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 "memory/allocation.inline.hpp"
28 #include "memory/resourceArea.hpp"
29 #include "oops/markOop.hpp"
30 #include "oops/oop.inline.hpp"
31 #include "runtime/atomic.hpp"
32 #include "runtime/handles.inline.hpp"
33 #include "runtime/interfaceSupport.inline.hpp"
34 #include "runtime/mutexLocker.hpp"
35 #include "runtime/objectMonitor.hpp"
36 #include "runtime/objectMonitor.inline.hpp"
37 #include "runtime/orderAccess.inline.hpp"
38 #include "runtime/osThread.hpp"
39 #include "runtime/safepointMechanism.inline.hpp"
40 #include "runtime/stubRoutines.hpp"
41 #include "runtime/thread.inline.hpp"
42 #include "services/threadService.hpp"
43 #include "trace/tracing.hpp"
44 #include "trace/traceMacros.hpp"
45 #include "utilities/dtrace.hpp"
46 #include "utilities/macros.hpp"
47 #include "utilities/preserveException.hpp"
48
49 #ifdef DTRACE_ENABLED
50
51 // Only bother with this argument setup if dtrace is available
52 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
53
54
55 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
56 char* bytes = NULL; \
57 int len = 0; \
58 jlong jtid = SharedRuntime::get_java_tid(thread); \
59 Symbol* klassname = ((oop)obj)->klass()->name(); \
60 if (klassname != NULL) { \
61 bytes = (char*)klassname->bytes(); \
62 len = klassname->utf8_length(); \
63 }
64
65 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
66 { \
67 if (DTraceMonitorProbes) { \
68 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
69 HOTSPOT_MONITOR_WAIT(jtid, \
70 (monitor), bytes, len, (millis)); \
71 } \
72 }
73
74 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
75 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
76 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
77 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
78 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
79
80 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
81 { \
82 if (DTraceMonitorProbes) { \
83 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
84 HOTSPOT_MONITOR_##probe(jtid, \
85 (uintptr_t)(monitor), bytes, len); \
86 } \
87 }
88
89 #else // ndef DTRACE_ENABLED
90
91 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
92 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
93
94 #endif // ndef DTRACE_ENABLED
95
96 // Tunables ...
97 // The knob* variables are effectively final. Once set they should
98 // never be modified hence. Consider using __read_mostly with GCC.
99
100 int ObjectMonitor::Knob_ExitRelease = 0;
101 int ObjectMonitor::Knob_Verbose = 0;
102 int ObjectMonitor::Knob_VerifyInUse = 0;
103 int ObjectMonitor::Knob_VerifyMatch = 0;
104 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
105 static int Knob_ReportSettings = 0;
106
107 static int Knob_SpinBase = 0; // Floor AKA SpinMin
108 static int Knob_SpinBackOff = 0; // spin-loop backoff
109 static int Knob_CASPenalty = -1; // Penalty for failed CAS
110 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
111 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
112 static int Knob_SpinEarly = 1;
113 static int Knob_SuccEnabled = 1; // futile wake throttling
114 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
115 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
116 static int Knob_Bonus = 100; // spin success bonus
117 static int Knob_BonusB = 100; // spin success bonus
118 static int Knob_Penalty = 200; // spin failure penalty
119 static int Knob_Poverty = 1000;
120 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
121 static int Knob_FixedSpin = 0;
122 static int Knob_OState = 3; // Spinner checks thread state of _owner
123 static int Knob_UsePause = 1;
124 static int Knob_ExitPolicy = 0;
125 static int Knob_PreSpin = 10; // 20-100 likely better
126 static int Knob_ResetEvent = 0;
127 static int BackOffMask = 0;
128
129 static int Knob_FastHSSEC = 0;
130 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
131 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
132 static volatile int InitDone = 0;
133
134 // -----------------------------------------------------------------------------
135 // Theory of operations -- Monitors lists, thread residency, etc:
136 //
137 // * A thread acquires ownership of a monitor by successfully
138 // CAS()ing the _owner field from null to non-null.
139 //
140 // * Invariant: A thread appears on at most one monitor list --
141 // cxq, EntryList or WaitSet -- at any one time.
142 //
143 // * Contending threads "push" themselves onto the cxq with CAS
144 // and then spin/park.
145 //
146 // * After a contending thread eventually acquires the lock it must
147 // dequeue itself from either the EntryList or the cxq.
148 //
149 // * The exiting thread identifies and unparks an "heir presumptive"
150 // tentative successor thread on the EntryList. Critically, the
151 // exiting thread doesn't unlink the successor thread from the EntryList.
152 // After having been unparked, the wakee will recontend for ownership of
153 // the monitor. The successor (wakee) will either acquire the lock or
154 // re-park itself.
155 //
156 // Succession is provided for by a policy of competitive handoff.
157 // The exiting thread does _not_ grant or pass ownership to the
158 // successor thread. (This is also referred to as "handoff" succession").
159 // Instead the exiting thread releases ownership and possibly wakes
160 // a successor, so the successor can (re)compete for ownership of the lock.
161 // If the EntryList is empty but the cxq is populated the exiting
162 // thread will drain the cxq into the EntryList. It does so by
163 // by detaching the cxq (installing null with CAS) and folding
164 // the threads from the cxq into the EntryList. The EntryList is
165 // doubly linked, while the cxq is singly linked because of the
166 // CAS-based "push" used to enqueue recently arrived threads (RATs).
167 //
168 // * Concurrency invariants:
169 //
170 // -- only the monitor owner may access or mutate the EntryList.
171 // The mutex property of the monitor itself protects the EntryList
172 // from concurrent interference.
173 // -- Only the monitor owner may detach the cxq.
174 //
175 // * The monitor entry list operations avoid locks, but strictly speaking
176 // they're not lock-free. Enter is lock-free, exit is not.
177 // For a description of 'Methods and apparatus providing non-blocking access
178 // to a resource,' see U.S. Pat. No. 7844973.
179 //
180 // * The cxq can have multiple concurrent "pushers" but only one concurrent
181 // detaching thread. This mechanism is immune from the ABA corruption.
182 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
183 //
184 // * Taken together, the cxq and the EntryList constitute or form a
185 // single logical queue of threads stalled trying to acquire the lock.
186 // We use two distinct lists to improve the odds of a constant-time
187 // dequeue operation after acquisition (in the ::enter() epilogue) and
188 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
189 // A key desideratum is to minimize queue & monitor metadata manipulation
190 // that occurs while holding the monitor lock -- that is, we want to
191 // minimize monitor lock holds times. Note that even a small amount of
192 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
193 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
194 // locks and monitor metadata.
195 //
196 // Cxq points to the set of Recently Arrived Threads attempting entry.
197 // Because we push threads onto _cxq with CAS, the RATs must take the form of
198 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
199 // the unlocking thread notices that EntryList is null but _cxq is != null.
200 //
201 // The EntryList is ordered by the prevailing queue discipline and
202 // can be organized in any convenient fashion, such as a doubly-linked list or
203 // a circular doubly-linked list. Critically, we want insert and delete operations
204 // to operate in constant-time. If we need a priority queue then something akin
205 // to Solaris' sleepq would work nicely. Viz.,
206 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
207 // Queue discipline is enforced at ::exit() time, when the unlocking thread
208 // drains the cxq into the EntryList, and orders or reorders the threads on the
209 // EntryList accordingly.
210 //
211 // Barring "lock barging", this mechanism provides fair cyclic ordering,
212 // somewhat similar to an elevator-scan.
213 //
214 // * The monitor synchronization subsystem avoids the use of native
215 // synchronization primitives except for the narrow platform-specific
216 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
217 // the semantics of park-unpark. Put another way, this monitor implementation
218 // depends only on atomic operations and park-unpark. The monitor subsystem
219 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
220 // underlying OS manages the READY<->RUN transitions.
221 //
222 // * Waiting threads reside on the WaitSet list -- wait() puts
223 // the caller onto the WaitSet.
224 //
225 // * notify() or notifyAll() simply transfers threads from the WaitSet to
226 // either the EntryList or cxq. Subsequent exit() operations will
227 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
228 // it's likely the notifyee would simply impale itself on the lock held
229 // by the notifier.
230 //
231 // * An interesting alternative is to encode cxq as (List,LockByte) where
232 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
233 // variable, like _recursions, in the scheme. The threads or Events that form
234 // the list would have to be aligned in 256-byte addresses. A thread would
235 // try to acquire the lock or enqueue itself with CAS, but exiting threads
236 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
237 // Note that is is *not* word-tearing, but it does presume that full-word
238 // CAS operations are coherent with intermix with STB operations. That's true
239 // on most common processors.
240 //
241 // * See also http://blogs.sun.com/dave
242
243
244 void* ObjectMonitor::operator new (size_t size) throw() {
245 return AllocateHeap(size, mtInternal);
246 }
247 void* ObjectMonitor::operator new[] (size_t size) throw() {
248 return operator new (size);
249 }
250 void ObjectMonitor::operator delete(void* p) {
251 FreeHeap(p);
252 }
253 void ObjectMonitor::operator delete[] (void *p) {
254 operator delete(p);
255 }
256
257 // -----------------------------------------------------------------------------
258 // Enter support
259
260 void ObjectMonitor::enter(TRAPS) {
261 // The following code is ordered to check the most common cases first
262 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
263 Thread * const Self = THREAD;
264
265 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
266 if (cur == NULL) {
267 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
268 assert(_recursions == 0, "invariant");
269 assert(_owner == Self, "invariant");
270 return;
271 }
272
273 if (cur == Self) {
274 // TODO-FIXME: check for integer overflow! BUGID 6557169.
275 _recursions++;
276 return;
277 }
278
279 if (Self->is_lock_owned ((address)cur)) {
280 assert(_recursions == 0, "internal state error");
281 _recursions = 1;
282 // Commute owner from a thread-specific on-stack BasicLockObject address to
283 // a full-fledged "Thread *".
284 _owner = Self;
285 return;
286 }
287
288 // We've encountered genuine contention.
289 assert(Self->_Stalled == 0, "invariant");
290 Self->_Stalled = intptr_t(this);
291
292 // Try one round of spinning *before* enqueueing Self
293 // and before going through the awkward and expensive state
294 // transitions. The following spin is strictly optional ...
295 // Note that if we acquire the monitor from an initial spin
296 // we forgo posting JVMTI events and firing DTRACE probes.
297 if (Knob_SpinEarly && TrySpin (Self) > 0) {
298 assert(_owner == Self, "invariant");
299 assert(_recursions == 0, "invariant");
300 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
301 Self->_Stalled = 0;
302 return;
303 }
304
305 assert(_owner != Self, "invariant");
306 assert(_succ != Self, "invariant");
307 assert(Self->is_Java_thread(), "invariant");
308 JavaThread * jt = (JavaThread *) Self;
309 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
310 assert(jt->thread_state() != _thread_blocked, "invariant");
311 assert(this->object() != NULL, "invariant");
312 assert(_count >= 0, "invariant");
313
314 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
315 // Ensure the object-monitor relationship remains stable while there's contention.
