1 /*
2 * Copyright (c) 1998, 2016, 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/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/atomic.inline.hpp"
31 #include "runtime/handles.inline.hpp"
32 #include "runtime/interfaceSupport.hpp"
33 #include "runtime/mutexLocker.hpp"
34 #include "runtime/objectMonitor.hpp"
35 #include "runtime/objectMonitor.inline.hpp"
36 #include "runtime/orderAccess.inline.hpp"
37 #include "runtime/osThread.hpp"
38 #include "runtime/stubRoutines.hpp"
39 #include "runtime/thread.inline.hpp"
40 #include "services/threadService.hpp"
41 #include "trace/tracing.hpp"
42 #include "trace/traceMacros.hpp"
43 #include "utilities/dtrace.hpp"
44 #include "utilities/macros.hpp"
45 #include "utilities/preserveException.hpp"
46
47 #ifdef DTRACE_ENABLED
48
49 // Only bother with this argument setup if dtrace is available
50 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
51
52
53 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
54 char* bytes = NULL; \
55 int len = 0; \
56 jlong jtid = SharedRuntime::get_java_tid(thread); \
57 Symbol* klassname = ((oop)obj)->klass()->name(); \
58 if (klassname != NULL) { \
59 bytes = (char*)klassname->bytes(); \
60 len = klassname->utf8_length(); \
61 }
62
63 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
64 { \
65 if (DTraceMonitorProbes) { \
66 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
67 HOTSPOT_MONITOR_WAIT(jtid, \
68 (monitor), bytes, len, (millis)); \
69 } \
70 }
71
72 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
73 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
74 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
75 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
76 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
77
78 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
79 { \
80 if (DTraceMonitorProbes) { \
81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
82 HOTSPOT_MONITOR_##probe(jtid, \
83 (uintptr_t)(monitor), bytes, len); \
84 } \
85 }
86
87 #else // ndef DTRACE_ENABLED
88
89 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
90 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
91
92 #endif // ndef DTRACE_ENABLED
93
94 // Tunables ...
95 // The knob* variables are effectively final. Once set they should
96 // never be modified hence. Consider using __read_mostly with GCC.
97
98 int ObjectMonitor::Knob_ExitRelease = 0;
99 int ObjectMonitor::Knob_Verbose = 0;
100 int ObjectMonitor::Knob_VerifyInUse = 0;
101 int ObjectMonitor::Knob_VerifyMatch = 0;
102 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
103 static int Knob_LogSpins = 0; // enable jvmstat tally for spins
104 static int Knob_HandOff = 0;
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 #define TrySpin TrySpin_VaryDuration
135
136 // -----------------------------------------------------------------------------
137 // Theory of operations -- Monitors lists, thread residency, etc:
138 //
139 // * A thread acquires ownership of a monitor by successfully
140 // CAS()ing the _owner field from null to non-null.
141 //
142 // * Invariant: A thread appears on at most one monitor list --
143 // cxq, EntryList or WaitSet -- at any one time.
144 //
145 // * Contending threads "push" themselves onto the cxq with CAS
146 // and then spin/park.
147 //
148 // * After a contending thread eventually acquires the lock it must
149 // dequeue itself from either the EntryList or the cxq.
150 //
151 // * The exiting thread identifies and unparks an "heir presumptive"
152 // tentative successor thread on the EntryList. Critically, the
153 // exiting thread doesn't unlink the successor thread from the EntryList.
154 // After having been unparked, the wakee will recontend for ownership of
155 // the monitor. The successor (wakee) will either acquire the lock or
156 // re-park itself.
157 //
158 // Succession is provided for by a policy of competitive handoff.
159 // The exiting thread does _not_ grant or pass ownership to the
160 // successor thread. (This is also referred to as "handoff" succession").
161 // Instead the exiting thread releases ownership and possibly wakes
162 // a successor, so the successor can (re)compete for ownership of the lock.
163 // If the EntryList is empty but the cxq is populated the exiting
164 // thread will drain the cxq into the EntryList. It does so by
165 // by detaching the cxq (installing null with CAS) and folding
166 // the threads from the cxq into the EntryList. The EntryList is
167 // doubly linked, while the cxq is singly linked because of the
168 // CAS-based "push" used to enqueue recently arrived threads (RATs).
169 //
170 // * Concurrency invariants:
171 //
172 // -- only the monitor owner may access or mutate the EntryList.
173 // The mutex property of the monitor itself protects the EntryList
174 // from concurrent interference.
175 // -- Only the monitor owner may detach the cxq.
176 //
177 // * The monitor entry list operations avoid locks, but strictly speaking
178 // they're not lock-free. Enter is lock-free, exit is not.
179 // For a description of 'Methods and apparatus providing non-blocking access
180 // to a resource,' see U.S. Pat. No. 7844973.
181 //
182 // * The cxq can have multiple concurrent "pushers" but only one concurrent
183 // detaching thread. This mechanism is immune from the ABA corruption.
184 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
185 //
186 // * Taken together, the cxq and the EntryList constitute or form a
187 // single logical queue of threads stalled trying to acquire the lock.
188 // We use two distinct lists to improve the odds of a constant-time
189 // dequeue operation after acquisition (in the ::enter() epilogue) and
190 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
191 // A key desideratum is to minimize queue & monitor metadata manipulation
192 // that occurs while holding the monitor lock -- that is, we want to
193 // minimize monitor lock holds times. Note that even a small amount of
194 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
195 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
196 // locks and monitor metadata.
197 //
198 // Cxq points to the set of Recently Arrived Threads attempting entry.
199 // Because we push threads onto _cxq with CAS, the RATs must take the form of
200 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
201 // the unlocking thread notices that EntryList is null but _cxq is != null.
202 //
203 // The EntryList is ordered by the prevailing queue discipline and
204 // can be organized in any convenient fashion, such as a doubly-linked list or
205 // a circular doubly-linked list. Critically, we want insert and delete operations
206 // to operate in constant-time. If we need a priority queue then something akin
207 // to Solaris' sleepq would work nicely. Viz.,
208 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
209 // Queue discipline is enforced at ::exit() time, when the unlocking thread
210 // drains the cxq into the EntryList, and orders or reorders the threads on the
211 // EntryList accordingly.
212 //
213 // Barring "lock barging", this mechanism provides fair cyclic ordering,
214 // somewhat similar to an elevator-scan.
215 //
216 // * The monitor synchronization subsystem avoids the use of native
217 // synchronization primitives except for the narrow platform-specific
218 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
219 // the semantics of park-unpark. Put another way, this monitor implementation
220 // depends only on atomic operations and park-unpark. The monitor subsystem
221 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
222 // underlying OS manages the READY<->RUN transitions.
223 //
224 // * Waiting threads reside on the WaitSet list -- wait() puts
225 // the caller onto the WaitSet.
226 //
227 // * notify() or notifyAll() simply transfers threads from the WaitSet to
228 // either the EntryList or cxq. Subsequent exit() operations will
229 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
230 // it's likely the notifyee would simply impale itself on the lock held
231 // by the notifier.
232 //
233 // * An interesting alternative is to encode cxq as (List,LockByte) where
234 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
235 // variable, like _recursions, in the scheme. The threads or Events that form
236 // the list would have to be aligned in 256-byte addresses. A thread would
237 // try to acquire the lock or enqueue itself with CAS, but exiting threads
238 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
239 // Note that is is *not* word-tearing, but it does presume that full-word
240 // CAS operations are coherent with intermix with STB operations. That's true
241 // on most common processors.
242 //
243 // * See also http://blogs.sun.com/dave
244
245
246 // -----------------------------------------------------------------------------
247 // Enter support
248
249 void ObjectMonitor::enter(TRAPS) {
250 // The following code is ordered to check the most common cases first
251 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
252 Thread * const Self = THREAD;
253
254 void * cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL);
255 if (cur == NULL) {
256 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
257 assert(_recursions == 0, "invariant");
258 assert(_owner == Self, "invariant");
259 return;
260 }
261
262 if (cur == Self) {
263 // TODO-FIXME: check for integer overflow! BUGID 6557169.
264 _recursions++;
265 return;
266 }
267
268 if (Self->is_lock_owned ((address)cur)) {
269 assert(_recursions == 0, "internal state error");
270 _recursions = 1;
271 // Commute owner from a thread-specific on-stack BasicLockObject address to
272 // a full-fledged "Thread *".
273 _owner = Self;
274 return;
275 }
276
277 // We've encountered genuine contention.
278 assert(Self->_Stalled == 0, "invariant");
279 Self->_Stalled = intptr_t(this);
280
281 // Try one round of spinning *before* enqueueing Self
282 // and before going through the awkward and expensive state
283 // transitions. The following spin is strictly optional ...
284 // Note that if we acquire the monitor from an initial spin
285 // we forgo posting JVMTI events and firing DTRACE probes.
286 if (Knob_SpinEarly && TrySpin (Self) > 0) {
287 assert(_owner == Self, "invariant");
288 assert(_recursions == 0, "invariant");
289 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
290 Self->_Stalled = 0;
291 return;
292 }
293
294 assert(_owner != Self, "invariant");
295 assert(_succ != Self, "invariant");
296 assert(Self->is_Java_thread(), "invariant");
297 JavaThread * jt = (JavaThread *) Self;
298 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
299 assert(jt->thread_state() != _thread_blocked, "invariant");
300 assert(this->object() != NULL, "invariant");
301 assert(_count >= 0, "invariant");
302
303 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
304 // Ensure the object-monitor relationship remains stable while there's contention.
305 Atomic::inc(&_count);
306
307 EventJavaMonitorEnter event;
308
309 { // Change java thread status to indicate blocked on monitor enter.
310 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
311
312 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
313 if (JvmtiExport::should_post_monitor_contended_enter()) {
314 JvmtiExport::post_monitor_contended_enter(jt, this);
315
316 // The current thread does not yet own the monitor and does not
317 // yet appear on any queues that would get it made the successor.
318 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
319 // handler cannot accidentally consume an unpark() meant for the
320 // ParkEvent associated with this ObjectMonitor.
321 }
322
323 OSThreadContendState osts(Self->osthread());
324 ThreadBlockInVM tbivm(jt);
325
326 Self->set_current_pending_monitor(this);
327
328 // TODO-FIXME: change the following for(;;) loop to straight-line code.
