1 /*
2 * Copyright (c) 1998, 2018, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "jfr/jfrEvents.hpp"
28 #include "jfr/support/jfrThreadId.hpp"
29 #include "memory/allocation.inline.hpp"
30 #include "memory/resourceArea.hpp"
31 #include "oops/markOop.hpp"
32 #include "oops/oop.inline.hpp"
33 #include "runtime/atomic.hpp"
34 #include "runtime/handles.inline.hpp"
35 #include "runtime/interfaceSupport.inline.hpp"
36 #include "runtime/mutexLocker.hpp"
37 #include "runtime/objectMonitor.hpp"
38 #include "runtime/objectMonitor.inline.hpp"
39 #include "runtime/orderAccess.hpp"
40 #include "runtime/osThread.hpp"
41 #include "runtime/safepointMechanism.inline.hpp"
42 #include "runtime/sharedRuntime.hpp"
43 #include "runtime/stubRoutines.hpp"
44 #include "runtime/thread.inline.hpp"
45 #include "services/threadService.hpp"
46 #include "utilities/dtrace.hpp"
47 #include "utilities/macros.hpp"
48 #include "utilities/preserveException.hpp"
49 #if INCLUDE_JFR
50 #include "jfr/support/jfrFlush.hpp"
51 #endif
52
53 #ifdef DTRACE_ENABLED
54
55 // Only bother with this argument setup if dtrace is available
56 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
57
58
59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
60 char* bytes = NULL; \
61 int len = 0; \
62 jlong jtid = SharedRuntime::get_java_tid(thread); \
63 Symbol* klassname = ((oop)obj)->klass()->name(); \
64 if (klassname != NULL) { \
65 bytes = (char*)klassname->bytes(); \
66 len = klassname->utf8_length(); \
67 }
68
69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
70 { \
71 if (DTraceMonitorProbes) { \
72 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
73 HOTSPOT_MONITOR_WAIT(jtid, \
74 (monitor), bytes, len, (millis)); \
75 } \
76 }
77
78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
83
84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
85 { \
86 if (DTraceMonitorProbes) { \
87 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
88 HOTSPOT_MONITOR_##probe(jtid, \
89 (uintptr_t)(monitor), bytes, len); \
90 } \
91 }
92
93 #else // ndef DTRACE_ENABLED
94
95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
97
98 #endif // ndef DTRACE_ENABLED
99
100 // Tunables ...
101 // The knob* variables are effectively final. Once set they should
102 // never be modified hence. Consider using __read_mostly with GCC.
103
104 int ObjectMonitor::Knob_ExitRelease = 0;
105 int ObjectMonitor::Knob_InlineNotify = 1;
106 int ObjectMonitor::Knob_Verbose = 0;
107 int ObjectMonitor::Knob_VerifyInUse = 0;
108 int ObjectMonitor::Knob_VerifyMatch = 0;
109 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
110
111 static int Knob_ReportSettings = 0;
112 static int Knob_SpinBase = 0; // Floor AKA SpinMin
113 static int Knob_SpinBackOff = 0; // spin-loop backoff
114 static int Knob_CASPenalty = -1; // Penalty for failed CAS
115 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
116 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
117 static int Knob_SpinEarly = 1;
118 static int Knob_SuccEnabled = 1; // futile wake throttling
119 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
120 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
121 static int Knob_Bonus = 100; // spin success bonus
122 static int Knob_BonusB = 100; // spin success bonus
123 static int Knob_Penalty = 200; // spin failure penalty
124 static int Knob_Poverty = 1000;
125 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
126 static int Knob_FixedSpin = 0;
127 static int Knob_OState = 3; // Spinner checks thread state of _owner
128 static int Knob_UsePause = 1;
129 static int Knob_ExitPolicy = 0;
130 static int Knob_PreSpin = 10; // 20-100 likely better
131 static int Knob_ResetEvent = 0;
132 static int BackOffMask = 0;
133
134 static int Knob_FastHSSEC = 0;
135 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
136 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
137 static volatile int InitDone = 0;
138
139 // -----------------------------------------------------------------------------
140 // Theory of operations -- Monitors lists, thread residency, etc:
141 //
142 // * A thread acquires ownership of a monitor by successfully
143 // CAS()ing the _owner field from null to non-null.
144 //
145 // * Invariant: A thread appears on at most one monitor list --
146 // cxq, EntryList or WaitSet -- at any one time.
147 //
148 // * Contending threads "push" themselves onto the cxq with CAS
149 // and then spin/park.
150 //
151 // * After a contending thread eventually acquires the lock it must
152 // dequeue itself from either the EntryList or the cxq.
153 //
154 // * The exiting thread identifies and unparks an "heir presumptive"
155 // tentative successor thread on the EntryList. Critically, the
156 // exiting thread doesn't unlink the successor thread from the EntryList.
157 // After having been unparked, the wakee will recontend for ownership of
158 // the monitor. The successor (wakee) will either acquire the lock or
159 // re-park itself.
160 //
161 // Succession is provided for by a policy of competitive handoff.
162 // The exiting thread does _not_ grant or pass ownership to the
163 // successor thread. (This is also referred to as "handoff" succession").
164 // Instead the exiting thread releases ownership and possibly wakes
165 // a successor, so the successor can (re)compete for ownership of the lock.
166 // If the EntryList is empty but the cxq is populated the exiting
167 // thread will drain the cxq into the EntryList. It does so by
168 // by detaching the cxq (installing null with CAS) and folding
169 // the threads from the cxq into the EntryList. The EntryList is
170 // doubly linked, while the cxq is singly linked because of the
171 // CAS-based "push" used to enqueue recently arrived threads (RATs).
172 //
173 // * Concurrency invariants:
174 //
175 // -- only the monitor owner may access or mutate the EntryList.
176 // The mutex property of the monitor itself protects the EntryList
177 // from concurrent interference.
178 // -- Only the monitor owner may detach the cxq.
179 //
180 // * The monitor entry list operations avoid locks, but strictly speaking
181 // they're not lock-free. Enter is lock-free, exit is not.
182 // For a description of 'Methods and apparatus providing non-blocking access
183 // to a resource,' see U.S. Pat. No. 7844973.
184 //
185 // * The cxq can have multiple concurrent "pushers" but only one concurrent
186 // detaching thread. This mechanism is immune from the ABA corruption.
187 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
188 //
189 // * Taken together, the cxq and the EntryList constitute or form a
190 // single logical queue of threads stalled trying to acquire the lock.
191 // We use two distinct lists to improve the odds of a constant-time
192 // dequeue operation after acquisition (in the ::enter() epilogue) and
193 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
194 // A key desideratum is to minimize queue & monitor metadata manipulation
195 // that occurs while holding the monitor lock -- that is, we want to
196 // minimize monitor lock holds times. Note that even a small amount of
197 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
198 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
199 // locks and monitor metadata.
200 //
201 // Cxq points to the set of Recently Arrived Threads attempting entry.
202 // Because we push threads onto _cxq with CAS, the RATs must take the form of
203 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
204 // the unlocking thread notices that EntryList is null but _cxq is != null.
205 //
206 // The EntryList is ordered by the prevailing queue discipline and
207 // can be organized in any convenient fashion, such as a doubly-linked list or
208 // a circular doubly-linked list. Critically, we want insert and delete operations
209 // to operate in constant-time. If we need a priority queue then something akin
210 // to Solaris' sleepq would work nicely. Viz.,
211 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
212 // Queue discipline is enforced at ::exit() time, when the unlocking thread
213 // drains the cxq into the EntryList, and orders or reorders the threads on the
214 // EntryList accordingly.
215 //
216 // Barring "lock barging", this mechanism provides fair cyclic ordering,
217 // somewhat similar to an elevator-scan.
218 //
219 // * The monitor synchronization subsystem avoids the use of native
220 // synchronization primitives except for the narrow platform-specific
221 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
222 // the semantics of park-unpark. Put another way, this monitor implementation
223 // depends only on atomic operations and park-unpark. The monitor subsystem
224 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
225 // underlying OS manages the READY<->RUN transitions.
226 //
227 // * Waiting threads reside on the WaitSet list -- wait() puts
228 // the caller onto the WaitSet.
229 //
230 // * notify() or notifyAll() simply transfers threads from the WaitSet to
231 // either the EntryList or cxq. Subsequent exit() operations will
232 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
233 // it's likely the notifyee would simply impale itself on the lock held
234 // by the notifier.
235 //
236 // * An interesting alternative is to encode cxq as (List,LockByte) where
237 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
238 // variable, like _recursions, in the scheme. The threads or Events that form
239 // the list would have to be aligned in 256-byte addresses. A thread would
240 // try to acquire the lock or enqueue itself with CAS, but exiting threads
241 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
242 // Note that is is *not* word-tearing, but it does presume that full-word
243 // CAS operations are coherent with intermix with STB operations. That's true
244 // on most common processors.
245 //
246 // * See also http://blogs.sun.com/dave
247
248
249 void* ObjectMonitor::operator new (size_t size) throw() {
250 return AllocateHeap(size, mtInternal);
251 }
252 void* ObjectMonitor::operator new[] (size_t size) throw() {
253 return operator new (size);
254 }
255 void ObjectMonitor::operator delete(void* p) {
256 FreeHeap(p);
257 }
258 void ObjectMonitor::operator delete[] (void *p) {
259 operator delete(p);
260 }
261
262 // -----------------------------------------------------------------------------
263 // Enter support
264
265 void ObjectMonitor::enter(TRAPS) {
266 // The following code is ordered to check the most common cases first
267 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
268 Thread * const Self = THREAD;
269
270 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
271 if (cur == NULL) {
272 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
273 assert(_recursions == 0, "invariant");
274 assert(_owner == Self, "invariant");
275 return;
276 }
277
278 if (cur == Self) {
279 // TODO-FIXME: check for integer overflow! BUGID 6557169.
280 _recursions++;
281 return;
282 }
283
284 if (Self->is_lock_owned ((address)cur)) {
285 assert(_recursions == 0, "internal state error");
286 _recursions = 1;
287 // Commute owner from a thread-specific on-stack BasicLockObject address to
288 // a full-fledged "Thread *".
289 _owner = Self;
290 return;
291 }
292
293 // We've encountered genuine contention.
294 assert(Self->_Stalled == 0, "invariant");
295 Self->_Stalled = intptr_t(this);
296
297 // Try one round of spinning *before* enqueueing Self
298 // and before going through the awkward and expensive state
299 // transitions. The following spin is strictly optional ...
300 // Note that if we acquire the monitor from an initial spin
301 // we forgo posting JVMTI events and firing DTRACE probes.
