1 /*
2 * Copyright (c) 1998, 2013, 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/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/osThread.hpp"
36 #include "runtime/stubRoutines.hpp"
37 #include "runtime/thread.inline.hpp"
38 #include "services/threadService.hpp"
39 #include "trace/tracing.hpp"
40 #include "trace/traceMacros.hpp"
41 #include "utilities/dtrace.hpp"
42 #include "utilities/macros.hpp"
43 #include "utilities/preserveException.hpp"
44 #ifdef TARGET_OS_FAMILY_linux
45 # include "os_linux.inline.hpp"
46 #endif
47 #ifdef TARGET_OS_FAMILY_solaris
48 # include "os_solaris.inline.hpp"
49 #endif
50 #ifdef TARGET_OS_FAMILY_windows
51 # include "os_windows.inline.hpp"
52 #endif
53 #ifdef TARGET_OS_FAMILY_bsd
54 # include "os_bsd.inline.hpp"
55 #endif
56
57 #if defined(__GNUC__) && !defined(IA64)
58 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
59 #define ATTR __attribute__((noinline))
60 #else
61 #define ATTR
62 #endif
63
64
65 #ifdef DTRACE_ENABLED
66
67 // Only bother with this argument setup if dtrace is available
68 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
69
70
71 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
72 char* bytes = NULL; \
73 int len = 0; \
74 jlong jtid = SharedRuntime::get_java_tid(thread); \
75 Symbol* klassname = ((oop)obj)->klass()->name(); \
76 if (klassname != NULL) { \
77 bytes = (char*)klassname->bytes(); \
78 len = klassname->utf8_length(); \
79 }
80
81 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
82 { \
83 if (DTraceMonitorProbes) { \
84 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
85 HOTSPOT_MONITOR_WAIT(jtid, \
86 (monitor), bytes, len, (millis)); \
87 } \
88 }
89
90 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
91 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
92 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
93 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
94 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
95
96 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
97 { \
98 if (DTraceMonitorProbes) { \
99 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
100 HOTSPOT_MONITOR_##probe(jtid, \
101 (uintptr_t)(monitor), bytes, len); \
102 } \
103 }
104
105 #else // ndef DTRACE_ENABLED
106
107 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
108 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
109
110 #endif // ndef DTRACE_ENABLED
111
112 // Tunables ...
113 // The knob* variables are effectively final. Once set they should
114 // never be modified hence. Consider using __read_mostly with GCC.
115
116 int ObjectMonitor::Knob_Verbose = 0 ;
117 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
118 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
119 static int Knob_HandOff = 0 ;
120 static int Knob_ReportSettings = 0 ;
121
122 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
123 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
124 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
125 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
126 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
127 static int Knob_SpinEarly = 1 ;
128 static int Knob_SuccEnabled = 1 ; // futile wake throttling
129 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
130 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
131 static int Knob_Bonus = 100 ; // spin success bonus
132 static int Knob_BonusB = 100 ; // spin success bonus
133 static int Knob_Penalty = 200 ; // spin failure penalty
134 static int Knob_Poverty = 1000 ;
135 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
136 static int Knob_FixedSpin = 0 ;
137 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
138 static int Knob_UsePause = 1 ;
139 static int Knob_ExitPolicy = 0 ;
140 static int Knob_PreSpin = 10 ; // 20-100 likely better
141 static int Knob_ResetEvent = 0 ;
142 static int BackOffMask = 0 ;
143
144 static int Knob_FastHSSEC = 0 ;
145 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
146 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
147 static volatile int InitDone = 0 ;
148
149 #define TrySpin TrySpin_VaryDuration
150
151 // -----------------------------------------------------------------------------
152 // Theory of operations -- Monitors lists, thread residency, etc:
153 //
154 // * A thread acquires ownership of a monitor by successfully
155 // CAS()ing the _owner field from null to non-null.
156 //
157 // * Invariant: A thread appears on at most one monitor list --
158 // cxq, EntryList or WaitSet -- at any one time.
159 //
160 // * Contending threads "push" themselves onto the cxq with CAS
161 // and then spin/park.
162 //
163 // * After a contending thread eventually acquires the lock it must
164 // dequeue itself from either the EntryList or the cxq.
165 //
166 // * The exiting thread identifies and unparks an "heir presumptive"
167 // tentative successor thread on the EntryList. Critically, the
168 // exiting thread doesn't unlink the successor thread from the EntryList.
169 // After having been unparked, the wakee will recontend for ownership of
170 // the monitor. The successor (wakee) will either acquire the lock or
171 // re-park itself.
172 //
173 // Succession is provided for by a policy of competitive handoff.
174 // The exiting thread does _not_ grant or pass ownership to the
175 // successor thread. (This is also referred to as "handoff" succession").
176 // Instead the exiting thread releases ownership and possibly wakes
177 // a successor, so the successor can (re)compete for ownership of the lock.
178 // If the EntryList is empty but the cxq is populated the exiting
179 // thread will drain the cxq into the EntryList. It does so by
180 // by detaching the cxq (installing null with CAS) and folding
181 // the threads from the cxq into the EntryList. The EntryList is
182 // doubly linked, while the cxq is singly linked because of the
183 // CAS-based "push" used to enqueue recently arrived threads (RATs).
184 //
185 // * Concurrency invariants:
186 //
187 // -- only the monitor owner may access or mutate the EntryList.
188 // The mutex property of the monitor itself protects the EntryList
189 // from concurrent interference.
190 // -- Only the monitor owner may detach the cxq.
191 //
192 // * The monitor entry list operations avoid locks, but strictly speaking
193 // they're not lock-free. Enter is lock-free, exit is not.
194 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
195 //
196 // * The cxq can have multiple concurrent "pushers" but only one concurrent
197 // detaching thread. This mechanism is immune from the ABA corruption.
198 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
199 //
200 // * Taken together, the cxq and the EntryList constitute or form a
201 // single logical queue of threads stalled trying to acquire the lock.
202 // We use two distinct lists to improve the odds of a constant-time
203 // dequeue operation after acquisition (in the ::enter() epilogue) and
204 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
205 // A key desideratum is to minimize queue & monitor metadata manipulation
206 // that occurs while holding the monitor lock -- that is, we want to
207 // minimize monitor lock holds times. Note that even a small amount of
208 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
209 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
210 // locks and monitor metadata.
211 //
212 // Cxq points to the the set of Recently Arrived Threads attempting entry.
213 // Because we push threads onto _cxq with CAS, the RATs must take the form of
214 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
215 // the unlocking thread notices that EntryList is null but _cxq is != null.
216 //
217 // The EntryList is ordered by the prevailing queue discipline and
218 // can be organized in any convenient fashion, such as a doubly-linked list or
219 // a circular doubly-linked list. Critically, we want insert and delete operations
220 // to operate in constant-time. If we need a priority queue then something akin
221 // to Solaris' sleepq would work nicely. Viz.,
222 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
223 // Queue discipline is enforced at ::exit() time, when the unlocking thread
224 // drains the cxq into the EntryList, and orders or reorders the threads on the
225 // EntryList accordingly.
226 //
227 // Barring "lock barging", this mechanism provides fair cyclic ordering,
228 // somewhat similar to an elevator-scan.
229 //
230 // * The monitor synchronization subsystem avoids the use of native
231 // synchronization primitives except for the narrow platform-specific
232 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
233 // the semantics of park-unpark. Put another way, this monitor implementation
234 // depends only on atomic operations and park-unpark. The monitor subsystem
235 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
236 // underlying OS manages the READY<->RUN transitions.
237 //
238 // * Waiting threads reside on the WaitSet list -- wait() puts
239 // the caller onto the WaitSet.
240 //
241 // * notify() or notifyAll() simply transfers threads from the WaitSet to
242 // either the EntryList or cxq. Subsequent exit() operations will
243 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
244 // it's likely the notifyee would simply impale itself on the lock held
245 // by the notifier.
246 //
247 // * An interesting alternative is to encode cxq as (List,LockByte) where
248 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
249 // variable, like _recursions, in the scheme. The threads or Events that form
250 // the list would have to be aligned in 256-byte addresses. A thread would
251 // try to acquire the lock or enqueue itself with CAS, but exiting threads
252 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
253 // Note that is is *not* word-tearing, but it does presume that full-word
254 // CAS operations are coherent with intermix with STB operations. That's true
255 // on most common processors.
256 //
257 // * See also http://blogs.sun.com/dave
258
259
260 // -----------------------------------------------------------------------------
261 // Enter support
262
263 bool ObjectMonitor::try_enter(Thread* THREAD) {
264 if (THREAD != _owner) {
265 if (THREAD->is_lock_owned ((address)_owner)) {
266 assert(_recursions == 0, "internal state error");
267 _owner = THREAD ;
268 _recursions = 1 ;
269 OwnerIsThread = 1 ;
270 return true;
271 }
272 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
273 return false;
274 }
275 return true;
276 } else {
277 _recursions++;
278 return true;
279 }
280 }
281
282 void ATTR ObjectMonitor::enter(TRAPS) {
283 // The following code is ordered to check the most common cases first
284 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
285 Thread * const Self = THREAD ;
286 void * cur ;
287
288 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
289 if (cur == NULL) {
290 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
291 assert (_recursions == 0 , "invariant") ;
292 assert (_owner == Self, "invariant") ;
293 // CONSIDER: set or assert OwnerIsThread == 1
294 return ;
295 }
296
297 if (cur == Self) {
298 // TODO-FIXME: check for integer overflow! BUGID 6557169.
299 _recursions ++ ;
300 return ;
301 }
302
303 if (Self->is_lock_owned ((address)cur)) {
304 assert (_recursions == 0, "internal state error");
305 _recursions = 1 ;
306 // Commute owner from a thread-specific on-stack BasicLockObject address to
307 // a full-fledged "Thread *".
308 _owner = Self ;
309 OwnerIsThread = 1 ;
310 return ;
311 }
312
313 // We've encountered genuine contention.
314 assert (Self->_Stalled == 0, "invariant") ;
315 Self->_Stalled = intptr_t(this) ;
316
317 // Try one round of spinning *before* enqueueing Self
318 // and before going through the awkward and expensive state
319 // transitions. The following spin is strictly optional ...
320 // Note that if we acquire the monitor from an initial spin
321 // we forgo posting JVMTI events and firing DTRACE probes.