316 Atomic::inc(&_count);
317
318 EventJavaMonitorEnter event;
319
320 { // Change java thread status to indicate blocked on monitor enter.
321 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
322
323 Self->set_current_pending_monitor(this);
324
325 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
326 if (JvmtiExport::should_post_monitor_contended_enter()) {
327 JvmtiExport::post_monitor_contended_enter(jt, this);
328
329 // The current thread does not yet own the monitor and does not
330 // yet appear on any queues that would get it made the successor.
331 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
332 // handler cannot accidentally consume an unpark() meant for the
333 // ParkEvent associated with this ObjectMonitor.
334 }
335
336 OSThreadContendState osts(Self->osthread());
337 ThreadBlockInVM tbivm(jt);
338
339 // TODO-FIXME: change the following for(;;) loop to straight-line code.
340 for (;;) {
341 jt->set_suspend_equivalent();
342 // cleared by handle_special_suspend_equivalent_condition()
343 // or java_suspend_self()
344
345 EnterI(THREAD);
346
347 if (!ExitSuspendEquivalent(jt)) break;
348
349 // We have acquired the contended monitor, but while we were
350 // waiting another thread suspended us. We don't want to enter
351 // the monitor while suspended because that would surprise the
352 // thread that suspended us.
353 //
354 _recursions = 0;
355 _succ = NULL;
356 exit(false, Self);
357
358 jt->java_suspend_self();
359 }
360 Self->set_current_pending_monitor(NULL);
361
362 // We cleared the pending monitor info since we've just gotten past
363 // the enter-check-for-suspend dance and we now own the monitor free
364 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
365 // destructor can go to a safepoint at the end of this block. If we
366 // do a thread dump during that safepoint, then this thread will show
367 // as having "-locked" the monitor, but the OS and java.lang.Thread
368 // states will still report that the thread is blocked trying to
369 // acquire it.
370 }
371
372 Atomic::dec(&_count);
373 assert(_count >= 0, "invariant");
374 Self->_Stalled = 0;
375
376 // Must either set _recursions = 0 or ASSERT _recursions == 0.
377 assert(_recursions == 0, "invariant");
378 assert(_owner == Self, "invariant");
379 assert(_succ != Self, "invariant");
380 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
381
382 // The thread -- now the owner -- is back in vm mode.
383 // Report the glorious news via TI,DTrace and jvmstat.
384 // The probe effect is non-trivial. All the reportage occurs
385 // while we hold the monitor, increasing the length of the critical
386 // section. Amdahl's parallel speedup law comes vividly into play.
387 //
388 // Another option might be to aggregate the events (thread local or
389 // per-monitor aggregation) and defer reporting until a more opportune
390 // time -- such as next time some thread encounters contention but has
391 // yet to acquire the lock. While spinning that thread could
392 // spinning we could increment JVMStat counters, etc.
393
394 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
395 if (JvmtiExport::should_post_monitor_contended_entered()) {
396 JvmtiExport::post_monitor_contended_entered(jt, this);
397
398 // The current thread already owns the monitor and is not going to
399 // call park() for the remainder of the monitor enter protocol. So
400 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
401 // event handler consumed an unpark() issued by the thread that
402 // just exited the monitor.
403 }
404
405 if (event.should_commit()) {
406 event.set_monitorClass(((oop)this->object())->klass());
407 event.set_previousOwner((TYPE_THREAD)_previous_owner_tid);
408 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
409 event.commit();
410 }
411
412 OM_PERFDATA_OP(ContendedLockAttempts, inc());
413 }
414
415
416 // Caveat: TryLock() is not necessarily serializing if it returns failure.
417 // Callers must compensate as needed.
418
419 int ObjectMonitor::TryLock(Thread * Self) {
420 void * own = _owner;
421 if (own != NULL) return 0;
422 if (Atomic::replace_if_null(Self, &_owner)) {
423 // Either guarantee _recursions == 0 or set _recursions = 0.
424 assert(_recursions == 0, "invariant");
425 assert(_owner == Self, "invariant");
426 return 1;
427 }
428 // The lock had been free momentarily, but we lost the race to the lock.
429 // Interference -- the CAS failed.
430 // We can either return -1 or retry.
431 // Retry doesn't make as much sense because the lock was just acquired.
432 return -1;
433 }
434
435 #define MAX_RECHECK_INTERVAL 1000
436
437 void ObjectMonitor::EnterI(TRAPS) {
438 Thread * const Self = THREAD;
439 assert(Self->is_Java_thread(), "invariant");
440 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
441
442 // Try the lock - TATAS
443 if (TryLock (Self) > 0) {
444 assert(_succ != Self, "invariant");
445 assert(_owner == Self, "invariant");
446 assert(_Responsible != Self, "invariant");
447 return;
448 }
449
450 DeferredInitialize();
451
452 // We try one round of spinning *before* enqueueing Self.
453 //
454 // If the _owner is ready but OFFPROC we could use a YieldTo()
455 // operation to donate the remainder of this thread's quantum
456 // to the owner. This has subtle but beneficial affinity
457 // effects.
458
459 if (TrySpin (Self) > 0) {
460 assert(_owner == Self, "invariant");
461 assert(_succ != Self, "invariant");
462 assert(_Responsible != Self, "invariant");
463 return;
464 }
465
466 // The Spin failed -- Enqueue and park the thread ...
467 assert(_succ != Self, "invariant");
468 assert(_owner != Self, "invariant");
469 assert(_Responsible != Self, "invariant");
470
471 // Enqueue "Self" on ObjectMonitor's _cxq.
472 //
473 // Node acts as a proxy for Self.
474 // As an aside, if were to ever rewrite the synchronization code mostly
475 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
476 // Java objects. This would avoid awkward lifecycle and liveness issues,
477 // as well as eliminate a subset of ABA issues.
478 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
479
480 ObjectWaiter node(Self);
481 Self->_ParkEvent->reset();
482 node._prev = (ObjectWaiter *) 0xBAD;
483 node.TState = ObjectWaiter::TS_CXQ;
484
485 // Push "Self" onto the front of the _cxq.
486 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
487 // Note that spinning tends to reduce the rate at which threads
488 // enqueue and dequeue on EntryList|cxq.
489 ObjectWaiter * nxt;
490 for (;;) {
491 node._next = nxt = _cxq;
492 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
493
494 // Interference - the CAS failed because _cxq changed. Just retry.
495 // As an optional optimization we retry the lock.
496 if (TryLock (Self) > 0) {
497 assert(_succ != Self, "invariant");
498 assert(_owner == Self, "invariant");
499 assert(_Responsible != Self, "invariant");
500 return;
501 }
502 }
503
504 // Check for cxq|EntryList edge transition to non-null. This indicates
505 // the onset of contention. While contention persists exiting threads
506 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
507 // operations revert to the faster 1-0 mode. This enter operation may interleave
508 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
509 // arrange for one of the contending thread to use a timed park() operations
510 // to detect and recover from the race. (Stranding is form of progress failure
511 // where the monitor is unlocked but all the contending threads remain parked).
512 // That is, at least one of the contended threads will periodically poll _owner.
513 // One of the contending threads will become the designated "Responsible" thread.
514 // The Responsible thread uses a timed park instead of a normal indefinite park
515 // operation -- it periodically wakes and checks for and recovers from potential
516 // strandings admitted by 1-0 exit operations. We need at most one Responsible
517 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
518 // be responsible for a monitor.
519 //
520 // Currently, one of the contended threads takes on the added role of "Responsible".
521 // A viable alternative would be to use a dedicated "stranding checker" thread
522 // that periodically iterated over all the threads (or active monitors) and unparked
523 // successors where there was risk of stranding. This would help eliminate the
524 // timer scalability issues we see on some platforms as we'd only have one thread
525 // -- the checker -- parked on a timer.
526
527 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
528 // Try to assume the role of responsible thread for the monitor.
529 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
530 Atomic::replace_if_null(Self, &_Responsible);
531 }
532
533 // The lock might have been released while this thread was occupied queueing
534 // itself onto _cxq. To close the race and avoid "stranding" and
535 // progress-liveness failure we must resample-retry _owner before parking.
536 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
537 // In this case the ST-MEMBAR is accomplished with CAS().
538 //
539 // TODO: Defer all thread state transitions until park-time.
540 // Since state transitions are heavy and inefficient we'd like
541 // to defer the state transitions until absolutely necessary,
542 // and in doing so avoid some transitions ...
543
544 TEVENT(Inflated enter - Contention);
545 int nWakeups = 0;
546 int recheckInterval = 1;
547
548 for (;;) {
549
550 if (TryLock(Self) > 0) break;
551 assert(_owner != Self, "invariant");
552
553 if ((SyncFlags & 2) && _Responsible == NULL) {
554 Atomic::replace_if_null(Self, &_Responsible);
555 }
556
557 // park self
558 if (_Responsible == Self || (SyncFlags & 1)) {
559 TEVENT(Inflated enter - park TIMED);
560 Self->_ParkEvent->park((jlong) recheckInterval);
561 // Increase the recheckInterval, but clamp the value.
562 recheckInterval *= 8;
563 if (recheckInterval > MAX_RECHECK_INTERVAL) {
564 recheckInterval = MAX_RECHECK_INTERVAL;
565 }
566 } else {
567 TEVENT(Inflated enter - park UNTIMED);
568 Self->_ParkEvent->park();
569 }
570
571 if (TryLock(Self) > 0) break;
572
573 // The lock is still contested.
574 // Keep a tally of the # of futile wakeups.
575 // Note that the counter is not protected by a lock or updated by atomics.
576 // That is by design - we trade "lossy" counters which are exposed to
577 // races during updates for a lower probe effect.
578 TEVENT(Inflated enter - Futile wakeup);
579 // This PerfData object can be used in parallel with a safepoint.
580 // See the work around in PerfDataManager::destroy().
581 OM_PERFDATA_OP(FutileWakeups, inc());
582 ++nWakeups;
583
584 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
585 // We can defer clearing _succ until after the spin completes
586 // TrySpin() must tolerate being called with _succ == Self.
587 // Try yet another round of adaptive spinning.
588 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
589
590 // We can find that we were unpark()ed and redesignated _succ while
591 // we were spinning. That's harmless. If we iterate and call park(),
592 // park() will consume the event and return immediately and we'll
593 // just spin again. This pattern can repeat, leaving _succ to simply
594 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
595 // Alternately, we can sample fired() here, and if set, forgo spinning
596 // in the next iteration.
597
598 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
599 Self->_ParkEvent->reset();
600 OrderAccess::fence();
601 }
602 if (_succ == Self) _succ = NULL;
603
604 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
605 OrderAccess::fence();
606 }
607
608 // Egress :
609 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
610 // Normally we'll find Self on the EntryList .
611 // From the perspective of the lock owner (this thread), the
612 // EntryList is stable and cxq is prepend-only.
613 // The head of cxq is volatile but the interior is stable.
614 // In addition, Self.TState is stable.
615
616 assert(_owner == Self, "invariant");
617 assert(object() != NULL, "invariant");
618 // I'd like to write:
619 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
620 // but as we're at a safepoint that's not safe.