302 if (Knob_SpinEarly && TrySpin (Self) > 0) {
303 assert(_owner == Self, "invariant");
304 assert(_recursions == 0, "invariant");
305 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
306 Self->_Stalled = 0;
307 return;
308 }
309
310 assert(_owner != Self, "invariant");
311 assert(_succ != Self, "invariant");
312 assert(Self->is_Java_thread(), "invariant");
313 JavaThread * jt = (JavaThread *) Self;
314 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
315 assert(jt->thread_state() != _thread_blocked, "invariant");
316 assert(this->object() != NULL, "invariant");
317 assert(_count >= 0, "invariant");
318
319 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
320 // Ensure the object-monitor relationship remains stable while there's contention.
321 Atomic::inc(&_count);
322
323 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
324 EventJavaMonitorEnter event;
325 if (event.should_commit()) {
326 event.set_monitorClass(((oop)this->object())->klass());
327 event.set_address((uintptr_t)(this->object_addr()));
328 }
329
330 { // Change java thread status to indicate blocked on monitor enter.
331 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
332
333 Self->set_current_pending_monitor(this);
334
335 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
336 if (JvmtiExport::should_post_monitor_contended_enter()) {
337 JvmtiExport::post_monitor_contended_enter(jt, this);
338
339 // The current thread does not yet own the monitor and does not
340 // yet appear on any queues that would get it made the successor.
341 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
342 // handler cannot accidentally consume an unpark() meant for the
343 // ParkEvent associated with this ObjectMonitor.
344 }
345
346 OSThreadContendState osts(Self->osthread());
347 ThreadBlockInVM tbivm(jt);
348
349 // TODO-FIXME: change the following for(;;) loop to straight-line code.
350 for (;;) {
351 jt->set_suspend_equivalent();
352 // cleared by handle_special_suspend_equivalent_condition()
353 // or java_suspend_self()
354
355 EnterI(THREAD);
356
357 if (!ExitSuspendEquivalent(jt)) break;
358
359 // We have acquired the contended monitor, but while we were
360 // waiting another thread suspended us. We don't want to enter
361 // the monitor while suspended because that would surprise the
362 // thread that suspended us.
363 //
364 _recursions = 0;
365 _succ = NULL;
366 exit(false, Self);
367
368 jt->java_suspend_self();
369 }
370 Self->set_current_pending_monitor(NULL);
371
372 // We cleared the pending monitor info since we've just gotten past
373 // the enter-check-for-suspend dance and we now own the monitor free
374 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
375 // destructor can go to a safepoint at the end of this block. If we
376 // do a thread dump during that safepoint, then this thread will show
377 // as having "-locked" the monitor, but the OS and java.lang.Thread
378 // states will still report that the thread is blocked trying to
379 // acquire it.
380 }
381
382 Atomic::dec(&_count);
383 assert(_count >= 0, "invariant");
384 Self->_Stalled = 0;
385
386 // Must either set _recursions = 0 or ASSERT _recursions == 0.
387 assert(_recursions == 0, "invariant");
388 assert(_owner == Self, "invariant");
389 assert(_succ != Self, "invariant");
390 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
391
392 // The thread -- now the owner -- is back in vm mode.
393 // Report the glorious news via TI,DTrace and jvmstat.
394 // The probe effect is non-trivial. All the reportage occurs
395 // while we hold the monitor, increasing the length of the critical
396 // section. Amdahl's parallel speedup law comes vividly into play.
397 //
398 // Another option might be to aggregate the events (thread local or
399 // per-monitor aggregation) and defer reporting until a more opportune
400 // time -- such as next time some thread encounters contention but has
401 // yet to acquire the lock. While spinning that thread could
402 // spinning we could increment JVMStat counters, etc.
403
404 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
405 if (JvmtiExport::should_post_monitor_contended_entered()) {
406 JvmtiExport::post_monitor_contended_entered(jt, this);
407
408 // The current thread already owns the monitor and is not going to
409 // call park() for the remainder of the monitor enter protocol. So
410 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
411 // event handler consumed an unpark() issued by the thread that
412 // just exited the monitor.
413 }
414 if (event.should_commit()) {
415 event.set_previousOwner((uintptr_t)_previous_owner_tid);
416 event.commit();
417 }
418 OM_PERFDATA_OP(ContendedLockAttempts, inc());
419 }
420
421 // Caveat: TryLock() is not necessarily serializing if it returns failure.
422 // Callers must compensate as needed.
423
424 int ObjectMonitor::TryLock(Thread * Self) {
425 void * own = _owner;
426 if (own != NULL) return 0;
427 if (Atomic::replace_if_null(Self, &_owner)) {
428 // Either guarantee _recursions == 0 or set _recursions = 0.
429 assert(_recursions == 0, "invariant");
430 assert(_owner == Self, "invariant");
431 return 1;
432 }
433 // The lock had been free momentarily, but we lost the race to the lock.
434 // Interference -- the CAS failed.
435 // We can either return -1 or retry.
436 // Retry doesn't make as much sense because the lock was just acquired.
437 return -1;
438 }
439
440 #define MAX_RECHECK_INTERVAL 1000
441
442 void ObjectMonitor::EnterI(TRAPS) {
443 Thread * const Self = THREAD;
444 assert(Self->is_Java_thread(), "invariant");
445 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
446
447 // Try the lock - TATAS
448 if (TryLock (Self) > 0) {
449 assert(_succ != Self, "invariant");
450 assert(_owner == Self, "invariant");
451 assert(_Responsible != Self, "invariant");
452 return;
453 }
454
455 DeferredInitialize();
456
457 // We try one round of spinning *before* enqueueing Self.
458 //
459 // If the _owner is ready but OFFPROC we could use a YieldTo()
460 // operation to donate the remainder of this thread's quantum
461 // to the owner. This has subtle but beneficial affinity
462 // effects.
463
464 if (TrySpin (Self) > 0) {
465 assert(_owner == Self, "invariant");
466 assert(_succ != Self, "invariant");
467 assert(_Responsible != Self, "invariant");
468 return;
469 }
470
471 // The Spin failed -- Enqueue and park the thread ...
472 assert(_succ != Self, "invariant");
473 assert(_owner != Self, "invariant");
474 assert(_Responsible != Self, "invariant");
475
476 // Enqueue "Self" on ObjectMonitor's _cxq.
477 //
478 // Node acts as a proxy for Self.
479 // As an aside, if were to ever rewrite the synchronization code mostly
480 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
481 // Java objects. This would avoid awkward lifecycle and liveness issues,
482 // as well as eliminate a subset of ABA issues.
483 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
484
485 ObjectWaiter node(Self);
486 Self->_ParkEvent->reset();
487 node._prev = (ObjectWaiter *) 0xBAD;
488 node.TState = ObjectWaiter::TS_CXQ;
489
490 // Push "Self" onto the front of the _cxq.
491 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
492 // Note that spinning tends to reduce the rate at which threads
493 // enqueue and dequeue on EntryList|cxq.
494 ObjectWaiter * nxt;
495 for (;;) {
496 node._next = nxt = _cxq;
497 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
498
499 // Interference - the CAS failed because _cxq changed. Just retry.
500 // As an optional optimization we retry the lock.
501 if (TryLock (Self) > 0) {
502 assert(_succ != Self, "invariant");
503 assert(_owner == Self, "invariant");
504 assert(_Responsible != Self, "invariant");
505 return;
506 }
507 }
508
509 // Check for cxq|EntryList edge transition to non-null. This indicates
510 // the onset of contention. While contention persists exiting threads
511 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
512 // operations revert to the faster 1-0 mode. This enter operation may interleave
513 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
514 // arrange for one of the contending thread to use a timed park() operations
515 // to detect and recover from the race. (Stranding is form of progress failure
516 // where the monitor is unlocked but all the contending threads remain parked).
517 // That is, at least one of the contended threads will periodically poll _owner.
518 // One of the contending threads will become the designated "Responsible" thread.
519 // The Responsible thread uses a timed park instead of a normal indefinite park
520 // operation -- it periodically wakes and checks for and recovers from potential
521 // strandings admitted by 1-0 exit operations. We need at most one Responsible
522 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
523 // be responsible for a monitor.
524 //
525 // Currently, one of the contended threads takes on the added role of "Responsible".
526 // A viable alternative would be to use a dedicated "stranding checker" thread
527 // that periodically iterated over all the threads (or active monitors) and unparked
528 // successors where there was risk of stranding. This would help eliminate the
529 // timer scalability issues we see on some platforms as we'd only have one thread
530 // -- the checker -- parked on a timer.
531
532 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
533 // Try to assume the role of responsible thread for the monitor.
534 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
535 Atomic::replace_if_null(Self, &_Responsible);
536 }
537
538 // The lock might have been released while this thread was occupied queueing
539 // itself onto _cxq. To close the race and avoid "stranding" and
540 // progress-liveness failure we must resample-retry _owner before parking.
541 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
542 // In this case the ST-MEMBAR is accomplished with CAS().
543 //
544 // TODO: Defer all thread state transitions until park-time.
545 // Since state transitions are heavy and inefficient we'd like
546 // to defer the state transitions until absolutely necessary,
547 // and in doing so avoid some transitions ...
548
549 TEVENT(Inflated enter - Contention);
550 int nWakeups = 0;
551 int recheckInterval = 1;
552
553 for (;;) {
554
555 if (TryLock(Self) > 0) break;
556 assert(_owner != Self, "invariant");
557
558 if ((SyncFlags & 2) && _Responsible == NULL) {
559 Atomic::replace_if_null(Self, &_Responsible);
560 }
561
562 // park self
563 if (_Responsible == Self || (SyncFlags & 1)) {
564 TEVENT(Inflated enter - park TIMED);
565 Self->_ParkEvent->park((jlong) recheckInterval);
566 // Increase the recheckInterval, but clamp the value.
567 recheckInterval *= 8;
568 if (recheckInterval > MAX_RECHECK_INTERVAL) {
569 recheckInterval = MAX_RECHECK_INTERVAL;
570 }
571 } else {
572 TEVENT(Inflated enter - park UNTIMED);
573 Self->_ParkEvent->park();
574 }
575
576 if (TryLock(Self) > 0) break;
577
578 // The lock is still contested.
579 // Keep a tally of the # of futile wakeups.
580 // Note that the counter is not protected by a lock or updated by atomics.
581 // That is by design - we trade "lossy" counters which are exposed to
582 // races during updates for a lower probe effect.
583 TEVENT(Inflated enter - Futile wakeup);
584 // This PerfData object can be used in parallel with a safepoint.
585 // See the work around in PerfDataManager::destroy().
586 OM_PERFDATA_OP(FutileWakeups, inc());
587 ++nWakeups;
588
589 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
590 // We can defer clearing _succ until after the spin completes
591 // TrySpin() must tolerate being called with _succ == Self.
592 // Try yet another round of adaptive spinning.
593 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
594
595 // We can find that we were unpark()ed and redesignated _succ while
596 // we were spinning. That's harmless. If we iterate and call park(),
597 // park() will consume the event and return immediately and we'll
598 // just spin again. This pattern can repeat, leaving _succ to simply
599 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
600 // Alternately, we can sample fired() here, and if set, forgo spinning
601 // in the next iteration.