322 if (Knob_SpinEarly && TrySpin (Self) > 0) {
323 assert (_owner == Self , "invariant") ;
324 assert (_recursions == 0 , "invariant") ;
325 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
326 Self->_Stalled = 0 ;
327 return ;
328 }
329
330 assert (_owner != Self , "invariant") ;
331 assert (_succ != Self , "invariant") ;
332 assert (Self->is_Java_thread() , "invariant") ;
333 JavaThread * jt = (JavaThread *) Self ;
334 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
335 assert (jt->thread_state() != _thread_blocked , "invariant") ;
336 assert (this->object() != NULL , "invariant") ;
337 assert (_count >= 0, "invariant") ;
338
339 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
340 // Ensure the object-monitor relationship remains stable while there's contention.
341 Atomic::inc_ptr(&_count);
342
343 EventJavaMonitorEnter event;
344
345 { // Change java thread status to indicate blocked on monitor enter.
346 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
347
348 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
349 if (JvmtiExport::should_post_monitor_contended_enter()) {
350 JvmtiExport::post_monitor_contended_enter(jt, this);
351 }
352
353 OSThreadContendState osts(Self->osthread());
354 ThreadBlockInVM tbivm(jt);
355
356 Self->set_current_pending_monitor(this);
357
358 // TODO-FIXME: change the following for(;;) loop to straight-line code.
359 for (;;) {
360 jt->set_suspend_equivalent();
361 // cleared by handle_special_suspend_equivalent_condition()
362 // or java_suspend_self()
363
364 EnterI (THREAD) ;
365
366 if (!ExitSuspendEquivalent(jt)) break ;
367
368 //
369 // We have acquired the contended monitor, but while we were
370 // waiting another thread suspended us. We don't want to enter
371 // the monitor while suspended because that would surprise the
372 // thread that suspended us.
373 //
374 _recursions = 0 ;
375 _succ = NULL ;
376 exit (false, Self) ;
377
378 jt->java_suspend_self();
379 }
380 Self->set_current_pending_monitor(NULL);
381 }
382
383 Atomic::dec_ptr(&_count);
384 assert (_count >= 0, "invariant") ;
385 Self->_Stalled = 0 ;
386
387 // Must either set _recursions = 0 or ASSERT _recursions == 0.
388 assert (_recursions == 0 , "invariant") ;
389 assert (_owner == Self , "invariant") ;
390 assert (_succ != Self , "invariant") ;
391 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
392
393 // The thread -- now the owner -- is back in vm mode.
394 // Report the glorious news via TI,DTrace and jvmstat.
395 // The probe effect is non-trivial. All the reportage occurs
396 // while we hold the monitor, increasing the length of the critical
397 // section. Amdahl's parallel speedup law comes vividly into play.
398 //
399 // Another option might be to aggregate the events (thread local or
400 // per-monitor aggregation) and defer reporting until a more opportune
401 // time -- such as next time some thread encounters contention but has
402 // yet to acquire the lock. While spinning that thread could
403 // spinning we could increment JVMStat counters, etc.
404
405 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
406 if (JvmtiExport::should_post_monitor_contended_entered()) {
407 JvmtiExport::post_monitor_contended_entered(jt, this);
408 }
409
410 if (event.should_commit()) {
411 event.set_klass(((oop)this->object())->klass());
412 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
413 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
414 event.commit();
415 }
416
417 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
418 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
419 }
420 }
421
422
423 // Caveat: TryLock() is not necessarily serializing if it returns failure.
424 // Callers must compensate as needed.
425
426 int ObjectMonitor::TryLock (Thread * Self) {
427 for (;;) {
428 void * own = _owner ;
429 if (own != NULL) return 0 ;
430 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
431 // Either guarantee _recursions == 0 or set _recursions = 0.
432 assert (_recursions == 0, "invariant") ;
433 assert (_owner == Self, "invariant") ;
434 // CONSIDER: set or assert that OwnerIsThread == 1
435 return 1 ;
436 }
437 // The lock had been free momentarily, but we lost the race to the lock.
438 // Interference -- the CAS failed.
439 // We can either return -1 or retry.
440 // Retry doesn't make as much sense because the lock was just acquired.
441 if (true) return -1 ;
442 }
443 }
444
445 void ATTR ObjectMonitor::EnterI (TRAPS) {
446 Thread * Self = THREAD ;
447 assert (Self->is_Java_thread(), "invariant") ;
448 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
449
450 // Try the lock - TATAS
451 if (TryLock (Self) > 0) {
452 assert (_succ != Self , "invariant") ;
453 assert (_owner == Self , "invariant") ;
454 assert (_Responsible != Self , "invariant") ;
455 return ;
456 }
457
458 DeferredInitialize () ;
459
460 // We try one round of spinning *before* enqueueing Self.
461 //
462 // If the _owner is ready but OFFPROC we could use a YieldTo()
463 // operation to donate the remainder of this thread's quantum
464 // to the owner. This has subtle but beneficial affinity
465 // effects.
466
467 if (TrySpin (Self) > 0) {
468 assert (_owner == Self , "invariant") ;
469 assert (_succ != Self , "invariant") ;
470 assert (_Responsible != Self , "invariant") ;
471 return ;
472 }
473
474 // The Spin failed -- Enqueue and park the thread ...
475 assert (_succ != Self , "invariant") ;
476 assert (_owner != Self , "invariant") ;
477 assert (_Responsible != Self , "invariant") ;
478
479 // Enqueue "Self" on ObjectMonitor's _cxq.
480 //
481 // Node acts as a proxy for Self.
482 // As an aside, if were to ever rewrite the synchronization code mostly
483 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
484 // Java objects. This would avoid awkward lifecycle and liveness issues,
485 // as well as eliminate a subset of ABA issues.
486 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
487 //
488
489 ObjectWaiter node(Self) ;
490 Self->_ParkEvent->reset() ;
491 node._prev = (ObjectWaiter *) 0xBAD ;
492 node.TState = ObjectWaiter::TS_CXQ ;
493
494 // Push "Self" onto the front of the _cxq.
495 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
496 // Note that spinning tends to reduce the rate at which threads
497 // enqueue and dequeue on EntryList|cxq.
498 ObjectWaiter * nxt ;
499 for (;;) {
500 node._next = nxt = _cxq ;
501 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
502
503 // Interference - the CAS failed because _cxq changed. Just retry.
504 // As an optional optimization we retry the lock.
505 if (TryLock (Self) > 0) {
506 assert (_succ != Self , "invariant") ;
507 assert (_owner == Self , "invariant") ;
508 assert (_Responsible != Self , "invariant") ;
509 return ;
510 }
511 }
512
513 // Check for cxq|EntryList edge transition to non-null. This indicates
514 // the onset of contention. While contention persists exiting threads
515 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
516 // operations revert to the faster 1-0 mode. This enter operation may interleave
517 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
518 // arrange for one of the contending thread to use a timed park() operations
519 // to detect and recover from the race. (Stranding is form of progress failure
520 // where the monitor is unlocked but all the contending threads remain parked).
521 // That is, at least one of the contended threads will periodically poll _owner.
522 // One of the contending threads will become the designated "Responsible" thread.
523 // The Responsible thread uses a timed park instead of a normal indefinite park
524 // operation -- it periodically wakes and checks for and recovers from potential
525 // strandings admitted by 1-0 exit operations. We need at most one Responsible
526 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
527 // be responsible for a monitor.
528 //
529 // Currently, one of the contended threads takes on the added role of "Responsible".
530 // A viable alternative would be to use a dedicated "stranding checker" thread
531 // that periodically iterated over all the threads (or active monitors) and unparked
532 // successors where there was risk of stranding. This would help eliminate the
533 // timer scalability issues we see on some platforms as we'd only have one thread
534 // -- the checker -- parked on a timer.
535
536 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
537 // Try to assume the role of responsible thread for the monitor.
538 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
539 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
540 }
541
542 // The lock have been released while this thread was occupied queueing
543 // itself onto _cxq. To close the race and avoid "stranding" and
544 // progress-liveness failure we must resample-retry _owner before parking.
545 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
546 // In this case the ST-MEMBAR is accomplished with CAS().
547 //
548 // TODO: Defer all thread state transitions until park-time.
549 // Since state transitions are heavy and inefficient we'd like
550 // to defer the state transitions until absolutely necessary,
551 // and in doing so avoid some transitions ...
552
553 TEVENT (Inflated enter - Contention) ;
554 int nWakeups = 0 ;
555 int RecheckInterval = 1 ;
556
557 for (;;) {
558
559 if (TryLock (Self) > 0) break ;
560 assert (_owner != Self, "invariant") ;
561
562 if ((SyncFlags & 2) && _Responsible == NULL) {
563 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
564 }
565
566 // park self
567 if (_Responsible == Self || (SyncFlags & 1)) {
568 TEVENT (Inflated enter - park TIMED) ;
569 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
570 // Increase the RecheckInterval, but clamp the value.
571 RecheckInterval *= 8 ;
572 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
573 } else {
574 TEVENT (Inflated enter - park UNTIMED) ;
575 Self->_ParkEvent->park() ;
576 }
577
578 if (TryLock(Self) > 0) break ;
579
580 // The lock is still contested.
581 // Keep a tally of the # of futile wakeups.
582 // Note that the counter is not protected by a lock or updated by atomics.
583 // That is by design - we trade "lossy" counters which are exposed to
584 // races during updates for a lower probe effect.
585 TEVENT (Inflated enter - Futile wakeup) ;
586 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
587 ObjectMonitor::_sync_FutileWakeups->inc() ;
588 }
589 ++ nWakeups ;
590
591 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
592 // We can defer clearing _succ until after the spin completes
593 // TrySpin() must tolerate being called with _succ == Self.
594 // Try yet another round of adaptive spinning.
595 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
596
597 // We can find that we were unpark()ed and redesignated _succ while
598 // we were spinning. That's harmless. If we iterate and call park(),
599 // park() will consume the event and return immediately and we'll
600 // just spin again. This pattern can repeat, leaving _succ to simply
601 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
602 // Alternately, we can sample fired() here, and if set, forgo spinning
603 // in the next iteration.
604
605 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
606 Self->_ParkEvent->reset() ;
607 OrderAccess::fence() ;
608 }
609 if (_succ == Self) _succ = NULL ;
610
611 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
612 OrderAccess::fence() ;
613 }
614
615 // Egress :
616 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
617 // Normally we'll find Self on the EntryList .
618 // From the perspective of the lock owner (this thread), the
619 // EntryList is stable and cxq is prepend-only.
620 // The head of cxq is volatile but the interior is stable.
