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 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
48 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
49 #define NOINLINE __attribute__((noinline))
50 #else
51 #define NOINLINE
52 #endif
53
54
55 #ifdef DTRACE_ENABLED
56
57 // Only bother with this argument setup if dtrace is available
58 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
59
60
61 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
62 char* bytes = NULL; \
63 int len = 0; \
64 jlong jtid = SharedRuntime::get_java_tid(thread); \
65 Symbol* klassname = ((oop)obj)->klass()->name(); \
66 if (klassname != NULL) { \
67 bytes = (char*)klassname->bytes(); \
68 len = klassname->utf8_length(); \
69 }
70
71 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
72 { \
73 if (DTraceMonitorProbes) { \
74 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
75 HOTSPOT_MONITOR_WAIT(jtid, \
76 (monitor), bytes, len, (millis)); \
77 } \
78 }
79
80 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
81 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
82 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
83 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
84 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
85
86 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
87 { \
88 if (DTraceMonitorProbes) { \
89 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
90 HOTSPOT_MONITOR_##probe(jtid, \
91 (uintptr_t)(monitor), bytes, len); \
92 } \
93 }
94
95 #else // ndef DTRACE_ENABLED
96
97 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
98 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
99
100 #endif // ndef DTRACE_ENABLED
101
102 // Tunables ...
103 // The knob* variables are effectively final. Once set they should
104 // never be modified hence. Consider using __read_mostly with GCC.
105
106 int ObjectMonitor::Knob_ExitRelease = 0;
107 int ObjectMonitor::Knob_Verbose = 0;
108 int ObjectMonitor::Knob_VerifyInUse = 0;
109 int ObjectMonitor::Knob_VerifyMatch = 0;
110 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
111 static int Knob_LogSpins = 0; // enable jvmstat tally for spins
112 static int Knob_HandOff = 0;
113 static int Knob_ReportSettings = 0;
114
115 static int Knob_SpinBase = 0; // Floor AKA SpinMin
116 static int Knob_SpinBackOff = 0; // spin-loop backoff
117 static int Knob_CASPenalty = -1; // Penalty for failed CAS
118 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
119 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
120 static int Knob_SpinEarly = 1;
121 static int Knob_SuccEnabled = 1; // futile wake throttling
122 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
123 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
124 static int Knob_Bonus = 100; // spin success bonus
125 static int Knob_BonusB = 100; // spin success bonus
126 static int Knob_Penalty = 200; // spin failure penalty
127 static int Knob_Poverty = 1000;
128 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
129 static int Knob_FixedSpin = 0;
130 static int Knob_OState = 3; // Spinner checks thread state of _owner
131 static int Knob_UsePause = 1;
132 static int Knob_ExitPolicy = 0;
133 static int Knob_PreSpin = 10; // 20-100 likely better
134 static int Knob_ResetEvent = 0;
135 static int BackOffMask = 0;
136
137 static int Knob_FastHSSEC = 0;
138 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
139 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
140 static volatile int InitDone = 0;
141
142 #define TrySpin TrySpin_VaryDuration
143
144 // -----------------------------------------------------------------------------
145 // Theory of operations -- Monitors lists, thread residency, etc:
146 //
147 // * A thread acquires ownership of a monitor by successfully
148 // CAS()ing the _owner field from null to non-null.
149 //
150 // * Invariant: A thread appears on at most one monitor list --
151 // cxq, EntryList or WaitSet -- at any one time.
152 //
153 // * Contending threads "push" themselves onto the cxq with CAS
154 // and then spin/park.
155 //
156 // * After a contending thread eventually acquires the lock it must
157 // dequeue itself from either the EntryList or the cxq.
158 //
159 // * The exiting thread identifies and unparks an "heir presumptive"
160 // tentative successor thread on the EntryList. Critically, the
161 // exiting thread doesn't unlink the successor thread from the EntryList.
162 // After having been unparked, the wakee will recontend for ownership of
163 // the monitor. The successor (wakee) will either acquire the lock or
164 // re-park itself.
165 //
166 // Succession is provided for by a policy of competitive handoff.
167 // The exiting thread does _not_ grant or pass ownership to the
168 // successor thread. (This is also referred to as "handoff" succession").
169 // Instead the exiting thread releases ownership and possibly wakes
170 // a successor, so the successor can (re)compete for ownership of the lock.
171 // If the EntryList is empty but the cxq is populated the exiting
172 // thread will drain the cxq into the EntryList. It does so by
173 // by detaching the cxq (installing null with CAS) and folding
174 // the threads from the cxq into the EntryList. The EntryList is
175 // doubly linked, while the cxq is singly linked because of the
176 // CAS-based "push" used to enqueue recently arrived threads (RATs).
177 //
178 // * Concurrency invariants:
179 //
180 // -- only the monitor owner may access or mutate the EntryList.
181 // The mutex property of the monitor itself protects the EntryList
182 // from concurrent interference.
183 // -- Only the monitor owner may detach the cxq.
184 //
185 // * The monitor entry list operations avoid locks, but strictly speaking
186 // they're not lock-free. Enter is lock-free, exit is not.
187 // For a description of 'Methods and apparatus providing non-blocking access
188 // to a resource,' see U.S. Pat. No. 7844973.
189 //
190 // * The cxq can have multiple concurrent "pushers" but only one concurrent
191 // detaching thread. This mechanism is immune from the ABA corruption.
192 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
193 //
194 // * Taken together, the cxq and the EntryList constitute or form a
195 // single logical queue of threads stalled trying to acquire the lock.
196 // We use two distinct lists to improve the odds of a constant-time
197 // dequeue operation after acquisition (in the ::enter() epilogue) and
198 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
199 // A key desideratum is to minimize queue & monitor metadata manipulation
200 // that occurs while holding the monitor lock -- that is, we want to
201 // minimize monitor lock holds times. Note that even a small amount of
202 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
203 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
204 // locks and monitor metadata.
205 //
206 // Cxq points to the set of Recently Arrived Threads attempting entry.
207 // Because we push threads onto _cxq with CAS, the RATs must take the form of
208 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
209 // the unlocking thread notices that EntryList is null but _cxq is != null.
210 //
211 // The EntryList is ordered by the prevailing queue discipline and
212 // can be organized in any convenient fashion, such as a doubly-linked list or
213 // a circular doubly-linked list. Critically, we want insert and delete operations
214 // to operate in constant-time. If we need a priority queue then something akin
215 // to Solaris' sleepq would work nicely. Viz.,
216 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
217 // Queue discipline is enforced at ::exit() time, when the unlocking thread
218 // drains the cxq into the EntryList, and orders or reorders the threads on the
219 // EntryList accordingly.
220 //
221 // Barring "lock barging", this mechanism provides fair cyclic ordering,
222 // somewhat similar to an elevator-scan.
223 //
224 // * The monitor synchronization subsystem avoids the use of native
225 // synchronization primitives except for the narrow platform-specific
226 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
227 // the semantics of park-unpark. Put another way, this monitor implementation
228 // depends only on atomic operations and park-unpark. The monitor subsystem
229 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
230 // underlying OS manages the READY<->RUN transitions.
231 //
232 // * Waiting threads reside on the WaitSet list -- wait() puts
233 // the caller onto the WaitSet.
234 //
235 // * notify() or notifyAll() simply transfers threads from the WaitSet to
236 // either the EntryList or cxq. Subsequent exit() operations will
237 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
238 // it's likely the notifyee would simply impale itself on the lock held
239 // by the notifier.
240 //
241 // * An interesting alternative is to encode cxq as (List,LockByte) where
242 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
243 // variable, like _recursions, in the scheme. The threads or Events that form
244 // the list would have to be aligned in 256-byte addresses. A thread would
245 // try to acquire the lock or enqueue itself with CAS, but exiting threads
246 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
247 // Note that is is *not* word-tearing, but it does presume that full-word
248 // CAS operations are coherent with intermix with STB operations. That's true
249 // on most common processors.
250 //
251 // * See also http://blogs.sun.com/dave
252
253
254 // -----------------------------------------------------------------------------
255 // Enter support
256
257 void NOINLINE ObjectMonitor::enter(TRAPS) {
258 // The following code is ordered to check the most common cases first
259 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
260 Thread * const Self = THREAD;
261
262 void * cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL);
263 if (cur == NULL) {
264 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
265 assert(_recursions == 0, "invariant");
266 assert(_owner == Self, "invariant");
267 return;
268 }
269
270 if (cur == Self) {
271 // TODO-FIXME: check for integer overflow! BUGID 6557169.
272 _recursions++;
273 return;
274 }
275
276 if (Self->is_lock_owned ((address)cur)) {
277 assert(_recursions == 0, "internal state error");
278 _recursions = 1;
279 // Commute owner from a thread-specific on-stack BasicLockObject address to
280 // a full-fledged "Thread *".
281 _owner = Self;
282 return;
283 }
284
285 // We've encountered genuine contention.
286 assert(Self->_Stalled == 0, "invariant");
287 Self->_Stalled = intptr_t(this);
288
289 // Try one round of spinning *before* enqueueing Self
290 // and before going through the awkward and expensive state
291 // transitions. The following spin is strictly optional ...
292 // Note that if we acquire the monitor from an initial spin
293 // we forgo posting JVMTI events and firing DTRACE probes.
294 if (Knob_SpinEarly && TrySpin (Self) > 0) {
295 assert(_owner == Self, "invariant");
296 assert(_recursions == 0, "invariant");
297 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
298 Self->_Stalled = 0;
299 return;
300 }
301
302 assert(_owner != Self, "invariant");
303 assert(_succ != Self, "invariant");
304 assert(Self->is_Java_thread(), "invariant");
305 JavaThread * jt = (JavaThread *) Self;
306 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
307 assert(jt->thread_state() != _thread_blocked, "invariant");
308 assert(this->object() != NULL, "invariant");
309 assert(_count >= 0, "invariant");
310
311 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
312 // Ensure the object-monitor relationship remains stable while there's contention.
313 Atomic::inc(&_count);
314
315 EventJavaMonitorEnter event;
316
317 { // Change java thread status to indicate blocked on monitor enter.
318 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
319
320 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
321 if (JvmtiExport::should_post_monitor_contended_enter()) {
322 JvmtiExport::post_monitor_contended_enter(jt, this);
323
324 // The current thread does not yet own the monitor and does not
325 // yet appear on any queues that would get it made the successor.
326 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
327 // handler cannot accidentally consume an unpark() meant for the
328 // ParkEvent associated with this ObjectMonitor.
329 }
330
331 OSThreadContendState osts(Self->osthread());
332 ThreadBlockInVM tbivm(jt);
333
334 Self->set_current_pending_monitor(this);
335
336 // TODO-FIXME: change the following for(;;) loop to straight-line code.
337 for (;;) {
338 jt->set_suspend_equivalent();
339 // cleared by handle_special_suspend_equivalent_condition()
340 // or java_suspend_self()
341
342 EnterI(THREAD);
343
344 if (!ExitSuspendEquivalent(jt)) break;
345
346 // We have acquired the contended monitor, but while we were
347 // waiting another thread suspended us. We don't want to enter
348 // the monitor while suspended because that would surprise the
349 // thread that suspended us.