602
603 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
604 Self->_ParkEvent->reset();
605 OrderAccess::fence();
606 }
607 if (_succ == Self) _succ = NULL;
608
609 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
610 OrderAccess::fence();
611 }
612
613 // Egress :
614 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
615 // Normally we'll find Self on the EntryList .
616 // From the perspective of the lock owner (this thread), the
617 // EntryList is stable and cxq is prepend-only.
618 // The head of cxq is volatile but the interior is stable.
619 // In addition, Self.TState is stable.
620
621 assert(_owner == Self, "invariant");
622 assert(object() != NULL, "invariant");
623 // I'd like to write:
624 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
625 // but as we're at a safepoint that's not safe.
626
627 UnlinkAfterAcquire(Self, &node);
628 if (_succ == Self) _succ = NULL;
629
630 assert(_succ != Self, "invariant");
631 if (_Responsible == Self) {
632 _Responsible = NULL;
633 OrderAccess::fence(); // Dekker pivot-point
634
635 // We may leave threads on cxq|EntryList without a designated
636 // "Responsible" thread. This is benign. When this thread subsequently
637 // exits the monitor it can "see" such preexisting "old" threads --
638 // threads that arrived on the cxq|EntryList before the fence, above --
639 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
640 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
641 // non-null and elect a new "Responsible" timer thread.
642 //
643 // This thread executes:
644 // ST Responsible=null; MEMBAR (in enter epilogue - here)
645 // LD cxq|EntryList (in subsequent exit)
646 //
647 // Entering threads in the slow/contended path execute:
648 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
649 // The (ST cxq; MEMBAR) is accomplished with CAS().
650 //
651 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
652 // exit operation from floating above the ST Responsible=null.
653 }
654
655 // We've acquired ownership with CAS().
656 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
657 // But since the CAS() this thread may have also stored into _succ,
658 // EntryList, cxq or Responsible. These meta-data updates must be
659 // visible __before this thread subsequently drops the lock.
660 // Consider what could occur if we didn't enforce this constraint --
661 // STs to monitor meta-data and user-data could reorder with (become
662 // visible after) the ST in exit that drops ownership of the lock.
663 // Some other thread could then acquire the lock, but observe inconsistent
664 // or old monitor meta-data and heap data. That violates the JMM.
665 // To that end, the 1-0 exit() operation must have at least STST|LDST
666 // "release" barrier semantics. Specifically, there must be at least a
667 // STST|LDST barrier in exit() before the ST of null into _owner that drops
668 // the lock. The barrier ensures that changes to monitor meta-data and data
669 // protected by the lock will be visible before we release the lock, and
670 // therefore before some other thread (CPU) has a chance to acquire the lock.
671 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
672 //
673 // Critically, any prior STs to _succ or EntryList must be visible before
674 // the ST of null into _owner in the *subsequent* (following) corresponding
675 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
676 // execute a serializing instruction.
677
678 if (SyncFlags & 8) {
679 OrderAccess::fence();
680 }
681 return;
682 }
683
684 // ReenterI() is a specialized inline form of the latter half of the
685 // contended slow-path from EnterI(). We use ReenterI() only for
686 // monitor reentry in wait().
687 //
688 // In the future we should reconcile EnterI() and ReenterI(), adding
689 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
690 // loop accordingly.
691
692 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
693 assert(Self != NULL, "invariant");
694 assert(SelfNode != NULL, "invariant");
695 assert(SelfNode->_thread == Self, "invariant");
696 assert(_waiters > 0, "invariant");
697 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
698 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
699 JavaThread * jt = (JavaThread *) Self;
700
701 int nWakeups = 0;
702 for (;;) {
703 ObjectWaiter::TStates v = SelfNode->TState;
704 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
705 assert(_owner != Self, "invariant");
706
707 if (TryLock(Self) > 0) break;
708 if (TrySpin(Self) > 0) break;
709
710 TEVENT(Wait Reentry - parking);
711
712 // State transition wrappers around park() ...
713 // ReenterI() wisely defers state transitions until
714 // it's clear we must park the thread.
715 {
716 OSThreadContendState osts(Self->osthread());
717 ThreadBlockInVM tbivm(jt);
718
719 // cleared by handle_special_suspend_equivalent_condition()
720 // or java_suspend_self()
721 jt->set_suspend_equivalent();
722 if (SyncFlags & 1) {
723 Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL);
724 } else {
725 Self->_ParkEvent->park();
726 }
727
728 // were we externally suspended while we were waiting?
729 for (;;) {
730 if (!ExitSuspendEquivalent(jt)) break;
731 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
732 jt->java_suspend_self();
733 jt->set_suspend_equivalent();
734 }
735 }
736
737 // Try again, but just so we distinguish between futile wakeups and
738 // successful wakeups. The following test isn't algorithmically
739 // necessary, but it helps us maintain sensible statistics.
740 if (TryLock(Self) > 0) break;
741
742 // The lock is still contested.
743 // Keep a tally of the # of futile wakeups.
744 // Note that the counter is not protected by a lock or updated by atomics.
745 // That is by design - we trade "lossy" counters which are exposed to
746 // races during updates for a lower probe effect.
747 TEVENT(Wait Reentry - futile wakeup);
748 ++nWakeups;
749
750 // Assuming this is not a spurious wakeup we'll normally
751 // find that _succ == Self.
752 if (_succ == Self) _succ = NULL;
753
754 // Invariant: after clearing _succ a contending thread
755 // *must* retry _owner before parking.
756 OrderAccess::fence();
757
758 // This PerfData object can be used in parallel with a safepoint.
759 // See the work around in PerfDataManager::destroy().
760 OM_PERFDATA_OP(FutileWakeups, inc());
761 }
762
763 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
764 // Normally we'll find Self on the EntryList.
765 // Unlinking from the EntryList is constant-time and atomic-free.
766 // From the perspective of the lock owner (this thread), the
767 // EntryList is stable and cxq is prepend-only.
768 // The head of cxq is volatile but the interior is stable.
769 // In addition, Self.TState is stable.
770
771 assert(_owner == Self, "invariant");
772 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
773 UnlinkAfterAcquire(Self, SelfNode);
774 if (_succ == Self) _succ = NULL;
775 assert(_succ != Self, "invariant");
776 SelfNode->TState = ObjectWaiter::TS_RUN;
777 OrderAccess::fence(); // see comments at the end of EnterI()
778 }
779
780 // By convention we unlink a contending thread from EntryList|cxq immediately
781 // after the thread acquires the lock in ::enter(). Equally, we could defer
782 // unlinking the thread until ::exit()-time.
783
784 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
785 assert(_owner == Self, "invariant");
786 assert(SelfNode->_thread == Self, "invariant");
787
788 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
789 // Normal case: remove Self from the DLL EntryList .
790 // This is a constant-time operation.
791 ObjectWaiter * nxt = SelfNode->_next;
792 ObjectWaiter * prv = SelfNode->_prev;
793 if (nxt != NULL) nxt->_prev = prv;
794 if (prv != NULL) prv->_next = nxt;
795 if (SelfNode == _EntryList) _EntryList = nxt;
796 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
797 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
798 TEVENT(Unlink from EntryList);
799 } else {
800 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
801 // Inopportune interleaving -- Self is still on the cxq.
802 // This usually means the enqueue of self raced an exiting thread.
803 // Normally we'll find Self near the front of the cxq, so
804 // dequeueing is typically fast. If needbe we can accelerate
805 // this with some MCS/CHL-like bidirectional list hints and advisory
806 // back-links so dequeueing from the interior will normally operate
807 // in constant-time.
808 // Dequeue Self from either the head (with CAS) or from the interior
809 // with a linear-time scan and normal non-atomic memory operations.
810 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
811 // and then unlink Self from EntryList. We have to drain eventually,
812 // so it might as well be now.
813
814 ObjectWaiter * v = _cxq;
815 assert(v != NULL, "invariant");
816 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
817 // The CAS above can fail from interference IFF a "RAT" arrived.
818 // In that case Self must be in the interior and can no longer be
819 // at the head of cxq.
820 if (v == SelfNode) {
821 assert(_cxq != v, "invariant");
822 v = _cxq; // CAS above failed - start scan at head of list
823 }
824 ObjectWaiter * p;
825 ObjectWaiter * q = NULL;
826 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
827 q = p;
828 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
829 }
830 assert(v != SelfNode, "invariant");
831 assert(p == SelfNode, "Node not found on cxq");
832 assert(p != _cxq, "invariant");
833 assert(q != NULL, "invariant");
834 assert(q->_next == p, "invariant");
835 q->_next = p->_next;
836 }
837 TEVENT(Unlink from cxq);
838 }
839
840 #ifdef ASSERT
841 // Diagnostic hygiene ...
842 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
843 SelfNode->_next = (ObjectWaiter *) 0xBAD;
844 SelfNode->TState = ObjectWaiter::TS_RUN;
845 #endif
846 }
847
848 // -----------------------------------------------------------------------------
849 // Exit support
850 //
851 // exit()
852 // ~~~~~~
853 // Note that the collector can't reclaim the objectMonitor or deflate
854 // the object out from underneath the thread calling ::exit() as the
855 // thread calling ::exit() never transitions to a stable state.
856 // This inhibits GC, which in turn inhibits asynchronous (and
857 // inopportune) reclamation of "this".
858 //
859 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
860 // There's one exception to the claim above, however. EnterI() can call
861 // exit() to drop a lock if the acquirer has been externally suspended.
862 // In that case exit() is called with _thread_state as _thread_blocked,
863 // but the monitor's _count field is > 0, which inhibits reclamation.
864 //
865 // 1-0 exit
866 // ~~~~~~~~
867 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
868 // the fast-path operators have been optimized so the common ::exit()
869 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
870 // The code emitted by fast_unlock() elides the usual MEMBAR. This
871 // greatly improves latency -- MEMBAR and CAS having considerable local
872 // latency on modern processors -- but at the cost of "stranding". Absent the
873 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
874 // ::enter() path, resulting in the entering thread being stranding
875 // and a progress-liveness failure. Stranding is extremely rare.
876 // We use timers (timed park operations) & periodic polling to detect
877 // and recover from stranding. Potentially stranded threads periodically
878 // wake up and poll the lock. See the usage of the _Responsible variable.
879 //
880 // The CAS() in enter provides for safety and exclusion, while the CAS or
881 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
882 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
883 // We detect and recover from stranding with timers.
884 //
885 // If a thread transiently strands it'll park until (a) another
886 // thread acquires the lock and then drops the lock, at which time the
887 // exiting thread will notice and unpark the stranded thread, or, (b)
888 // the timer expires. If the lock is high traffic then the stranding latency
889 // will be low due to (a). If the lock is low traffic then the odds of
890 // stranding are lower, although the worst-case stranding latency
891 // is longer. Critically, we don't want to put excessive load in the
892 // platform's timer subsystem. We want to minimize both the timer injection
893 // rate (timers created/sec) as well as the number of timers active at
894 // any one time. (more precisely, we want to minimize timer-seconds, which is
895 // the integral of the # of active timers at any instant over time).
