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 = try_set_owner_from(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 set_owner_from_BasicLock(cur, Self); // Convert from BasicLock* to Thread*.
264 return;
265 }
266
267 // We've encountered genuine contention.
268 assert(Self->_Stalled == 0, "invariant");
269 Self->_Stalled = intptr_t(this);
270
271 // Try one round of spinning *before* enqueueing Self
272 // and before going through the awkward and expensive state
273 // transitions. The following spin is strictly optional ...
274 // Note that if we acquire the monitor from an initial spin
275 // we forgo posting JVMTI events and firing DTRACE probes.
276 if (TrySpin(Self) > 0) {
277 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner));
278 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
279 assert(((oop)object())->mark() == markWord::encode(this),
280 "object mark must match encoded this: mark=" INTPTR_FORMAT
281 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(),
282 markWord::encode(this).value());
283 Self->_Stalled = 0;
284 return;
285 }
286
287 assert(_owner != Self, "invariant");
288 assert(_succ != Self, "invariant");
289 assert(Self->is_Java_thread(), "invariant");
290 JavaThread * jt = (JavaThread *) Self;
291 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
292 assert(jt->thread_state() != _thread_blocked, "invariant");
293 assert(this->object() != NULL, "invariant");
294 assert(_contentions >= 0, "invariant");
295
296 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
297 // Ensure the object-monitor relationship remains stable while there's contention.
298 Atomic::inc(&_contentions);
299
300 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
301 EventJavaMonitorEnter event;
302 if (event.should_commit()) {
303 event.set_monitorClass(((oop)this->object())->klass());
304 event.set_address((uintptr_t)(this->object_addr()));
305 }
306
307 { // Change java thread status to indicate blocked on monitor enter.
308 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
309
310 Self->set_current_pending_monitor(this);
311
312 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
313 if (JvmtiExport::should_post_monitor_contended_enter()) {
314 JvmtiExport::post_monitor_contended_enter(jt, this);
315
316 // The current thread does not yet own the monitor and does not
317 // yet appear on any queues that would get it made the successor.
318 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
319 // handler cannot accidentally consume an unpark() meant for the
320 // ParkEvent associated with this ObjectMonitor.
321 }
322
323 OSThreadContendState osts(Self->osthread());
324 ThreadBlockInVM tbivm(jt);
325
326 // TODO-FIXME: change the following for(;;) loop to straight-line code.
327 for (;;) {
328 jt->set_suspend_equivalent();
329 // cleared by handle_special_suspend_equivalent_condition()
330 // or java_suspend_self()
331
332 EnterI(THREAD);
333
334 if (!ExitSuspendEquivalent(jt)) break;
335
336 // We have acquired the contended monitor, but while we were
337 // waiting another thread suspended us. We don't want to enter
338 // the monitor while suspended because that would surprise the
339 // thread that suspended us.
340 //
341 _recursions = 0;
342 _succ = NULL;
343 exit(false, Self);
344
345 jt->java_suspend_self();
346 }
347 Self->set_current_pending_monitor(NULL);
348
349 // We cleared the pending monitor info since we've just gotten past
350 // the enter-check-for-suspend dance and we now own the monitor free
351 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
352 // destructor can go to a safepoint at the end of this block. If we
353 // do a thread dump during that safepoint, then this thread will show
354 // as having "-locked" the monitor, but the OS and java.lang.Thread
355 // states will still report that the thread is blocked trying to
356 // acquire it.
357 }
358
359 Atomic::dec(&_contentions);
360 assert(_contentions >= 0, "invariant");
361 Self->_Stalled = 0;
362
363 // Must either set _recursions = 0 or ASSERT _recursions == 0.
364 assert(_recursions == 0, "invariant");
365 assert(_owner == Self, "invariant");
366 assert(_succ != Self, "invariant");
367 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
368
369 // The thread -- now the owner -- is back in vm mode.
370 // Report the glorious news via TI,DTrace and jvmstat.
371 // The probe effect is non-trivial. All the reportage occurs
372 // while we hold the monitor, increasing the length of the critical
373 // section. Amdahl's parallel speedup law comes vividly into play.
374 //
375 // Another option might be to aggregate the events (thread local or
376 // per-monitor aggregation) and defer reporting until a more opportune
377 // time -- such as next time some thread encounters contention but has
378 // yet to acquire the lock. While spinning that thread could
379 // spinning we could increment JVMStat counters, etc.
380
381 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
382 if (JvmtiExport::should_post_monitor_contended_entered()) {
383 JvmtiExport::post_monitor_contended_entered(jt, this);
384
385 // The current thread already owns the monitor and is not going to
386 // call park() for the remainder of the monitor enter protocol. So
387 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
388 // event handler consumed an unpark() issued by the thread that
389 // just exited the monitor.
390 }
391 if (event.should_commit()) {
392 event.set_previousOwner((uintptr_t)_previous_owner_tid);
393 event.commit();
394 }
395 OM_PERFDATA_OP(ContendedLockAttempts, inc());
396 }
397
398 // Caveat: TryLock() is not necessarily serializing if it returns failure.
399 // Callers must compensate as needed.
400
401 int ObjectMonitor::TryLock(Thread * Self) {
402 void * own = _owner;
403 if (own != NULL) return 0;
404 if (try_set_owner_from(NULL, Self) == NULL) {
405 assert(_recursions == 0, "invariant");
406 return 1;
407 }
408 // The lock had been free momentarily, but we lost the race to the lock.
409 // Interference -- the CAS failed.
410 // We can either return -1 or retry.
411 // Retry doesn't make as much sense because the lock was just acquired.
412 return -1;
413 }
414
415 // Convert the fields used by is_busy() to a string that can be
416 // used for diagnostic output.
417 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
418 ss->print("is_busy: contentions=%d, waiters=%d, owner=" INTPTR_FORMAT
419 ", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, _contentions,
420 _waiters, p2i(_owner), p2i(_cxq), p2i(_EntryList));
421 return ss->base();
422 }
423
424 #define MAX_RECHECK_INTERVAL 1000
425
426 void ObjectMonitor::EnterI(TRAPS) {
427 Thread * const Self = THREAD;
428 assert(Self->is_Java_thread(), "invariant");
429 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
430
431 // Try the lock - TATAS
432 if (TryLock (Self) > 0) {
433 assert(_succ != Self, "invariant");
434 assert(_owner == Self, "invariant");
435 assert(_Responsible != Self, "invariant");
436 return;
437 }
438
439 assert(InitDone, "Unexpectedly not initialized");
440
441 // We try one round of spinning *before* enqueueing Self.
442 //
443 // If the _owner is ready but OFFPROC we could use a YieldTo()
444 // operation to donate the remainder of this thread's quantum
445 // to the owner. This has subtle but beneficial affinity
446 // effects.
447
448 if (TrySpin(Self) > 0) {
449 assert(_owner == Self, "invariant");
450 assert(_succ != Self, "invariant");
451 assert(_Responsible != Self, "invariant");
452 return;
453 }
454
455 // The Spin failed -- Enqueue and park the thread ...
456 assert(_succ != Self, "invariant");
457 assert(_owner != Self, "invariant");
458 assert(_Responsible != Self, "invariant");
459
460 // Enqueue "Self" on ObjectMonitor's _cxq.
461 //
462 // Node acts as a proxy for Self.