329 for (;;) {
330 jt->set_suspend_equivalent();
331 // cleared by handle_special_suspend_equivalent_condition()
332 // or java_suspend_self()
333
334 EnterI(THREAD);
335
336 if (!ExitSuspendEquivalent(jt)) break;
337
338 // We have acquired the contended monitor, but while we were
339 // waiting another thread suspended us. We don't want to enter
340 // the monitor while suspended because that would surprise the
341 // thread that suspended us.
342 //
343 _recursions = 0;
344 _succ = NULL;
345 exit(false, Self);
346
347 jt->java_suspend_self();
348 }
349 Self->set_current_pending_monitor(NULL);
350
351 // We cleared the pending monitor info since we've just gotten past
352 // the enter-check-for-suspend dance and we now own the monitor free
353 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
354 // destructor can go to a safepoint at the end of this block. If we
355 // do a thread dump during that safepoint, then this thread will show
356 // as having "-locked" the monitor, but the OS and java.lang.Thread
357 // states will still report that the thread is blocked trying to
358 // acquire it.
359 }
360
361 Atomic::dec(&_count);
362 assert(_count >= 0, "invariant");
363 Self->_Stalled = 0;
364
365 // Must either set _recursions = 0 or ASSERT _recursions == 0.
366 assert(_recursions == 0, "invariant");
367 assert(_owner == Self, "invariant");
368 assert(_succ != Self, "invariant");
369 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
370
371 // The thread -- now the owner -- is back in vm mode.
372 // Report the glorious news via TI,DTrace and jvmstat.
373 // The probe effect is non-trivial. All the reportage occurs
374 // while we hold the monitor, increasing the length of the critical
375 // section. Amdahl's parallel speedup law comes vividly into play.
376 //
377 // Another option might be to aggregate the events (thread local or
378 // per-monitor aggregation) and defer reporting until a more opportune
379 // time -- such as next time some thread encounters contention but has
380 // yet to acquire the lock. While spinning that thread could
381 // spinning we could increment JVMStat counters, etc.
382
383 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
384 if (JvmtiExport::should_post_monitor_contended_entered()) {
385 JvmtiExport::post_monitor_contended_entered(jt, this);
386
387 // The current thread already owns the monitor and is not going to
388 // call park() for the remainder of the monitor enter protocol. So
389 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
390 // event handler consumed an unpark() issued by the thread that
391 // just exited the monitor.
392 }
393
394 if (event.should_commit()) {
395 event.set_klass(((oop)this->object())->klass());
396 event.set_previousOwner((TYPE_THREAD)_previous_owner_tid);
397 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
398 event.commit();
399 }
400
401 OM_PERFDATA_OP(ContendedLockAttempts, inc());
402 }
403
404
405 // Caveat: TryLock() is not necessarily serializing if it returns failure.
406 // Callers must compensate as needed.
407
408 int ObjectMonitor::TryLock(Thread * Self) {
409 void * own = _owner;
410 if (own != NULL) return 0;
411 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
412 // Either guarantee _recursions == 0 or set _recursions = 0.
413 assert(_recursions == 0, "invariant");
414 assert(_owner == Self, "invariant");
415 return 1;
416 }
417 // The lock had been free momentarily, but we lost the race to the lock.
418 // Interference -- the CAS failed.
419 // We can either return -1 or retry.
420 // Retry doesn't make as much sense because the lock was just acquired.
421 return -1;
422 }
423
424 #define MAX_RECHECK_INTERVAL 1000
425
426 void ObjectMonitor::EnterI(TRAPS) {
427 Thread * const Self = THREAD;
428 assert(Self->is_Java_thread(), "invariant");
429 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
430
431 // Try the lock - TATAS
432 if (TryLock (Self) > 0) {
433 assert(_succ != Self, "invariant");
434 assert(_owner == Self, "invariant");
435 assert(_Responsible != Self, "invariant");
436 return;
437 }
438
439 DeferredInitialize();
440
441 // We try one round of spinning *before* enqueueing Self.
442 //
443 // If the _owner is ready but OFFPROC we could use a YieldTo()
444 // operation to donate the remainder of this thread's quantum
445 // to the owner. This has subtle but beneficial affinity
446 // effects.
447
448 if (TrySpin (Self) > 0) {
449 assert(_owner == Self, "invariant");
450 assert(_succ != Self, "invariant");
451 assert(_Responsible != Self, "invariant");
452 return;
453 }
454
455 // The Spin failed -- Enqueue and park the thread ...
456 assert(_succ != Self, "invariant");
457 assert(_owner != Self, "invariant");
458 assert(_Responsible != Self, "invariant");
459
460 // Enqueue "Self" on ObjectMonitor's _cxq.
461 //
462 // Node acts as a proxy for Self.
463 // As an aside, if were to ever rewrite the synchronization code mostly
464 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
465 // Java objects. This would avoid awkward lifecycle and liveness issues,
466 // as well as eliminate a subset of ABA issues.
467 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
468
469 ObjectWaiter node(Self);
470 Self->_ParkEvent->reset();
471 node._prev = (ObjectWaiter *) 0xBAD;
472 node.TState = ObjectWaiter::TS_CXQ;
473
474 // Push "Self" onto the front of the _cxq.
475 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
476 // Note that spinning tends to reduce the rate at which threads
477 // enqueue and dequeue on EntryList|cxq.
478 ObjectWaiter * nxt;
479 for (;;) {
480 node._next = nxt = _cxq;
481 if (Atomic::cmpxchg_ptr(&node, &_cxq, nxt) == nxt) break;
482
483 // Interference - the CAS failed because _cxq changed. Just retry.
484 // As an optional optimization we retry the lock.
485 if (TryLock (Self) > 0) {
486 assert(_succ != Self, "invariant");
487 assert(_owner == Self, "invariant");
488 assert(_Responsible != Self, "invariant");
489 return;
490 }
491 }
492
493 // Check for cxq|EntryList edge transition to non-null. This indicates
494 // the onset of contention. While contention persists exiting threads
495 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
496 // operations revert to the faster 1-0 mode. This enter operation may interleave
497 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
498 // arrange for one of the contending thread to use a timed park() operations
499 // to detect and recover from the race. (Stranding is form of progress failure
500 // where the monitor is unlocked but all the contending threads remain parked).
501 // That is, at least one of the contended threads will periodically poll _owner.
502 // One of the contending threads will become the designated "Responsible" thread.
503 // The Responsible thread uses a timed park instead of a normal indefinite park
504 // operation -- it periodically wakes and checks for and recovers from potential
505 // strandings admitted by 1-0 exit operations. We need at most one Responsible
506 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
507 // be responsible for a monitor.
508 //
509 // Currently, one of the contended threads takes on the added role of "Responsible".
510 // A viable alternative would be to use a dedicated "stranding checker" thread
511 // that periodically iterated over all the threads (or active monitors) and unparked
512 // successors where there was risk of stranding. This would help eliminate the
513 // timer scalability issues we see on some platforms as we'd only have one thread
514 // -- the checker -- parked on a timer.
515
516 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
517 // Try to assume the role of responsible thread for the monitor.
518 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
519 Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
520 }
521
522 // The lock might have been released while this thread was occupied queueing
523 // itself onto _cxq. To close the race and avoid "stranding" and
524 // progress-liveness failure we must resample-retry _owner before parking.
525 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
526 // In this case the ST-MEMBAR is accomplished with CAS().
527 //
528 // TODO: Defer all thread state transitions until park-time.
529 // Since state transitions are heavy and inefficient we'd like
530 // to defer the state transitions until absolutely necessary,
531 // and in doing so avoid some transitions ...
532
533 TEVENT(Inflated enter - Contention);
534 int nWakeups = 0;
535 int recheckInterval = 1;
536
537 for (;;) {
538
539 if (TryLock(Self) > 0) break;
540 assert(_owner != Self, "invariant");
541
542 if ((SyncFlags & 2) && _Responsible == NULL) {
543 Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
544 }
545
546 // park self
547 if (_Responsible == Self || (SyncFlags & 1)) {
548 TEVENT(Inflated enter - park TIMED);
549 Self->_ParkEvent->park((jlong) recheckInterval);
550 // Increase the recheckInterval, but clamp the value.
551 recheckInterval *= 8;
552 if (recheckInterval > MAX_RECHECK_INTERVAL) {
553 recheckInterval = MAX_RECHECK_INTERVAL;
554 }
555 } else {
556 TEVENT(Inflated enter - park UNTIMED);
557 Self->_ParkEvent->park();
558 }
559
560 if (TryLock(Self) > 0) break;
561
562 // The lock is still contested.
563 // Keep a tally of the # of futile wakeups.
564 // Note that the counter is not protected by a lock or updated by atomics.
565 // That is by design - we trade "lossy" counters which are exposed to
566 // races during updates for a lower probe effect.
567 TEVENT(Inflated enter - Futile wakeup);
568 // This PerfData object can be used in parallel with a safepoint.
569 // See the work around in PerfDataManager::destroy().
570 OM_PERFDATA_OP(FutileWakeups, inc());
571 ++nWakeups;
572
573 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
574 // We can defer clearing _succ until after the spin completes
575 // TrySpin() must tolerate being called with _succ == Self.
576 // Try yet another round of adaptive spinning.
577 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
578
579 // We can find that we were unpark()ed and redesignated _succ while
580 // we were spinning. That's harmless. If we iterate and call park(),
581 // park() will consume the event and return immediately and we'll
582 // just spin again. This pattern can repeat, leaving _succ to simply
583 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
584 // Alternately, we can sample fired() here, and if set, forgo spinning
585 // in the next iteration.
586
587 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
588 Self->_ParkEvent->reset();
589 OrderAccess::fence();
590 }
591 if (_succ == Self) _succ = NULL;
592
593 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
594 OrderAccess::fence();
595 }
596
597 // Egress :
598 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
599 // Normally we'll find Self on the EntryList .
600 // From the perspective of the lock owner (this thread), the
601 // EntryList is stable and cxq is prepend-only.
602 // The head of cxq is volatile but the interior is stable.