896 // Both impinge on OS scalability. Given that, at most one thread parked on
897 // a monitor will use a timer.
898 //
899 // There is also the risk of a futile wake-up. If we drop the lock
900 // another thread can reacquire the lock immediately, and we can
901 // then wake a thread unnecessarily. This is benign, and we've
902 // structured the code so the windows are short and the frequency
903 // of such futile wakups is low.
904
905 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
906 Thread * const Self = THREAD;
907 if (THREAD != _owner) {
908 if (THREAD->is_lock_owned((address) _owner)) {
909 // Transmute _owner from a BasicLock pointer to a Thread address.
910 // We don't need to hold _mutex for this transition.
911 // Non-null to Non-null is safe as long as all readers can
912 // tolerate either flavor.
913 assert(_recursions == 0, "invariant");
914 _owner = THREAD;
915 _recursions = 0;
916 } else {
917 // Apparent unbalanced locking ...
918 // Naively we'd like to throw IllegalMonitorStateException.
919 // As a practical matter we can neither allocate nor throw an
920 // exception as ::exit() can be called from leaf routines.
921 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
922 // Upon deeper reflection, however, in a properly run JVM the only
923 // way we should encounter this situation is in the presence of
924 // unbalanced JNI locking. TODO: CheckJNICalls.
925 // See also: CR4414101
926 TEVENT(Exit - Throw IMSX);
927 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
928 return;
929 }
930 }
931
932 if (_recursions != 0) {
933 _recursions--; // this is simple recursive enter
934 TEVENT(Inflated exit - recursive);
935 return;
936 }
937
938 // Invariant: after setting Responsible=null an thread must execute
939 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
940 if ((SyncFlags & 4) == 0) {
941 _Responsible = NULL;
942 }
943
944 #if INCLUDE_JFR
945 // get the owner's thread id for the MonitorEnter event
946 // if it is enabled and the thread isn't suspended
947 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
948 _previous_owner_tid = JFR_THREAD_ID(Self);
949 }
950 #endif
951
952 for (;;) {
953 assert(THREAD == _owner, "invariant");
954
955 if (Knob_ExitPolicy == 0) {
956 // release semantics: prior loads and stores from within the critical section
957 // must not float (reorder) past the following store that drops the lock.
958 // On SPARC that requires MEMBAR #loadstore|#storestore.
959 // But of course in TSO #loadstore|#storestore is not required.
960 // I'd like to write one of the following:
961 // A. OrderAccess::release() ; _owner = NULL
962 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
963 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
964 // store into a _dummy variable. That store is not needed, but can result
965 // in massive wasteful coherency traffic on classic SMP systems.
966 // Instead, I use release_store(), which is implemented as just a simple
967 // ST on x64, x86 and SPARC.
968 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
969 OrderAccess::storeload(); // See if we need to wake a successor
970 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
971 TEVENT(Inflated exit - simple egress);
972 return;
973 }
974 TEVENT(Inflated exit - complex egress);
975 // Other threads are blocked trying to acquire the lock.
976
977 // Normally the exiting thread is responsible for ensuring succession,
978 // but if other successors are ready or other entering threads are spinning
979 // then this thread can simply store NULL into _owner and exit without
980 // waking a successor. The existence of spinners or ready successors
981 // guarantees proper succession (liveness). Responsibility passes to the
982 // ready or running successors. The exiting thread delegates the duty.
983 // More precisely, if a successor already exists this thread is absolved
984 // of the responsibility of waking (unparking) one.
985 //
986 // The _succ variable is critical to reducing futile wakeup frequency.
987 // _succ identifies the "heir presumptive" thread that has been made
988 // ready (unparked) but that has not yet run. We need only one such
989 // successor thread to guarantee progress.
990 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
991 // section 3.3 "Futile Wakeup Throttling" for details.
992 //
993 // Note that spinners in Enter() also set _succ non-null.
994 // In the current implementation spinners opportunistically set
995 // _succ so that exiting threads might avoid waking a successor.
996 // Another less appealing alternative would be for the exiting thread
997 // to drop the lock and then spin briefly to see if a spinner managed
998 // to acquire the lock. If so, the exiting thread could exit
999 // immediately without waking a successor, otherwise the exiting
1000 // thread would need to dequeue and wake a successor.
1001 // (Note that we'd need to make the post-drop spin short, but no
1002 // shorter than the worst-case round-trip cache-line migration time.
1003 // The dropped lock needs to become visible to the spinner, and then
1004 // the acquisition of the lock by the spinner must become visible to
1005 // the exiting thread).
1006
1007 // It appears that an heir-presumptive (successor) must be made ready.
1008 // Only the current lock owner can manipulate the EntryList or
1009 // drain _cxq, so we need to reacquire the lock. If we fail
1010 // to reacquire the lock the responsibility for ensuring succession
1011 // falls to the new owner.
1012 //
1013 if (!Atomic::replace_if_null(THREAD, &_owner)) {
1014 return;
1015 }
1016 TEVENT(Exit - Reacquired);
1017 } else {
1018 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1019 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
1020 OrderAccess::storeload();
1021 // Ratify the previously observed values.
1022 if (_cxq == NULL || _succ != NULL) {
1023 TEVENT(Inflated exit - simple egress);
1024 return;
1025 }
1026
1027 // inopportune interleaving -- the exiting thread (this thread)
1028 // in the fast-exit path raced an entering thread in the slow-enter
1029 // path.
1030 // We have two choices:
1031 // A. Try to reacquire the lock.
1032 // If the CAS() fails return immediately, otherwise
1033 // we either restart/rerun the exit operation, or simply
1034 // fall-through into the code below which wakes a successor.
1035 // B. If the elements forming the EntryList|cxq are TSM
1036 // we could simply unpark() the lead thread and return
1037 // without having set _succ.
1038 if (!Atomic::replace_if_null(THREAD, &_owner)) {
1039 TEVENT(Inflated exit - reacquired succeeded);
1040 return;
1041 }
1042 TEVENT(Inflated exit - reacquired failed);
1043 } else {
1044 TEVENT(Inflated exit - complex egress);
1045 }
1046 }
1047
1048 guarantee(_owner == THREAD, "invariant");
1049
1050 ObjectWaiter * w = NULL;
1051 int QMode = Knob_QMode;
1052
1053 if (QMode == 2 && _cxq != NULL) {
1054 // QMode == 2 : cxq has precedence over EntryList.
1055 // Try to directly wake a successor from the cxq.
1056 // If successful, the successor will need to unlink itself from cxq.
1057 w = _cxq;
1058 assert(w != NULL, "invariant");
1059 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1060 ExitEpilog(Self, w);
1061 return;
1062 }
1063
1064 if (QMode == 3 && _cxq != NULL) {
1065 // Aggressively drain cxq into EntryList at the first opportunity.
1066 // This policy ensure that recently-run threads live at the head of EntryList.
1067 // Drain _cxq into EntryList - bulk transfer.
1068 // First, detach _cxq.
1069 // The following loop is tantamount to: w = swap(&cxq, NULL)
1070 w = _cxq;
1071 for (;;) {
1072 assert(w != NULL, "Invariant");
1073 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1074 if (u == w) break;
1075 w = u;
1076 }
1077 assert(w != NULL, "invariant");
1078
1079 ObjectWaiter * q = NULL;
1080 ObjectWaiter * p;
1081 for (p = w; p != NULL; p = p->_next) {
1082 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1083 p->TState = ObjectWaiter::TS_ENTER;
1084 p->_prev = q;
1085 q = p;
1086 }
1087
1088 // Append the RATs to the EntryList
1089 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1090 ObjectWaiter * Tail;
1091 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1092 Tail = Tail->_next)
1093 /* empty */;
1094 if (Tail == NULL) {
1095 _EntryList = w;
1096 } else {
1097 Tail->_next = w;
1098 w->_prev = Tail;
1099 }
1100
1101 // Fall thru into code that tries to wake a successor from EntryList
1102 }
1103
1104 if (QMode == 4 && _cxq != NULL) {
1105 // Aggressively drain cxq into EntryList at the first opportunity.
1106 // This policy ensure that recently-run threads live at the head of EntryList.
1107
1108 // Drain _cxq into EntryList - bulk transfer.
1109 // First, detach _cxq.
1110 // The following loop is tantamount to: w = swap(&cxq, NULL)
1111 w = _cxq;
1112 for (;;) {
1113 assert(w != NULL, "Invariant");
1114 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1115 if (u == w) break;
1116 w = u;
1117 }
1118 assert(w != NULL, "invariant");
1119
1120 ObjectWaiter * q = NULL;
1121 ObjectWaiter * p;
1122 for (p = w; p != NULL; p = p->_next) {
1123 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1124 p->TState = ObjectWaiter::TS_ENTER;
1125 p->_prev = q;
1126 q = p;
1127 }
1128
1129 // Prepend the RATs to the EntryList
1130 if (_EntryList != NULL) {
1131 q->_next = _EntryList;
1132 _EntryList->_prev = q;
1133 }
1134 _EntryList = w;
1135
1136 // Fall thru into code that tries to wake a successor from EntryList
1137 }
1138
1139 w = _EntryList;
1140 if (w != NULL) {
1141 // I'd like to write: guarantee (w->_thread != Self).
1142 // But in practice an exiting thread may find itself on the EntryList.
1143 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1144 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1145 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1146 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1147 // release the lock "O". T2 resumes immediately after the ST of null into
1148 // _owner, above. T2 notices that the EntryList is populated, so it
1149 // reacquires the lock and then finds itself on the EntryList.
1150 // Given all that, we have to tolerate the circumstance where "w" is
1151 // associated with Self.
1152 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1153 ExitEpilog(Self, w);
1154 return;
1155 }
1156
1157 // If we find that both _cxq and EntryList are null then just
1158 // re-run the exit protocol from the top.
1159 w = _cxq;
1160 if (w == NULL) continue;
1161
1162 // Drain _cxq into EntryList - bulk transfer.
1163 // First, detach _cxq.
1164 // The following loop is tantamount to: w = swap(&cxq, NULL)
1165 for (;;) {
1166 assert(w != NULL, "Invariant");
1167 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1168 if (u == w) break;
1169 w = u;
1170 }
1171 TEVENT(Inflated exit - drain cxq into EntryList);
1172
1173 assert(w != NULL, "invariant");
1174 assert(_EntryList == NULL, "invariant");
1175
1176 // Convert the LIFO SLL anchored by _cxq into a DLL.
1177 // The list reorganization step operates in O(LENGTH(w)) time.