621
622 UnlinkAfterAcquire(Self, &node);
623 if (_succ == Self) _succ = NULL;
624
625 assert(_succ != Self, "invariant");
626 if (_Responsible == Self) {
627 _Responsible = NULL;
628 OrderAccess::fence(); // Dekker pivot-point
629
630 // We may leave threads on cxq|EntryList without a designated
631 // "Responsible" thread. This is benign. When this thread subsequently
632 // exits the monitor it can "see" such preexisting "old" threads --
633 // threads that arrived on the cxq|EntryList before the fence, above --
634 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
635 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
636 // non-null and elect a new "Responsible" timer thread.
637 //
638 // This thread executes:
639 // ST Responsible=null; MEMBAR (in enter epilogue - here)
640 // LD cxq|EntryList (in subsequent exit)
641 //
642 // Entering threads in the slow/contended path execute:
643 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
644 // The (ST cxq; MEMBAR) is accomplished with CAS().
645 //
646 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
647 // exit operation from floating above the ST Responsible=null.
648 }
649
650 // We've acquired ownership with CAS().
651 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
652 // But since the CAS() this thread may have also stored into _succ,
653 // EntryList, cxq or Responsible. These meta-data updates must be
654 // visible __before this thread subsequently drops the lock.
655 // Consider what could occur if we didn't enforce this constraint --
656 // STs to monitor meta-data and user-data could reorder with (become
657 // visible after) the ST in exit that drops ownership of the lock.
658 // Some other thread could then acquire the lock, but observe inconsistent
659 // or old monitor meta-data and heap data. That violates the JMM.
660 // To that end, the 1-0 exit() operation must have at least STST|LDST
661 // "release" barrier semantics. Specifically, there must be at least a
662 // STST|LDST barrier in exit() before the ST of null into _owner that drops
663 // the lock. The barrier ensures that changes to monitor meta-data and data
664 // protected by the lock will be visible before we release the lock, and
665 // therefore before some other thread (CPU) has a chance to acquire the lock.
666 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
667 //
668 // Critically, any prior STs to _succ or EntryList must be visible before
669 // the ST of null into _owner in the *subsequent* (following) corresponding
670 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
671 // execute a serializing instruction.
672
673 if (SyncFlags & 8) {
674 OrderAccess::fence();
675 }
676 return;
677 }
678
679 // ReenterI() is a specialized inline form of the latter half of the
680 // contended slow-path from EnterI(). We use ReenterI() only for
681 // monitor reentry in wait().
682 //
683 // In the future we should reconcile EnterI() and ReenterI(), adding
684 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
685 // loop accordingly.
686
687 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
688 assert(Self != NULL, "invariant");
689 assert(SelfNode != NULL, "invariant");
690 assert(SelfNode->_thread == Self, "invariant");
691 assert(_waiters > 0, "invariant");
692 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
693 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
694 JavaThread * jt = (JavaThread *) Self;
695
696 int nWakeups = 0;
697 for (;;) {
698 ObjectWaiter::TStates v = SelfNode->TState;
699 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
700 assert(_owner != Self, "invariant");
701
702 if (TryLock(Self) > 0) break;
703 if (TrySpin(Self) > 0) break;
704
705 TEVENT(Wait Reentry - parking);
706
707 // State transition wrappers around park() ...
708 // ReenterI() wisely defers state transitions until
709 // it's clear we must park the thread.
710 {
711 OSThreadContendState osts(Self->osthread());
712 ThreadBlockInVM tbivm(jt);
713
714 // cleared by handle_special_suspend_equivalent_condition()
715 // or java_suspend_self()
716 jt->set_suspend_equivalent();
717 if (SyncFlags & 1) {
718 Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL);
719 } else {
720 Self->_ParkEvent->park();
721 }
722
723 // were we externally suspended while we were waiting?
724 for (;;) {
725 if (!ExitSuspendEquivalent(jt)) break;
726 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
727 jt->java_suspend_self();
728 jt->set_suspend_equivalent();
729 }
730 }
731
732 // Try again, but just so we distinguish between futile wakeups and
733 // successful wakeups. The following test isn't algorithmically
734 // necessary, but it helps us maintain sensible statistics.
735 if (TryLock(Self) > 0) break;
736
737 // The lock is still contested.
738 // Keep a tally of the # of futile wakeups.
739 // Note that the counter is not protected by a lock or updated by atomics.
740 // That is by design - we trade "lossy" counters which are exposed to
741 // races during updates for a lower probe effect.
742 TEVENT(Wait Reentry - futile wakeup);
743 ++nWakeups;
744
745 // Assuming this is not a spurious wakeup we'll normally
746 // find that _succ == Self.
747 if (_succ == Self) _succ = NULL;
748
749 // Invariant: after clearing _succ a contending thread
750 // *must* retry _owner before parking.
751 OrderAccess::fence();
752
753 // This PerfData object can be used in parallel with a safepoint.
754 // See the work around in PerfDataManager::destroy().
755 OM_PERFDATA_OP(FutileWakeups, inc());
756 }
757
758 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
759 // Normally we'll find Self on the EntryList.
760 // Unlinking from the EntryList is constant-time and atomic-free.
761 // From the perspective of the lock owner (this thread), the
762 // EntryList is stable and cxq is prepend-only.
763 // The head of cxq is volatile but the interior is stable.
764 // In addition, Self.TState is stable.
765
766 assert(_owner == Self, "invariant");
767 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
768 UnlinkAfterAcquire(Self, SelfNode);
769 if (_succ == Self) _succ = NULL;
770 assert(_succ != Self, "invariant");
771 SelfNode->TState = ObjectWaiter::TS_RUN;
772 OrderAccess::fence(); // see comments at the end of EnterI()
773 }
774
775 // By convention we unlink a contending thread from EntryList|cxq immediately
776 // after the thread acquires the lock in ::enter(). Equally, we could defer
777 // unlinking the thread until ::exit()-time.
778
779 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
780 assert(_owner == Self, "invariant");
781 assert(SelfNode->_thread == Self, "invariant");
782
783 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
784 // Normal case: remove Self from the DLL EntryList .
785 // This is a constant-time operation.
786 ObjectWaiter * nxt = SelfNode->_next;
787 ObjectWaiter * prv = SelfNode->_prev;
788 if (nxt != NULL) nxt->_prev = prv;
789 if (prv != NULL) prv->_next = nxt;
790 if (SelfNode == _EntryList) _EntryList = nxt;
791 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
792 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
793 TEVENT(Unlink from EntryList);
794 } else {
795 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
796 // Inopportune interleaving -- Self is still on the cxq.
797 // This usually means the enqueue of self raced an exiting thread.
798 // Normally we'll find Self near the front of the cxq, so
799 // dequeueing is typically fast. If needbe we can accelerate
800 // this with some MCS/CHL-like bidirectional list hints and advisory
801 // back-links so dequeueing from the interior will normally operate
802 // in constant-time.
803 // Dequeue Self from either the head (with CAS) or from the interior
804 // with a linear-time scan and normal non-atomic memory operations.
805 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
806 // and then unlink Self from EntryList. We have to drain eventually,
807 // so it might as well be now.
808
809 ObjectWaiter * v = _cxq;
810 assert(v != NULL, "invariant");
811 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
812 // The CAS above can fail from interference IFF a "RAT" arrived.
813 // In that case Self must be in the interior and can no longer be
814 // at the head of cxq.
815 if (v == SelfNode) {
816 assert(_cxq != v, "invariant");
817 v = _cxq; // CAS above failed - start scan at head of list
818 }
819 ObjectWaiter * p;
820 ObjectWaiter * q = NULL;
821 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
822 q = p;
823 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
824 }
825 assert(v != SelfNode, "invariant");
826 assert(p == SelfNode, "Node not found on cxq");
827 assert(p != _cxq, "invariant");
828 assert(q != NULL, "invariant");
829 assert(q->_next == p, "invariant");
830 q->_next = p->_next;
831 }
832 TEVENT(Unlink from cxq);
833 }
834
835 #ifdef ASSERT
836 // Diagnostic hygiene ...
837 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
838 SelfNode->_next = (ObjectWaiter *) 0xBAD;
839 SelfNode->TState = ObjectWaiter::TS_RUN;
840 #endif
841 }
842
843 // -----------------------------------------------------------------------------
844 // Exit support
845 //
846 // exit()
847 // ~~~~~~
848 // Note that the collector can't reclaim the objectMonitor or deflate
849 // the object out from underneath the thread calling ::exit() as the
850 // thread calling ::exit() never transitions to a stable state.
851 // This inhibits GC, which in turn inhibits asynchronous (and
852 // inopportune) reclamation of "this".
853 //
854 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
855 // There's one exception to the claim above, however. EnterI() can call
856 // exit() to drop a lock if the acquirer has been externally suspended.
857 // In that case exit() is called with _thread_state as _thread_blocked,
858 // but the monitor's _count field is > 0, which inhibits reclamation.
859 //
860 // 1-0 exit
861 // ~~~~~~~~
862 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
863 // the fast-path operators have been optimized so the common ::exit()
864 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
865 // The code emitted by fast_unlock() elides the usual MEMBAR. This
866 // greatly improves latency -- MEMBAR and CAS having considerable local
867 // latency on modern processors -- but at the cost of "stranding". Absent the
868 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
869 // ::enter() path, resulting in the entering thread being stranding
870 // and a progress-liveness failure. Stranding is extremely rare.
871 // We use timers (timed park operations) & periodic polling to detect
872 // and recover from stranding. Potentially stranded threads periodically
873 // wake up and poll the lock. See the usage of the _Responsible variable.
874 //
875 // The CAS() in enter provides for safety and exclusion, while the CAS or
876 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
877 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
878 // We detect and recover from stranding with timers.
879 //
880 // If a thread transiently strands it'll park until (a) another
881 // thread acquires the lock and then drops the lock, at which time the
882 // exiting thread will notice and unpark the stranded thread, or, (b)
883 // the timer expires. If the lock is high traffic then the stranding latency
884 // will be low due to (a). If the lock is low traffic then the odds of
885 // stranding are lower, although the worst-case stranding latency
886 // is longer. Critically, we don't want to put excessive load in the
887 // platform's timer subsystem. We want to minimize both the timer injection
888 // rate (timers created/sec) as well as the number of timers active at
889 // any one time. (more precisely, we want to minimize timer-seconds, which is
890 // the integral of the # of active timers at any instant over time).
891 // Both impinge on OS scalability. Given that, at most one thread parked on
892 // a monitor will use a timer.
893 //
894 // There is also the risk of a futile wake-up. If we drop the lock
895 // another thread can reacquire the lock immediately, and we can
896 // then wake a thread unnecessarily. This is benign, and we've
897 // structured the code so the windows are short and the frequency
898 // of such futile wakups is low.
899
900 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
901 Thread * const Self = THREAD;
902 if (THREAD != _owner) {
903 if (THREAD->is_lock_owned((address) _owner)) {
904 // Transmute _owner from a BasicLock pointer to a Thread address.