603 // In addition, Self.TState is stable.
604
605 assert(_owner == Self, "invariant");
606 assert(object() != NULL, "invariant");
607 // I'd like to write:
608 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
609 // but as we're at a safepoint that's not safe.
610
611 UnlinkAfterAcquire(Self, &node);
612 if (_succ == Self) _succ = NULL;
613
614 assert(_succ != Self, "invariant");
615 if (_Responsible == Self) {
616 _Responsible = NULL;
617 OrderAccess::fence(); // Dekker pivot-point
618
619 // We may leave threads on cxq|EntryList without a designated
620 // "Responsible" thread. This is benign. When this thread subsequently
621 // exits the monitor it can "see" such preexisting "old" threads --
622 // threads that arrived on the cxq|EntryList before the fence, above --
623 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
624 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
625 // non-null and elect a new "Responsible" timer thread.
626 //
627 // This thread executes:
628 // ST Responsible=null; MEMBAR (in enter epilogue - here)
629 // LD cxq|EntryList (in subsequent exit)
630 //
631 // Entering threads in the slow/contended path execute:
632 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
633 // The (ST cxq; MEMBAR) is accomplished with CAS().
634 //
635 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
636 // exit operation from floating above the ST Responsible=null.
637 }
638
639 // We've acquired ownership with CAS().
640 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
641 // But since the CAS() this thread may have also stored into _succ,
642 // EntryList, cxq or Responsible. These meta-data updates must be
643 // visible __before this thread subsequently drops the lock.
644 // Consider what could occur if we didn't enforce this constraint --
645 // STs to monitor meta-data and user-data could reorder with (become
646 // visible after) the ST in exit that drops ownership of the lock.
647 // Some other thread could then acquire the lock, but observe inconsistent
648 // or old monitor meta-data and heap data. That violates the JMM.
649 // To that end, the 1-0 exit() operation must have at least STST|LDST
650 // "release" barrier semantics. Specifically, there must be at least a
651 // STST|LDST barrier in exit() before the ST of null into _owner that drops
652 // the lock. The barrier ensures that changes to monitor meta-data and data
653 // protected by the lock will be visible before we release the lock, and
654 // therefore before some other thread (CPU) has a chance to acquire the lock.
655 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
656 //
657 // Critically, any prior STs to _succ or EntryList must be visible before
658 // the ST of null into _owner in the *subsequent* (following) corresponding
659 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
660 // execute a serializing instruction.
661
662 if (SyncFlags & 8) {
663 OrderAccess::fence();
664 }
665 return;
666 }
667
668 // ReenterI() is a specialized inline form of the latter half of the
669 // contended slow-path from EnterI(). We use ReenterI() only for
670 // monitor reentry in wait().
671 //
672 // In the future we should reconcile EnterI() and ReenterI(), adding
673 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
674 // loop accordingly.
675
676 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
677 assert(Self != NULL, "invariant");
678 assert(SelfNode != NULL, "invariant");
679 assert(SelfNode->_thread == Self, "invariant");
680 assert(_waiters > 0, "invariant");
681 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
682 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
683 JavaThread * jt = (JavaThread *) Self;
684
685 int nWakeups = 0;
686 for (;;) {
687 ObjectWaiter::TStates v = SelfNode->TState;
688 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
689 assert(_owner != Self, "invariant");
690
691 if (TryLock(Self) > 0) break;
692 if (TrySpin(Self) > 0) break;
693
694 TEVENT(Wait Reentry - parking);
695
696 // State transition wrappers around park() ...
697 // ReenterI() wisely defers state transitions until
698 // it's clear we must park the thread.
699 {
700 OSThreadContendState osts(Self->osthread());
701 ThreadBlockInVM tbivm(jt);
702
703 // cleared by handle_special_suspend_equivalent_condition()
704 // or java_suspend_self()
705 jt->set_suspend_equivalent();
706 if (SyncFlags & 1) {
707 Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL);
708 } else {
709 Self->_ParkEvent->park();
710 }
711
712 // were we externally suspended while we were waiting?
713 for (;;) {
714 if (!ExitSuspendEquivalent(jt)) break;
715 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
716 jt->java_suspend_self();
717 jt->set_suspend_equivalent();
718 }
719 }
720
721 // Try again, but just so we distinguish between futile wakeups and
722 // successful wakeups. The following test isn't algorithmically
723 // necessary, but it helps us maintain sensible statistics.
724 if (TryLock(Self) > 0) break;
725
726 // The lock is still contested.
727 // Keep a tally of the # of futile wakeups.
728 // Note that the counter is not protected by a lock or updated by atomics.
729 // That is by design - we trade "lossy" counters which are exposed to
730 // races during updates for a lower probe effect.
731 TEVENT(Wait Reentry - futile wakeup);
732 ++nWakeups;
733
734 // Assuming this is not a spurious wakeup we'll normally
735 // find that _succ == Self.
736 if (_succ == Self) _succ = NULL;
737
738 // Invariant: after clearing _succ a contending thread
739 // *must* retry _owner before parking.
740 OrderAccess::fence();
741
742 // This PerfData object can be used in parallel with a safepoint.
743 // See the work around in PerfDataManager::destroy().
744 OM_PERFDATA_OP(FutileWakeups, inc());
745 }
746
747 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
748 // Normally we'll find Self on the EntryList.
749 // Unlinking from the EntryList is constant-time and atomic-free.
750 // From the perspective of the lock owner (this thread), the
751 // EntryList is stable and cxq is prepend-only.
752 // The head of cxq is volatile but the interior is stable.
753 // In addition, Self.TState is stable.
754
755 assert(_owner == Self, "invariant");
756 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
757 UnlinkAfterAcquire(Self, SelfNode);
758 if (_succ == Self) _succ = NULL;
759 assert(_succ != Self, "invariant");
760 SelfNode->TState = ObjectWaiter::TS_RUN;
761 OrderAccess::fence(); // see comments at the end of EnterI()
762 }
763
764 // By convention we unlink a contending thread from EntryList|cxq immediately
765 // after the thread acquires the lock in ::enter(). Equally, we could defer
766 // unlinking the thread until ::exit()-time.
767
768 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
769 assert(_owner == Self, "invariant");
770 assert(SelfNode->_thread == Self, "invariant");
771
772 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
773 // Normal case: remove Self from the DLL EntryList .
774 // This is a constant-time operation.
775 ObjectWaiter * nxt = SelfNode->_next;
776 ObjectWaiter * prv = SelfNode->_prev;
777 if (nxt != NULL) nxt->_prev = prv;
778 if (prv != NULL) prv->_next = nxt;
779 if (SelfNode == _EntryList) _EntryList = nxt;
780 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
781 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
782 TEVENT(Unlink from EntryList);
783 } else {
784 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
785 // Inopportune interleaving -- Self is still on the cxq.
786 // This usually means the enqueue of self raced an exiting thread.
787 // Normally we'll find Self near the front of the cxq, so
788 // dequeueing is typically fast. If needbe we can accelerate
789 // this with some MCS/CHL-like bidirectional list hints and advisory
790 // back-links so dequeueing from the interior will normally operate
791 // in constant-time.
792 // Dequeue Self from either the head (with CAS) or from the interior
793 // with a linear-time scan and normal non-atomic memory operations.
794 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
795 // and then unlink Self from EntryList. We have to drain eventually,
796 // so it might as well be now.
797
798 ObjectWaiter * v = _cxq;
799 assert(v != NULL, "invariant");
800 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
801 // The CAS above can fail from interference IFF a "RAT" arrived.
802 // In that case Self must be in the interior and can no longer be
803 // at the head of cxq.
804 if (v == SelfNode) {
805 assert(_cxq != v, "invariant");
806 v = _cxq; // CAS above failed - start scan at head of list
807 }
808 ObjectWaiter * p;
809 ObjectWaiter * q = NULL;
810 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
811 q = p;
812 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
813 }
814 assert(v != SelfNode, "invariant");
815 assert(p == SelfNode, "Node not found on cxq");
816 assert(p != _cxq, "invariant");
817 assert(q != NULL, "invariant");
818 assert(q->_next == p, "invariant");
819 q->_next = p->_next;
820 }
821 TEVENT(Unlink from cxq);
822 }
823
824 #ifdef ASSERT
825 // Diagnostic hygiene ...
826 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
827 SelfNode->_next = (ObjectWaiter *) 0xBAD;
828 SelfNode->TState = ObjectWaiter::TS_RUN;
829 #endif
830 }
831
832 // -----------------------------------------------------------------------------
833 // Exit support
834 //
835 // exit()
836 // ~~~~~~
837 // Note that the collector can't reclaim the objectMonitor or deflate
838 // the object out from underneath the thread calling ::exit() as the
839 // thread calling ::exit() never transitions to a stable state.
840 // This inhibits GC, which in turn inhibits asynchronous (and
841 // inopportune) reclamation of "this".
842 //
843 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
844 // There's one exception to the claim above, however. EnterI() can call
845 // exit() to drop a lock if the acquirer has been externally suspended.
846 // In that case exit() is called with _thread_state as _thread_blocked,
847 // but the monitor's _count field is > 0, which inhibits reclamation.
848 //
849 // 1-0 exit
850 // ~~~~~~~~
851 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
852 // the fast-path operators have been optimized so the common ::exit()
853 // operation is 1-0. See i486.ad fast_unlock(), for instance.
854 // The code emitted by fast_unlock() elides the usual MEMBAR. This
855 // greatly improves latency -- MEMBAR and CAS having considerable local
856 // latency on modern processors -- but at the cost of "stranding". Absent the
857 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
858 // ::enter() path, resulting in the entering thread being stranding
859 // and a progress-liveness failure. Stranding is extremely rare.
860 // We use timers (timed park operations) & periodic polling to detect
861 // and recover from stranding. Potentially stranded threads periodically
862 // wake up and poll the lock. See the usage of the _Responsible variable.
863 //
864 // The CAS() in enter provides for safety and exclusion, while the CAS or
865 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
866 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
867 // We detect and recover from stranding with timers.
