1 /*
2 * Copyright (c) 1998, 2012, 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 "utilities/dtrace.hpp"
40 #include "utilities/preserveException.hpp"
41 #ifdef TARGET_OS_FAMILY_linux
42 # include "os_linux.inline.hpp"
43 #endif
44 #ifdef TARGET_OS_FAMILY_solaris
45 # include "os_solaris.inline.hpp"
46 #endif
47 #ifdef TARGET_OS_FAMILY_windows
48 # include "os_windows.inline.hpp"
49 #endif
50 #ifdef TARGET_OS_FAMILY_bsd
51 # include "os_bsd.inline.hpp"
52 #endif
53
54 #if defined(__GNUC__) && !defined(IA64)
55 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
56 #define ATTR __attribute__((noinline))
57 #else
58 #define ATTR
59 #endif
60
61
62 #ifdef DTRACE_ENABLED
63
64 // Only bother with this argument setup if dtrace is available
65 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
66
67
68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
69 char* bytes = NULL; \
70 int len = 0; \
71 jlong jtid = SharedRuntime::get_java_tid(thread); \
72 Symbol* klassname = ((oop)obj)->klass()->name(); \
73 if (klassname != NULL) { \
74 bytes = (char*)klassname->bytes(); \
75 len = klassname->utf8_length(); \
76 }
77
78 #ifndef USDT2
79
80 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
81 jlong, uintptr_t, char*, int);
82 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
83 jlong, uintptr_t, char*, int);
84 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
85 jlong, uintptr_t, char*, int);
86 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
87 jlong, uintptr_t, char*, int);
88 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
89 jlong, uintptr_t, char*, int);
90
91 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
92 { \
93 if (DTraceMonitorProbes) { \
94 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
95 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
96 (monitor), bytes, len, (millis)); \
97 } \
98 }
99
100 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
101 { \
102 if (DTraceMonitorProbes) { \
103 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
104 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
105 (uintptr_t)(monitor), bytes, len); \
106 } \
107 }
108
109 #else /* USDT2 */
110
111 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
112 { \
113 if (DTraceMonitorProbes) { \
114 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
115 HOTSPOT_MONITOR_WAIT(jtid, \
116 (monitor), bytes, len, (millis)); \
117 } \
118 }
119
120 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
121 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
122 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
123 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
124 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
125
126 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
127 { \
128 if (DTraceMonitorProbes) { \
129 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
130 HOTSPOT_MONITOR_##probe(jtid, \
131 (uintptr_t)(monitor), bytes, len); \
132 } \
133 }
134
135 #endif /* USDT2 */
136 #else // ndef DTRACE_ENABLED
137
138 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
139 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
140
141 #endif // ndef DTRACE_ENABLED
142
143 // Tunables ...
144 // The knob* variables are effectively final. Once set they should
145 // never be modified hence. Consider using __read_mostly with GCC.
146
147 int ObjectMonitor::Knob_Verbose = 0 ;
148 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
149 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
150 static int Knob_HandOff = 0 ;
151 static int Knob_ReportSettings = 0 ;
152
153 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
154 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
155 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
156 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
157 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
158 static int Knob_SpinEarly = 1 ;
159 static int Knob_SuccEnabled = 1 ; // futile wake throttling
160 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
161 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
162 static int Knob_Bonus = 100 ; // spin success bonus
163 static int Knob_BonusB = 100 ; // spin success bonus
164 static int Knob_Penalty = 200 ; // spin failure penalty
165 static int Knob_Poverty = 1000 ;
166 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
167 static int Knob_FixedSpin = 0 ;
168 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
169 static int Knob_UsePause = 1 ;
170 static int Knob_ExitPolicy = 0 ;
171 static int Knob_PreSpin = 10 ; // 20-100 likely better
172 static int Knob_ResetEvent = 0 ;
173 static int BackOffMask = 0 ;
174
175 static int Knob_FastHSSEC = 0 ;
176 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
177 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
178 static volatile int InitDone = 0 ;
179
180 #define TrySpin TrySpin_VaryDuration
181
182 // -----------------------------------------------------------------------------
183 // Theory of operations -- Monitors lists, thread residency, etc:
184 //
185 // * A thread acquires ownership of a monitor by successfully
186 // CAS()ing the _owner field from null to non-null.
187 //
188 // * Invariant: A thread appears on at most one monitor list --
189 // cxq, EntryList or WaitSet -- at any one time.
190 //
191 // * Contending threads "push" themselves onto the cxq with CAS
192 // and then spin/park.
193 //
194 // * After a contending thread eventually acquires the lock it must
195 // dequeue itself from either the EntryList or the cxq.
196 //
197 // * The exiting thread identifies and unparks an "heir presumptive"
198 // tentative successor thread on the EntryList. Critically, the
199 // exiting thread doesn't unlink the successor thread from the EntryList.
200 // After having been unparked, the wakee will recontend for ownership of
201 // the monitor. The successor (wakee) will either acquire the lock or
202 // re-park itself.
203 //
204 // Succession is provided for by a policy of competitive handoff.
205 // The exiting thread does _not_ grant or pass ownership to the
206 // successor thread. (This is also referred to as "handoff" succession").
207 // Instead the exiting thread releases ownership and possibly wakes
208 // a successor, so the successor can (re)compete for ownership of the lock.
209 // If the EntryList is empty but the cxq is populated the exiting
210 // thread will drain the cxq into the EntryList. It does so by
211 // by detaching the cxq (installing null with CAS) and folding
212 // the threads from the cxq into the EntryList. The EntryList is
213 // doubly linked, while the cxq is singly linked because of the
214 // CAS-based "push" used to enqueue recently arrived threads (RATs).
215 //
216 // * Concurrency invariants:
217 //
218 // -- only the monitor owner may access or mutate the EntryList.
219 // The mutex property of the monitor itself protects the EntryList
220 // from concurrent interference.
221 // -- Only the monitor owner may detach the cxq.
222 //
223 // * The monitor entry list operations avoid locks, but strictly speaking
224 // they're not lock-free. Enter is lock-free, exit is not.
225 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
226 //
227 // * The cxq can have multiple concurrent "pushers" but only one concurrent
228 // detaching thread. This mechanism is immune from the ABA corruption.
229 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
230 //
231 // * Taken together, the cxq and the EntryList constitute or form a
232 // single logical queue of threads stalled trying to acquire the lock.
233 // We use two distinct lists to improve the odds of a constant-time
234 // dequeue operation after acquisition (in the ::enter() epilog) and
235 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
236 // A key desideratum is to minimize queue & monitor metadata manipulation
237 // that occurs while holding the monitor lock -- that is, we want to
238 // minimize monitor lock holds times. Note that even a small amount of
239 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
240 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
241 // locks and monitor metadata.
242 //
243 // Cxq points to the the set of Recently Arrived Threads attempting entry.
244 // Because we push threads onto _cxq with CAS, the RATs must take the form of
245 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
246 // the unlocking thread notices that EntryList is null but _cxq is != null.
247 //
248 // The EntryList is ordered by the prevailing queue discipline and
249 // can be organized in any convenient fashion, such as a doubly-linked list or
250 // a circular doubly-linked list. Critically, we want insert and delete operations
251 // to operate in constant-time. If we need a priority queue then something akin
252 // to Solaris' sleepq would work nicely. Viz.,
253 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
254 // Queue discipline is enforced at ::exit() time, when the unlocking thread
255 // drains the cxq into the EntryList, and orders or reorders the threads on the
256 // EntryList accordingly.
257 //
258 // Barring "lock barging", this mechanism provides fair cyclic ordering,
259 // somewhat similar to an elevator-scan.
260 //
261 // * The monitor synchronization subsystem avoids the use of native
262 // synchronization primitives except for the narrow platform-specific
263 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
264 // the semantics of park-unpark. Put another way, this monitor implementation
265 // depends only on atomic operations and park-unpark. The monitor subsystem
266 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
267 // underlying OS manages the READY<->RUN transitions.
268 //
269 // * Waiting threads reside on the WaitSet list -- wait() puts
270 // the caller onto the WaitSet.
271 //
272 // * notify() or notifyAll() simply transfers threads from the WaitSet to
273 // either the EntryList or cxq. Subsequent exit() operations will
274 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
275 // it's likely the notifyee would simply impale itself on the lock held
276 // by the notifier.
277 //
278 // * An interesting alternative is to encode cxq as (List,LockByte) where
279 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
280 // variable, like _recursions, in the scheme. The threads or Events that form
281 // the list would have to be aligned in 256-byte addresses. A thread would
282 // try to acquire the lock or enqueue itself with CAS, but exiting threads
283 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
284 // Note that is is *not* word-tearing, but it does presume that full-word
285 // CAS operations are coherent with intermix with STB operations. That's true
286 // on most common processors.
287 //
288 // * See also http://blogs.sun.com/dave
289
290
291 // -----------------------------------------------------------------------------
292 // Enter support
293
294 bool ObjectMonitor::try_enter(Thread* THREAD) {
295 if (THREAD != _owner) {
296 if (THREAD->is_lock_owned ((address)_owner)) {
297 assert(_recursions == 0, "internal state error");
298 _owner = THREAD ;
299 _recursions = 1 ;
300 OwnerIsThread = 1 ;
301 return true;
302 }
303 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
304 return false;
305 }
306 return true;
307 } else {
308 _recursions++;
309 return true;
310 }
311 }
312
313 void ATTR ObjectMonitor::enter(TRAPS) {
314 // The following code is ordered to check the most common cases first
315 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
316 Thread * const Self = THREAD ;
317 void * cur ;
318
319 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
320 if (cur == NULL) {
321 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
322 assert (_recursions == 0 , "invariant") ;
323 assert (_owner == Self, "invariant") ;
324 // CONSIDER: set or assert OwnerIsThread == 1
325 return ;
326 }
327
328 if (cur == Self) {
329 // TODO-FIXME: check for integer overflow! BUGID 6557169.
330 _recursions ++ ;
331 return ;
332 }
333
334 if (Self->is_lock_owned ((address)cur)) {
335 assert (_recursions == 0, "internal state error");
336 _recursions = 1 ;
337 // Commute owner from a thread-specific on-stack BasicLockObject address to
338 // a full-fledged "Thread *".
339 _owner = Self ;
340 OwnerIsThread = 1 ;
341 return ;
342 }
343
344 // We've encountered genuine contention.
345 assert (Self->_Stalled == 0, "invariant") ;
346 Self->_Stalled = intptr_t(this) ;
347
348 // Try one round of spinning *before* enqueueing Self
349 // and before going through the awkward and expensive state
350 // transitions. The following spin is strictly optional ...
351 // Note that if we acquire the monitor from an initial spin
352 // we forgo posting JVMTI events and firing DTRACE probes.