1178 // It's critical that this step operate quickly as
1179 // "Self" still holds the outer-lock, restricting parallelism
1180 // and effectively lengthening the critical section.
1181 // Invariant: s chases t chases u.
1182 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1183 // we have faster access to the tail.
1184
1185 if (QMode == 1) {
1186 // QMode == 1 : drain cxq to EntryList, reversing order
1187 // We also reverse the order of the list.
1188 ObjectWaiter * s = NULL;
1189 ObjectWaiter * t = w;
1190 ObjectWaiter * u = NULL;
1191 while (t != NULL) {
1192 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1193 t->TState = ObjectWaiter::TS_ENTER;
1194 u = t->_next;
1195 t->_prev = u;
1196 t->_next = s;
1197 s = t;
1198 t = u;
1199 }
1200 _EntryList = s;
1201 assert(s != NULL, "invariant");
1202 } else {
1203 // QMode == 0 or QMode == 2
1204 _EntryList = w;
1205 ObjectWaiter * q = NULL;
1206 ObjectWaiter * p;
1207 for (p = w; p != NULL; p = p->_next) {
1208 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1209 p->TState = ObjectWaiter::TS_ENTER;
1210 p->_prev = q;
1211 q = p;
1212 }
1213 }
1214
1215 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1216 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1217
1218 // See if we can abdicate to a spinner instead of waking a thread.
1219 // A primary goal of the implementation is to reduce the
1220 // context-switch rate.
1221 if (_succ != NULL) continue;
1222
1223 w = _EntryList;
1224 if (w != NULL) {
1225 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1226 ExitEpilog(Self, w);
1227 return;
1228 }
1229 }
1230 }
1231
1232 // ExitSuspendEquivalent:
1233 // A faster alternate to handle_special_suspend_equivalent_condition()
1234 //
1235 // handle_special_suspend_equivalent_condition() unconditionally
1236 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1237 // operations have high latency. Note that in ::enter() we call HSSEC
1238 // while holding the monitor, so we effectively lengthen the critical sections.
1239 //
1240 // There are a number of possible solutions:
1241 //
1242 // A. To ameliorate the problem we might also defer state transitions
1243 // to as late as possible -- just prior to parking.
1244 // Given that, we'd call HSSEC after having returned from park(),
1245 // but before attempting to acquire the monitor. This is only a
1246 // partial solution. It avoids calling HSSEC while holding the
1247 // monitor (good), but it still increases successor reacquisition latency --
1248 // the interval between unparking a successor and the time the successor
1249 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1250 // If we use this technique we can also avoid EnterI()-exit() loop
1251 // in ::enter() where we iteratively drop the lock and then attempt
1252 // to reacquire it after suspending.
1253 //
1254 // B. In the future we might fold all the suspend bits into a
1255 // composite per-thread suspend flag and then update it with CAS().
1256 // Alternately, a Dekker-like mechanism with multiple variables
1257 // would suffice:
1258 // ST Self->_suspend_equivalent = false
1259 // MEMBAR
1260 // LD Self_>_suspend_flags
1261 //
1262 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1263 // with a more efficient implementation, the need to use "FastHSSEC" has
1264 // decreased. - Dave
1265
1266
1267 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1268 const int Mode = Knob_FastHSSEC;
1269 if (Mode && !jSelf->is_external_suspend()) {
1270 assert(jSelf->is_suspend_equivalent(), "invariant");
1271 jSelf->clear_suspend_equivalent();
1272 if (2 == Mode) OrderAccess::storeload();
1273 if (!jSelf->is_external_suspend()) return false;
1274 // We raced a suspension -- fall thru into the slow path
1275 TEVENT(ExitSuspendEquivalent - raced);
1276 jSelf->set_suspend_equivalent();
1277 }
1278 return jSelf->handle_special_suspend_equivalent_condition();
1279 }
1280
1281
1282 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1283 assert(_owner == Self, "invariant");
1284
1285 // Exit protocol:
1286 // 1. ST _succ = wakee
1287 // 2. membar #loadstore|#storestore;
1288 // 2. ST _owner = NULL
1289 // 3. unpark(wakee)
1290
1291 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1292 ParkEvent * Trigger = Wakee->_event;
1293
1294 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1295 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1296 // out-of-scope (non-extant).
1297 Wakee = NULL;
1298
1299 // Drop the lock
1300 OrderAccess::release_store(&_owner, (void*)NULL);
1301 OrderAccess::fence(); // ST _owner vs LD in unpark()
1302
1303 if (SafepointMechanism::poll(Self)) {
1304 TEVENT(unpark before SAFEPOINT);
1305 }
1306
1307 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1308 Trigger->unpark();
1309
1310 // Maintain stats and report events to JVMTI
1311 OM_PERFDATA_OP(Parks, inc());
1312 }
1313
1314
1315 // -----------------------------------------------------------------------------
1316 // Class Loader deadlock handling.
1317 //
1318 // complete_exit exits a lock returning recursion count
1319 // complete_exit/reenter operate as a wait without waiting
1320 // complete_exit requires an inflated monitor
1321 // The _owner field is not always the Thread addr even with an
1322 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1323 // thread due to contention.
1324 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1325 Thread * const Self = THREAD;
1326 assert(Self->is_Java_thread(), "Must be Java thread!");
1327 JavaThread *jt = (JavaThread *)THREAD;
1328
1329 DeferredInitialize();
1330
1331 if (THREAD != _owner) {
1332 if (THREAD->is_lock_owned ((address)_owner)) {
1333 assert(_recursions == 0, "internal state error");
1334 _owner = THREAD; // Convert from basiclock addr to Thread addr
1335 _recursions = 0;
1336 }
1337 }
1338
1339 guarantee(Self == _owner, "complete_exit not owner");
1340 intptr_t save = _recursions; // record the old recursion count
1341 _recursions = 0; // set the recursion level to be 0
1342 exit(true, Self); // exit the monitor
1343 guarantee(_owner != Self, "invariant");
1344 return save;
1345 }
1346
1347 // reenter() enters a lock and sets recursion count
1348 // complete_exit/reenter operate as a wait without waiting
1349 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1350 Thread * const Self = THREAD;
1351 assert(Self->is_Java_thread(), "Must be Java thread!");
1352 JavaThread *jt = (JavaThread *)THREAD;
1353
1354 guarantee(_owner != Self, "reenter already owner");
1355 enter(THREAD); // enter the monitor
1356 guarantee(_recursions == 0, "reenter recursion");
1357 _recursions = recursions;
1358 return;
1359 }
1360
1361
1362 // -----------------------------------------------------------------------------
1363 // A macro is used below because there may already be a pending
1364 // exception which should not abort the execution of the routines
1365 // which use this (which is why we don't put this into check_slow and
1366 // call it with a CHECK argument).
1367
1368 #define CHECK_OWNER() \
1369 do { \
1370 if (THREAD != _owner) { \
1371 if (THREAD->is_lock_owned((address) _owner)) { \
1372 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1373 _recursions = 0; \
1374 } else { \
1375 TEVENT(Throw IMSX); \
1376 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1377 } \
1378 } \
1379 } while (false)
1380
1381 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1382 // TODO-FIXME: remove check_slow() -- it's likely dead.
1383
1384 void ObjectMonitor::check_slow(TRAPS) {
1385 TEVENT(check_slow - throw IMSX);
1386 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1387 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1388 }
1389
1390 static int Adjust(volatile int * adr, int dx) {
1391 int v;
1392 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1393 return v;
1394 }
1395
1396 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1397 ObjectMonitor* monitor,
1398 jlong notifier_tid,
1399 jlong timeout,
1400 bool timedout) {
1401 assert(event != NULL, "invariant");
1402 assert(monitor != NULL, "invariant");
1403 event->set_monitorClass(((oop)monitor->object())->klass());
1404 event->set_timeout(timeout);
1405 event->set_address((uintptr_t)monitor->object_addr());
1406 event->set_notifier(notifier_tid);
1407 event->set_timedOut(timedout);
1408 event->commit();
1409 }
1410
1411 // -----------------------------------------------------------------------------
1412 // Wait/Notify/NotifyAll
1413 //
1414 // Note: a subset of changes to ObjectMonitor::wait()
1415 // will need to be replicated in complete_exit
1416 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1417 Thread * const Self = THREAD;
1418 assert(Self->is_Java_thread(), "Must be Java thread!");
1419 JavaThread *jt = (JavaThread *)THREAD;
1420
1421 DeferredInitialize();
1422
1423 // Throw IMSX or IEX.
1424 CHECK_OWNER();
1425
1426 EventJavaMonitorWait event;
1427
1428 // check for a pending interrupt
1429 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1430 // post monitor waited event. Note that this is past-tense, we are done waiting.
1431 if (JvmtiExport::should_post_monitor_waited()) {
1432 // Note: 'false' parameter is passed here because the
1433 // wait was not timed out due to thread interrupt.
1434 JvmtiExport::post_monitor_waited(jt, this, false);
1435
1436 // In this short circuit of the monitor wait protocol, the
1437 // current thread never drops ownership of the monitor and
1438 // never gets added to the wait queue so the current thread
1439 // cannot be made the successor. This means that the
1440 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1441 // consume an unpark() meant for the ParkEvent associated with
1442 // this ObjectMonitor.
1443 }
1444 if (event.should_commit()) {
1445 post_monitor_wait_event(&event, this, 0, millis, false);
1446 }
1447 TEVENT(Wait - Throw IEX);
1448 THROW(vmSymbols::java_lang_InterruptedException());
1449 return;
1450 }
1451
1452 TEVENT(Wait);
1453
1454 assert(Self->_Stalled == 0, "invariant");
1455 Self->_Stalled = intptr_t(this);
1456 jt->set_current_waiting_monitor(this);
1457
1458 // create a node to be put into the queue
1459 // Critically, after we reset() the event but prior to park(), we must check
1460 // for a pending interrupt.
1461 ObjectWaiter node(Self);
1462 node.TState = ObjectWaiter::TS_WAIT;
1463 Self->_ParkEvent->reset();
1464 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1465
1466 // Enter the waiting queue, which is a circular doubly linked list in this case
1467 // but it could be a priority queue or any data structure.
1468 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1469 // by the the owner of the monitor *except* in the case where park()
1470 // returns because of a timeout of interrupt. Contention is exceptionally rare
1471 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1472
1473 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1474 AddWaiter(&node);
1475 Thread::SpinRelease(&_WaitSetLock);
1476
1477 if ((SyncFlags & 4) == 0) {
1478 _Responsible = NULL;
1479 }
1480 intptr_t save = _recursions; // record the old recursion count
1481 _waiters++; // increment the number of waiters
1482 _recursions = 0; // set the recursion level to be 1
1483 exit(true, Self); // exit the monitor
1484 guarantee(_owner != Self, "invariant");
1485
1486 // The thread is on the WaitSet list - now park() it.
1487 // On MP systems it's conceivable that a brief spin before we park
1488 // could be profitable.