868 //
869 // If a thread transiently strands it'll park until (a) another
870 // thread acquires the lock and then drops the lock, at which time the
871 // exiting thread will notice and unpark the stranded thread, or, (b)
872 // the timer expires. If the lock is high traffic then the stranding latency
873 // will be low due to (a). If the lock is low traffic then the odds of
874 // stranding are lower, although the worst-case stranding latency
875 // is longer. Critically, we don't want to put excessive load in the
876 // platform's timer subsystem. We want to minimize both the timer injection
877 // rate (timers created/sec) as well as the number of timers active at
878 // any one time. (more precisely, we want to minimize timer-seconds, which is
879 // the integral of the # of active timers at any instant over time).
880 // Both impinge on OS scalability. Given that, at most one thread parked on
881 // a monitor will use a timer.
882 //
883 // There is also the risk of a futile wake-up. If we drop the lock
884 // another thread can reacquire the lock immediately, and we can
885 // then wake a thread unnecessarily. This is benign, and we've
886 // structured the code so the windows are short and the frequency
887 // of such futile wakups is low.
888
889 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
890 Thread * const Self = THREAD;
891 if (THREAD != _owner) {
892 if (THREAD->is_lock_owned((address) _owner)) {
893 // Transmute _owner from a BasicLock pointer to a Thread address.
894 // We don't need to hold _mutex for this transition.
895 // Non-null to Non-null is safe as long as all readers can
896 // tolerate either flavor.
897 assert(_recursions == 0, "invariant");
898 _owner = THREAD;
899 _recursions = 0;
900 } else {
901 // Apparent unbalanced locking ...
902 // Naively we'd like to throw IllegalMonitorStateException.
903 // As a practical matter we can neither allocate nor throw an
904 // exception as ::exit() can be called from leaf routines.
905 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
906 // Upon deeper reflection, however, in a properly run JVM the only
907 // way we should encounter this situation is in the presence of
908 // unbalanced JNI locking. TODO: CheckJNICalls.
909 // See also: CR4414101
910 TEVENT(Exit - Throw IMSX);
911 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
912 return;
913 }
914 }
915
916 if (_recursions != 0) {
917 _recursions--; // this is simple recursive enter
918 TEVENT(Inflated exit - recursive);
919 return;
920 }
921
922 // Invariant: after setting Responsible=null an thread must execute
923 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
924 if ((SyncFlags & 4) == 0) {
925 _Responsible = NULL;
926 }
927
928 #if INCLUDE_TRACE
929 // get the owner's thread id for the MonitorEnter event
930 // if it is enabled and the thread isn't suspended
931 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
932 _previous_owner_tid = THREAD_TRACE_ID(Self);
933 }
934 #endif
935
936 for (;;) {
937 assert(THREAD == _owner, "invariant");
938
939
940 if (Knob_ExitPolicy == 0) {
941 // release semantics: prior loads and stores from within the critical section
942 // must not float (reorder) past the following store that drops the lock.
943 // On SPARC that requires MEMBAR #loadstore|#storestore.
944 // But of course in TSO #loadstore|#storestore is not required.
945 // I'd like to write one of the following:
946 // A. OrderAccess::release() ; _owner = NULL
947 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
948 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
949 // store into a _dummy variable. That store is not needed, but can result
950 // in massive wasteful coherency traffic on classic SMP systems.
951 // Instead, I use release_store(), which is implemented as just a simple
952 // ST on x64, x86 and SPARC.
953 OrderAccess::release_store_ptr(&_owner, NULL); // drop the lock
954 OrderAccess::storeload(); // See if we need to wake a successor
955 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
956 TEVENT(Inflated exit - simple egress);
957 return;
958 }
959 TEVENT(Inflated exit - complex egress);
960 // Other threads are blocked trying to acquire the lock.
961
962 // Normally the exiting thread is responsible for ensuring succession,
963 // but if other successors are ready or other entering threads are spinning
964 // then this thread can simply store NULL into _owner and exit without
965 // waking a successor. The existence of spinners or ready successors
966 // guarantees proper succession (liveness). Responsibility passes to the
967 // ready or running successors. The exiting thread delegates the duty.
968 // More precisely, if a successor already exists this thread is absolved
969 // of the responsibility of waking (unparking) one.
970 //
971 // The _succ variable is critical to reducing futile wakeup frequency.
972 // _succ identifies the "heir presumptive" thread that has been made
973 // ready (unparked) but that has not yet run. We need only one such
974 // successor thread to guarantee progress.
975 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
976 // section 3.3 "Futile Wakeup Throttling" for details.
977 //
978 // Note that spinners in Enter() also set _succ non-null.
979 // In the current implementation spinners opportunistically set
980 // _succ so that exiting threads might avoid waking a successor.
981 // Another less appealing alternative would be for the exiting thread
982 // to drop the lock and then spin briefly to see if a spinner managed
983 // to acquire the lock. If so, the exiting thread could exit
984 // immediately without waking a successor, otherwise the exiting
985 // thread would need to dequeue and wake a successor.
986 // (Note that we'd need to make the post-drop spin short, but no
987 // shorter than the worst-case round-trip cache-line migration time.
988 // The dropped lock needs to become visible to the spinner, and then
989 // the acquisition of the lock by the spinner must become visible to
990 // the exiting thread).
991
992 // It appears that an heir-presumptive (successor) must be made ready.
993 // Only the current lock owner can manipulate the EntryList or
994 // drain _cxq, so we need to reacquire the lock. If we fail
995 // to reacquire the lock the responsibility for ensuring succession
996 // falls to the new owner.
997 //
998 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
999 return;
1000 }
1001 TEVENT(Exit - Reacquired);
1002 } else {
1003 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1004 OrderAccess::release_store_ptr(&_owner, NULL); // drop the lock
1005 OrderAccess::storeload();
1006 // Ratify the previously observed values.
1007 if (_cxq == NULL || _succ != NULL) {
1008 TEVENT(Inflated exit - simple egress);
1009 return;
1010 }
1011
1012 // inopportune interleaving -- the exiting thread (this thread)
1013 // in the fast-exit path raced an entering thread in the slow-enter
1014 // path.
1015 // We have two choices:
1016 // A. Try to reacquire the lock.
1017 // If the CAS() fails return immediately, otherwise
1018 // we either restart/rerun the exit operation, or simply
1019 // fall-through into the code below which wakes a successor.
1020 // B. If the elements forming the EntryList|cxq are TSM
1021 // we could simply unpark() the lead thread and return
1022 // without having set _succ.
1023 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1024 TEVENT(Inflated exit - reacquired succeeded);
1025 return;
1026 }
1027 TEVENT(Inflated exit - reacquired failed);
1028 } else {
1029 TEVENT(Inflated exit - complex egress);
1030 }
1031 }
1032
1033 guarantee(_owner == THREAD, "invariant");
1034
1035 ObjectWaiter * w = NULL;
1036 int QMode = Knob_QMode;
1037
1038 if (QMode == 2 && _cxq != NULL) {
1039 // QMode == 2 : cxq has precedence over EntryList.
1040 // Try to directly wake a successor from the cxq.
1041 // If successful, the successor will need to unlink itself from cxq.
1042 w = _cxq;
1043 assert(w != NULL, "invariant");
1044 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1045 ExitEpilog(Self, w);
1046 return;
1047 }
1048
1049 if (QMode == 3 && _cxq != NULL) {
1050 // Aggressively drain cxq into EntryList at the first opportunity.
1051 // This policy ensure that recently-run threads live at the head of EntryList.
1052 // Drain _cxq into EntryList - bulk transfer.
1053 // First, detach _cxq.
1054 // The following loop is tantamount to: w = swap(&cxq, NULL)
1055 w = _cxq;
1056 for (;;) {
1057 assert(w != NULL, "Invariant");
1058 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1059 if (u == w) break;
1060 w = u;
1061 }
1062 assert(w != NULL, "invariant");
1063
1064 ObjectWaiter * q = NULL;
1065 ObjectWaiter * p;
1066 for (p = w; p != NULL; p = p->_next) {
1067 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1068 p->TState = ObjectWaiter::TS_ENTER;
1069 p->_prev = q;
1070 q = p;
1071 }
1072
1073 // Append the RATs to the EntryList
1074 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1075 ObjectWaiter * Tail;
1076 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1077 Tail = Tail->_next)
1078 /* empty */;
1079 if (Tail == NULL) {
1080 _EntryList = w;
1081 } else {
1082 Tail->_next = w;
1083 w->_prev = Tail;
1084 }
1085
1086 // Fall thru into code that tries to wake a successor from EntryList
1087 }
1088
1089 if (QMode == 4 && _cxq != NULL) {
1090 // Aggressively drain cxq into EntryList at the first opportunity.
1091 // This policy ensure that recently-run threads live at the head of EntryList.
1092
1093 // Drain _cxq into EntryList - bulk transfer.
1094 // First, detach _cxq.
1095 // The following loop is tantamount to: w = swap(&cxq, NULL)
1096 w = _cxq;
1097 for (;;) {
1098 assert(w != NULL, "Invariant");
1099 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1100 if (u == w) break;
1101 w = u;
1102 }
1103 assert(w != NULL, "invariant");
1104
1105 ObjectWaiter * q = NULL;
1106 ObjectWaiter * p;
1107 for (p = w; p != NULL; p = p->_next) {
1108 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1109 p->TState = ObjectWaiter::TS_ENTER;
1110 p->_prev = q;
1111 q = p;
1112 }
1113
1114 // Prepend the RATs to the EntryList
1115 if (_EntryList != NULL) {
1116 q->_next = _EntryList;
1117 _EntryList->_prev = q;
1118 }
1119 _EntryList = w;
1120
1121 // Fall thru into code that tries to wake a successor from EntryList
1122 }
1123
1124 w = _EntryList;
1125 if (w != NULL) {
1126 // I'd like to write: guarantee (w->_thread != Self).
1127 // But in practice an exiting thread may find itself on the EntryList.
1128 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1129 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1130 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1131 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1132 // release the lock "O". T2 resumes immediately after the ST of null into
1133 // _owner, above. T2 notices that the EntryList is populated, so it
1134 // reacquires the lock and then finds itself on the EntryList.
1135 // Given all that, we have to tolerate the circumstance where "w" is
1136 // associated with Self.