353 if (Knob_SpinEarly && TrySpin (Self) > 0) {
354 assert (_owner == Self , "invariant") ;
355 assert (_recursions == 0 , "invariant") ;
356 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
357 Self->_Stalled = 0 ;
358 return ;
359 }
360
361 assert (_owner != Self , "invariant") ;
362 assert (_succ != Self , "invariant") ;
363 assert (Self->is_Java_thread() , "invariant") ;
364 JavaThread * jt = (JavaThread *) Self ;
365 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
366 assert (jt->thread_state() != _thread_blocked , "invariant") ;
367 assert (this->object() != NULL , "invariant") ;
368 assert (_count >= 0, "invariant") ;
369
370 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
371 // Ensure the object-monitor relationship remains stable while there's contention.
372 Atomic::inc_ptr(&_count);
373
374 { // Change java thread status to indicate blocked on monitor enter.
375 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
376
377 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
378 if (JvmtiExport::should_post_monitor_contended_enter()) {
379 JvmtiExport::post_monitor_contended_enter(jt, this);
380 }
381
382 OSThreadContendState osts(Self->osthread());
383 ThreadBlockInVM tbivm(jt);
384
385 Self->set_current_pending_monitor(this);
386
387 // TODO-FIXME: change the following for(;;) loop to straight-line code.
388 for (;;) {
389 jt->set_suspend_equivalent();
390 // cleared by handle_special_suspend_equivalent_condition()
391 // or java_suspend_self()
392
393 EnterI (THREAD) ;
394
395 if (!ExitSuspendEquivalent(jt)) break ;
396
397 //
398 // We have acquired the contended monitor, but while we were
399 // waiting another thread suspended us. We don't want to enter
400 // the monitor while suspended because that would surprise the
401 // thread that suspended us.
402 //
403 _recursions = 0 ;
404 _succ = NULL ;
405 exit (Self) ;
406
407 jt->java_suspend_self();
408 }
409 Self->set_current_pending_monitor(NULL);
410 }
411
412 Atomic::dec_ptr(&_count);
413 assert (_count >= 0, "invariant") ;
414 Self->_Stalled = 0 ;
415
416 // Must either set _recursions = 0 or ASSERT _recursions == 0.
417 assert (_recursions == 0 , "invariant") ;
418 assert (_owner == Self , "invariant") ;
419 assert (_succ != Self , "invariant") ;
420 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
421
422 // The thread -- now the owner -- is back in vm mode.
423 // Report the glorious news via TI,DTrace and jvmstat.
424 // The probe effect is non-trivial. All the reportage occurs
425 // while we hold the monitor, increasing the length of the critical
426 // section. Amdahl's parallel speedup law comes vividly into play.
427 //
428 // Another option might be to aggregate the events (thread local or
429 // per-monitor aggregation) and defer reporting until a more opportune
430 // time -- such as next time some thread encounters contention but has
431 // yet to acquire the lock. While spinning that thread could
432 // spinning we could increment JVMStat counters, etc.
433
434 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
435 if (JvmtiExport::should_post_monitor_contended_entered()) {
436 JvmtiExport::post_monitor_contended_entered(jt, this);
437 }
438 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
439 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
440 }
441 }
442
443
444 // Caveat: TryLock() is not necessarily serializing if it returns failure.
445 // Callers must compensate as needed.
446
447 int ObjectMonitor::TryLock (Thread * Self) {
448 for (;;) {
449 void * own = _owner ;
450 if (own != NULL) return 0 ;
451 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
452 // Either guarantee _recursions == 0 or set _recursions = 0.
453 assert (_recursions == 0, "invariant") ;
454 assert (_owner == Self, "invariant") ;
455 // CONSIDER: set or assert that OwnerIsThread == 1
456 return 1 ;
457 }
458 // The lock had been free momentarily, but we lost the race to the lock.
459 // Interference -- the CAS failed.
460 // We can either return -1 or retry.
461 // Retry doesn't make as much sense because the lock was just acquired.
462 if (true) return -1 ;
463 }
464 }
465
466 void ATTR ObjectMonitor::EnterI (TRAPS) {
467 Thread * Self = THREAD ;
468 assert (Self->is_Java_thread(), "invariant") ;
469 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
470
471 // Try the lock - TATAS
472 if (TryLock (Self) > 0) {
473 assert (_succ != Self , "invariant") ;
474 assert (_owner == Self , "invariant") ;
475 assert (_Responsible != Self , "invariant") ;
476 return ;
477 }
478
479 DeferredInitialize () ;
480
481 // We try one round of spinning *before* enqueueing Self.
482 //
483 // If the _owner is ready but OFFPROC we could use a YieldTo()
484 // operation to donate the remainder of this thread's quantum
485 // to the owner. This has subtle but beneficial affinity
486 // effects.
487
488 if (TrySpin (Self) > 0) {
489 assert (_owner == Self , "invariant") ;
490 assert (_succ != Self , "invariant") ;
491 assert (_Responsible != Self , "invariant") ;
492 return ;
493 }
494
495 // The Spin failed -- Enqueue and park the thread ...
496 assert (_succ != Self , "invariant") ;
497 assert (_owner != Self , "invariant") ;
498 assert (_Responsible != Self , "invariant") ;
499
500 // Enqueue "Self" on ObjectMonitor's _cxq.
501 //
502 // Node acts as a proxy for Self.
503 // As an aside, if were to ever rewrite the synchronization code mostly
504 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
505 // Java objects. This would avoid awkward lifecycle and liveness issues,
506 // as well as eliminate a subset of ABA issues.
507 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
508 //
509
510 ObjectWaiter node(Self) ;
511 Self->_ParkEvent->reset() ;
512 node._prev = (ObjectWaiter *) 0xBAD ;
513 node.TState = ObjectWaiter::TS_CXQ ;
514
515 // Push "Self" onto the front of the _cxq.
516 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
517 // Note that spinning tends to reduce the rate at which threads
518 // enqueue and dequeue on EntryList|cxq.
519 ObjectWaiter * nxt ;
520 for (;;) {
521 node._next = nxt = _cxq ;
522 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
523
524 // Interference - the CAS failed because _cxq changed. Just retry.
525 // As an optional optimization we retry the lock.
526 if (TryLock (Self) > 0) {
527 assert (_succ != Self , "invariant") ;
528 assert (_owner == Self , "invariant") ;
529 assert (_Responsible != Self , "invariant") ;
530 return ;
531 }
532 }
533
534 // Check for cxq|EntryList edge transition to non-null. This indicates
535 // the onset of contention. While contention persists exiting threads
536 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
537 // operations revert to the faster 1-0 mode. This enter operation may interleave
538 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
539 // arrange for one of the contending thread to use a timed park() operations
540 // to detect and recover from the race. (Stranding is form of progress failure
541 // where the monitor is unlocked but all the contending threads remain parked).
542 // That is, at least one of the contended threads will periodically poll _owner.
543 // One of the contending threads will become the designated "Responsible" thread.
544 // The Responsible thread uses a timed park instead of a normal indefinite park
545 // operation -- it periodically wakes and checks for and recovers from potential
546 // strandings admitted by 1-0 exit operations. We need at most one Responsible
547 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
548 // be responsible for a monitor.
549 //
550 // Currently, one of the contended threads takes on the added role of "Responsible".
551 // A viable alternative would be to use a dedicated "stranding checker" thread
552 // that periodically iterated over all the threads (or active monitors) and unparked
553 // successors where there was risk of stranding. This would help eliminate the
554 // timer scalability issues we see on some platforms as we'd only have one thread
555 // -- the checker -- parked on a timer.
556
557 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
558 // Try to assume the role of responsible thread for the monitor.
559 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
560 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
561 }
562
563 // The lock have been released while this thread was occupied queueing
564 // itself onto _cxq. To close the race and avoid "stranding" and
565 // progress-liveness failure we must resample-retry _owner before parking.
566 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
567 // In this case the ST-MEMBAR is accomplished with CAS().
568 //
569 // TODO: Defer all thread state transitions until park-time.
570 // Since state transitions are heavy and inefficient we'd like
571 // to defer the state transitions until absolutely necessary,
572 // and in doing so avoid some transitions ...
573
574 TEVENT (Inflated enter - Contention) ;
575 int nWakeups = 0 ;
576 int RecheckInterval = 1 ;
577
578 for (;;) {
579
580 if (TryLock (Self) > 0) break ;
581 assert (_owner != Self, "invariant") ;
582
583 if ((SyncFlags & 2) && _Responsible == NULL) {
584 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
585 }
586
587 // park self
588 if (_Responsible == Self || (SyncFlags & 1)) {
589 TEVENT (Inflated enter - park TIMED) ;
590 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
591 // Increase the RecheckInterval, but clamp the value.
592 RecheckInterval *= 8 ;
593 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
594 } else {
595 TEVENT (Inflated enter - park UNTIMED) ;
596 Self->_ParkEvent->park() ;
597 }
598
599 if (TryLock(Self) > 0) break ;
600
601 // The lock is still contested.
602 // Keep a tally of the # of futile wakeups.
603 // Note that the counter is not protected by a lock or updated by atomics.
604 // That is by design - we trade "lossy" counters which are exposed to
605 // races during updates for a lower probe effect.
606 TEVENT (Inflated enter - Futile wakeup) ;
607 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
608 ObjectMonitor::_sync_FutileWakeups->inc() ;
609 }
610 ++ nWakeups ;
611
612 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
613 // We can defer clearing _succ until after the spin completes
614 // TrySpin() must tolerate being called with _succ == Self.
615 // Try yet another round of adaptive spinning.
616 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
617
618 // We can find that we were unpark()ed and redesignated _succ while
619 // we were spinning. That's harmless. If we iterate and call park(),
620 // park() will consume the event and return immediately and we'll
621 // just spin again. This pattern can repeat, leaving _succ to simply
622 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
623 // Alternately, we can sample fired() here, and if set, forgo spinning
624 // in the next iteration.
625
626 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
627 Self->_ParkEvent->reset() ;
628 OrderAccess::fence() ;
629 }
630 if (_succ == Self) _succ = NULL ;
631
632 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
633 OrderAccess::fence() ;
634 }
635
636 // Egress :
637 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
638 // Normally we'll find Self on the EntryList .
639 // From the perspective of the lock owner (this thread), the
640 // EntryList is stable and cxq is prepend-only.
641 // The head of cxq is volatile but the interior is stable.
642 // In addition, Self.TState is stable.
643
644 assert (_owner == Self , "invariant") ;
645 assert (object() != NULL , "invariant") ;
646 // I'd like to write:
647 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
648 // but as we're at a safepoint that's not safe.
649
650 UnlinkAfterAcquire (Self, &node) ;
651 if (_succ == Self) _succ = NULL ;
652
653 assert (_succ != Self, "invariant") ;
654 if (_Responsible == Self) {
655 _Responsible = NULL ;
656 // Dekker pivot-point.
657 // Consider OrderAccess::storeload() here
658
659 // We may leave threads on cxq|EntryList without a designated
660 // "Responsible" thread. This is benign. When this thread subsequently
661 // exits the monitor it can "see" such preexisting "old" threads --
662 // threads that arrived on the cxq|EntryList before the fence, above --
663 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
664 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
665 // non-null and elect a new "Responsible" timer thread.
