.
469 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
470
471 ObjectWaiter node(Self);
472 Self->_ParkEvent->reset();
473 node._prev = (ObjectWaiter *) 0xBAD;
474 node.TState = ObjectWaiter::TS_CXQ;
475
476 // Push "Self" onto the front of the _cxq.
477 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
478 // Note that spinning tends to reduce the rate at which threads
479 // enqueue and dequeue on EntryList|cxq.
480 ObjectWaiter * nxt;
481 for (;;) {
482 node._next = nxt = _cxq;
483 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
484
485 // Interference - the CAS failed because _cxq changed. Just retry.
486 // As an optional optimization we retry the lock.
487 if (TryLock (Self) > 0) {
488 assert(_succ != Self, "invariant");
489 assert(_owner == Self, "invariant");
490 assert(_Responsible != Self, "invariant");
491 return;
492 }
493 }
494
495 // Check for cxq|EntryList edge transition to non-null. This indicates
496 // the onset of contention. While contention persists exiting threads
497 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
498 // operations revert to the faster 1-0 mode. This enter operation may interleave
499 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
500 // arrange for one of the contending thread to use a timed park() operations
501 // to detect and recover from the race. (Stranding is form of progress failure
502 // where the monitor is unlocked but all the contending threads remain parked).
503 // That is, at least one of the contended threads will periodically poll _owner.
504 // One of the contending threads will become the designated "Responsible" thread.
505 // The Responsible thread uses a timed park instead of a normal indefinite park
506 // operation -- it periodically wakes and checks for and recovers from potential
507 // strandings admitted by 1-0 exit operations. We need at most one Responsible
508 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
509 // be responsible for a monitor.
510 //
511 // Currently, one of the contended threads takes on the added role of "Responsible".
512 // A viable alternative would be to use a dedicated "stranding checker" thread
513 // that periodically iterated over all the threads (or active monitors) and unparked
514 // successors where there was risk of stranding. This would help eliminate the
515 // timer scalability issues we see on some platforms as we'd only have one thread
516 // -- the checker -- parked on a timer.
517
518 if (nxt == NULL && _EntryList == NULL) {
519 // Try to assume the role of responsible thread for the monitor.
520 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
521 Atomic::replace_if_null(&_Responsible, Self);
522 }
523
524 // The lock might have been released while this thread was occupied queueing
525 // itself onto _cxq. To close the race and avoid "stranding" and
526 // progress-liveness failure we must resample-retry _owner before parking.
527 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
528 // In this case the ST-MEMBAR is accomplished with CAS().
529 //
530 // TODO: Defer all thread state transitions until park-time.
531 // Since state transitions are heavy and inefficient we'd like
532 // to defer the state transitions until absolutely necessary,
533 // and in doing so avoid some transitions ...
534
535 int nWakeups = 0;
536 int recheckInterval = 1;
537
538 for (;;) {
539
540 if (TryLock(Self) > 0) break;
541 assert(_owner != Self, "invariant");
542
543 // park self
544 if (_Responsible == Self) {
545 Self->_ParkEvent->park((jlong) recheckInterval);
546 // Increase the recheckInterval, but clamp the value.
547 recheckInterval *= 8;
548 if (recheckInterval > MAX_RECHECK_INTERVAL) {
549 recheckInterval = MAX_RECHECK_INTERVAL;
550 }
551 } else {
552 Self->_ParkEvent->park();
553 }
554
555 if (TryLock(Self) > 0) break;
556
557 // The lock is still contested.
558 // Keep a tally of the # of futile wakeups.
559 // Note that the counter is not protected by a lock or updated by atomics.
560 // That is by design - we trade "lossy" counters which are exposed to
561 // races during updates for a lower probe effect.
562
563 // This PerfData object can be used in parallel with a safepoint.
564 // See the work around in PerfDataManager::destroy().
565 OM_PERFDATA_OP(FutileWakeups, inc());
566 ++nWakeups;
567
568 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
569 // We can defer clearing _succ until after the spin completes
570 // TrySpin() must tolerate being called with _succ == Self.
571 // Try yet another round of adaptive spinning.
572 if (TrySpin(Self) > 0) break;
573
574 // We can find that we were unpark()ed and redesignated _succ while
575 // we were spinning. That's harmless. If we iterate and call park(),
576 // park() will consume the event and return immediately and we'll
577 // just spin again. This pattern can repeat, leaving _succ to simply
578 // spin on a CPU.
579
580 if (_succ == Self) _succ = NULL;
581
582 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
583 OrderAccess::fence();
584 }
585
586 // Egress :
587 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
588 // Normally we'll find Self on the EntryList .
589 // From the perspective of the lock owner (this thread), the
590 // EntryList is stable and cxq is prepend-only.
591 // The head of cxq is volatile but the interior is stable.
592 // In addition, Self.TState is stable.
593
594 assert(_owner == Self, "invariant");
595 assert(object() != NULL, "invariant");
596 // I'd like to write:
597 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ;
598 // but as we're at a safepoint that's not safe.
599
600 UnlinkAfterAcquire(Self, &node);
601 if (_succ == Self) _succ = NULL;
602
603 assert(_succ != Self, "invariant");
604 if (_Responsible == Self) {
605 _Responsible = NULL;
606 OrderAccess::fence(); // Dekker pivot-point
607
608 // We may leave threads on cxq|EntryList without a designated
609 // "Responsible" thread. This is benign. When this thread subsequently
610 // exits the monitor it can "see" such preexisting "old" threads --
611 // threads that arrived on the cxq|EntryList before the fence, above --
612 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
613 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
614 // non-null and elect a new "Responsible" timer thread.
615 //
616 // This thread executes:
617 // ST Responsible=null; MEMBAR (in enter epilogue - here)
618 // LD cxq|EntryList (in subsequent exit)
619 //
620 // Entering threads in the slow/contended path execute:
621 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
622 // The (ST cxq; MEMBAR) is accomplished with CAS().
623 //
624 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
625 // exit operation from floating above the ST Responsible=null.
626 }
627
628 // We've acquired ownership with CAS().
629 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
630 // But since the CAS() this thread may have also stored into _succ,
631 // EntryList, cxq or Responsible. These meta-data updates must be
632 // visible __before this thread subsequently drops the lock.
633 // Consider what could occur if we didn't enforce this constraint --
634 // STs to monitor meta-data and user-data could reorder with (become
635 // visible after) the ST in exit that drops ownership of the lock.
636 // Some other thread could then acquire the lock, but observe inconsistent
637 // or old monitor meta-data and heap data. That violates the JMM.
638 // To that end, the 1-0 exit() operation must have at least STST|LDST
639 // "release" barrier semantics. Specifically, there must be at least a
640 // STST|LDST barrier in exit() before the ST of null into _owner that drops
641 // the lock. The barrier ensures that changes to monitor meta-data and data
642 // protected by the lock will be visible before we release the lock, and
643 // therefore before some other thread (CPU) has a chance to acquire the lock.
644 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
645 //
646 // Critically, any prior STs to _succ or EntryList must be visible before
647 // the ST of null into _owner in the *subsequent* (following) corresponding
648 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
649 // execute a serializing instruction.
