1 /*
2 * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/osThread.hpp"
36 #include "runtime/stubRoutines.hpp"
37 #include "runtime/thread.inline.hpp"
38 #include "services/threadService.hpp"
39 #include "trace/tracing.hpp"
40 #include "trace/traceMacros.hpp"
41 #include "utilities/dtrace.hpp"
42 #include "utilities/macros.hpp"
43 #include "utilities/preserveException.hpp"
44 #ifdef TARGET_OS_FAMILY_linux
45 # include "os_linux.inline.hpp"
46 #endif
47 #ifdef TARGET_OS_FAMILY_solaris
48 # include "os_solaris.inline.hpp"
49 #endif
50 #ifdef TARGET_OS_FAMILY_windows
51 # include "os_windows.inline.hpp"
52 #endif
53 #ifdef TARGET_OS_FAMILY_bsd
54 # include "os_bsd.inline.hpp"
55 #endif
56
57 #if defined(__GNUC__) && !defined(IA64)
58 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
59 #define ATTR __attribute__((noinline))
60 #else
61 #define ATTR
62 #endif
63
64
65 #ifdef DTRACE_ENABLED
66
67 // Only bother with this argument setup if dtrace is available
68 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
69
70
71 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
72 char* bytes = NULL; \
73 int len = 0; \
74 jlong jtid = SharedRuntime::get_java_tid(thread); \
75 Symbol* klassname = ((oop)obj)->klass()->name(); \
76 if (klassname != NULL) { \
77 bytes = (char*)klassname->bytes(); \
78 len = klassname->utf8_length(); \
79 }
80
81 #ifndef USDT2
82
83 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
84 jlong, uintptr_t, char*, int);
85 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
86 jlong, uintptr_t, char*, int);
87 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
88 jlong, uintptr_t, char*, int);
89 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
90 jlong, uintptr_t, char*, int);
91 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
92 jlong, uintptr_t, char*, int);
93
94 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
95 { \
96 if (DTraceMonitorProbes) { \
97 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
98 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
99 (monitor), bytes, len, (millis)); \
100 } \
101 }
102
103 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
104 { \
105 if (DTraceMonitorProbes) { \
106 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
107 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
108 (uintptr_t)(monitor), bytes, len); \
109 } \
110 }
111
112 #else /* USDT2 */
113
114 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
115 { \
116 if (DTraceMonitorProbes) { \
117 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
118 HOTSPOT_MONITOR_WAIT(jtid, \
119 (monitor), bytes, len, (millis)); \
120 } \
121 }
122
123 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
124 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
125 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
126 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
127 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
128
129 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
130 { \
131 if (DTraceMonitorProbes) { \
132 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
133 HOTSPOT_MONITOR_##probe(jtid, \
134 (uintptr_t)(monitor), bytes, len); \
135 } \
136 }
137
138 #endif /* USDT2 */
139 #else // ndef DTRACE_ENABLED
140
141 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
142 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
143
144 #endif // ndef DTRACE_ENABLED
145
146 // Tunables ...
147 // The knob* variables are effectively final. Once set they should
148 // never be modified hence. Consider using __read_mostly with GCC.
149
150 int ObjectMonitor::Knob_Verbose = 0 ;
151 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
152 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
153 static int Knob_HandOff = 0 ;
154 static int Knob_ReportSettings = 0 ;
155
156 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
157 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
158 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
159 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
160 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
161 static int Knob_SpinEarly = 1 ;
162 static int Knob_SuccEnabled = 1 ; // futile wake throttling
163 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
164 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
165 static int Knob_Bonus = 100 ; // spin success bonus
166 static int Knob_BonusB = 100 ; // spin success bonus
167 static int Knob_Penalty = 200 ; // spin failure penalty
168 static int Knob_Poverty = 1000 ;
169 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
170 static int Knob_FixedSpin = 0 ;
171 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
172 static int Knob_UsePause = 1 ;
173 static int Knob_ExitPolicy = 0 ;
174 static int Knob_PreSpin = 10 ; // 20-100 likely better
175 static int Knob_ResetEvent = 0 ;
176 static int BackOffMask = 0 ;
177
178 static int Knob_FastHSSEC = 0 ;
179 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
180 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
181 static volatile int InitDone = 0 ;
182
183 #define TrySpin TrySpin_VaryDuration
184
185 // -----------------------------------------------------------------------------
186 // Theory of operations -- Monitors lists, thread residency, etc:
187 //
188 // * A thread acquires ownership of a monitor by successfully
189 // CAS()ing the _owner field from null to non-null.
190 //
191 // * Invariant: A thread appears on at most one monitor list --
192 // cxq, EntryList or WaitSet -- at any one time.
193 //
194 // * Contending threads "push" themselves onto the cxq with CAS
195 // and then spin/park.
196 //
197 // * After a contending thread eventually acquires the lock it must
198 // dequeue itself from either the EntryList or the cxq.
199 //
200 // * The exiting thread identifies and unparks an "heir presumptive"
201 // tentative successor thread on the EntryList. Critically, the
202 // exiting thread doesn't unlink the successor thread from the EntryList.
203 // After having been unparked, the wakee will recontend for ownership of
204 // the monitor. The successor (wakee) will either acquire the lock or
205 // re-park itself.
206 //
207 // Succession is provided for by a policy of competitive handoff.
208 // The exiting thread does _not_ grant or pass ownership to the
209 // successor thread. (This is also referred to as "handoff" succession").
210 // Instead the exiting thread releases ownership and possibly wakes
211 // a successor, so the successor can (re)compete for ownership of the lock.
212 // If the EntryList is empty but the cxq is populated the exiting
213 // thread will drain the cxq into the EntryList. It does so by
214 // by detaching the cxq (installing null with CAS) and folding
215 // the threads from the cxq into the EntryList. The EntryList is
216 // doubly linked, while the cxq is singly linked because of the
217 // CAS-based "push" used to enqueue recently arrived threads (RATs).
218 //
219 // * Concurrency invariants:
220 //
221 // -- only the monitor owner may access or mutate the EntryList.
222 // The mutex property of the monitor itself protects the EntryList
223 // from concurrent interference.
224 // -- Only the monitor owner may detach the cxq.
225 //
226 // * The monitor entry list operations avoid locks, but strictly speaking
227 // they're not lock-free. Enter is lock-free, exit is not.
228 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
229 //
230 // * The cxq can have multiple concurrent "pushers" but only one concurrent
231 // detaching thread. This mechanism is immune from the ABA corruption.
232 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
233 //
234 // * Taken together, the cxq and the EntryList constitute or form a
235 // single logical queue of threads stalled trying to acquire the lock.
236 // We use two distinct lists to improve the odds of a constant-time
237 // dequeue operation after acquisition (in the ::enter() epilogue) and
238 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
239 // A key desideratum is to minimize queue & monitor metadata manipulation
240 // that occurs while holding the monitor lock -- that is, we want to
241 // minimize monitor lock holds times. Note that even a small amount of
242 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
243 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
244 // locks and monitor metadata.
245 //
246 // Cxq points to the the set of Recently Arrived Threads attempting entry.
247 // Because we push threads onto _cxq with CAS, the RATs must take the form of
248 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
249 // the unlocking thread notices that EntryList is null but _cxq is != null.
250 //
251 // The EntryList is ordered by the prevailing queue discipline and
252 // can be organized in any convenient fashion, such as a doubly-linked list or
253 // a circular doubly-linked list. Critically, we want insert and delete operations
254 // to operate in constant-time. If we need a priority queue then something akin
255 // to Solaris' sleepq would work nicely. Viz.,
256 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
257 // Queue discipline is enforced at ::exit() time, when the unlocking thread
258 // drains the cxq into the EntryList, and orders or reorders the threads on the
259 // EntryList accordingly.