350 //
351 _recursions = 0;
352 _succ = NULL;
353 exit(false, Self);
354
355 jt->java_suspend_self();
356 }
357 Self->set_current_pending_monitor(NULL);
358
359 // We cleared the pending monitor info since we've just gotten past
360 // the enter-check-for-suspend dance and we now own the monitor free
361 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
362 // destructor can go to a safepoint at the end of this block. If we
363 // do a thread dump during that safepoint, then this thread will show
364 // as having "-locked" the monitor, but the OS and java.lang.Thread
365 // states will still report that the thread is blocked trying to
366 // acquire it.
367 }
368
369 Atomic::dec(&_count);
370 assert(_count >= 0, "invariant");
371 Self->_Stalled = 0;
372
373 // Must either set _recursions = 0 or ASSERT _recursions == 0.
374 assert(_recursions == 0, "invariant");
375 assert(_owner == Self, "invariant");
376 assert(_succ != Self, "invariant");
377 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
378
379 // The thread -- now the owner -- is back in vm mode.
380 // Report the glorious news via TI,DTrace and jvmstat.
381 // The probe effect is non-trivial. All the reportage occurs
382 // while we hold the monitor, increasing the length of the critical
383 // section. Amdahl's parallel speedup law comes vividly into play.
384 //
385 // Another option might be to aggregate the events (thread local or
386 // per-monitor aggregation) and defer reporting until a more opportune
387 // time -- such as next time some thread encounters contention but has
388 // yet to acquire the lock. While spinning that thread could
389 // spinning we could increment JVMStat counters, etc.
390
391 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
392 if (JvmtiExport::should_post_monitor_contended_entered()) {
393 JvmtiExport::post_monitor_contended_entered(jt, this);
394
395 // The current thread already owns the monitor and is not going to
396 // call park() for the remainder of the monitor enter protocol. So
397 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
398 // event handler consumed an unpark() issued by the thread that
399 // just exited the monitor.
400 }
401
402 if (event.should_commit()) {
403 event.set_klass(((oop)this->object())->klass());
404 event.set_previousOwner((TYPE_THREAD)_previous_owner_tid);
405 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
406 event.commit();
407 }
408
409 OM_PERFDATA_OP(ContendedLockAttempts, inc());
410 }
411
412
413 // Caveat: TryLock() is not necessarily serializing if it returns failure.
414 // Callers must compensate as needed.
415
416 int ObjectMonitor::TryLock(Thread * Self) {
417 void * own = _owner;
418 if (own != NULL) return 0;
419 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
420 // Either guarantee _recursions == 0 or set _recursions = 0.
421 assert(_recursions == 0, "invariant");
422 assert(_owner == Self, "invariant");
423 return 1;
424 }
425 // The lock had been free momentarily, but we lost the race to the lock.
426 // Interference -- the CAS failed.
427 // We can either return -1 or retry.
428 // Retry doesn't make as much sense because the lock was just acquired.
429 return -1;
430 }
431
432 #define MAX_RECHECK_INTERVAL 1000
433
434 void NOINLINE ObjectMonitor::EnterI(TRAPS) {
435 Thread * const Self = THREAD;
436 assert(Self->is_Java_thread(), "invariant");
437 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
438
439 // Try the lock - TATAS
440 if (TryLock (Self) > 0) {
441 assert(_succ != Self, "invariant");
442 assert(_owner == Self, "invariant");
443 assert(_Responsible != Self, "invariant");
444 return;
445 }
446
447 DeferredInitialize();
448
449 // We try one round of spinning *before* enqueueing Self.
450 //
451 // If the _owner is ready but OFFPROC we could use a YieldTo()
452 // operation to donate the remainder of this thread's quantum
453 // to the owner. This has subtle but beneficial affinity
454 // effects.
455
456 if (TrySpin (Self) > 0) {
457 assert(_owner == Self, "invariant");
458 assert(_succ != Self, "invariant");
459 assert(_Responsible != Self, "invariant");
460 return;
461 }
462
463 // The Spin failed -- Enqueue and park the thread ...
464 assert(_succ != Self, "invariant");
465 assert(_owner != Self, "invariant");
466 assert(_Responsible != Self, "invariant");
467
468 // Enqueue "Self" on ObjectMonitor's _cxq.
469 //
470 // Node acts as a proxy for Self.
471 // As an aside, if were to ever rewrite the synchronization code mostly
472 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
473 // Java objects. This would avoid awkward lifecycle and liveness issues,
474 // as well as eliminate a subset of ABA issues.
475 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
476
477 ObjectWaiter node(Self);
478 Self->_ParkEvent->reset();
479 node._prev = (ObjectWaiter *) 0xBAD;
480 node.TState = ObjectWaiter::TS_CXQ;
481
482 // Push "Self" onto the front of the _cxq.
483 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
484 // Note that spinning tends to reduce the rate at which threads
485 // enqueue and dequeue on EntryList|cxq.
486 ObjectWaiter * nxt;
487 for (;;) {
488 node._next = nxt = _cxq;
489 if (Atomic::cmpxchg_ptr(&node, &_cxq, nxt) == nxt) break;
490
491 // Interference - the CAS failed because _cxq changed. Just retry.
492 // As an optional optimization we retry the lock.
493 if (TryLock (Self) > 0) {
494 assert(_succ != Self, "invariant");
495 assert(_owner == Self, "invariant");
496 assert(_Responsible != Self, "invariant");
497 return;
498 }
499 }
500
501 // Check for cxq|EntryList edge transition to non-null. This indicates
502 // the onset of contention. While contention persists exiting threads
503 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
504 // operations revert to the faster 1-0 mode. This enter operation may interleave
505 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
506 // arrange for one of the contending thread to use a timed park() operations
507 // to detect and recover from the race. (Stranding is form of progress failure
508 // where the monitor is unlocked but all the contending threads remain parked).
509 // That is, at least one of the contended threads will periodically poll _owner.
510 // One of the contending threads will become the designated "Responsible" thread.
511 // The Responsible thread uses a timed park instead of a normal indefinite park
512 // operation -- it periodically wakes and checks for and recovers from potential
513 // strandings admitted by 1-0 exit operations. We need at most one Responsible
514 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
515 // be responsible for a monitor.
516 //
517 // Currently, one of the contended threads takes on the added role of "Responsible".
518 // A viable alternative would be to use a dedicated "stranding checker" thread
519 // that periodically iterated over all the threads (or active monitors) and unparked
520 // successors where there was risk of stranding. This would help eliminate the
521 // timer scalability issues we see on some platforms as we'd only have one thread
522 // -- the checker -- parked on a timer.
523
524 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
525 // Try to assume the role of responsible thread for the monitor.
526 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
527 Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
528 }
529
530 // The lock might have been released while this thread was occupied queueing
531 // itself onto _cxq. To close the race and avoid "stranding" and
532 // progress-liveness failure we must resample-retry _owner before parking.
533 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
534 // In this case the ST-MEMBAR is accomplished with CAS().
535 //
536 // TODO: Defer all thread state transitions until park-time.
537 // Since state transitions are heavy and inefficient we'd like
538 // to defer the state transitions until absolutely necessary,
539 // and in doing so avoid some transitions ...
540
541 TEVENT(Inflated enter - Contention);
542 int nWakeups = 0;
543 int recheckInterval = 1;
544
545 for (;;) {
546
547 if (TryLock(Self) > 0) break;
548 assert(_owner != Self, "invariant");
549
550 if ((SyncFlags & 2) && _Responsible == NULL) {
551 Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
552 }
553
554 // park self
555 if (_Responsible == Self || (SyncFlags & 1)) {
556 TEVENT(Inflated enter - park TIMED);
557 Self->_ParkEvent->park((jlong) recheckInterval);
558 // Increase the recheckInterval, but clamp the value.
559 recheckInterval *= 8;
560 if (recheckInterval > MAX_RECHECK_INTERVAL) {
561 recheckInterval = MAX_RECHECK_INTERVAL;
562 }
563 } else {
564 TEVENT(Inflated enter - park UNTIMED);
565 Self->_ParkEvent->park();
566 }
567
568 if (TryLock(Self) > 0) break;
569
570 // The lock is still contested.
571 // Keep a tally of the # of futile wakeups.
572 // Note that the counter is not protected by a lock or updated by atomics.
573 // That is by design - we trade "lossy" counters which are exposed to
574 // races during updates for a lower probe effect.
575 TEVENT(Inflated enter - Futile wakeup);
576 // This PerfData object can be used in parallel with a safepoint.
577 // See the work around in PerfDataManager::destroy().
578 OM_PERFDATA_OP(FutileWakeups, inc());
579 ++nWakeups;
580
581 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
582 // We can defer clearing _succ until after the spin completes
583 // TrySpin() must tolerate being called with _succ == Self.
584 // Try yet another round of adaptive spinning.
585 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
586
587 // We can find that we were unpark()ed and redesignated _succ while
588 // we were spinning. That's harmless. If we iterate and call park(),
589 // park() will consume the event and return immediately and we'll
590 // just spin again. This pattern can repeat, leaving _succ to simply
591 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
592 // Alternately, we can sample fired() here, and if set, forgo spinning
593 // in the next iteration.
594
595 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
596 Self->_ParkEvent->reset();
597 OrderAccess::fence();
598 }
599 if (_succ == Self) _succ = NULL;
600
601 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
602 OrderAccess::fence();
603 }
604
605 // Egress :
606 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
607 // Normally we'll find Self on the EntryList .
608 // From the perspective of the lock owner (this thread), the
609 // EntryList is stable and cxq is prepend-only.
610 // The head of cxq is volatile but the interior is stable.
611 // In addition, Self.TState is stable.
612
613 assert(_owner == Self, "invariant");
614 assert(object() != NULL, "invariant");
615 // I'd like to write:
616 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
617 // but as we're at a safepoint that's not safe.
618
619 UnlinkAfterAcquire(Self, &node);
620 if (_succ == Self) _succ = NULL;
621
622 assert(_succ != Self, "invariant");
623 if (_Responsible == Self) {
624 _Responsible = NULL;
625 OrderAccess::fence(); // Dekker pivot-point
626
627 // We may leave threads on cxq|EntryList without a designated
628 // "Responsible" thread. This is benign. When this thread subsequently
629 // exits the monitor it can "see" such preexisting "old" threads --
630 // threads that arrived on the cxq|EntryList before the fence, above --
631 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
632 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
633 // non-null and elect a new "Responsible" timer thread.
634 //
635 // This thread executes:
636 // ST Responsible=null; MEMBAR (in enter epilogue - here)
637 // LD cxq|EntryList (in subsequent exit)
638 //
639 // Entering threads in the slow/contended path execute:
640 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
641 // The (ST cxq; MEMBAR) is accomplished with CAS().
642 //
643 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
644 // exit operation from floating above the ST Responsible=null.
645 }
646
647 // We've acquired ownership with CAS().
648 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
649 // But since the CAS() this thread may have also stored into _succ,
650 // EntryList, cxq or Responsible. These meta-data updates must be
651 // visible __before this thread subsequently drops the lock.