905 // We don't need to hold _mutex for this transition.
906 // Non-null to Non-null is safe as long as all readers can
907 // tolerate either flavor.
908 assert(_recursions == 0, "invariant");
909 _owner = THREAD;
910 _recursions = 0;
911 } else {
912 // Apparent unbalanced locking ...
913 // Naively we'd like to throw IllegalMonitorStateException.
914 // As a practical matter we can neither allocate nor throw an
915 // exception as ::exit() can be called from leaf routines.
916 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
917 // Upon deeper reflection, however, in a properly run JVM the only
918 // way we should encounter this situation is in the presence of
919 // unbalanced JNI locking. TODO: CheckJNICalls.
920 // See also: CR4414101
921 TEVENT(Exit - Throw IMSX);
922 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
923 return;
924 }
925 }
926
927 if (_recursions != 0) {
928 _recursions--; // this is simple recursive enter
929 TEVENT(Inflated exit - recursive);
930 return;
931 }
932
933 // Invariant: after setting Responsible=null an thread must execute
934 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
935 if ((SyncFlags & 4) == 0) {
936 _Responsible = NULL;
937 }
938
939 #if INCLUDE_TRACE
940 // get the owner's thread id for the MonitorEnter event
941 // if it is enabled and the thread isn't suspended
942 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
943 _previous_owner_tid = THREAD_TRACE_ID(Self);
944 }
945 #endif
946
947 for (;;) {
948 assert(THREAD == _owner, "invariant");
949
950 if (Knob_ExitPolicy == 0) {
951 // release semantics: prior loads and stores from within the critical section
952 // must not float (reorder) past the following store that drops the lock.
953 // On SPARC that requires MEMBAR #loadstore|#storestore.
954 // But of course in TSO #loadstore|#storestore is not required.
955 // I'd like to write one of the following:
956 // A. OrderAccess::release() ; _owner = NULL
957 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
958 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
959 // store into a _dummy variable. That store is not needed, but can result
960 // in massive wasteful coherency traffic on classic SMP systems.
961 // Instead, I use release_store(), which is implemented as just a simple
962 // ST on x64, x86 and SPARC.
963 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
964 OrderAccess::storeload(); // See if we need to wake a successor
965 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
966 TEVENT(Inflated exit - simple egress);
967 return;
968 }
969 TEVENT(Inflated exit - complex egress);
970 // Other threads are blocked trying to acquire the lock.
971
972 // Normally the exiting thread is responsible for ensuring succession,
973 // but if other successors are ready or other entering threads are spinning
974 // then this thread can simply store NULL into _owner and exit without
975 // waking a successor. The existence of spinners or ready successors
976 // guarantees proper succession (liveness). Responsibility passes to the
977 // ready or running successors. The exiting thread delegates the duty.
978 // More precisely, if a successor already exists this thread is absolved
979 // of the responsibility of waking (unparking) one.
980 //
981 // The _succ variable is critical to reducing futile wakeup frequency.
982 // _succ identifies the "heir presumptive" thread that has been made
983 // ready (unparked) but that has not yet run. We need only one such
984 // successor thread to guarantee progress.
985 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
986 // section 3.3 "Futile Wakeup Throttling" for details.
987 //
988 // Note that spinners in Enter() also set _succ non-null.
989 // In the current implementation spinners opportunistically set
990 // _succ so that exiting threads might avoid waking a successor.
991 // Another less appealing alternative would be for the exiting thread
992 // to drop the lock and then spin briefly to see if a spinner managed
993 // to acquire the lock. If so, the exiting thread could exit
994 // immediately without waking a successor, otherwise the exiting
995 // thread would need to dequeue and wake a successor.
996 // (Note that we'd need to make the post-drop spin short, but no
997 // shorter than the worst-case round-trip cache-line migration time.
998 // The dropped lock needs to become visible to the spinner, and then
999 // the acquisition of the lock by the spinner must become visible to
1000 // the exiting thread).
1001
1002 // It appears that an heir-presumptive (successor) must be made ready.
1003 // Only the current lock owner can manipulate the EntryList or
1004 // drain _cxq, so we need to reacquire the lock. If we fail
1005 // to reacquire the lock the responsibility for ensuring succession
1006 // falls to the new owner.
1007 //
1008 if (!Atomic::replace_if_null(THREAD, &_owner)) {
1009 return;
1010 }
1011 TEVENT(Exit - Reacquired);
1012 } else {
1013 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1014 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
1015 OrderAccess::storeload();
1016 // Ratify the previously observed values.
1017 if (_cxq == NULL || _succ != NULL) {
1018 TEVENT(Inflated exit - simple egress);
1019 return;
1020 }
1021
1022 // inopportune interleaving -- the exiting thread (this thread)
1023 // in the fast-exit path raced an entering thread in the slow-enter
1024 // path.
1025 // We have two choices:
1026 // A. Try to reacquire the lock.
1027 // If the CAS() fails return immediately, otherwise
1028 // we either restart/rerun the exit operation, or simply
1029 // fall-through into the code below which wakes a successor.
1030 // B. If the elements forming the EntryList|cxq are TSM
1031 // we could simply unpark() the lead thread and return
1032 // without having set _succ.
1033 if (!Atomic::replace_if_null(THREAD, &_owner)) {
1034 TEVENT(Inflated exit - reacquired succeeded);
1035 return;
1036 }
1037 TEVENT(Inflated exit - reacquired failed);
1038 } else {
1039 TEVENT(Inflated exit - complex egress);
1040 }
1041 }
1042
1043 guarantee(_owner == THREAD, "invariant");
1044
1045 ObjectWaiter * w = NULL;
1046 int QMode = Knob_QMode;
1047
1048 if (QMode == 2 && _cxq != NULL) {
1049 // QMode == 2 : cxq has precedence over EntryList.
1050 // Try to directly wake a successor from the cxq.
1051 // If successful, the successor will need to unlink itself from cxq.
1052 w = _cxq;
1053 assert(w != NULL, "invariant");
1054 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1055 ExitEpilog(Self, w);
1056 return;
1057 }
1058
1059 if (QMode == 3 && _cxq != NULL) {
1060 // Aggressively drain cxq into EntryList at the first opportunity.
1061 // This policy ensure that recently-run threads live at the head of EntryList.
1062 // Drain _cxq into EntryList - bulk transfer.
1063 // First, detach _cxq.
1064 // The following loop is tantamount to: w = swap(&cxq, NULL)
1065 w = _cxq;
1066 for (;;) {
1067 assert(w != NULL, "Invariant");
1068 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1069 if (u == w) break;
1070 w = u;
1071 }
1072 assert(w != NULL, "invariant");
1073
1074 ObjectWaiter * q = NULL;
1075 ObjectWaiter * p;
1076 for (p = w; p != NULL; p = p->_next) {
1077 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1078 p->TState = ObjectWaiter::TS_ENTER;
1079 p->_prev = q;
1080 q = p;
1081 }
1082
1083 // Append the RATs to the EntryList
1084 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1085 ObjectWaiter * Tail;
1086 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1087 Tail = Tail->_next)
1088 /* empty */;
1089 if (Tail == NULL) {
1090 _EntryList = w;
1091 } else {
1092 Tail->_next = w;
1093 w->_prev = Tail;
1094 }
1095
1096 // Fall thru into code that tries to wake a successor from EntryList
1097 }
1098
1099 if (QMode == 4 && _cxq != NULL) {
1100 // Aggressively drain cxq into EntryList at the first opportunity.
1101 // This policy ensure that recently-run threads live at the head of EntryList.
1102
1103 // Drain _cxq into EntryList - bulk transfer.
1104 // First, detach _cxq.
1105 // The following loop is tantamount to: w = swap(&cxq, NULL)
1106 w = _cxq;
1107 for (;;) {
1108 assert(w != NULL, "Invariant");
1109 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1110 if (u == w) break;
1111 w = u;
1112 }
1113 assert(w != NULL, "invariant");
1114
1115 ObjectWaiter * q = NULL;
1116 ObjectWaiter * p;
1117 for (p = w; p != NULL; p = p->_next) {
1118 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1119 p->TState = ObjectWaiter::TS_ENTER;
1120 p->_prev = q;
1121 q = p;
1122 }
1123
1124 // Prepend the RATs to the EntryList
1125 if (_EntryList != NULL) {
1126 q->_next = _EntryList;
1127 _EntryList->_prev = q;
1128 }
1129 _EntryList = w;
1130
1131 // Fall thru into code that tries to wake a successor from EntryList
1132 }
1133
1134 w = _EntryList;
1135 if (w != NULL) {
1136 // I'd like to write: guarantee (w->_thread != Self).
1137 // But in practice an exiting thread may find itself on the EntryList.
1138 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1139 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1140 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1141 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1142 // release the lock "O". T2 resumes immediately after the ST of null into
1143 // _owner, above. T2 notices that the EntryList is populated, so it
1144 // reacquires the lock and then finds itself on the EntryList.
1145 // Given all that, we have to tolerate the circumstance where "w" is
1146 // associated with Self.
1147 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1148 ExitEpilog(Self, w);
1149 return;
1150 }
1151
1152 // If we find that both _cxq and EntryList are null then just
1153 // re-run the exit protocol from the top.
1154 w = _cxq;
1155 if (w == NULL) continue;
1156
1157 // Drain _cxq into EntryList - bulk transfer.
1158 // First, detach _cxq.
1159 // The following loop is tantamount to: w = swap(&cxq, NULL)
1160 for (;;) {
1161 assert(w != NULL, "Invariant");
1162 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1163 if (u == w) break;
1164 w = u;
1165 }
1166 TEVENT(Inflated exit - drain cxq into EntryList);
1167
1168 assert(w != NULL, "invariant");
1169 assert(_EntryList == NULL, "invariant");
1170
1171 // Convert the LIFO SLL anchored by _cxq into a DLL.
1172 // The list reorganization step operates in O(LENGTH(w)) time.
1173 // It's critical that this step operate quickly as
1174 // "Self" still holds the outer-lock, restricting parallelism
1175 // and effectively lengthening the critical section.
1176 // Invariant: s chases t chases u.
1177 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1178 // we have faster access to the tail.
1179
1180 if (QMode == 1) {
1181 // QMode == 1 : drain cxq to EntryList, reversing order
1182 // We also reverse the order of the list.
1183 ObjectWaiter * s = NULL;
1184 ObjectWaiter * t = w;
1185 ObjectWaiter * u = NULL;
1186 while (t != NULL) {
1187 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1188 t->TState = ObjectWaiter::TS_ENTER;
1189 u = t->_next;
1190 t->_prev = u;
1191 t->_next = s;
1192 s = t;
1193 t = u;
1194 }
1195 _EntryList = s;
1196 assert(s != NULL, "invariant");
1197 } else {
1198 // QMode == 0 or QMode == 2
1199 _EntryList = w;
1200 ObjectWaiter * q = NULL;
1201 ObjectWaiter * p;
1202 for (p = w; p != NULL; p = p->_next) {
1203 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1204 p->TState = ObjectWaiter::TS_ENTER;
1205 p->_prev = q;
1206 q = p;
1207 }
1208 }
1209
1210 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1211 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1212
1213 // See if we can abdicate to a spinner instead of waking a thread.