621 // In addition, Self.TState is stable.
622
623 assert (_owner == Self , "invariant") ;
624 assert (object() != NULL , "invariant") ;
625 // I'd like to write:
626 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
627 // but as we're at a safepoint that's not safe.
628
629 UnlinkAfterAcquire (Self, &node) ;
630 if (_succ == Self) _succ = NULL ;
631
632 assert (_succ != Self, "invariant") ;
633 if (_Responsible == Self) {
634 _Responsible = NULL ;
635 OrderAccess::fence(); // Dekker pivot-point
636
637 // We may leave threads on cxq|EntryList without a designated
638 // "Responsible" thread. This is benign. When this thread subsequently
639 // exits the monitor it can "see" such preexisting "old" threads --
640 // threads that arrived on the cxq|EntryList before the fence, above --
641 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
642 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
643 // non-null and elect a new "Responsible" timer thread.
644 //
645 // This thread executes:
646 // ST Responsible=null; MEMBAR (in enter epilogue - here)
647 // LD cxq|EntryList (in subsequent exit)
648 //
649 // Entering threads in the slow/contended path execute:
650 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
651 // The (ST cxq; MEMBAR) is accomplished with CAS().
652 //
653 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
654 // exit operation from floating above the ST Responsible=null.
655 }
656
657 // We've acquired ownership with CAS().
658 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
659 // But since the CAS() this thread may have also stored into _succ,
660 // EntryList, cxq or Responsible. These meta-data updates must be
661 // visible __before this thread subsequently drops the lock.
662 // Consider what could occur if we didn't enforce this constraint --
663 // STs to monitor meta-data and user-data could reorder with (become
664 // visible after) the ST in exit that drops ownership of the lock.
665 // Some other thread could then acquire the lock, but observe inconsistent
666 // or old monitor meta-data and heap data. That violates the JMM.
667 // To that end, the 1-0 exit() operation must have at least STST|LDST
668 // "release" barrier semantics. Specifically, there must be at least a
669 // STST|LDST barrier in exit() before the ST of null into _owner that drops
670 // the lock. The barrier ensures that changes to monitor meta-data and data
671 // protected by the lock will be visible before we release the lock, and
672 // therefore before some other thread (CPU) has a chance to acquire the lock.
673 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
674 //
675 // Critically, any prior STs to _succ or EntryList must be visible before
676 // the ST of null into _owner in the *subsequent* (following) corresponding
677 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
678 // execute a serializing instruction.
679
680 if (SyncFlags & 8) {
681 OrderAccess::fence() ;
682 }
683 return ;
684 }
685
686 // ReenterI() is a specialized inline form of the latter half of the
687 // contended slow-path from EnterI(). We use ReenterI() only for
688 // monitor reentry in wait().
689 //
690 // In the future we should reconcile EnterI() and ReenterI(), adding
691 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
692 // loop accordingly.
693
694 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
695 assert (Self != NULL , "invariant") ;
696 assert (SelfNode != NULL , "invariant") ;
697 assert (SelfNode->_thread == Self , "invariant") ;
698 assert (_waiters > 0 , "invariant") ;
699 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
700 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
701 JavaThread * jt = (JavaThread *) Self ;
702
703 int nWakeups = 0 ;
704 for (;;) {
705 ObjectWaiter::TStates v = SelfNode->TState ;
706 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
707 assert (_owner != Self, "invariant") ;
708
709 if (TryLock (Self) > 0) break ;
710 if (TrySpin (Self) > 0) break ;
711
712 TEVENT (Wait Reentry - parking) ;
713
714 // State transition wrappers around park() ...
715 // ReenterI() wisely defers state transitions until
716 // it's clear we must park the thread.
717 {
718 OSThreadContendState osts(Self->osthread());
719 ThreadBlockInVM tbivm(jt);
720
721 // cleared by handle_special_suspend_equivalent_condition()
722 // or java_suspend_self()
723 jt->set_suspend_equivalent();
724 if (SyncFlags & 1) {
725 Self->_ParkEvent->park ((jlong)1000) ;
726 } else {
727 Self->_ParkEvent->park () ;
728 }
729
730 // were we externally suspended while we were waiting?
731 for (;;) {
732 if (!ExitSuspendEquivalent (jt)) break ;
733 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
734 jt->java_suspend_self();
735 jt->set_suspend_equivalent();
736 }
737 }
738
739 // Try again, but just so we distinguish between futile wakeups and
740 // successful wakeups. The following test isn't algorithmically
741 // necessary, but it helps us maintain sensible statistics.
742 if (TryLock(Self) > 0) break ;
743
744 // The lock is still contested.
745 // Keep a tally of the # of futile wakeups.
746 // Note that the counter is not protected by a lock or updated by atomics.
747 // That is by design - we trade "lossy" counters which are exposed to
748 // races during updates for a lower probe effect.
749 TEVENT (Wait Reentry - futile wakeup) ;
750 ++ nWakeups ;
751
752 // Assuming this is not a spurious wakeup we'll normally
753 // find that _succ == Self.
754 if (_succ == Self) _succ = NULL ;
755
756 // Invariant: after clearing _succ a contending thread
757 // *must* retry _owner before parking.
758 OrderAccess::fence() ;
759
760 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
761 ObjectMonitor::_sync_FutileWakeups->inc() ;
762 }
763 }
764
765 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
766 // Normally we'll find Self on the EntryList.
767 // Unlinking from the EntryList is constant-time and atomic-free.
768 // From the perspective of the lock owner (this thread), the
769 // EntryList is stable and cxq is prepend-only.
770 // The head of cxq is volatile but the interior is stable.
771 // In addition, Self.TState is stable.
772
773 assert (_owner == Self, "invariant") ;
774 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
775 UnlinkAfterAcquire (Self, SelfNode) ;
776 if (_succ == Self) _succ = NULL ;
777 assert (_succ != Self, "invariant") ;
778 SelfNode->TState = ObjectWaiter::TS_RUN ;
779 OrderAccess::fence() ; // see comments at the end of EnterI()
780 }
781
782 // after the thread acquires the lock in ::enter(). Equally, we could defer
783 // unlinking the thread until ::exit()-time.
784
785 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
786 {
787 assert (_owner == Self, "invariant") ;
788 assert (SelfNode->_thread == Self, "invariant") ;
789
790 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
791 // Normal case: remove Self from the DLL EntryList .
792 // This is a constant-time operation.
793 ObjectWaiter * nxt = SelfNode->_next ;
794 ObjectWaiter * prv = SelfNode->_prev ;
795 if (nxt != NULL) nxt->_prev = prv ;
796 if (prv != NULL) prv->_next = nxt ;
797 if (SelfNode == _EntryList ) _EntryList = nxt ;
798 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
799 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
800 TEVENT (Unlink from EntryList) ;
801 } else {
802 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
803 // Inopportune interleaving -- Self is still on the cxq.
804 // This usually means the enqueue of self raced an exiting thread.
805 // Normally we'll find Self near the front of the cxq, so
806 // dequeueing is typically fast. If needbe we can accelerate
807 // this with some MCS/CHL-like bidirectional list hints and advisory
808 // back-links so dequeueing from the interior will normally operate
809 // in constant-time.
810 // Dequeue Self from either the head (with CAS) or from the interior
811 // with a linear-time scan and normal non-atomic memory operations.
812 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
813 // and then unlink Self from EntryList. We have to drain eventually,
814 // so it might as well be now.
815
816 ObjectWaiter * v = _cxq ;
817 assert (v != NULL, "invariant") ;
818 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
819 // The CAS above can fail from interference IFF a "RAT" arrived.
820 // In that case Self must be in the interior and can no longer be
821 // at the head of cxq.
822 if (v == SelfNode) {
823 assert (_cxq != v, "invariant") ;
824 v = _cxq ; // CAS above failed - start scan at head of list
825 }
826 ObjectWaiter * p ;
827 ObjectWaiter * q = NULL ;
828 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
829 q = p ;
830 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
831 }
832 assert (v != SelfNode, "invariant") ;
833 assert (p == SelfNode, "Node not found on cxq") ;
834 assert (p != _cxq, "invariant") ;
835 assert (q != NULL, "invariant") ;
836 assert (q->_next == p, "invariant") ;
837 q->_next = p->_next ;
838 }
839 TEVENT (Unlink from cxq) ;
840 }
841
842 // Diagnostic hygiene ...
843 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
844 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
845 SelfNode->TState = ObjectWaiter::TS_RUN ;
846 }
847
848 // -----------------------------------------------------------------------------
849 // Exit support
850 //
851 // exit()
852 // ~~~~~~
853 // Note that the collector can't reclaim the objectMonitor or deflate
854 // the object out from underneath the thread calling ::exit() as the
855 // thread calling ::exit() never transitions to a stable state.
856 // This inhibits GC, which in turn inhibits asynchronous (and
857 // inopportune) reclamation of "this".
858 //
859 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
860 // There's one exception to the claim above, however. EnterI() can call
861 // exit() to drop a lock if the acquirer has been externally suspended.
862 // In that case exit() is called with _thread_state as _thread_blocked,
863 // but the monitor's _count field is > 0, which inhibits reclamation.
864 //
865 // 1-0 exit
866 // ~~~~~~~~
867 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
868 // the fast-path operators have been optimized so the common ::exit()
869 // operation is 1-0. See i486.ad fast_unlock(), for instance.
870 // The code emitted by fast_unlock() elides the usual MEMBAR. This
871 // greatly improves latency -- MEMBAR and CAS having considerable local
872 // latency on modern processors -- but at the cost of "stranding". Absent the
873 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
874 // ::enter() path, resulting in the entering thread being stranding
875 // and a progress-liveness failure. Stranding is extremely rare.
876 // We use timers (timed park operations) & periodic polling to detect
877 // and recover from stranding. Potentially stranded threads periodically
878 // wake up and poll the lock. See the usage of the _Responsible variable.
879 //
880 // The CAS() in enter provides for safety and exclusion, while the CAS or
881 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
882 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
883 // We detect and recover from stranding with timers.
884 //
885 // If a thread transiently strands it'll park until (a) another
886 // thread acquires the lock and then drops the lock, at which time the
887 // exiting thread will notice and unpark the stranded thread, or, (b)
888 // the timer expires. If the lock is high traffic then the stranding latency
889 // will be low due to (a). If the lock is low traffic then the odds of
890 // stranding are lower, although the worst-case stranding latency
891 // is longer. Critically, we don't want to put excessive load in the
892 // platform's timer subsystem. We want to minimize both the timer injection
893 // rate (timers created/sec) as well as the number of timers active at
894 // any one time. (more precisely, we want to minimize timer-seconds, which is
895 // the integral of the # of active timers at any instant over time).