650
651 return;
652 }
653
654 // ReenterI() is a specialized inline form of the latter half of the
655 // contended slow-path from EnterI(). We use ReenterI() only for
656 // monitor reentry in wait().
657 //
658 // In the future we should reconcile EnterI() and ReenterI().
659
660 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
661 assert(Self != NULL, "invariant");
662 assert(SelfNode != NULL, "invariant");
663 assert(SelfNode->_thread == Self, "invariant");
664 assert(_waiters > 0, "invariant");
665 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
666 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
667 JavaThread * jt = (JavaThread *) Self;
668
669 int nWakeups = 0;
670 for (;;) {
671 ObjectWaiter::TStates v = SelfNode->TState;
672 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
673 assert(_owner != Self, "invariant");
674
675 if (TryLock(Self) > 0) break;
676 if (TrySpin(Self) > 0) break;
677
678 // State transition wrappers around park() ...
679 // ReenterI() wisely defers state transitions until
680 // it's clear we must park the thread.
681 {
682 OSThreadContendState osts(Self->osthread());
683 ThreadBlockInVM tbivm(jt);
684
685 // cleared by handle_special_suspend_equivalent_condition()
686 // or java_suspend_self()
687 jt->set_suspend_equivalent();
688 Self->_ParkEvent->park();
689
690 // were we externally suspended while we were waiting?
691 for (;;) {
692 if (!ExitSuspendEquivalent(jt)) break;
693 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
694 jt->java_suspend_self();
695 jt->set_suspend_equivalent();
696 }
697 }
698
699 // Try again, but just so we distinguish between futile wakeups and
700 // successful wakeups. The following test isn't algorithmically
701 // necessary, but it helps us maintain sensible statistics.
702 if (TryLock(Self) > 0) break;
703
704 // The lock is still contested.
705 // Keep a tally of the # of futile wakeups.
706 // Note that the counter is not protected by a lock or updated by atomics.
707 // That is by design - we trade "lossy" counters which are exposed to
708 // races during updates for a lower probe effect.
709 ++nWakeups;
710
711 // Assuming this is not a spurious wakeup we'll normally
712 // find that _succ == Self.
713 if (_succ == Self) _succ = NULL;
714
715 // Invariant: after clearing _succ a contending thread
716 // *must* retry _owner before parking.
717 OrderAccess::fence();
718
719 // This PerfData object can be used in parallel with a safepoint.
720 // See the work around in PerfDataManager::destroy().
721 OM_PERFDATA_OP(FutileWakeups, inc());
722 }
723
724 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
725 // Normally we'll find Self on the EntryList.
726 // Unlinking from the EntryList is constant-time and atomic-free.
727 // From the perspective of the lock owner (this thread), the
728 // EntryList is stable and cxq is prepend-only.
729 // The head of cxq is volatile but the interior is stable.
730 // In addition, Self.TState is stable.
731
732 assert(_owner == Self, "invariant");
733 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
734 UnlinkAfterAcquire(Self, SelfNode);
735 if (_succ == Self) _succ = NULL;
736 assert(_succ != Self, "invariant");
737 SelfNode->TState = ObjectWaiter::TS_RUN;
738 OrderAccess::fence(); // see comments at the end of EnterI()
739 }
740
741 // By convention we unlink a contending thread from EntryList|cxq immediately
742 // after the thread acquires the lock in ::enter(). Equally, we could defer
743 // unlinking the thread until ::exit()-time.
744
745 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
746 assert(_owner == Self, "invariant");
747 assert(SelfNode->_thread == Self, "invariant");
748
749 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
750 // Normal case: remove Self from the DLL EntryList .
751 // This is a constant-time operation.
752 ObjectWaiter * nxt = SelfNode->_next;
753 ObjectWaiter * prv = SelfNode->_prev;
754 if (nxt != NULL) nxt->_prev = prv;
755 if (prv != NULL) prv->_next = nxt;
756 if (SelfNode == _EntryList) _EntryList = nxt;
757 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
758 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
759 } else {
760 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
761 // Inopportune interleaving -- Self is still on the cxq.
762 // This usually means the enqueue of self raced an exiting thread.
763 // Normally we'll find Self near the front of the cxq, so
764 // dequeueing is typically fast. If needbe we can accelerate
765 // this with some MCS/CHL-like bidirectional list hints and advisory
766 // back-links so dequeueing from the interior will normally operate
767 // in constant-time.
768 // Dequeue Self from either the head (with CAS) or from the interior
769 // with a linear-time scan and normal non-atomic memory operations.
770 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
771 // and then unlink Self from EntryList. We have to drain eventually,
772 // so it might as well be now.
773
774 ObjectWaiter * v = _cxq;
775 assert(v != NULL, "invariant");
776 if (v != SelfNode || Atomic::cmpxchg(&_cxq, v, SelfNode->_next) != v) {
777 // The CAS above can fail from interference IFF a "RAT" arrived.
778 // In that case Self must be in the interior and can no longer be
779 // at the head of cxq.
780 if (v == SelfNode) {
781 assert(_cxq != v, "invariant");
782 v = _cxq; // CAS above failed - start scan at head of list
783 }
784 ObjectWaiter * p;
785 ObjectWaiter * q = NULL;
786 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
787 q = p;
788 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
789 }
790 assert(v != SelfNode, "invariant");
791 assert(p == SelfNode, "Node not found on cxq");
792 assert(p != _cxq, "invariant");
793 assert(q != NULL, "invariant");
794 assert(q->_next == p, "invariant");
795 q->_next = p->_next;
796 }
797 }
798
799 #ifdef ASSERT
800 // Diagnostic hygiene ...
801 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
802 SelfNode->_next = (ObjectWaiter *) 0xBAD;
803 SelfNode->TState = ObjectWaiter::TS_RUN;
804 #endif
805 }
806
807 // -----------------------------------------------------------------------------
808 // Exit support
809 //
810 // exit()
811 // ~~~~~~
812 // Note that the collector can't reclaim the objectMonitor or deflate
813 // the object out from underneath the thread calling ::exit() as the
814 // thread calling ::exit() never transitions to a stable state.
815 // This inhibits GC, which in turn inhibits asynchronous (and
816 // inopportune) reclamation of "this".
817 //
818 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
819 // There's one exception to the claim above, however. EnterI() can call
820 // exit() to drop a lock if the acquirer has been externally suspended.
821 // In that case exit() is called with _thread_state as _thread_blocked,
822 // but the monitor's _contentions field is > 0, which inhibits reclamation.
823 //
824 // 1-0 exit
825 // ~~~~~~~~
826 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
827 // the fast-path operators have been optimized so the common ::exit()
828 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
829 // The code emitted by fast_unlock() elides the usual MEMBAR. This
830 // greatly improves latency -- MEMBAR and CAS having considerable local
831 // latency on modern processors -- but at the cost of "stranding". Absent the
832 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
833 // ::enter() path, resulting in the entering thread being stranding
834 // and a progress-liveness failure. Stranding is extremely rare.
835 // We use timers (timed park operations) & periodic polling to detect
836 // and recover from stranding. Potentially stranded threads periodically
837 // wake up and poll the lock. See the usage of the _Responsible variable.
838 //
839 // The CAS() in enter provides for safety and exclusion, while the CAS or
840 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
841 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
842 // We detect and recover from stranding with timers.