1137 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1138 ExitEpilog(Self, w);
1139 return;
1140 }
1141
1142 // If we find that both _cxq and EntryList are null then just
1143 // re-run the exit protocol from the top.
1144 w = _cxq;
1145 if (w == NULL) continue;
1146
1147 // Drain _cxq into EntryList - bulk transfer.
1148 // First, detach _cxq.
1149 // The following loop is tantamount to: w = swap(&cxq, NULL)
1150 for (;;) {
1151 assert(w != NULL, "Invariant");
1152 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1153 if (u == w) break;
1154 w = u;
1155 }
1156 TEVENT(Inflated exit - drain cxq into EntryList);
1157
1158 assert(w != NULL, "invariant");
1159 assert(_EntryList == NULL, "invariant");
1160
1161 // Convert the LIFO SLL anchored by _cxq into a DLL.
1162 // The list reorganization step operates in O(LENGTH(w)) time.
1163 // It's critical that this step operate quickly as
1164 // "Self" still holds the outer-lock, restricting parallelism
1165 // and effectively lengthening the critical section.
1166 // Invariant: s chases t chases u.
1167 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1168 // we have faster access to the tail.
1169
1170 if (QMode == 1) {
1171 // QMode == 1 : drain cxq to EntryList, reversing order
1172 // We also reverse the order of the list.
1173 ObjectWaiter * s = NULL;
1174 ObjectWaiter * t = w;
1175 ObjectWaiter * u = NULL;
1176 while (t != NULL) {
1177 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1178 t->TState = ObjectWaiter::TS_ENTER;
1179 u = t->_next;
1180 t->_prev = u;
1181 t->_next = s;
1182 s = t;
1183 t = u;
1184 }
1185 _EntryList = s;
1186 assert(s != NULL, "invariant");
1187 } else {
1188 // QMode == 0 or QMode == 2
1189 _EntryList = w;
1190 ObjectWaiter * q = NULL;
1191 ObjectWaiter * p;
1192 for (p = w; p != NULL; p = p->_next) {
1193 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1194 p->TState = ObjectWaiter::TS_ENTER;
1195 p->_prev = q;
1196 q = p;
1197 }
1198 }
1199
1200 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1201 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1202
1203 // See if we can abdicate to a spinner instead of waking a thread.
1204 // A primary goal of the implementation is to reduce the
1205 // context-switch rate.
1206 if (_succ != NULL) continue;
1207
1208 w = _EntryList;
1209 if (w != NULL) {
1210 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1211 ExitEpilog(Self, w);
1212 return;
1213 }
1214 }
1215 }
1216
1217 // ExitSuspendEquivalent:
1218 // A faster alternate to handle_special_suspend_equivalent_condition()
1219 //
1220 // handle_special_suspend_equivalent_condition() unconditionally
1221 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1222 // operations have high latency. Note that in ::enter() we call HSSEC
1223 // while holding the monitor, so we effectively lengthen the critical sections.
1224 //
1225 // There are a number of possible solutions:
1226 //
1227 // A. To ameliorate the problem we might also defer state transitions
1228 // to as late as possible -- just prior to parking.
1229 // Given that, we'd call HSSEC after having returned from park(),
1230 // but before attempting to acquire the monitor. This is only a
1231 // partial solution. It avoids calling HSSEC while holding the
1232 // monitor (good), but it still increases successor reacquisition latency --
1233 // the interval between unparking a successor and the time the successor
1234 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1235 // If we use this technique we can also avoid EnterI()-exit() loop
1236 // in ::enter() where we iteratively drop the lock and then attempt
1237 // to reacquire it after suspending.
1238 //
1239 // B. In the future we might fold all the suspend bits into a
1240 // composite per-thread suspend flag and then update it with CAS().
1241 // Alternately, a Dekker-like mechanism with multiple variables
1242 // would suffice:
1243 // ST Self->_suspend_equivalent = false
1244 // MEMBAR
1245 // LD Self_>_suspend_flags
1246 //
1247 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1248 // with a more efficient implementation, the need to use "FastHSSEC" has
1249 // decreased. - Dave
1250
1251
1252 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1253 const int Mode = Knob_FastHSSEC;
1254 if (Mode && !jSelf->is_external_suspend()) {
1255 assert(jSelf->is_suspend_equivalent(), "invariant");
1256 jSelf->clear_suspend_equivalent();
1257 if (2 == Mode) OrderAccess::storeload();
1258 if (!jSelf->is_external_suspend()) return false;
1259 // We raced a suspension -- fall thru into the slow path
1260 TEVENT(ExitSuspendEquivalent - raced);
1261 jSelf->set_suspend_equivalent();
1262 }
1263 return jSelf->handle_special_suspend_equivalent_condition();
1264 }
1265
1266
1267 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1268 assert(_owner == Self, "invariant");
1269
1270 // Exit protocol:
1271 // 1. ST _succ = wakee
1272 // 2. membar #loadstore|#storestore;
1273 // 2. ST _owner = NULL
1274 // 3. unpark(wakee)
1275
1276 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1277 ParkEvent * Trigger = Wakee->_event;
1278
1279 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1280 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1281 // out-of-scope (non-extant).
1282 Wakee = NULL;
1283
1284 // Drop the lock
1285 OrderAccess::release_store_ptr(&_owner, NULL);
1286 OrderAccess::fence(); // ST _owner vs LD in unpark()
1287
1288 if (SafepointSynchronize::do_call_back()) {
1289 TEVENT(unpark before SAFEPOINT);
1290 }
1291
1292 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1293 Trigger->unpark();
1294
1295 // Maintain stats and report events to JVMTI
1296 OM_PERFDATA_OP(Parks, inc());
1297 }
1298
1299
1300 // -----------------------------------------------------------------------------
1301 // Class Loader deadlock handling.
1302 //
1303 // complete_exit exits a lock returning recursion count
1304 // complete_exit/reenter operate as a wait without waiting
1305 // complete_exit requires an inflated monitor
1306 // The _owner field is not always the Thread addr even with an
1307 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1308 // thread due to contention.
1309 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1310 Thread * const Self = THREAD;
1311 assert(Self->is_Java_thread(), "Must be Java thread!");
1312 JavaThread *jt = (JavaThread *)THREAD;
1313
1314 DeferredInitialize();
1315
1316 if (THREAD != _owner) {
1317 if (THREAD->is_lock_owned ((address)_owner)) {
1318 assert(_recursions == 0, "internal state error");
1319 _owner = THREAD; // Convert from basiclock addr to Thread addr
1320 _recursions = 0;
1321 }
1322 }
1323
1324 guarantee(Self == _owner, "complete_exit not owner");
1325 intptr_t save = _recursions; // record the old recursion count
1326 _recursions = 0; // set the recursion level to be 0
1327 exit(true, Self); // exit the monitor
1328 guarantee(_owner != Self, "invariant");
1329 return save;
1330 }
1331
1332 // reenter() enters a lock and sets recursion count
1333 // complete_exit/reenter operate as a wait without waiting
1334 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1335 Thread * const Self = THREAD;
1336 assert(Self->is_Java_thread(), "Must be Java thread!");
1337 JavaThread *jt = (JavaThread *)THREAD;
1338
1339 guarantee(_owner != Self, "reenter already owner");
1340 enter(THREAD); // enter the monitor
1341 guarantee(_recursions == 0, "reenter recursion");
1342 _recursions = recursions;
1343 return;
1344 }
1345
1346
1347 // -----------------------------------------------------------------------------
1348 // A macro is used below because there may already be a pending
1349 // exception which should not abort the execution of the routines
1350 // which use this (which is why we don't put this into check_slow and
1351 // call it with a CHECK argument).
1352
1353 #define CHECK_OWNER() \
1354 do { \
1355 if (THREAD != _owner) { \
1356 if (THREAD->is_lock_owned((address) _owner)) { \
1357 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1358 _recursions = 0; \
1359 } else { \
1360 TEVENT(Throw IMSX); \
1361 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1362 } \
1363 } \
1364 } while (false)
1365
1366 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1367 // TODO-FIXME: remove check_slow() -- it's likely dead.
1368
1369 void ObjectMonitor::check_slow(TRAPS) {
1370 TEVENT(check_slow - throw IMSX);
1371 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1372 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1373 }
1374
1375 static int Adjust(volatile int * adr, int dx) {
1376 int v;
1377 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1378 return v;
1379 }
1380
1381 // helper method for posting a monitor wait event
1382 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1383 jlong notifier_tid,
1384 jlong timeout,
1385 bool timedout) {
1386 assert(event != NULL, "invariant");
1387 event->set_klass(((oop)this->object())->klass());
1388 event->set_timeout(timeout);
1389 event->set_address((TYPE_ADDRESS)this->object_addr());
1390 event->set_notifier(notifier_tid);
1391 event->set_timedOut(timedout);
1392 event->commit();
1393 }
1394
1395 // -----------------------------------------------------------------------------
1396 // Wait/Notify/NotifyAll
1397 //
1398 // Note: a subset of changes to ObjectMonitor::wait()
1399 // will need to be replicated in complete_exit
1400 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1401 Thread * const Self = THREAD;
1402 assert(Self->is_Java_thread(), "Must be Java thread!");
1403 JavaThread *jt = (JavaThread *)THREAD;
1404
1405 DeferredInitialize();
1406
1407 // Throw IMSX or IEX.
1408 CHECK_OWNER();
1409
1410 EventJavaMonitorWait event;
1411
1412 // check for a pending interrupt
1413 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1414 // post monitor waited event. Note that this is past-tense, we are done waiting.
1415 if (JvmtiExport::should_post_monitor_waited()) {
1416 // Note: 'false' parameter is passed here because the
1417 // wait was not timed out due to thread interrupt.
1418 JvmtiExport::post_monitor_waited(jt, this, false);
1419
1420 // In this short circuit of the monitor wait protocol, the
1421 // current thread never drops ownership of the monitor and
1422 // never gets added to the wait queue so the current thread
1423 // cannot be made the successor. This means that the
1424 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1425 // consume an unpark() meant for the ParkEvent associated with
1426 // this ObjectMonitor.