463 // As an aside, if were to ever rewrite the synchronization code mostly
464 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
465 // Java objects. This would avoid awkward lifecycle and liveness issues,
466 // as well as eliminate a subset of ABA issues.
467 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
468
469 ObjectWaiter node(Self);
470 Self->_ParkEvent->reset();
471 node._prev = (ObjectWaiter *) 0xBAD;
472 node.TState = ObjectWaiter::TS_CXQ;
473
474 // Push "Self" onto the front of the _cxq.
475 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
476 // Note that spinning tends to reduce the rate at which threads
477 // enqueue and dequeue on EntryList|cxq.
478 ObjectWaiter * nxt;
479 for (;;) {
480 node._next = nxt = _cxq;
481 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
482
483 // Interference - the CAS failed because _cxq changed. Just retry.
484 // As an optional optimization we retry the lock.
485 if (TryLock (Self) > 0) {
486 assert(_succ != Self, "invariant");
487 assert(_owner == Self, "invariant");
488 assert(_Responsible != Self, "invariant");
489 return;
490 }
491 }
492
493 // Check for cxq|EntryList edge transition to non-null. This indicates
494 // the onset of contention. While contention persists exiting threads
495 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
496 // operations revert to the faster 1-0 mode. This enter operation may interleave
497 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
498 // arrange for one of the contending thread to use a timed park() operations
499 // to detect and recover from the race. (Stranding is form of progress failure
500 // where the monitor is unlocked but all the contending threads remain parked).
501 // That is, at least one of the contended threads will periodically poll _owner.
502 // One of the contending threads will become the designated "Responsible" thread.
503 // The Responsible thread uses a timed park instead of a normal indefinite park
504 // operation -- it periodically wakes and checks for and recovers from potential
505 // strandings admitted by 1-0 exit operations. We need at most one Responsible
506 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
507 // be responsible for a monitor.
508 //
509 // Currently, one of the contended threads takes on the added role of "Responsible".
510 // A viable alternative would be to use a dedicated "stranding checker" thread
511 // that periodically iterated over all the threads (or active monitors) and unparked
512 // successors where there was risk of stranding. This would help eliminate the
513 // timer scalability issues we see on some platforms as we'd only have one thread
514 // -- the checker -- parked on a timer.
515
516 if (nxt == NULL && _EntryList == NULL) {
517 // Try to assume the role of responsible thread for the monitor.
518 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
519 Atomic::replace_if_null(&_Responsible, Self);
520 }
521
522 // The lock might have been released while this thread was occupied queueing
523 // itself onto _cxq. To close the race and avoid "stranding" and
524 // progress-liveness failure we must resample-retry _owner before parking.
525 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
526 // In this case the ST-MEMBAR is accomplished with CAS().
527 //
528 // TODO: Defer all thread state transitions until park-time.
529 // Since state transitions are heavy and inefficient we'd like
530 // to defer the state transitions until absolutely necessary,
531 // and in doing so avoid some transitions ...
532
533 int nWakeups = 0;
534 int recheckInterval = 1;
535
536 for (;;) {
537
538 if (TryLock(Self) > 0) break;
539 assert(_owner != Self, "invariant");
540
541 // park self
542 if (_Responsible == Self) {
543 Self->_ParkEvent->park((jlong) recheckInterval);
544 // Increase the recheckInterval, but clamp the value.
545 recheckInterval *= 8;
546 if (recheckInterval > MAX_RECHECK_INTERVAL) {
547 recheckInterval = MAX_RECHECK_INTERVAL;
548 }
549 } else {
550 Self->_ParkEvent->park();
551 }
552
553 if (TryLock(Self) > 0) break;
554
555 // The lock is still contested.
556 // Keep a tally of the # of futile wakeups.
557 // Note that the counter is not protected by a lock or updated by atomics.
558 // That is by design - we trade "lossy" counters which are exposed to
559 // races during updates for a lower probe effect.
560
561 // This PerfData object can be used in parallel with a safepoint.
562 // See the work around in PerfDataManager::destroy().
563 OM_PERFDATA_OP(FutileWakeups, inc());
564 ++nWakeups;
565
566 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
567 // We can defer clearing _succ until after the spin completes
568 // TrySpin() must tolerate being called with _succ == Self.
569 // Try yet another round of adaptive spinning.
570 if (TrySpin(Self) > 0) break;
571
572 // We can find that we were unpark()ed and redesignated _succ while
573 // we were spinning. That's harmless. If we iterate and call park(),
574 // park() will consume the event and return immediately and we'll
575 // just spin again. This pattern can repeat, leaving _succ to simply
576 // spin on a CPU.
577
578 if (_succ == Self) _succ = NULL;
579
580 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
581 OrderAccess::fence();
582 }
583
584 // Egress :
585 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
586 // Normally we'll find Self on the EntryList .
587 // From the perspective of the lock owner (this thread), the
588 // EntryList is stable and cxq is prepend-only.
589 // The head of cxq is volatile but the interior is stable.
590 // In addition, Self.TState is stable.
591
592 assert(_owner == Self, "invariant");
593 assert(object() != NULL, "invariant");
594 // I'd like to write:
595 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ;
596 // but as we're at a safepoint that's not safe.
597
598 UnlinkAfterAcquire(Self, &node);
599 if (_succ == Self) _succ = NULL;
600
601 assert(_succ != Self, "invariant");
602 if (_Responsible == Self) {
603 _Responsible = NULL;
604 OrderAccess::fence(); // Dekker pivot-point
605
606 // We may leave threads on cxq|EntryList without a designated
607 // "Responsible" thread. This is benign. When this thread subsequently
608 // exits the monitor it can "see" such preexisting "old" threads --
609 // threads that arrived on the cxq|EntryList before the fence, above --
610 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
611 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
612 // non-null and elect a new "Responsible" timer thread.
613 //
614 // This thread executes:
615 // ST Responsible=null; MEMBAR (in enter epilogue - here)
616 // LD cxq|EntryList (in subsequent exit)
617 //
618 // Entering threads in the slow/contended path execute:
619 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
620 // The (ST cxq; MEMBAR) is accomplished with CAS().
621 //
622 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
623 // exit operation from floating above the ST Responsible=null.
624 }
625
626 // We've acquired ownership with CAS().
627 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
628 // But since the CAS() this thread may have also stored into _succ,
629 // EntryList, cxq or Responsible. These meta-data updates must be
630 // visible __before this thread subsequently drops the lock.
631 // Consider what could occur if we didn't enforce this constraint --
632 // STs to monitor meta-data and user-data could reorder with (become
633 // visible after) the ST in exit that drops ownership of the lock.
634 // Some other thread could then acquire the lock, but observe inconsistent
635 // or old monitor meta-data and heap data. That violates the JMM.
636 // To that end, the 1-0 exit() operation must have at least STST|LDST
637 // "release" barrier semantics. Specifically, there must be at least a
638 // STST|LDST barrier in exit() before the ST of null into _owner that drops
639 // the lock. The barrier ensures that changes to monitor meta-data and data
640 // protected by the lock will be visible before we release the lock, and
641 // therefore before some other thread (CPU) has a chance to acquire the lock.
642 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
643 //
644 // Critically, any prior STs to _succ or EntryList must be visible before
645 // the ST of null into _owner in the *subsequent* (following) corresponding
646 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
647 // execute a serializing instruction.
648
649 return;
650 }
651
652 // ReenterI() is a specialized inline form of the latter half of the
653 // contended slow-path from EnterI(). We use ReenterI() only for
654 // monitor reentry in wait().