666 //
667 // This thread executes:
668 // ST Responsible=null; MEMBAR (in enter epilog - here)
669 // LD cxq|EntryList (in subsequent exit)
670 //
671 // Entering threads in the slow/contended path execute:
672 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
673 // The (ST cxq; MEMBAR) is accomplished with CAS().
674 //
675 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
676 // exit operation from floating above the ST Responsible=null.
677 //
678 // In *practice* however, EnterI() is always followed by some atomic
679 // operation such as the decrement of _count in ::enter(). Those atomics
680 // obviate the need for the explicit MEMBAR, above.
681 }
682
683 // We've acquired ownership with CAS().
684 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
685 // But since the CAS() this thread may have also stored into _succ,
686 // EntryList, cxq or Responsible. These meta-data updates must be
687 // visible __before this thread subsequently drops the lock.
688 // Consider what could occur if we didn't enforce this constraint --
689 // STs to monitor meta-data and user-data could reorder with (become
690 // visible after) the ST in exit that drops ownership of the lock.
691 // Some other thread could then acquire the lock, but observe inconsistent
692 // or old monitor meta-data and heap data. That violates the JMM.
693 // To that end, the 1-0 exit() operation must have at least STST|LDST
694 // "release" barrier semantics. Specifically, there must be at least a
695 // STST|LDST barrier in exit() before the ST of null into _owner that drops
696 // the lock. The barrier ensures that changes to monitor meta-data and data
697 // protected by the lock will be visible before we release the lock, and
698 // therefore before some other thread (CPU) has a chance to acquire the lock.
699 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
700 //
701 // Critically, any prior STs to _succ or EntryList must be visible before
702 // the ST of null into _owner in the *subsequent* (following) corresponding
703 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
704 // execute a serializing instruction.
705
706 if (SyncFlags & 8) {
707 OrderAccess::fence() ;
708 }
709 return ;
710 }
711
712 // ReenterI() is a specialized inline form of the latter half of the
713 // contended slow-path from EnterI(). We use ReenterI() only for
714 // monitor reentry in wait().
715 //
716 // In the future we should reconcile EnterI() and ReenterI(), adding
717 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
718 // loop accordingly.
719
720 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
721 assert (Self != NULL , "invariant") ;
722 assert (SelfNode != NULL , "invariant") ;
723 assert (SelfNode->_thread == Self , "invariant") ;
724 assert (_waiters > 0 , "invariant") ;
725 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
726 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
727 JavaThread * jt = (JavaThread *) Self ;
728
729 int nWakeups = 0 ;
730 for (;;) {
731 ObjectWaiter::TStates v = SelfNode->TState ;
732 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
733 assert (_owner != Self, "invariant") ;
734
735 if (TryLock (Self) > 0) break ;
736 if (TrySpin (Self) > 0) break ;
737
738 TEVENT (Wait Reentry - parking) ;
739
740 // State transition wrappers around park() ...
741 // ReenterI() wisely defers state transitions until
742 // it's clear we must park the thread.
743 {
744 OSThreadContendState osts(Self->osthread());
745 ThreadBlockInVM tbivm(jt);
746
747 // cleared by handle_special_suspend_equivalent_condition()
748 // or java_suspend_self()
749 jt->set_suspend_equivalent();
750 if (SyncFlags & 1) {
751 Self->_ParkEvent->park ((jlong)1000) ;
752 } else {
753 Self->_ParkEvent->park () ;
754 }
755
756 // were we externally suspended while we were waiting?
757 for (;;) {
758 if (!ExitSuspendEquivalent (jt)) break ;
759 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
760 jt->java_suspend_self();
761 jt->set_suspend_equivalent();
762 }
763 }
764
765 // Try again, but just so we distinguish between futile wakeups and
766 // successful wakeups. The following test isn't algorithmically
767 // necessary, but it helps us maintain sensible statistics.
768 if (TryLock(Self) > 0) break ;
769
770 // The lock is still contested.
771 // Keep a tally of the # of futile wakeups.
772 // Note that the counter is not protected by a lock or updated by atomics.
773 // That is by design - we trade "lossy" counters which are exposed to
774 // races during updates for a lower probe effect.
775 TEVENT (Wait Reentry - futile wakeup) ;
776 ++ nWakeups ;
777
778 // Assuming this is not a spurious wakeup we'll normally
779 // find that _succ == Self.
780 if (_succ == Self) _succ = NULL ;
781
782 // Invariant: after clearing _succ a contending thread
783 // *must* retry _owner before parking.
784 OrderAccess::fence() ;
785
786 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
787 ObjectMonitor::_sync_FutileWakeups->inc() ;
788 }
789 }
790
791 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
792 // Normally we'll find Self on the EntryList.
793 // Unlinking from the EntryList is constant-time and atomic-free.
794 // From the perspective of the lock owner (this thread), the
795 // EntryList is stable and cxq is prepend-only.
796 // The head of cxq is volatile but the interior is stable.
797 // In addition, Self.TState is stable.
798
799 assert (_owner == Self, "invariant") ;
800 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
801 UnlinkAfterAcquire (Self, SelfNode) ;
802 if (_succ == Self) _succ = NULL ;
803 assert (_succ != Self, "invariant") ;
804 SelfNode->TState = ObjectWaiter::TS_RUN ;
805 OrderAccess::fence() ; // see comments at the end of EnterI()
806 }
807
808 // after the thread acquires the lock in ::enter(). Equally, we could defer
809 // unlinking the thread until ::exit()-time.
810
811 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
812 {
813 assert (_owner == Self, "invariant") ;
814 assert (SelfNode->_thread == Self, "invariant") ;
815
816 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
817 // Normal case: remove Self from the DLL EntryList .
818 // This is a constant-time operation.
819 ObjectWaiter * nxt = SelfNode->_next ;
820 ObjectWaiter * prv = SelfNode->_prev ;
821 if (nxt != NULL) nxt->_prev = prv ;
822 if (prv != NULL) prv->_next = nxt ;
823 if (SelfNode == _EntryList ) _EntryList = nxt ;
824 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
825 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
826 TEVENT (Unlink from EntryList) ;
827 } else {
828 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
829 // Inopportune interleaving -- Self is still on the cxq.
830 // This usually means the enqueue of self raced an exiting thread.
831 // Normally we'll find Self near the front of the cxq, so
832 // dequeueing is typically fast. If needbe we can accelerate
833 // this with some MCS/CHL-like bidirectional list hints and advisory
834 // back-links so dequeueing from the interior will normally operate
835 // in constant-time.
836 // Dequeue Self from either the head (with CAS) or from the interior
837 // with a linear-time scan and normal non-atomic memory operations.
838 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
839 // and then unlink Self from EntryList. We have to drain eventually,
840 // so it might as well be now.
841
842 ObjectWaiter * v = _cxq ;
843 assert (v != NULL, "invariant") ;
844 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
845 // The CAS above can fail from interference IFF a "RAT" arrived.
846 // In that case Self must be in the interior and can no longer be
847 // at the head of cxq.
848 if (v == SelfNode) {
849 assert (_cxq != v, "invariant") ;
850 v = _cxq ; // CAS above failed - start scan at head of list
851 }
852 ObjectWaiter * p ;
853 ObjectWaiter * q = NULL ;
854 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
855 q = p ;
856 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
857 }
858 assert (v != SelfNode, "invariant") ;
859 assert (p == SelfNode, "Node not found on cxq") ;
860 assert (p != _cxq, "invariant") ;
861 assert (q != NULL, "invariant") ;
862 assert (q->_next == p, "invariant") ;
863 q->_next = p->_next ;
864 }
865 TEVENT (Unlink from cxq) ;
866 }
867
868 // Diagnostic hygiene ...
869 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
870 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
871 SelfNode->TState = ObjectWaiter::TS_RUN ;
872 }
873
874 // -----------------------------------------------------------------------------
875 // Exit support
876 //
877 // exit()
878 // ~~~~~~
879 // Note that the collector can't reclaim the objectMonitor or deflate
880 // the object out from underneath the thread calling ::exit() as the
881 // thread calling ::exit() never transitions to a stable state.
882 // This inhibits GC, which in turn inhibits asynchronous (and
883 // inopportune) reclamation of "this".
884 //
885 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
886 // There's one exception to the claim above, however. EnterI() can call
887 // exit() to drop a lock if the acquirer has been externally suspended.
888 // In that case exit() is called with _thread_state as _thread_blocked,
889 // but the monitor's _count field is > 0, which inhibits reclamation.
890 //
891 // 1-0 exit
892 // ~~~~~~~~
893 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
894 // the fast-path operators have been optimized so the common ::exit()
895 // operation is 1-0. See i486.ad fast_unlock(), for instance.
896 // The code emitted by fast_unlock() elides the usual MEMBAR. This
897 // greatly improves latency -- MEMBAR and CAS having considerable local
898 // latency on modern processors -- but at the cost of "stranding". Absent the
899 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
900 // ::enter() path, resulting in the entering thread being stranding
901 // and a progress-liveness failure. Stranding is extremely rare.
902 // We use timers (timed park operations) & periodic polling to detect
903 // and recover from stranding. Potentially stranded threads periodically
904 // wake up and poll the lock. See the usage of the _Responsible variable.
905 //
906 // The CAS() in enter provides for safety and exclusion, while the CAS or
907 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
908 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
909 // We detect and recover from stranding with timers.
910 //
911 // If a thread transiently strands it'll park until (a) another
912 // thread acquires the lock and then drops the lock, at which time the
913 // exiting thread will notice and unpark the stranded thread, or, (b)
914 // the timer expires. If the lock is high traffic then the stranding latency
915 // will be low due to (a). If the lock is low traffic then the odds of
916 // stranding are lower, although the worst-case stranding latency
917 // is longer. Critically, we don't want to put excessive load in the
918 // platform's timer subsystem. We want to minimize both the timer injection
919 // rate (timers created/sec) as well as the number of timers active at
920 // any one time. (more precisely, we want to minimize timer-seconds, which is
921 // the integral of the # of active timers at any instant over time).
922 // Both impinge on OS scalability. Given that, at most one thread parked on
923 // a monitor will use a timer.
924
925 void ATTR ObjectMonitor::exit(TRAPS) {
926 Thread * Self = THREAD ;
927 if (THREAD != _owner) {
928 if (THREAD->is_lock_owned((address) _owner)) {
929 // Transmute _owner from a BasicLock pointer to a Thread address.
930 // We don't need to hold _mutex for this transition.
931 // Non-null to Non-null is safe as long as all readers can
932 // tolerate either flavor.
933 assert (_recursions == 0, "invariant") ;
934 _owner = THREAD ;
935 _recursions = 0 ;
936 OwnerIsThread = 1 ;
937 } else {
938 // NOTE: we need to handle unbalanced monitor enter/exit
939 // in native code by throwing an exception.
940 // TODO: Throw an IllegalMonitorStateException ?