843 //
844 // If a thread transiently strands it'll park until (a) another
845 // thread acquires the lock and then drops the lock, at which time the
846 // exiting thread will notice and unpark the stranded thread, or, (b)
847 // the timer expires. If the lock is high traffic then the stranding latency
848 // will be low due to (a). If the lock is low traffic then the odds of
849 // stranding are lower, although the worst-case stranding latency
850 // is longer. Critically, we don't want to put excessive load in the
851 // platform's timer subsystem. We want to minimize both the timer injection
852 // rate (timers created/sec) as well as the number of timers active at
853 // any one time. (more precisely, we want to minimize timer-seconds, which is
854 // the integral of the # of active timers at any instant over time).
855 // Both impinge on OS scalability. Given that, at most one thread parked on
856 // a monitor will use a timer.
857 //
858 // There is also the risk of a futile wake-up. If we drop the lock
859 // another thread can reacquire the lock immediately, and we can
860 // then wake a thread unnecessarily. This is benign, and we've
861 // structured the code so the windows are short and the frequency
862 // of such futile wakups is low.
863
864 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
865 Thread * const Self = THREAD;
866 if (THREAD != _owner) {
867 if (THREAD->is_lock_owned((address) _owner)) {
868 // Transmute _owner from a BasicLock pointer to a Thread address.
869 // We don't need to hold _mutex for this transition.
870 // Non-null to Non-null is safe as long as all readers can
871 // tolerate either flavor.
872 assert(_recursions == 0, "invariant");
873 _owner = THREAD;
874 _recursions = 0;
875 } else {
876 // Apparent unbalanced locking ...
877 // Naively we'd like to throw IllegalMonitorStateException.
878 // As a practical matter we can neither allocate nor throw an
879 // exception as ::exit() can be called from leaf routines.
880 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
881 // Upon deeper reflection, however, in a properly run JVM the only
882 // way we should encounter this situation is in the presence of
883 // unbalanced JNI locking. TODO: CheckJNICalls.
884 // See also: CR4414101
885 #ifdef ASSERT
886 LogStreamHandle(Error, monitorinflation) lsh;
887 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
888 " is exiting an ObjectMonitor it does not own.", p2i(THREAD));
889 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
890 print_debug_style_on(&lsh);
891 #endif
892 assert(false, "Non-balanced monitor enter/exit!");
893 return;
894 }
895 }
896
897 if (_recursions != 0) {
898 _recursions--; // this is simple recursive enter
899 return;
900 }
901
902 // Invariant: after setting Responsible=null an thread must execute
903 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
904 _Responsible = NULL;
905
906 #if INCLUDE_JFR
907 // get the owner's thread id for the MonitorEnter event
908 // if it is enabled and the thread isn't suspended
909 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
910 _previous_owner_tid = JFR_THREAD_ID(Self);
911 }
912 #endif
913
914 for (;;) {
915 assert(THREAD == _owner, "invariant");
916
917 // release semantics: prior loads and stores from within the critical section
918 // must not float (reorder) past the following store that drops the lock.
919 Atomic::release_store(&_owner, (void*)NULL); // drop the lock
920 OrderAccess::storeload(); // See if we need to wake a successor
921 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
922 return;
923 }
924 // Other threads are blocked trying to acquire the lock.
925
926 // Normally the exiting thread is responsible for ensuring succession,
927 // but if other successors are ready or other entering threads are spinning
928 // then this thread can simply store NULL into _owner and exit without
929 // waking a successor. The existence of spinners or ready successors
930 // guarantees proper succession (liveness). Responsibility passes to the
931 // ready or running successors. The exiting thread delegates the duty.
932 // More precisely, if a successor already exists this thread is absolved
933 // of the responsibility of waking (unparking) one.
934 //
935 // The _succ variable is critical to reducing futile wakeup frequency.
936 // _succ identifies the "heir presumptive" thread that has been made
937 // ready (unparked) but that has not yet run. We need only one such
938 // successor thread to guarantee progress.
939 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
940 // section 3.3 "Futile Wakeup Throttling" for details.
941 //
942 // Note that spinners in Enter() also set _succ non-null.
943 // In the current implementation spinners opportunistically set
944 // _succ so that exiting threads might avoid waking a successor.
945 // Another less appealing alternative would be for the exiting thread
946 // to drop the lock and then spin briefly to see if a spinner managed
947 // to acquire the lock. If so, the exiting thread could exit
948 // immediately without waking a successor, otherwise the exiting
949 // thread would need to dequeue and wake a successor.
950 // (Note that we'd need to make the post-drop spin short, but no
951 // shorter than the worst-case round-trip cache-line migration time.
952 // The dropped lock needs to become visible to the spinner, and then
953 // the acquisition of the lock by the spinner must become visible to
954 // the exiting thread).
955
956 // It appears that an heir-presumptive (successor) must be made ready.
957 // Only the current lock owner can manipulate the EntryList or
958 // drain _cxq, so we need to reacquire the lock. If we fail
959 // to reacquire the lock the responsibility for ensuring succession
960 // falls to the new owner.
961 //
962 if (!Atomic::replace_if_null(&_owner, THREAD)) {
963 return;
964 }
965
966 guarantee(_owner == THREAD, "invariant");
967
968 ObjectWaiter * w = NULL;
969
970 w = _EntryList;
971 if (w != NULL) {
972 // I'd like to write: guarantee (w->_thread != Self).
973 // But in practice an exiting thread may find itself on the EntryList.
974 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
975 // then calls exit(). Exit release the lock by setting O._owner to NULL.
976 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
977 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
978 // release the lock "O". T2 resumes immediately after the ST of null into
979 // _owner, above. T2 notices that the EntryList is populated, so it
980 // reacquires the lock and then finds itself on the EntryList.
981 // Given all that, we have to tolerate the circumstance where "w" is
982 // associated with Self.
983 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
984 ExitEpilog(Self, w);
985 return;
986 }
987
988 // If we find that both _cxq and EntryList are null then just
989 // re-run the exit protocol from the top.
990 w = _cxq;
991 if (w == NULL) continue;
992
993 // Drain _cxq into EntryList - bulk transfer.
994 // First, detach _cxq.
995 // The following loop is tantamount to: w = swap(&cxq, NULL)
996 for (;;) {
997 assert(w != NULL, "Invariant");
998 ObjectWaiter * u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)NULL);
999 if (u == w) break;
1000 w = u;
1001 }
1002
1003 assert(w != NULL, "invariant");
1004 assert(_EntryList == NULL, "invariant");
1005
1006 // Convert the LIFO SLL anchored by _cxq into a DLL.
1007 // The list reorganization step operates in O(LENGTH(w)) time.
1008 // It's critical that this step operate quickly as
1009 // "Self" still holds the outer-lock, restricting parallelism
1010 // and effectively lengthening the critical section.
1011 // Invariant: s chases t chases u.
1012 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1013 // we have faster access to the tail.
1014
1015 _EntryList = w;
1016 ObjectWaiter * q = NULL;
1017 ObjectWaiter * p;
1018 for (p = w; p != NULL; p = p->_next) {
1019 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1020 p->TState = ObjectWaiter::TS_ENTER;
1021 p->_prev = q;
1022 q = p;
1023 }
1024
1025 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1026 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1027
1028 // See if we can abdicate to a spinner instead of waking a thread.