260 //
261 // Barring "lock barging", this mechanism provides fair cyclic ordering,
262 // somewhat similar to an elevator-scan.
263 //
264 // * The monitor synchronization subsystem avoids the use of native
265 // synchronization primitives except for the narrow platform-specific
266 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
267 // the semantics of park-unpark. Put another way, this monitor implementation
268 // depends only on atomic operations and park-unpark. The monitor subsystem
269 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
270 // underlying OS manages the READY<->RUN transitions.
271 //
272 // * Waiting threads reside on the WaitSet list -- wait() puts
273 // the caller onto the WaitSet.
274 //
275 // * notify() or notifyAll() simply transfers threads from the WaitSet to
276 // either the EntryList or cxq. Subsequent exit() operations will
277 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
278 // it's likely the notifyee would simply impale itself on the lock held
279 // by the notifier.
280 //
281 // * An interesting alternative is to encode cxq as (List,LockByte) where
282 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
283 // variable, like _recursions, in the scheme. The threads or Events that form
284 // the list would have to be aligned in 256-byte addresses. A thread would
285 // try to acquire the lock or enqueue itself with CAS, but exiting threads
286 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
287 // Note that is is *not* word-tearing, but it does presume that full-word
288 // CAS operations are coherent with intermix with STB operations. That's true
289 // on most common processors.
290 //
291 // * See also http://blogs.sun.com/dave
292
293
294 // -----------------------------------------------------------------------------
295 // Enter support
296
297 bool ObjectMonitor::try_enter(Thread* THREAD) {
298 if (THREAD != _owner) {
299 if (THREAD->is_lock_owned ((address)_owner)) {
300 assert(_recursions == 0, "internal state error");
301 _owner = THREAD ;
302 _recursions = 1 ;
303 OwnerIsThread = 1 ;
304 return true;
305 }
306 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
307 return false;
308 }
309 return true;
310 } else {
311 _recursions++;
312 return true;
313 }
314 }
315
316 void ATTR ObjectMonitor::enter(TRAPS) {
317 // The following code is ordered to check the most common cases first
318 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
319 Thread * const Self = THREAD ;
320 void * cur ;
321
322 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
323 if (cur == NULL) {
324 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
325 assert (_recursions == 0 , "invariant") ;
326 assert (_owner == Self, "invariant") ;
327 // CONSIDER: set or assert OwnerIsThread == 1
328 return ;
329 }
330
331 if (cur == Self) {
332 // TODO-FIXME: check for integer overflow! BUGID 6557169.
333 _recursions ++ ;
334 return ;
335 }
336
337 if (Self->is_lock_owned ((address)cur)) {
338 assert (_recursions == 0, "internal state error");
339 _recursions = 1 ;
340 // Commute owner from a thread-specific on-stack BasicLockObject address to
341 // a full-fledged "Thread *".
342 _owner = Self ;
343 OwnerIsThread = 1 ;
344 return ;
345 }
346
347 // We've encountered genuine contention.
348 assert (Self->_Stalled == 0, "invariant") ;
349 Self->_Stalled = intptr_t(this) ;
350
351 // Try one round of spinning *before* enqueueing Self
352 // and before going through the awkward and expensive state
353 // transitions. The following spin is strictly optional ...
354 // Note that if we acquire the monitor from an initial spin
355 // we forgo posting JVMTI events and firing DTRACE probes.
356 if (Knob_SpinEarly && TrySpin (Self) > 0) {
357 assert (_owner == Self , "invariant") ;
358 assert (_recursions == 0 , "invariant") ;
359 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
360 Self->_Stalled = 0 ;
361 return ;
362 }
363
364 assert (_owner != Self , "invariant") ;
365 assert (_succ != Self , "invariant") ;
366 assert (Self->is_Java_thread() , "invariant") ;
367 JavaThread * jt = (JavaThread *) Self ;
368 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
369 assert (jt->thread_state() != _thread_blocked , "invariant") ;
370 assert (this->object() != NULL , "invariant") ;
371 assert (_count >= 0, "invariant") ;
372
373 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
374 // Ensure the object-monitor relationship remains stable while there's contention.
375 Atomic::inc_ptr(&_count);
376
377 EventJavaMonitorEnter event;
378
379 { // Change java thread status to indicate blocked on monitor enter.
380 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
381
382 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
383 if (JvmtiExport::should_post_monitor_contended_enter()) {
384 JvmtiExport::post_monitor_contended_enter(jt, this);
385 }
386
387 OSThreadContendState osts(Self->osthread());
388 ThreadBlockInVM tbivm(jt);
389
390 Self->set_current_pending_monitor(this);
391
392 // TODO-FIXME: change the following for(;;) loop to straight-line code.
393 for (;;) {
394 jt->set_suspend_equivalent();
395 // cleared by handle_special_suspend_equivalent_condition()
396 // or java_suspend_self()
397
398 EnterI (THREAD) ;
399
400 if (!ExitSuspendEquivalent(jt)) break ;
401
402 //
403 // We have acquired the contended monitor, but while we were
404 // waiting another thread suspended us. We don't want to enter
405 // the monitor while suspended because that would surprise the
406 // thread that suspended us.
407 //
408 _recursions = 0 ;
409 _succ = NULL ;
410 exit (false, Self) ;
411
412 jt->java_suspend_self();
413 }
414 Self->set_current_pending_monitor(NULL);
415 }
416
417 Atomic::dec_ptr(&_count);
418 assert (_count >= 0, "invariant") ;
419 Self->_Stalled = 0 ;
420
421 // Must either set _recursions = 0 or ASSERT _recursions == 0.
422 assert (_recursions == 0 , "invariant") ;
423 assert (_owner == Self , "invariant") ;
424 assert (_succ != Self , "invariant") ;
425 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
426
427 // The thread -- now the owner -- is back in vm mode.
428 // Report the glorious news via TI,DTrace and jvmstat.
429 // The probe effect is non-trivial. All the reportage occurs
430 // while we hold the monitor, increasing the length of the critical
431 // section. Amdahl's parallel speedup law comes vividly into play.
432 //
433 // Another option might be to aggregate the events (thread local or
434 // per-monitor aggregation) and defer reporting until a more opportune
435 // time -- such as next time some thread encounters contention but has
436 // yet to acquire the lock. While spinning that thread could
437 // spinning we could increment JVMStat counters, etc.
438
439 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
440 if (JvmtiExport::should_post_monitor_contended_entered()) {
441 JvmtiExport::post_monitor_contended_entered(jt, this);
442 }
443
444 if (event.should_commit()) {
445 event.set_klass(((oop)this->object())->klass());
446 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
447 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
448 event.commit();
449 }
450
451 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
452 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
453 }
454 }
455
456
457 // Caveat: TryLock() is not necessarily serializing if it returns failure.
458 // Callers must compensate as needed.
459
460 int ObjectMonitor::TryLock (Thread * Self) {
461 for (;;) {
462 void * own = _owner ;
463 if (own != NULL) return 0 ;
464 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
465 // Either guarantee _recursions == 0 or set _recursions = 0.
466 assert (_recursions == 0, "invariant") ;
467 assert (_owner == Self, "invariant") ;
468 // CONSIDER: set or assert that OwnerIsThread == 1
469 return 1 ;
470 }
471 // The lock had been free momentarily, but we lost the race to the lock.
472 // Interference -- the CAS failed.
473 // We can either return -1 or retry.
474 // Retry doesn't make as much sense because the lock was just acquired.
475 if (true) return -1 ;
476 }
477 }
478
479 void ATTR ObjectMonitor::EnterI (TRAPS) {
480 Thread * Self = THREAD ;
481 assert (Self->is_Java_thread(), "invariant") ;
482 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
483
484 // Try the lock - TATAS
485 if (TryLock (Self) > 0) {
486 assert (_succ != Self , "invariant") ;
487 assert (_owner == Self , "invariant") ;
488 assert (_Responsible != Self , "invariant") ;
489 return ;
490 }
491
492 DeferredInitialize () ;
493
494 // We try one round of spinning *before* enqueueing Self.