652 // Consider what could occur if we didn't enforce this constraint --
653 // STs to monitor meta-data and user-data could reorder with (become
654 // visible after) the ST in exit that drops ownership of the lock.
655 // Some other thread could then acquire the lock, but observe inconsistent
656 // or old monitor meta-data and heap data. That violates the JMM.
657 // To that end, the 1-0 exit() operation must have at least STST|LDST
658 // "release" barrier semantics. Specifically, there must be at least a
659 // STST|LDST barrier in exit() before the ST of null into _owner that drops
660 // the lock. The barrier ensures that changes to monitor meta-data and data
661 // protected by the lock will be visible before we release the lock, and
662 // therefore before some other thread (CPU) has a chance to acquire the lock.
663 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
664 //
665 // Critically, any prior STs to _succ or EntryList must be visible before
666 // the ST of null into _owner in the *subsequent* (following) corresponding
667 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
668 // execute a serializing instruction.
669
670 if (SyncFlags & 8) {
671 OrderAccess::fence();
672 }
673 return;
674 }
675
676 // ReenterI() is a specialized inline form of the latter half of the
677 // contended slow-path from EnterI(). We use ReenterI() only for
678 // monitor reentry in wait().
679 //
680 // In the future we should reconcile EnterI() and ReenterI(), adding
681 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
682 // loop accordingly.
683
684 void NOINLINE ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
685 assert(Self != NULL, "invariant");
686 assert(SelfNode != NULL, "invariant");
687 assert(SelfNode->_thread == Self, "invariant");
688 assert(_waiters > 0, "invariant");
689 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
690 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
691 JavaThread * jt = (JavaThread *) Self;
692
693 int nWakeups = 0;
694 for (;;) {
695 ObjectWaiter::TStates v = SelfNode->TState;
696 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
697 assert(_owner != Self, "invariant");
698
699 if (TryLock(Self) > 0) break;
700 if (TrySpin(Self) > 0) break;
701
702 TEVENT(Wait Reentry - parking);
703
704 // State transition wrappers around park() ...
705 // ReenterI() wisely defers state transitions until
706 // it's clear we must park the thread.
707 {
708 OSThreadContendState osts(Self->osthread());
709 ThreadBlockInVM tbivm(jt);
710
711 // cleared by handle_special_suspend_equivalent_condition()
712 // or java_suspend_self()
713 jt->set_suspend_equivalent();
714 if (SyncFlags & 1) {
715 Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL);
716 } else {
717 Self->_ParkEvent->park();
718 }
719
720 // were we externally suspended while we were waiting?
721 for (;;) {
722 if (!ExitSuspendEquivalent(jt)) break;
723 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
724 jt->java_suspend_self();
725 jt->set_suspend_equivalent();
726 }
727 }
728
729 // Try again, but just so we distinguish between futile wakeups and
730 // successful wakeups. The following test isn't algorithmically
731 // necessary, but it helps us maintain sensible statistics.
732 if (TryLock(Self) > 0) break;
733
734 // The lock is still contested.
735 // Keep a tally of the # of futile wakeups.
736 // Note that the counter is not protected by a lock or updated by atomics.
737 // That is by design - we trade "lossy" counters which are exposed to
738 // races during updates for a lower probe effect.
739 TEVENT(Wait Reentry - futile wakeup);
740 ++nWakeups;
741
742 // Assuming this is not a spurious wakeup we'll normally
743 // find that _succ == Self.
744 if (_succ == Self) _succ = NULL;
745
746 // Invariant: after clearing _succ a contending thread
747 // *must* retry _owner before parking.
748 OrderAccess::fence();
749
750 // This PerfData object can be used in parallel with a safepoint.
751 // See the work around in PerfDataManager::destroy().
752 OM_PERFDATA_OP(FutileWakeups, inc());
753 }
754
755 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
756 // Normally we'll find Self on the EntryList.
757 // Unlinking from the EntryList is constant-time and atomic-free.
758 // From the perspective of the lock owner (this thread), the
759 // EntryList is stable and cxq is prepend-only.
760 // The head of cxq is volatile but the interior is stable.
761 // In addition, Self.TState is stable.
762
763 assert(_owner == Self, "invariant");
764 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
765 UnlinkAfterAcquire(Self, SelfNode);
766 if (_succ == Self) _succ = NULL;
767 assert(_succ != Self, "invariant");
768 SelfNode->TState = ObjectWaiter::TS_RUN;
769 OrderAccess::fence(); // see comments at the end of EnterI()
770 }
771
772 // By convention we unlink a contending thread from EntryList|cxq immediately
773 // after the thread acquires the lock in ::enter(). Equally, we could defer
774 // unlinking the thread until ::exit()-time.
775
776 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
777 assert(_owner == Self, "invariant");
778 assert(SelfNode->_thread == Self, "invariant");
779
780 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
781 // Normal case: remove Self from the DLL EntryList .
782 // This is a constant-time operation.
783 ObjectWaiter * nxt = SelfNode->_next;
784 ObjectWaiter * prv = SelfNode->_prev;
785 if (nxt != NULL) nxt->_prev = prv;
786 if (prv != NULL) prv->_next = nxt;
787 if (SelfNode == _EntryList) _EntryList = nxt;
788 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
789 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
790 TEVENT(Unlink from EntryList);
791 } else {
792 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
793 // Inopportune interleaving -- Self is still on the cxq.
794 // This usually means the enqueue of self raced an exiting thread.
795 // Normally we'll find Self near the front of the cxq, so
796 // dequeueing is typically fast. If needbe we can accelerate
797 // this with some MCS/CHL-like bidirectional list hints and advisory
798 // back-links so dequeueing from the interior will normally operate
799 // in constant-time.
800 // Dequeue Self from either the head (with CAS) or from the interior
801 // with a linear-time scan and normal non-atomic memory operations.
802 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
803 // and then unlink Self from EntryList. We have to drain eventually,
804 // so it might as well be now.
805
806 ObjectWaiter * v = _cxq;
807 assert(v != NULL, "invariant");
808 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
809 // The CAS above can fail from interference IFF a "RAT" arrived.
810 // In that case Self must be in the interior and can no longer be
811 // at the head of cxq.
812 if (v == SelfNode) {
813 assert(_cxq != v, "invariant");
814 v = _cxq; // CAS above failed - start scan at head of list
815 }
816 ObjectWaiter * p;
817 ObjectWaiter * q = NULL;
818 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
819 q = p;
820 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
821 }
822 assert(v != SelfNode, "invariant");
823 assert(p == SelfNode, "Node not found on cxq");
824 assert(p != _cxq, "invariant");
825 assert(q != NULL, "invariant");
826 assert(q->_next == p, "invariant");
827 q->_next = p->_next;
828 }
829 TEVENT(Unlink from cxq);
830 }
831
832 #ifdef ASSERT
833 // Diagnostic hygiene ...
834 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
835 SelfNode->_next = (ObjectWaiter *) 0xBAD;
836 SelfNode->TState = ObjectWaiter::TS_RUN;
837 #endif
838 }
839
840 // -----------------------------------------------------------------------------
841 // Exit support
842 //
843 // exit()
844 // ~~~~~~
845 // Note that the collector can't reclaim the objectMonitor or deflate
846 // the object out from underneath the thread calling ::exit() as the
847 // thread calling ::exit() never transitions to a stable state.
848 // This inhibits GC, which in turn inhibits asynchronous (and
849 // inopportune) reclamation of "this".
850 //
851 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
852 // There's one exception to the claim above, however. EnterI() can call
853 // exit() to drop a lock if the acquirer has been externally suspended.
854 // In that case exit() is called with _thread_state as _thread_blocked,
855 // but the monitor's _count field is > 0, which inhibits reclamation.
856 //
857 // 1-0 exit
858 // ~~~~~~~~
859 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
860 // the fast-path operators have been optimized so the common ::exit()
861 // operation is 1-0. See i486.ad fast_unlock(), for instance.
862 // The code emitted by fast_unlock() elides the usual MEMBAR. This
863 // greatly improves latency -- MEMBAR and CAS having considerable local
864 // latency on modern processors -- but at the cost of "stranding". Absent the
865 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
866 // ::enter() path, resulting in the entering thread being stranding
867 // and a progress-liveness failure. Stranding is extremely rare.
868 // We use timers (timed park operations) & periodic polling to detect
869 // and recover from stranding. Potentially stranded threads periodically
870 // wake up and poll the lock. See the usage of the _Responsible variable.
871 //
872 // The CAS() in enter provides for safety and exclusion, while the CAS or
873 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
874 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
875 // We detect and recover from stranding with timers.
876 //
877 // If a thread transiently strands it'll park until (a) another
878 // thread acquires the lock and then drops the lock, at which time the
879 // exiting thread will notice and unpark the stranded thread, or, (b)
880 // the timer expires. If the lock is high traffic then the stranding latency
881 // will be low due to (a). If the lock is low traffic then the odds of
882 // stranding are lower, although the worst-case stranding latency
883 // is longer. Critically, we don't want to put excessive load in the
884 // platform's timer subsystem. We want to minimize both the timer injection
885 // rate (timers created/sec) as well as the number of timers active at
886 // any one time. (more precisely, we want to minimize timer-seconds, which is
887 // the integral of the # of active timers at any instant over time).
888 // Both impinge on OS scalability. Given that, at most one thread parked on
889 // a monitor will use a timer.
890 //
891 // There is also the risk of a futile wake-up. If we drop the lock
892 // another thread can reacquire the lock immediately, and we can
893 // then wake a thread unnecessarily. This is benign, and we've
894 // structured the code so the windows are short and the frequency
895 // of such futile wakups is low.
896
897 void NOINLINE ObjectMonitor::exit(bool not_suspended, TRAPS) {
898 Thread * const Self = THREAD;
899 if (THREAD != _owner) {
900 if (THREAD->is_lock_owned((address) _owner)) {
901 // Transmute _owner from a BasicLock pointer to a Thread address.
902 // We don't need to hold _mutex for this transition.
903 // Non-null to Non-null is safe as long as all readers can
904 // tolerate either flavor.
905 assert(_recursions == 0, "invariant");
906 _owner = THREAD;
907 _recursions = 0;
908 } else {
909 // Apparent unbalanced locking ...
910 // Naively we'd like to throw IllegalMonitorStateException.
911 // As a practical matter we can neither allocate nor throw an
912 // exception as ::exit() can be called from leaf routines.
913 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
914 // Upon deeper reflection, however, in a properly run JVM the only
915 // way we should encounter this situation is in the presence of
916 // unbalanced JNI locking. TODO: CheckJNICalls.
917 // See also: CR4414101
918 TEVENT(Exit - Throw IMSX);
919 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
920 return;
921 }
922 }
923
924 if (_recursions != 0) {
925 _recursions--; // this is simple recursive enter
926 TEVENT(Inflated exit - recursive);
927 return;
928 }
929
930 // Invariant: after setting Responsible=null an thread must execute
931 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
932 if ((SyncFlags & 4) == 0) {
933 _Responsible = NULL;
934 }
935
936 #if INCLUDE_TRACE
937 // get the owner's thread id for the MonitorEnter event
938 // if it is enabled and the thread isn't suspended
939 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
940 _previous_owner_tid = THREAD_TRACE_ID(Self);
941 }
942 #endif
943
944 for (;;) {
945 assert(THREAD == _owner, "invariant");
946
947
948 if (Knob_ExitPolicy == 0) {
949 // release semantics: prior loads and stores from within the critical section
950 // must not float (reorder) past the following store that drops the lock.