655 //
656 // In the future we should reconcile EnterI() and ReenterI().
657
658 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
659 assert(Self != NULL, "invariant");
660 assert(SelfNode != NULL, "invariant");
661 assert(SelfNode->_thread == Self, "invariant");
662 assert(_waiters > 0, "invariant");
663 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
664 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
665 JavaThread * jt = (JavaThread *) Self;
666
667 int nWakeups = 0;
668 for (;;) {
669 ObjectWaiter::TStates v = SelfNode->TState;
670 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
671 assert(_owner != Self, "invariant");
672
673 if (TryLock(Self) > 0) break;
674 if (TrySpin(Self) > 0) break;
675
676 // State transition wrappers around park() ...
677 // ReenterI() wisely defers state transitions until
678 // it's clear we must park the thread.
679 {
680 OSThreadContendState osts(Self->osthread());
681 ThreadBlockInVM tbivm(jt);
682
683 // cleared by handle_special_suspend_equivalent_condition()
684 // or java_suspend_self()
685 jt->set_suspend_equivalent();
686 Self->_ParkEvent->park();
687
688 // were we externally suspended while we were waiting?
689 for (;;) {
690 if (!ExitSuspendEquivalent(jt)) break;
691 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
692 jt->java_suspend_self();
693 jt->set_suspend_equivalent();
694 }
695 }
696
697 // Try again, but just so we distinguish between futile wakeups and
698 // successful wakeups. The following test isn't algorithmically
699 // necessary, but it helps us maintain sensible statistics.
700 if (TryLock(Self) > 0) break;
701
702 // The lock is still contested.
703 // Keep a tally of the # of futile wakeups.
704 // Note that the counter is not protected by a lock or updated by atomics.
705 // That is by design - we trade "lossy" counters which are exposed to
706 // races during updates for a lower probe effect.
707 ++nWakeups;
708
709 // Assuming this is not a spurious wakeup we'll normally
710 // find that _succ == Self.
711 if (_succ == Self) _succ = NULL;
712
713 // Invariant: after clearing _succ a contending thread
714 // *must* retry _owner before parking.
715 OrderAccess::fence();
716
717 // This PerfData object can be used in parallel with a safepoint.
718 // See the work around in PerfDataManager::destroy().
719 OM_PERFDATA_OP(FutileWakeups, inc());
720 }
721
722 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
723 // Normally we'll find Self on the EntryList.
724 // Unlinking from the EntryList is constant-time and atomic-free.
725 // From the perspective of the lock owner (this thread), the
726 // EntryList is stable and cxq is prepend-only.
727 // The head of cxq is volatile but the interior is stable.
728 // In addition, Self.TState is stable.
729
730 assert(_owner == Self, "invariant");
731 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
732 UnlinkAfterAcquire(Self, SelfNode);
733 if (_succ == Self) _succ = NULL;
734 assert(_succ != Self, "invariant");
735 SelfNode->TState = ObjectWaiter::TS_RUN;
736 OrderAccess::fence(); // see comments at the end of EnterI()
737 }
738
739 // By convention we unlink a contending thread from EntryList|cxq immediately
740 // after the thread acquires the lock in ::enter(). Equally, we could defer
741 // unlinking the thread until ::exit()-time.
742
743 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
744 assert(_owner == Self, "invariant");
745 assert(SelfNode->_thread == Self, "invariant");
746
747 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
748 // Normal case: remove Self from the DLL EntryList .
749 // This is a constant-time operation.
750 ObjectWaiter * nxt = SelfNode->_next;
751 ObjectWaiter * prv = SelfNode->_prev;
752 if (nxt != NULL) nxt->_prev = prv;
753 if (prv != NULL) prv->_next = nxt;
754 if (SelfNode == _EntryList) _EntryList = nxt;
755 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
756 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
757 } else {
758 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
759 // Inopportune interleaving -- Self is still on the cxq.
760 // This usually means the enqueue of self raced an exiting thread.
761 // Normally we'll find Self near the front of the cxq, so
762 // dequeueing is typically fast. If needbe we can accelerate
763 // this with some MCS/CHL-like bidirectional list hints and advisory
764 // back-links so dequeueing from the interior will normally operate
765 // in constant-time.
766 // Dequeue Self from either the head (with CAS) or from the interior
767 // with a linear-time scan and normal non-atomic memory operations.
768 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
769 // and then unlink Self from EntryList. We have to drain eventually,
770 // so it might as well be now.
771
772 ObjectWaiter * v = _cxq;
773 assert(v != NULL, "invariant");
774 if (v != SelfNode || Atomic::cmpxchg(&_cxq, v, SelfNode->_next) != v) {
775 // The CAS above can fail from interference IFF a "RAT" arrived.
776 // In that case Self must be in the interior and can no longer be
777 // at the head of cxq.
778 if (v == SelfNode) {
779 assert(_cxq != v, "invariant");
780 v = _cxq; // CAS above failed - start scan at head of list
781 }
782 ObjectWaiter * p;
783 ObjectWaiter * q = NULL;
784 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
785 q = p;
786 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
787 }
788 assert(v != SelfNode, "invariant");
789 assert(p == SelfNode, "Node not found on cxq");
790 assert(p != _cxq, "invariant");
791 assert(q != NULL, "invariant");
792 assert(q->_next == p, "invariant");
793 q->_next = p->_next;
794 }
795 }
796
797 #ifdef ASSERT
798 // Diagnostic hygiene ...
799 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
800 SelfNode->_next = (ObjectWaiter *) 0xBAD;
801 SelfNode->TState = ObjectWaiter::TS_RUN;
802 #endif
803 }
804
805 // -----------------------------------------------------------------------------
806 // Exit support
807 //
808 // exit()
809 // ~~~~~~
810 // Note that the collector can't reclaim the objectMonitor or deflate
811 // the object out from underneath the thread calling ::exit() as the
812 // thread calling ::exit() never transitions to a stable state.
813 // This inhibits GC, which in turn inhibits asynchronous (and
814 // inopportune) reclamation of "this".
815 //
816 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
817 // There's one exception to the claim above, however. EnterI() can call
818 // exit() to drop a lock if the acquirer has been externally suspended.
819 // In that case exit() is called with _thread_state as _thread_blocked,
820 // but the monitor's _contentions field is > 0, which inhibits reclamation.
821 //
822 // 1-0 exit
823 // ~~~~~~~~
824 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
825 // the fast-path operators have been optimized so the common ::exit()
826 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
827 // The code emitted by fast_unlock() elides the usual MEMBAR. This
828 // greatly improves latency -- MEMBAR and CAS having considerable local
829 // latency on modern processors -- but at the cost of "stranding". Absent the
830 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
831 // ::enter() path, resulting in the entering thread being stranding
832 // and a progress-liveness failure. Stranding is extremely rare.
833 // We use timers (timed park operations) & periodic polling to detect
834 // and recover from stranding. Potentially stranded threads periodically
835 // wake up and poll the lock. See the usage of the _Responsible variable.
836 //
837 // The CAS() in enter provides for safety and exclusion, while the CAS or
838 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
839 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
840 // We detect and recover from stranding with timers.
