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