951 // On SPARC that requires MEMBAR #loadstore|#storestore.
952 // But of course in TSO #loadstore|#storestore is not required.
953 // I'd like to write one of the following:
954 // A. OrderAccess::release() ; _owner = NULL
955 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
956 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
957 // store into a _dummy variable. That store is not needed, but can result
958 // in massive wasteful coherency traffic on classic SMP systems.
959 // Instead, I use release_store(), which is implemented as just a simple
960 // ST on x64, x86 and SPARC.
961 OrderAccess::release_store_ptr(&_owner, NULL); // drop the lock
962 OrderAccess::storeload(); // See if we need to wake a successor
963 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
964 TEVENT(Inflated exit - simple egress);
965 return;
966 }
967 TEVENT(Inflated exit - complex egress);
968 // Other threads are blocked trying to acquire the lock.
969
970 // Normally the exiting thread is responsible for ensuring succession,
971 // but if other successors are ready or other entering threads are spinning
972 // then this thread can simply store NULL into _owner and exit without
973 // waking a successor. The existence of spinners or ready successors
974 // guarantees proper succession (liveness). Responsibility passes to the
975 // ready or running successors. The exiting thread delegates the duty.
976 // More precisely, if a successor already exists this thread is absolved
977 // of the responsibility of waking (unparking) one.
978 //
979 // The _succ variable is critical to reducing futile wakeup frequency.
980 // _succ identifies the "heir presumptive" thread that has been made
981 // ready (unparked) but that has not yet run. We need only one such
982 // successor thread to guarantee progress.
983 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
984 // section 3.3 "Futile Wakeup Throttling" for details.
985 //
986 // Note that spinners in Enter() also set _succ non-null.
987 // In the current implementation spinners opportunistically set
988 // _succ so that exiting threads might avoid waking a successor.
989 // Another less appealing alternative would be for the exiting thread
990 // to drop the lock and then spin briefly to see if a spinner managed
991 // to acquire the lock. If so, the exiting thread could exit
992 // immediately without waking a successor, otherwise the exiting
993 // thread would need to dequeue and wake a successor.
994 // (Note that we'd need to make the post-drop spin short, but no
995 // shorter than the worst-case round-trip cache-line migration time.
996 // The dropped lock needs to become visible to the spinner, and then
997 // the acquisition of the lock by the spinner must become visible to
998 // the exiting thread).
999
1000 // It appears that an heir-presumptive (successor) must be made ready.
1001 // Only the current lock owner can manipulate the EntryList or
1002 // drain _cxq, so we need to reacquire the lock. If we fail
1003 // to reacquire the lock the responsibility for ensuring succession
1004 // falls to the new owner.
1005 //
1006 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1007 return;
1008 }
1009 TEVENT(Exit - Reacquired);
1010 } else {
1011 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1012 OrderAccess::release_store_ptr(&_owner, NULL); // drop the lock
1013 OrderAccess::storeload();
1014 // Ratify the previously observed values.
1015 if (_cxq == NULL || _succ != NULL) {
1016 TEVENT(Inflated exit - simple egress);
1017 return;
1018 }
1019
1020 // inopportune interleaving -- the exiting thread (this thread)
1021 // in the fast-exit path raced an entering thread in the slow-enter
1022 // path.
1023 // We have two choices:
1024 // A. Try to reacquire the lock.
1025 // If the CAS() fails return immediately, otherwise
1026 // we either restart/rerun the exit operation, or simply
1027 // fall-through into the code below which wakes a successor.
1028 // B. If the elements forming the EntryList|cxq are TSM
1029 // we could simply unpark() the lead thread and return
1030 // without having set _succ.
1031 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1032 TEVENT(Inflated exit - reacquired succeeded);
1033 return;
1034 }
1035 TEVENT(Inflated exit - reacquired failed);
1036 } else {
1037 TEVENT(Inflated exit - complex egress);
1038 }
1039 }
1040
1041 guarantee(_owner == THREAD, "invariant");
1042
1043 ObjectWaiter * w = NULL;
1044 int QMode = Knob_QMode;
1045
1046 if (QMode == 2 && _cxq != NULL) {
1047 // QMode == 2 : cxq has precedence over EntryList.
1048 // Try to directly wake a successor from the cxq.
1049 // If successful, the successor will need to unlink itself from cxq.
1050 w = _cxq;
1051 assert(w != NULL, "invariant");
1052 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1053 ExitEpilog(Self, w);
1054 return;
1055 }
1056
1057 if (QMode == 3 && _cxq != NULL) {
1058 // Aggressively drain cxq into EntryList at the first opportunity.
1059 // This policy ensure that recently-run threads live at the head of EntryList.
1060 // Drain _cxq into EntryList - bulk transfer.
1061 // First, detach _cxq.
1062 // The following loop is tantamount to: w = swap(&cxq, NULL)
1063 w = _cxq;
1064 for (;;) {
1065 assert(w != NULL, "Invariant");
1066 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1067 if (u == w) break;
1068 w = u;
1069 }
1070 assert(w != NULL, "invariant");
1071
1072 ObjectWaiter * q = NULL;
1073 ObjectWaiter * p;
1074 for (p = w; p != NULL; p = p->_next) {
1075 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1076 p->TState = ObjectWaiter::TS_ENTER;
1077 p->_prev = q;
1078 q = p;
1079 }
1080
1081 // Append the RATs to the EntryList
1082 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1083 ObjectWaiter * Tail;
1084 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1085 Tail = Tail->_next)
1086 /* empty */;
1087 if (Tail == NULL) {
1088 _EntryList = w;
1089 } else {
1090 Tail->_next = w;
1091 w->_prev = Tail;
1092 }
1093
1094 // Fall thru into code that tries to wake a successor from EntryList
1095 }
1096
1097 if (QMode == 4 && _cxq != NULL) {
1098 // Aggressively drain cxq into EntryList at the first opportunity.
1099 // This policy ensure that recently-run threads live at the head of EntryList.
1100
1101 // Drain _cxq into EntryList - bulk transfer.
1102 // First, detach _cxq.
1103 // The following loop is tantamount to: w = swap(&cxq, NULL)
1104 w = _cxq;
1105 for (;;) {
1106 assert(w != NULL, "Invariant");
1107 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1108 if (u == w) break;
1109 w = u;
1110 }
1111 assert(w != NULL, "invariant");
1112
1113 ObjectWaiter * q = NULL;
1114 ObjectWaiter * p;
1115 for (p = w; p != NULL; p = p->_next) {
1116 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1117 p->TState = ObjectWaiter::TS_ENTER;
1118 p->_prev = q;
1119 q = p;
1120 }
1121
1122 // Prepend the RATs to the EntryList
1123 if (_EntryList != NULL) {
1124 q->_next = _EntryList;
1125 _EntryList->_prev = q;
1126 }
1127 _EntryList = w;
1128
1129 // Fall thru into code that tries to wake a successor from EntryList
1130 }
1131
1132 w = _EntryList;
1133 if (w != NULL) {
1134 // I'd like to write: guarantee (w->_thread != Self).
1135 // But in practice an exiting thread may find itself on the EntryList.
1136 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1137 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1138 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1139 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1140 // release the lock "O". T2 resumes immediately after the ST of null into
1141 // _owner, above. T2 notices that the EntryList is populated, so it
1142 // reacquires the lock and then finds itself on the EntryList.
1143 // Given all that, we have to tolerate the circumstance where "w" is
1144 // associated with Self.
1145 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1146 ExitEpilog(Self, w);
1147 return;
1148 }
1149
1150 // If we find that both _cxq and EntryList are null then just
1151 // re-run the exit protocol from the top.
1152 w = _cxq;
1153 if (w == NULL) continue;
1154
1155 // Drain _cxq into EntryList - bulk transfer.
1156 // First, detach _cxq.
1157 // The following loop is tantamount to: w = swap(&cxq, NULL)
1158 for (;;) {
1159 assert(w != NULL, "Invariant");
1160 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1161 if (u == w) break;
1162 w = u;
1163 }
1164 TEVENT(Inflated exit - drain cxq into EntryList);
1165
1166 assert(w != NULL, "invariant");
1167 assert(_EntryList == NULL, "invariant");
1168
1169 // Convert the LIFO SLL anchored by _cxq into a DLL.
1170 // The list reorganization step operates in O(LENGTH(w)) time.
1171 // It's critical that this step operate quickly as
1172 // "Self" still holds the outer-lock, restricting parallelism
1173 // and effectively lengthening the critical section.
1174 // Invariant: s chases t chases u.
1175 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1176 // we have faster access to the tail.
1177
1178 if (QMode == 1) {
1179 // QMode == 1 : drain cxq to EntryList, reversing order
1180 // We also reverse the order of the list.
1181 ObjectWaiter * s = NULL;
1182 ObjectWaiter * t = w;
1183 ObjectWaiter * u = NULL;
1184 while (t != NULL) {
1185 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1186 t->TState = ObjectWaiter::TS_ENTER;
1187 u = t->_next;
1188 t->_prev = u;
1189 t->_next = s;
1190 s = t;
1191 t = u;
1192 }
1193 _EntryList = s;
1194 assert(s != NULL, "invariant");
1195 } else {
1196 // QMode == 0 or QMode == 2
1197 _EntryList = w;
1198 ObjectWaiter * q = NULL;
1199 ObjectWaiter * p;
1200 for (p = w; p != NULL; p = p->_next) {
1201 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1202 p->TState = ObjectWaiter::TS_ENTER;
1203 p->_prev = q;
1204 q = p;
1205 }
1206 }
1207
1208 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1209 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1210
1211 // See if we can abdicate to a spinner instead of waking a thread.
1212 // A primary goal of the implementation is to reduce the
1213 // context-switch rate.
1214 if (_succ != NULL) continue;
1215
1216 w = _EntryList;
1217 if (w != NULL) {
1218 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1219 ExitEpilog(Self, w);
1220 return;
1221 }
1222 }
1223 }
1224
1225 // ExitSuspendEquivalent:
1226 // A faster alternate to handle_special_suspend_equivalent_condition()
1227 //
1228 // handle_special_suspend_equivalent_condition() unconditionally
1229 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1230 // operations have high latency. Note that in ::enter() we call HSSEC
1231 // while holding the monitor, so we effectively lengthen the critical sections.
1232 //
1233 // There are a number of possible solutions:
1234 //
1235 // A. To ameliorate the problem we might also defer state transitions
1236 // to as late as possible -- just prior to parking.
1237 // Given that, we'd call HSSEC after having returned from park(),
1238 // but before attempting to acquire the monitor. This is only a
1239 // partial solution. It avoids calling HSSEC while holding the
1240 // monitor (good), but it still increases successor reacquisition latency --
1241 // the interval between unparking a successor and the time the successor
1242 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1243 // If we use this technique we can also avoid EnterI()-exit() loop
1244 // in ::enter() where we iteratively drop the lock and then attempt
1245 // to reacquire it after suspending.