841 //
842 // If a thread transiently strands it'll park until (a) another
843 // thread acquires the lock and then drops the lock, at which time the
844 // exiting thread will notice and unpark the stranded thread, or, (b)
845 // the timer expires. If the lock is high traffic then the stranding latency
846 // will be low due to (a). If the lock is low traffic then the odds of
847 // stranding are lower, although the worst-case stranding latency
848 // is longer. Critically, we don't want to put excessive load in the
849 // platform's timer subsystem. We want to minimize both the timer injection
850 // rate (timers created/sec) as well as the number of timers active at
851 // any one time. (more precisely, we want to minimize timer-seconds, which is
852 // the integral of the # of active timers at any instant over time).
853 // Both impinge on OS scalability. Given that, at most one thread parked on
854 // a monitor will use a timer.
855 //
856 // There is also the risk of a futile wake-up. If we drop the lock
857 // another thread can reacquire the lock immediately, and we can
858 // then wake a thread unnecessarily. This is benign, and we've
859 // structured the code so the windows are short and the frequency
860 // of such futile wakups is low.
861
862 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
863 Thread * const Self = THREAD;
864 if (THREAD != _owner) {
865 void* cur = _owner;
866 if (THREAD->is_lock_owned((address)cur)) {
867 assert(_recursions == 0, "invariant");
868 set_owner_from_BasicLock(cur, Self); // Convert from BasicLock* to Thread*.
869 _recursions = 0;
870 } else {
871 // Apparent unbalanced locking ...
872 // Naively we'd like to throw IllegalMonitorStateException.
873 // As a practical matter we can neither allocate nor throw an
874 // exception as ::exit() can be called from leaf routines.
875 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
876 // Upon deeper reflection, however, in a properly run JVM the only
877 // way we should encounter this situation is in the presence of
878 // unbalanced JNI locking. TODO: CheckJNICalls.
879 // See also: CR4414101
880 #ifdef ASSERT
881 LogStreamHandle(Error, monitorinflation) lsh;
882 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
883 " is exiting an ObjectMonitor it does not own.", p2i(THREAD));
884 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
885 print_debug_style_on(&lsh);
886 #endif
887 assert(false, "Non-balanced monitor enter/exit!");
888 return;
889 }
890 }
891
892 if (_recursions != 0) {
893 _recursions--; // this is simple recursive enter
894 return;
895 }
896
897 // Invariant: after setting Responsible=null an thread must execute
898 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
899 _Responsible = NULL;
900
901 #if INCLUDE_JFR
902 // get the owner's thread id for the MonitorEnter event
903 // if it is enabled and the thread isn't suspended
904 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
905 _previous_owner_tid = JFR_THREAD_ID(Self);
906 }
907 #endif
908
909 for (;;) {
910 assert(THREAD == _owner, "invariant");
911
912 // Drop the lock.
913 // release semantics: prior loads and stores from within the critical section
914 // must not float (reorder) past the following store that drops the lock.
915 // Uses a storeload to separate release_store(owner) from the
916 // successor check. The try_set_owner() below uses cmpxchg() so
917 // we get the fence down there.
918 release_clear_owner(Self);
919 OrderAccess::storeload();
920
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 (try_set_owner_from(NULL, Self) != NULL) {
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 // Uses a fence to separate release_store(owner) from the LD in unpark().
1096 release_clear_owner(Self);
1097 OrderAccess::fence();
1098
1099 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1100 Trigger->unpark();
1101
1102 // Maintain stats and report events to JVMTI
1103 OM_PERFDATA_OP(Parks, inc());
1104 }
1105
1106
1107 // -----------------------------------------------------------------------------
1108 // Class Loader deadlock handling.
1109 //
1110 // complete_exit exits a lock returning recursion count
1111 // complete_exit/reenter operate as a wait without waiting
1112 // complete_exit requires an inflated monitor
1113 // The _owner field is not always the Thread addr even with an
1114 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1115 // thread due to contention.
1116 intx ObjectMonitor::complete_exit(TRAPS) {
1117 Thread * const Self = THREAD;
1118 assert(Self->is_Java_thread(), "Must be Java thread!");
1119 JavaThread *jt = (JavaThread *)THREAD;
1120
1121 assert(InitDone, "Unexpectedly not initialized");
1122
1123 if (THREAD != _owner) {
1124 void* cur = _owner;
1125 if (THREAD->is_lock_owned((address)cur)) {
1126 assert(_recursions == 0, "internal state error");
1127 set_owner_from_BasicLock(cur, Self); // Convert from BasicLock* to Thread*.
1128 _recursions = 0;
1129 }
1130 }
1131
1132 guarantee(Self == _owner, "complete_exit not owner");
1133 intx save = _recursions; // record the old recursion count
1134 _recursions = 0; // set the recursion level to be 0
1135 exit(true, Self); // exit the monitor
1136 guarantee(_owner != Self, "invariant");
1137 return save;
1138 }
1139
1140 // reenter() enters a lock and sets recursion count
1141 // complete_exit/reenter operate as a wait without waiting
1142 void ObjectMonitor::reenter(intx recursions, TRAPS) {
1143 Thread * const Self = THREAD;
1144 assert(Self->is_Java_thread(), "Must be Java thread!");
1145 JavaThread *jt = (JavaThread *)THREAD;
1146
1147 guarantee(_owner != Self, "reenter already owner");
1148 enter(THREAD); // enter the monitor
1149 guarantee(_recursions == 0, "reenter recursion");
1150 _recursions = recursions;
1151 return;
1152 }
1153
1154 // Checks that the current THREAD owns this monitor and causes an
1155 // immediate return if it doesn't. We don't use the CHECK macro
1156 // because we want the IMSE to be the only exception that is thrown
1157 // from the call site when false is returned. Any other pending
1158 // exception is ignored.
1159 #define CHECK_OWNER() \
1160 do { \
1161 if (!check_owner(THREAD)) { \
1162 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1163 return; \
1164 } \
1165 } while (false)
1166
1167 // Returns true if the specified thread owns the ObjectMonitor.
1168 // Otherwise returns false and throws IllegalMonitorStateException
1169 // (IMSE). If there is a pending exception and the specified thread
1170 // is not the owner, that exception will be replaced by the IMSE.
1171 bool ObjectMonitor::check_owner(Thread* THREAD) {
1172 if (_owner == THREAD) {
1173 return true;
1174 }
1175 void* cur = _owner;
1176 if (THREAD->is_lock_owned((address)cur)) {
1177 set_owner_from_BasicLock(cur, THREAD); // Convert from BasicLock* to Thread*.
1178 _recursions = 0;
1179 return true;
1180 }
1181 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1182 "current thread is not owner", false);
1183 }
1184
1185 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1186 ObjectMonitor* monitor,
1187 jlong notifier_tid,
1188 jlong timeout,
1189 bool timedout) {
1190 assert(event != NULL, "invariant");
1191 assert(monitor != NULL, "invariant");
1192 event->set_monitorClass(((oop)monitor->object())->klass());
1193 event->set_timeout(timeout);
1194 event->set_address((uintptr_t)monitor->object_addr());
1195 event->set_notifier(notifier_tid);
1196 event->set_timedOut(timedout);
1197 event->commit();
1198 }
1199
1200 // -----------------------------------------------------------------------------
1201 // Wait/Notify/NotifyAll
1202 //
1203 // Note: a subset of changes to ObjectMonitor::wait()
1204 // will need to be replicated in complete_exit
1205 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1206 Thread * const Self = THREAD;
1207 assert(Self->is_Java_thread(), "Must be Java thread!");
1208 JavaThread *jt = (JavaThread *)THREAD;
1209
1210 assert(InitDone, "Unexpectedly not initialized");
1211
1212 CHECK_OWNER(); // Throws IMSE if not owner.
1213
1214 EventJavaMonitorWait event;
1215
1216 // check for a pending interrupt
1217 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1218 // post monitor waited event. Note that this is past-tense, we are done waiting.