1427 }
1428 if (event.should_commit()) {
1429 post_monitor_wait_event(&event, 0, millis, false);
1430 }
1431 TEVENT(Wait - Throw IEX);
1432 THROW(vmSymbols::java_lang_InterruptedException());
1433 return;
1434 }
1435
1436 TEVENT(Wait);
1437
1438 assert(Self->_Stalled == 0, "invariant");
1439 Self->_Stalled = intptr_t(this);
1440 jt->set_current_waiting_monitor(this);
1441
1442 // create a node to be put into the queue
1443 // Critically, after we reset() the event but prior to park(), we must check
1444 // for a pending interrupt.
1445 ObjectWaiter node(Self);
1446 node.TState = ObjectWaiter::TS_WAIT;
1447 Self->_ParkEvent->reset();
1448 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1449
1450 // Enter the waiting queue, which is a circular doubly linked list in this case
1451 // but it could be a priority queue or any data structure.
1452 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1453 // by the the owner of the monitor *except* in the case where park()
1454 // returns because of a timeout of interrupt. Contention is exceptionally rare
1455 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1456
1457 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1458 AddWaiter(&node);
1459 Thread::SpinRelease(&_WaitSetLock);
1460
1461 if ((SyncFlags & 4) == 0) {
1462 _Responsible = NULL;
1463 }
1464 intptr_t save = _recursions; // record the old recursion count
1465 _waiters++; // increment the number of waiters
1466 _recursions = 0; // set the recursion level to be 1
1467 exit(true, Self); // exit the monitor
1468 guarantee(_owner != Self, "invariant");
1469
1470 // The thread is on the WaitSet list - now park() it.
1471 // On MP systems it's conceivable that a brief spin before we park
1472 // could be profitable.
1473 //
1474 // TODO-FIXME: change the following logic to a loop of the form
1475 // while (!timeout && !interrupted && _notified == 0) park()
1476
1477 int ret = OS_OK;
1478 int WasNotified = 0;
1479 { // State transition wrappers
1480 OSThread* osthread = Self->osthread();
1481 OSThreadWaitState osts(osthread, true);
1482 {
1483 ThreadBlockInVM tbivm(jt);
1484 // Thread is in thread_blocked state and oop access is unsafe.
1485 jt->set_suspend_equivalent();
1486
1487 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1488 // Intentionally empty
1489 } else if (node._notified == 0) {
1490 if (millis <= 0) {
1491 Self->_ParkEvent->park();
1492 } else {
1493 ret = Self->_ParkEvent->park(millis);
1494 }
1495 }
1496
1497 // were we externally suspended while we were waiting?
1498 if (ExitSuspendEquivalent (jt)) {
1499 // TODO-FIXME: add -- if succ == Self then succ = null.
1500 jt->java_suspend_self();
1501 }
1502
1503 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1504
1505 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1506 // from the WaitSet to the EntryList.
1507 // See if we need to remove Node from the WaitSet.
1508 // We use double-checked locking to avoid grabbing _WaitSetLock
1509 // if the thread is not on the wait queue.
1510 //
1511 // Note that we don't need a fence before the fetch of TState.
1512 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1513 // written by the is thread. (perhaps the fetch might even be satisfied
1514 // by a look-aside into the processor's own store buffer, although given
1515 // the length of the code path between the prior ST and this load that's
1516 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1517 // then we'll acquire the lock and then re-fetch a fresh TState value.
1518 // That is, we fail toward safety.
1519
1520 if (node.TState == ObjectWaiter::TS_WAIT) {
1521 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1522 if (node.TState == ObjectWaiter::TS_WAIT) {
1523 DequeueSpecificWaiter(&node); // unlink from WaitSet
1524 assert(node._notified == 0, "invariant");
1525 node.TState = ObjectWaiter::TS_RUN;
1526 }
1527 Thread::SpinRelease(&_WaitSetLock);
1528 }
1529
1530 // The thread is now either on off-list (TS_RUN),
1531 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1532 // The Node's TState variable is stable from the perspective of this thread.
1533 // No other threads will asynchronously modify TState.
1534 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1535 OrderAccess::loadload();
1536 if (_succ == Self) _succ = NULL;
1537 WasNotified = node._notified;
1538
1539 // Reentry phase -- reacquire the monitor.
1540 // re-enter contended monitor after object.wait().
1541 // retain OBJECT_WAIT state until re-enter successfully completes
1542 // Thread state is thread_in_vm and oop access is again safe,
1543 // although the raw address of the object may have changed.
1544 // (Don't cache naked oops over safepoints, of course).
1545
1546 // post monitor waited event. Note that this is past-tense, we are done waiting.
1547 if (JvmtiExport::should_post_monitor_waited()) {
1548 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1549
1550 if (node._notified != 0 && _succ == Self) {
1551 // In this part of the monitor wait-notify-reenter protocol it
1552 // is possible (and normal) for another thread to do a fastpath
1553 // monitor enter-exit while this thread is still trying to get
1554 // to the reenter portion of the protocol.
1555 //
1556 // The ObjectMonitor was notified and the current thread is
1557 // the successor which also means that an unpark() has already
1558 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1559 // consume the unpark() that was done when the successor was
1560 // set because the same ParkEvent is shared between Java
1561 // monitors and JVM/TI RawMonitors (for now).
1562 //
1563 // We redo the unpark() to ensure forward progress, i.e., we
1564 // don't want all pending threads hanging (parked) with none
1565 // entering the unlocked monitor.
1566 node._event->unpark();
1567 }
1568 }
1569
1570 if (event.should_commit()) {
1571 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1572 }
1573
1574 OrderAccess::fence();
1575
1576 assert(Self->_Stalled != 0, "invariant");
1577 Self->_Stalled = 0;
1578
1579 assert(_owner != Self, "invariant");
1580 ObjectWaiter::TStates v = node.TState;
1581 if (v == ObjectWaiter::TS_RUN) {
1582 enter(Self);
1583 } else {
1584 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1585 ReenterI(Self, &node);
1586 node.wait_reenter_end(this);
1587 }
1588
1589 // Self has reacquired the lock.
1590 // Lifecycle - the node representing Self must not appear on any queues.
1591 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1592 // want residual elements associated with this thread left on any lists.
1593 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1594 assert(_owner == Self, "invariant");
1595 assert(_succ != Self, "invariant");
1596 } // OSThreadWaitState()
1597
1598 jt->set_current_waiting_monitor(NULL);
1599
1600 guarantee(_recursions == 0, "invariant");
1601 _recursions = save; // restore the old recursion count
1602 _waiters--; // decrement the number of waiters
1603
1604 // Verify a few postconditions
1605 assert(_owner == Self, "invariant");
1606 assert(_succ != Self, "invariant");
1607 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1608
1609 if (SyncFlags & 32) {
1610 OrderAccess::fence();
1611 }
1612
1613 // check if the notification happened
1614 if (!WasNotified) {
1615 // no, it could be timeout or Thread.interrupt() or both
1616 // check for interrupt event, otherwise it is timeout
1617 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1618 TEVENT(Wait - throw IEX from epilog);
1619 THROW(vmSymbols::java_lang_InterruptedException());
1620 }
1621 }
1622
1623 // NOTE: Spurious wake up will be consider as timeout.
1624 // Monitor notify has precedence over thread interrupt.
1625 }
1626
1627
1628 // Consider:
1629 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1630 // then instead of transferring a thread from the WaitSet to the EntryList
1631 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1632
1633 void ObjectMonitor::INotify(Thread * Self) {
1634 const int policy = Knob_MoveNotifyee;
1635
1636 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1637 ObjectWaiter * iterator = DequeueWaiter();
1638 if (iterator != NULL) {
1639 TEVENT(Notify1 - Transfer);
1640 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1641 guarantee(iterator->_notified == 0, "invariant");
1642 // Disposition - what might we do with iterator ?
1643 // a. add it directly to the EntryList - either tail (policy == 1)
1644 // or head (policy == 0).
1645 // b. push it onto the front of the _cxq (policy == 2).
1646 // For now we use (b).
1647 if (policy != 4) {
1648 iterator->TState = ObjectWaiter::TS_ENTER;
1649 }
1650 iterator->_notified = 1;
1651 iterator->_notifier_tid = THREAD_TRACE_ID(Self);
1652
1653 ObjectWaiter * list = _EntryList;
1654 if (list != NULL) {
1655 assert(list->_prev == NULL, "invariant");
1656 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1657 assert(list != iterator, "invariant");
1658 }
1659
1660 if (policy == 0) { // prepend to EntryList
1661 if (list == NULL) {
1662 iterator->_next = iterator->_prev = NULL;
1663 _EntryList = iterator;
1664 } else {
1665 list->_prev = iterator;
1666 iterator->_next = list;
1667 iterator->_prev = NULL;
1668 _EntryList = iterator;
1669 }
1670 } else if (policy == 1) { // append to EntryList
1671 if (list == NULL) {
1672 iterator->_next = iterator->_prev = NULL;
1673 _EntryList = iterator;
1674 } else {
1675 // CONSIDER: finding the tail currently requires a linear-time walk of
1676 // the EntryList. We can make tail access constant-time by converting to
1677 // a CDLL instead of using our current DLL.
1678 ObjectWaiter * tail;
1679 for (tail = list; tail->_next != NULL; tail = tail->_next) /* empty */;
1680 assert(tail != NULL && tail->_next == NULL, "invariant");
1681 tail->_next = iterator;
1682 iterator->_prev = tail;
1683 iterator->_next = NULL;
1684 }
1685 } else if (policy == 2) { // prepend to cxq
1686 if (list == NULL) {
1687 iterator->_next = iterator->_prev = NULL;
1688 _EntryList = iterator;
1689 } else {
1690 iterator->TState = ObjectWaiter::TS_CXQ;
1691 for (;;) {
1692 ObjectWaiter * front = _cxq;
1693 iterator->_next = front;
1694 if (Atomic::cmpxchg_ptr(iterator, &_cxq, front) == front) {
1695 break;
1696 }
1697 }
1698 }
1699 } else if (policy == 3) { // append to cxq
1700 iterator->TState = ObjectWaiter::TS_CXQ;
1701 for (;;) {
1702 ObjectWaiter * tail = _cxq;
1703 if (tail == NULL) {
1704 iterator->_next = NULL;
1705 if (Atomic::cmpxchg_ptr(iterator, &_cxq, NULL) == NULL) {
1706 break;
1707 }
1708 } else {
1709 while (tail->_next != NULL) tail = tail->_next;
1710 tail->_next = iterator;
1711 iterator->_prev = tail;
1712 iterator->_next = NULL;
1713 break;
1714 }
1715 }
1716 } else {
1717 ParkEvent * ev = iterator->_event;
1718 iterator->TState = ObjectWaiter::TS_RUN;
1719 OrderAccess::fence();
1720 ev->unpark();
1721 }
1722
1723 // _WaitSetLock protects the wait queue, not the EntryList. We could
1724 // move the add-to-EntryList operation, above, outside the critical section
1725 // protected by _WaitSetLock. In practice that's not useful. With the
1726 // exception of wait() timeouts and interrupts the monitor owner
1727 // is the only thread that grabs _WaitSetLock. There's almost no contention
1728 // on _WaitSetLock so it's not profitable to reduce the length of the
1729 // critical section.