1246 //
1247 // B. In the future we might fold all the suspend bits into a
1248 // composite per-thread suspend flag and then update it with CAS().
1249 // Alternately, a Dekker-like mechanism with multiple variables
1250 // would suffice:
1251 // ST Self->_suspend_equivalent = false
1252 // MEMBAR
1253 // LD Self_>_suspend_flags
1254 //
1255 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1256 // with a more efficient implementation, the need to use "FastHSSEC" has
1257 // decreased. - Dave
1258
1259
1260 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1261 const int Mode = Knob_FastHSSEC;
1262 if (Mode && !jSelf->is_external_suspend()) {
1263 assert(jSelf->is_suspend_equivalent(), "invariant");
1264 jSelf->clear_suspend_equivalent();
1265 if (2 == Mode) OrderAccess::storeload();
1266 if (!jSelf->is_external_suspend()) return false;
1267 // We raced a suspension -- fall thru into the slow path
1268 TEVENT(ExitSuspendEquivalent - raced);
1269 jSelf->set_suspend_equivalent();
1270 }
1271 return jSelf->handle_special_suspend_equivalent_condition();
1272 }
1273
1274
1275 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1276 assert(_owner == Self, "invariant");
1277
1278 // Exit protocol:
1279 // 1. ST _succ = wakee
1280 // 2. membar #loadstore|#storestore;
1281 // 2. ST _owner = NULL
1282 // 3. unpark(wakee)
1283
1284 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1285 ParkEvent * Trigger = Wakee->_event;
1286
1287 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1288 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1289 // out-of-scope (non-extant).
1290 Wakee = NULL;
1291
1292 // Drop the lock
1293 OrderAccess::release_store_ptr(&_owner, NULL);
1294 OrderAccess::fence(); // ST _owner vs LD in unpark()
1295
1296 if (SafepointSynchronize::do_call_back()) {
1297 TEVENT(unpark before SAFEPOINT);
1298 }
1299
1300 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1301 Trigger->unpark();
1302
1303 // Maintain stats and report events to JVMTI
1304 OM_PERFDATA_OP(Parks, inc());
1305 }
1306
1307
1308 // -----------------------------------------------------------------------------
1309 // Class Loader deadlock handling.
1310 //
1311 // complete_exit exits a lock returning recursion count
1312 // complete_exit/reenter operate as a wait without waiting
1313 // complete_exit requires an inflated monitor
1314 // The _owner field is not always the Thread addr even with an
1315 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1316 // thread due to contention.
1317 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1318 Thread * const Self = THREAD;
1319 assert(Self->is_Java_thread(), "Must be Java thread!");
1320 JavaThread *jt = (JavaThread *)THREAD;
1321
1322 DeferredInitialize();
1323
1324 if (THREAD != _owner) {
1325 if (THREAD->is_lock_owned ((address)_owner)) {
1326 assert(_recursions == 0, "internal state error");
1327 _owner = THREAD; // Convert from basiclock addr to Thread addr
1328 _recursions = 0;
1329 }
1330 }
1331
1332 guarantee(Self == _owner, "complete_exit not owner");
1333 intptr_t save = _recursions; // record the old recursion count
1334 _recursions = 0; // set the recursion level to be 0
1335 exit(true, Self); // exit the monitor
1336 guarantee(_owner != Self, "invariant");
1337 return save;
1338 }
1339
1340 // reenter() enters a lock and sets recursion count
1341 // complete_exit/reenter operate as a wait without waiting
1342 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1343 Thread * const Self = THREAD;
1344 assert(Self->is_Java_thread(), "Must be Java thread!");
1345 JavaThread *jt = (JavaThread *)THREAD;
1346
1347 guarantee(_owner != Self, "reenter already owner");
1348 enter(THREAD); // enter the monitor
1349 guarantee(_recursions == 0, "reenter recursion");
1350 _recursions = recursions;
1351 return;
1352 }
1353
1354
1355 // -----------------------------------------------------------------------------
1356 // A macro is used below because there may already be a pending
1357 // exception which should not abort the execution of the routines
1358 // which use this (which is why we don't put this into check_slow and
1359 // call it with a CHECK argument).
1360
1361 #define CHECK_OWNER() \
1362 do { \
1363 if (THREAD != _owner) { \
1364 if (THREAD->is_lock_owned((address) _owner)) { \
1365 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1366 _recursions = 0; \
1367 } else { \
1368 TEVENT(Throw IMSX); \
1369 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1370 } \
1371 } \
1372 } while (false)
1373
1374 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1375 // TODO-FIXME: remove check_slow() -- it's likely dead.
1376
1377 void ObjectMonitor::check_slow(TRAPS) {
1378 TEVENT(check_slow - throw IMSX);
1379 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1380 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1381 }
1382
1383 static int Adjust(volatile int * adr, int dx) {
1384 int v;
1385 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1386 return v;
1387 }
1388
1389 // helper method for posting a monitor wait event
1390 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1391 jlong notifier_tid,
1392 jlong timeout,
1393 bool timedout) {
1394 assert(event != NULL, "invariant");
1395 event->set_klass(((oop)this->object())->klass());
1396 event->set_timeout(timeout);
1397 event->set_address((TYPE_ADDRESS)this->object_addr());
1398 event->set_notifier(notifier_tid);
1399 event->set_timedOut(timedout);
1400 event->commit();
1401 }
1402
1403 // -----------------------------------------------------------------------------
1404 // Wait/Notify/NotifyAll
1405 //
1406 // Note: a subset of changes to ObjectMonitor::wait()
1407 // will need to be replicated in complete_exit
1408 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1409 Thread * const Self = THREAD;
1410 assert(Self->is_Java_thread(), "Must be Java thread!");
1411 JavaThread *jt = (JavaThread *)THREAD;
1412
1413 DeferredInitialize();
1414
1415 // Throw IMSX or IEX.
1416 CHECK_OWNER();
1417
1418 EventJavaMonitorWait event;
1419
1420 // check for a pending interrupt
1421 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1422 // post monitor waited event. Note that this is past-tense, we are done waiting.
1423 if (JvmtiExport::should_post_monitor_waited()) {
1424 // Note: 'false' parameter is passed here because the
1425 // wait was not timed out due to thread interrupt.
1426 JvmtiExport::post_monitor_waited(jt, this, false);
1427
1428 // In this short circuit of the monitor wait protocol, the
1429 // current thread never drops ownership of the monitor and
1430 // never gets added to the wait queue so the current thread
1431 // cannot be made the successor. This means that the
1432 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1433 // consume an unpark() meant for the ParkEvent associated with
1434 // this ObjectMonitor.
1435 }
1436 if (event.should_commit()) {
1437 post_monitor_wait_event(&event, 0, millis, false);
1438 }
1439 TEVENT(Wait - Throw IEX);
1440 THROW(vmSymbols::java_lang_InterruptedException());
1441 return;
1442 }
1443
1444 TEVENT(Wait);
1445
1446 assert(Self->_Stalled == 0, "invariant");
1447 Self->_Stalled = intptr_t(this);
1448 jt->set_current_waiting_monitor(this);
1449
1450 // create a node to be put into the queue
1451 // Critically, after we reset() the event but prior to park(), we must check
1452 // for a pending interrupt.
1453 ObjectWaiter node(Self);
1454 node.TState = ObjectWaiter::TS_WAIT;
1455 Self->_ParkEvent->reset();
1456 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1457
1458 // Enter the waiting queue, which is a circular doubly linked list in this case
1459 // but it could be a priority queue or any data structure.
1460 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1461 // by the the owner of the monitor *except* in the case where park()
1462 // returns because of a timeout of interrupt. Contention is exceptionally rare
1463 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1464
1465 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1466 AddWaiter(&node);
1467 Thread::SpinRelease(&_WaitSetLock);
1468
1469 if ((SyncFlags & 4) == 0) {
1470 _Responsible = NULL;
1471 }
1472 intptr_t save = _recursions; // record the old recursion count
1473 _waiters++; // increment the number of waiters
1474 _recursions = 0; // set the recursion level to be 1
1475 exit(true, Self); // exit the monitor
1476 guarantee(_owner != Self, "invariant");
1477
1478 // The thread is on the WaitSet list - now park() it.
1479 // On MP systems it's conceivable that a brief spin before we park
1480 // could be profitable.
1481 //
1482 // TODO-FIXME: change the following logic to a loop of the form
1483 // while (!timeout && !interrupted && _notified == 0) park()
1484
1485 int ret = OS_OK;
1486 int WasNotified = 0;
1487 { // State transition wrappers
1488 OSThread* osthread = Self->osthread();
1489 OSThreadWaitState osts(osthread, true);
1490 {
1491 ThreadBlockInVM tbivm(jt);
1492 // Thread is in thread_blocked state and oop access is unsafe.
1493 jt->set_suspend_equivalent();
1494
1495 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1496 // Intentionally empty
1497 } else if (node._notified == 0) {
1498 if (millis <= 0) {
1499 Self->_ParkEvent->park();
1500 } else {
1501 ret = Self->_ParkEvent->park(millis);
1502 }
1503 }
1504
1505 // were we externally suspended while we were waiting?
1506 if (ExitSuspendEquivalent (jt)) {
1507 // TODO-FIXME: add -- if succ == Self then succ = null.
1508 jt->java_suspend_self();
1509 }
1510
1511 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1512
1513 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1514 // from the WaitSet to the EntryList.
1515 // See if we need to remove Node from the WaitSet.
1516 // We use double-checked locking to avoid grabbing _WaitSetLock
1517 // if the thread is not on the wait queue.
1518 //
1519 // Note that we don't need a fence before the fetch of TState.
1520 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1521 // written by the is thread. (perhaps the fetch might even be satisfied
1522 // by a look-aside into the processor's own store buffer, although given
1523 // the length of the code path between the prior ST and this load that's
1524 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1525 // then we'll acquire the lock and then re-fetch a fresh TState value.
1526 // That is, we fail toward safety.
1527
1528 if (node.TState == ObjectWaiter::TS_WAIT) {
1529 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1530 if (node.TState == ObjectWaiter::TS_WAIT) {
1531 DequeueSpecificWaiter(&node); // unlink from WaitSet
1532 assert(node._notified == 0, "invariant");
1533 node.TState = ObjectWaiter::TS_RUN;
1534 }
1535 Thread::SpinRelease(&_WaitSetLock);
1536 }
1537
1538 // The thread is now either on off-list (TS_RUN),
1539 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1540 // The Node's TState variable is stable from the perspective of this thread.
1541 // No other threads will asynchronously modify TState.
1542 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1543 OrderAccess::loadload();
1544 if (_succ == Self) _succ = NULL;
1545 WasNotified = node._notified;
1546
1547 // Reentry phase -- reacquire the monitor.
1548 // re-enter contended monitor after object.wait().
1549 // retain OBJECT_WAIT state until re-enter successfully completes
1550 // Thread state is thread_in_vm and oop access is again safe,
1551 // although the raw address of the object may have changed.
1552 // (Don't cache naked oops over safepoints, of course).
1553
1554 // post monitor waited event. Note that this is past-tense, we are done waiting.