1219 if (JvmtiExport::should_post_monitor_waited()) {
1220 // Note: 'false' parameter is passed here because the
1221 // wait was not timed out due to thread interrupt.
1222 JvmtiExport::post_monitor_waited(jt, this, false);
1223
1224 // In this short circuit of the monitor wait protocol, the
1225 // current thread never drops ownership of the monitor and
1226 // never gets added to the wait queue so the current thread
1227 // cannot be made the successor. This means that the
1228 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1229 // consume an unpark() meant for the ParkEvent associated with
1230 // this ObjectMonitor.
1231 }
1232 if (event.should_commit()) {
1233 post_monitor_wait_event(&event, this, 0, millis, false);
1234 }
1235 THROW(vmSymbols::java_lang_InterruptedException());
1236 return;
1237 }
1238
1239 assert(Self->_Stalled == 0, "invariant");
1240 Self->_Stalled = intptr_t(this);
1241 jt->set_current_waiting_monitor(this);
1242
1243 // create a node to be put into the queue
1244 // Critically, after we reset() the event but prior to park(), we must check
1245 // for a pending interrupt.
1246 ObjectWaiter node(Self);
1247 node.TState = ObjectWaiter::TS_WAIT;
1248 Self->_ParkEvent->reset();
1249 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1250
1251 // Enter the waiting queue, which is a circular doubly linked list in this case
1252 // but it could be a priority queue or any data structure.
1253 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1254 // by the the owner of the monitor *except* in the case where park()
1255 // returns because of a timeout of interrupt. Contention is exceptionally rare
1256 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1257
1258 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1259 AddWaiter(&node);
1260 Thread::SpinRelease(&_WaitSetLock);
1261
1262 _Responsible = NULL;
1263
1264 intx save = _recursions; // record the old recursion count
1265 _waiters++; // increment the number of waiters
1266 _recursions = 0; // set the recursion level to be 1
1267 exit(true, Self); // exit the monitor
1268 guarantee(_owner != Self, "invariant");
1269
1270 // The thread is on the WaitSet list - now park() it.
1271 // On MP systems it's conceivable that a brief spin before we park
1272 // could be profitable.
1273 //
1274 // TODO-FIXME: change the following logic to a loop of the form
1275 // while (!timeout && !interrupted && _notified == 0) park()
1276
1277 int ret = OS_OK;
1278 int WasNotified = 0;
1279
1280 // Need to check interrupt state whilst still _thread_in_vm
1281 bool interrupted = interruptible && jt->is_interrupted(false);
1282
1283 { // State transition wrappers
1284 OSThread* osthread = Self->osthread();
1285 OSThreadWaitState osts(osthread, true);
1286 {
1287 ThreadBlockInVM tbivm(jt);
1288 // Thread is in thread_blocked state and oop access is unsafe.
1289 jt->set_suspend_equivalent();
1290
1291 if (interrupted || HAS_PENDING_EXCEPTION) {
1292 // Intentionally empty
1293 } else if (node._notified == 0) {
1294 if (millis <= 0) {
1295 Self->_ParkEvent->park();
1296 } else {
1297 ret = Self->_ParkEvent->park(millis);
1298 }
1299 }
1300
1301 // were we externally suspended while we were waiting?
1302 if (ExitSuspendEquivalent (jt)) {
1303 // TODO-FIXME: add -- if succ == Self then succ = null.
1304 jt->java_suspend_self();
1305 }
1306
1307 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1308
1309 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1310 // from the WaitSet to the EntryList.
1311 // See if we need to remove Node from the WaitSet.
1312 // We use double-checked locking to avoid grabbing _WaitSetLock
1313 // if the thread is not on the wait queue.
1314 //
1315 // Note that we don't need a fence before the fetch of TState.
1316 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1317 // written by the is thread. (perhaps the fetch might even be satisfied
1318 // by a look-aside into the processor's own store buffer, although given
1319 // the length of the code path between the prior ST and this load that's
1320 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1321 // then we'll acquire the lock and then re-fetch a fresh TState value.
1322 // That is, we fail toward safety.
1323
1324 if (node.TState == ObjectWaiter::TS_WAIT) {
1325 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1326 if (node.TState == ObjectWaiter::TS_WAIT) {
1327 DequeueSpecificWaiter(&node); // unlink from WaitSet
1328 assert(node._notified == 0, "invariant");
1329 node.TState = ObjectWaiter::TS_RUN;
1330 }
1331 Thread::SpinRelease(&_WaitSetLock);
1332 }
1333
1334 // The thread is now either on off-list (TS_RUN),
1335 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1336 // The Node's TState variable is stable from the perspective of this thread.
1337 // No other threads will asynchronously modify TState.
1338 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1339 OrderAccess::loadload();
1340 if (_succ == Self) _succ = NULL;
1341 WasNotified = node._notified;
1342
1343 // Reentry phase -- reacquire the monitor.
1344 // re-enter contended monitor after object.wait().
1345 // retain OBJECT_WAIT state until re-enter successfully completes
1346 // Thread state is thread_in_vm and oop access is again safe,
1347 // although the raw address of the object may have changed.
1348 // (Don't cache naked oops over safepoints, of course).
1349
1350 // post monitor waited event. Note that this is past-tense, we are done waiting.
1351 if (JvmtiExport::should_post_monitor_waited()) {
1352 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1353
1354 if (node._notified != 0 && _succ == Self) {
1355 // In this part of the monitor wait-notify-reenter protocol it
1356 // is possible (and normal) for another thread to do a fastpath
1357 // monitor enter-exit while this thread is still trying to get
1358 // to the reenter portion of the protocol.
1359 //
1360 // The ObjectMonitor was notified and the current thread is
1361 // the successor which also means that an unpark() has already
1362 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1363 // consume the unpark() that was done when the successor was
1364 // set because the same ParkEvent is shared between Java
1365 // monitors and JVM/TI RawMonitors (for now).
1366 //
1367 // We redo the unpark() to ensure forward progress, i.e., we
1368 // don't want all pending threads hanging (parked) with none
1369 // entering the unlocked monitor.
1370 node._event->unpark();
1371 }
1372 }
1373
1374 if (event.should_commit()) {
1375 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1376 }
1377
1378 OrderAccess::fence();
1379
1380 assert(Self->_Stalled != 0, "invariant");
1381 Self->_Stalled = 0;
1382
1383 assert(_owner != Self, "invariant");
1384 ObjectWaiter::TStates v = node.TState;
1385 if (v == ObjectWaiter::TS_RUN) {
1386 enter(Self);
1387 } else {
1388 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1389 ReenterI(Self, &node);
1390 node.wait_reenter_end(this);
1391 }
1392
1393 // Self has reacquired the lock.
1394 // Lifecycle - the node representing Self must not appear on any queues.
1395 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1396 // want residual elements associated with this thread left on any lists.