1489 //
1490 // TODO-FIXME: change the following logic to a loop of the form
1491 // while (!timeout && !interrupted && _notified == 0) park()
1492
1493 int ret = OS_OK;
1494 int WasNotified = 0;
1495 { // State transition wrappers
1496 OSThread* osthread = Self->osthread();
1497 OSThreadWaitState osts(osthread, true);
1498 {
1499 ThreadBlockInVM tbivm(jt);
1500 // Thread is in thread_blocked state and oop access is unsafe.
1501 jt->set_suspend_equivalent();
1502
1503 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1504 // Intentionally empty
1505 } else if (node._notified == 0) {
1506 if (millis <= 0) {
1507 Self->_ParkEvent->park();
1508 } else {
1509 ret = Self->_ParkEvent->park(millis);
1510 }
1511 }
1512
1513 // were we externally suspended while we were waiting?
1514 if (ExitSuspendEquivalent (jt)) {
1515 // TODO-FIXME: add -- if succ == Self then succ = null.
1516 jt->java_suspend_self();
1517 }
1518
1519 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1520
1521 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1522 // from the WaitSet to the EntryList.
1523 // See if we need to remove Node from the WaitSet.
1524 // We use double-checked locking to avoid grabbing _WaitSetLock
1525 // if the thread is not on the wait queue.
1526 //
1527 // Note that we don't need a fence before the fetch of TState.
1528 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1529 // written by the is thread. (perhaps the fetch might even be satisfied
1530 // by a look-aside into the processor's own store buffer, although given
1531 // the length of the code path between the prior ST and this load that's
1532 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1533 // then we'll acquire the lock and then re-fetch a fresh TState value.
1534 // That is, we fail toward safety.
1535
1536 if (node.TState == ObjectWaiter::TS_WAIT) {
1537 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1538 if (node.TState == ObjectWaiter::TS_WAIT) {
1539 DequeueSpecificWaiter(&node); // unlink from WaitSet
1540 assert(node._notified == 0, "invariant");
1541 node.TState = ObjectWaiter::TS_RUN;
1542 }
1543 Thread::SpinRelease(&_WaitSetLock);
1544 }
1545
1546 // The thread is now either on off-list (TS_RUN),
1547 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1548 // The Node's TState variable is stable from the perspective of this thread.
1549 // No other threads will asynchronously modify TState.
1550 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1551 OrderAccess::loadload();
1552 if (_succ == Self) _succ = NULL;
1553 WasNotified = node._notified;
1554
1555 // Reentry phase -- reacquire the monitor.
1556 // re-enter contended monitor after object.wait().
1557 // retain OBJECT_WAIT state until re-enter successfully completes
1558 // Thread state is thread_in_vm and oop access is again safe,
1559 // although the raw address of the object may have changed.
1560 // (Don't cache naked oops over safepoints, of course).
1561
1562 // post monitor waited event. Note that this is past-tense, we are done waiting.
1563 if (JvmtiExport::should_post_monitor_waited()) {
1564 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1565
1566 if (node._notified != 0 && _succ == Self) {
1567 // In this part of the monitor wait-notify-reenter protocol it
1568 // is possible (and normal) for another thread to do a fastpath
1569 // monitor enter-exit while this thread is still trying to get
1570 // to the reenter portion of the protocol.
1571 //
1572 // The ObjectMonitor was notified and the current thread is
1573 // the successor which also means that an unpark() has already
1574 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1575 // consume the unpark() that was done when the successor was
1576 // set because the same ParkEvent is shared between Java
1577 // monitors and JVM/TI RawMonitors (for now).
1578 //
1579 // We redo the unpark() to ensure forward progress, i.e., we
1580 // don't want all pending threads hanging (parked) with none
1581 // entering the unlocked monitor.
1582 node._event->unpark();
1583 }
1584 }
1585
1586 if (event.should_commit()) {
1587 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1588 }
1589
1590 OrderAccess::fence();
1591
1592 assert(Self->_Stalled != 0, "invariant");
1593 Self->_Stalled = 0;
1594
1595 assert(_owner != Self, "invariant");
1596 ObjectWaiter::TStates v = node.TState;
1597 if (v == ObjectWaiter::TS_RUN) {
1598 enter(Self);
1599 } else {
1600 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1601 ReenterI(Self, &node);
1602 node.wait_reenter_end(this);
1603 }
1604
1605 // Self has reacquired the lock.
1606 // Lifecycle - the node representing Self must not appear on any queues.
1607 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1608 // want residual elements associated with this thread left on any lists.
1609 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1610 assert(_owner == Self, "invariant");
1611 assert(_succ != Self, "invariant");
1612 } // OSThreadWaitState()
1613
1614 jt->set_current_waiting_monitor(NULL);
1615
1616 guarantee(_recursions == 0, "invariant");
1617 _recursions = save; // restore the old recursion count
1618 _waiters--; // decrement the number of waiters
1619
1620 // Verify a few postconditions
1621 assert(_owner == Self, "invariant");
1622 assert(_succ != Self, "invariant");
1623 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1624
1625 if (SyncFlags & 32) {
1626 OrderAccess::fence();
1627 }
1628
1629 // check if the notification happened
1630 if (!WasNotified) {
1631 // no, it could be timeout or Thread.interrupt() or both
1632 // check for interrupt event, otherwise it is timeout
1633 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1634 TEVENT(Wait - throw IEX from epilog);
1635 THROW(vmSymbols::java_lang_InterruptedException());
1636 }
1637 }
1638
1639 // NOTE: Spurious wake up will be consider as timeout.
1640 // Monitor notify has precedence over thread interrupt.
1641 }
1642
1643
1644 // Consider:
1645 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1646 // then instead of transferring a thread from the WaitSet to the EntryList
1647 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1648
1649 void ObjectMonitor::INotify(Thread * Self) {
1650 const int policy = Knob_MoveNotifyee;
1651
1652 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1653 ObjectWaiter * iterator = DequeueWaiter();
1654 if (iterator != NULL) {
1655 TEVENT(Notify1 - Transfer);
1656 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1657 guarantee(iterator->_notified == 0, "invariant");
1658 // Disposition - what might we do with iterator ?
1659 // a. add it directly to the EntryList - either tail (policy == 1)
1660 // or head (policy == 0).
1661 // b. push it onto the front of the _cxq (policy == 2).
1662 // For now we use (b).
1663 if (policy != 4) {
1664 iterator->TState = ObjectWaiter::TS_ENTER;
1665 }
1666 iterator->_notified = 1;
1667 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1668
1669 ObjectWaiter * list = _EntryList;
1670 if (list != NULL) {
1671 assert(list->_prev == NULL, "invariant");
1672 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1673 assert(list != iterator, "invariant");
1674 }
1675
1676 if (policy == 0) { // prepend to EntryList
1677 if (list == NULL) {
1678 iterator->_next = iterator->_prev = NULL;
1679 _EntryList = iterator;
1680 } else {
1681 list->_prev = iterator;
1682 iterator->_next = list;
1683 iterator->_prev = NULL;
1684 _EntryList = iterator;
1685 }
1686 } else if (policy == 1) { // append to EntryList
1687 if (list == NULL) {
1688 iterator->_next = iterator->_prev = NULL;
1689 _EntryList = iterator;
1690 } else {
1691 // CONSIDER: finding the tail currently requires a linear-time walk of
1692 // the EntryList. We can make tail access constant-time by converting to
1693 // a CDLL instead of using our current DLL.
1694 ObjectWaiter * tail;
1695 for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1696 assert(tail != NULL && tail->_next == NULL, "invariant");
1697 tail->_next = iterator;
1698 iterator->_prev = tail;
1699 iterator->_next = NULL;
1700 }
1701 } else if (policy == 2) { // prepend to cxq
1702 if (list == NULL) {
1703 iterator->_next = iterator->_prev = NULL;
1704 _EntryList = iterator;
1705 } else {
1706 iterator->TState = ObjectWaiter::TS_CXQ;
1707 for (;;) {
1708 ObjectWaiter * front = _cxq;
1709 iterator->_next = front;
1710 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1711 break;
1712 }
1713 }
1714 }
1715 } else if (policy == 3) { // append to cxq
1716 iterator->TState = ObjectWaiter::TS_CXQ;
1717 for (;;) {
1718 ObjectWaiter * tail = _cxq;
1719 if (tail == NULL) {
1720 iterator->_next = NULL;
1721 if (Atomic::replace_if_null(iterator, &_cxq)) {
1722 break;
1723 }
1724 } else {
1725 while (tail->_next != NULL) tail = tail->_next;
1726 tail->_next = iterator;
1727 iterator->_prev = tail;
1728 iterator->_next = NULL;
1729 break;
1730 }
1731 }
1732 } else {
1733 ParkEvent * ev = iterator->_event;
1734 iterator->TState = ObjectWaiter::TS_RUN;
1735 OrderAccess::fence();
1736 ev->unpark();
1737 }
1738
1739 // _WaitSetLock protects the wait queue, not the EntryList. We could
1740 // move the add-to-EntryList operation, above, outside the critical section
1741 // protected by _WaitSetLock. In practice that's not useful. With the
1742 // exception of wait() timeouts and interrupts the monitor owner
1743 // is the only thread that grabs _WaitSetLock. There's almost no contention
1744 // on _WaitSetLock so it's not profitable to reduce the length of the
1745 // critical section.
1746
1747 if (policy < 4) {
1748 iterator->wait_reenter_begin(this);
1749 }
1750 }
1751 Thread::SpinRelease(&_WaitSetLock);
1752 }
1753
1754 // Consider: a not-uncommon synchronization bug is to use notify() when
1755 // notifyAll() is more appropriate, potentially resulting in stranded
1756 // threads; this is one example of a lost wakeup. A useful diagnostic
1757 // option is to force all notify() operations to behave as notifyAll().
1758 //
1759 // Note: We can also detect many such problems with a "minimum wait".
1760 // When the "minimum wait" is set to a small non-zero timeout value
1761 // and the program does not hang whereas it did absent "minimum wait",
1762 // that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses
1763 // a "minimum wait" for all park() operations; see the recheckInterval
1764 // variable and MAX_RECHECK_INTERVAL.