1555 if (JvmtiExport::should_post_monitor_waited()) {
1556 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1557
1558 if (node._notified != 0 && _succ == Self) {
1559 // In this part of the monitor wait-notify-reenter protocol it
1560 // is possible (and normal) for another thread to do a fastpath
1561 // monitor enter-exit while this thread is still trying to get
1562 // to the reenter portion of the protocol.
1563 //
1564 // The ObjectMonitor was notified and the current thread is
1565 // the successor which also means that an unpark() has already
1566 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1567 // consume the unpark() that was done when the successor was
1568 // set because the same ParkEvent is shared between Java
1569 // monitors and JVM/TI RawMonitors (for now).
1570 //
1571 // We redo the unpark() to ensure forward progress, i.e., we
1572 // don't want all pending threads hanging (parked) with none
1573 // entering the unlocked monitor.
1574 node._event->unpark();
1575 }
1576 }
1577
1578 if (event.should_commit()) {
1579 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1580 }
1581
1582 OrderAccess::fence();
1583
1584 assert(Self->_Stalled != 0, "invariant");
1585 Self->_Stalled = 0;
1586
1587 assert(_owner != Self, "invariant");
1588 ObjectWaiter::TStates v = node.TState;
1589 if (v == ObjectWaiter::TS_RUN) {
1590 enter(Self);
1591 } else {
1592 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1593 ReenterI(Self, &node);
1594 node.wait_reenter_end(this);
1595 }
1596
1597 // Self has reacquired the lock.
1598 // Lifecycle - the node representing Self must not appear on any queues.
1599 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1600 // want residual elements associated with this thread left on any lists.
1601 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1602 assert(_owner == Self, "invariant");
1603 assert(_succ != Self, "invariant");
1604 } // OSThreadWaitState()
1605
1606 jt->set_current_waiting_monitor(NULL);
1607
1608 guarantee(_recursions == 0, "invariant");
1609 _recursions = save; // restore the old recursion count
1610 _waiters--; // decrement the number of waiters
1611
1612 // Verify a few postconditions
1613 assert(_owner == Self, "invariant");
1614 assert(_succ != Self, "invariant");
1615 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1616
1617 if (SyncFlags & 32) {
1618 OrderAccess::fence();
1619 }
1620
1621 // check if the notification happened
1622 if (!WasNotified) {
1623 // no, it could be timeout or Thread.interrupt() or both
1624 // check for interrupt event, otherwise it is timeout
1625 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1626 TEVENT(Wait - throw IEX from epilog);
1627 THROW(vmSymbols::java_lang_InterruptedException());
1628 }
1629 }
1630
1631 // NOTE: Spurious wake up will be consider as timeout.
1632 // Monitor notify has precedence over thread interrupt.
1633 }
1634
1635
1636 // Consider:
1637 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1638 // then instead of transferring a thread from the WaitSet to the EntryList
1639 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1640
1641 void ObjectMonitor::INotify(Thread * Self) {
1642 const int policy = Knob_MoveNotifyee;
1643
1644 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1645 ObjectWaiter * iterator = DequeueWaiter();
1646 if (iterator != NULL) {
1647 TEVENT(Notify1 - Transfer);
1648 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1649 guarantee(iterator->_notified == 0, "invariant");
1650 // Disposition - what might we do with iterator ?
1651 // a. add it directly to the EntryList - either tail (policy == 1)
1652 // or head (policy == 0).
1653 // b. push it onto the front of the _cxq (policy == 2).
1654 // For now we use (b).
1655 if (policy != 4) {
1656 iterator->TState = ObjectWaiter::TS_ENTER;
1657 }
1658 iterator->_notified = 1;
1659 iterator->_notifier_tid = THREAD_TRACE_ID(Self);
1660
1661 ObjectWaiter * list = _EntryList;
1662 if (list != NULL) {
1663 assert(list->_prev == NULL, "invariant");
1664 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1665 assert(list != iterator, "invariant");
1666 }
1667
1668 if (policy == 0) { // prepend to EntryList
1669 if (list == NULL) {
1670 iterator->_next = iterator->_prev = NULL;
1671 _EntryList = iterator;
1672 } else {
1673 list->_prev = iterator;
1674 iterator->_next = list;
1675 iterator->_prev = NULL;
1676 _EntryList = iterator;
1677 }
1678 } else if (policy == 1) { // append to EntryList
1679 if (list == NULL) {
1680 iterator->_next = iterator->_prev = NULL;
1681 _EntryList = iterator;
1682 } else {
1683 // CONSIDER: finding the tail currently requires a linear-time walk of
1684 // the EntryList. We can make tail access constant-time by converting to
1685 // a CDLL instead of using our current DLL.
1686 ObjectWaiter * tail;
1687 for (tail = list; tail->_next != NULL; tail = tail->_next) /* empty */;
1688 assert(tail != NULL && tail->_next == NULL, "invariant");
1689 tail->_next = iterator;
1690 iterator->_prev = tail;
1691 iterator->_next = NULL;
1692 }
1693 } else if (policy == 2) { // prepend to cxq
1694 if (list == NULL) {
1695 iterator->_next = iterator->_prev = NULL;
1696 _EntryList = iterator;
1697 } else {
1698 iterator->TState = ObjectWaiter::TS_CXQ;
1699 for (;;) {
1700 ObjectWaiter * front = _cxq;
1701 iterator->_next = front;
1702 if (Atomic::cmpxchg_ptr(iterator, &_cxq, front) == front) {
1703 break;
1704 }
1705 }
1706 }
1707 } else if (policy == 3) { // append to cxq
1708 iterator->TState = ObjectWaiter::TS_CXQ;
1709 for (;;) {
1710 ObjectWaiter * tail = _cxq;
1711 if (tail == NULL) {
1712 iterator->_next = NULL;
1713 if (Atomic::cmpxchg_ptr(iterator, &_cxq, NULL) == NULL) {
1714 break;
1715 }
1716 } else {
1717 while (tail->_next != NULL) tail = tail->_next;
1718 tail->_next = iterator;
1719 iterator->_prev = tail;
1720 iterator->_next = NULL;
1721 break;
1722 }
1723 }
1724 } else {
1725 ParkEvent * ev = iterator->_event;
1726 iterator->TState = ObjectWaiter::TS_RUN;
1727 OrderAccess::fence();
1728 ev->unpark();
1729 }
1730
1731 // _WaitSetLock protects the wait queue, not the EntryList. We could
1732 // move the add-to-EntryList operation, above, outside the critical section
1733 // protected by _WaitSetLock. In practice that's not useful. With the
1734 // exception of wait() timeouts and interrupts the monitor owner
1735 // is the only thread that grabs _WaitSetLock. There's almost no contention
1736 // on _WaitSetLock so it's not profitable to reduce the length of the
1737 // critical section.
1738
1739 if (policy < 4) {
1740 iterator->wait_reenter_begin(this);
1741 }
1742 }
1743 Thread::SpinRelease(&_WaitSetLock);
1744 }
1745
1746 // Consider: a not-uncommon synchronization bug is to use notify() when
1747 // notifyAll() is more appropriate, potentially resulting in stranded
1748 // threads; this is one example of a lost wakeup. A useful diagnostic
1749 // option is to force all notify() operations to behave as notifyAll().
1750 //
1751 // Note: We can also detect many such problems with a "minimum wait".
1752 // When the "minimum wait" is set to a small non-zero timeout value
1753 // and the program does not hang whereas it did absent "minimum wait",
1754 // that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses
1755 // a "minimum wait" for all park() operations; see the recheckInterval
1756 // variable and MAX_RECHECK_INTERVAL.
1757
1758 void ObjectMonitor::notify(TRAPS) {
1759 CHECK_OWNER();
1760 if (_WaitSet == NULL) {
1761 TEVENT(Empty-Notify);
1762 return;
1763 }
1764 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1765 INotify(THREAD);
1766 OM_PERFDATA_OP(Notifications, inc(1));
1767 }
1768
1769
1770 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1771 // from the waitset to the EntryList. This could be done more efficiently with a
1772 // single bulk transfer but in practice it's not time-critical. Beware too,
1773 // that in prepend-mode we invert the order of the waiters. Let's say that the
1774 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1775 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1776
1777 void ObjectMonitor::notifyAll(TRAPS) {
1778 CHECK_OWNER();
1779 if (_WaitSet == NULL) {
1780 TEVENT(Empty-NotifyAll);
1781 return;
1782 }
1783
1784 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1785 int tally = 0;
1786 while (_WaitSet != NULL) {
1787 tally++;
1788 INotify(THREAD);
1789 }
1790
1791 OM_PERFDATA_OP(Notifications, inc(tally));
1792 }
1793
1794 // -----------------------------------------------------------------------------
1795 // Adaptive Spinning Support
1796 //
1797 // Adaptive spin-then-block - rational spinning
1798 //
1799 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1800 // algorithm. On high order SMP systems it would be better to start with
1801 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1802 // a contending thread could enqueue itself on the cxq and then spin locally
1803 // on a thread-specific variable such as its ParkEvent._Event flag.
1804 // That's left as an exercise for the reader. Note that global spinning is
1805 // not problematic on Niagara, as the L2 cache serves the interconnect and
1806 // has both low latency and massive bandwidth.
1807 //
1808 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1809 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1810 // (duration) or we can fix the count at approximately the duration of
1811 // a context switch and vary the frequency. Of course we could also
1812 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1813 // For a description of 'Adaptive spin-then-block mutual exclusion in
1814 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1815 //
1816 // This implementation varies the duration "D", where D varies with
1817 // the success rate of recent spin attempts. (D is capped at approximately
1818 // length of a round-trip context switch). The success rate for recent
1819 // spin attempts is a good predictor of the success rate of future spin
1820 // attempts. The mechanism adapts automatically to varying critical
1821 // section length (lock modality), system load and degree of parallelism.
1822 // D is maintained per-monitor in _SpinDuration and is initialized
1823 // optimistically. Spin frequency is fixed at 100%.
1824 //
1825 // Note that _SpinDuration is volatile, but we update it without locks
1826 // or atomics. The code is designed so that _SpinDuration stays within
1827 // a reasonable range even in the presence of races. The arithmetic
1828 // operations on _SpinDuration are closed over the domain of legal values,
1829 // so at worst a race will install and older but still legal value.
1830 // At the very worst this introduces some apparent non-determinism.
1831 // We might spin when we shouldn't or vice-versa, but since the spin
1832 // count are relatively short, even in the worst case, the effect is harmless.
1833 //
1834 // Care must be taken that a low "D" value does not become an
1835 // an absorbing state. Transient spinning failures -- when spinning
1836 // is overall profitable -- should not cause the system to converge
1837 // on low "D" values. We want spinning to be stable and predictable
1838 // and fairly responsive to change and at the same time we don't want
1839 // it to oscillate, become metastable, be "too" non-deterministic,
1840 // or converge on or enter undesirable stable absorbing states.
1841 //
1842 // We implement a feedback-based control system -- using past behavior
1843 // to predict future behavior. We face two issues: (a) if the
1844 // input signal is random then the spin predictor won't provide optimal
1845 // results, and (b) if the signal frequency is too high then the control
1846 // system, which has some natural response lag, will "chase" the signal.