495 //
496 // If the _owner is ready but OFFPROC we could use a YieldTo()
497 // operation to donate the remainder of this thread's quantum
498 // to the owner. This has subtle but beneficial affinity
499 // effects.
500
501 if (TrySpin (Self) > 0) {
502 assert (_owner == Self , "invariant") ;
503 assert (_succ != Self , "invariant") ;
504 assert (_Responsible != Self , "invariant") ;
505 return ;
506 }
507
508 // The Spin failed -- Enqueue and park the thread ...
509 assert (_succ != Self , "invariant") ;
510 assert (_owner != Self , "invariant") ;
511 assert (_Responsible != Self , "invariant") ;
512
513 // Enqueue "Self" on ObjectMonitor's _cxq.
514 //
515 // Node acts as a proxy for Self.
516 // As an aside, if were to ever rewrite the synchronization code mostly
517 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
518 // Java objects. This would avoid awkward lifecycle and liveness issues,
519 // as well as eliminate a subset of ABA issues.
520 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
521 //
522
523 ObjectWaiter node(Self) ;
524 Self->_ParkEvent->reset() ;
525 node._prev = (ObjectWaiter *) 0xBAD ;
526 node.TState = ObjectWaiter::TS_CXQ ;
527
528 // Push "Self" onto the front of the _cxq.
529 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
530 // Note that spinning tends to reduce the rate at which threads
531 // enqueue and dequeue on EntryList|cxq.
532 ObjectWaiter * nxt ;
533 for (;;) {
534 node._next = nxt = _cxq ;
535 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
536
537 // Interference - the CAS failed because _cxq changed. Just retry.
538 // As an optional optimization we retry the lock.
539 if (TryLock (Self) > 0) {
540 assert (_succ != Self , "invariant") ;
541 assert (_owner == Self , "invariant") ;
542 assert (_Responsible != Self , "invariant") ;
543 return ;
544 }
545 }
546
547 // Check for cxq|EntryList edge transition to non-null. This indicates
548 // the onset of contention. While contention persists exiting threads
549 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
550 // operations revert to the faster 1-0 mode. This enter operation may interleave
551 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
552 // arrange for one of the contending thread to use a timed park() operations
553 // to detect and recover from the race. (Stranding is form of progress failure
554 // where the monitor is unlocked but all the contending threads remain parked).
555 // That is, at least one of the contended threads will periodically poll _owner.
556 // One of the contending threads will become the designated "Responsible" thread.
557 // The Responsible thread uses a timed park instead of a normal indefinite park
558 // operation -- it periodically wakes and checks for and recovers from potential
559 // strandings admitted by 1-0 exit operations. We need at most one Responsible
560 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
561 // be responsible for a monitor.
562 //
563 // Currently, one of the contended threads takes on the added role of "Responsible".
564 // A viable alternative would be to use a dedicated "stranding checker" thread
565 // that periodically iterated over all the threads (or active monitors) and unparked
566 // successors where there was risk of stranding. This would help eliminate the
567 // timer scalability issues we see on some platforms as we'd only have one thread
568 // -- the checker -- parked on a timer.
569
570 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
571 // Try to assume the role of responsible thread for the monitor.
572 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
573 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
574 }
575
576 // The lock have been released while this thread was occupied queueing
577 // itself onto _cxq. To close the race and avoid "stranding" and
578 // progress-liveness failure we must resample-retry _owner before parking.
579 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
580 // In this case the ST-MEMBAR is accomplished with CAS().
581 //
582 // TODO: Defer all thread state transitions until park-time.
583 // Since state transitions are heavy and inefficient we'd like
584 // to defer the state transitions until absolutely necessary,
585 // and in doing so avoid some transitions ...
586
587 TEVENT (Inflated enter - Contention) ;
588 int nWakeups = 0 ;
589 int RecheckInterval = 1 ;
590
591 for (;;) {
592
593 if (TryLock (Self) > 0) break ;
594 assert (_owner != Self, "invariant") ;
595
596 if ((SyncFlags & 2) && _Responsible == NULL) {
597 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
598 }
599
600 // park self
601 if (_Responsible == Self || (SyncFlags & 1)) {
602 TEVENT (Inflated enter - park TIMED) ;
603 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
604 // Increase the RecheckInterval, but clamp the value.
605 RecheckInterval *= 8 ;
606 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
607 } else {
608 TEVENT (Inflated enter - park UNTIMED) ;
609 Self->_ParkEvent->park() ;
610 }
611
612 if (TryLock(Self) > 0) break ;
613
614 // The lock is still contested.
615 // Keep a tally of the # of futile wakeups.
616 // Note that the counter is not protected by a lock or updated by atomics.
617 // That is by design - we trade "lossy" counters which are exposed to
618 // races during updates for a lower probe effect.
619 TEVENT (Inflated enter - Futile wakeup) ;
620 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
621 ObjectMonitor::_sync_FutileWakeups->inc() ;
622 }
623 ++ nWakeups ;
624
625 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
626 // We can defer clearing _succ until after the spin completes
627 // TrySpin() must tolerate being called with _succ == Self.
628 // Try yet another round of adaptive spinning.
629 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
630
631 // We can find that we were unpark()ed and redesignated _succ while
632 // we were spinning. That's harmless. If we iterate and call park(),
633 // park() will consume the event and return immediately and we'll
634 // just spin again. This pattern can repeat, leaving _succ to simply
635 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
636 // Alternately, we can sample fired() here, and if set, forgo spinning
637 // in the next iteration.
638
639 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
640 Self->_ParkEvent->reset() ;
641 OrderAccess::fence() ;
642 }
643 if (_succ == Self) _succ = NULL ;
644
645 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
646 OrderAccess::fence() ;
647 }
648
649 // Egress :
650 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
651 // Normally we'll find Self on the EntryList .
652 // From the perspective of the lock owner (this thread), the
653 // EntryList is stable and cxq is prepend-only.
654 // The head of cxq is volatile but the interior is stable.
655 // In addition, Self.TState is stable.
656
657 assert (_owner == Self , "invariant") ;
658 assert (object() != NULL , "invariant") ;
659 // I'd like to write:
660 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
661 // but as we're at a safepoint that's not safe.
662
663 UnlinkAfterAcquire (Self, &node) ;
664 if (_succ == Self) _succ = NULL ;
665
666 assert (_succ != Self, "invariant") ;
667 if (_Responsible == Self) {
668 _Responsible = NULL ;
669 OrderAccess::fence(); // Dekker pivot-point
670
671 // We may leave threads on cxq|EntryList without a designated
672 // "Responsible" thread. This is benign. When this thread subsequently
673 // exits the monitor it can "see" such preexisting "old" threads --
674 // threads that arrived on the cxq|EntryList before the fence, above --
675 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
676 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
677 // non-null and elect a new "Responsible" timer thread.
678 //
679 // This thread executes:
680 // ST Responsible=null; MEMBAR (in enter epilogue - here)
681 // LD cxq|EntryList (in subsequent exit)
682 //
683 // Entering threads in the slow/contended path execute:
684 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
685 // The (ST cxq; MEMBAR) is accomplished with CAS().
686 //
687 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
688 // exit operation from floating above the ST Responsible=null.
689 }
690
691 // We've acquired ownership with CAS().
692 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
693 // But since the CAS() this thread may have also stored into _succ,
694 // EntryList, cxq or Responsible. These meta-data updates must be
695 // visible __before this thread subsequently drops the lock.
696 // Consider what could occur if we didn't enforce this constraint --
697 // STs to monitor meta-data and user-data could reorder with (become
698 // visible after) the ST in exit that drops ownership of the lock.