1765
1766 void ObjectMonitor::notify(TRAPS) {
1767 CHECK_OWNER();
1768 if (_WaitSet == NULL) {
1769 TEVENT(Empty-Notify);
1770 return;
1771 }
1772 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1773 INotify(THREAD);
1774 OM_PERFDATA_OP(Notifications, inc(1));
1775 }
1776
1777
1778 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1779 // from the waitset to the EntryList. This could be done more efficiently with a
1780 // single bulk transfer but in practice it's not time-critical. Beware too,
1781 // that in prepend-mode we invert the order of the waiters. Let's say that the
1782 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1783 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1784
1785 void ObjectMonitor::notifyAll(TRAPS) {
1786 CHECK_OWNER();
1787 if (_WaitSet == NULL) {
1788 TEVENT(Empty-NotifyAll);
1789 return;
1790 }
1791
1792 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1793 int tally = 0;
1794 while (_WaitSet != NULL) {
1795 tally++;
1796 INotify(THREAD);
1797 }
1798
1799 OM_PERFDATA_OP(Notifications, inc(tally));
1800 }
1801
1802 // -----------------------------------------------------------------------------
1803 // Adaptive Spinning Support
1804 //
1805 // Adaptive spin-then-block - rational spinning
1806 //
1807 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1808 // algorithm. On high order SMP systems it would be better to start with
1809 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1810 // a contending thread could enqueue itself on the cxq and then spin locally
1811 // on a thread-specific variable such as its ParkEvent._Event flag.
1812 // That's left as an exercise for the reader. Note that global spinning is
1813 // not problematic on Niagara, as the L2 cache serves the interconnect and
1814 // has both low latency and massive bandwidth.
1815 //
1816 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1817 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1818 // (duration) or we can fix the count at approximately the duration of
1819 // a context switch and vary the frequency. Of course we could also
1820 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1821 // For a description of 'Adaptive spin-then-block mutual exclusion in
1822 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1823 //
1824 // This implementation varies the duration "D", where D varies with
1825 // the success rate of recent spin attempts. (D is capped at approximately
1826 // length of a round-trip context switch). The success rate for recent
1827 // spin attempts is a good predictor of the success rate of future spin
1828 // attempts. The mechanism adapts automatically to varying critical
1829 // section length (lock modality), system load and degree of parallelism.
1830 // D is maintained per-monitor in _SpinDuration and is initialized
1831 // optimistically. Spin frequency is fixed at 100%.
1832 //
1833 // Note that _SpinDuration is volatile, but we update it without locks
1834 // or atomics. The code is designed so that _SpinDuration stays within
1835 // a reasonable range even in the presence of races. The arithmetic
1836 // operations on _SpinDuration are closed over the domain of legal values,
1837 // so at worst a race will install and older but still legal value.
1838 // At the very worst this introduces some apparent non-determinism.
1839 // We might spin when we shouldn't or vice-versa, but since the spin
1840 // count are relatively short, even in the worst case, the effect is harmless.
1841 //
1842 // Care must be taken that a low "D" value does not become an
1843 // an absorbing state. Transient spinning failures -- when spinning
1844 // is overall profitable -- should not cause the system to converge
1845 // on low "D" values. We want spinning to be stable and predictable
1846 // and fairly responsive to change and at the same time we don't want
1847 // it to oscillate, become metastable, be "too" non-deterministic,
1848 // or converge on or enter undesirable stable absorbing states.
1849 //
1850 // We implement a feedback-based control system -- using past behavior
1851 // to predict future behavior. We face two issues: (a) if the
1852 // input signal is random then the spin predictor won't provide optimal
1853 // results, and (b) if the signal frequency is too high then the control
1854 // system, which has some natural response lag, will "chase" the signal.
1855 // (b) can arise from multimodal lock hold times. Transient preemption
1856 // can also result in apparent bimodal lock hold times.
1857 // Although sub-optimal, neither condition is particularly harmful, as
1858 // in the worst-case we'll spin when we shouldn't or vice-versa.
1859 // The maximum spin duration is rather short so the failure modes aren't bad.
1860 // To be conservative, I've tuned the gain in system to bias toward
1861 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1862 // "rings" or oscillates between spinning and not spinning. This happens
1863 // when spinning is just on the cusp of profitability, however, so the
1864 // situation is not dire. The state is benign -- there's no need to add
1865 // hysteresis control to damp the transition rate between spinning and
1866 // not spinning.
1867
1868 // Spinning: Fixed frequency (100%), vary duration
1869 int ObjectMonitor::TrySpin(Thread * Self) {
1870 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1871 int ctr = Knob_FixedSpin;
1872 if (ctr != 0) {
1873 while (--ctr >= 0) {
1874 if (TryLock(Self) > 0) return 1;
1875 SpinPause();
1876 }
1877 return 0;
1878 }
1879
1880 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1881 if (TryLock(Self) > 0) {
1882 // Increase _SpinDuration ...
1883 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1884 // Raising _SpurDuration to the poverty line is key.
1885 int x = _SpinDuration;
1886 if (x < Knob_SpinLimit) {
1887 if (x < Knob_Poverty) x = Knob_Poverty;
1888 _SpinDuration = x + Knob_BonusB;
1889 }
1890 return 1;
1891 }
1892 SpinPause();
1893 }
1894
1895 // Admission control - verify preconditions for spinning
1896 //
1897 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1898 // becoming an absorbing state. Put another way, we spin briefly to
1899 // sample, just in case the system load, parallelism, contention, or lock
1900 // modality changed.
1901 //
1902 // Consider the following alternative:
1903 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1904 // spin attempt. "Periodically" might mean after a tally of
1905 // the # of failed spin attempts (or iterations) reaches some threshold.
1906 // This takes us into the realm of 1-out-of-N spinning, where we
1907 // hold the duration constant but vary the frequency.
1908
1909 ctr = _SpinDuration;
1910 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1911 if (ctr <= 0) return 0;
1912
1913 if (Knob_SuccRestrict && _succ != NULL) return 0;
1914 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1915 TEVENT(Spin abort - notrunnable [TOP]);
1916 return 0;
1917 }
1918
1919 int MaxSpin = Knob_MaxSpinners;
1920 if (MaxSpin >= 0) {
1921 if (_Spinner > MaxSpin) {
1922 TEVENT(Spin abort -- too many spinners);
1923 return 0;
1924 }
1925 // Slightly racy, but benign ...
1926 Adjust(&_Spinner, 1);
1927 }
1928
1929 // We're good to spin ... spin ingress.
1930 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1931 // when preparing to LD...CAS _owner, etc and the CAS is likely
1932 // to succeed.
1933 int hits = 0;
1934 int msk = 0;
1935 int caspty = Knob_CASPenalty;
1936 int oxpty = Knob_OXPenalty;
1937 int sss = Knob_SpinSetSucc;
1938 if (sss && _succ == NULL) _succ = Self;
1939 Thread * prv = NULL;
1940
1941 // There are three ways to exit the following loop:
1942 // 1. A successful spin where this thread has acquired the lock.
1943 // 2. Spin failure with prejudice
1944 // 3. Spin failure without prejudice
1945
1946 while (--ctr >= 0) {
1947
1948 // Periodic polling -- Check for pending GC
1949 // Threads may spin while they're unsafe.
1950 // We don't want spinning threads to delay the JVM from reaching
1951 // a stop-the-world safepoint or to steal cycles from GC.
1952 // If we detect a pending safepoint we abort in order that
1953 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1954 // this thread, if safe, doesn't steal cycles from GC.
1955 // This is in keeping with the "no loitering in runtime" rule.
1956 // We periodically check to see if there's a safepoint pending.
1957 if ((ctr & 0xFF) == 0) {
1958 if (SafepointMechanism::poll(Self)) {
1959 TEVENT(Spin: safepoint);
1960 goto Abort; // abrupt spin egress
1961 }
1962 if (Knob_UsePause & 1) SpinPause();
1963 }
1964
1965 if (Knob_UsePause & 2) SpinPause();
1966
1967 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
1968 // This is useful on classic SMP systems, but is of less utility on
1969 // N1-style CMT platforms.
1970 //
1971 // Trade-off: lock acquisition latency vs coherency bandwidth.
1972 // Lock hold times are typically short. A histogram
1973 // of successful spin attempts shows that we usually acquire
1974 // the lock early in the spin. That suggests we want to
1975 // sample _owner frequently in the early phase of the spin,
1976 // but then back-off and sample less frequently as the spin
1977 // progresses. The back-off makes a good citizen on SMP big
1978 // SMP systems. Oversampling _owner can consume excessive
1979 // coherency bandwidth. Relatedly, if we _oversample _owner we
1980 // can inadvertently interfere with the the ST m->owner=null.
1981 // executed by the lock owner.
1982 if (ctr & msk) continue;
1983 ++hits;
1984 if ((hits & 0xF) == 0) {
1985 // The 0xF, above, corresponds to the exponent.
1986 // Consider: (msk+1)|msk
1987 msk = ((msk << 2)|3) & BackOffMask;
1988 }
1989
1990 // Probe _owner with TATAS
1991 // If this thread observes the monitor transition or flicker
1992 // from locked to unlocked to locked, then the odds that this
1993 // thread will acquire the lock in this spin attempt go down
1994 // considerably. The same argument applies if the CAS fails
1995 // or if we observe _owner change from one non-null value to
1996 // another non-null value. In such cases we might abort
1997 // the spin without prejudice or apply a "penalty" to the
1998 // spin count-down variable "ctr", reducing it by 100, say.
1999
2000 Thread * ox = (Thread *) _owner;
2001 if (ox == NULL) {
2002 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
2003 if (ox == NULL) {
2004 // The CAS succeeded -- this thread acquired ownership
2005 // Take care of some bookkeeping to exit spin state.
2006 if (sss && _succ == Self) {
2007 _succ = NULL;
2008 }
2009 if (MaxSpin > 0) Adjust(&_Spinner, -1);
2010
2011 // Increase _SpinDuration :
2012 // The spin was successful (profitable) so we tend toward
2013 // longer spin attempts in the future.
2014 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2015 // If we acquired the lock early in the spin cycle it
2016 // makes sense to increase _SpinDuration proportionally.
2017 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2018 int x = _SpinDuration;
2019 if (x < Knob_SpinLimit) {
2020 if (x < Knob_Poverty) x = Knob_Poverty;
2021 _SpinDuration = x + Knob_Bonus;
2022 }
2023 return 1;
2024 }
2025
2026 // The CAS failed ... we can take any of the following actions:
2027 // * penalize: ctr -= Knob_CASPenalty
2028 // * exit spin with prejudice -- goto Abort;
2029 // * exit spin without prejudice.
2030 // * Since CAS is high-latency, retry again immediately.
2031 prv = ox;
2032 TEVENT(Spin: cas failed);
2033 if (caspty == -2) break;
2034 if (caspty == -1) goto Abort;
2035 ctr -= caspty;
2036 continue;
2037 }
2038
2039 // Did lock ownership change hands ?
2040 if (ox != prv && prv != NULL) {
2041 TEVENT(spin: Owner changed)
2042 if (oxpty == -2) break;
2043 if (oxpty == -1) goto Abort;
2044 ctr -= oxpty;
2045 }
2046 prv = ox;
2047
2048 // Abort the spin if the owner is not executing.
2049 // The owner must be executing in order to drop the lock.
2050 // Spinning while the owner is OFFPROC is idiocy.