1214 // A primary goal of the implementation is to reduce the
1215 // context-switch rate.
1216 if (_succ != NULL) continue;
1217
1218 w = _EntryList;
1219 if (w != NULL) {
1220 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1221 ExitEpilog(Self, w);
1222 return;
1223 }
1224 }
1225 }
1226
1227 // ExitSuspendEquivalent:
1228 // A faster alternate to handle_special_suspend_equivalent_condition()
1229 //
1230 // handle_special_suspend_equivalent_condition() unconditionally
1231 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1232 // operations have high latency. Note that in ::enter() we call HSSEC
1233 // while holding the monitor, so we effectively lengthen the critical sections.
1234 //
1235 // There are a number of possible solutions:
1236 //
1237 // A. To ameliorate the problem we might also defer state transitions
1238 // to as late as possible -- just prior to parking.
1239 // Given that, we'd call HSSEC after having returned from park(),
1240 // but before attempting to acquire the monitor. This is only a
1241 // partial solution. It avoids calling HSSEC while holding the
1242 // monitor (good), but it still increases successor reacquisition latency --
1243 // the interval between unparking a successor and the time the successor
1244 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1245 // If we use this technique we can also avoid EnterI()-exit() loop
1246 // in ::enter() where we iteratively drop the lock and then attempt
1247 // to reacquire it after suspending.
1248 //
1249 // B. In the future we might fold all the suspend bits into a
1250 // composite per-thread suspend flag and then update it with CAS().
1251 // Alternately, a Dekker-like mechanism with multiple variables
1252 // would suffice:
1253 // ST Self->_suspend_equivalent = false
1254 // MEMBAR
1255 // LD Self_>_suspend_flags
1256 //
1257 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1258 // with a more efficient implementation, the need to use "FastHSSEC" has
1259 // decreased. - Dave
1260
1261
1262 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1263 const int Mode = Knob_FastHSSEC;
1264 if (Mode && !jSelf->is_external_suspend()) {
1265 assert(jSelf->is_suspend_equivalent(), "invariant");
1266 jSelf->clear_suspend_equivalent();
1267 if (2 == Mode) OrderAccess::storeload();
1268 if (!jSelf->is_external_suspend()) return false;
1269 // We raced a suspension -- fall thru into the slow path
1270 TEVENT(ExitSuspendEquivalent - raced);
1271 jSelf->set_suspend_equivalent();
1272 }
1273 return jSelf->handle_special_suspend_equivalent_condition();
1274 }
1275
1276
1277 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1278 assert(_owner == Self, "invariant");
1279
1280 // Exit protocol:
1281 // 1. ST _succ = wakee
1282 // 2. membar #loadstore|#storestore;
1283 // 2. ST _owner = NULL
1284 // 3. unpark(wakee)
1285
1286 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1287 ParkEvent * Trigger = Wakee->_event;
1288
1289 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1290 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1291 // out-of-scope (non-extant).
1292 Wakee = NULL;
1293
1294 // Drop the lock
1295 OrderAccess::release_store(&_owner, (void*)NULL);
1296 OrderAccess::fence(); // ST _owner vs LD in unpark()
1297
1298 if (SafepointMechanism::poll(Self)) {
1299 TEVENT(unpark before SAFEPOINT);
1300 }
1301
1302 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1303 Trigger->unpark();
1304
1305 // Maintain stats and report events to JVMTI
1306 OM_PERFDATA_OP(Parks, inc());
1307 }
1308
1309
1310 // -----------------------------------------------------------------------------
1311 // Class Loader deadlock handling.
1312 //
1313 // complete_exit exits a lock returning recursion count
1314 // complete_exit/reenter operate as a wait without waiting
1315 // complete_exit requires an inflated monitor
1316 // The _owner field is not always the Thread addr even with an
1317 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1318 // thread due to contention.
1319 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1320 Thread * const Self = THREAD;
1321 assert(Self->is_Java_thread(), "Must be Java thread!");
1322 JavaThread *jt = (JavaThread *)THREAD;
1323
1324 DeferredInitialize();
1325
1326 if (THREAD != _owner) {
1327 if (THREAD->is_lock_owned ((address)_owner)) {
1328 assert(_recursions == 0, "internal state error");
1329 _owner = THREAD; // Convert from basiclock addr to Thread addr
1330 _recursions = 0;
1331 }
1332 }
1333
1334 guarantee(Self == _owner, "complete_exit not owner");
1335 intptr_t save = _recursions; // record the old recursion count
1336 _recursions = 0; // set the recursion level to be 0
1337 exit(true, Self); // exit the monitor
1338 guarantee(_owner != Self, "invariant");
1339 return save;
1340 }
1341
1342 // reenter() enters a lock and sets recursion count
1343 // complete_exit/reenter operate as a wait without waiting
1344 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1345 Thread * const Self = THREAD;
1346 assert(Self->is_Java_thread(), "Must be Java thread!");
1347 JavaThread *jt = (JavaThread *)THREAD;
1348
1349 guarantee(_owner != Self, "reenter already owner");
1350 enter(THREAD); // enter the monitor
1351 guarantee(_recursions == 0, "reenter recursion");
1352 _recursions = recursions;
1353 return;
1354 }
1355
1356
1357 // -----------------------------------------------------------------------------
1358 // A macro is used below because there may already be a pending
1359 // exception which should not abort the execution of the routines
1360 // which use this (which is why we don't put this into check_slow and
1361 // call it with a CHECK argument).
1362
1363 #define CHECK_OWNER() \
1364 do { \
1365 if (THREAD != _owner) { \
1366 if (THREAD->is_lock_owned((address) _owner)) { \
1367 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1368 _recursions = 0; \
1369 } else { \
1370 TEVENT(Throw IMSX); \
1371 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1372 } \
1373 } \
1374 } while (false)
1375
1376 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1377 // TODO-FIXME: remove check_slow() -- it's likely dead.
1378
1379 void ObjectMonitor::check_slow(TRAPS) {
1380 TEVENT(check_slow - throw IMSX);
1381 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1382 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1383 }
1384
1385 static int Adjust(volatile int * adr, int dx) {
1386 int v;
1387 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1388 return v;
1389 }
1390
1391 // helper method for posting a monitor wait event
1392 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1393 jlong notifier_tid,
1394 jlong timeout,
1395 bool timedout) {
1396 assert(event != NULL, "invariant");
1397 event->set_monitorClass(((oop)this->object())->klass());
1398 event->set_timeout(timeout);
1399 event->set_address((TYPE_ADDRESS)this->object_addr());
1400 event->set_notifier(notifier_tid);
1401 event->set_timedOut(timedout);
1402 event->commit();
1403 }
1404
1405 // -----------------------------------------------------------------------------
1406 // Wait/Notify/NotifyAll
1407 //
1408 // Note: a subset of changes to ObjectMonitor::wait()
1409 // will need to be replicated in complete_exit
1410 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1411 Thread * const Self = THREAD;
1412 assert(Self->is_Java_thread(), "Must be Java thread!");
1413 JavaThread *jt = (JavaThread *)THREAD;
1414
1415 DeferredInitialize();
1416
1417 // Throw IMSX or IEX.
1418 CHECK_OWNER();
1419
1420 EventJavaMonitorWait event;
1421
1422 // check for a pending interrupt
1423 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1424 // post monitor waited event. Note that this is past-tense, we are done waiting.
1425 if (JvmtiExport::should_post_monitor_waited()) {
1426 // Note: 'false' parameter is passed here because the
1427 // wait was not timed out due to thread interrupt.
1428 JvmtiExport::post_monitor_waited(jt, this, false);
1429
1430 // In this short circuit of the monitor wait protocol, the
1431 // current thread never drops ownership of the monitor and
1432 // never gets added to the wait queue so the current thread
1433 // cannot be made the successor. This means that the
1434 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1435 // consume an unpark() meant for the ParkEvent associated with
1436 // this ObjectMonitor.
1437 }
1438 if (event.should_commit()) {
1439 post_monitor_wait_event(&event, 0, millis, false);
1440 }
1441 TEVENT(Wait - Throw IEX);
1442 THROW(vmSymbols::java_lang_InterruptedException());
1443 return;
1444 }
1445
1446 TEVENT(Wait);
1447
1448 assert(Self->_Stalled == 0, "invariant");
1449 Self->_Stalled = intptr_t(this);
1450 jt->set_current_waiting_monitor(this);
1451
1452 // create a node to be put into the queue
1453 // Critically, after we reset() the event but prior to park(), we must check
1454 // for a pending interrupt.
1455 ObjectWaiter node(Self);
1456 node.TState = ObjectWaiter::TS_WAIT;
1457 Self->_ParkEvent->reset();
1458 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1459
1460 // Enter the waiting queue, which is a circular doubly linked list in this case
1461 // but it could be a priority queue or any data structure.
1462 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1463 // by the the owner of the monitor *except* in the case where park()
1464 // returns because of a timeout of interrupt. Contention is exceptionally rare
1465 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1466
1467 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1468 AddWaiter(&node);
1469 Thread::SpinRelease(&_WaitSetLock);
1470
1471 if ((SyncFlags & 4) == 0) {
1472 _Responsible = NULL;
1473 }
1474 intptr_t save = _recursions; // record the old recursion count
1475 _waiters++; // increment the number of waiters
1476 _recursions = 0; // set the recursion level to be 1
1477 exit(true, Self); // exit the monitor
1478 guarantee(_owner != Self, "invariant");
1479
1480 // The thread is on the WaitSet list - now park() it.
1481 // On MP systems it's conceivable that a brief spin before we park
1482 // could be profitable.
1483 //
1484 // TODO-FIXME: change the following logic to a loop of the form
1485 // while (!timeout && !interrupted && _notified == 0) park()
1486
1487 int ret = OS_OK;
1488 int WasNotified = 0;
1489 { // State transition wrappers
1490 OSThread* osthread = Self->osthread();
1491 OSThreadWaitState osts(osthread, true);
1492 {
1493 ThreadBlockInVM tbivm(jt);
1494 // Thread is in thread_blocked state and oop access is unsafe.
1495 jt->set_suspend_equivalent();
1496
1497 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1498 // Intentionally empty
1499 } else if (node._notified == 0) {
1500 if (millis <= 0) {
1501 Self->_ParkEvent->park();
1502 } else {
1503 ret = Self->_ParkEvent->park(millis);
1504 }
1505 }
1506
1507 // were we externally suspended while we were waiting?
1508 if (ExitSuspendEquivalent (jt)) {
1509 // TODO-FIXME: add -- if succ == Self then succ = null.
1510 jt->java_suspend_self();
1511 }
1512
1513 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1514
1515 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1516 // from the WaitSet to the EntryList.
1517 // See if we need to remove Node from the WaitSet.
1518 // We use double-checked locking to avoid grabbing _WaitSetLock
1519 // if the thread is not on the wait queue.
1520 //
1521 // Note that we don't need a fence before the fetch of TState.