896 // Both impinge on OS scalability. Given that, at most one thread parked on
897 // a monitor will use a timer.
898
899 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
900 Thread * Self = THREAD ;
901 if (THREAD != _owner) {
902 if (THREAD->is_lock_owned((address) _owner)) {
903 // Transmute _owner from a BasicLock pointer to a Thread address.
904 // We don't need to hold _mutex for this transition.
905 // Non-null to Non-null is safe as long as all readers can
906 // tolerate either flavor.
907 assert (_recursions == 0, "invariant") ;
908 _owner = THREAD ;
909 _recursions = 0 ;
910 OwnerIsThread = 1 ;
911 } else {
912 // NOTE: we need to handle unbalanced monitor enter/exit
913 // in native code by throwing an exception.
914 // TODO: Throw an IllegalMonitorStateException ?
915 TEVENT (Exit - Throw IMSX) ;
916 assert(false, "Non-balanced monitor enter/exit!");
917 if (false) {
918 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
919 }
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 = SharedRuntime::get_java_tid(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
969 // Normally the exiting thread is responsible for ensuring succession,
970 // but if other successors are ready or other entering threads are spinning
971 // then this thread can simply store NULL into _owner and exit without
972 // waking a successor. The existence of spinners or ready successors
973 // guarantees proper succession (liveness). Responsibility passes to the
974 // ready or running successors. The exiting thread delegates the duty.
975 // More precisely, if a successor already exists this thread is absolved
976 // of the responsibility of waking (unparking) one.
977 //
978 // The _succ variable is critical to reducing futile wakeup frequency.
979 // _succ identifies the "heir presumptive" thread that has been made
980 // ready (unparked) but that has not yet run. We need only one such
981 // successor thread to guarantee progress.
982 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
983 // section 3.3 "Futile Wakeup Throttling" for details.
984 //
985 // Note that spinners in Enter() also set _succ non-null.
986 // In the current implementation spinners opportunistically set
987 // _succ so that exiting threads might avoid waking a successor.
988 // Another less appealing alternative would be for the exiting thread
989 // to drop the lock and then spin briefly to see if a spinner managed
990 // to acquire the lock. If so, the exiting thread could exit
991 // immediately without waking a successor, otherwise the exiting
992 // thread would need to dequeue and wake a successor.
993 // (Note that we'd need to make the post-drop spin short, but no
994 // shorter than the worst-case round-trip cache-line migration time.
995 // The dropped lock needs to become visible to the spinner, and then
996 // the acquisition of the lock by the spinner must become visible to
997 // the exiting thread).
998 //
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 ; Tail = Tail->_next) ;
1085 if (Tail == NULL) {
1086 _EntryList = w ;
1087 } else {
1088 Tail->_next = w ;
1089 w->_prev = Tail ;
1090 }
1091
1092 // Fall thru into code that tries to wake a successor from EntryList
1093 }
1094
1095 if (QMode == 4 && _cxq != NULL) {
1096 // Aggressively drain cxq into EntryList at the first opportunity.
1097 // This policy ensure that recently-run threads live at the head of EntryList.
1098
1099 // Drain _cxq into EntryList - bulk transfer.
1100 // First, detach _cxq.
1101 // The following loop is tantamount to: w = swap (&cxq, NULL)
1102 w = _cxq ;
1103 for (;;) {
1104 assert (w != NULL, "Invariant") ;
1105 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1106 if (u == w) break ;
1107 w = u ;
1108 }
1109 assert (w != NULL , "invariant") ;
1110
1111 ObjectWaiter * q = NULL ;
1112 ObjectWaiter * p ;
1113 for (p = w ; p != NULL ; p = p->_next) {
1114 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1115 p->TState = ObjectWaiter::TS_ENTER ;
1116 p->_prev = q ;
1117 q = p ;
1118 }
1119
1120 // Prepend the RATs to the EntryList
1121 if (_EntryList != NULL) {
1122 q->_next = _EntryList ;
1123 _EntryList->_prev = q ;
1124 }
1125 _EntryList = w ;
1126
1127 // Fall thru into code that tries to wake a successor from EntryList
1128 }
1129
1130 w = _EntryList ;
1131 if (w != NULL) {
1132 // I'd like to write: guarantee (w->_thread != Self).
1133 // But in practice an exiting thread may find itself on the EntryList.
1134 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1135 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1136 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1137 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1138 // release the lock "O". T2 resumes immediately after the ST of null into
1139 // _owner, above. T2 notices that the EntryList is populated, so it
1140 // reacquires the lock and then finds itself on the EntryList.
1141 // Given all that, we have to tolerate the circumstance where "w" is
1142 // associated with Self.
1143 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1144 ExitEpilog (Self, w) ;
1145 return ;
1146 }
1147
1148 // If we find that both _cxq and EntryList are null then just
1149 // re-run the exit protocol from the top.
1150 w = _cxq ;
1151 if (w == NULL) continue ;
1152
1153 // Drain _cxq into EntryList - bulk transfer.
1154 // First, detach _cxq.
1155 // The following loop is tantamount to: w = swap (&cxq, NULL)
1156 for (;;) {
1157 assert (w != NULL, "Invariant") ;
1158 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1159 if (u == w) break ;
1160 w = u ;
1161 }
1162 TEVENT (Inflated exit - drain cxq into EntryList) ;
1163
1164 assert (w != NULL , "invariant") ;
1165 assert (_EntryList == NULL , "invariant") ;
1166
1167 // Convert the LIFO SLL anchored by _cxq into a DLL.
1168 // The list reorganization step operates in O(LENGTH(w)) time.
1169 // It's critical that this step operate quickly as
1170 // "Self" still holds the outer-lock, restricting parallelism
1171 // and effectively lengthening the critical section.
1172 // Invariant: s chases t chases u.
1173 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1174 // we have faster access to the tail.
1175
1176 if (QMode == 1) {
1177 // QMode == 1 : drain cxq to EntryList, reversing order
1178 // We also reverse the order of the list.
1179 ObjectWaiter * s = NULL ;
1180 ObjectWaiter * t = w ;
1181 ObjectWaiter * u = NULL ;
1182 while (t != NULL) {
1183 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1184 t->TState = ObjectWaiter::TS_ENTER ;
1185 u = t->_next ;
1186 t->_prev = u ;
1187 t->_next = s ;
1188 s = t;
1189 t = u ;
1190 }
1191 _EntryList = s ;
1192 assert (s != NULL, "invariant") ;
1193 } else {
1194 // QMode == 0 or QMode == 2
1195 _EntryList = w ;
1196 ObjectWaiter * q = NULL ;
1197 ObjectWaiter * p ;
1198 for (p = w ; p != NULL ; p = p->_next) {
1199 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1200 p->TState = ObjectWaiter::TS_ENTER ;
1201 p->_prev = q ;
1202 q = p ;
1203 }
1204 }
1205
1206 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1207 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1208
1209 // See if we can abdicate to a spinner instead of waking a thread.
1210 // A primary goal of the implementation is to reduce the
1211 // context-switch rate.
1212 if (_succ != NULL) continue;
1213
1214 w = _EntryList ;
1215 if (w != NULL) {
1216 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1217 ExitEpilog (Self, w) ;
1218 return ;
1219 }
1220 }
1221 }
1222
1223 // ExitSuspendEquivalent:
1224 // A faster alternate to handle_special_suspend_equivalent_condition()
1225 //
1226 // handle_special_suspend_equivalent_condition() unconditionally
1227 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1228 // operations have high latency. Note that in ::enter() we call HSSEC
1229 // while holding the monitor, so we effectively lengthen the critical sections.
1230 //
1231 // There are a number of possible solutions:
1232 //
1233 // A. To ameliorate the problem we might also defer state transitions
1234 // to as late as possible -- just prior to parking.
1235 // Given that, we'd call HSSEC after having returned from park(),
1236 // but before attempting to acquire the monitor. This is only a
1237 // partial solution. It avoids calling HSSEC while holding the
1238 // monitor (good), but it still increases successor reacquisition latency --
1239 // the interval between unparking a successor and the time the successor
1240 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1241 // If we use this technique we can also avoid EnterI()-exit() loop
1242 // in ::enter() where we iteratively drop the lock and then attempt
1243 // to reacquire it after suspending.
1244 //
1245 // B. In the future we might fold all the suspend bits into a
1246 // composite per-thread suspend flag and then update it with CAS().
1247 // Alternately, a Dekker-like mechanism with multiple variables
1248 // would suffice:
1249 // ST Self->_suspend_equivalent = false
1250 // MEMBAR
1251 // LD Self_>_suspend_flags
1252 //
1253
1254
1255 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1256 int Mode = Knob_FastHSSEC ;
1257 if (Mode && !jSelf->is_external_suspend()) {
1258 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1259 jSelf->clear_suspend_equivalent() ;
1260 if (2 == Mode) OrderAccess::storeload() ;
1261 if (!jSelf->is_external_suspend()) return false ;
1262 // We raced a suspension -- fall thru into the slow path
1263 TEVENT (ExitSuspendEquivalent - raced) ;
1264 jSelf->set_suspend_equivalent() ;
1265 }
1266 return jSelf->handle_special_suspend_equivalent_condition() ;
1267 }
1268
1269
1270 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1271 assert (_owner == Self, "invariant") ;
1272
1273 // Exit protocol:
1274 // 1. ST _succ = wakee
1275 // 2. membar #loadstore|#storestore;
1276 // 2. ST _owner = NULL
1277 // 3. unpark(wakee)
1278
1279 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1280 ParkEvent * Trigger = Wakee->_event ;
1281
1282 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1283 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1284 // out-of-scope (non-extant).
1285 Wakee = NULL ;
1286
1287 // Drop the lock
1288 OrderAccess::release_store_ptr (&_owner, NULL) ;
1289 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1290
1291 if (SafepointSynchronize::do_call_back()) {
1292 TEVENT (unpark before SAFEPOINT) ;
1293 }
1294
1295 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1296 Trigger->unpark() ;
1297
1298 // Maintain stats and report events to JVMTI
1299 if (ObjectMonitor::_sync_Parks != NULL) {
1300 ObjectMonitor::_sync_Parks->inc() ;
1301 }
1302 }
1303
1304
1305 // -----------------------------------------------------------------------------
1306 // Class Loader deadlock handling.