699 // Some other thread could then acquire the lock, but observe inconsistent
700 // or old monitor meta-data and heap data. That violates the JMM.
701 // To that end, the 1-0 exit() operation must have at least STST|LDST
702 // "release" barrier semantics. Specifically, there must be at least a
703 // STST|LDST barrier in exit() before the ST of null into _owner that drops
704 // the lock. The barrier ensures that changes to monitor meta-data and data
705 // protected by the lock will be visible before we release the lock, and
706 // therefore before some other thread (CPU) has a chance to acquire the lock.
707 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
708 //
709 // Critically, any prior STs to _succ or EntryList must be visible before
710 // the ST of null into _owner in the *subsequent* (following) corresponding
711 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
712 // execute a serializing instruction.
713
714 if (SyncFlags & 8) {
715 OrderAccess::fence() ;
716 }
717 return ;
718 }
719
720 // ReenterI() is a specialized inline form of the latter half of the
721 // contended slow-path from EnterI(). We use ReenterI() only for
722 // monitor reentry in wait().
723 //
724 // In the future we should reconcile EnterI() and ReenterI(), adding
725 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
726 // loop accordingly.
727
728 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
729 assert (Self != NULL , "invariant") ;
730 assert (SelfNode != NULL , "invariant") ;
731 assert (SelfNode->_thread == Self , "invariant") ;
732 assert (_waiters > 0 , "invariant") ;
733 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
734 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
735 JavaThread * jt = (JavaThread *) Self ;
736
737 int nWakeups = 0 ;
738 for (;;) {
739 ObjectWaiter::TStates v = SelfNode->TState ;
740 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
741 assert (_owner != Self, "invariant") ;
742
743 if (TryLock (Self) > 0) break ;
744 if (TrySpin (Self) > 0) break ;
745
746 TEVENT (Wait Reentry - parking) ;
747
748 // State transition wrappers around park() ...
749 // ReenterI() wisely defers state transitions until
750 // it's clear we must park the thread.
751 {
752 OSThreadContendState osts(Self->osthread());
753 ThreadBlockInVM tbivm(jt);
754
755 // cleared by handle_special_suspend_equivalent_condition()
756 // or java_suspend_self()
757 jt->set_suspend_equivalent();
758 if (SyncFlags & 1) {
759 Self->_ParkEvent->park ((jlong)1000) ;
760 } else {
761 Self->_ParkEvent->park () ;
762 }
763
764 // were we externally suspended while we were waiting?
765 for (;;) {
766 if (!ExitSuspendEquivalent (jt)) break ;
767 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
768 jt->java_suspend_self();
769 jt->set_suspend_equivalent();
770 }
771 }
772
773 // Try again, but just so we distinguish between futile wakeups and
774 // successful wakeups. The following test isn't algorithmically
775 // necessary, but it helps us maintain sensible statistics.
776 if (TryLock(Self) > 0) break ;
777
778 // The lock is still contested.
779 // Keep a tally of the # of futile wakeups.
780 // Note that the counter is not protected by a lock or updated by atomics.
781 // That is by design - we trade "lossy" counters which are exposed to
782 // races during updates for a lower probe effect.
783 TEVENT (Wait Reentry - futile wakeup) ;
784 ++ nWakeups ;
785
786 // Assuming this is not a spurious wakeup we'll normally
787 // find that _succ == Self.
788 if (_succ == Self) _succ = NULL ;
789
790 // Invariant: after clearing _succ a contending thread
791 // *must* retry _owner before parking.
792 OrderAccess::fence() ;
793
794 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
795 ObjectMonitor::_sync_FutileWakeups->inc() ;
796 }
797 }
798
799 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
800 // Normally we'll find Self on the EntryList.
801 // Unlinking from the EntryList is constant-time and atomic-free.
802 // From the perspective of the lock owner (this thread), the
803 // EntryList is stable and cxq is prepend-only.
804 // The head of cxq is volatile but the interior is stable.
805 // In addition, Self.TState is stable.
806
807 assert (_owner == Self, "invariant") ;
808 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
809 UnlinkAfterAcquire (Self, SelfNode) ;
810 if (_succ == Self) _succ = NULL ;
811 assert (_succ != Self, "invariant") ;
812 SelfNode->TState = ObjectWaiter::TS_RUN ;
813 OrderAccess::fence() ; // see comments at the end of EnterI()
814 }
815
816 // after the thread acquires the lock in ::enter(). Equally, we could defer
817 // unlinking the thread until ::exit()-time.
818
819 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
820 {
821 assert (_owner == Self, "invariant") ;
822 assert (SelfNode->_thread == Self, "invariant") ;
823
824 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
825 // Normal case: remove Self from the DLL EntryList .
826 // This is a constant-time operation.
827 ObjectWaiter * nxt = SelfNode->_next ;
828 ObjectWaiter * prv = SelfNode->_prev ;
829 if (nxt != NULL) nxt->_prev = prv ;
830 if (prv != NULL) prv->_next = nxt ;
831 if (SelfNode == _EntryList ) _EntryList = nxt ;
832 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
833 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
834 TEVENT (Unlink from EntryList) ;
835 } else {
836 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
837 // Inopportune interleaving -- Self is still on the cxq.
838 // This usually means the enqueue of self raced an exiting thread.
839 // Normally we'll find Self near the front of the cxq, so
840 // dequeueing is typically fast. If needbe we can accelerate
841 // this with some MCS/CHL-like bidirectional list hints and advisory
842 // back-links so dequeueing from the interior will normally operate
843 // in constant-time.
844 // Dequeue Self from either the head (with CAS) or from the interior
845 // with a linear-time scan and normal non-atomic memory operations.
846 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
847 // and then unlink Self from EntryList. We have to drain eventually,
848 // so it might as well be now.
849
850 ObjectWaiter * v = _cxq ;
851 assert (v != NULL, "invariant") ;
852 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
853 // The CAS above can fail from interference IFF a "RAT" arrived.
854 // In that case Self must be in the interior and can no longer be
855 // at the head of cxq.
856 if (v == SelfNode) {
857 assert (_cxq != v, "invariant") ;
858 v = _cxq ; // CAS above failed - start scan at head of list
859 }
860 ObjectWaiter * p ;
861 ObjectWaiter * q = NULL ;
862 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
863 q = p ;
864 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
865 }
866 assert (v != SelfNode, "invariant") ;
867 assert (p == SelfNode, "Node not found on cxq") ;
868 assert (p != _cxq, "invariant") ;
869 assert (q != NULL, "invariant") ;
870 assert (q->_next == p, "invariant") ;
871 q->_next = p->_next ;
872 }
873 TEVENT (Unlink from cxq) ;
874 }
875
876 // Diagnostic hygiene ...
877 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
878 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
879 SelfNode->TState = ObjectWaiter::TS_RUN ;
880 }
881
882 // -----------------------------------------------------------------------------
883 // Exit support
884 //
885 // exit()
886 // ~~~~~~
887 // Note that the collector can't reclaim the objectMonitor or deflate
888 // the object out from underneath the thread calling ::exit() as the
889 // thread calling ::exit() never transitions to a stable state.
890 // This inhibits GC, which in turn inhibits asynchronous (and
891 // inopportune) reclamation of "this".
892 //
893 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
894 // There's one exception to the claim above, however. EnterI() can call
895 // exit() to drop a lock if the acquirer has been externally suspended.
896 // In that case exit() is called with _thread_state as _thread_blocked,
897 // but the monitor's _count field is > 0, which inhibits reclamation.
898 //
899 // 1-0 exit
900 // ~~~~~~~~
901 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
902 // the fast-path operators have been optimized so the common ::exit()
903 // operation is 1-0. See i486.ad fast_unlock(), for instance.