1029 // A primary goal of the implementation is to reduce the
1030 // context-switch rate.
1031 if (_succ != NULL) continue;
1032
1033 w = _EntryList;
1034 if (w != NULL) {
1035 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1036 ExitEpilog(Self, w);
1037 return;
1038 }
1039 }
1040 }
1041
1042 // ExitSuspendEquivalent:
1043 // A faster alternate to handle_special_suspend_equivalent_condition()
1044 //
1045 // handle_special_suspend_equivalent_condition() unconditionally
1046 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1047 // operations have high latency. Note that in ::enter() we call HSSEC
1048 // while holding the monitor, so we effectively lengthen the critical sections.
1049 //
1050 // There are a number of possible solutions:
1051 //
1052 // A. To ameliorate the problem we might also defer state transitions
1053 // to as late as possible -- just prior to parking.
1054 // Given that, we'd call HSSEC after having returned from park(),
1055 // but before attempting to acquire the monitor. This is only a
1056 // partial solution. It avoids calling HSSEC while holding the
1057 // monitor (good), but it still increases successor reacquisition latency --
1058 // the interval between unparking a successor and the time the successor
1059 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1060 // If we use this technique we can also avoid EnterI()-exit() loop
1061 // in ::enter() where we iteratively drop the lock and then attempt
1062 // to reacquire it after suspending.
1063 //
1064 // B. In the future we might fold all the suspend bits into a
1065 // composite per-thread suspend flag and then update it with CAS().
1066 // Alternately, a Dekker-like mechanism with multiple variables
1067 // would suffice:
1068 // ST Self->_suspend_equivalent = false
1069 // MEMBAR
1070 // LD Self_>_suspend_flags
1071
1072 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1073 return jSelf->handle_special_suspend_equivalent_condition();
1074 }
1075
1076
1077 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1078 assert(_owner == Self, "invariant");
1079
1080 // Exit protocol:
1081 // 1. ST _succ = wakee
1082 // 2. membar #loadstore|#storestore;
1083 // 2. ST _owner = NULL
1084 // 3. unpark(wakee)
1085
1086 _succ = Wakee->_thread;
1087 ParkEvent * Trigger = Wakee->_event;
1088
1089 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1090 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1091 // out-of-scope (non-extant).
1092 Wakee = NULL;
1093
1094 // Drop the lock
1095 Atomic::release_store(&_owner, (void*)NULL);
1096 OrderAccess::fence(); // ST _owner vs LD in unpark()
1097
1098 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1099 Trigger->unpark();
1100
1101 // Maintain stats and report events to JVMTI
1102 OM_PERFDATA_OP(Parks, inc());
1103 }
1104
1105
1106 // -----------------------------------------------------------------------------
1107 // Class Loader deadlock handling.
1108 //
1109 // complete_exit exits a lock returning recursion count
1110 // complete_exit/reenter operate as a wait without waiting
1111 // complete_exit requires an inflated monitor
1112 // The _owner field is not always the Thread addr even with an
1113 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1114 // thread due to contention.
1115 intx ObjectMonitor::complete_exit(TRAPS) {
1116 Thread * const Self = THREAD;
1117 assert(Self->is_Java_thread(), "Must be Java thread!");
1118 JavaThread *jt = (JavaThread *)THREAD;
1119
1120 assert(InitDone, "Unexpectedly not initialized");
1121
1122 if (THREAD != _owner) {
1123 if (THREAD->is_lock_owned ((address)_owner)) {
1124 assert(_recursions == 0, "internal state error");
1125 _owner = THREAD; // Convert from basiclock addr to Thread addr
1126 _recursions = 0;
1127 }
1128 }
1129
1130 guarantee(Self == _owner, "complete_exit not owner");
1131 intx save = _recursions; // record the old recursion count
1132 _recursions = 0; // set the recursion level to be 0
1133 exit(true, Self); // exit the monitor
1134 guarantee(_owner != Self, "invariant");
1135 return save;
1136 }
1137
1138 // reenter() enters a lock and sets recursion count
1139 // complete_exit/reenter operate as a wait without waiting
1140 void ObjectMonitor::reenter(intx recursions, TRAPS) {
1141 Thread * const Self = THREAD;
1142 assert(Self->is_Java_thread(), "Must be Java thread!");
1143 JavaThread *jt = (JavaThread *)THREAD;
1144
1145 guarantee(_owner != Self, "reenter already owner");
1146 enter(THREAD); // enter the monitor
1147 guarantee(_recursions == 0, "reenter recursion");
1148 _recursions = recursions;
1149 return;
1150 }
1151
1152 // Checks that the current THREAD owns this monitor and causes an
1153 // immediate return if it doesn't. We don't use the CHECK macro
1154 // because we want the IMSE to be the only exception that is thrown
1155 // from the call site when false is returned. Any other pending
1156 // exception is ignored.
1157 #define CHECK_OWNER() \
1158 do { \
1159 if (!check_owner(THREAD)) { \
1160 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1161 return; \
1162 } \
1163 } while (false)
1164
1165 // Returns true if the specified thread owns the ObjectMonitor.
1166 // Otherwise returns false and throws IllegalMonitorStateException
1167 // (IMSE). If there is a pending exception and the specified thread
1168 // is not the owner, that exception will be replaced by the IMSE.
1169 bool ObjectMonitor::check_owner(Thread* THREAD) {
1170 if (_owner == THREAD) {
1171 return true;
1172 }
1173 if (THREAD->is_lock_owned((address)_owner)) {
1174 _owner = THREAD; // convert from BasicLock addr to Thread addr
1175 _recursions = 0;
1176 return true;
1177 }
1178 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1179 "current thread is not owner", false);
1180 }
1181
1182 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1183 ObjectMonitor* monitor,
1184 jlong notifier_tid,
1185 jlong timeout,
1186 bool timedout) {
1187 assert(event != NULL, "invariant");
1188 assert(monitor != NULL, "invariant");
1189 event->set_monitorClass(((oop)monitor->object())->klass());
1190 event->set_timeout(timeout);
1191 event->set_address((uintptr_t)monitor->object_addr());
1192 event->set_notifier(notifier_tid);
1193 event->set_timedOut(timedout);
1194 event->commit();
1195 }
1196
1197 // -----------------------------------------------------------------------------
1198 // Wait/Notify/NotifyAll
1199 //
1200 // Note: a subset of changes to ObjectMonitor::wait()
1201 // will need to be replicated in complete_exit
1202 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1203 Thread * const Self = THREAD;
1204 assert(Self->is_Java_thread(), "Must be Java thread!");
1205 JavaThread *jt = (JavaThread *)THREAD;
1206
1207 assert(InitDone, "Unexpectedly not initialized");
1208
1209 CHECK_OWNER(); // Throws IMSE if not owner.
1210
1211 EventJavaMonitorWait event;
1212
1213 // check for a pending interrupt
1214 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1215 // post monitor waited event. Note that this is past-tense, we are done waiting.
1216 if (JvmtiExport::should_post_monitor_waited()) {
1217 // Note: 'false' parameter is passed here because the
1218 // wait was not timed out due to thread interrupt.