1307 //
1308 // complete_exit exits a lock returning recursion count
1309 // complete_exit/reenter operate as a wait without waiting
1310 // complete_exit requires an inflated monitor
1311 // The _owner field is not always the Thread addr even with an
1312 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1313 // thread due to contention.
1314 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1315 Thread * const Self = THREAD;
1316 assert(Self->is_Java_thread(), "Must be Java thread!");
1317 JavaThread *jt = (JavaThread *)THREAD;
1318
1319 DeferredInitialize();
1320
1321 if (THREAD != _owner) {
1322 if (THREAD->is_lock_owned ((address)_owner)) {
1323 assert(_recursions == 0, "internal state error");
1324 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1325 _recursions = 0 ;
1326 OwnerIsThread = 1 ;
1327 }
1328 }
1329
1330 guarantee(Self == _owner, "complete_exit not owner");
1331 intptr_t save = _recursions; // record the old recursion count
1332 _recursions = 0; // set the recursion level to be 0
1333 exit (true, Self) ; // exit the monitor
1334 guarantee (_owner != Self, "invariant");
1335 return save;
1336 }
1337
1338 // reenter() enters a lock and sets recursion count
1339 // complete_exit/reenter operate as a wait without waiting
1340 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1341 Thread * const Self = THREAD;
1342 assert(Self->is_Java_thread(), "Must be Java thread!");
1343 JavaThread *jt = (JavaThread *)THREAD;
1344
1345 guarantee(_owner != Self, "reenter already owner");
1346 enter (THREAD); // enter the monitor
1347 guarantee (_recursions == 0, "reenter recursion");
1348 _recursions = recursions;
1349 return;
1350 }
1351
1352
1353 // -----------------------------------------------------------------------------
1354 // A macro is used below because there may already be a pending
1355 // exception which should not abort the execution of the routines
1356 // which use this (which is why we don't put this into check_slow and
1357 // call it with a CHECK argument).
1358
1359 #define CHECK_OWNER() \
1360 do { \
1361 if (THREAD != _owner) { \
1362 if (THREAD->is_lock_owned((address) _owner)) { \
1363 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1364 _recursions = 0; \
1365 OwnerIsThread = 1 ; \
1366 } else { \
1367 TEVENT (Throw IMSX) ; \
1368 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1369 } \
1370 } \
1371 } while (false)
1372
1373 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1374 // TODO-FIXME: remove check_slow() -- it's likely dead.
1375
1376 void ObjectMonitor::check_slow(TRAPS) {
1377 TEVENT (check_slow - throw IMSX) ;
1378 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1379 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1380 }
1381
1382 static int Adjust (volatile int * adr, int dx) {
1383 int v ;
1384 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1385 return v ;
1386 }
1387
1388 // helper method for posting a monitor wait event
1389 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1390 jlong notifier_tid,
1391 jlong timeout,
1392 bool timedout) {
1393 event->set_klass(((oop)this->object())->klass());
1394 event->set_timeout((TYPE_ULONG)timeout);
1395 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1396 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1397 event->set_timedOut((TYPE_BOOLEAN)timedout);
1398 event->commit();
1399 }
1400
1401 // -----------------------------------------------------------------------------
1402 // Wait/Notify/NotifyAll
1403 //
1404 // Note: a subset of changes to ObjectMonitor::wait()
1405 // will need to be replicated in complete_exit above
1406 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1407 Thread * const Self = THREAD ;
1408 assert(Self->is_Java_thread(), "Must be Java thread!");
1409 JavaThread *jt = (JavaThread *)THREAD;
1410
1411 DeferredInitialize () ;
1412
1413 // Throw IMSX or IEX.
1414 CHECK_OWNER();
1415
1416 EventJavaMonitorWait event;
1417
1418 // check for a pending interrupt
1419 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1420 // post monitor waited event. Note that this is past-tense, we are done waiting.
1421 if (JvmtiExport::should_post_monitor_waited()) {
1422 // Note: 'false' parameter is passed here because the
1423 // wait was not timed out due to thread interrupt.
1424 JvmtiExport::post_monitor_waited(jt, this, false);
1425 }
1426 if (event.should_commit()) {
1427 post_monitor_wait_event(&event, 0, millis, false);
1428 }
1429 TEVENT (Wait - Throw IEX) ;
1430 THROW(vmSymbols::java_lang_InterruptedException());
1431 return ;
1432 }
1433
1434 TEVENT (Wait) ;
1435
1436 assert (Self->_Stalled == 0, "invariant") ;
1437 Self->_Stalled = intptr_t(this) ;
1438 jt->set_current_waiting_monitor(this);
1439
1440 // create a node to be put into the queue
1441 // Critically, after we reset() the event but prior to park(), we must check
1442 // for a pending interrupt.
1443 ObjectWaiter node(Self);
1444 node.TState = ObjectWaiter::TS_WAIT ;
1445 Self->_ParkEvent->reset() ;
1446 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1447
1448 // Enter the waiting queue, which is a circular doubly linked list in this case
1449 // but it could be a priority queue or any data structure.
1450 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1451 // by the the owner of the monitor *except* in the case where park()
1452 // returns because of a timeout of interrupt. Contention is exceptionally rare
1453 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1454
1455 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1456 AddWaiter (&node) ;
1457 Thread::SpinRelease (&_WaitSetLock) ;
1458
1459 if ((SyncFlags & 4) == 0) {
1460 _Responsible = NULL ;
1461 }
1462 intptr_t save = _recursions; // record the old recursion count
1463 _waiters++; // increment the number of waiters
1464 _recursions = 0; // set the recursion level to be 1
1465 exit (true, Self) ; // exit the monitor
1466 guarantee (_owner != Self, "invariant") ;
1467
1468 // As soon as the ObjectMonitor's ownership is dropped in the exit()
1469 // call above, another thread can enter() the ObjectMonitor, do the
1470 // notify(), and exit() the ObjectMonitor. If the other thread's
1471 // exit() call chooses this thread as the successor and the unpark()
1472 // call happens to occur while this thread is posting a
1473 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
1474 // handler using RawMonitors and consuming the unpark().
1475 //
1476 // To avoid the problem, we re-post the event. This does no harm
1477 // even if the original unpark() was not consumed because we are the
1478 // chosen successor for this monitor.
1479 if (node._notified != 0 && _succ == Self) {
1480 node._event->unpark();
1481 }
1482
1483 // The thread is on the WaitSet list - now park() it.
1484 // On MP systems it's conceivable that a brief spin before we park
1485 // could be profitable.
1486 //
1487 // TODO-FIXME: change the following logic to a loop of the form
1488 // while (!timeout && !interrupted && _notified == 0) park()
1489
1490 int ret = OS_OK ;
1491 int WasNotified = 0 ;
1492 { // State transition wrappers
1493 OSThread* osthread = Self->osthread();
1494 OSThreadWaitState osts(osthread, true);
1495 {
1496 ThreadBlockInVM tbivm(jt);
1497 // Thread is in thread_blocked state and oop access is unsafe.
1498 jt->set_suspend_equivalent();
1499
1500 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1501 // Intentionally empty
1502 } else
1503 if (node._notified == 0) {
1504 if (millis <= 0) {
1505 Self->_ParkEvent->park () ;
1506 } else {
1507 ret = Self->_ParkEvent->park (millis) ;
1508 }
1509 }
1510
1511 // were we externally suspended while we were waiting?
1512 if (ExitSuspendEquivalent (jt)) {
1513 // TODO-FIXME: add -- if succ == Self then succ = null.
1514 jt->java_suspend_self();
1515 }
1516
1517 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1518
1519
1520 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1521 // from the WaitSet to the EntryList.
1522 // See if we need to remove Node from the WaitSet.
1523 // We use double-checked locking to avoid grabbing _WaitSetLock
1524 // if the thread is not on the wait queue.
1525 //
1526 // Note that we don't need a fence before the fetch of TState.
1527 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1528 // written by the is thread. (perhaps the fetch might even be satisfied
1529 // by a look-aside into the processor's own store buffer, although given
1530 // the length of the code path between the prior ST and this load that's
1531 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1532 // then we'll acquire the lock and then re-fetch a fresh TState value.
1533 // That is, we fail toward safety.
1534
1535 if (node.TState == ObjectWaiter::TS_WAIT) {
1536 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1537 if (node.TState == ObjectWaiter::TS_WAIT) {
1538 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1539 assert(node._notified == 0, "invariant");
1540 node.TState = ObjectWaiter::TS_RUN ;
1541 }
1542 Thread::SpinRelease (&_WaitSetLock) ;
1543 }
1544
1545 // The thread is now either on off-list (TS_RUN),
1546 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1547 // The Node's TState variable is stable from the perspective of this thread.
1548 // No other threads will asynchronously modify TState.
1549 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1550 OrderAccess::loadload() ;
1551 if (_succ == Self) _succ = NULL ;
1552 WasNotified = node._notified ;
1553
1554 // Reentry phase -- reacquire the monitor.
1555 // re-enter contended monitor after object.wait().
1556 // retain OBJECT_WAIT state until re-enter successfully completes
1557 // Thread state is thread_in_vm and oop access is again safe,
1558 // although the raw address of the object may have changed.
1559 // (Don't cache naked oops over safepoints, of course).
1560
1561 // post monitor waited event. Note that this is past-tense, we are done waiting.
1562 if (JvmtiExport::should_post_monitor_waited()) {
1563 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1564 }
1565
1566 if (event.should_commit()) {
1567 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1568 }
1569
1570 OrderAccess::fence() ;
1571
1572 assert (Self->_Stalled != 0, "invariant") ;
1573 Self->_Stalled = 0 ;
1574
1575 assert (_owner != Self, "invariant") ;
1576 ObjectWaiter::TStates v = node.TState ;
1577 if (v == ObjectWaiter::TS_RUN) {
1578 enter (Self) ;
1579 } else {
1580 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1581 ReenterI (Self, &node) ;
1582 node.wait_reenter_end(this);
1583 }
1584
1585 // Self has reacquired the lock.
1586 // Lifecycle - the node representing Self must not appear on any queues.
1587 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1588 // want residual elements associated with this thread left on any lists.