1847 // (b) can arise from multimodal lock hold times. Transient preemption
1848 // can also result in apparent bimodal lock hold times.
1849 // Although sub-optimal, neither condition is particularly harmful, as
1850 // in the worst-case we'll spin when we shouldn't or vice-versa.
1851 // The maximum spin duration is rather short so the failure modes aren't bad.
1852 // To be conservative, I've tuned the gain in system to bias toward
1853 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1854 // "rings" or oscillates between spinning and not spinning. This happens
1855 // when spinning is just on the cusp of profitability, however, so the
1856 // situation is not dire. The state is benign -- there's no need to add
1857 // hysteresis control to damp the transition rate between spinning and
1858 // not spinning.
1859
1860 intptr_t ObjectMonitor::SpinCallbackArgument = 0;
1861 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL;
1862
1863 // Spinning: Fixed frequency (100%), vary duration
1864
1865
1866 int ObjectMonitor::TrySpin_VaryDuration(Thread * Self) {
1867 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1868 int ctr = Knob_FixedSpin;
1869 if (ctr != 0) {
1870 while (--ctr >= 0) {
1871 if (TryLock(Self) > 0) return 1;
1872 SpinPause();
1873 }
1874 return 0;
1875 }
1876
1877 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1878 if (TryLock(Self) > 0) {
1879 // Increase _SpinDuration ...
1880 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1881 // Raising _SpurDuration to the poverty line is key.
1882 int x = _SpinDuration;
1883 if (x < Knob_SpinLimit) {
1884 if (x < Knob_Poverty) x = Knob_Poverty;
1885 _SpinDuration = x + Knob_BonusB;
1886 }
1887 return 1;
1888 }
1889 SpinPause();
1890 }
1891
1892 // Admission control - verify preconditions for spinning
1893 //
1894 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1895 // becoming an absorbing state. Put another way, we spin briefly to
1896 // sample, just in case the system load, parallelism, contention, or lock
1897 // modality changed.
1898 //
1899 // Consider the following alternative:
1900 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1901 // spin attempt. "Periodically" might mean after a tally of
1902 // the # of failed spin attempts (or iterations) reaches some threshold.
1903 // This takes us into the realm of 1-out-of-N spinning, where we
1904 // hold the duration constant but vary the frequency.
1905
1906 ctr = _SpinDuration;
1907 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1908 if (ctr <= 0) return 0;
1909
1910 if (Knob_SuccRestrict && _succ != NULL) return 0;
1911 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1912 TEVENT(Spin abort - notrunnable [TOP]);
1913 return 0;
1914 }
1915
1916 int MaxSpin = Knob_MaxSpinners;
1917 if (MaxSpin >= 0) {
1918 if (_Spinner > MaxSpin) {
1919 TEVENT(Spin abort -- too many spinners);
1920 return 0;
1921 }
1922 // Slightly racy, but benign ...
1923 Adjust(&_Spinner, 1);
1924 }
1925
1926 // We're good to spin ... spin ingress.
1927 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1928 // when preparing to LD...CAS _owner, etc and the CAS is likely
1929 // to succeed.
1930 int hits = 0;
1931 int msk = 0;
1932 int caspty = Knob_CASPenalty;
1933 int oxpty = Knob_OXPenalty;
1934 int sss = Knob_SpinSetSucc;
1935 if (sss && _succ == NULL) _succ = Self;
1936 Thread * prv = NULL;
1937
1938 // There are three ways to exit the following loop:
1939 // 1. A successful spin where this thread has acquired the lock.
1940 // 2. Spin failure with prejudice
1941 // 3. Spin failure without prejudice
1942
1943 while (--ctr >= 0) {
1944
1945 // Periodic polling -- Check for pending GC
1946 // Threads may spin while they're unsafe.
1947 // We don't want spinning threads to delay the JVM from reaching
1948 // a stop-the-world safepoint or to steal cycles from GC.
1949 // If we detect a pending safepoint we abort in order that
1950 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1951 // this thread, if safe, doesn't steal cycles from GC.
1952 // This is in keeping with the "no loitering in runtime" rule.
1953 // We periodically check to see if there's a safepoint pending.
1954 if ((ctr & 0xFF) == 0) {
1955 if (SafepointSynchronize::do_call_back()) {
1956 TEVENT(Spin: safepoint);
1957 goto Abort; // abrupt spin egress
1958 }
1959 if (Knob_UsePause & 1) SpinPause();
1960
1961 int (*scb)(intptr_t,int) = SpinCallbackFunction;
1962 if (hits > 50 && scb != NULL) {
1963 int abend = (*scb)(SpinCallbackArgument, 0);
1964 }
1965 }
1966
1967 if (Knob_UsePause & 2) SpinPause();
1968
1969 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
1970 // This is useful on classic SMP systems, but is of less utility on
1971 // N1-style CMT platforms.
1972 //
1973 // Trade-off: lock acquisition latency vs coherency bandwidth.
1974 // Lock hold times are typically short. A histogram
1975 // of successful spin attempts shows that we usually acquire
1976 // the lock early in the spin. That suggests we want to
1977 // sample _owner frequently in the early phase of the spin,
1978 // but then back-off and sample less frequently as the spin
1979 // progresses. The back-off makes a good citizen on SMP big
1980 // SMP systems. Oversampling _owner can consume excessive
1981 // coherency bandwidth. Relatedly, if we _oversample _owner we
1982 // can inadvertently interfere with the the ST m->owner=null.
1983 // executed by the lock owner.
1984 if (ctr & msk) continue;
1985 ++hits;
1986 if ((hits & 0xF) == 0) {
1987 // The 0xF, above, corresponds to the exponent.
1988 // Consider: (msk+1)|msk
1989 msk = ((msk << 2)|3) & BackOffMask;
1990 }
1991
1992 // Probe _owner with TATAS
1993 // If this thread observes the monitor transition or flicker
1994 // from locked to unlocked to locked, then the odds that this
1995 // thread will acquire the lock in this spin attempt go down
1996 // considerably. The same argument applies if the CAS fails
1997 // or if we observe _owner change from one non-null value to
1998 // another non-null value. In such cases we might abort
1999 // the spin without prejudice or apply a "penalty" to the
2000 // spin count-down variable "ctr", reducing it by 100, say.
2001
2002 Thread * ox = (Thread *) _owner;
2003 if (ox == NULL) {
2004 ox = (Thread *) Atomic::cmpxchg_ptr(Self, &_owner, NULL);
2005 if (ox == NULL) {
2006 // The CAS succeeded -- this thread acquired ownership
2007 // Take care of some bookkeeping to exit spin state.
2008 if (sss && _succ == Self) {
2009 _succ = NULL;
2010 }
2011 if (MaxSpin > 0) Adjust(&_Spinner, -1);
2012
2013 // Increase _SpinDuration :
2014 // The spin was successful (profitable) so we tend toward
2015 // longer spin attempts in the future.
2016 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2017 // If we acquired the lock early in the spin cycle it
2018 // makes sense to increase _SpinDuration proportionally.
2019 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2020 int x = _SpinDuration;
2021 if (x < Knob_SpinLimit) {
2022 if (x < Knob_Poverty) x = Knob_Poverty;
2023 _SpinDuration = x + Knob_Bonus;
2024 }
2025 return 1;
2026 }
2027
2028 // The CAS failed ... we can take any of the following actions:
2029 // * penalize: ctr -= Knob_CASPenalty
2030 // * exit spin with prejudice -- goto Abort;
2031 // * exit spin without prejudice.
2032 // * Since CAS is high-latency, retry again immediately.
2033 prv = ox;
2034 TEVENT(Spin: cas failed);
2035 if (caspty == -2) break;
2036 if (caspty == -1) goto Abort;
2037 ctr -= caspty;
2038 continue;
2039 }
2040
2041 // Did lock ownership change hands ?
2042 if (ox != prv && prv != NULL) {
2043 TEVENT(spin: Owner changed)
2044 if (oxpty == -2) break;
2045 if (oxpty == -1) goto Abort;
2046 ctr -= oxpty;
2047 }
2048 prv = ox;
2049
2050 // Abort the spin if the owner is not executing.
2051 // The owner must be executing in order to drop the lock.
2052 // Spinning while the owner is OFFPROC is idiocy.
2053 // Consider: ctr -= RunnablePenalty ;
2054 if (Knob_OState && NotRunnable (Self, ox)) {
2055 TEVENT(Spin abort - notrunnable);
2056 goto Abort;
2057 }
2058 if (sss && _succ == NULL) _succ = Self;
2059 }
2060
2061 // Spin failed with prejudice -- reduce _SpinDuration.
2062 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2063 // AIMD is globally stable.
2064 TEVENT(Spin failure);
2065 {
2066 int x = _SpinDuration;
2067 if (x > 0) {
2068 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2069 // This is globally sample and tends to damp the response.
2070 x -= Knob_Penalty;
2071 if (x < 0) x = 0;
2072 _SpinDuration = x;
2073 }
2074 }
2075
2076 Abort:
2077 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2078 if (sss && _succ == Self) {
2079 _succ = NULL;
2080 // Invariant: after setting succ=null a contending thread
2081 // must recheck-retry _owner before parking. This usually happens
2082 // in the normal usage of TrySpin(), but it's safest
2083 // to make TrySpin() as foolproof as possible.
2084 OrderAccess::fence();
2085 if (TryLock(Self) > 0) return 1;
2086 }
2087 return 0;
2088 }
2089
2090 // NotRunnable() -- informed spinning
2091 //
2092 // Don't bother spinning if the owner is not eligible to drop the lock.
2093 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2094 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2095 // The thread must be runnable in order to drop the lock in timely fashion.
2096 // If the _owner is not runnable then spinning will not likely be
2097 // successful (profitable).
2098 //
2099 // Beware -- the thread referenced by _owner could have died
2100 // so a simply fetch from _owner->_thread_state might trap.
2101 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2102 // Because of the lifecycle issues the schedctl and _thread_state values
2103 // observed by NotRunnable() might be garbage. NotRunnable must
2104 // tolerate this and consider the observed _thread_state value
2105 // as advisory.
2106 //
2107 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2108 // a thread pointer.
2109 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2110 // with the LSB of _owner. Another option would be to probablistically probe
2111 // the putative _owner->TypeTag value.
2112 //
2113 // Checking _thread_state isn't perfect. Even if the thread is
2114 // in_java it might be blocked on a page-fault or have been preempted
2115 // and sitting on a ready/dispatch queue. _thread state in conjunction
2116 // with schedctl.sc_state gives us a good picture of what the
2117 // thread is doing, however.
2118 //
2119 // TODO: check schedctl.sc_state.
2120 // We'll need to use SafeFetch32() to read from the schedctl block.
2121 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2122 //
2123 // The return value from NotRunnable() is *advisory* -- the
2124 // result is based on sampling and is not necessarily coherent.
2125 // The caller must tolerate false-negative and false-positive errors.
2126 // Spinning, in general, is probabilistic anyway.
2127
2128
2129 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2130 // Check ox->TypeTag == 2BAD.
2131 if (ox == NULL) return 0;
2132
2133 // Avoid transitive spinning ...