904 // The code emitted by fast_unlock() elides the usual MEMBAR. This
905 // greatly improves latency -- MEMBAR and CAS having considerable local
906 // latency on modern processors -- but at the cost of "stranding". Absent the
907 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
908 // ::enter() path, resulting in the entering thread being stranding
909 // and a progress-liveness failure. Stranding is extremely rare.
910 // We use timers (timed park operations) & periodic polling to detect
911 // and recover from stranding. Potentially stranded threads periodically
912 // wake up and poll the lock. See the usage of the _Responsible variable.
913 //
914 // The CAS() in enter provides for safety and exclusion, while the CAS or
915 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
916 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
917 // We detect and recover from stranding with timers.
918 //
919 // If a thread transiently strands it'll park until (a) another
920 // thread acquires the lock and then drops the lock, at which time the
921 // exiting thread will notice and unpark the stranded thread, or, (b)
922 // the timer expires. If the lock is high traffic then the stranding latency
923 // will be low due to (a). If the lock is low traffic then the odds of
924 // stranding are lower, although the worst-case stranding latency
925 // is longer. Critically, we don't want to put excessive load in the
926 // platform's timer subsystem. We want to minimize both the timer injection
927 // rate (timers created/sec) as well as the number of timers active at
928 // any one time. (more precisely, we want to minimize timer-seconds, which is
929 // the integral of the # of active timers at any instant over time).
930 // Both impinge on OS scalability. Given that, at most one thread parked on
931 // a monitor will use a timer.
932
933 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
934 Thread * Self = THREAD ;
935 if (THREAD != _owner) {
936 if (THREAD->is_lock_owned((address) _owner)) {
937 // Transmute _owner from a BasicLock pointer to a Thread address.
938 // We don't need to hold _mutex for this transition.
939 // Non-null to Non-null is safe as long as all readers can
940 // tolerate either flavor.
941 assert (_recursions == 0, "invariant") ;
942 _owner = THREAD ;
943 _recursions = 0 ;
944 OwnerIsThread = 1 ;
945 } else {
946 // NOTE: we need to handle unbalanced monitor enter/exit
947 // in native code by throwing an exception.
948 // TODO: Throw an IllegalMonitorStateException ?
949 TEVENT (Exit - Throw IMSX) ;
950 assert(false, "Non-balanced monitor enter/exit!");
951 if (false) {
952 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
953 }
954 return;
955 }
956 }
957
958 if (_recursions != 0) {
959 _recursions--; // this is simple recursive enter
960 TEVENT (Inflated exit - recursive) ;
961 return ;
962 }
963
964 // Invariant: after setting Responsible=null an thread must execute
965 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
966 if ((SyncFlags & 4) == 0) {
967 _Responsible = NULL ;
968 }
969
970 #if INCLUDE_TRACE
971 // get the owner's thread id for the MonitorEnter event
972 // if it is enabled and the thread isn't suspended
973 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
974 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
975 }
976 #endif
977
978 for (;;) {
979 assert (THREAD == _owner, "invariant") ;
980
981
982 if (Knob_ExitPolicy == 0) {
983 // release semantics: prior loads and stores from within the critical section
984 // must not float (reorder) past the following store that drops the lock.
985 // On SPARC that requires MEMBAR #loadstore|#storestore.
986 // But of course in TSO #loadstore|#storestore is not required.
987 // I'd like to write one of the following:
988 // A. OrderAccess::release() ; _owner = NULL
989 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
990 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
991 // store into a _dummy variable. That store is not needed, but can result
992 // in massive wasteful coherency traffic on classic SMP systems.
993 // Instead, I use release_store(), which is implemented as just a simple
994 // ST on x64, x86 and SPARC.
995 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
996 OrderAccess::storeload() ; // See if we need to wake a successor
997 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
998 TEVENT (Inflated exit - simple egress) ;
999 return ;
1000 }
1001 TEVENT (Inflated exit - complex egress) ;
1002
1003 // Normally the exiting thread is responsible for ensuring succession,
1004 // but if other successors are ready or other entering threads are spinning
1005 // then this thread can simply store NULL into _owner and exit without
1006 // waking a successor. The existence of spinners or ready successors
1007 // guarantees proper succession (liveness). Responsibility passes to the
1008 // ready or running successors. The exiting thread delegates the duty.
1009 // More precisely, if a successor already exists this thread is absolved
1010 // of the responsibility of waking (unparking) one.
1011 //
1012 // The _succ variable is critical to reducing futile wakeup frequency.
1013 // _succ identifies the "heir presumptive" thread that has been made
1014 // ready (unparked) but that has not yet run. We need only one such
1015 // successor thread to guarantee progress.
1016 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1017 // section 3.3 "Futile Wakeup Throttling" for details.
1018 //
1019 // Note that spinners in Enter() also set _succ non-null.
1020 // In the current implementation spinners opportunistically set
1021 // _succ so that exiting threads might avoid waking a successor.
1022 // Another less appealing alternative would be for the exiting thread
1023 // to drop the lock and then spin briefly to see if a spinner managed
1024 // to acquire the lock. If so, the exiting thread could exit
1025 // immediately without waking a successor, otherwise the exiting
1026 // thread would need to dequeue and wake a successor.
1027 // (Note that we'd need to make the post-drop spin short, but no
1028 // shorter than the worst-case round-trip cache-line migration time.
1029 // The dropped lock needs to become visible to the spinner, and then
1030 // the acquisition of the lock by the spinner must become visible to
1031 // the exiting thread).
1032 //
1033
1034 // It appears that an heir-presumptive (successor) must be made ready.
1035 // Only the current lock owner can manipulate the EntryList or
1036 // drain _cxq, so we need to reacquire the lock. If we fail
1037 // to reacquire the lock the responsibility for ensuring succession
1038 // falls to the new owner.
1039 //
1040 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1041 return ;
1042 }
1043 TEVENT (Exit - Reacquired) ;
1044 } else {
1045 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1046 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1047 OrderAccess::storeload() ;
1048 // Ratify the previously observed values.
1049 if (_cxq == NULL || _succ != NULL) {
1050 TEVENT (Inflated exit - simple egress) ;
1051 return ;
1052 }
1053
1054 // inopportune interleaving -- the exiting thread (this thread)
1055 // in the fast-exit path raced an entering thread in the slow-enter
1056 // path.
1057 // We have two choices:
1058 // A. Try to reacquire the lock.
1059 // If the CAS() fails return immediately, otherwise
1060 // we either restart/rerun the exit operation, or simply
1061 // fall-through into the code below which wakes a successor.
1062 // B. If the elements forming the EntryList|cxq are TSM
1063 // we could simply unpark() the lead thread and return
1064 // without having set _succ.
1065 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1066 TEVENT (Inflated exit - reacquired succeeded) ;
1067 return ;
1068 }
1069 TEVENT (Inflated exit - reacquired failed) ;
1070 } else {
1071 TEVENT (Inflated exit - complex egress) ;
1072 }
1073 }
1074
1075 guarantee (_owner == THREAD, "invariant") ;
1076
1077 ObjectWaiter * w = NULL ;
1078 int QMode = Knob_QMode ;
1079
1080 if (QMode == 2 && _cxq != NULL) {
1081 // QMode == 2 : cxq has precedence over EntryList.
1082 // Try to directly wake a successor from the cxq.
1083 // If successful, the successor will need to unlink itself from cxq.
1084 w = _cxq ;
1085 assert (w != NULL, "invariant") ;
1086 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1087 ExitEpilog (Self, w) ;
1088 return ;
1089 }
1090
1091 if (QMode == 3 && _cxq != NULL) {
1092 // Aggressively drain cxq into EntryList at the first opportunity.
1093 // This policy ensure that recently-run threads live at the head of EntryList.
1094 // Drain _cxq into EntryList - bulk transfer.
1095 // First, detach _cxq.