1589 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1590 assert (_owner == Self, "invariant") ;
1591 assert (_succ != Self , "invariant") ;
1592 } // OSThreadWaitState()
1593
1594 jt->set_current_waiting_monitor(NULL);
1595
1596 guarantee (_recursions == 0, "invariant") ;
1597 _recursions = save; // restore the old recursion count
1598 _waiters--; // decrement the number of waiters
1599
1600 // Verify a few postconditions
1601 assert (_owner == Self , "invariant") ;
1602 assert (_succ != Self , "invariant") ;
1603 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1604
1605 if (SyncFlags & 32) {
1606 OrderAccess::fence() ;
1607 }
1608
1609 // check if the notification happened
1610 if (!WasNotified) {
1611 // no, it could be timeout or Thread.interrupt() or both
1612 // check for interrupt event, otherwise it is timeout
1613 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1614 TEVENT (Wait - throw IEX from epilog) ;
1615 THROW(vmSymbols::java_lang_InterruptedException());
1616 }
1617 }
1618
1619 // NOTE: Spurious wake up will be consider as timeout.
1620 // Monitor notify has precedence over thread interrupt.
1621 }
1622
1623
1624 // Consider:
1625 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1626 // then instead of transferring a thread from the WaitSet to the EntryList
1627 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1628
1629 void ObjectMonitor::notify(TRAPS) {
1630 CHECK_OWNER();
1631 if (_WaitSet == NULL) {
1632 TEVENT (Empty-Notify) ;
1633 return ;
1634 }
1635 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1636
1637 int Policy = Knob_MoveNotifyee ;
1638
1639 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1640 ObjectWaiter * iterator = DequeueWaiter() ;
1641 if (iterator != NULL) {
1642 TEVENT (Notify1 - Transfer) ;
1643 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1644 guarantee (iterator->_notified == 0, "invariant") ;
1645 if (Policy != 4) {
1646 iterator->TState = ObjectWaiter::TS_ENTER ;
1647 }
1648 iterator->_notified = 1 ;
1649 Thread * Self = THREAD;
1650 iterator->_notifier_tid = Self->osthread()->thread_id();
1651
1652 ObjectWaiter * List = _EntryList ;
1653 if (List != NULL) {
1654 assert (List->_prev == NULL, "invariant") ;
1655 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1656 assert (List != iterator, "invariant") ;
1657 }
1658
1659 if (Policy == 0) { // prepend to EntryList
1660 if (List == NULL) {
1661 iterator->_next = iterator->_prev = NULL ;
1662 _EntryList = iterator ;
1663 } else {
1664 List->_prev = iterator ;
1665 iterator->_next = List ;
1666 iterator->_prev = NULL ;
1667 _EntryList = iterator ;
1668 }
1669 } else
1670 if (Policy == 1) { // append to EntryList
1671 if (List == NULL) {
1672 iterator->_next = iterator->_prev = NULL ;
1673 _EntryList = iterator ;
1674 } else {
1675 // CONSIDER: finding the tail currently requires a linear-time walk of
1676 // the EntryList. We can make tail access constant-time by converting to
1677 // a CDLL instead of using our current DLL.
1678 ObjectWaiter * Tail ;
1679 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1680 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1681 Tail->_next = iterator ;
1682 iterator->_prev = Tail ;
1683 iterator->_next = NULL ;
1684 }
1685 } else
1686 if (Policy == 2) { // prepend to cxq
1687 // prepend to cxq
1688 if (List == NULL) {
1689 iterator->_next = iterator->_prev = NULL ;
1690 _EntryList = iterator ;
1691 } else {
1692 iterator->TState = ObjectWaiter::TS_CXQ ;
1693 for (;;) {
1694 ObjectWaiter * Front = _cxq ;
1695 iterator->_next = Front ;
1696 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1697 break ;
1698 }
1699 }
1700 }
1701 } else
1702 if (Policy == 3) { // append to cxq
1703 iterator->TState = ObjectWaiter::TS_CXQ ;
1704 for (;;) {
1705 ObjectWaiter * Tail ;
1706 Tail = _cxq ;
1707 if (Tail == NULL) {
1708 iterator->_next = NULL ;
1709 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1710 break ;
1711 }
1712 } else {
1713 while (Tail->_next != NULL) Tail = Tail->_next ;
1714 Tail->_next = iterator ;
1715 iterator->_prev = Tail ;
1716 iterator->_next = NULL ;
1717 break ;
1718 }
1719 }
1720 } else {
1721 ParkEvent * ev = iterator->_event ;
1722 iterator->TState = ObjectWaiter::TS_RUN ;
1723 OrderAccess::fence() ;
1724 ev->unpark() ;
1725 }
1726
1727 if (Policy < 4) {
1728 iterator->wait_reenter_begin(this);
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
1740 Thread::SpinRelease (&_WaitSetLock) ;
1741
1742 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1743 ObjectMonitor::_sync_Notifications->inc() ;
1744 }
1745 }
1746
1747
1748 void ObjectMonitor::notifyAll(TRAPS) {
1749 CHECK_OWNER();
1750 ObjectWaiter* iterator;
1751 if (_WaitSet == NULL) {
1752 TEVENT (Empty-NotifyAll) ;
1753 return ;
1754 }
1755 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1756
1757 int Policy = Knob_MoveNotifyee ;
1758 int Tally = 0 ;
1759 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1760
1761 for (;;) {
1762 iterator = DequeueWaiter () ;
1763 if (iterator == NULL) break ;
1764 TEVENT (NotifyAll - Transfer1) ;
1765 ++Tally ;
1766
1767 // Disposition - what might we do with iterator ?
1768 // a. add it directly to the EntryList - either tail or head.
1769 // b. push it onto the front of the _cxq.
1770 // For now we use (a).
1771
1772 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1773 guarantee (iterator->_notified == 0, "invariant") ;
1774 iterator->_notified = 1 ;
1775 Thread * Self = THREAD;
1776 iterator->_notifier_tid = Self->osthread()->thread_id();
1777 if (Policy != 4) {
1778 iterator->TState = ObjectWaiter::TS_ENTER ;
1779 }
1780
1781 ObjectWaiter * List = _EntryList ;
1782 if (List != NULL) {
1783 assert (List->_prev == NULL, "invariant") ;
1784 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1785 assert (List != iterator, "invariant") ;
1786 }
1787
1788 if (Policy == 0) { // prepend to EntryList
1789 if (List == NULL) {
1790 iterator->_next = iterator->_prev = NULL ;
1791 _EntryList = iterator ;
1792 } else {
1793 List->_prev = iterator ;
1794 iterator->_next = List ;
1795 iterator->_prev = NULL ;
1796 _EntryList = iterator ;
1797 }
1798 } else
1799 if (Policy == 1) { // append to EntryList
1800 if (List == NULL) {
1801 iterator->_next = iterator->_prev = NULL ;
1802 _EntryList = iterator ;
1803 } else {
1804 // CONSIDER: finding the tail currently requires a linear-time walk of
1805 // the EntryList. We can make tail access constant-time by converting to
1806 // a CDLL instead of using our current DLL.
1807 ObjectWaiter * Tail ;
1808 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1809 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1810 Tail->_next = iterator ;
1811 iterator->_prev = Tail ;
1812 iterator->_next = NULL ;
1813 }
1814 } else
1815 if (Policy == 2) { // prepend to cxq
1816 // prepend to cxq
1817 iterator->TState = ObjectWaiter::TS_CXQ ;
1818 for (;;) {
1819 ObjectWaiter * Front = _cxq ;
1820 iterator->_next = Front ;
1821 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1822 break ;
1823 }
1824 }
1825 } else
1826 if (Policy == 3) { // append to cxq
1827 iterator->TState = ObjectWaiter::TS_CXQ ;
1828 for (;;) {
1829 ObjectWaiter * Tail ;
1830 Tail = _cxq ;
1831 if (Tail == NULL) {
1832 iterator->_next = NULL ;
1833 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1834 break ;
1835 }
1836 } else {
1837 while (Tail->_next != NULL) Tail = Tail->_next ;
1838 Tail->_next = iterator ;
1839 iterator->_prev = Tail ;
1840 iterator->_next = NULL ;
1841 break ;
1842 }
1843 }
1844 } else {
1845 ParkEvent * ev = iterator->_event ;
1846 iterator->TState = ObjectWaiter::TS_RUN ;
1847 OrderAccess::fence() ;
1848 ev->unpark() ;
1849 }
1850
1851 if (Policy < 4) {
1852 iterator->wait_reenter_begin(this);
1853 }
1854
1855 // _WaitSetLock protects the wait queue, not the EntryList. We could
1856 // move the add-to-EntryList operation, above, outside the critical section
1857 // protected by _WaitSetLock. In practice that's not useful. With the
1858 // exception of wait() timeouts and interrupts the monitor owner
1859 // is the only thread that grabs _WaitSetLock. There's almost no contention
1860 // on _WaitSetLock so it's not profitable to reduce the length of the
1861 // critical section.
1862 }
1863
1864 Thread::SpinRelease (&_WaitSetLock) ;
1865
1866 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1867 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1868 }
1869 }
1870
1871 // -----------------------------------------------------------------------------
1872 // Adaptive Spinning Support
1873 //
1874 // Adaptive spin-then-block - rational spinning
1875 //
1876 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1877 // algorithm. On high order SMP systems it would be better to start with
1878 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1879 // a contending thread could enqueue itself on the cxq and then spin locally
1880 // on a thread-specific variable such as its ParkEvent._Event flag.
1881 // That's left as an exercise for the reader. Note that global spinning is
1882 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1883 // low latency and massive bandwidth.
1884 //
1885 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1886 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1887 // (duration) or we can fix the count at approximately the duration of
1888 // a context switch and vary the frequency. Of course we could also
1889 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1890 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1891 //
1892 // This implementation varies the duration "D", where D varies with
1893 // the success rate of recent spin attempts. (D is capped at approximately
1894 // length of a round-trip context switch). The success rate for recent
1895 // spin attempts is a good predictor of the success rate of future spin
1896 // attempts. The mechanism adapts automatically to varying critical
1897 // section length (lock modality), system load and degree of parallelism.
1898 // D is maintained per-monitor in _SpinDuration and is initialized
1899 // optimistically. Spin frequency is fixed at 100%.
1900 //
1901 // Note that _SpinDuration is volatile, but we update it without locks
1902 // or atomics. The code is designed so that _SpinDuration stays within
1903 // a reasonable range even in the presence of races. The arithmetic
1904 // operations on _SpinDuration are closed over the domain of legal values,
1905 // so at worst a race will install and older but still legal value.
1906 // At the very worst this introduces some apparent non-determinism.