941 TEVENT (Exit - Throw IMSX) ;
942 assert(false, "Non-balanced monitor enter/exit!");
943 if (false) {
944 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
945 }
946 return;
947 }
948 }
949
950 if (_recursions != 0) {
951 _recursions--; // this is simple recursive enter
952 TEVENT (Inflated exit - recursive) ;
953 return ;
954 }
955
956 // Invariant: after setting Responsible=null an thread must execute
957 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
958 if ((SyncFlags & 4) == 0) {
959 _Responsible = NULL ;
960 }
961
962 for (;;) {
963 assert (THREAD == _owner, "invariant") ;
964
965
966 if (Knob_ExitPolicy == 0) {
967 // release semantics: prior loads and stores from within the critical section
968 // must not float (reorder) past the following store that drops the lock.
969 // On SPARC that requires MEMBAR #loadstore|#storestore.
970 // But of course in TSO #loadstore|#storestore is not required.
971 // I'd like to write one of the following:
972 // A. OrderAccess::release() ; _owner = NULL
973 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
974 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
975 // store into a _dummy variable. That store is not needed, but can result
976 // in massive wasteful coherency traffic on classic SMP systems.
977 // Instead, I use release_store(), which is implemented as just a simple
978 // ST on x64, x86 and SPARC.
979 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
980 OrderAccess::storeload() ; // See if we need to wake a successor
981 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
982 TEVENT (Inflated exit - simple egress) ;
983 return ;
984 }
985 TEVENT (Inflated exit - complex egress) ;
986
987 // Normally the exiting thread is responsible for ensuring succession,
988 // but if other successors are ready or other entering threads are spinning
989 // then this thread can simply store NULL into _owner and exit without
990 // waking a successor. The existence of spinners or ready successors
991 // guarantees proper succession (liveness). Responsibility passes to the
992 // ready or running successors. The exiting thread delegates the duty.
993 // More precisely, if a successor already exists this thread is absolved
994 // of the responsibility of waking (unparking) one.
995 //
996 // The _succ variable is critical to reducing futile wakeup frequency.
997 // _succ identifies the "heir presumptive" thread that has been made
998 // ready (unparked) but that has not yet run. We need only one such
999 // successor thread to guarantee progress.
1000 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1001 // section 3.3 "Futile Wakeup Throttling" for details.
1002 //
1003 // Note that spinners in Enter() also set _succ non-null.
1004 // In the current implementation spinners opportunistically set
1005 // _succ so that exiting threads might avoid waking a successor.
1006 // Another less appealing alternative would be for the exiting thread
1007 // to drop the lock and then spin briefly to see if a spinner managed
1008 // to acquire the lock. If so, the exiting thread could exit
1009 // immediately without waking a successor, otherwise the exiting
1010 // thread would need to dequeue and wake a successor.
1011 // (Note that we'd need to make the post-drop spin short, but no
1012 // shorter than the worst-case round-trip cache-line migration time.
1013 // The dropped lock needs to become visible to the spinner, and then
1014 // the acquisition of the lock by the spinner must become visible to
1015 // the exiting thread).
1016 //
1017
1018 // It appears that an heir-presumptive (successor) must be made ready.
1019 // Only the current lock owner can manipulate the EntryList or
1020 // drain _cxq, so we need to reacquire the lock. If we fail
1021 // to reacquire the lock the responsibility for ensuring succession
1022 // falls to the new owner.
1023 //
1024 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1025 return ;
1026 }
1027 TEVENT (Exit - Reacquired) ;
1028 } else {
1029 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1030 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1031 OrderAccess::storeload() ;
1032 // Ratify the previously observed values.
1033 if (_cxq == NULL || _succ != NULL) {
1034 TEVENT (Inflated exit - simple egress) ;
1035 return ;
1036 }
1037
1038 // inopportune interleaving -- the exiting thread (this thread)
1039 // in the fast-exit path raced an entering thread in the slow-enter
1040 // path.
1041 // We have two choices:
1042 // A. Try to reacquire the lock.
1043 // If the CAS() fails return immediately, otherwise
1044 // we either restart/rerun the exit operation, or simply
1045 // fall-through into the code below which wakes a successor.
1046 // B. If the elements forming the EntryList|cxq are TSM
1047 // we could simply unpark() the lead thread and return
1048 // without having set _succ.
1049 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1050 TEVENT (Inflated exit - reacquired succeeded) ;
1051 return ;
1052 }
1053 TEVENT (Inflated exit - reacquired failed) ;
1054 } else {
1055 TEVENT (Inflated exit - complex egress) ;
1056 }
1057 }
1058
1059 guarantee (_owner == THREAD, "invariant") ;
1060
1061 ObjectWaiter * w = NULL ;
1062 int QMode = Knob_QMode ;
1063
1064 if (QMode == 2 && _cxq != NULL) {
1065 // QMode == 2 : cxq has precedence over EntryList.
1066 // Try to directly wake a successor from the cxq.
1067 // If successful, the successor will need to unlink itself from cxq.
1068 w = _cxq ;
1069 assert (w != NULL, "invariant") ;
1070 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1071 ExitEpilog (Self, w) ;
1072 return ;
1073 }
1074
1075 if (QMode == 3 && _cxq != NULL) {
1076 // Aggressively drain cxq into EntryList at the first opportunity.
1077 // This policy ensure that recently-run threads live at the head of EntryList.
1078 // Drain _cxq into EntryList - bulk transfer.
1079 // First, detach _cxq.
1080 // The following loop is tantamount to: w = swap (&cxq, NULL)
1081 w = _cxq ;
1082 for (;;) {
1083 assert (w != NULL, "Invariant") ;
1084 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1085 if (u == w) break ;
1086 w = u ;
1087 }
1088 assert (w != NULL , "invariant") ;
1089
1090 ObjectWaiter * q = NULL ;
1091 ObjectWaiter * p ;
1092 for (p = w ; p != NULL ; p = p->_next) {
1093 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1094 p->TState = ObjectWaiter::TS_ENTER ;
1095 p->_prev = q ;
1096 q = p ;
1097 }
1098
1099 // Append the RATs to the EntryList
1100 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1101 ObjectWaiter * Tail ;
1102 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1103 if (Tail == NULL) {
1104 _EntryList = w ;
1105 } else {
1106 Tail->_next = w ;
1107 w->_prev = Tail ;
1108 }
1109
1110 // Fall thru into code that tries to wake a successor from EntryList
1111 }
1112
1113 if (QMode == 4 && _cxq != NULL) {
1114 // Aggressively drain cxq into EntryList at the first opportunity.
1115 // This policy ensure that recently-run threads live at the head of EntryList.
1116
1117 // Drain _cxq into EntryList - bulk transfer.
1118 // First, detach _cxq.
1119 // The following loop is tantamount to: w = swap (&cxq, NULL)
1120 w = _cxq ;
1121 for (;;) {
1122 assert (w != NULL, "Invariant") ;
1123 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1124 if (u == w) break ;
1125 w = u ;
1126 }
1127 assert (w != NULL , "invariant") ;
1128
1129 ObjectWaiter * q = NULL ;
1130 ObjectWaiter * p ;
1131 for (p = w ; p != NULL ; p = p->_next) {
1132 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1133 p->TState = ObjectWaiter::TS_ENTER ;
1134 p->_prev = q ;
1135 q = p ;
1136 }
1137
1138 // Prepend the RATs to the EntryList
1139 if (_EntryList != NULL) {
1140 q->_next = _EntryList ;
1141 _EntryList->_prev = q ;
1142 }
1143 _EntryList = w ;
1144
1145 // Fall thru into code that tries to wake a successor from EntryList
1146 }
1147
1148 w = _EntryList ;
1149 if (w != NULL) {
1150 // I'd like to write: guarantee (w->_thread != Self).
1151 // But in practice an exiting thread may find itself on the EntryList.
1152 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1153 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1154 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1155 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1156 // release the lock "O". T2 resumes immediately after the ST of null into
1157 // _owner, above. T2 notices that the EntryList is populated, so it
1158 // reacquires the lock and then finds itself on the EntryList.
1159 // Given all that, we have to tolerate the circumstance where "w" is
1160 // associated with Self.
1161 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1162 ExitEpilog (Self, w) ;
1163 return ;
1164 }
1165
1166 // If we find that both _cxq and EntryList are null then just
1167 // re-run the exit protocol from the top.
1168 w = _cxq ;
1169 if (w == NULL) continue ;
1170
1171 // Drain _cxq into EntryList - bulk transfer.
1172 // First, detach _cxq.
1173 // The following loop is tantamount to: w = swap (&cxq, NULL)
1174 for (;;) {
1175 assert (w != NULL, "Invariant") ;
1176 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1177 if (u == w) break ;
1178 w = u ;
1179 }
1180 TEVENT (Inflated exit - drain cxq into EntryList) ;
1181
1182 assert (w != NULL , "invariant") ;
1183 assert (_EntryList == NULL , "invariant") ;
1184
1185 // Convert the LIFO SLL anchored by _cxq into a DLL.
1186 // The list reorganization step operates in O(LENGTH(w)) time.
1187 // It's critical that this step operate quickly as
1188 // "Self" still holds the outer-lock, restricting parallelism
1189 // and effectively lengthening the critical section.
1190 // Invariant: s chases t chases u.
1191 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1192 // we have faster access to the tail.
1193
1194 if (QMode == 1) {
1195 // QMode == 1 : drain cxq to EntryList, reversing order
1196 // We also reverse the order of the list.
1197 ObjectWaiter * s = NULL ;
1198 ObjectWaiter * t = w ;
1199 ObjectWaiter * u = NULL ;
1200 while (t != NULL) {
1201 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1202 t->TState = ObjectWaiter::TS_ENTER ;
1203 u = t->_next ;
1204 t->_prev = u ;
1205 t->_next = s ;
1206 s = t;
1207 t = u ;
1208 }
1209 _EntryList = s ;
1210 assert (s != NULL, "invariant") ;
1211 } else {
1212 // QMode == 0 or QMode == 2
1213 _EntryList = w ;
1214 ObjectWaiter * q = NULL ;
1215 ObjectWaiter * p ;
1216 for (p = w ; p != NULL ; p = p->_next) {
1217 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1218 p->TState = ObjectWaiter::TS_ENTER ;
1219 p->_prev = q ;
1220 q = p ;
1221 }
1222 }
1223
1224 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1225 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1226
1227 // See if we can abdicate to a spinner instead of waking a thread.
1228 // A primary goal of the implementation is to reduce the
1229 // context-switch rate.
1230 if (_succ != NULL) continue;
1231
1232 w = _EntryList ;
1233 if (w != NULL) {
1234 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1235 ExitEpilog (Self, w) ;
1236 return ;
1237 }
1238 }
1239 }
1240
1241 // ExitSuspendEquivalent:
1242 // A faster alternate to handle_special_suspend_equivalent_condition()
1243 //
1244 // handle_special_suspend_equivalent_condition() unconditionally
1245 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1246 // operations have high latency. Note that in ::enter() we call HSSEC
1247 // while holding the monitor, so we effectively lengthen the critical sections.