1096 // The following loop is tantamount to: w = swap (&cxq, NULL)
1097 w = _cxq ;
1098 for (;;) {
1099 assert (w != NULL, "Invariant") ;
1100 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1101 if (u == w) break ;
1102 w = u ;
1103 }
1104 assert (w != NULL , "invariant") ;
1105
1106 ObjectWaiter * q = NULL ;
1107 ObjectWaiter * p ;
1108 for (p = w ; p != NULL ; p = p->_next) {
1109 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1110 p->TState = ObjectWaiter::TS_ENTER ;
1111 p->_prev = q ;
1112 q = p ;
1113 }
1114
1115 // Append the RATs to the EntryList
1116 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1117 ObjectWaiter * Tail ;
1118 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1119 if (Tail == NULL) {
1120 _EntryList = w ;
1121 } else {
1122 Tail->_next = w ;
1123 w->_prev = Tail ;
1124 }
1125
1126 // Fall thru into code that tries to wake a successor from EntryList
1127 }
1128
1129 if (QMode == 4 && _cxq != NULL) {
1130 // Aggressively drain cxq into EntryList at the first opportunity.
1131 // This policy ensure that recently-run threads live at the head of EntryList.
1132
1133 // Drain _cxq into EntryList - bulk transfer.
1134 // First, detach _cxq.
1135 // The following loop is tantamount to: w = swap (&cxq, NULL)
1136 w = _cxq ;
1137 for (;;) {
1138 assert (w != NULL, "Invariant") ;
1139 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1140 if (u == w) break ;
1141 w = u ;
1142 }
1143 assert (w != NULL , "invariant") ;
1144
1145 ObjectWaiter * q = NULL ;
1146 ObjectWaiter * p ;
1147 for (p = w ; p != NULL ; p = p->_next) {
1148 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1149 p->TState = ObjectWaiter::TS_ENTER ;
1150 p->_prev = q ;
1151 q = p ;
1152 }
1153
1154 // Prepend the RATs to the EntryList
1155 if (_EntryList != NULL) {
1156 q->_next = _EntryList ;
1157 _EntryList->_prev = q ;
1158 }
1159 _EntryList = w ;
1160
1161 // Fall thru into code that tries to wake a successor from EntryList
1162 }
1163
1164 w = _EntryList ;
1165 if (w != NULL) {
1166 // I'd like to write: guarantee (w->_thread != Self).
1167 // But in practice an exiting thread may find itself on the EntryList.
1168 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1169 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1170 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1171 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1172 // release the lock "O". T2 resumes immediately after the ST of null into
1173 // _owner, above. T2 notices that the EntryList is populated, so it
1174 // reacquires the lock and then finds itself on the EntryList.
1175 // Given all that, we have to tolerate the circumstance where "w" is
1176 // associated with Self.
1177 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1178 ExitEpilog (Self, w) ;
1179 return ;
1180 }
1181
1182 // If we find that both _cxq and EntryList are null then just
1183 // re-run the exit protocol from the top.
1184 w = _cxq ;
1185 if (w == NULL) continue ;
1186
1187 // Drain _cxq into EntryList - bulk transfer.
1188 // First, detach _cxq.
1189 // The following loop is tantamount to: w = swap (&cxq, NULL)
1190 for (;;) {
1191 assert (w != NULL, "Invariant") ;
1192 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1193 if (u == w) break ;
1194 w = u ;
1195 }
1196 TEVENT (Inflated exit - drain cxq into EntryList) ;
1197
1198 assert (w != NULL , "invariant") ;
1199 assert (_EntryList == NULL , "invariant") ;
1200
1201 // Convert the LIFO SLL anchored by _cxq into a DLL.
1202 // The list reorganization step operates in O(LENGTH(w)) time.
1203 // It's critical that this step operate quickly as
1204 // "Self" still holds the outer-lock, restricting parallelism
1205 // and effectively lengthening the critical section.
1206 // Invariant: s chases t chases u.
1207 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1208 // we have faster access to the tail.
1209
1210 if (QMode == 1) {
1211 // QMode == 1 : drain cxq to EntryList, reversing order
1212 // We also reverse the order of the list.
1213 ObjectWaiter * s = NULL ;
1214 ObjectWaiter * t = w ;
1215 ObjectWaiter * u = NULL ;
1216 while (t != NULL) {
1217 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1218 t->TState = ObjectWaiter::TS_ENTER ;
1219 u = t->_next ;
1220 t->_prev = u ;
1221 t->_next = s ;
1222 s = t;
1223 t = u ;
1224 }
1225 _EntryList = s ;
1226 assert (s != NULL, "invariant") ;
1227 } else {
1228 // QMode == 0 or QMode == 2
1229 _EntryList = w ;
1230 ObjectWaiter * q = NULL ;
1231 ObjectWaiter * p ;
1232 for (p = w ; p != NULL ; p = p->_next) {
1233 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1234 p->TState = ObjectWaiter::TS_ENTER ;
1235 p->_prev = q ;
1236 q = p ;
1237 }
1238 }
1239
1240 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1241 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1242
1243 // See if we can abdicate to a spinner instead of waking a thread.
1244 // A primary goal of the implementation is to reduce the
1245 // context-switch rate.
1246 if (_succ != NULL) continue;
1247
1248 w = _EntryList ;
1249 if (w != NULL) {
1250 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1251 ExitEpilog (Self, w) ;
1252 return ;
1253 }
1254 }
1255 }
1256
1257 // ExitSuspendEquivalent:
1258 // A faster alternate to handle_special_suspend_equivalent_condition()
1259 //
1260 // handle_special_suspend_equivalent_condition() unconditionally
1261 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1262 // operations have high latency. Note that in ::enter() we call HSSEC
1263 // while holding the monitor, so we effectively lengthen the critical sections.
1264 //
1265 // There are a number of possible solutions:
1266 //
1267 // A. To ameliorate the problem we might also defer state transitions
1268 // to as late as possible -- just prior to parking.
1269 // Given that, we'd call HSSEC after having returned from park(),
1270 // but before attempting to acquire the monitor. This is only a
1271 // partial solution. It avoids calling HSSEC while holding the
1272 // monitor (good), but it still increases successor reacquisition latency --
1273 // the interval between unparking a successor and the time the successor
1274 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1275 // If we use this technique we can also avoid EnterI()-exit() loop
1276 // in ::enter() where we iteratively drop the lock and then attempt
1277 // to reacquire it after suspending.
1278 //
1279 // B. In the future we might fold all the suspend bits into a
1280 // composite per-thread suspend flag and then update it with CAS().
1281 // Alternately, a Dekker-like mechanism with multiple variables
1282 // would suffice:
1283 // ST Self->_suspend_equivalent = false
1284 // MEMBAR
1285 // LD Self_>_suspend_flags
1286 //
1287
1288
1289 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1290 int Mode = Knob_FastHSSEC ;
1291 if (Mode && !jSelf->is_external_suspend()) {
1292 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1293 jSelf->clear_suspend_equivalent() ;
1294 if (2 == Mode) OrderAccess::storeload() ;
1295 if (!jSelf->is_external_suspend()) return false ;
1296 // We raced a suspension -- fall thru into the slow path
1297 TEVENT (ExitSuspendEquivalent - raced) ;
1298 jSelf->set_suspend_equivalent() ;
1299 }
1300 return jSelf->handle_special_suspend_equivalent_condition() ;
1301 }
1302
1303
1304 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1305 assert (_owner == Self, "invariant") ;
1306
1307 // Exit protocol:
1308 // 1. ST _succ = wakee
1309 // 2. membar #loadstore|#storestore;
1310 // 2. ST _owner = NULL
1311 // 3. unpark(wakee)
1312
1313 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1314 ParkEvent * Trigger = Wakee->_event ;
1315
1316 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1317 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1318 // out-of-scope (non-extant).