1219 JvmtiExport::post_monitor_waited(jt, this, false);
1220
1221 // In this short circuit of the monitor wait protocol, the
1222 // current thread never drops ownership of the monitor and
1223 // never gets added to the wait queue so the current thread
1224 // cannot be made the successor. This means that the
1225 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1226 // consume an unpark() meant for the ParkEvent associated with
1227 // this ObjectMonitor.
1228 }
1229 if (event.should_commit()) {
1230 post_monitor_wait_event(&event, this, 0, millis, false);
1231 }
1232 THROW(vmSymbols::java_lang_InterruptedException());
1233 return;
1234 }
1235
1236 assert(Self->_Stalled == 0, "invariant");
1237 Self->_Stalled = intptr_t(this);
1238 jt->set_current_waiting_monitor(this);
1239
1240 // create a node to be put into the queue
1241 // Critically, after we reset() the event but prior to park(), we must check
1242 // for a pending interrupt.
1243 ObjectWaiter node(Self);
1244 node.TState = ObjectWaiter::TS_WAIT;
1245 Self->_ParkEvent->reset();
1246 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1247
1248 // Enter the waiting queue, which is a circular doubly linked list in this case
1249 // but it could be a priority queue or any data structure.
1250 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1251 // by the the owner of the monitor *except* in the case where park()
1252 // returns because of a timeout of interrupt. Contention is exceptionally rare
1253 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1254
1255 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1256 AddWaiter(&node);
1257 Thread::SpinRelease(&_WaitSetLock);
1258
1259 _Responsible = NULL;
1260
1261 intx save = _recursions; // record the old recursion count
1262 _waiters++; // increment the number of waiters
1263 _recursions = 0; // set the recursion level to be 1
1264 exit(true, Self); // exit the monitor
1265 guarantee(_owner != Self, "invariant");
1266
1267 // The thread is on the WaitSet list - now park() it.
1268 // On MP systems it's conceivable that a brief spin before we park
1269 // could be profitable.
1270 //
1271 // TODO-FIXME: change the following logic to a loop of the form
1272 // while (!timeout && !interrupted && _notified == 0) park()
1273
1274 int ret = OS_OK;
1275 int WasNotified = 0;
1276
1277 // Need to check interrupt state whilst still _thread_in_vm
1278 bool interrupted = interruptible && jt->is_interrupted(false);
1279
1280 { // State transition wrappers
1281 OSThread* osthread = Self->osthread();
1282 OSThreadWaitState osts(osthread, true);
1283 {
1284 ThreadBlockInVM tbivm(jt);
1285 // Thread is in thread_blocked state and oop access is unsafe.
1286 jt->set_suspend_equivalent();
1287
1288 if (interrupted || HAS_PENDING_EXCEPTION) {
1289 // Intentionally empty
1290 } else if (node._notified == 0) {
1291 if (millis <= 0) {
1292 Self->_ParkEvent->park();
1293 } else {
1294 ret = Self->_ParkEvent->park(millis);
1295 }
1296 }
1297
1298 // were we externally suspended while we were waiting?
1299 if (ExitSuspendEquivalent (jt)) {
1300 // TODO-FIXME: add -- if succ == Self then succ = null.
1301 jt->java_suspend_self();
1302 }
1303
1304 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1305
1306 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1307 // from the WaitSet to the EntryList.
1308 // See if we need to remove Node from the WaitSet.
1309 // We use double-checked locking to avoid grabbing _WaitSetLock
1310 // if the thread is not on the wait queue.
1311 //
1312 // Note that we don't need a fence before the fetch of TState.
1313 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1314 // written by the is thread. (perhaps the fetch might even be satisfied
1315 // by a look-aside into the processor's own store buffer, although given
1316 // the length of the code path between the prior ST and this load that's
1317 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1318 // then we'll acquire the lock and then re-fetch a fresh TState value.
1319 // That is, we fail toward safety.
1320
1321 if (node.TState == ObjectWaiter::TS_WAIT) {
1322 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1323 if (node.TState == ObjectWaiter::TS_WAIT) {
1324 DequeueSpecificWaiter(&node); // unlink from WaitSet
1325 assert(node._notified == 0, "invariant");
1326 node.TState = ObjectWaiter::TS_RUN;
1327 }
1328 Thread::SpinRelease(&_WaitSetLock);
1329 }
1330
1331 // The thread is now either on off-list (TS_RUN),
1332 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1333 // The Node's TState variable is stable from the perspective of this thread.
1334 // No other threads will asynchronously modify TState.
1335 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1336 OrderAccess::loadload();
1337 if (_succ == Self) _succ = NULL;
1338 WasNotified = node._notified;
1339
1340 // Reentry phase -- reacquire the monitor.
1341 // re-enter contended monitor after object.wait().
1342 // retain OBJECT_WAIT state until re-enter successfully completes
1343 // Thread state is thread_in_vm and oop access is again safe,
1344 // although the raw address of the object may have changed.
1345 // (Don't cache naked oops over safepoints, of course).
1346
1347 // post monitor waited event. Note that this is past-tense, we are done waiting.
1348 if (JvmtiExport::should_post_monitor_waited()) {
1349 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1350
1351 if (node._notified != 0 && _succ == Self) {
1352 // In this part of the monitor wait-notify-reenter protocol it
1353 // is possible (and normal) for another thread to do a fastpath
1354 // monitor enter-exit while this thread is still trying to get
1355 // to the reenter portion of the protocol.
1356 //
1357 // The ObjectMonitor was notified and the current thread is
1358 // the successor which also means that an unpark() has already
1359 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1360 // consume the unpark() that was done when the successor was
1361 // set because the same ParkEvent is shared between Java
1362 // monitors and JVM/TI RawMonitors (for now).
1363 //
1364 // We redo the unpark() to ensure forward progress, i.e., we
1365 // don't want all pending threads hanging (parked) with none
1366 // entering the unlocked monitor.
1367 node._event->unpark();
1368 }
1369 }
1370
1371 if (event.should_commit()) {
1372 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1373 }
1374
1375 OrderAccess::fence();
1376
1377 assert(Self->_Stalled != 0, "invariant");
1378 Self->_Stalled = 0;
1379
1380 assert(_owner != Self, "invariant");
1381 ObjectWaiter::TStates v = node.TState;
1382 if (v == ObjectWaiter::TS_RUN) {
1383 enter(Self);
1384 } else {
1385 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1386 ReenterI(Self, &node);
1387 node.wait_reenter_end(this);
1388 }
1389
1390 // Self has reacquired the lock.
1391 // Lifecycle - the node representing Self must not appear on any queues.
1392 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1393 // want residual elements associated with this thread left on any lists.
1394 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1395 assert(_owner == Self, "invariant");
1396 assert(_succ != Self, "invariant");
1397 } // OSThreadWaitState()
1398
1399 jt->set_current_waiting_monitor(NULL);
1400
1401 guarantee(_recursions == 0, "invariant");
1402 _recursions = save; // restore the old recursion count
1403 _waiters--; // decrement the number of waiters
1404
1405 // Verify a few postconditions
1406 assert(_owner == Self, "invariant");
1407 assert(_succ != Self, "invariant");
1408 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
1409
1410 // check if the notification happened
1411 if (!WasNotified) {
1412 // no, it could be timeout or Thread.interrupt() or both
1413 // check for interrupt event, otherwise it is timeout
1414 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1415 THROW(vmSymbols::java_lang_InterruptedException());
1416 }
1417 }
1418
1419 // NOTE: Spurious wake up will be consider as timeout.