1248 //
1249 // There are a number of possible solutions:
1250 //
1251 // A. To ameliorate the problem we might also defer state transitions
1252 // to as late as possible -- just prior to parking.
1253 // Given that, we'd call HSSEC after having returned from park(),
1254 // but before attempting to acquire the monitor. This is only a
1255 // partial solution. It avoids calling HSSEC while holding the
1256 // monitor (good), but it still increases successor reacquisition latency --
1257 // the interval between unparking a successor and the time the successor
1258 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1259 // If we use this technique we can also avoid EnterI()-exit() loop
1260 // in ::enter() where we iteratively drop the lock and then attempt
1261 // to reacquire it after suspending.
1262 //
1263 // B. In the future we might fold all the suspend bits into a
1264 // composite per-thread suspend flag and then update it with CAS().
1265 // Alternately, a Dekker-like mechanism with multiple variables
1266 // would suffice:
1267 // ST Self->_suspend_equivalent = false
1268 // MEMBAR
1269 // LD Self_>_suspend_flags
1270 //
1271
1272
1273 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1274 int Mode = Knob_FastHSSEC ;
1275 if (Mode && !jSelf->is_external_suspend()) {
1276 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1277 jSelf->clear_suspend_equivalent() ;
1278 if (2 == Mode) OrderAccess::storeload() ;
1279 if (!jSelf->is_external_suspend()) return false ;
1280 // We raced a suspension -- fall thru into the slow path
1281 TEVENT (ExitSuspendEquivalent - raced) ;
1282 jSelf->set_suspend_equivalent() ;
1283 }
1284 return jSelf->handle_special_suspend_equivalent_condition() ;
1285 }
1286
1287
1288 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1289 assert (_owner == Self, "invariant") ;
1290
1291 // Exit protocol:
1292 // 1. ST _succ = wakee
1293 // 2. membar #loadstore|#storestore;
1294 // 2. ST _owner = NULL
1295 // 3. unpark(wakee)
1296
1297 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1298 ParkEvent * Trigger = Wakee->_event ;
1299
1300 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1301 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1302 // out-of-scope (non-extant).
1303 Wakee = NULL ;
1304
1305 // Drop the lock
1306 OrderAccess::release_store_ptr (&_owner, NULL) ;
1307 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1308
1309 if (SafepointSynchronize::do_call_back()) {
1310 TEVENT (unpark before SAFEPOINT) ;
1311 }
1312
1313 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1314 Trigger->unpark() ;
1315
1316 // Maintain stats and report events to JVMTI
1317 if (ObjectMonitor::_sync_Parks != NULL) {
1318 ObjectMonitor::_sync_Parks->inc() ;
1319 }
1320 }
1321
1322
1323 // -----------------------------------------------------------------------------
1324 // Class Loader deadlock handling.
1325 //
1326 // complete_exit exits a lock returning recursion count
1327 // complete_exit/reenter operate as a wait without waiting
1328 // complete_exit requires an inflated monitor
1329 // The _owner field is not always the Thread addr even with an
1330 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1331 // thread due to contention.
1332 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1333 Thread * const Self = THREAD;
1334 assert(Self->is_Java_thread(), "Must be Java thread!");
1335 JavaThread *jt = (JavaThread *)THREAD;
1336
1337 DeferredInitialize();
1338
1339 if (THREAD != _owner) {
1340 if (THREAD->is_lock_owned ((address)_owner)) {
1341 assert(_recursions == 0, "internal state error");
1342 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1343 _recursions = 0 ;
1344 OwnerIsThread = 1 ;
1345 }
1346 }
1347
1348 guarantee(Self == _owner, "complete_exit not owner");
1349 intptr_t save = _recursions; // record the old recursion count
1350 _recursions = 0; // set the recursion level to be 0
1351 exit (Self) ; // exit the monitor
1352 guarantee (_owner != Self, "invariant");
1353 return save;
1354 }
1355
1356 // reenter() enters a lock and sets recursion count
1357 // complete_exit/reenter operate as a wait without waiting
1358 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1359 Thread * const Self = THREAD;
1360 assert(Self->is_Java_thread(), "Must be Java thread!");
1361 JavaThread *jt = (JavaThread *)THREAD;
1362
1363 guarantee(_owner != Self, "reenter already owner");
1364 enter (THREAD); // enter the monitor
1365 guarantee (_recursions == 0, "reenter recursion");
1366 _recursions = recursions;
1367 return;
1368 }
1369
1370
1371 // -----------------------------------------------------------------------------
1372 // A macro is used below because there may already be a pending
1373 // exception which should not abort the execution of the routines
1374 // which use this (which is why we don't put this into check_slow and
1375 // call it with a CHECK argument).
1376
1377 #define CHECK_OWNER() \
1378 do { \
1379 if (THREAD != _owner) { \
1380 if (THREAD->is_lock_owned((address) _owner)) { \
1381 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1382 _recursions = 0; \
1383 OwnerIsThread = 1 ; \
1384 } else { \
1385 TEVENT (Throw IMSX) ; \
1386 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1387 } \
1388 } \
1389 } while (false)
1390
1391 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1392 // TODO-FIXME: remove check_slow() -- it's likely dead.
1393
1394 void ObjectMonitor::check_slow(TRAPS) {
1395 TEVENT (check_slow - throw IMSX) ;
1396 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1397 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1398 }
1399
1400 static int Adjust (volatile int * adr, int dx) {
1401 int v ;
1402 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1403 return v ;
1404 }
1405 // -----------------------------------------------------------------------------
1406 // Wait/Notify/NotifyAll
1407 //
1408 // Note: a subset of changes to ObjectMonitor::wait()
1409 // will need to be replicated in complete_exit above
1410 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1411 Thread * const Self = THREAD ;
1412 assert(Self->is_Java_thread(), "Must be Java thread!");
1413 JavaThread *jt = (JavaThread *)THREAD;
1414
1415 DeferredInitialize () ;
1416
1417 // Throw IMSX or IEX.
1418 CHECK_OWNER();
1419
1420 // check for a pending interrupt
1421 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1422 // post monitor waited event. Note that this is past-tense, we are done waiting.
1423 if (JvmtiExport::should_post_monitor_waited()) {
1424 // Note: 'false' parameter is passed here because the
1425 // wait was not timed out due to thread interrupt.
1426 JvmtiExport::post_monitor_waited(jt, this, false);
1427 }
1428 TEVENT (Wait - Throw IEX) ;
1429 THROW(vmSymbols::java_lang_InterruptedException());
1430 return ;
1431 }
1432 TEVENT (Wait) ;
1433
1434 assert (Self->_Stalled == 0, "invariant") ;
1435 Self->_Stalled = intptr_t(this) ;
1436 jt->set_current_waiting_monitor(this);
1437
1438 // create a node to be put into the queue
1439 // Critically, after we reset() the event but prior to park(), we must check
1440 // for a pending interrupt.
1441 ObjectWaiter node(Self);
1442 node.TState = ObjectWaiter::TS_WAIT ;
1443 Self->_ParkEvent->reset() ;
1444 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1445
1446 // Enter the waiting queue, which is a circular doubly linked list in this case
1447 // but it could be a priority queue or any data structure.
1448 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1449 // by the the owner of the monitor *except* in the case where park()
1450 // returns because of a timeout of interrupt. Contention is exceptionally rare
1451 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1452
1453 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1454 AddWaiter (&node) ;
1455 Thread::SpinRelease (&_WaitSetLock) ;
1456
1457 if ((SyncFlags & 4) == 0) {
1458 _Responsible = NULL ;
1459 }
1460 intptr_t save = _recursions; // record the old recursion count
1461 _waiters++; // increment the number of waiters
1462 _recursions = 0; // set the recursion level to be 1
1463 exit (Self) ; // exit the monitor
1464 guarantee (_owner != Self, "invariant") ;
1465
1466 // As soon as the ObjectMonitor's ownership is dropped in the exit()
1467 // call above, another thread can enter() the ObjectMonitor, do the
1468 // notify(), and exit() the ObjectMonitor. If the other thread's
1469 // exit() call chooses this thread as the successor and the unpark()
1470 // call happens to occur while this thread is posting a
1471 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
1472 // handler using RawMonitors and consuming the unpark().
1473 //
1474 // To avoid the problem, we re-post the event. This does no harm
1475 // even if the original unpark() was not consumed because we are the
1476 // chosen successor for this monitor.
1477 if (node._notified != 0 && _succ == Self) {
1478 node._event->unpark();
1479 }
1480
1481 // The thread is on the WaitSet list - now park() it.
1482 // On MP systems it's conceivable that a brief spin before we park
1483 // could be profitable.
1484 //
1485 // TODO-FIXME: change the following logic to a loop of the form
1486 // while (!timeout && !interrupted && _notified == 0) park()
1487
1488 int ret = OS_OK ;
1489 int WasNotified = 0 ;
1490 { // State transition wrappers
1491 OSThread* osthread = Self->osthread();
1492 OSThreadWaitState osts(osthread, true);
1493 {
1494 ThreadBlockInVM tbivm(jt);
1495 // Thread is in thread_blocked state and oop access is unsafe.
1496 jt->set_suspend_equivalent();
1497
1498 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1499 // Intentionally empty
1500 } else
1501 if (node._notified == 0) {
1502 if (millis <= 0) {
1503 Self->_ParkEvent->park () ;
1504 } else {
1505 ret = Self->_ParkEvent->park (millis) ;
1506 }
1507 }
1508
1509 // were we externally suspended while we were waiting?
1510 if (ExitSuspendEquivalent (jt)) {
1511 // TODO-FIXME: add -- if succ == Self then succ = null.
1512 jt->java_suspend_self();
1513 }
1514
1515 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1516
1517
1518 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1519 // from the WaitSet to the EntryList.
1520 // See if we need to remove Node from the WaitSet.
1521 // We use double-checked locking to avoid grabbing _WaitSetLock
1522 // if the thread is not on the wait queue.
1523 //
1524 // Note that we don't need a fence before the fetch of TState.
1525 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1526 // written by the is thread. (perhaps the fetch might even be satisfied
1527 // by a look-aside into the processor's own store buffer, although given
1528 // the length of the code path between the prior ST and this load that's
1529 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1530 // then we'll acquire the lock and then re-fetch a fresh TState value.
1531 // That is, we fail toward safety.
1532
1533 if (node.TState == ObjectWaiter::TS_WAIT) {
1534 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1535 if (node.TState == ObjectWaiter::TS_WAIT) {
1536 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1537 assert(node._notified == 0, "invariant");
1538 node.TState = ObjectWaiter::TS_RUN ;
1539 }
1540 Thread::SpinRelease (&_WaitSetLock) ;
1541 }
1542
1543 // The thread is now either on off-list (TS_RUN),
1544 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1545 // The Node's TState variable is stable from the perspective of this thread.
1546 // No other threads will asynchronously modify TState.
1547 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1548 OrderAccess::loadload() ;
1549 if (_succ == Self) _succ = NULL ;
1550 WasNotified = node._notified ;
1551
1552 // Reentry phase -- reacquire the monitor.