1907 // We might spin when we shouldn't or vice-versa, but since the spin
1908 // count are relatively short, even in the worst case, the effect is harmless.
1909 //
1910 // Care must be taken that a low "D" value does not become an
1911 // an absorbing state. Transient spinning failures -- when spinning
1912 // is overall profitable -- should not cause the system to converge
1913 // on low "D" values. We want spinning to be stable and predictable
1914 // and fairly responsive to change and at the same time we don't want
1915 // it to oscillate, become metastable, be "too" non-deterministic,
1916 // or converge on or enter undesirable stable absorbing states.
1917 //
1918 // We implement a feedback-based control system -- using past behavior
1919 // to predict future behavior. We face two issues: (a) if the
1920 // input signal is random then the spin predictor won't provide optimal
1921 // results, and (b) if the signal frequency is too high then the control
1922 // system, which has some natural response lag, will "chase" the signal.
1923 // (b) can arise from multimodal lock hold times. Transient preemption
1924 // can also result in apparent bimodal lock hold times.
1925 // Although sub-optimal, neither condition is particularly harmful, as
1926 // in the worst-case we'll spin when we shouldn't or vice-versa.
1927 // The maximum spin duration is rather short so the failure modes aren't bad.
1928 // To be conservative, I've tuned the gain in system to bias toward
1929 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1930 // "rings" or oscillates between spinning and not spinning. This happens
1931 // when spinning is just on the cusp of profitability, however, so the
1932 // situation is not dire. The state is benign -- there's no need to add
1933 // hysteresis control to damp the transition rate between spinning and
1934 // not spinning.
1935 //
1936
1937 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1938 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1939
1940 // Spinning: Fixed frequency (100%), vary duration
1941
1942
1943 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1944
1945 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1946 int ctr = Knob_FixedSpin ;
1947 if (ctr != 0) {
1948 while (--ctr >= 0) {
1949 if (TryLock (Self) > 0) return 1 ;
1950 SpinPause () ;
1951 }
1952 return 0 ;
1953 }
1954
1955 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1956 if (TryLock(Self) > 0) {
1957 // Increase _SpinDuration ...
1958 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1959 // Raising _SpurDuration to the poverty line is key.
1960 int x = _SpinDuration ;
1961 if (x < Knob_SpinLimit) {
1962 if (x < Knob_Poverty) x = Knob_Poverty ;
1963 _SpinDuration = x + Knob_BonusB ;
1964 }
1965 return 1 ;
1966 }
1967 SpinPause () ;
1968 }
1969
1970 // Admission control - verify preconditions for spinning
1971 //
1972 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1973 // becoming an absorbing state. Put another way, we spin briefly to
1974 // sample, just in case the system load, parallelism, contention, or lock
1975 // modality changed.
1976 //
1977 // Consider the following alternative:
1978 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1979 // spin attempt. "Periodically" might mean after a tally of
1980 // the # of failed spin attempts (or iterations) reaches some threshold.
1981 // This takes us into the realm of 1-out-of-N spinning, where we
1982 // hold the duration constant but vary the frequency.
1983
1984 ctr = _SpinDuration ;
1985 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
1986 if (ctr <= 0) return 0 ;
1987
1988 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
1989 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1990 TEVENT (Spin abort - notrunnable [TOP]);
1991 return 0 ;
1992 }
1993
1994 int MaxSpin = Knob_MaxSpinners ;
1995 if (MaxSpin >= 0) {
1996 if (_Spinner > MaxSpin) {
1997 TEVENT (Spin abort -- too many spinners) ;
1998 return 0 ;
1999 }
2000 // Slightly racy, but benign ...
2001 Adjust (&_Spinner, 1) ;
2002 }
2003
2004 // We're good to spin ... spin ingress.
2005 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2006 // when preparing to LD...CAS _owner, etc and the CAS is likely
2007 // to succeed.
2008 int hits = 0 ;
2009 int msk = 0 ;
2010 int caspty = Knob_CASPenalty ;
2011 int oxpty = Knob_OXPenalty ;
2012 int sss = Knob_SpinSetSucc ;
2013 if (sss && _succ == NULL ) _succ = Self ;
2014 Thread * prv = NULL ;
2015
2016 // There are three ways to exit the following loop:
2017 // 1. A successful spin where this thread has acquired the lock.
2018 // 2. Spin failure with prejudice
2019 // 3. Spin failure without prejudice
2020
2021 while (--ctr >= 0) {
2022
2023 // Periodic polling -- Check for pending GC
2024 // Threads may spin while they're unsafe.
2025 // We don't want spinning threads to delay the JVM from reaching
2026 // a stop-the-world safepoint or to steal cycles from GC.
2027 // If we detect a pending safepoint we abort in order that
2028 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2029 // this thread, if safe, doesn't steal cycles from GC.
2030 // This is in keeping with the "no loitering in runtime" rule.
2031 // We periodically check to see if there's a safepoint pending.
2032 if ((ctr & 0xFF) == 0) {
2033 if (SafepointSynchronize::do_call_back()) {
2034 TEVENT (Spin: safepoint) ;
2035 goto Abort ; // abrupt spin egress
2036 }
2037 if (Knob_UsePause & 1) SpinPause () ;
2038
2039 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2040 if (hits > 50 && scb != NULL) {
2041 int abend = (*scb)(SpinCallbackArgument, 0) ;
2042 }
2043 }
2044
2045 if (Knob_UsePause & 2) SpinPause() ;
2046
2047 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2048 // This is useful on classic SMP systems, but is of less utility on
2049 // N1-style CMT platforms.
2050 //
2051 // Trade-off: lock acquisition latency vs coherency bandwidth.
2052 // Lock hold times are typically short. A histogram
2053 // of successful spin attempts shows that we usually acquire
2054 // the lock early in the spin. That suggests we want to
2055 // sample _owner frequently in the early phase of the spin,
2056 // but then back-off and sample less frequently as the spin
2057 // progresses. The back-off makes a good citizen on SMP big
2058 // SMP systems. Oversampling _owner can consume excessive
2059 // coherency bandwidth. Relatedly, if we _oversample _owner we
2060 // can inadvertently interfere with the the ST m->owner=null.
2061 // executed by the lock owner.
2062 if (ctr & msk) continue ;
2063 ++hits ;
2064 if ((hits & 0xF) == 0) {
2065 // The 0xF, above, corresponds to the exponent.
2066 // Consider: (msk+1)|msk
2067 msk = ((msk << 2)|3) & BackOffMask ;
2068 }
2069
2070 // Probe _owner with TATAS
2071 // If this thread observes the monitor transition or flicker
2072 // from locked to unlocked to locked, then the odds that this
2073 // thread will acquire the lock in this spin attempt go down
2074 // considerably. The same argument applies if the CAS fails
2075 // or if we observe _owner change from one non-null value to
2076 // another non-null value. In such cases we might abort
2077 // the spin without prejudice or apply a "penalty" to the
2078 // spin count-down variable "ctr", reducing it by 100, say.
2079
2080 Thread * ox = (Thread *) _owner ;
2081 if (ox == NULL) {
2082 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2083 if (ox == NULL) {
2084 // The CAS succeeded -- this thread acquired ownership
2085 // Take care of some bookkeeping to exit spin state.
2086 if (sss && _succ == Self) {
2087 _succ = NULL ;
2088 }
2089 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2090
2091 // Increase _SpinDuration :
2092 // The spin was successful (profitable) so we tend toward
2093 // longer spin attempts in the future.
2094 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2095 // If we acquired the lock early in the spin cycle it
2096 // makes sense to increase _SpinDuration proportionally.
2097 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2098 int x = _SpinDuration ;
2099 if (x < Knob_SpinLimit) {
2100 if (x < Knob_Poverty) x = Knob_Poverty ;
2101 _SpinDuration = x + Knob_Bonus ;
2102 }
2103 return 1 ;
2104 }
2105
2106 // The CAS failed ... we can take any of the following actions:
2107 // * penalize: ctr -= Knob_CASPenalty
2108 // * exit spin with prejudice -- goto Abort;
2109 // * exit spin without prejudice.
2110 // * Since CAS is high-latency, retry again immediately.
2111 prv = ox ;
2112 TEVENT (Spin: cas failed) ;
2113 if (caspty == -2) break ;
2114 if (caspty == -1) goto Abort ;
2115 ctr -= caspty ;
2116 continue ;
2117 }
2118
2119 // Did lock ownership change hands ?
2120 if (ox != prv && prv != NULL ) {
2121 TEVENT (spin: Owner changed)
2122 if (oxpty == -2) break ;
2123 if (oxpty == -1) goto Abort ;
2124 ctr -= oxpty ;
2125 }
2126 prv = ox ;
2127
2128 // Abort the spin if the owner is not executing.
2129 // The owner must be executing in order to drop the lock.
2130 // Spinning while the owner is OFFPROC is idiocy.
2131 // Consider: ctr -= RunnablePenalty ;
2132 if (Knob_OState && NotRunnable (Self, ox)) {
2133 TEVENT (Spin abort - notrunnable);
2134 goto Abort ;
2135 }
2136 if (sss && _succ == NULL ) _succ = Self ;
2137 }
2138
2139 // Spin failed with prejudice -- reduce _SpinDuration.
2140 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2141 // AIMD is globally stable.
2142 TEVENT (Spin failure) ;
2143 {
2144 int x = _SpinDuration ;
2145 if (x > 0) {
2146 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2147 // This is globally sample and tends to damp the response.
2148 x -= Knob_Penalty ;
2149 if (x < 0) x = 0 ;
2150 _SpinDuration = x ;
2151 }
2152 }
2153
2154 Abort:
2155 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2156 if (sss && _succ == Self) {
2157 _succ = NULL ;
2158 // Invariant: after setting succ=null a contending thread
2159 // must recheck-retry _owner before parking. This usually happens
2160 // in the normal usage of TrySpin(), but it's safest
2161 // to make TrySpin() as foolproof as possible.
2162 OrderAccess::fence() ;
2163 if (TryLock(Self) > 0) return 1 ;
2164 }
2165 return 0 ;
2166 }
2167
2168 // NotRunnable() -- informed spinning
2169 //
2170 // Don't bother spinning if the owner is not eligible to drop the lock.
2171 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2172 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2173 // The thread must be runnable in order to drop the lock in timely fashion.
2174 // If the _owner is not runnable then spinning will not likely be
2175 // successful (profitable).
2176 //
2177 // Beware -- the thread referenced by _owner could have died
2178 // so a simply fetch from _owner->_thread_state might trap.