1397 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1398 assert(_owner == Self, "invariant");
1399 assert(_succ != Self, "invariant");
1400 } // OSThreadWaitState()
1401
1402 jt->set_current_waiting_monitor(NULL);
1403
1404 guarantee(_recursions == 0, "invariant");
1405 _recursions = save; // restore the old recursion count
1406 _waiters--; // decrement the number of waiters
1407
1408 // Verify a few postconditions
1409 assert(_owner == Self, "invariant");
1410 assert(_succ != Self, "invariant");
1411 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
1412
1413 // check if the notification happened
1414 if (!WasNotified) {
1415 // no, it could be timeout or Thread.interrupt() or both
1416 // check for interrupt event, otherwise it is timeout
1417 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1418 THROW(vmSymbols::java_lang_InterruptedException());
1419 }
1420 }
1421
1422 // NOTE: Spurious wake up will be consider as timeout.
1423 // Monitor notify has precedence over thread interrupt.
1424 }
1425
1426
1427 // Consider:
1428 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1429 // then instead of transferring a thread from the WaitSet to the EntryList
1430 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1431
1432 void ObjectMonitor::INotify(Thread * Self) {
1433 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1434 ObjectWaiter * iterator = DequeueWaiter();
1435 if (iterator != NULL) {
1436 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1437 guarantee(iterator->_notified == 0, "invariant");
1438 // Disposition - what might we do with iterator ?
1439 // a. add it directly to the EntryList - either tail (policy == 1)
1440 // or head (policy == 0).
1441 // b. push it onto the front of the _cxq (policy == 2).
1442 // For now we use (b).
1443
1444 iterator->TState = ObjectWaiter::TS_ENTER;
1445
1446 iterator->_notified = 1;
1447 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1448
1449 ObjectWaiter * list = _EntryList;
1450 if (list != NULL) {
1451 assert(list->_prev == NULL, "invariant");
1452 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1453 assert(list != iterator, "invariant");
1454 }
1455
1456 // prepend to cxq
1457 if (list == NULL) {
1458 iterator->_next = iterator->_prev = NULL;
1459 _EntryList = iterator;
1460 } else {
1461 iterator->TState = ObjectWaiter::TS_CXQ;
1462 for (;;) {
1463 ObjectWaiter * front = _cxq;
1464 iterator->_next = front;
1465 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1466 break;
1467 }
1468 }
1469 }
1470
1471 // _WaitSetLock protects the wait queue, not the EntryList. We could
1472 // move the add-to-EntryList operation, above, outside the critical section
1473 // protected by _WaitSetLock. In practice that's not useful. With the
1474 // exception of wait() timeouts and interrupts the monitor owner
1475 // is the only thread that grabs _WaitSetLock. There's almost no contention
1476 // on _WaitSetLock so it's not profitable to reduce the length of the
1477 // critical section.
1478
1479 iterator->wait_reenter_begin(this);
1480 }
1481 Thread::SpinRelease(&_WaitSetLock);
1482 }
1483
1484 // Consider: a not-uncommon synchronization bug is to use notify() when
1485 // notifyAll() is more appropriate, potentially resulting in stranded
1486 // threads; this is one example of a lost wakeup. A useful diagnostic
1487 // option is to force all notify() operations to behave as notifyAll().
1488 //
1489 // Note: We can also detect many such problems with a "minimum wait".
1490 // When the "minimum wait" is set to a small non-zero timeout value
1491 // and the program does not hang whereas it did absent "minimum wait",
1492 // that suggests a lost wakeup bug.
1493
1494 void ObjectMonitor::notify(TRAPS) {
1495 CHECK_OWNER(); // Throws IMSE if not owner.
1496 if (_WaitSet == NULL) {
1497 return;
1498 }
1499 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1500 INotify(THREAD);
1501 OM_PERFDATA_OP(Notifications, inc(1));
1502 }
1503
1504
1505 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1506 // from the waitset to the EntryList. This could be done more efficiently with a
1507 // single bulk transfer but in practice it's not time-critical. Beware too,
1508 // that in prepend-mode we invert the order of the waiters. Let's say that the
1509 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1510 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1511
1512 void ObjectMonitor::notifyAll(TRAPS) {
1513 CHECK_OWNER(); // Throws IMSE if not owner.
1514 if (_WaitSet == NULL) {
1515 return;
1516 }
1517
1518 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1519 int tally = 0;
1520 while (_WaitSet != NULL) {
1521 tally++;
1522 INotify(THREAD);
1523 }
1524
1525 OM_PERFDATA_OP(Notifications, inc(tally));
1526 }
1527
1528 // -----------------------------------------------------------------------------
1529 // Adaptive Spinning Support
1530 //
1531 // Adaptive spin-then-block - rational spinning
1532 //
1533 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1534 // algorithm. On high order SMP systems it would be better to start with
1535 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1536 // a contending thread could enqueue itself on the cxq and then spin locally
1537 // on a thread-specific variable such as its ParkEvent._Event flag.
1538 // That's left as an exercise for the reader. Note that global spinning is
1539 // not problematic on Niagara, as the L2 cache serves the interconnect and
1540 // has both low latency and massive bandwidth.
1541 //
1542 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1543 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1544 // (duration) or we can fix the count at approximately the duration of
1545 // a context switch and vary the frequency. Of course we could also
1546 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1547 // For a description of 'Adaptive spin-then-block mutual exclusion in
1548 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1549 //
1550 // This implementation varies the duration "D", where D varies with
1551 // the success rate of recent spin attempts. (D is capped at approximately
1552 // length of a round-trip context switch). The success rate for recent
1553 // spin attempts is a good predictor of the success rate of future spin
1554 // attempts. The mechanism adapts automatically to varying critical
1555 // section length (lock modality), system load and degree of parallelism.
1556 // D is maintained per-monitor in _SpinDuration and is initialized
1557 // optimistically. Spin frequency is fixed at 100%.
1558 //
1559 // Note that _SpinDuration is volatile, but we update it without locks
1560 // or atomics. The code is designed so that _SpinDuration stays within
1561 // a reasonable range even in the presence of races. The arithmetic
1562 // operations on _SpinDuration are closed over the domain of legal values,
1563 // so at worst a race will install and older but still legal value.
1564 // At the very worst this introduces some apparent non-determinism.
1565 // We might spin when we shouldn't or vice-versa, but since the spin
1566 // count are relatively short, even in the worst case, the effect is harmless.
1567 //
1568 // Care must be taken that a low "D" value does not become an
1569 // an absorbing state. Transient spinning failures -- when spinning
1570 // is overall profitable -- should not cause the system to converge
1571 // on low "D" values. We want spinning to be stable and predictable
1572 // and fairly responsive to change and at the same time we don't want
1573 // it to oscillate, become metastable, be "too" non-deterministic,
1574 // or converge on or enter undesirable stable absorbing states.
1575 //
1576 // We implement a feedback-based control system -- using past behavior
1577 // to predict future behavior. We face two issues: (a) if the
1578 // input signal is random then the spin predictor won't provide optimal
1579 // results, and (b) if the signal frequency is too high then the control
1580 // system, which has some natural response lag, will "chase" the signal.
1581 // (b) can arise from multimodal lock hold times. Transient preemption
1582 // can also result in apparent bimodal lock hold times.
1583 // Although sub-optimal, neither condition is particularly harmful, as
1584 // in the worst-case we'll spin when we shouldn't or vice-versa.
1585 // The maximum spin duration is rather short so the failure modes aren't bad.