1553 // re-enter contended monitor after object.wait().
1554 // retain OBJECT_WAIT state until re-enter successfully completes
1555 // Thread state is thread_in_vm and oop access is again safe,
1556 // although the raw address of the object may have changed.
1557 // (Don't cache naked oops over safepoints, of course).
1558
1559 // post monitor waited event. Note that this is past-tense, we are done waiting.
1560 if (JvmtiExport::should_post_monitor_waited()) {
1561 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1562 }
1563 OrderAccess::fence() ;
1564
1565 assert (Self->_Stalled != 0, "invariant") ;
1566 Self->_Stalled = 0 ;
1567
1568 assert (_owner != Self, "invariant") ;
1569 ObjectWaiter::TStates v = node.TState ;
1570 if (v == ObjectWaiter::TS_RUN) {
1571 enter (Self) ;
1572 } else {
1573 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1574 ReenterI (Self, &node) ;
1575 node.wait_reenter_end(this);
1576 }
1577
1578 // Self has reacquired the lock.
1579 // Lifecycle - the node representing Self must not appear on any queues.
1580 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1581 // want residual elements associated with this thread left on any lists.
1582 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1583 assert (_owner == Self, "invariant") ;
1584 assert (_succ != Self , "invariant") ;
1585 } // OSThreadWaitState()
1586
1587 jt->set_current_waiting_monitor(NULL);
1588
1589 guarantee (_recursions == 0, "invariant") ;
1590 _recursions = save; // restore the old recursion count
1591 _waiters--; // decrement the number of waiters
1592
1593 // Verify a few postconditions
1594 assert (_owner == Self , "invariant") ;
1595 assert (_succ != Self , "invariant") ;
1596 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1597
1598 if (SyncFlags & 32) {
1599 OrderAccess::fence() ;
1600 }
1601
1602 // check if the notification happened
1603 if (!WasNotified) {
1604 // no, it could be timeout or Thread.interrupt() or both
1605 // check for interrupt event, otherwise it is timeout
1606 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1607 TEVENT (Wait - throw IEX from epilog) ;
1608 THROW(vmSymbols::java_lang_InterruptedException());
1609 }
1610 }
1611
1612 // NOTE: Spurious wake up will be consider as timeout.
1613 // Monitor notify has precedence over thread interrupt.
1614 }
1615
1616
1617 // Consider:
1618 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1619 // then instead of transferring a thread from the WaitSet to the EntryList
1620 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1621
1622 void ObjectMonitor::notify(TRAPS) {
1623 CHECK_OWNER();
1624 if (_WaitSet == NULL) {
1625 TEVENT (Empty-Notify) ;
1626 return ;
1627 }
1628 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1629
1630 int Policy = Knob_MoveNotifyee ;
1631
1632 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1633 ObjectWaiter * iterator = DequeueWaiter() ;
1634 if (iterator != NULL) {
1635 TEVENT (Notify1 - Transfer) ;
1636 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1637 guarantee (iterator->_notified == 0, "invariant") ;
1638 if (Policy != 4) {
1639 iterator->TState = ObjectWaiter::TS_ENTER ;
1640 }
1641 iterator->_notified = 1 ;
1642
1643 ObjectWaiter * List = _EntryList ;
1644 if (List != NULL) {
1645 assert (List->_prev == NULL, "invariant") ;
1646 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1647 assert (List != iterator, "invariant") ;
1648 }
1649
1650 if (Policy == 0) { // prepend to EntryList
1651 if (List == NULL) {
1652 iterator->_next = iterator->_prev = NULL ;
1653 _EntryList = iterator ;
1654 } else {
1655 List->_prev = iterator ;
1656 iterator->_next = List ;
1657 iterator->_prev = NULL ;
1658 _EntryList = iterator ;
1659 }
1660 } else
1661 if (Policy == 1) { // append to EntryList
1662 if (List == NULL) {
1663 iterator->_next = iterator->_prev = NULL ;
1664 _EntryList = iterator ;
1665 } else {
1666 // CONSIDER: finding the tail currently requires a linear-time walk of
1667 // the EntryList. We can make tail access constant-time by converting to
1668 // a CDLL instead of using our current DLL.
1669 ObjectWaiter * Tail ;
1670 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1671 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1672 Tail->_next = iterator ;
1673 iterator->_prev = Tail ;
1674 iterator->_next = NULL ;
1675 }
1676 } else
1677 if (Policy == 2) { // prepend to cxq
1678 // prepend to cxq
1679 if (List == NULL) {
1680 iterator->_next = iterator->_prev = NULL ;
1681 _EntryList = iterator ;
1682 } else {
1683 iterator->TState = ObjectWaiter::TS_CXQ ;
1684 for (;;) {
1685 ObjectWaiter * Front = _cxq ;
1686 iterator->_next = Front ;
1687 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1688 break ;
1689 }
1690 }
1691 }
1692 } else
1693 if (Policy == 3) { // append to cxq
1694 iterator->TState = ObjectWaiter::TS_CXQ ;
1695 for (;;) {
1696 ObjectWaiter * Tail ;
1697 Tail = _cxq ;
1698 if (Tail == NULL) {
1699 iterator->_next = NULL ;
1700 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1701 break ;
1702 }
1703 } else {
1704 while (Tail->_next != NULL) Tail = Tail->_next ;
1705 Tail->_next = iterator ;
1706 iterator->_prev = Tail ;
1707 iterator->_next = NULL ;
1708 break ;
1709 }
1710 }
1711 } else {
1712 ParkEvent * ev = iterator->_event ;
1713 iterator->TState = ObjectWaiter::TS_RUN ;
1714 OrderAccess::fence() ;
1715 ev->unpark() ;
1716 }
1717
1718 if (Policy < 4) {
1719 iterator->wait_reenter_begin(this);
1720 }
1721
1722 // _WaitSetLock protects the wait queue, not the EntryList. We could
1723 // move the add-to-EntryList operation, above, outside the critical section
1724 // protected by _WaitSetLock. In practice that's not useful. With the
1725 // exception of wait() timeouts and interrupts the monitor owner
1726 // is the only thread that grabs _WaitSetLock. There's almost no contention
1727 // on _WaitSetLock so it's not profitable to reduce the length of the
1728 // critical section.
1729 }
1730
1731 Thread::SpinRelease (&_WaitSetLock) ;
1732
1733 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1734 ObjectMonitor::_sync_Notifications->inc() ;
1735 }
1736 }
1737
1738
1739 void ObjectMonitor::notifyAll(TRAPS) {
1740 CHECK_OWNER();
1741 ObjectWaiter* iterator;
1742 if (_WaitSet == NULL) {
1743 TEVENT (Empty-NotifyAll) ;
1744 return ;
1745 }
1746 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1747
1748 int Policy = Knob_MoveNotifyee ;
1749 int Tally = 0 ;
1750 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1751
1752 for (;;) {
1753 iterator = DequeueWaiter () ;
1754 if (iterator == NULL) break ;
1755 TEVENT (NotifyAll - Transfer1) ;
1756 ++Tally ;
1757
1758 // Disposition - what might we do with iterator ?
1759 // a. add it directly to the EntryList - either tail or head.
1760 // b. push it onto the front of the _cxq.
1761 // For now we use (a).
1762
1763 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1764 guarantee (iterator->_notified == 0, "invariant") ;
1765 iterator->_notified = 1 ;
1766 if (Policy != 4) {
1767 iterator->TState = ObjectWaiter::TS_ENTER ;
1768 }
1769
1770 ObjectWaiter * List = _EntryList ;
1771 if (List != NULL) {
1772 assert (List->_prev == NULL, "invariant") ;
1773 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1774 assert (List != iterator, "invariant") ;
1775 }
1776
1777 if (Policy == 0) { // prepend to EntryList
1778 if (List == NULL) {
1779 iterator->_next = iterator->_prev = NULL ;
1780 _EntryList = iterator ;
1781 } else {
1782 List->_prev = iterator ;
1783 iterator->_next = List ;
1784 iterator->_prev = NULL ;
1785 _EntryList = iterator ;
1786 }
1787 } else
1788 if (Policy == 1) { // append to EntryList
1789 if (List == NULL) {
1790 iterator->_next = iterator->_prev = NULL ;
1791 _EntryList = iterator ;
1792 } else {
1793 // CONSIDER: finding the tail currently requires a linear-time walk of
1794 // the EntryList. We can make tail access constant-time by converting to
1795 // a CDLL instead of using our current DLL.
1796 ObjectWaiter * Tail ;
1797 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1798 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1799 Tail->_next = iterator ;
1800 iterator->_prev = Tail ;
1801 iterator->_next = NULL ;
1802 }
1803 } else
1804 if (Policy == 2) { // prepend to cxq
1805 // prepend to cxq
1806 iterator->TState = ObjectWaiter::TS_CXQ ;
1807 for (;;) {
1808 ObjectWaiter * Front = _cxq ;
1809 iterator->_next = Front ;
1810 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1811 break ;
1812 }
1813 }
1814 } else
1815 if (Policy == 3) { // append to cxq
1816 iterator->TState = ObjectWaiter::TS_CXQ ;
1817 for (;;) {
1818 ObjectWaiter * Tail ;
1819 Tail = _cxq ;
1820 if (Tail == NULL) {
1821 iterator->_next = NULL ;
1822 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1823 break ;
1824 }
1825 } else {
1826 while (Tail->_next != NULL) Tail = Tail->_next ;
1827 Tail->_next = iterator ;
1828 iterator->_prev = Tail ;
1829 iterator->_next = NULL ;
1830 break ;
1831 }
1832 }
1833 } else {
1834 ParkEvent * ev = iterator->_event ;
1835 iterator->TState = ObjectWaiter::TS_RUN ;
1836 OrderAccess::fence() ;
1837 ev->unpark() ;
1838 }
1839
1840 if (Policy < 4) {
1841 iterator->wait_reenter_begin(this);
1842 }
1843
1844 // _WaitSetLock protects the wait queue, not the EntryList. We could
1845 // move the add-to-EntryList operation, above, outside the critical section
1846 // protected by _WaitSetLock. In practice that's not useful. With the
1847 // exception of wait() timeouts and interrupts the monitor owner
1848 // is the only thread that grabs _WaitSetLock. There's almost no contention
1849 // on _WaitSetLock so it's not profitable to reduce the length of the
1850 // critical section.
1851 }
1852
1853 Thread::SpinRelease (&_WaitSetLock) ;
1854
1855 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1856 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1857 }
1858 }
1859
1860 // -----------------------------------------------------------------------------
1861 // Adaptive Spinning Support
1862 //
1863 // Adaptive spin-then-block - rational spinning
1864 //
1865 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1866 // algorithm. On high order SMP systems it would be better to start with
1867 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1868 // a contending thread could enqueue itself on the cxq and then spin locally
1869 // on a thread-specific variable such as its ParkEvent._Event flag.
1870 // That's left as an exercise for the reader. Note that global spinning is
1871 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1872 // low latency and massive bandwidth.