2134 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2135 // Immediately after T1 acquires L it's possible that T2, also
2136 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2137 // This occurs transiently after T1 acquired L but before
2138 // T1 managed to clear T1.Stalled. T2 does not need to abort
2139 // its spin in this circumstance.
2140 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2141
2142 if (BlockedOn == 1) return 1;
2143 if (BlockedOn != 0) {
2144 return BlockedOn != intptr_t(this) && _owner == ox;
2145 }
2146
2147 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2148 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2149 // consider also: jst != _thread_in_Java -- but that's overspecific.
2150 return jst == _thread_blocked || jst == _thread_in_native;
2151 }
2152
2153
2154 // -----------------------------------------------------------------------------
2155 // WaitSet management ...
2156
2157 ObjectWaiter::ObjectWaiter(Thread* thread) {
2158 _next = NULL;
2159 _prev = NULL;
2160 _notified = 0;
2161 TState = TS_RUN;
2162 _thread = thread;
2163 _event = thread->_ParkEvent;
2164 _active = false;
2165 assert(_event != NULL, "invariant");
2166 }
2167
2168 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2169 JavaThread *jt = (JavaThread *)this->_thread;
2170 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2171 }
2172
2173 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2174 JavaThread *jt = (JavaThread *)this->_thread;
2175 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2176 }
2177
2178 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2179 assert(node != NULL, "should not add NULL node");
2180 assert(node->_prev == NULL, "node already in list");
2181 assert(node->_next == NULL, "node already in list");
2182 // put node at end of queue (circular doubly linked list)
2183 if (_WaitSet == NULL) {
2184 _WaitSet = node;
2185 node->_prev = node;
2186 node->_next = node;
2187 } else {
2188 ObjectWaiter* head = _WaitSet;
2189 ObjectWaiter* tail = head->_prev;
2190 assert(tail->_next == head, "invariant check");
2191 tail->_next = node;
2192 head->_prev = node;
2193 node->_next = head;
2194 node->_prev = tail;
2195 }
2196 }
2197
2198 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2199 // dequeue the very first waiter
2200 ObjectWaiter* waiter = _WaitSet;
2201 if (waiter) {
2202 DequeueSpecificWaiter(waiter);
2203 }
2204 return waiter;
2205 }
2206
2207 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2208 assert(node != NULL, "should not dequeue NULL node");
2209 assert(node->_prev != NULL, "node already removed from list");
2210 assert(node->_next != NULL, "node already removed from list");
2211 // when the waiter has woken up because of interrupt,
2212 // timeout or other spurious wake-up, dequeue the
2213 // waiter from waiting list
2214 ObjectWaiter* next = node->_next;
2215 if (next == node) {
2216 assert(node->_prev == node, "invariant check");
2217 _WaitSet = NULL;
2218 } else {
2219 ObjectWaiter* prev = node->_prev;
2220 assert(prev->_next == node, "invariant check");
2221 assert(next->_prev == node, "invariant check");
2222 next->_prev = prev;
2223 prev->_next = next;
2224 if (_WaitSet == node) {
2225 _WaitSet = next;
2226 }
2227 }
2228 node->_next = NULL;
2229 node->_prev = NULL;
2230 }
2231
2232 // -----------------------------------------------------------------------------
2233 // PerfData support
2234 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2235 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2236 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2237 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL;
2238 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2239 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL;
2240 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL;
2241 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL;
2242 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL;
2243 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL;
2244 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL;
2245 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL;
2246 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL;
2247 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL;
2248 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL;
2249 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2250 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2251 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2252
2253 // One-shot global initialization for the sync subsystem.
2254 // We could also defer initialization and initialize on-demand
2255 // the first time we call inflate(). Initialization would
2256 // be protected - like so many things - by the MonitorCache_lock.
2257
2258 void ObjectMonitor::Initialize() {
2259 static int InitializationCompleted = 0;
2260 assert(InitializationCompleted == 0, "invariant");
2261 InitializationCompleted = 1;
2262 if (UsePerfData) {
2263 EXCEPTION_MARK;
2264 #define NEWPERFCOUNTER(n) \
2265 { \
2266 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2267 CHECK); \
2268 }
2269 #define NEWPERFVARIABLE(n) \
2270 { \
2271 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2272 CHECK); \
2273 }
2274 NEWPERFCOUNTER(_sync_Inflations);
2275 NEWPERFCOUNTER(_sync_Deflations);
2276 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2277 NEWPERFCOUNTER(_sync_FutileWakeups);
2278 NEWPERFCOUNTER(_sync_Parks);
2279 NEWPERFCOUNTER(_sync_EmptyNotifications);
2280 NEWPERFCOUNTER(_sync_Notifications);
2281 NEWPERFCOUNTER(_sync_SlowEnter);
2282 NEWPERFCOUNTER(_sync_SlowExit);
2283 NEWPERFCOUNTER(_sync_SlowNotify);
2284 NEWPERFCOUNTER(_sync_SlowNotifyAll);
2285 NEWPERFCOUNTER(_sync_FailedSpins);
2286 NEWPERFCOUNTER(_sync_SuccessfulSpins);
2287 NEWPERFCOUNTER(_sync_PrivateA);
2288 NEWPERFCOUNTER(_sync_PrivateB);
2289 NEWPERFCOUNTER(_sync_MonInCirculation);
2290 NEWPERFCOUNTER(_sync_MonScavenged);
2291 NEWPERFVARIABLE(_sync_MonExtant);
2292 #undef NEWPERFCOUNTER
2293 #undef NEWPERFVARIABLE
2294 }
2295 }
2296
2297 static char * kvGet(char * kvList, const char * Key) {
2298 if (kvList == NULL) return NULL;
2299 size_t n = strlen(Key);
2300 char * Search;
2301 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2302 if (strncmp (Search, Key, n) == 0) {
2303 if (Search[n] == '=') return Search + n + 1;
2304 if (Search[n] == 0) return(char *) "1";
2305 }
2306 }
2307 return NULL;
2308 }
2309
2310 static int kvGetInt(char * kvList, const char * Key, int Default) {
2311 char * v = kvGet(kvList, Key);
2312 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2313 if (Knob_ReportSettings && v != NULL) {
2314 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ;
2315 tty->flush();
2316 }
2317 return rslt;
2318 }
2319
2320 void ObjectMonitor::DeferredInitialize() {
2321 if (InitDone > 0) return;
2322 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2323 while (InitDone != 1) /* empty */;
2324 return;
2325 }
2326
2327 // One-shot global initialization ...
2328 // The initialization is idempotent, so we don't need locks.
2329 // In the future consider doing this via os::init_2().
2330 // SyncKnobs consist of <Key>=<Value> pairs in the style
2331 // of environment variables. Start by converting ':' to NUL.
2332
2333 if (SyncKnobs == NULL) SyncKnobs = "";
2334
2335 size_t sz = strlen(SyncKnobs);
2336 char * knobs = (char *) malloc(sz + 2);
2337 if (knobs == NULL) {
2338 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2339 guarantee(0, "invariant");
2340 }
2341 strcpy(knobs, SyncKnobs);
2342 knobs[sz+1] = 0;
2343 for (char * p = knobs; *p; p++) {
2344 if (*p == ':') *p = 0;
2345 }
2346
2347 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2348 SETKNOB(ReportSettings);
2349 SETKNOB(ExitRelease);
2350 SETKNOB(Verbose);
2351 SETKNOB(VerifyInUse);
2352 SETKNOB(VerifyMatch);
2353 SETKNOB(FixedSpin);
2354 SETKNOB(SpinLimit);
2355 SETKNOB(SpinBase);
2356 SETKNOB(SpinBackOff);
2357 SETKNOB(CASPenalty);
2358 SETKNOB(OXPenalty);
2359 SETKNOB(LogSpins);
2360 SETKNOB(SpinSetSucc);
2361 SETKNOB(SuccEnabled);
2362 SETKNOB(SuccRestrict);
2363 SETKNOB(Penalty);
2364 SETKNOB(Bonus);
2365 SETKNOB(BonusB);
2366 SETKNOB(Poverty);
2367 SETKNOB(SpinAfterFutile);
2368 SETKNOB(UsePause);
2369 SETKNOB(SpinEarly);
2370 SETKNOB(OState);
2371 SETKNOB(MaxSpinners);
2372 SETKNOB(PreSpin);
2373 SETKNOB(ExitPolicy);
2374 SETKNOB(QMode);
2375 SETKNOB(ResetEvent);
2376 SETKNOB(MoveNotifyee);
2377 SETKNOB(FastHSSEC);
2378 #undef SETKNOB
2379
2380 if (Knob_Verbose) {
2381 sanity_checks();
2382 }
2383
2384 if (os::is_MP()) {
2385 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2386 if (Knob_ReportSettings) {
2387 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask);
2388 }
2389 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2390 } else {
2391 Knob_SpinLimit = 0;
2392 Knob_SpinBase = 0;
2393 Knob_PreSpin = 0;
2394 Knob_FixedSpin = -1;
2395 }
2396
2397 if (Knob_LogSpins == 0) {
2398 ObjectMonitor::_sync_FailedSpins = NULL;
2399 }
2400
2401 free(knobs);
2402 OrderAccess::fence();
2403 InitDone = 1;
2404 }
2405
2406 void ObjectMonitor::sanity_checks() {
2407 int error_cnt = 0;
2408 int warning_cnt = 0;
2409 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2410
2411 if (verbose) {
2412 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2413 sizeof(ObjectMonitor));
2414 tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT,
2415 sizeof(PaddedEnd<ObjectMonitor>));
2416 }
2417
2418 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2419 if (verbose) {
2420 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2421 }
2422
2423 ObjectMonitor dummy;
2424 u_char *addr_begin = (u_char*)&dummy;
2425 u_char *addr_header = (u_char*)&dummy._header;
2426 u_char *addr_owner = (u_char*)&dummy._owner;
2427
2428 uint offset_header = (uint)(addr_header - addr_begin);
2429 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2430
2431 uint offset_owner = (uint)(addr_owner - addr_begin);
2432 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2433
2434 if ((uint)(addr_header - addr_begin) != 0) {
2435 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2436 error_cnt++;
2437 }
2438
2439 if (cache_line_size != 0) {
2440 // We were able to determine the L1 data cache line size so
2441 // do some cache line specific sanity checks
2442
2443 if ((offset_owner - offset_header) < cache_line_size) {
2444 tty->print_cr("WARNING: the _header and _owner fields are closer "
2445 "than a cache line which permits false sharing.");
2446 warning_cnt++;
2447 }
2448
2449 if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) {
2450 tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a "
2451 "multiple of a cache line which permits false sharing.");
2452 warning_cnt++;
2453 }
2454 }
2455
2456 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2457 &warning_cnt);
2458
2459 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2460 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2461 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2462 }
2463
2464 guarantee(error_cnt == 0,
2465 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2466 }
2467
2468 #ifndef PRODUCT
2469 void ObjectMonitor::verify() {
2470 }
2471
2472 void ObjectMonitor::print() {
2473 }
2474
2475 void ObjectMonitor_test() {
2476 ObjectMonitor::sanity_checks();
2477 }
2478 #endif