1522 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1523 // written by the is thread. (perhaps the fetch might even be satisfied
1524 // by a look-aside into the processor's own store buffer, although given
1525 // the length of the code path between the prior ST and this load that's
1526 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1527 // then we'll acquire the lock and then re-fetch a fresh TState value.
1528 // That is, we fail toward safety.
1529
1530 if (node.TState == ObjectWaiter::TS_WAIT) {
1531 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1532 if (node.TState == ObjectWaiter::TS_WAIT) {
1533 DequeueSpecificWaiter(&node); // unlink from WaitSet
1534 assert(node._notified == 0, "invariant");
1535 node.TState = ObjectWaiter::TS_RUN;
1536 }
1537 Thread::SpinRelease(&_WaitSetLock);
1538 }
1539
1540 // The thread is now either on off-list (TS_RUN),
1541 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1542 // The Node's TState variable is stable from the perspective of this thread.
1543 // No other threads will asynchronously modify TState.
1544 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1545 OrderAccess::loadload();
1546 if (_succ == Self) _succ = NULL;
1547 WasNotified = node._notified;
1548
1549 // Reentry phase -- reacquire the monitor.
1550 // re-enter contended monitor after object.wait().
1551 // retain OBJECT_WAIT state until re-enter successfully completes
1552 // Thread state is thread_in_vm and oop access is again safe,
1553 // although the raw address of the object may have changed.
1554 // (Don't cache naked oops over safepoints, of course).
1555
1556 // post monitor waited event. Note that this is past-tense, we are done waiting.
1557 if (JvmtiExport::should_post_monitor_waited()) {
1558 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1559
1560 if (node._notified != 0 && _succ == Self) {
1561 // In this part of the monitor wait-notify-reenter protocol it
1562 // is possible (and normal) for another thread to do a fastpath
1563 // monitor enter-exit while this thread is still trying to get
1564 // to the reenter portion of the protocol.
1565 //
1566 // The ObjectMonitor was notified and the current thread is
1567 // the successor which also means that an unpark() has already
1568 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1569 // consume the unpark() that was done when the successor was
1570 // set because the same ParkEvent is shared between Java
1571 // monitors and JVM/TI RawMonitors (for now).
1572 //
1573 // We redo the unpark() to ensure forward progress, i.e., we
1574 // don't want all pending threads hanging (parked) with none
1575 // entering the unlocked monitor.
1576 node._event->unpark();
1577 }
1578 }
1579
1580 if (event.should_commit()) {
1581 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1582 }
1583
1584 OrderAccess::fence();
1585
1586 assert(Self->_Stalled != 0, "invariant");
1587 Self->_Stalled = 0;
1588
1589 assert(_owner != Self, "invariant");
1590 ObjectWaiter::TStates v = node.TState;
1591 if (v == ObjectWaiter::TS_RUN) {
1592 enter(Self);
1593 } else {
1594 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1595 ReenterI(Self, &node);
1596 node.wait_reenter_end(this);
1597 }
1598
1599 // Self has reacquired the lock.
1600 // Lifecycle - the node representing Self must not appear on any queues.
1601 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1602 // want residual elements associated with this thread left on any lists.
1603 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1604 assert(_owner == Self, "invariant");
1605 assert(_succ != Self, "invariant");
1606 } // OSThreadWaitState()
1607
1608 jt->set_current_waiting_monitor(NULL);
1609
1610 guarantee(_recursions == 0, "invariant");
1611 _recursions = save; // restore the old recursion count
1612 _waiters--; // decrement the number of waiters
1613
1614 // Verify a few postconditions
1615 assert(_owner == Self, "invariant");
1616 assert(_succ != Self, "invariant");
1617 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1618
1619 if (SyncFlags & 32) {
1620 OrderAccess::fence();
1621 }
1622
1623 // check if the notification happened
1624 if (!WasNotified) {
1625 // no, it could be timeout or Thread.interrupt() or both
1626 // check for interrupt event, otherwise it is timeout
1627 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1628 TEVENT(Wait - throw IEX from epilog);
1629 THROW(vmSymbols::java_lang_InterruptedException());
1630 }
1631 }
1632
1633 // NOTE: Spurious wake up will be consider as timeout.
1634 // Monitor notify has precedence over thread interrupt.
1635 }
1636
1637
1638 // Consider:
1639 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1640 // then instead of transferring a thread from the WaitSet to the EntryList
1641 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1642
1643 void ObjectMonitor::INotify(Thread * Self) {
1644 const int policy = Knob_MoveNotifyee;
1645
1646 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1647 ObjectWaiter * iterator = DequeueWaiter();
1648 if (iterator != NULL) {
1649 TEVENT(Notify1 - Transfer);
1650 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1651 guarantee(iterator->_notified == 0, "invariant");
1652 // Disposition - what might we do with iterator ?
1653 // a. add it directly to the EntryList - either tail (policy == 1)
1654 // or head (policy == 0).
1655 // b. push it onto the front of the _cxq (policy == 2).
1656 // For now we use (b).
1657 if (policy != 4) {
1658 iterator->TState = ObjectWaiter::TS_ENTER;
1659 }
1660 iterator->_notified = 1;
1661 iterator->_notifier_tid = THREAD_TRACE_ID(Self);
1662
1663 ObjectWaiter * list = _EntryList;
1664 if (list != NULL) {
1665 assert(list->_prev == NULL, "invariant");
1666 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1667 assert(list != iterator, "invariant");
1668 }
1669
1670 if (policy == 0) { // prepend to EntryList
1671 if (list == NULL) {
1672 iterator->_next = iterator->_prev = NULL;
1673 _EntryList = iterator;
1674 } else {
1675 list->_prev = iterator;
1676 iterator->_next = list;
1677 iterator->_prev = NULL;
1678 _EntryList = iterator;
1679 }
1680 } else if (policy == 1) { // append to EntryList
1681 if (list == NULL) {
1682 iterator->_next = iterator->_prev = NULL;
1683 _EntryList = iterator;
1684 } else {
1685 // CONSIDER: finding the tail currently requires a linear-time walk of
1686 // the EntryList. We can make tail access constant-time by converting to
1687 // a CDLL instead of using our current DLL.
1688 ObjectWaiter * tail;
1689 for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1690 assert(tail != NULL && tail->_next == NULL, "invariant");
1691 tail->_next = iterator;
1692 iterator->_prev = tail;
1693 iterator->_next = NULL;
1694 }
1695 } else if (policy == 2) { // prepend to cxq
1696 if (list == NULL) {
1697 iterator->_next = iterator->_prev = NULL;
1698 _EntryList = iterator;
1699 } else {
1700 iterator->TState = ObjectWaiter::TS_CXQ;
1701 for (;;) {
1702 ObjectWaiter * front = _cxq;
1703 iterator->_next = front;
1704 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1705 break;
1706 }
1707 }
1708 }
1709 } else if (policy == 3) { // append to cxq
1710 iterator->TState = ObjectWaiter::TS_CXQ;
1711 for (;;) {
1712 ObjectWaiter * tail = _cxq;
1713 if (tail == NULL) {
1714 iterator->_next = NULL;
1715 if (Atomic::replace_if_null(iterator, &_cxq)) {
1716 break;
1717 }
1718 } else {
1719 while (tail->_next != NULL) tail = tail->_next;
1720 tail->_next = iterator;
1721 iterator->_prev = tail;
1722 iterator->_next = NULL;
1723 break;
1724 }
1725 }
1726 } else {
1727 ParkEvent * ev = iterator->_event;
1728 iterator->TState = ObjectWaiter::TS_RUN;
1729 OrderAccess::fence();
1730 ev->unpark();
1731 }
1732
1733 // _WaitSetLock protects the wait queue, not the EntryList. We could
1734 // move the add-to-EntryList operation, above, outside the critical section
1735 // protected by _WaitSetLock. In practice that's not useful. With the
1736 // exception of wait() timeouts and interrupts the monitor owner
1737 // is the only thread that grabs _WaitSetLock. There's almost no contention
1738 // on _WaitSetLock so it's not profitable to reduce the length of the
1739 // critical section.
1740
1741 if (policy < 4) {
1742 iterator->wait_reenter_begin(this);
1743 }
1744 }
1745 Thread::SpinRelease(&_WaitSetLock);
1746 }
1747
1748 // Consider: a not-uncommon synchronization bug is to use notify() when
1749 // notifyAll() is more appropriate, potentially resulting in stranded
1750 // threads; this is one example of a lost wakeup. A useful diagnostic
1751 // option is to force all notify() operations to behave as notifyAll().
1752 //
1753 // Note: We can also detect many such problems with a "minimum wait".
1754 // When the "minimum wait" is set to a small non-zero timeout value
1755 // and the program does not hang whereas it did absent "minimum wait",
1756 // that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses
1757 // a "minimum wait" for all park() operations; see the recheckInterval
1758 // variable and MAX_RECHECK_INTERVAL.
1759
1760 void ObjectMonitor::notify(TRAPS) {
1761 CHECK_OWNER();
1762 if (_WaitSet == NULL) {
1763 TEVENT(Empty-Notify);
1764 return;
1765 }
1766 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1767 INotify(THREAD);
1768 OM_PERFDATA_OP(Notifications, inc(1));
1769 }
1770
1771
1772 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1773 // from the waitset to the EntryList. This could be done more efficiently with a
1774 // single bulk transfer but in practice it's not time-critical. Beware too,
1775 // that in prepend-mode we invert the order of the waiters. Let's say that the
1776 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1777 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1778
1779 void ObjectMonitor::notifyAll(TRAPS) {
1780 CHECK_OWNER();
1781 if (_WaitSet == NULL) {
1782 TEVENT(Empty-NotifyAll);
1783 return;
1784 }
1785
1786 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1787 int tally = 0;
1788 while (_WaitSet != NULL) {
1789 tally++;
1790 INotify(THREAD);
1791 }
1792
1793 OM_PERFDATA_OP(Notifications, inc(tally));
1794 }
1795
1796 // -----------------------------------------------------------------------------
1797 // Adaptive Spinning Support
1798 //
1799 // Adaptive spin-then-block - rational spinning
1800 //
1801 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1802 // algorithm. On high order SMP systems it would be better to start with
1803 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1804 // a contending thread could enqueue itself on the cxq and then spin locally
1805 // on a thread-specific variable such as its ParkEvent._Event flag.
1806 // That's left as an exercise for the reader. Note that global spinning is
1807 // not problematic on Niagara, as the L2 cache serves the interconnect and
1808 // has both low latency and massive bandwidth.
1809 //
1810 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1811 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1812 // (duration) or we can fix the count at approximately the duration of
1813 // a context switch and vary the frequency. Of course we could also
1814 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1815 // For a description of 'Adaptive spin-then-block mutual exclusion in
1816 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1817 //
1818 // This implementation varies the duration "D", where D varies with
1819 // the success rate of recent spin attempts. (D is capped at approximately
1820 // length of a round-trip context switch). The success rate for recent
1821 // spin attempts is a good predictor of the success rate of future spin
1822 // attempts. The mechanism adapts automatically to varying critical
1823 // section length (lock modality), system load and degree of parallelism.