1873 //
1874 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1875 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1876 // (duration) or we can fix the count at approximately the duration of
1877 // a context switch and vary the frequency. Of course we could also
1878 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1879 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1880 //
1881 // This implementation varies the duration "D", where D varies with
1882 // the success rate of recent spin attempts. (D is capped at approximately
1883 // length of a round-trip context switch). The success rate for recent
1884 // spin attempts is a good predictor of the success rate of future spin
1885 // attempts. The mechanism adapts automatically to varying critical
1886 // section length (lock modality), system load and degree of parallelism.
1887 // D is maintained per-monitor in _SpinDuration and is initialized
1888 // optimistically. Spin frequency is fixed at 100%.
1889 //
1890 // Note that _SpinDuration is volatile, but we update it without locks
1891 // or atomics. The code is designed so that _SpinDuration stays within
1892 // a reasonable range even in the presence of races. The arithmetic
1893 // operations on _SpinDuration are closed over the domain of legal values,
1894 // so at worst a race will install and older but still legal value.
1895 // At the very worst this introduces some apparent non-determinism.
1896 // We might spin when we shouldn't or vice-versa, but since the spin
1897 // count are relatively short, even in the worst case, the effect is harmless.
1898 //
1899 // Care must be taken that a low "D" value does not become an
1900 // an absorbing state. Transient spinning failures -- when spinning
1901 // is overall profitable -- should not cause the system to converge
1902 // on low "D" values. We want spinning to be stable and predictable
1903 // and fairly responsive to change and at the same time we don't want
1904 // it to oscillate, become metastable, be "too" non-deterministic,
1905 // or converge on or enter undesirable stable absorbing states.
1906 //
1907 // We implement a feedback-based control system -- using past behavior
1908 // to predict future behavior. We face two issues: (a) if the
1909 // input signal is random then the spin predictor won't provide optimal
1910 // results, and (b) if the signal frequency is too high then the control
1911 // system, which has some natural response lag, will "chase" the signal.
1912 // (b) can arise from multimodal lock hold times. Transient preemption
1913 // can also result in apparent bimodal lock hold times.
1914 // Although sub-optimal, neither condition is particularly harmful, as
1915 // in the worst-case we'll spin when we shouldn't or vice-versa.
1916 // The maximum spin duration is rather short so the failure modes aren't bad.
1917 // To be conservative, I've tuned the gain in system to bias toward
1918 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1919 // "rings" or oscillates between spinning and not spinning. This happens
1920 // when spinning is just on the cusp of profitability, however, so the
1921 // situation is not dire. The state is benign -- there's no need to add
1922 // hysteresis control to damp the transition rate between spinning and
1923 // not spinning.
1924 //
1925
1926 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1927 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1928
1929 // Spinning: Fixed frequency (100%), vary duration
1930
1931
1932 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1933
1934 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1935 int ctr = Knob_FixedSpin ;
1936 if (ctr != 0) {
1937 while (--ctr >= 0) {
1938 if (TryLock (Self) > 0) return 1 ;
1939 SpinPause () ;
1940 }
1941 return 0 ;
1942 }
1943
1944 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1945 if (TryLock(Self) > 0) {
1946 // Increase _SpinDuration ...
1947 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1948 // Raising _SpurDuration to the poverty line is key.
1949 int x = _SpinDuration ;
1950 if (x < Knob_SpinLimit) {
1951 if (x < Knob_Poverty) x = Knob_Poverty ;
1952 _SpinDuration = x + Knob_BonusB ;
1953 }
1954 return 1 ;
1955 }
1956 SpinPause () ;
1957 }
1958
1959 // Admission control - verify preconditions for spinning
1960 //
1961 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1962 // becoming an absorbing state. Put another way, we spin briefly to
1963 // sample, just in case the system load, parallelism, contention, or lock
1964 // modality changed.
1965 //
1966 // Consider the following alternative:
1967 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1968 // spin attempt. "Periodically" might mean after a tally of
1969 // the # of failed spin attempts (or iterations) reaches some threshold.
1970 // This takes us into the realm of 1-out-of-N spinning, where we
1971 // hold the duration constant but vary the frequency.
1972
1973 ctr = _SpinDuration ;
1974 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
1975 if (ctr <= 0) return 0 ;
1976
1977 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
1978 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1979 TEVENT (Spin abort - notrunnable [TOP]);
1980 return 0 ;
1981 }
1982
1983 int MaxSpin = Knob_MaxSpinners ;
1984 if (MaxSpin >= 0) {
1985 if (_Spinner > MaxSpin) {
1986 TEVENT (Spin abort -- too many spinners) ;
1987 return 0 ;
1988 }
1989 // Slighty racy, but benign ...
1990 Adjust (&_Spinner, 1) ;
1991 }
1992
1993 // We're good to spin ... spin ingress.
1994 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1995 // when preparing to LD...CAS _owner, etc and the CAS is likely
1996 // to succeed.
1997 int hits = 0 ;
1998 int msk = 0 ;
1999 int caspty = Knob_CASPenalty ;
2000 int oxpty = Knob_OXPenalty ;
2001 int sss = Knob_SpinSetSucc ;
2002 if (sss && _succ == NULL ) _succ = Self ;
2003 Thread * prv = NULL ;
2004
2005 // There are three ways to exit the following loop:
2006 // 1. A successful spin where this thread has acquired the lock.
2007 // 2. Spin failure with prejudice
2008 // 3. Spin failure without prejudice
2009
2010 while (--ctr >= 0) {
2011
2012 // Periodic polling -- Check for pending GC
2013 // Threads may spin while they're unsafe.
2014 // We don't want spinning threads to delay the JVM from reaching
2015 // a stop-the-world safepoint or to steal cycles from GC.
2016 // If we detect a pending safepoint we abort in order that
2017 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2018 // this thread, if safe, doesn't steal cycles from GC.
2019 // This is in keeping with the "no loitering in runtime" rule.
2020 // We periodically check to see if there's a safepoint pending.
2021 if ((ctr & 0xFF) == 0) {
2022 if (SafepointSynchronize::do_call_back()) {
2023 TEVENT (Spin: safepoint) ;
2024 goto Abort ; // abrupt spin egress
2025 }
2026 if (Knob_UsePause & 1) SpinPause () ;
2027
2028 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2029 if (hits > 50 && scb != NULL) {
2030 int abend = (*scb)(SpinCallbackArgument, 0) ;
2031 }
2032 }
2033
2034 if (Knob_UsePause & 2) SpinPause() ;
2035
2036 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2037 // This is useful on classic SMP systems, but is of less utility on
2038 // N1-style CMT platforms.
2039 //
2040 // Trade-off: lock acquisition latency vs coherency bandwidth.
2041 // Lock hold times are typically short. A histogram
2042 // of successful spin attempts shows that we usually acquire
2043 // the lock early in the spin. That suggests we want to
2044 // sample _owner frequently in the early phase of the spin,
2045 // but then back-off and sample less frequently as the spin
2046 // progresses. The back-off makes a good citizen on SMP big
2047 // SMP systems. Oversampling _owner can consume excessive
2048 // coherency bandwidth. Relatedly, if we _oversample _owner we
2049 // can inadvertently interfere with the the ST m->owner=null.
2050 // executed by the lock owner.
2051 if (ctr & msk) continue ;
2052 ++hits ;
2053 if ((hits & 0xF) == 0) {
2054 // The 0xF, above, corresponds to the exponent.
2055 // Consider: (msk+1)|msk
2056 msk = ((msk << 2)|3) & BackOffMask ;
2057 }
2058
2059 // Probe _owner with TATAS
2060 // If this thread observes the monitor transition or flicker
2061 // from locked to unlocked to locked, then the odds that this
2062 // thread will acquire the lock in this spin attempt go down
2063 // considerably. The same argument applies if the CAS fails
2064 // or if we observe _owner change from one non-null value to
2065 // another non-null value. In such cases we might abort
2066 // the spin without prejudice or apply a "penalty" to the
2067 // spin count-down variable "ctr", reducing it by 100, say.
2068
2069 Thread * ox = (Thread *) _owner ;
2070 if (ox == NULL) {
2071 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2072 if (ox == NULL) {
2073 // The CAS succeeded -- this thread acquired ownership
2074 // Take care of some bookkeeping to exit spin state.
2075 if (sss && _succ == Self) {
2076 _succ = NULL ;
2077 }
2078 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2079
2080 // Increase _SpinDuration :
2081 // The spin was successful (profitable) so we tend toward
2082 // longer spin attempts in the future.
2083 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2084 // If we acquired the lock early in the spin cycle it
2085 // makes sense to increase _SpinDuration proportionally.
2086 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2087 int x = _SpinDuration ;
2088 if (x < Knob_SpinLimit) {
2089 if (x < Knob_Poverty) x = Knob_Poverty ;
2090 _SpinDuration = x + Knob_Bonus ;
2091 }
2092 return 1 ;
2093 }
2094
2095 // The CAS failed ... we can take any of the following actions:
2096 // * penalize: ctr -= Knob_CASPenalty
2097 // * exit spin with prejudice -- goto Abort;
2098 // * exit spin without prejudice.
2099 // * Since CAS is high-latency, retry again immediately.
2100 prv = ox ;
2101 TEVENT (Spin: cas failed) ;
2102 if (caspty == -2) break ;
2103 if (caspty == -1) goto Abort ;
2104 ctr -= caspty ;
2105 continue ;
2106 }
2107
2108 // Did lock ownership change hands ?
2109 if (ox != prv && prv != NULL ) {
2110 TEVENT (spin: Owner changed)
2111 if (oxpty == -2) break ;
2112 if (oxpty == -1) goto Abort ;
2113 ctr -= oxpty ;
2114 }
2115 prv = ox ;
2116
2117 // Abort the spin if the owner is not executing.
2118 // The owner must be executing in order to drop the lock.
2119 // Spinning while the owner is OFFPROC is idiocy.
2120 // Consider: ctr -= RunnablePenalty ;
2121 if (Knob_OState && NotRunnable (Self, ox)) {
2122 TEVENT (Spin abort - notrunnable);
2123 goto Abort ;
2124 }
2125 if (sss && _succ == NULL ) _succ = Self ;
2126 }
2127
2128 // Spin failed with prejudice -- reduce _SpinDuration.
2129 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2130 // AIMD is globally stable.
2131 TEVENT (Spin failure) ;
2132 {
2133 int x = _SpinDuration ;
2134 if (x > 0) {
2135 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2136 // This is globally sample and tends to damp the response.
2137 x -= Knob_Penalty ;
2138 if (x < 0) x = 0 ;
2139 _SpinDuration = x ;
2140 }
2141 }
2142
2143 Abort:
2144 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2145 if (sss && _succ == Self) {
2146 _succ = NULL ;
2147 // Invariant: after setting succ=null a contending thread
2148 // must recheck-retry _owner before parking. This usually happens
2149 // in the normal usage of TrySpin(), but it's safest
2150 // to make TrySpin() as foolproof as possible.