1319 Wakee = NULL ;
1320
1321 // Drop the lock
1322 OrderAccess::release_store_ptr (&_owner, NULL) ;
1323 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1324
1325 if (SafepointSynchronize::do_call_back()) {
1326 TEVENT (unpark before SAFEPOINT) ;
1327 }
1328
1329 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1330 Trigger->unpark() ;
1331
1332 // Maintain stats and report events to JVMTI
1333 if (ObjectMonitor::_sync_Parks != NULL) {
1334 ObjectMonitor::_sync_Parks->inc() ;
1335 }
1336 }
1337
1338
1339 // -----------------------------------------------------------------------------
1340 // Class Loader deadlock handling.
1341 //
1342 // complete_exit exits a lock returning recursion count
1343 // complete_exit/reenter operate as a wait without waiting
1344 // complete_exit requires an inflated monitor
1345 // The _owner field is not always the Thread addr even with an
1346 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1347 // thread due to contention.
1348 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1349 Thread * const Self = THREAD;
1350 assert(Self->is_Java_thread(), "Must be Java thread!");
1351 JavaThread *jt = (JavaThread *)THREAD;
1352
1353 DeferredInitialize();
1354
1355 if (THREAD != _owner) {
1356 if (THREAD->is_lock_owned ((address)_owner)) {
1357 assert(_recursions == 0, "internal state error");
1358 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1359 _recursions = 0 ;
1360 OwnerIsThread = 1 ;
1361 }
1362 }
1363
1364 guarantee(Self == _owner, "complete_exit not owner");
1365 intptr_t save = _recursions; // record the old recursion count
1366 _recursions = 0; // set the recursion level to be 0
1367 exit (true, Self) ; // exit the monitor
1368 guarantee (_owner != Self, "invariant");
1369 return save;
1370 }
1371
1372 // reenter() enters a lock and sets recursion count
1373 // complete_exit/reenter operate as a wait without waiting
1374 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1375 Thread * const Self = THREAD;
1376 assert(Self->is_Java_thread(), "Must be Java thread!");
1377 JavaThread *jt = (JavaThread *)THREAD;
1378
1379 guarantee(_owner != Self, "reenter already owner");
1380 enter (THREAD); // enter the monitor
1381 guarantee (_recursions == 0, "reenter recursion");
1382 _recursions = recursions;
1383 return;
1384 }
1385
1386
1387 // -----------------------------------------------------------------------------
1388 // A macro is used below because there may already be a pending
1389 // exception which should not abort the execution of the routines
1390 // which use this (which is why we don't put this into check_slow and
1391 // call it with a CHECK argument).
1392
1393 #define CHECK_OWNER() \
1394 do { \
1395 if (THREAD != _owner) { \
1396 if (THREAD->is_lock_owned((address) _owner)) { \
1397 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1398 _recursions = 0; \
1399 OwnerIsThread = 1 ; \
1400 } else { \
1401 TEVENT (Throw IMSX) ; \
1402 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1403 } \
1404 } \
1405 } while (false)
1406
1407 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1408 // TODO-FIXME: remove check_slow() -- it's likely dead.
1409
1410 void ObjectMonitor::check_slow(TRAPS) {
1411 TEVENT (check_slow - throw IMSX) ;
1412 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1413 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1414 }
1415
1416 static int Adjust (volatile int * adr, int dx) {
1417 int v ;
1418 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1419 return v ;
1420 }
1421
1422 // helper method for posting a monitor wait event
1423 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1424 jlong notifier_tid,
1425 jlong timeout,
1426 bool timedout) {
1427 event->set_klass(((oop)this->object())->klass());
1428 event->set_timeout((TYPE_ULONG)timeout);
1429 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1430 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1431 event->set_timedOut((TYPE_BOOLEAN)timedout);
1432 event->commit();
1433 }
1434
1435 // -----------------------------------------------------------------------------
1436 // Wait/Notify/NotifyAll
1437 //
1438 // Note: a subset of changes to ObjectMonitor::wait()
1439 // will need to be replicated in complete_exit above
1440 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1441 Thread * const Self = THREAD ;
1442 assert(Self->is_Java_thread(), "Must be Java thread!");
1443 JavaThread *jt = (JavaThread *)THREAD;
1444
1445 DeferredInitialize () ;
1446
1447 // Throw IMSX or IEX.
1448 CHECK_OWNER();
1449
1450 EventJavaMonitorWait event;
1451
1452 // check for a pending interrupt
1453 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1454 // post monitor waited event. Note that this is past-tense, we are done waiting.
1455 if (JvmtiExport::should_post_monitor_waited()) {
1456 // Note: 'false' parameter is passed here because the
1457 // wait was not timed out due to thread interrupt.
1458 JvmtiExport::post_monitor_waited(jt, this, false);
1459 }
1460 if (event.should_commit()) {
1461 post_monitor_wait_event(&event, 0, millis, false);
1462 }
1463 TEVENT (Wait - Throw IEX) ;
1464 THROW(vmSymbols::java_lang_InterruptedException());
1465 return ;
1466 }
1467
1468 TEVENT (Wait) ;
1469
1470 assert (Self->_Stalled == 0, "invariant") ;
1471 Self->_Stalled = intptr_t(this) ;
1472 jt->set_current_waiting_monitor(this);
1473
1474 // create a node to be put into the queue
1475 // Critically, after we reset() the event but prior to park(), we must check
1476 // for a pending interrupt.
1477 ObjectWaiter node(Self);
1478 node.TState = ObjectWaiter::TS_WAIT ;
1479 Self->_ParkEvent->reset() ;
1480 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1481
1482 // Enter the waiting queue, which is a circular doubly linked list in this case
1483 // but it could be a priority queue or any data structure.
1484 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1485 // by the the owner of the monitor *except* in the case where park()
1486 // returns because of a timeout of interrupt. Contention is exceptionally rare
1487 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1488
1489 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1490 AddWaiter (&node) ;
1491 Thread::SpinRelease (&_WaitSetLock) ;
1492
1493 if ((SyncFlags & 4) == 0) {
1494 _Responsible = NULL ;
1495 }
1496 intptr_t save = _recursions; // record the old recursion count
1497 _waiters++; // increment the number of waiters
1498 _recursions = 0; // set the recursion level to be 1
1499 exit (true, Self) ; // exit the monitor
1500 guarantee (_owner != Self, "invariant") ;
1501
1502 // As soon as the ObjectMonitor's ownership is dropped in the exit()
1503 // call above, another thread can enter() the ObjectMonitor, do the
1504 // notify(), and exit() the ObjectMonitor. If the other thread's
1505 // exit() call chooses this thread as the successor and the unpark()
1506 // call happens to occur while this thread is posting a
1507 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
1508 // handler using RawMonitors and consuming the unpark().
1509 //
1510 // To avoid the problem, we re-post the event. This does no harm
1511 // even if the original unpark() was not consumed because we are the
1512 // chosen successor for this monitor.
1513 if (node._notified != 0 && _succ == Self) {
1514 node._event->unpark();
1515 }
1516
1517 // The thread is on the WaitSet list - now park() it.
1518 // On MP systems it's conceivable that a brief spin before we park
1519 // could be profitable.
1520 //
1521 // TODO-FIXME: change the following logic to a loop of the form
1522 // while (!timeout && !interrupted && _notified == 0) park()
1523
1524 int ret = OS_OK ;
1525 int WasNotified = 0 ;
1526 { // State transition wrappers
1527 OSThread* osthread = Self->osthread();
1528 OSThreadWaitState osts(osthread, true);
1529 {
1530 ThreadBlockInVM tbivm(jt);
1531 // Thread is in thread_blocked state and oop access is unsafe.