1730
1731 if (policy < 4) {
1732 iterator->wait_reenter_begin(this);
1733 }
1734 }
1735 Thread::SpinRelease(&_WaitSetLock);
1736 }
1737
1738 // Consider: a not-uncommon synchronization bug is to use notify() when
1739 // notifyAll() is more appropriate, potentially resulting in stranded
1740 // threads; this is one example of a lost wakeup. A useful diagnostic
1741 // option is to force all notify() operations to behave as notifyAll().
1742 //
1743 // Note: We can also detect many such problems with a "minimum wait".
1744 // When the "minimum wait" is set to a small non-zero timeout value
1745 // and the program does not hang whereas it did absent "minimum wait",
1746 // that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses
1747 // a "minimum wait" for all park() operations; see the recheckInterval
1748 // variable and MAX_RECHECK_INTERVAL.
1749
1750 void ObjectMonitor::notify(TRAPS) {
1751 CHECK_OWNER();
1752 if (_WaitSet == NULL) {
1753 TEVENT(Empty-Notify);
1754 return;
1755 }
1756 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1757 INotify(THREAD);
1758 OM_PERFDATA_OP(Notifications, inc(1));
1759 }
1760
1761
1762 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1763 // from the waitset to the EntryList. This could be done more efficiently with a
1764 // single bulk transfer but in practice it's not time-critical. Beware too,
1765 // that in prepend-mode we invert the order of the waiters. Let's say that the
1766 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1767 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1768
1769 void ObjectMonitor::notifyAll(TRAPS) {
1770 CHECK_OWNER();
1771 if (_WaitSet == NULL) {
1772 TEVENT(Empty-NotifyAll);
1773 return;
1774 }
1775
1776 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1777 int tally = 0;
1778 while (_WaitSet != NULL) {
1779 tally++;
1780 INotify(THREAD);
1781 }
1782
1783 OM_PERFDATA_OP(Notifications, inc(tally));
1784 }
1785
1786 // -----------------------------------------------------------------------------
1787 // Adaptive Spinning Support
1788 //
1789 // Adaptive spin-then-block - rational spinning
1790 //
1791 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1792 // algorithm. On high order SMP systems it would be better to start with
1793 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1794 // a contending thread could enqueue itself on the cxq and then spin locally
1795 // on a thread-specific variable such as its ParkEvent._Event flag.
1796 // That's left as an exercise for the reader. Note that global spinning is
1797 // not problematic on Niagara, as the L2 cache serves the interconnect and
1798 // has both low latency and massive bandwidth.
1799 //
1800 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1801 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1802 // (duration) or we can fix the count at approximately the duration of
1803 // a context switch and vary the frequency. Of course we could also
1804 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1805 // For a description of 'Adaptive spin-then-block mutual exclusion in
1806 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1807 //
1808 // This implementation varies the duration "D", where D varies with
1809 // the success rate of recent spin attempts. (D is capped at approximately
1810 // length of a round-trip context switch). The success rate for recent
1811 // spin attempts is a good predictor of the success rate of future spin
1812 // attempts. The mechanism adapts automatically to varying critical
1813 // section length (lock modality), system load and degree of parallelism.
1814 // D is maintained per-monitor in _SpinDuration and is initialized
1815 // optimistically. Spin frequency is fixed at 100%.
1816 //
1817 // Note that _SpinDuration is volatile, but we update it without locks
1818 // or atomics. The code is designed so that _SpinDuration stays within
1819 // a reasonable range even in the presence of races. The arithmetic
1820 // operations on _SpinDuration are closed over the domain of legal values,
1821 // so at worst a race will install and older but still legal value.
1822 // At the very worst this introduces some apparent non-determinism.
1823 // We might spin when we shouldn't or vice-versa, but since the spin
1824 // count are relatively short, even in the worst case, the effect is harmless.
1825 //
1826 // Care must be taken that a low "D" value does not become an
1827 // an absorbing state. Transient spinning failures -- when spinning
1828 // is overall profitable -- should not cause the system to converge
1829 // on low "D" values. We want spinning to be stable and predictable
1830 // and fairly responsive to change and at the same time we don't want
1831 // it to oscillate, become metastable, be "too" non-deterministic,
1832 // or converge on or enter undesirable stable absorbing states.
1833 //
1834 // We implement a feedback-based control system -- using past behavior
1835 // to predict future behavior. We face two issues: (a) if the
1836 // input signal is random then the spin predictor won't provide optimal
1837 // results, and (b) if the signal frequency is too high then the control
1838 // system, which has some natural response lag, will "chase" the signal.
1839 // (b) can arise from multimodal lock hold times. Transient preemption
1840 // can also result in apparent bimodal lock hold times.
1841 // Although sub-optimal, neither condition is particularly harmful, as
1842 // in the worst-case we'll spin when we shouldn't or vice-versa.
1843 // The maximum spin duration is rather short so the failure modes aren't bad.
1844 // To be conservative, I've tuned the gain in system to bias toward
1845 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1846 // "rings" or oscillates between spinning and not spinning. This happens
1847 // when spinning is just on the cusp of profitability, however, so the
1848 // situation is not dire. The state is benign -- there's no need to add
1849 // hysteresis control to damp the transition rate between spinning and
1850 // not spinning.
1851
1852 intptr_t ObjectMonitor::SpinCallbackArgument = 0;
1853 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL;
1854
1855 // Spinning: Fixed frequency (100%), vary duration
1856
1857
1858 int ObjectMonitor::TrySpin_VaryDuration(Thread * Self) {
1859 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1860 int ctr = Knob_FixedSpin;
1861 if (ctr != 0) {
1862 while (--ctr >= 0) {
1863 if (TryLock(Self) > 0) return 1;
1864 SpinPause();
1865 }
1866 return 0;
1867 }
1868
1869 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1870 if (TryLock(Self) > 0) {
1871 // Increase _SpinDuration ...
1872 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1873 // Raising _SpurDuration to the poverty line is key.
1874 int x = _SpinDuration;
1875 if (x < Knob_SpinLimit) {
1876 if (x < Knob_Poverty) x = Knob_Poverty;
1877 _SpinDuration = x + Knob_BonusB;
1878 }
1879 return 1;
1880 }
1881 SpinPause();
1882 }
1883
1884 // Admission control - verify preconditions for spinning
1885 //
1886 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1887 // becoming an absorbing state. Put another way, we spin briefly to
1888 // sample, just in case the system load, parallelism, contention, or lock
1889 // modality changed.
1890 //
1891 // Consider the following alternative:
1892 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1893 // spin attempt. "Periodically" might mean after a tally of
1894 // the # of failed spin attempts (or iterations) reaches some threshold.
1895 // This takes us into the realm of 1-out-of-N spinning, where we
1896 // hold the duration constant but vary the frequency.
1897
1898 ctr = _SpinDuration;
1899 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1900 if (ctr <= 0) return 0;
1901
1902 if (Knob_SuccRestrict && _succ != NULL) return 0;
1903 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1904 TEVENT(Spin abort - notrunnable [TOP]);
1905 return 0;
1906 }
1907
1908 int MaxSpin = Knob_MaxSpinners;
1909 if (MaxSpin >= 0) {
1910 if (_Spinner > MaxSpin) {
1911 TEVENT(Spin abort -- too many spinners);
1912 return 0;
1913 }
1914 // Slightly racy, but benign ...
1915 Adjust(&_Spinner, 1);
1916 }
1917
1918 // We're good to spin ... spin ingress.
1919 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1920 // when preparing to LD...CAS _owner, etc and the CAS is likely
1921 // to succeed.
1922 int hits = 0;
1923 int msk = 0;
1924 int caspty = Knob_CASPenalty;
1925 int oxpty = Knob_OXPenalty;
1926 int sss = Knob_SpinSetSucc;
1927 if (sss && _succ == NULL) _succ = Self;
1928 Thread * prv = NULL;
1929
1930 // There are three ways to exit the following loop:
1931 // 1. A successful spin where this thread has acquired the lock.
1932 // 2. Spin failure with prejudice
1933 // 3. Spin failure without prejudice
1934
1935 while (--ctr >= 0) {
1936
1937 // Periodic polling -- Check for pending GC
1938 // Threads may spin while they're unsafe.
1939 // We don't want spinning threads to delay the JVM from reaching
1940 // a stop-the-world safepoint or to steal cycles from GC.
1941 // If we detect a pending safepoint we abort in order that
1942 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1943 // this thread, if safe, doesn't steal cycles from GC.
1944 // This is in keeping with the "no loitering in runtime" rule.
1945 // We periodically check to see if there's a safepoint pending.