1586 // To be conservative, I've tuned the gain in system to bias toward
1587 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1588 // "rings" or oscillates between spinning and not spinning. This happens
1589 // when spinning is just on the cusp of profitability, however, so the
1590 // situation is not dire. The state is benign -- there's no need to add
1591 // hysteresis control to damp the transition rate between spinning and
1592 // not spinning.
1593
1594 // Spinning: Fixed frequency (100%), vary duration
1595 int ObjectMonitor::TrySpin(Thread * Self) {
1596 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1597 int ctr = Knob_FixedSpin;
1598 if (ctr != 0) {
1599 while (--ctr >= 0) {
1600 if (TryLock(Self) > 0) return 1;
1601 SpinPause();
1602 }
1603 return 0;
1604 }
1605
1606 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1607 if (TryLock(Self) > 0) {
1608 // Increase _SpinDuration ...
1609 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1610 // Raising _SpurDuration to the poverty line is key.
1611 int x = _SpinDuration;
1612 if (x < Knob_SpinLimit) {
1613 if (x < Knob_Poverty) x = Knob_Poverty;
1614 _SpinDuration = x + Knob_BonusB;
1615 }
1616 return 1;
1617 }
1618 SpinPause();
1619 }
1620
1621 // Admission control - verify preconditions for spinning
1622 //
1623 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1624 // becoming an absorbing state. Put another way, we spin briefly to
1625 // sample, just in case the system load, parallelism, contention, or lock
1626 // modality changed.
1627 //
1628 // Consider the following alternative:
1629 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1630 // spin attempt. "Periodically" might mean after a tally of
1631 // the # of failed spin attempts (or iterations) reaches some threshold.
1632 // This takes us into the realm of 1-out-of-N spinning, where we
1633 // hold the duration constant but vary the frequency.
1634
1635 ctr = _SpinDuration;
1636 if (ctr <= 0) return 0;
1637
1638 if (NotRunnable(Self, (Thread *) _owner)) {
1639 return 0;
1640 }
1641
1642 // We're good to spin ... spin ingress.
1643 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1644 // when preparing to LD...CAS _owner, etc and the CAS is likely
1645 // to succeed.
1646 if (_succ == NULL) {
1647 _succ = Self;
1648 }
1649 Thread * prv = NULL;
1650
1651 // There are three ways to exit the following loop:
1652 // 1. A successful spin where this thread has acquired the lock.
1653 // 2. Spin failure with prejudice
1654 // 3. Spin failure without prejudice
1655
1656 while (--ctr >= 0) {
1657
1658 // Periodic polling -- Check for pending GC
1659 // Threads may spin while they're unsafe.
1660 // We don't want spinning threads to delay the JVM from reaching
1661 // a stop-the-world safepoint or to steal cycles from GC.
1662 // If we detect a pending safepoint we abort in order that
1663 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1664 // this thread, if safe, doesn't steal cycles from GC.
1665 // This is in keeping with the "no loitering in runtime" rule.
1666 // We periodically check to see if there's a safepoint pending.
1667 if ((ctr & 0xFF) == 0) {
1668 if (SafepointMechanism::should_block(Self)) {
1669 goto Abort; // abrupt spin egress
1670 }
1671 SpinPause();
1672 }
1673
1674 // Probe _owner with TATAS
1675 // If this thread observes the monitor transition or flicker
1676 // from locked to unlocked to locked, then the odds that this
1677 // thread will acquire the lock in this spin attempt go down
1678 // considerably. The same argument applies if the CAS fails
1679 // or if we observe _owner change from one non-null value to
1680 // another non-null value. In such cases we might abort
1681 // the spin without prejudice or apply a "penalty" to the
1682 // spin count-down variable "ctr", reducing it by 100, say.
1683
1684 Thread * ox = (Thread *) _owner;
1685 if (ox == NULL) {
1686 ox = (Thread*)try_set_owner_from(NULL, Self);
1687 if (ox == NULL) {
1688 // The CAS succeeded -- this thread acquired ownership
1689 // Take care of some bookkeeping to exit spin state.
1690 if (_succ == Self) {
1691 _succ = NULL;
1692 }
1693
1694 // Increase _SpinDuration :
1695 // The spin was successful (profitable) so we tend toward
1696 // longer spin attempts in the future.
1697 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1698 // If we acquired the lock early in the spin cycle it
1699 // makes sense to increase _SpinDuration proportionally.
1700 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1701 int x = _SpinDuration;
1702 if (x < Knob_SpinLimit) {
1703 if (x < Knob_Poverty) x = Knob_Poverty;
1704 _SpinDuration = x + Knob_Bonus;
1705 }
1706 return 1;
1707 }
1708
1709 // The CAS failed ... we can take any of the following actions:
1710 // * penalize: ctr -= CASPenalty
1711 // * exit spin with prejudice -- goto Abort;
1712 // * exit spin without prejudice.
1713 // * Since CAS is high-latency, retry again immediately.
1714 prv = ox;
1715 goto Abort;
1716 }
1717
1718 // Did lock ownership change hands ?
1719 if (ox != prv && prv != NULL) {
1720 goto Abort;
1721 }
1722 prv = ox;
1723
1724 // Abort the spin if the owner is not executing.
1725 // The owner must be executing in order to drop the lock.
1726 // Spinning while the owner is OFFPROC is idiocy.
1727 // Consider: ctr -= RunnablePenalty ;
1728 if (NotRunnable(Self, ox)) {
1729 goto Abort;
1730 }
1731 if (_succ == NULL) {
1732 _succ = Self;
1733 }
1734 }
1735
1736 // Spin failed with prejudice -- reduce _SpinDuration.
1737 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1738 // AIMD is globally stable.
1739 {
1740 int x = _SpinDuration;
1741 if (x > 0) {
1742 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1743 // This is globally sample and tends to damp the response.
1744 x -= Knob_Penalty;
1745 if (x < 0) x = 0;
1746 _SpinDuration = x;
1747 }
1748 }
1749
1750 Abort:
1751 if (_succ == Self) {
1752 _succ = NULL;
1753 // Invariant: after setting succ=null a contending thread
1754 // must recheck-retry _owner before parking. This usually happens
1755 // in the normal usage of TrySpin(), but it's safest
1756 // to make TrySpin() as foolproof as possible.
1757 OrderAccess::fence();
1758 if (TryLock(Self) > 0) return 1;
1759 }
1760 return 0;
1761 }
1762
1763 // NotRunnable() -- informed spinning
1764 //
1765 // Don't bother spinning if the owner is not eligible to drop the lock.
1766 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1767 // The thread must be runnable in order to drop the lock in timely fashion.
1768 // If the _owner is not runnable then spinning will not likely be
1769 // successful (profitable).
1770 //
1771 // Beware -- the thread referenced by _owner could have died
1772 // so a simply fetch from _owner->_thread_state might trap.
1773 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1774 // Because of the lifecycle issues, the _thread_state values
1775 // observed by NotRunnable() might be garbage. NotRunnable must
1776 // tolerate this and consider the observed _thread_state value
1777 // as advisory.
1778 //
1779 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1780 // a thread pointer.
1781 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1782 // with the LSB of _owner. Another option would be to probabilistically probe
1783 // the putative _owner->TypeTag value.
1784 //
1785 // Checking _thread_state isn't perfect. Even if the thread is
1786 // in_java it might be blocked on a page-fault or have been preempted
1787 // and sitting on a ready/dispatch queue.