2179 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2180 // Because of the lifecycle issues the schedctl and _thread_state values
2181 // observed by NotRunnable() might be garbage. NotRunnable must
2182 // tolerate this and consider the observed _thread_state value
2183 // as advisory.
2184 //
2185 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2186 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2187 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2188 // with the LSB of _owner. Another option would be to probablistically probe
2189 // the putative _owner->TypeTag value.
2190 //
2191 // Checking _thread_state isn't perfect. Even if the thread is
2192 // in_java it might be blocked on a page-fault or have been preempted
2193 // and sitting on a ready/dispatch queue. _thread state in conjunction
2194 // with schedctl.sc_state gives us a good picture of what the
2195 // thread is doing, however.
2196 //
2197 // TODO: check schedctl.sc_state.
2198 // We'll need to use SafeFetch32() to read from the schedctl block.
2199 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2200 //
2201 // The return value from NotRunnable() is *advisory* -- the
2202 // result is based on sampling and is not necessarily coherent.
2203 // The caller must tolerate false-negative and false-positive errors.
2204 // Spinning, in general, is probabilistic anyway.
2205
2206
2207 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2208 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2209 if (!OwnerIsThread) return 0 ;
2210
2211 if (ox == NULL) return 0 ;
2212
2213 // Avoid transitive spinning ...
2214 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2215 // Immediately after T1 acquires L it's possible that T2, also
2216 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2217 // This occurs transiently after T1 acquired L but before
2218 // T1 managed to clear T1.Stalled. T2 does not need to abort
2219 // its spin in this circumstance.
2220 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2221
2222 if (BlockedOn == 1) return 1 ;
2223 if (BlockedOn != 0) {
2224 return BlockedOn != intptr_t(this) && _owner == ox ;
2225 }
2226
2227 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2228 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2229 // consider also: jst != _thread_in_Java -- but that's overspecific.
2230 return jst == _thread_blocked || jst == _thread_in_native ;
2231 }
2232
2233
2234 // -----------------------------------------------------------------------------
2235 // WaitSet management ...
2236
2237 ObjectWaiter::ObjectWaiter(Thread* thread) {
2238 _next = NULL;
2239 _prev = NULL;
2240 _notified = 0;
2241 TState = TS_RUN ;
2242 _thread = thread;
2243 _event = thread->_ParkEvent ;
2244 _active = false;
2245 assert (_event != NULL, "invariant") ;
2246 }
2247
2248 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2249 JavaThread *jt = (JavaThread *)this->_thread;
2250 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2251 }
2252
2253 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2254 JavaThread *jt = (JavaThread *)this->_thread;
2255 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2256 }
2257
2258 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2259 assert(node != NULL, "should not dequeue NULL node");
2260 assert(node->_prev == NULL, "node already in list");
2261 assert(node->_next == NULL, "node already in list");
2262 // put node at end of queue (circular doubly linked list)
2263 if (_WaitSet == NULL) {
2264 _WaitSet = node;
2265 node->_prev = node;
2266 node->_next = node;
2267 } else {
2268 ObjectWaiter* head = _WaitSet ;
2269 ObjectWaiter* tail = head->_prev;
2270 assert(tail->_next == head, "invariant check");
2271 tail->_next = node;
2272 head->_prev = node;
2273 node->_next = head;
2274 node->_prev = tail;
2275 }
2276 }
2277
2278 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2279 // dequeue the very first waiter
2280 ObjectWaiter* waiter = _WaitSet;
2281 if (waiter) {
2282 DequeueSpecificWaiter(waiter);
2283 }
2284 return waiter;
2285 }
2286
2287 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2288 assert(node != NULL, "should not dequeue NULL node");
2289 assert(node->_prev != NULL, "node already removed from list");
2290 assert(node->_next != NULL, "node already removed from list");
2291 // when the waiter has woken up because of interrupt,
2292 // timeout or other spurious wake-up, dequeue the
2293 // waiter from waiting list
2294 ObjectWaiter* next = node->_next;
2295 if (next == node) {
2296 assert(node->_prev == node, "invariant check");
2297 _WaitSet = NULL;
2298 } else {
2299 ObjectWaiter* prev = node->_prev;
2300 assert(prev->_next == node, "invariant check");
2301 assert(next->_prev == node, "invariant check");
2302 next->_prev = prev;
2303 prev->_next = next;
2304 if (_WaitSet == node) {
2305 _WaitSet = next;
2306 }
2307 }
2308 node->_next = NULL;
2309 node->_prev = NULL;
2310 }
2311
2312 // -----------------------------------------------------------------------------
2313 // PerfData support
2314 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2315 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2316 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2317 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2318 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2319 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2320 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2321 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2322 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2323 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2324 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2325 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2326 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2327 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2328 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2329 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2330 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2331 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2332
2333 // One-shot global initialization for the sync subsystem.
2334 // We could also defer initialization and initialize on-demand
2335 // the first time we call inflate(). Initialization would
2336 // be protected - like so many things - by the MonitorCache_lock.
2337
2338 void ObjectMonitor::Initialize () {
2339 static int InitializationCompleted = 0 ;
2340 assert (InitializationCompleted == 0, "invariant") ;
2341 InitializationCompleted = 1 ;
2342 if (UsePerfData) {
2343 EXCEPTION_MARK ;
2344 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2345 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2346 NEWPERFCOUNTER(_sync_Inflations) ;
2347 NEWPERFCOUNTER(_sync_Deflations) ;
2348 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2349 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2350 NEWPERFCOUNTER(_sync_Parks) ;
2351 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2352 NEWPERFCOUNTER(_sync_Notifications) ;
2353 NEWPERFCOUNTER(_sync_SlowEnter) ;
2354 NEWPERFCOUNTER(_sync_SlowExit) ;
2355 NEWPERFCOUNTER(_sync_SlowNotify) ;
2356 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2357 NEWPERFCOUNTER(_sync_FailedSpins) ;
2358 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2359 NEWPERFCOUNTER(_sync_PrivateA) ;
2360 NEWPERFCOUNTER(_sync_PrivateB) ;
2361 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2362 NEWPERFCOUNTER(_sync_MonScavenged) ;
2363 NEWPERFVARIABLE(_sync_MonExtant) ;
2364 #undef NEWPERFCOUNTER
2365 }
2366 }
2367
2368
2369 // Compile-time asserts
2370 // When possible, it's better to catch errors deterministically at
2371 // compile-time than at runtime. The down-side to using compile-time
2372 // asserts is that error message -- often something about negative array
2373 // indices -- is opaque.
2374
2375 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2376
2377 void ObjectMonitor::ctAsserts() {
2378 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2379 }
2380
2381
2382 static char * kvGet (char * kvList, const char * Key) {
2383 if (kvList == NULL) return NULL ;
2384 size_t n = strlen (Key) ;
2385 char * Search ;
2386 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2387 if (strncmp (Search, Key, n) == 0) {
2388 if (Search[n] == '=') return Search + n + 1 ;
2389 if (Search[n] == 0) return (char *) "1" ;
2390 }
2391 }
2392 return NULL ;
2393 }
2394
2395 static int kvGetInt (char * kvList, const char * Key, int Default) {
2396 char * v = kvGet (kvList, Key) ;
2397 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2398 if (Knob_ReportSettings && v != NULL) {
2399 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2400 ::fflush (stdout) ;
2401 }
2402 return rslt ;
2403 }
2404
2405 void ObjectMonitor::DeferredInitialize () {
2406 if (InitDone > 0) return ;
2407 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2408 while (InitDone != 1) ;
2409 return ;
2410 }
2411
2412 // One-shot global initialization ...
2413 // The initialization is idempotent, so we don't need locks.
2414 // In the future consider doing this via os::init_2().
2415 // SyncKnobs consist of <Key>=<Value> pairs in the style
2416 // of environment variables. Start by converting ':' to NUL.
2417
2418 if (SyncKnobs == NULL) SyncKnobs = "" ;
2419
2420 size_t sz = strlen (SyncKnobs) ;
2421 char * knobs = (char *) malloc (sz + 2) ;
2422 if (knobs == NULL) {
2423 vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
2424 guarantee (0, "invariant") ;
2425 }
2426 strcpy (knobs, SyncKnobs) ;
2427 knobs[sz+1] = 0 ;
2428 for (char * p = knobs ; *p ; p++) {
2429 if (*p == ':') *p = 0 ;
2430 }
2431
2432 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2433 SETKNOB(ReportSettings) ;
2434 SETKNOB(Verbose) ;
2435 SETKNOB(FixedSpin) ;
2436 SETKNOB(SpinLimit) ;
2437 SETKNOB(SpinBase) ;
2438 SETKNOB(SpinBackOff);
2439 SETKNOB(CASPenalty) ;
2440 SETKNOB(OXPenalty) ;
2441 SETKNOB(LogSpins) ;
2442 SETKNOB(SpinSetSucc) ;
2443 SETKNOB(SuccEnabled) ;
2444 SETKNOB(SuccRestrict) ;
2445 SETKNOB(Penalty) ;
2446 SETKNOB(Bonus) ;
2447 SETKNOB(BonusB) ;
2448 SETKNOB(Poverty) ;
2449 SETKNOB(SpinAfterFutile) ;
2450 SETKNOB(UsePause) ;
2451 SETKNOB(SpinEarly) ;
2452 SETKNOB(OState) ;
2453 SETKNOB(MaxSpinners) ;
2454 SETKNOB(PreSpin) ;
2455 SETKNOB(ExitPolicy) ;
2456 SETKNOB(QMode);
2457 SETKNOB(ResetEvent) ;
2458 SETKNOB(MoveNotifyee) ;
2459 SETKNOB(FastHSSEC) ;
2460 #undef SETKNOB
2461
2462 if (os::is_MP()) {
2463 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2464 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2465 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2466 } else {
2467 Knob_SpinLimit = 0 ;
2468 Knob_SpinBase = 0 ;
2469 Knob_PreSpin = 0 ;
2470 Knob_FixedSpin = -1 ;
2471 }
2472
2473 if (Knob_LogSpins == 0) {
2474 ObjectMonitor::_sync_FailedSpins = NULL ;
2475 }
2476
2477 free (knobs) ;
2478 OrderAccess::fence() ;
2479 InitDone = 1 ;
2480 }
2481
2482 #ifndef PRODUCT
2483 void ObjectMonitor::verify() {
2484 }
2485
2486 void ObjectMonitor::print() {
2487 }
2488 #endif