2051 // Consider: ctr -= RunnablePenalty ;
2052 if (Knob_OState && NotRunnable (Self, ox)) {
2053 TEVENT(Spin abort - notrunnable);
2054 goto Abort;
2055 }
2056 if (sss && _succ == NULL) _succ = Self;
2057 }
2058
2059 // Spin failed with prejudice -- reduce _SpinDuration.
2060 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2061 // AIMD is globally stable.
2062 TEVENT(Spin failure);
2063 {
2064 int x = _SpinDuration;
2065 if (x > 0) {
2066 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2067 // This is globally sample and tends to damp the response.
2068 x -= Knob_Penalty;
2069 if (x < 0) x = 0;
2070 _SpinDuration = x;
2071 }
2072 }
2073
2074 Abort:
2075 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2076 if (sss && _succ == Self) {
2077 _succ = NULL;
2078 // Invariant: after setting succ=null a contending thread
2079 // must recheck-retry _owner before parking. This usually happens
2080 // in the normal usage of TrySpin(), but it's safest
2081 // to make TrySpin() as foolproof as possible.
2082 OrderAccess::fence();
2083 if (TryLock(Self) > 0) return 1;
2084 }
2085 return 0;
2086 }
2087
2088 // NotRunnable() -- informed spinning
2089 //
2090 // Don't bother spinning if the owner is not eligible to drop the lock.
2091 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2092 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2093 // The thread must be runnable in order to drop the lock in timely fashion.
2094 // If the _owner is not runnable then spinning will not likely be
2095 // successful (profitable).
2096 //
2097 // Beware -- the thread referenced by _owner could have died
2098 // so a simply fetch from _owner->_thread_state might trap.
2099 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2100 // Because of the lifecycle issues the schedctl and _thread_state values
2101 // observed by NotRunnable() might be garbage. NotRunnable must
2102 // tolerate this and consider the observed _thread_state value
2103 // as advisory.
2104 //
2105 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2106 // a thread pointer.
2107 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2108 // with the LSB of _owner. Another option would be to probablistically probe
2109 // the putative _owner->TypeTag value.
2110 //
2111 // Checking _thread_state isn't perfect. Even if the thread is
2112 // in_java it might be blocked on a page-fault or have been preempted
2113 // and sitting on a ready/dispatch queue. _thread state in conjunction
2114 // with schedctl.sc_state gives us a good picture of what the
2115 // thread is doing, however.
2116 //
2117 // TODO: check schedctl.sc_state.
2118 // We'll need to use SafeFetch32() to read from the schedctl block.
2119 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2120 //
2121 // The return value from NotRunnable() is *advisory* -- the
2122 // result is based on sampling and is not necessarily coherent.
2123 // The caller must tolerate false-negative and false-positive errors.
2124 // Spinning, in general, is probabilistic anyway.
2125
2126
2127 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2128 // Check ox->TypeTag == 2BAD.
2129 if (ox == NULL) return 0;
2130
2131 // Avoid transitive spinning ...
2132 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2133 // Immediately after T1 acquires L it's possible that T2, also
2134 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2135 // This occurs transiently after T1 acquired L but before
2136 // T1 managed to clear T1.Stalled. T2 does not need to abort
2137 // its spin in this circumstance.
2138 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2139
2140 if (BlockedOn == 1) return 1;
2141 if (BlockedOn != 0) {
2142 return BlockedOn != intptr_t(this) && _owner == ox;
2143 }
2144
2145 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2146 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2147 // consider also: jst != _thread_in_Java -- but that's overspecific.
2148 return jst == _thread_blocked || jst == _thread_in_native;
2149 }
2150
2151
2152 // -----------------------------------------------------------------------------
2153 // WaitSet management ...
2154
2155 ObjectWaiter::ObjectWaiter(Thread* thread) {
2156 _next = NULL;
2157 _prev = NULL;
2158 _notified = 0;
2159 _notifier_tid = 0;
2160 TState = TS_RUN;
2161 _thread = thread;
2162 _event = thread->_ParkEvent;
2163 _active = false;
2164 assert(_event != NULL, "invariant");
2165 }
2166
2167 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2168 JavaThread *jt = (JavaThread *)this->_thread;
2169 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2170 }
2171
2172 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2173 JavaThread *jt = (JavaThread *)this->_thread;
2174 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2175 }
2176
2177 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2178 assert(node != NULL, "should not add NULL node");
2179 assert(node->_prev == NULL, "node already in list");
2180 assert(node->_next == NULL, "node already in list");
2181 // put node at end of queue (circular doubly linked list)
2182 if (_WaitSet == NULL) {
2183 _WaitSet = node;
2184 node->_prev = node;
2185 node->_next = node;
2186 } else {
2187 ObjectWaiter* head = _WaitSet;
2188 ObjectWaiter* tail = head->_prev;
2189 assert(tail->_next == head, "invariant check");
2190 tail->_next = node;
2191 head->_prev = node;
2192 node->_next = head;
2193 node->_prev = tail;
2194 }
2195 }
2196
2197 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2198 // dequeue the very first waiter
2199 ObjectWaiter* waiter = _WaitSet;
2200 if (waiter) {
2201 DequeueSpecificWaiter(waiter);
2202 }
2203 return waiter;
2204 }
2205
2206 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2207 assert(node != NULL, "should not dequeue NULL node");
2208 assert(node->_prev != NULL, "node already removed from list");
2209 assert(node->_next != NULL, "node already removed from list");
2210 // when the waiter has woken up because of interrupt,
2211 // timeout or other spurious wake-up, dequeue the
2212 // waiter from waiting list
2213 ObjectWaiter* next = node->_next;
2214 if (next == node) {
2215 assert(node->_prev == node, "invariant check");
2216 _WaitSet = NULL;
2217 } else {
2218 ObjectWaiter* prev = node->_prev;
2219 assert(prev->_next == node, "invariant check");
2220 assert(next->_prev == node, "invariant check");
2221 next->_prev = prev;
2222 prev->_next = next;
2223 if (_WaitSet == node) {
2224 _WaitSet = next;
2225 }
2226 }
2227 node->_next = NULL;
2228 node->_prev = NULL;
2229 }
2230
2231 // -----------------------------------------------------------------------------
2232 // PerfData support
2233 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2234 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2235 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2236 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2237 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2238 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2239 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2240
2241 // One-shot global initialization for the sync subsystem.
2242 // We could also defer initialization and initialize on-demand
2243 // the first time we call inflate(). Initialization would
2244 // be protected - like so many things - by the MonitorCache_lock.
2245
2246 void ObjectMonitor::Initialize() {
2247 static int InitializationCompleted = 0;
2248 assert(InitializationCompleted == 0, "invariant");
2249 InitializationCompleted = 1;
2250 if (UsePerfData) {
2251 EXCEPTION_MARK;
2252 #define NEWPERFCOUNTER(n) \
2253 { \
2254 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2255 CHECK); \
2256 }
2257 #define NEWPERFVARIABLE(n) \
2258 { \
2259 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2260 CHECK); \
2261 }
2262 NEWPERFCOUNTER(_sync_Inflations);
2263 NEWPERFCOUNTER(_sync_Deflations);
2264 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2265 NEWPERFCOUNTER(_sync_FutileWakeups);
2266 NEWPERFCOUNTER(_sync_Parks);
2267 NEWPERFCOUNTER(_sync_Notifications);
2268 NEWPERFVARIABLE(_sync_MonExtant);
2269 #undef NEWPERFCOUNTER
2270 #undef NEWPERFVARIABLE
2271 }
2272 }
2273
2274 static char * kvGet(char * kvList, const char * Key) {
2275 if (kvList == NULL) return NULL;
2276 size_t n = strlen(Key);
2277 char * Search;
2278 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2279 if (strncmp (Search, Key, n) == 0) {
2280 if (Search[n] == '=') return Search + n + 1;
2281 if (Search[n] == 0) return(char *) "1";
2282 }
2283 }
2284 return NULL;
2285 }
2286
2287 static int kvGetInt(char * kvList, const char * Key, int Default) {
2288 char * v = kvGet(kvList, Key);
2289 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2290 if (Knob_ReportSettings && v != NULL) {
2291 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ;
2292 tty->flush();
2293 }
2294 return rslt;
2295 }
2296
2297 void ObjectMonitor::DeferredInitialize() {
2298 if (InitDone > 0) return;
2299 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2300 while (InitDone != 1) /* empty */;
2301 return;
2302 }
2303
2304 // One-shot global initialization ...
2305 // The initialization is idempotent, so we don't need locks.
2306 // In the future consider doing this via os::init_2().
2307 // SyncKnobs consist of <Key>=<Value> pairs in the style
2308 // of environment variables. Start by converting ':' to NUL.
2309
2310 if (SyncKnobs == NULL) SyncKnobs = "";
2311
2312 size_t sz = strlen(SyncKnobs);
2313 char * knobs = (char *) os::malloc(sz + 2, mtInternal);
2314 if (knobs == NULL) {
2315 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2316 guarantee(0, "invariant");
2317 }
2318 strcpy(knobs, SyncKnobs);
2319 knobs[sz+1] = 0;
2320 for (char * p = knobs; *p; p++) {
2321 if (*p == ':') *p = 0;
2322 }
2323
2324 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2325 SETKNOB(ReportSettings);
2326 SETKNOB(ExitRelease);
2327 SETKNOB(InlineNotify);
2328 SETKNOB(Verbose);
2329 SETKNOB(VerifyInUse);
2330 SETKNOB(VerifyMatch);
2331 SETKNOB(FixedSpin);
2332 SETKNOB(SpinLimit);
2333 SETKNOB(SpinBase);
2334 SETKNOB(SpinBackOff);
2335 SETKNOB(CASPenalty);
2336 SETKNOB(OXPenalty);
2337 SETKNOB(SpinSetSucc);
2338 SETKNOB(SuccEnabled);
2339 SETKNOB(SuccRestrict);
2340 SETKNOB(Penalty);
2341 SETKNOB(Bonus);
2342 SETKNOB(BonusB);
2343 SETKNOB(Poverty);
2344 SETKNOB(SpinAfterFutile);
2345 SETKNOB(UsePause);
2346 SETKNOB(SpinEarly);
2347 SETKNOB(OState);
2348 SETKNOB(MaxSpinners);
2349 SETKNOB(PreSpin);
2350 SETKNOB(ExitPolicy);
2351 SETKNOB(QMode);
2352 SETKNOB(ResetEvent);
2353 SETKNOB(MoveNotifyee);
2354 SETKNOB(FastHSSEC);
2355 #undef SETKNOB
2356
2357 if (os::is_MP()) {
2358 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2359 if (Knob_ReportSettings) {
2360 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask);
2361 }
2362 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2363 } else {
2364 Knob_SpinLimit = 0;
2365 Knob_SpinBase = 0;
2366 Knob_PreSpin = 0;
2367 Knob_FixedSpin = -1;
2368 }
2369
2370 os::free(knobs);
2371 OrderAccess::fence();
2372 InitDone = 1;
2373 }
2374