1946 if ((ctr & 0xFF) == 0) {
1947 if (SafepointSynchronize::do_call_back()) {
1948 TEVENT(Spin: safepoint);
1949 goto Abort; // abrupt spin egress
1950 }
1951 if (Knob_UsePause & 1) SpinPause();
1952
1953 int (*scb)(intptr_t,int) = SpinCallbackFunction;
1954 if (hits > 50 && scb != NULL) {
1955 int abend = (*scb)(SpinCallbackArgument, 0);
1956 }
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_ptr(Self, &_owner, 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 TState = TS_RUN;
2154 _thread = thread;
2155 _event = thread->_ParkEvent;
2156 _active = false;
2157 assert(_event != NULL, "invariant");
2158 }
2159
2160 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2161 JavaThread *jt = (JavaThread *)this->_thread;
2162 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2163 }
2164
2165 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2166 JavaThread *jt = (JavaThread *)this->_thread;
2167 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2168 }
2169
2170 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2171 assert(node != NULL, "should not add NULL node");
2172 assert(node->_prev == NULL, "node already in list");
2173 assert(node->_next == NULL, "node already in list");
2174 // put node at end of queue (circular doubly linked list)
2175 if (_WaitSet == NULL) {
2176 _WaitSet = node;
2177 node->_prev = node;
2178 node->_next = node;
2179 } else {
2180 ObjectWaiter* head = _WaitSet;
2181 ObjectWaiter* tail = head->_prev;
2182 assert(tail->_next == head, "invariant check");
2183 tail->_next = node;
2184 head->_prev = node;
2185 node->_next = head;
2186 node->_prev = tail;
2187 }
2188 }
2189
2190 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2191 // dequeue the very first waiter
2192 ObjectWaiter* waiter = _WaitSet;
2193 if (waiter) {
2194 DequeueSpecificWaiter(waiter);
2195 }
2196 return waiter;
2197 }
2198
2199 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2200 assert(node != NULL, "should not dequeue NULL node");
2201 assert(node->_prev != NULL, "node already removed from list");
2202 assert(node->_next != NULL, "node already removed from list");
2203 // when the waiter has woken up because of interrupt,
2204 // timeout or other spurious wake-up, dequeue the
2205 // waiter from waiting list
2206 ObjectWaiter* next = node->_next;
2207 if (next == node) {
2208 assert(node->_prev == node, "invariant check");
2209 _WaitSet = NULL;
2210 } else {
2211 ObjectWaiter* prev = node->_prev;
2212 assert(prev->_next == node, "invariant check");
2213 assert(next->_prev == node, "invariant check");
2214 next->_prev = prev;
2215 prev->_next = next;
2216 if (_WaitSet == node) {
2217 _WaitSet = next;
2218 }
2219 }
2220 node->_next = NULL;
2221 node->_prev = NULL;
2222 }
2223
2224 // -----------------------------------------------------------------------------
2225 // PerfData support
2226 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2227 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2228 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2229 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL;
2230 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2231 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL;
2232 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL;
2233 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL;
2234 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL;
2235 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL;
2236 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL;
2237 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL;
2238 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL;
2239 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL;
2240 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL;
2241 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2242 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2243 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2244
2245 // One-shot global initialization for the sync subsystem.
2246 // We could also defer initialization and initialize on-demand
2247 // the first time we call inflate(). Initialization would
2248 // be protected - like so many things - by the MonitorCache_lock.
2249
2250 void ObjectMonitor::Initialize() {
2251 static int InitializationCompleted = 0;
2252 assert(InitializationCompleted == 0, "invariant");
2253 InitializationCompleted = 1;
2254 if (UsePerfData) {
2255 EXCEPTION_MARK;
2256 #define NEWPERFCOUNTER(n) \
2257 { \
2258 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2259 CHECK); \
2260 }
2261 #define NEWPERFVARIABLE(n) \
2262 { \
2263 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2264 CHECK); \
2265 }
2266 NEWPERFCOUNTER(_sync_Inflations);
2267 NEWPERFCOUNTER(_sync_Deflations);
2268 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2269 NEWPERFCOUNTER(_sync_FutileWakeups);
2270 NEWPERFCOUNTER(_sync_Parks);
2271 NEWPERFCOUNTER(_sync_EmptyNotifications);
2272 NEWPERFCOUNTER(_sync_Notifications);
2273 NEWPERFCOUNTER(_sync_SlowEnter);
2274 NEWPERFCOUNTER(_sync_SlowExit);
2275 NEWPERFCOUNTER(_sync_SlowNotify);
2276 NEWPERFCOUNTER(_sync_SlowNotifyAll);
2277 NEWPERFCOUNTER(_sync_FailedSpins);
2278 NEWPERFCOUNTER(_sync_SuccessfulSpins);
2279 NEWPERFCOUNTER(_sync_PrivateA);
2280 NEWPERFCOUNTER(_sync_PrivateB);
2281 NEWPERFCOUNTER(_sync_MonInCirculation);
2282 NEWPERFCOUNTER(_sync_MonScavenged);
2283 NEWPERFVARIABLE(_sync_MonExtant);
2284 #undef NEWPERFCOUNTER
2285 #undef NEWPERFVARIABLE
2286 }
2287 }
2288
2289 static char * kvGet(char * kvList, const char * Key) {
2290 if (kvList == NULL) return NULL;
2291 size_t n = strlen(Key);
2292 char * Search;
2293 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2294 if (strncmp (Search, Key, n) == 0) {
2295 if (Search[n] == '=') return Search + n + 1;
2296 if (Search[n] == 0) return(char *) "1";
2297 }
2298 }
2299 return NULL;
2300 }
2301
2302 static int kvGetInt(char * kvList, const char * Key, int Default) {
2303 char * v = kvGet(kvList, Key);
2304 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2305 if (Knob_ReportSettings && v != NULL) {
2306 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ;
2307 tty->flush();
2308 }
2309 return rslt;
2310 }
2311
2312 void ObjectMonitor::DeferredInitialize() {
2313 if (InitDone > 0) return;
2314 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2315 while (InitDone != 1) /* empty */;
2316 return;
2317 }
2318
2319 // One-shot global initialization ...
2320 // The initialization is idempotent, so we don't need locks.
2321 // In the future consider doing this via os::init_2().
2322 // SyncKnobs consist of <Key>=<Value> pairs in the style
2323 // of environment variables. Start by converting ':' to NUL.
2324
2325 if (SyncKnobs == NULL) SyncKnobs = "";
2326
2327 size_t sz = strlen(SyncKnobs);
2328 char * knobs = (char *) malloc(sz + 2);
2329 if (knobs == NULL) {
2330 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2331 guarantee(0, "invariant");
2332 }
2333 strcpy(knobs, SyncKnobs);
2334 knobs[sz+1] = 0;
2335 for (char * p = knobs; *p; p++) {
2336 if (*p == ':') *p = 0;
2337 }
2338
2339 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2340 SETKNOB(ReportSettings);
2341 SETKNOB(ExitRelease);
2342 SETKNOB(Verbose);
2343 SETKNOB(VerifyInUse);
2344 SETKNOB(VerifyMatch);
2345 SETKNOB(FixedSpin);
2346 SETKNOB(SpinLimit);
2347 SETKNOB(SpinBase);
2348 SETKNOB(SpinBackOff);
2349 SETKNOB(CASPenalty);
2350 SETKNOB(OXPenalty);
2351 SETKNOB(LogSpins);
2352 SETKNOB(SpinSetSucc);
2353 SETKNOB(SuccEnabled);
2354 SETKNOB(SuccRestrict);
2355 SETKNOB(Penalty);
2356 SETKNOB(Bonus);
2357 SETKNOB(BonusB);
2358 SETKNOB(Poverty);
2359 SETKNOB(SpinAfterFutile);
2360 SETKNOB(UsePause);
2361 SETKNOB(SpinEarly);
2362 SETKNOB(OState);
2363 SETKNOB(MaxSpinners);
2364 SETKNOB(PreSpin);
2365 SETKNOB(ExitPolicy);
2366 SETKNOB(QMode);
2367 SETKNOB(ResetEvent);
2368 SETKNOB(MoveNotifyee);
2369 SETKNOB(FastHSSEC);
2370 #undef SETKNOB
2371
2372 if (Knob_Verbose) {
2373 sanity_checks();
2374 }
2375
2376 if (os::is_MP()) {
2377 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2378 if (Knob_ReportSettings) {
2379 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask);
2380 }
2381 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2382 } else {
2383 Knob_SpinLimit = 0;
2384 Knob_SpinBase = 0;
2385 Knob_PreSpin = 0;
2386 Knob_FixedSpin = -1;
2387 }
2388
2389 if (Knob_LogSpins == 0) {
2390 ObjectMonitor::_sync_FailedSpins = NULL;
2391 }
2392
2393 free(knobs);
2394 OrderAccess::fence();
2395 InitDone = 1;
2396 }
2397
2398 void ObjectMonitor::sanity_checks() {
2399 int error_cnt = 0;
2400 int warning_cnt = 0;
2401 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2402
2403 if (verbose) {
2404 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2405 sizeof(ObjectMonitor));
2406 tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT,
2407 sizeof(PaddedEnd<ObjectMonitor>));
2408 }
2409
2410 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2411 if (verbose) {
2412 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2413 }
2414
2415 ObjectMonitor dummy;
2416 u_char *addr_begin = (u_char*)&dummy;
2417 u_char *addr_header = (u_char*)&dummy._header;
2418 u_char *addr_owner = (u_char*)&dummy._owner;
2419
2420 uint offset_header = (uint)(addr_header - addr_begin);
2421 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2422
2423 uint offset_owner = (uint)(addr_owner - addr_begin);
2424 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2425
2426 if ((uint)(addr_header - addr_begin) != 0) {
2427 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2428 error_cnt++;
2429 }
2430
2431 if (cache_line_size != 0) {
2432 // We were able to determine the L1 data cache line size so
2433 // do some cache line specific sanity checks
2434
2435 if ((offset_owner - offset_header) < cache_line_size) {
2436 tty->print_cr("WARNING: the _header and _owner fields are closer "
2437 "than a cache line which permits false sharing.");
2438 warning_cnt++;
2439 }
2440
2441 if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) {
2442 tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a "
2443 "multiple of a cache line which permits false sharing.");
2444 warning_cnt++;
2445 }
2446 }
2447
2448 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2449 &warning_cnt);
2450
2451 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2452 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2453 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2454 }
2455
2456 guarantee(error_cnt == 0,
2457 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2458 }
2459
2460 #ifndef PRODUCT
2461 void ObjectMonitor::verify() {
2462 }
2463
2464 void ObjectMonitor::print() {
2465 }
2466
2467 void ObjectMonitor_test() {
2468 ObjectMonitor::sanity_checks();
2469 }
2470 #endif