1788 //
1789 // The return value from NotRunnable() is *advisory* -- the
1790 // result is based on sampling and is not necessarily coherent.
1791 // The caller must tolerate false-negative and false-positive errors.
1792 // Spinning, in general, is probabilistic anyway.
1793
1794
1795 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1796 // Check ox->TypeTag == 2BAD.
1797 if (ox == NULL) return 0;
1798
1799 // Avoid transitive spinning ...
1800 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
1801 // Immediately after T1 acquires L it's possible that T2, also
1802 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1803 // This occurs transiently after T1 acquired L but before
1804 // T1 managed to clear T1.Stalled. T2 does not need to abort
1805 // its spin in this circumstance.
1806 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1807
1808 if (BlockedOn == 1) return 1;
1809 if (BlockedOn != 0) {
1810 return BlockedOn != intptr_t(this) && _owner == ox;
1811 }
1812
1813 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1814 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1815 // consider also: jst != _thread_in_Java -- but that's overspecific.
1816 return jst == _thread_blocked || jst == _thread_in_native;
1817 }
1818
1819
1820 // -----------------------------------------------------------------------------
1821 // WaitSet management ...
1822
1823 ObjectWaiter::ObjectWaiter(Thread* thread) {
1824 _next = NULL;
1825 _prev = NULL;
1826 _notified = 0;
1827 _notifier_tid = 0;
1828 TState = TS_RUN;
1829 _thread = thread;
1830 _event = thread->_ParkEvent;
1831 _active = false;
1832 assert(_event != NULL, "invariant");
1833 }
1834
1835 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1836 JavaThread *jt = (JavaThread *)this->_thread;
1837 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1838 }
1839
1840 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1841 JavaThread *jt = (JavaThread *)this->_thread;
1842 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1843 }
1844
1845 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1846 assert(node != NULL, "should not add NULL node");
1847 assert(node->_prev == NULL, "node already in list");
1848 assert(node->_next == NULL, "node already in list");
1849 // put node at end of queue (circular doubly linked list)
1850 if (_WaitSet == NULL) {
1851 _WaitSet = node;
1852 node->_prev = node;
1853 node->_next = node;
1854 } else {
1855 ObjectWaiter* head = _WaitSet;
1856 ObjectWaiter* tail = head->_prev;
1857 assert(tail->_next == head, "invariant check");
1858 tail->_next = node;
1859 head->_prev = node;
1860 node->_next = head;
1861 node->_prev = tail;
1862 }
1863 }
1864
1865 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1866 // dequeue the very first waiter
1867 ObjectWaiter* waiter = _WaitSet;
1868 if (waiter) {
1869 DequeueSpecificWaiter(waiter);
1870 }
1871 return waiter;
1872 }
1873
1874 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1875 assert(node != NULL, "should not dequeue NULL node");
1876 assert(node->_prev != NULL, "node already removed from list");
1877 assert(node->_next != NULL, "node already removed from list");
1878 // when the waiter has woken up because of interrupt,
1879 // timeout or other spurious wake-up, dequeue the
1880 // waiter from waiting list
1881 ObjectWaiter* next = node->_next;
1882 if (next == node) {
1883 assert(node->_prev == node, "invariant check");
1884 _WaitSet = NULL;
1885 } else {
1886 ObjectWaiter* prev = node->_prev;
1887 assert(prev->_next == node, "invariant check");
1888 assert(next->_prev == node, "invariant check");
1889 next->_prev = prev;
1890 prev->_next = next;
1891 if (_WaitSet == node) {
1892 _WaitSet = next;
1893 }
1894 }
1895 node->_next = NULL;
1896 node->_prev = NULL;
1897 }
1898
1899 // -----------------------------------------------------------------------------
1900 // PerfData support
1901 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
1902 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
1903 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
1904 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
1905 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
1906 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
1907 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
1908
1909 // One-shot global initialization for the sync subsystem.
1910 // We could also defer initialization and initialize on-demand
1911 // the first time we call ObjectSynchronizer::inflate().
1912 // Initialization would be protected - like so many things - by
1913 // the MonitorCache_lock.
1914
1915 void ObjectMonitor::Initialize() {
1916 assert(!InitDone, "invariant");
1917
1918 if (!os::is_MP()) {
1919 Knob_SpinLimit = 0;
1920 Knob_PreSpin = 0;
1921 Knob_FixedSpin = -1;
1922 }
1923
1924 if (UsePerfData) {
1925 EXCEPTION_MARK;
1926 #define NEWPERFCOUNTER(n) \
1927 { \
1928 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
1929 CHECK); \
1930 }
1931 #define NEWPERFVARIABLE(n) \
1932 { \
1933 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
1934 CHECK); \
1935 }
1936 NEWPERFCOUNTER(_sync_Inflations);
1937 NEWPERFCOUNTER(_sync_Deflations);
1938 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
1939 NEWPERFCOUNTER(_sync_FutileWakeups);
1940 NEWPERFCOUNTER(_sync_Parks);
1941 NEWPERFCOUNTER(_sync_Notifications);
1942 NEWPERFVARIABLE(_sync_MonExtant);
1943 #undef NEWPERFCOUNTER
1944 #undef NEWPERFVARIABLE
1945 }
1946
1947 DEBUG_ONLY(InitDone = true;)
1948 }
1949
1950 void ObjectMonitor::print_on(outputStream* st) const {
1951 // The minimal things to print for markWord printing, more can be added for debugging and logging.
1952 st->print("{contentions=0x%08x,waiters=0x%08x"
1953 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
1954 contentions(), waiters(), recursions(),
1955 p2i(owner()));
1956 }
1957 void ObjectMonitor::print() const { print_on(tty); }
1958
1959 #ifdef ASSERT
1960 // Print the ObjectMonitor like a debugger would:
1961 //
1962 // (ObjectMonitor) 0x00007fdfb6012e40 = {
1963 // _header = 0x0000000000000001
1964 // _object = 0x000000070ff45fd0
1965 // _next_om = 0x0000000000000000
1966 // _pad_buf0 = {
1967 // [0] = '\0'
1968 // ...
1969 // [103] = '\0'
1970 // }
1971 // _owner = 0x0000000000000000
1972 // _previous_owner_tid = 0
1973 // _recursions = 0
1974 // _EntryList = 0x0000000000000000
1975 // _cxq = 0x0000000000000000
1976 // _succ = 0x0000000000000000
1977 // _Responsible = 0x0000000000000000
1978 // _Spinner = 0
1979 // _SpinDuration = 5000
1980 // _contentions = 0
1981 // _WaitSet = 0x0000700009756248
1982 // _waiters = 1
1983 // _WaitSetLock = 0
1984 // }
1985 //
1986 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
1987 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
1988 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
1989 st->print_cr(" _object = " INTPTR_FORMAT, p2i(_object));
1990 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(_next_om));
1991 st->print_cr(" _pad_buf0 = {");
1992 st->print_cr(" [0] = '\\0'");
1993 st->print_cr(" ...");
1994 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
1995 st->print_cr(" }");
1996 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(_owner));
1997 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid);
1998 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
1999 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2000 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2001 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2002 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2003 st->print_cr(" _Spinner = %d", _Spinner);
2004 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2005 st->print_cr(" _contentions = %d", _contentions);
2006 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2007 st->print_cr(" _waiters = %d", _waiters);
2008 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2009 st->print_cr("}");
2010 }
2011 #endif