2151 OrderAccess::fence() ;
2152 if (TryLock(Self) > 0) return 1 ;
2153 }
2154 return 0 ;
2155 }
2156
2157 // NotRunnable() -- informed spinning
2158 //
2159 // Don't bother spinning if the owner is not eligible to drop the lock.
2160 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2161 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2162 // The thread must be runnable in order to drop the lock in timely fashion.
2163 // If the _owner is not runnable then spinning will not likely be
2164 // successful (profitable).
2165 //
2166 // Beware -- the thread referenced by _owner could have died
2167 // so a simply fetch from _owner->_thread_state might trap.
2168 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2169 // Because of the lifecycle issues the schedctl and _thread_state values
2170 // observed by NotRunnable() might be garbage. NotRunnable must
2171 // tolerate this and consider the observed _thread_state value
2172 // as advisory.
2173 //
2174 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2175 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2176 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2177 // with the LSB of _owner. Another option would be to probablistically probe
2178 // the putative _owner->TypeTag value.
2179 //
2180 // Checking _thread_state isn't perfect. Even if the thread is
2181 // in_java it might be blocked on a page-fault or have been preempted
2182 // and sitting on a ready/dispatch queue. _thread state in conjunction
2183 // with schedctl.sc_state gives us a good picture of what the
2184 // thread is doing, however.
2185 //
2186 // TODO: check schedctl.sc_state.
2187 // We'll need to use SafeFetch32() to read from the schedctl block.
2188 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2189 //
2190 // The return value from NotRunnable() is *advisory* -- the
2191 // result is based on sampling and is not necessarily coherent.
2192 // The caller must tolerate false-negative and false-positive errors.
2193 // Spinning, in general, is probabilistic anyway.
2194
2195
2196 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2197 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2198 if (!OwnerIsThread) return 0 ;
2199
2200 if (ox == NULL) return 0 ;
2201
2202 // Avoid transitive spinning ...
2203 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2204 // Immediately after T1 acquires L it's possible that T2, also
2205 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2206 // This occurs transiently after T1 acquired L but before
2207 // T1 managed to clear T1.Stalled. T2 does not need to abort
2208 // its spin in this circumstance.
2209 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2210
2211 if (BlockedOn == 1) return 1 ;
2212 if (BlockedOn != 0) {
2213 return BlockedOn != intptr_t(this) && _owner == ox ;
2214 }
2215
2216 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2217 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2218 // consider also: jst != _thread_in_Java -- but that's overspecific.
2219 return jst == _thread_blocked || jst == _thread_in_native ;
2220 }
2221
2222
2223 // -----------------------------------------------------------------------------
2224 // WaitSet management ...
2225
2226 ObjectWaiter::ObjectWaiter(Thread* thread) {
2227 _next = NULL;
2228 _prev = NULL;
2229 _notified = 0;
2230 TState = TS_RUN ;
2231 _thread = thread;
2232 _event = thread->_ParkEvent ;
2233 _active = false;
2234 assert (_event != NULL, "invariant") ;
2235 }
2236
2237 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2238 JavaThread *jt = (JavaThread *)this->_thread;
2239 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2240 }
2241
2242 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2243 JavaThread *jt = (JavaThread *)this->_thread;
2244 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2245 }
2246
2247 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2248 assert(node != NULL, "should not dequeue NULL node");
2249 assert(node->_prev == NULL, "node already in list");
2250 assert(node->_next == NULL, "node already in list");
2251 // put node at end of queue (circular doubly linked list)
2252 if (_WaitSet == NULL) {
2253 _WaitSet = node;
2254 node->_prev = node;
2255 node->_next = node;
2256 } else {
2257 ObjectWaiter* head = _WaitSet ;
2258 ObjectWaiter* tail = head->_prev;
2259 assert(tail->_next == head, "invariant check");
2260 tail->_next = node;
2261 head->_prev = node;
2262 node->_next = head;
2263 node->_prev = tail;
2264 }
2265 }
2266
2267 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2268 // dequeue the very first waiter
2269 ObjectWaiter* waiter = _WaitSet;
2270 if (waiter) {
2271 DequeueSpecificWaiter(waiter);
2272 }
2273 return waiter;
2274 }
2275
2276 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2277 assert(node != NULL, "should not dequeue NULL node");
2278 assert(node->_prev != NULL, "node already removed from list");
2279 assert(node->_next != NULL, "node already removed from list");
2280 // when the waiter has woken up because of interrupt,
2281 // timeout or other spurious wake-up, dequeue the
2282 // waiter from waiting list
2283 ObjectWaiter* next = node->_next;
2284 if (next == node) {
2285 assert(node->_prev == node, "invariant check");
2286 _WaitSet = NULL;
2287 } else {
2288 ObjectWaiter* prev = node->_prev;
2289 assert(prev->_next == node, "invariant check");
2290 assert(next->_prev == node, "invariant check");
2291 next->_prev = prev;
2292 prev->_next = next;
2293 if (_WaitSet == node) {
2294 _WaitSet = next;
2295 }
2296 }
2297 node->_next = NULL;
2298 node->_prev = NULL;
2299 }
2300
2301 // -----------------------------------------------------------------------------
2302 // PerfData support
2303 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2304 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2305 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2306 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2307 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2308 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2309 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2310 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2311 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2312 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2313 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2314 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2315 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2316 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2317 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2318 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2319 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2320 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2321
2322 // One-shot global initialization for the sync subsystem.
2323 // We could also defer initialization and initialize on-demand
2324 // the first time we call inflate(). Initialization would
2325 // be protected - like so many things - by the MonitorCache_lock.
2326
2327 void ObjectMonitor::Initialize () {
2328 static int InitializationCompleted = 0 ;
2329 assert (InitializationCompleted == 0, "invariant") ;
2330 InitializationCompleted = 1 ;
2331 if (UsePerfData) {
2332 EXCEPTION_MARK ;
2333 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2334 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2335 NEWPERFCOUNTER(_sync_Inflations) ;
2336 NEWPERFCOUNTER(_sync_Deflations) ;
2337 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2338 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2339 NEWPERFCOUNTER(_sync_Parks) ;
2340 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2341 NEWPERFCOUNTER(_sync_Notifications) ;
2342 NEWPERFCOUNTER(_sync_SlowEnter) ;
2343 NEWPERFCOUNTER(_sync_SlowExit) ;
2344 NEWPERFCOUNTER(_sync_SlowNotify) ;
2345 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2346 NEWPERFCOUNTER(_sync_FailedSpins) ;
2347 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2348 NEWPERFCOUNTER(_sync_PrivateA) ;
2349 NEWPERFCOUNTER(_sync_PrivateB) ;
2350 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2351 NEWPERFCOUNTER(_sync_MonScavenged) ;
2352 NEWPERFVARIABLE(_sync_MonExtant) ;
2353 #undef NEWPERFCOUNTER
2354 }
2355 }
2356
2357
2358 // Compile-time asserts
2359 // When possible, it's better to catch errors deterministically at
2360 // compile-time than at runtime. The down-side to using compile-time
2361 // asserts is that error message -- often something about negative array
2362 // indices -- is opaque.
2363
2364 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2365
2366 void ObjectMonitor::ctAsserts() {
2367 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2368 }
2369
2370
2371 static char * kvGet (char * kvList, const char * Key) {
2372 if (kvList == NULL) return NULL ;
2373 size_t n = strlen (Key) ;
2374 char * Search ;
2375 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2376 if (strncmp (Search, Key, n) == 0) {
2377 if (Search[n] == '=') return Search + n + 1 ;
2378 if (Search[n] == 0) return (char *) "1" ;
2379 }
2380 }
2381 return NULL ;
2382 }
2383
2384 static int kvGetInt (char * kvList, const char * Key, int Default) {
2385 char * v = kvGet (kvList, Key) ;
2386 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2387 if (Knob_ReportSettings && v != NULL) {
2388 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2389 ::fflush (stdout) ;
2390 }
2391 return rslt ;
2392 }
2393
2394 void ObjectMonitor::DeferredInitialize () {
2395 if (InitDone > 0) return ;
2396 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2397 while (InitDone != 1) ;
2398 return ;
2399 }
2400
2401 // One-shot global initialization ...
2402 // The initialization is idempotent, so we don't need locks.
2403 // In the future consider doing this via os::init_2().
2404 // SyncKnobs consist of <Key>=<Value> pairs in the style
2405 // of environment variables. Start by converting ':' to NUL.
2406
2407 if (SyncKnobs == NULL) SyncKnobs = "" ;
2408
2409 size_t sz = strlen (SyncKnobs) ;
2410 char * knobs = (char *) malloc (sz + 2) ;
2411 if (knobs == NULL) {
2412 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2413 guarantee (0, "invariant") ;
2414 }
2415 strcpy (knobs, SyncKnobs) ;
2416 knobs[sz+1] = 0 ;
2417 for (char * p = knobs ; *p ; p++) {
2418 if (*p == ':') *p = 0 ;
2419 }
2420
2421 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2422 SETKNOB(ReportSettings) ;
2423 SETKNOB(Verbose) ;
2424 SETKNOB(FixedSpin) ;
2425 SETKNOB(SpinLimit) ;
2426 SETKNOB(SpinBase) ;
2427 SETKNOB(SpinBackOff);
2428 SETKNOB(CASPenalty) ;
2429 SETKNOB(OXPenalty) ;
2430 SETKNOB(LogSpins) ;
2431 SETKNOB(SpinSetSucc) ;
2432 SETKNOB(SuccEnabled) ;
2433 SETKNOB(SuccRestrict) ;
2434 SETKNOB(Penalty) ;
2435 SETKNOB(Bonus) ;
2436 SETKNOB(BonusB) ;
2437 SETKNOB(Poverty) ;
2438 SETKNOB(SpinAfterFutile) ;
2439 SETKNOB(UsePause) ;
2440 SETKNOB(SpinEarly) ;
2441 SETKNOB(OState) ;
2442 SETKNOB(MaxSpinners) ;
2443 SETKNOB(PreSpin) ;
2444 SETKNOB(ExitPolicy) ;
2445 SETKNOB(QMode);
2446 SETKNOB(ResetEvent) ;
2447 SETKNOB(MoveNotifyee) ;
2448 SETKNOB(FastHSSEC) ;
2449 #undef SETKNOB
2450
2451 if (os::is_MP()) {
2452 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2453 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2454 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2455 } else {
2456 Knob_SpinLimit = 0 ;
2457 Knob_SpinBase = 0 ;
2458 Knob_PreSpin = 0 ;
2459 Knob_FixedSpin = -1 ;
2460 }
2461
2462 if (Knob_LogSpins == 0) {
2463 ObjectMonitor::_sync_FailedSpins = NULL ;
2464 }
2465
2466 free (knobs) ;
2467 OrderAccess::fence() ;
2468 InitDone = 1 ;
2469 }
2470
2471 #ifndef PRODUCT
2472 void ObjectMonitor::verify() {
2473 }
2474
2475 void ObjectMonitor::print() {
2476 }
2477 #endif