1420 // Monitor notify has precedence over thread interrupt.
1421 }
1422
1423
1424 // Consider:
1425 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1426 // then instead of transferring a thread from the WaitSet to the EntryList
1427 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1428
1429 void ObjectMonitor::INotify(Thread * Self) {
1430 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1431 ObjectWaiter * iterator = DequeueWaiter();
1432 if (iterator != NULL) {
1433 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1434 guarantee(iterator->_notified == 0, "invariant");
1435 // Disposition - what might we do with iterator ?
1436 // a. add it directly to the EntryList - either tail (policy == 1)
1437 // or head (policy == 0).
1438 // b. push it onto the front of the _cxq (policy == 2).
1439 // For now we use (b).
1440
1441 iterator->TState = ObjectWaiter::TS_ENTER;
1442
1443 iterator->_notified = 1;
1444 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1445
1446 ObjectWaiter * list = _EntryList;
1447 if (list != NULL) {
1448 assert(list->_prev == NULL, "invariant");
1449 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1450 assert(list != iterator, "invariant");
1451 }
1452
1453 // prepend to cxq
1454 if (list == NULL) {
1455 iterator->_next = iterator->_prev = NULL;
1456 _EntryList = iterator;
1457 } else {
1458 iterator->TState = ObjectWaiter::TS_CXQ;
1459 for (;;) {
1460 ObjectWaiter * front = _cxq;
1461 iterator->_next = front;
1462 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1463 break;
1464 }
1465 }
1466 }
1467
1468 // _WaitSetLock protects the wait queue, not the EntryList. We could
1469 // move the add-to-EntryList operation, above, outside the critical section
1470 // protected by _WaitSetLock. In practice that's not useful. With the
1471 // exception of wait() timeouts and interrupts the monitor owner
1472 // is the only thread that grabs _WaitSetLock. There's almost no contention
1473 // on _WaitSetLock so it's not profitable to reduce the length of the
1474 // critical section.
1475
1476 iterator->wait_reenter_begin(this);
1477 }
1478 Thread::SpinRelease(&_WaitSetLock);
1479 }
1480
1481 // Consider: a not-uncommon synchronization bug is to use notify() when
1482 // notifyAll() is more appropriate, potentially resulting in stranded
1483 // threads; this is one example of a lost wakeup. A useful diagnostic
1484 // option is to force all notify() operations to behave as notifyAll().
1485 //
1486 // Note: We can also detect many such problems with a "minimum wait".
1487 // When the "minimum wait" is set to a small non-zero timeout value
1488 // and the program does not hang whereas it did absent "minimum wait",
1489 // that suggests a lost wakeup bug.
1490
1491 void ObjectMonitor::notify(TRAPS) {
1492 CHECK_OWNER(); // Throws IMSE if not owner.
1493 if (_WaitSet == NULL) {
1494 return;
1495 }
1496 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1497 INotify(THREAD);
1498 OM_PERFDATA_OP(Notifications, inc(1));
1499 }
1500
1501
1502 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1503 // from the waitset to the EntryList. This could be done more efficiently with a
1504 // single bulk transfer but in practice it's not time-critical. Beware too,
1505 // that in prepend-mode we invert the order of the waiters. Let's say that the
1506 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1507 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1508
1509 void ObjectMonitor::notifyAll(TRAPS) {
1510 CHECK_OWNER(); // Throws IMSE if not owner.
1511 if (_WaitSet == NULL) {
1512 return;
1513 }
1514
1515 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1516 int tally = 0;
1517 while (_WaitSet != NULL) {
1518 tally++;
1519 INotify(THREAD);
1520 }
1521
1522 OM_PERFDATA_OP(Notifications, inc(tally));
1523 }
1524
1525 // -----------------------------------------------------------------------------
1526 // Adaptive Spinning Support
1527 //
1528 // Adaptive spin-then-block - rational spinning
1529 //
1530 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1531 // algorithm. On high order SMP systems it would be better to start with
1532 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1533 // a contending thread could enqueue itself on the cxq and then spin locally
1534 // on a thread-specific variable such as its ParkEvent._Event flag.
1535 // That's left as an exercise for the reader. Note that global spinning is
1536 // not problematic on Niagara, as the L2 cache serves the interconnect and
1537 // has both low latency and massive bandwidth.
1538 //
1539 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1540 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1541 // (duration) or we can fix the count at approximately the duration of
1542 // a context switch and vary the frequency. Of course we could also
1543 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1544 // For a description of 'Adaptive spin-then-block mutual exclusion in
1545 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1546 //
1547 // This implementation varies the duration "D", where D varies with
1548 // the success rate of recent spin attempts. (D is capped at approximately
1549 // length of a round-trip context switch). The success rate for recent
1550 // spin attempts is a good predictor of the success rate of future spin
1551 // attempts. The mechanism adapts automatically to varying critical
1552 // section length (lock modality), system load and degree of parallelism.
1553 // D is maintained per-monitor in _SpinDuration and is initialized
1554 // optimistically. Spin frequency is fixed at 100%.
1555 //
1556 // Note that _SpinDuration is volatile, but we update it without locks
1557 // or atomics. The code is designed so that _SpinDuration stays within
1558 // a reasonable range even in the presence of races. The arithmetic
1559 // operations on _SpinDuration are closed over the domain of legal values,
1560 // so at worst a race will install and older but still legal value.
1561 // At the very worst this introduces some apparent non-determinism.
1562 // We might spin when we shouldn't or vice-versa, but since the spin
1563 // count are relatively short, even in the worst case, the effect is harmless.
1564 //
1565 // Care must be taken that a low "D" value does not become an
1566 // an absorbing state. Transient spinning failures -- when spinning
1567 // is overall profitable -- should not cause the system to converge
1568 // on low "D" values. We want spinning to be stable and predictable
1569 // and fairly responsive to change and at the same time we don't want
1570 // it to oscillate, become metastable, be "too" non-deterministic,
1571 // or converge on or enter undesirable stable absorbing states.
1572 //
1573 // We implement a feedback-based control system -- using past behavior
1574 // to predict future behavior. We face two issues: (a) if the
1575 // input signal is random then the spin predictor won't provide optimal
1576 // results, and (b) if the signal frequency is too high then the control
1577 // system, which has some natural response lag, will "chase" the signal.
1578 // (b) can arise from multimodal lock hold times. Transient preemption
1579 // can also result in apparent bimodal lock hold times.
1580 // Although sub-optimal, neither condition is particularly harmful, as
1581 // in the worst-case we'll spin when we shouldn't or vice-versa.
1582 // The maximum spin duration is rather short so the failure modes aren't bad.
1583 // To be conservative, I've tuned the gain in system to bias toward
1584 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1585 // "rings" or oscillates between spinning and not spinning. This happens
1586 // when spinning is just on the cusp of profitability, however, so the
1587 // situation is not dire. The state is benign -- there's no need to add
1588 // hysteresis control to damp the transition rate between spinning and
1589 // not spinning.