1824 // D is maintained per-monitor in _SpinDuration and is initialized
1825 // optimistically. Spin frequency is fixed at 100%.
1826 //
1827 // Note that _SpinDuration is volatile, but we update it without locks
1828 // or atomics. The code is designed so that _SpinDuration stays within
1829 // a reasonable range even in the presence of races. The arithmetic
1830 // operations on _SpinDuration are closed over the domain of legal values,
1831 // so at worst a race will install and older but still legal value.
1832 // At the very worst this introduces some apparent non-determinism.
1833 // We might spin when we shouldn't or vice-versa, but since the spin
1834 // count are relatively short, even in the worst case, the effect is harmless.
1835 //
1836 // Care must be taken that a low "D" value does not become an
1837 // an absorbing state. Transient spinning failures -- when spinning
1838 // is overall profitable -- should not cause the system to converge
1839 // on low "D" values. We want spinning to be stable and predictable
1840 // and fairly responsive to change and at the same time we don't want
1841 // it to oscillate, become metastable, be "too" non-deterministic,
1842 // or converge on or enter undesirable stable absorbing states.
1843 //
1844 // We implement a feedback-based control system -- using past behavior
1845 // to predict future behavior. We face two issues: (a) if the
1846 // input signal is random then the spin predictor won't provide optimal
1847 // results, and (b) if the signal frequency is too high then the control
1848 // system, which has some natural response lag, will "chase" the signal.
1849 // (b) can arise from multimodal lock hold times. Transient preemption
1850 // can also result in apparent bimodal lock hold times.
1851 // Although sub-optimal, neither condition is particularly harmful, as
1852 // in the worst-case we'll spin when we shouldn't or vice-versa.
1853 // The maximum spin duration is rather short so the failure modes aren't bad.
1854 // To be conservative, I've tuned the gain in system to bias toward
1855 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1856 // "rings" or oscillates between spinning and not spinning. This happens
1857 // when spinning is just on the cusp of profitability, however, so the
1858 // situation is not dire. The state is benign -- there's no need to add
1859 // hysteresis control to damp the transition rate between spinning and
1860 // not spinning.
1861
1862 // Spinning: Fixed frequency (100%), vary duration
1863 int ObjectMonitor::TrySpin(Thread * Self) {
1864 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1865 int ctr = Knob_FixedSpin;
1866 if (ctr != 0) {
1867 while (--ctr >= 0) {
1868 if (TryLock(Self) > 0) return 1;
1869 SpinPause();
1870 }
1871 return 0;
1872 }
1873
1874 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1875 if (TryLock(Self) > 0) {
1876 // Increase _SpinDuration ...
1877 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1878 // Raising _SpurDuration to the poverty line is key.
1879 int x = _SpinDuration;
1880 if (x < Knob_SpinLimit) {
1881 if (x < Knob_Poverty) x = Knob_Poverty;
1882 _SpinDuration = x + Knob_BonusB;
1883 }
1884 return 1;
1885 }
1886 SpinPause();
1887 }
1888
1889 // Admission control - verify preconditions for spinning
1890 //
1891 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1892 // becoming an absorbing state. Put another way, we spin briefly to
1893 // sample, just in case the system load, parallelism, contention, or lock
1894 // modality changed.
1895 //
1896 // Consider the following alternative:
1897 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1898 // spin attempt. "Periodically" might mean after a tally of
1899 // the # of failed spin attempts (or iterations) reaches some threshold.
1900 // This takes us into the realm of 1-out-of-N spinning, where we
1901 // hold the duration constant but vary the frequency.
1902
1903 ctr = _SpinDuration;
1904 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1905 if (ctr <= 0) return 0;
1906
1907 if (Knob_SuccRestrict && _succ != NULL) return 0;
1908 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1909 TEVENT(Spin abort - notrunnable [TOP]);
1910 return 0;
1911 }
1912
1913 int MaxSpin = Knob_MaxSpinners;
1914 if (MaxSpin >= 0) {
1915 if (_Spinner > MaxSpin) {
1916 TEVENT(Spin abort -- too many spinners);
1917 return 0;
1918 }
1919 // Slightly racy, but benign ...
1920 Adjust(&_Spinner, 1);
1921 }
1922
1923 // We're good to spin ... spin ingress.
1924 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1925 // when preparing to LD...CAS _owner, etc and the CAS is likely
1926 // to succeed.
1927 int hits = 0;
1928 int msk = 0;
1929 int caspty = Knob_CASPenalty;
1930 int oxpty = Knob_OXPenalty;
1931 int sss = Knob_SpinSetSucc;
1932 if (sss && _succ == NULL) _succ = Self;
1933 Thread * prv = NULL;
1934
1935 // There are three ways to exit the following loop:
1936 // 1. A successful spin where this thread has acquired the lock.
1937 // 2. Spin failure with prejudice
1938 // 3. Spin failure without prejudice
1939
1940 while (--ctr >= 0) {
1941
1942 // Periodic polling -- Check for pending GC
1943 // Threads may spin while they're unsafe.
1944 // We don't want spinning threads to delay the JVM from reaching
1945 // a stop-the-world safepoint or to steal cycles from GC.
1946 // If we detect a pending safepoint we abort in order that
1947 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1948 // this thread, if safe, doesn't steal cycles from GC.
1949 // This is in keeping with the "no loitering in runtime" rule.
1950 // We periodically check to see if there's a safepoint pending.
1951 if ((ctr & 0xFF) == 0) {
1952 if (SafepointMechanism::poll(Self)) {
1953 TEVENT(Spin: safepoint);
1954 goto Abort; // abrupt spin egress
1955 }
1956 if (Knob_UsePause & 1) SpinPause();
1957 }
1958
1959 if (Knob_UsePause & 2) SpinPause();
1960
1961 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
1962 // This is useful on classic SMP systems, but is of less utility on
1963 // N1-style CMT platforms.
1964 //
1965 // Trade-off: lock acquisition latency vs coherency bandwidth.
1966 // Lock hold times are typically short. A histogram
1967 // of successful spin attempts shows that we usually acquire
1968 // the lock early in the spin. That suggests we want to
1969 // sample _owner frequently in the early phase of the spin,
1970 // but then back-off and sample less frequently as the spin
1971 // progresses. The back-off makes a good citizen on SMP big
1972 // SMP systems. Oversampling _owner can consume excessive
1973 // coherency bandwidth. Relatedly, if we _oversample _owner we
1974 // can inadvertently interfere with the the ST m->owner=null.
1975 // executed by the lock owner.
1976 if (ctr & msk) continue;
1977 ++hits;
1978 if ((hits & 0xF) == 0) {
1979 // The 0xF, above, corresponds to the exponent.
1980 // Consider: (msk+1)|msk
1981 msk = ((msk << 2)|3) & BackOffMask;
1982 }
1983
1984 // Probe _owner with TATAS
1985 // If this thread observes the monitor transition or flicker
1986 // from locked to unlocked to locked, then the odds that this
1987 // thread will acquire the lock in this spin attempt go down
1988 // considerably. The same argument applies if the CAS fails
1989 // or if we observe _owner change from one non-null value to
1990 // another non-null value. In such cases we might abort
1991 // the spin without prejudice or apply a "penalty" to the
1992 // spin count-down variable "ctr", reducing it by 100, say.
1993
1994 Thread * ox = (Thread *) _owner;
1995 if (ox == NULL) {
1996 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1997 if (ox == NULL) {
1998 // The CAS succeeded -- this thread acquired ownership
1999 // Take care of some bookkeeping to exit spin state.
2000 if (sss && _succ == Self) {
2001 _succ = NULL;
2002 }
2003 if (MaxSpin > 0) Adjust(&_Spinner, -1);
2004
2005 // Increase _SpinDuration :
2006 // The spin was successful (profitable) so we tend toward
2007 // longer spin attempts in the future.
2008 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2009 // If we acquired the lock early in the spin cycle it
2010 // makes sense to increase _SpinDuration proportionally.
2011 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2012 int x = _SpinDuration;
2013 if (x < Knob_SpinLimit) {
2014 if (x < Knob_Poverty) x = Knob_Poverty;
2015 _SpinDuration = x + Knob_Bonus;
2016 }
2017 return 1;
2018 }
2019
2020 // The CAS failed ... we can take any of the following actions:
2021 // * penalize: ctr -= Knob_CASPenalty
2022 // * exit spin with prejudice -- goto Abort;
2023 // * exit spin without prejudice.
2024 // * Since CAS is high-latency, retry again immediately.
2025 prv = ox;
2026 TEVENT(Spin: cas failed);
2027 if (caspty == -2) break;
2028 if (caspty == -1) goto Abort;
2029 ctr -= caspty;
2030 continue;
2031 }
2032
2033 // Did lock ownership change hands ?
2034 if (ox != prv && prv != NULL) {
2035 TEVENT(spin: Owner changed)
2036 if (oxpty == -2) break;
2037 if (oxpty == -1) goto Abort;
2038 ctr -= oxpty;
2039 }
2040 prv = ox;
2041
2042 // Abort the spin if the owner is not executing.
2043 // The owner must be executing in order to drop the lock.
2044 // Spinning while the owner is OFFPROC is idiocy.
2045 // Consider: ctr -= RunnablePenalty ;
2046 if (Knob_OState && NotRunnable (Self, ox)) {
2047 TEVENT(Spin abort - notrunnable);
2048 goto Abort;
2049 }
2050 if (sss && _succ == NULL) _succ = Self;
2051 }
2052
2053 // Spin failed with prejudice -- reduce _SpinDuration.
2054 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2055 // AIMD is globally stable.
2056 TEVENT(Spin failure);
2057 {
2058 int x = _SpinDuration;
2059 if (x > 0) {
2060 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2061 // This is globally sample and tends to damp the response.
2062 x -= Knob_Penalty;
2063 if (x < 0) x = 0;
2064 _SpinDuration = x;
2065 }
2066 }
2067
2068 Abort:
2069 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2070 if (sss && _succ == Self) {
2071 _succ = NULL;
2072 // Invariant: after setting succ=null a contending thread
2073 // must recheck-retry _owner before parking. This usually happens
2074 // in the normal usage of TrySpin(), but it's safest
2075 // to make TrySpin() as foolproof as possible.
2076 OrderAccess::fence();
2077 if (TryLock(Self) > 0) return 1;
2078 }
2079 return 0;
2080 }
2081
2082 // NotRunnable() -- informed spinning
2083 //
2084 // Don't bother spinning if the owner is not eligible to drop the lock.
2085 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2086 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2087 // The thread must be runnable in order to drop the lock in timely fashion.
2088 // If the _owner is not runnable then spinning will not likely be
2089 // successful (profitable).
2090 //
2091 // Beware -- the thread referenced by _owner could have died
2092 // so a simply fetch from _owner->_thread_state might trap.
2093 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2094 // Because of the lifecycle issues the schedctl and _thread_state values
2095 // observed by NotRunnable() might be garbage. NotRunnable must
2096 // tolerate this and consider the observed _thread_state value
2097 // as advisory.
2098 //
2099 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2100 // a thread pointer.
2101 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2102 // with the LSB of _owner. Another option would be to probablistically probe
2103 // the putative _owner->TypeTag value.
2104 //
2105 // Checking _thread_state isn't perfect. Even if the thread is
2106 // in_java it might be blocked on a page-fault or have been preempted
2107 // and sitting on a ready/dispatch queue. _thread state in conjunction
2108 // with schedctl.sc_state gives us a good picture of what the
2109 // thread is doing, however.
2110 //
2111 // TODO: check schedctl.sc_state.
2112 // We'll need to use SafeFetch32() to read from the schedctl block.
2113 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2114 //
2115 // The return value from NotRunnable() is *advisory* -- the
2116 // result is based on sampling and is not necessarily coherent.
2117 // The caller must tolerate false-negative and false-positive errors.
2118 // Spinning, in general, is probabilistic anyway.
2119
2120
2121 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2122 // Check ox->TypeTag == 2BAD.
2123 if (ox == NULL) return 0;
2124
2125 // Avoid transitive spinning ...
2126 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2127 // Immediately after T1 acquires L it's possible that T2, also
2128 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2129 // This occurs transiently after T1 acquired L but before
2130 // T1 managed to clear T1.Stalled. T2 does not need to abort
2131 // its spin in this circumstance.
2132 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2133
2134 if (BlockedOn == 1) return 1;
2135 if (BlockedOn != 0) {
2136 return BlockedOn != intptr_t(this) && _owner == ox;
2137 }
2138
2139 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2140 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2141 // consider also: jst != _thread_in_Java -- but that's overspecific.
2142 return jst == _thread_blocked || jst == _thread_in_native;
2143 }
2144
2145
2146 // -----------------------------------------------------------------------------
2147 // WaitSet management ...
2148
2149 ObjectWaiter::ObjectWaiter(Thread* thread) {
2150 _next = NULL;
2151 _prev = NULL;
2152 _notified = 0;
2153 _notifier_tid = 0;
2154 TState = TS_RUN;
2155 _thread = thread;
2156 _event = thread->_ParkEvent;
2157 _active = false;
2158 assert(_event != NULL, "invariant");
2159 }
2160
2161 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2162 JavaThread *jt = (JavaThread *)this->_thread;
2163 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2164 }
2165
2166 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2167 JavaThread *jt = (JavaThread *)this->_thread;
2168 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2169 }
2170
2171 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2172 assert(node != NULL, "should not add NULL node");
2173 assert(node->_prev == NULL, "node already in list");
2174 assert(node->_next == NULL, "node already in list");
2175 // put node at end of queue (circular doubly linked list)
2176 if (_WaitSet == NULL) {
2177 _WaitSet = node;
2178 node->_prev = node;
2179 node->_next = node;
2180 } else {
2181 ObjectWaiter* head = _WaitSet;
2182 ObjectWaiter* tail = head->_prev;
2183 assert(tail->_next == head, "invariant check");
2184 tail->_next = node;
2185 head->_prev = node;
2186 node->_next = head;
2187 node->_prev = tail;
2188 }
2189 }
2190
2191 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2192 // dequeue the very first waiter
2193 ObjectWaiter* waiter = _WaitSet;
2194 if (waiter) {
2195 DequeueSpecificWaiter(waiter);
2196 }
2197 return waiter;
2198 }
2199
2200 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2201 assert(node != NULL, "should not dequeue NULL node");
2202 assert(node->_prev != NULL, "node already removed from list");
2203 assert(node->_next != NULL, "node already removed from list");
2204 // when the waiter has woken up because of interrupt,
2205 // timeout or other spurious wake-up, dequeue the
2206 // waiter from waiting list
2207 ObjectWaiter* next = node->_next;
2208 if (next == node) {
2209 assert(node->_prev == node, "invariant check");
2210 _WaitSet = NULL;
2211 } else {
2212 ObjectWaiter* prev = node->_prev;
2213 assert(prev->_next == node, "invariant check");
2214 assert(next->_prev == node, "invariant check");
2215 next->_prev = prev;
2216 prev->_next = next;
2217 if (_WaitSet == node) {
2218 _WaitSet = next;
2219 }
2220 }
2221 node->_next = NULL;
2222 node->_prev = NULL;
2223 }
2224
2225 // -----------------------------------------------------------------------------
2226 // PerfData support
2227 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2228 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2229 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2230 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2231 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2232 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2233 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2234
2235 // One-shot global initialization for the sync subsystem.
2236 // We could also defer initialization and initialize on-demand
2237 // the first time we call inflate(). Initialization would
2238 // be protected - like so many things - by the MonitorCache_lock.
2239
2240 void ObjectMonitor::Initialize() {
2241 static int InitializationCompleted = 0;
2242 assert(InitializationCompleted == 0, "invariant");
2243 InitializationCompleted = 1;
2244 if (UsePerfData) {
2245 EXCEPTION_MARK;
2246 #define NEWPERFCOUNTER(n) \
2247 { \
2248 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2249 CHECK); \
2250 }
2251 #define NEWPERFVARIABLE(n) \
2252 { \
2253 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2254 CHECK); \
2255 }
2256 NEWPERFCOUNTER(_sync_Inflations);
2257 NEWPERFCOUNTER(_sync_Deflations);
2258 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2259 NEWPERFCOUNTER(_sync_FutileWakeups);
2260 NEWPERFCOUNTER(_sync_Parks);
2261 NEWPERFCOUNTER(_sync_Notifications);
2262 NEWPERFVARIABLE(_sync_MonExtant);
2263 #undef NEWPERFCOUNTER
2264 #undef NEWPERFVARIABLE
2265 }
2266 }
2267
2268 static char * kvGet(char * kvList, const char * Key) {
2269 if (kvList == NULL) return NULL;
2270 size_t n = strlen(Key);
2271 char * Search;
2272 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2273 if (strncmp (Search, Key, n) == 0) {
2274 if (Search[n] == '=') return Search + n + 1;
2275 if (Search[n] == 0) return(char *) "1";
2276 }
2277 }
2278 return NULL;
2279 }
2280
2281 static int kvGetInt(char * kvList, const char * Key, int Default) {
2282 char * v = kvGet(kvList, Key);
2283 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2284 if (Knob_ReportSettings && v != NULL) {
2285 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ;
2286 tty->flush();
2287 }
2288 return rslt;
2289 }
2290
2291 void ObjectMonitor::DeferredInitialize() {
2292 if (InitDone > 0) return;
2293 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2294 while (InitDone != 1) /* empty */;
2295 return;
2296 }
2297
2298 // One-shot global initialization ...
2299 // The initialization is idempotent, so we don't need locks.
2300 // In the future consider doing this via os::init_2().
2301 // SyncKnobs consist of <Key>=<Value> pairs in the style
2302 // of environment variables. Start by converting ':' to NUL.
2303
2304 if (SyncKnobs == NULL) SyncKnobs = "";
2305
2306 size_t sz = strlen(SyncKnobs);
2307 char * knobs = (char *) os::malloc(sz + 2, mtInternal);
2308 if (knobs == NULL) {
2309 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2310 guarantee(0, "invariant");
2311 }
2312 strcpy(knobs, SyncKnobs);
2313 knobs[sz+1] = 0;
2314 for (char * p = knobs; *p; p++) {
2315 if (*p == ':') *p = 0;
2316 }
2317
2318 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2319 SETKNOB(ReportSettings);
2320 SETKNOB(ExitRelease);
2321 SETKNOB(Verbose);
2322 SETKNOB(VerifyInUse);
2323 SETKNOB(VerifyMatch);
2324 SETKNOB(FixedSpin);
2325 SETKNOB(SpinLimit);
2326 SETKNOB(SpinBase);
2327 SETKNOB(SpinBackOff);
2328 SETKNOB(CASPenalty);
2329 SETKNOB(OXPenalty);
2330 SETKNOB(SpinSetSucc);
2331 SETKNOB(SuccEnabled);
2332 SETKNOB(SuccRestrict);
2333 SETKNOB(Penalty);
2334 SETKNOB(Bonus);
2335 SETKNOB(BonusB);
2336 SETKNOB(Poverty);
2337 SETKNOB(SpinAfterFutile);
2338 SETKNOB(UsePause);
2339 SETKNOB(SpinEarly);
2340 SETKNOB(OState);
2341 SETKNOB(MaxSpinners);
2342 SETKNOB(PreSpin);
2343 SETKNOB(ExitPolicy);
2344 SETKNOB(QMode);
2345 SETKNOB(ResetEvent);
2346 SETKNOB(MoveNotifyee);
2347 SETKNOB(FastHSSEC);
2348 #undef SETKNOB
2349
2350 if (Knob_Verbose) {
2351 sanity_checks();
2352 }
2353
2354 if (os::is_MP()) {
2355 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2356 if (Knob_ReportSettings) {
2357 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask);
2358 }
2359 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2360 } else {
2361 Knob_SpinLimit = 0;
2362 Knob_SpinBase = 0;
2363 Knob_PreSpin = 0;
2364 Knob_FixedSpin = -1;
2365 }
2366
2367 os::free(knobs);
2368 OrderAccess::fence();
2369 InitDone = 1;
2370 }
2371
2372 void ObjectMonitor::sanity_checks() {
2373 int error_cnt = 0;
2374 int warning_cnt = 0;
2375 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2376
2377 if (verbose) {
2378 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2379 sizeof(ObjectMonitor));
2380 tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT,
2381 sizeof(PaddedEnd<ObjectMonitor>));
2382 }
2383
2384 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2385 if (verbose) {
2386 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2387 }
2388
2389 ObjectMonitor dummy;
2390 u_char *addr_begin = (u_char*)&dummy;
2391 u_char *addr_header = (u_char*)&dummy._header;
2392 u_char *addr_owner = (u_char*)&dummy._owner;
2393
2394 uint offset_header = (uint)(addr_header - addr_begin);
2395 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2396
2397 uint offset_owner = (uint)(addr_owner - addr_begin);
2398 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2399
2400 if ((uint)(addr_header - addr_begin) != 0) {
2401 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2402 error_cnt++;
2403 }
2404
2405 if (cache_line_size != 0) {
2406 // We were able to determine the L1 data cache line size so
2407 // do some cache line specific sanity checks
2408
2409 if ((offset_owner - offset_header) < cache_line_size) {
2410 tty->print_cr("WARNING: the _header and _owner fields are closer "
2411 "than a cache line which permits false sharing.");
2412 warning_cnt++;
2413 }
2414
2415 if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) {
2416 tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a "
2417 "multiple of a cache line which permits false sharing.");
2418 warning_cnt++;
2419 }
2420 }
2421
2422 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2423 &warning_cnt);
2424
2425 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2426 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2427 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2428 }
2429
2430 guarantee(error_cnt == 0,
2431 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2432 }
2433
2434 #ifndef PRODUCT
2435 void ObjectMonitor_test() {
2436 ObjectMonitor::sanity_checks();
2437 }
2438 #endif