1590
1591 // Spinning: Fixed frequency (100%), vary duration
1592 int ObjectMonitor::TrySpin(Thread * Self) {
1593 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1594 int ctr = Knob_FixedSpin;
1595 if (ctr != 0) {
1596 while (--ctr >= 0) {
1597 if (TryLock(Self) > 0) return 1;
1598 SpinPause();
1599 }
1600 return 0;
1601 }
1602
1603 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1604 if (TryLock(Self) > 0) {
1605 // Increase _SpinDuration ...
1606 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1607 // Raising _SpurDuration to the poverty line is key.
1608 int x = _SpinDuration;
1609 if (x < Knob_SpinLimit) {
1610 if (x < Knob_Poverty) x = Knob_Poverty;
1611 _SpinDuration = x + Knob_BonusB;
1612 }
1613 return 1;
1614 }
1615 SpinPause();
1616 }
1617
1618 // Admission control - verify preconditions for spinning
1619 //
1620 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1621 // becoming an absorbing state. Put another way, we spin briefly to
1622 // sample, just in case the system load, parallelism, contention, or lock
1623 // modality changed.
1624 //
1625 // Consider the following alternative:
1626 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1627 // spin attempt. "Periodically" might mean after a tally of
1628 // the # of failed spin attempts (or iterations) reaches some threshold.
1629 // This takes us into the realm of 1-out-of-N spinning, where we
1630 // hold the duration constant but vary the frequency.
1631
1632 ctr = _SpinDuration;
1633 if (ctr <= 0) return 0;
1634
1635 if (NotRunnable(Self, (Thread *) _owner)) {
1636 return 0;
1637 }
1638
1639 // We're good to spin ... spin ingress.
1640 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1641 // when preparing to LD...CAS _owner, etc and the CAS is likely
1642 // to succeed.
1643 if (_succ == NULL) {
1644 _succ = Self;
1645 }
1646 Thread * prv = NULL;
1647
1648 // There are three ways to exit the following loop:
1649 // 1. A successful spin where this thread has acquired the lock.
1650 // 2. Spin failure with prejudice
1651 // 3. Spin failure without prejudice
1652
1653 while (--ctr >= 0) {
1654
1655 // Periodic polling -- Check for pending GC
1656 // Threads may spin while they're unsafe.
1657 // We don't want spinning threads to delay the JVM from reaching
1658 // a stop-the-world safepoint or to steal cycles from GC.
1659 // If we detect a pending safepoint we abort in order that
1660 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1661 // this thread, if safe, doesn't steal cycles from GC.
1662 // This is in keeping with the "no loitering in runtime" rule.
1663 // We periodically check to see if there's a safepoint pending.
1664 if ((ctr & 0xFF) == 0) {
1665 if (SafepointMechanism::should_block(Self)) {
1666 goto Abort; // abrupt spin egress
1667 }
1668 SpinPause();
1669 }
1670
1671 // Probe _owner with TATAS
1672 // If this thread observes the monitor transition or flicker
1673 // from locked to unlocked to locked, then the odds that this
1674 // thread will acquire the lock in this spin attempt go down
1675 // considerably. The same argument applies if the CAS fails
1676 // or if we observe _owner change from one non-null value to
1677 // another non-null value. In such cases we might abort
1678 // the spin without prejudice or apply a "penalty" to the
1679 // spin count-down variable "ctr", reducing it by 100, say.
1680
1681 Thread * ox = (Thread *) _owner;
1682 if (ox == NULL) {
1683 ox = (Thread*)Atomic::cmpxchg(&_owner, (void*)NULL, Self);
1684 if (ox == NULL) {
1685 // The CAS succeeded -- this thread acquired ownership
1686 // Take care of some bookkeeping to exit spin state.
1687 if (_succ == Self) {
1688 _succ = NULL;
1689 }
1690
1691 // Increase _SpinDuration :
1692 // The spin was successful (profitable) so we tend toward
1693 // longer spin attempts in the future.
1694 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1695 // If we acquired the lock early in the spin cycle it
1696 // makes sense to increase _SpinDuration proportionally.
1697 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1698 int x = _SpinDuration;
1699 if (x < Knob_SpinLimit) {
1700 if (x < Knob_Poverty) x = Knob_Poverty;
1701 _SpinDuration = x + Knob_Bonus;
1702 }
1703 return 1;
1704 }
1705
1706 // The CAS failed ... we can take any of the following actions:
1707 // * penalize: ctr -= CASPenalty
1708 // * exit spin with prejudice -- goto Abort;
1709 // * exit spin without prejudice.
1710 // * Since CAS is high-latency, retry again immediately.
1711 prv = ox;
1712 goto Abort;
1713 }
1714
1715 // Did lock ownership change hands ?
1716 if (ox != prv && prv != NULL) {
1717 goto Abort;
1718 }
1719 prv = ox;
1720
1721 // Abort the spin if the owner is not executing.
1722 // The owner must be executing in order to drop the lock.
1723 // Spinning while the owner is OFFPROC is idiocy.
1724 // Consider: ctr -= RunnablePenalty ;
1725 if (NotRunnable(Self, ox)) {
1726 goto Abort;
1727 }
1728 if (_succ == NULL) {
1729 _succ = Self;
1730 }
1731 }
1732
1733 // Spin failed with prejudice -- reduce _SpinDuration.
1734 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1735 // AIMD is globally stable.
1736 {
1737 int x = _SpinDuration;
1738 if (x > 0) {
1739 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1740 // This is globally sample and tends to damp the response.
1741 x -= Knob_Penalty;
1742 if (x < 0) x = 0;
1743 _SpinDuration = x;
1744 }
1745 }
1746
1747 Abort:
1748 if (_succ == Self) {
1749 _succ = NULL;
1750 // Invariant: after setting succ=null a contending thread
1751 // must recheck-retry _owner before parking. This usually happens
1752 // in the normal usage of TrySpin(), but it's safest
1753 // to make TrySpin() as foolproof as possible.
1754 OrderAccess::fence();
1755 if (TryLock(Self) > 0) return 1;
1756 }
1757 return 0;
1758 }
1759
1760 // NotRunnable() -- informed spinning
1761 //
1762 // Don't bother spinning if the owner is not eligible to drop the lock.
1763 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1764 // The thread must be runnable in order to drop the lock in timely fashion.
1765 // If the _owner is not runnable then spinning will not likely be
1766 // successful (profitable).
1767 //
1768 // Beware -- the thread referenced by _owner could have died
1769 // so a simply fetch from _owner->_thread_state might trap.
1770 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1771 // Because of the lifecycle issues, the _thread_state values
1772 // observed by NotRunnable() might be garbage. NotRunnable must
1773 // tolerate this and consider the observed _thread_state value
1774 // as advisory.
1775 //
1776 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1777 // a thread pointer.
1778 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1779 // with the LSB of _owner. Another option would be to probabilistically probe
1780 // the putative _owner->TypeTag value.
1781 //
1782 // Checking _thread_state isn't perfect. Even if the thread is
1783 // in_java it might be blocked on a page-fault or have been preempted
1784 // and sitting on a ready/dispatch queue.
1785 //
1786 // The return value from NotRunnable() is *advisory* -- the
1787 // result is based on sampling and is not necessarily coherent.
1788 // The caller must tolerate false-negative and false-positive errors.
1789 // Spinning, in general, is probabilistic anyway.
1790
1791
1792 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1793 // Check ox->TypeTag == 2BAD.
1794 if (ox == NULL) return 0;
1795
1796 // Avoid transitive spinning ...
1797 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
1798 // Immediately after T1 acquires L it's possible that T2, also
1799 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1800 // This occurs transiently after T1 acquired L but before
1801 // T1 managed to clear T1.Stalled. T2 does not need to abort
1802 // its spin in this circumstance.
1803 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1804
1805 if (BlockedOn == 1) return 1;
1806 if (BlockedOn != 0) {
1807 return BlockedOn != intptr_t(this) && _owner == ox;
1808 }
1809
1810 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1811 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1812 // consider also: jst != _thread_in_Java -- but that's overspecific.
1813 return jst == _thread_blocked || jst == _thread_in_native;
1814 }
1815
1816
1817 // -----------------------------------------------------------------------------
1818 // WaitSet management ...
1819
1820 ObjectWaiter::ObjectWaiter(Thread* thread) {
1821 _next = NULL;
1822 _prev = NULL;
1823 _notified = 0;
1824 _notifier_tid = 0;
1825 TState = TS_RUN;
1826 _thread = thread;
1827 _event = thread->_ParkEvent;
1828 _active = false;
1829 assert(_event != NULL, "invariant");
1830 }
1831
1832 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1833 JavaThread *jt = (JavaThread *)this->_thread;
1834 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1835 }
1836
1837 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1838 JavaThread *jt = (JavaThread *)this->_thread;
1839 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1840 }
1841
1842 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1843 assert(node != NULL, "should not add NULL node");
1844 assert(node->_prev == NULL, "node already in list");
1845 assert(node->_next == NULL, "node already in list");
1846 // put node at end of queue (circular doubly linked list)
1847 if (_WaitSet == NULL) {
1848 _WaitSet = node;
1849 node->_prev = node;
1850 node->_next = node;
1851 } else {
1852 ObjectWaiter* head = _WaitSet;
1853 ObjectWaiter* tail = head->_prev;
1854 assert(tail->_next == head, "invariant check");
1855 tail->_next = node;
1856 head->_prev = node;
1857 node->_next = head;
1858 node->_prev = tail;
1859 }
1860 }
1861
1862 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1863 // dequeue the very first waiter
1864 ObjectWaiter* waiter = _WaitSet;
1865 if (waiter) {
1866 DequeueSpecificWaiter(waiter);
1867 }
1868 return waiter;
1869 }
1870
1871 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1872 assert(node != NULL, "should not dequeue NULL node");
1873 assert(node->_prev != NULL, "node already removed from list");
1874 assert(node->_next != NULL, "node already removed from list");
1875 // when the waiter has woken up because of interrupt,
1876 // timeout or other spurious wake-up, dequeue the
1877 // waiter from waiting list
1878 ObjectWaiter* next = node->_next;
1879 if (next == node) {
1880 assert(node->_prev == node, "invariant check");
1881 _WaitSet = NULL;
1882 } else {
1883 ObjectWaiter* prev = node->_prev;
1884 assert(prev->_next == node, "invariant check");
1885 assert(next->_prev == node, "invariant check");
1886 next->_prev = prev;
1887 prev->_next = next;
1888 if (_WaitSet == node) {
1889 _WaitSet = next;
1890 }
1891 }
1892 node->_next = NULL;
1893 node->_prev = NULL;
1894 }
1895
1896 // -----------------------------------------------------------------------------
1897 // PerfData support
1898 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
1899 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
1900 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
1901 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
1902 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
1903 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
1904 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
1905
1906 // One-shot global initialization for the sync subsystem.
1907 // We could also defer initialization and initialize on-demand
1908 // the first time we call ObjectSynchronizer::inflate().
1909 // Initialization would be protected - like so many things - by
1910 // the MonitorCache_lock.
1911
1912 void ObjectMonitor::Initialize() {
1913 assert(!InitDone, "invariant");
1914
1915 if (!os::is_MP()) {
1916 Knob_SpinLimit = 0;
1917 Knob_PreSpin = 0;
1918 Knob_FixedSpin = -1;
1919 }
1920
1921 if (UsePerfData) {
1922 EXCEPTION_MARK;
1923 #define NEWPERFCOUNTER(n) \
1924 { \
1925 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
1926 CHECK); \
1927 }
1928 #define NEWPERFVARIABLE(n) \
1929 { \
1930 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
1931 CHECK); \
1932 }
1933 NEWPERFCOUNTER(_sync_Inflations);
1934 NEWPERFCOUNTER(_sync_Deflations);
1935 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
1936 NEWPERFCOUNTER(_sync_FutileWakeups);
1937 NEWPERFCOUNTER(_sync_Parks);
1938 NEWPERFCOUNTER(_sync_Notifications);
1939 NEWPERFVARIABLE(_sync_MonExtant);
1940 #undef NEWPERFCOUNTER
1941 #undef NEWPERFVARIABLE
1942 }
1943
1944 DEBUG_ONLY(InitDone = true;)
1945 }
1946
1947 void ObjectMonitor::print_on(outputStream* st) const {
1948 // The minimal things to print for markWord printing, more can be added for debugging and logging.
1949 st->print("{contentions=0x%08x,waiters=0x%08x"
1950 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
1951 contentions(), waiters(), recursions(),
1952 p2i(owner()));
1953 }
1954 void ObjectMonitor::print() const { print_on(tty); }
1955
1956 #ifdef ASSERT
1957 // Print the ObjectMonitor like a debugger would:
1958 //
1959 // (ObjectMonitor) 0x00007fdfb6012e40 = {
1960 // _header = 0x0000000000000001
1961 // _object = 0x000000070ff45fd0
1962 // _next_om = 0x0000000000000000
1963 // _pad_buf0 = {
1964 // [0] = '\0'
1965 // ...
1966 // [103] = '\0'
1967 // }
1968 // _owner = 0x0000000000000000
1969 // _previous_owner_tid = 0
1970 // _recursions = 0
1971 // _EntryList = 0x0000000000000000
1972 // _cxq = 0x0000000000000000
1973 // _succ = 0x0000000000000000
1974 // _Responsible = 0x0000000000000000
1975 // _Spinner = 0
1976 // _SpinDuration = 5000
1977 // _contentions = 0
1978 // _WaitSet = 0x0000700009756248
1979 // _waiters = 1
1980 // _WaitSetLock = 0
1981 // }
1982 //
1983 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
1984 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
1985 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
1986 st->print_cr(" _object = " INTPTR_FORMAT, p2i(_object));
1987 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(_next_om));
1988 st->print_cr(" _pad_buf0 = {");
1989 st->print_cr(" [0] = '\\0'");
1990 st->print_cr(" ...");
1991 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
1992 st->print_cr(" }");
1993 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(_owner));
1994 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid);
1995 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
1996 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
1997 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
1998 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
1999 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2000 st->print_cr(" _Spinner = %d", _Spinner);
2001 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2002 st->print_cr(" _contentions = %d", _contentions);
2003 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2004 st->print_cr(" _waiters = %d", _waiters);
2005 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2006 st->print_cr("}");
2007 }
2008 #endif