1532 jt->set_suspend_equivalent();
1533
1534 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1535 // Intentionally empty
1536 } else
1537 if (node._notified == 0) {
1538 if (millis <= 0) {
1539 Self->_ParkEvent->park () ;
1540 } else {
1541 ret = Self->_ParkEvent->park (millis) ;
1542 }
1543 }
1544
1545 // were we externally suspended while we were waiting?
1546 if (ExitSuspendEquivalent (jt)) {
1547 // TODO-FIXME: add -- if succ == Self then succ = null.
1548 jt->java_suspend_self();
1549 }
1550
1551 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1552
1553
1554 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1555 // from the WaitSet to the EntryList.
1556 // See if we need to remove Node from the WaitSet.
1557 // We use double-checked locking to avoid grabbing _WaitSetLock
1558 // if the thread is not on the wait queue.
1559 //
1560 // Note that we don't need a fence before the fetch of TState.
1561 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1562 // written by the is thread. (perhaps the fetch might even be satisfied
1563 // by a look-aside into the processor's own store buffer, although given
1564 // the length of the code path between the prior ST and this load that's
1565 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1566 // then we'll acquire the lock and then re-fetch a fresh TState value.
1567 // That is, we fail toward safety.
1568
1569 if (node.TState == ObjectWaiter::TS_WAIT) {
1570 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1571 if (node.TState == ObjectWaiter::TS_WAIT) {
1572 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1573 assert(node._notified == 0, "invariant");
1574 node.TState = ObjectWaiter::TS_RUN ;
1575 }
1576 Thread::SpinRelease (&_WaitSetLock) ;
1577 }
1578
1579 // The thread is now either on off-list (TS_RUN),
1580 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1581 // The Node's TState variable is stable from the perspective of this thread.
1582 // No other threads will asynchronously modify TState.
1583 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1584 OrderAccess::loadload() ;
1585 if (_succ == Self) _succ = NULL ;
1586 WasNotified = node._notified ;
1587
1588 // Reentry phase -- reacquire the monitor.
1589 // re-enter contended monitor after object.wait().
1590 // retain OBJECT_WAIT state until re-enter successfully completes
1591 // Thread state is thread_in_vm and oop access is again safe,
1592 // although the raw address of the object may have changed.
1593 // (Don't cache naked oops over safepoints, of course).
1594
1595 // post monitor waited event. Note that this is past-tense, we are done waiting.
1596 if (JvmtiExport::should_post_monitor_waited()) {
1597 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1598 }
1599
1600 if (event.should_commit()) {
1601 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1602 }
1603
1604 OrderAccess::fence() ;
1605
1606 assert (Self->_Stalled != 0, "invariant") ;
1607 Self->_Stalled = 0 ;
1608
1609 assert (_owner != Self, "invariant") ;
1610 ObjectWaiter::TStates v = node.TState ;
1611 if (v == ObjectWaiter::TS_RUN) {
1612 enter (Self) ;
1613 } else {
1614 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1615 ReenterI (Self, &node) ;
1616 node.wait_reenter_end(this);
1617 }
1618
1619 // Self has reacquired the lock.
1620 // Lifecycle - the node representing Self must not appear on any queues.
1621 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1622 // want residual elements associated with this thread left on any lists.
1623 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1624 assert (_owner == Self, "invariant") ;
1625 assert (_succ != Self , "invariant") ;
1626 } // OSThreadWaitState()
1627
1628 jt->set_current_waiting_monitor(NULL);
1629
1630 guarantee (_recursions == 0, "invariant") ;
1631 _recursions = save; // restore the old recursion count
1632 _waiters--; // decrement the number of waiters
1633
1634 // Verify a few postconditions
1635 assert (_owner == Self , "invariant") ;
1636 assert (_succ != Self , "invariant") ;
1637 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1638
1639 if (SyncFlags & 32) {
1640 OrderAccess::fence() ;
1641 }
1642
1643 // check if the notification happened
1644 if (!WasNotified) {
1645 // no, it could be timeout or Thread.interrupt() or both
1646 // check for interrupt event, otherwise it is timeout
1647 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1648 TEVENT (Wait - throw IEX from epilog) ;
1649 THROW(vmSymbols::java_lang_InterruptedException());
1650 }
1651 }
1652
1653 // NOTE: Spurious wake up will be consider as timeout.
1654 // Monitor notify has precedence over thread interrupt.
1655 }
1656
1657
1658 // Consider:
1659 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1660 // then instead of transferring a thread from the WaitSet to the EntryList
1661 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1662
1663 void ObjectMonitor::notify(TRAPS) {
1664 CHECK_OWNER();
1665 if (_WaitSet == NULL) {
1666 TEVENT (Empty-Notify) ;
1667 return ;
1668 }
1669 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1670
1671 int Policy = Knob_MoveNotifyee ;
1672
1673 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1674 ObjectWaiter * iterator = DequeueWaiter() ;
1675 if (iterator != NULL) {
1676 TEVENT (Notify1 - Transfer) ;
1677 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1678 guarantee (iterator->_notified == 0, "invariant") ;
1679 if (Policy != 4) {
1680 iterator->TState = ObjectWaiter::TS_ENTER ;
1681 }
1682 iterator->_notified = 1 ;
1683 Thread * Self = THREAD;
1684 iterator->_notifier_tid = Self->osthread()->thread_id();
1685
1686 ObjectWaiter * List = _EntryList ;
1687 if (List != NULL) {
1688 assert (List->_prev == NULL, "invariant") ;
1689 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1690 assert (List != iterator, "invariant") ;
1691 }
1692
1693 if (Policy == 0) { // prepend to EntryList
1694 if (List == NULL) {
1695 iterator->_next = iterator->_prev = NULL ;
1696 _EntryList = iterator ;
1697 } else {
1698 List->_prev = iterator ;
1699 iterator->_next = List ;
1700 iterator->_prev = NULL ;
1701 _EntryList = iterator ;
1702 }
1703 } else
1704 if (Policy == 1) { // append to EntryList
1705 if (List == NULL) {
1706 iterator->_next = iterator->_prev = NULL ;
1707 _EntryList = iterator ;
1708 } else {
1709 // CONSIDER: finding the tail currently requires a linear-time walk of
1710 // the EntryList. We can make tail access constant-time by converting to
1711 // a CDLL instead of using our current DLL.
1712 ObjectWaiter * Tail ;
1713 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1714 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1715 Tail->_next = iterator ;
1716 iterator->_prev = Tail ;
1717 iterator->_next = NULL ;
1718 }
1719 } else
1720 if (Policy == 2) { // prepend to cxq
1721 // prepend to cxq
1722 if (List == NULL) {
1723 iterator->_next = iterator->_prev = NULL ;
1724 _EntryList = iterator ;
1725 } else {
1726 iterator->TState = ObjectWaiter::TS_CXQ ;
1727 for (;;) {
1728 ObjectWaiter * Front = _cxq ;
1729 iterator->_next = Front ;
1730 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1731 break ;
1732 }
1733 }
1734 }
1735 } else
1736 if (Policy == 3) { // append to cxq
1737 iterator->TState = ObjectWaiter::TS_CXQ ;
1738 for (;;) {
1739 ObjectWaiter * Tail ;
1740 Tail = _cxq ;
1741 if (Tail == NULL) {
1742 iterator->_next = NULL ;
1743 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1744 break ;
1745 }
1746 } else {
1747 while (Tail->_next != NULL) Tail = Tail->_next ;
1748 Tail->_next = iterator ;
1749 iterator->_prev = Tail ;
1750 iterator->_next = NULL ;
1751 break ;
1752 }
1753 }
1754 } else {
1755 ParkEvent * ev = iterator->_event ;
1756 iterator->TState = ObjectWaiter::TS_RUN ;
1757 OrderAccess::fence() ;
1758 ev->unpark() ;
1759 }
1760
1761 if (Policy < 4) {
1762 iterator->wait_reenter_begin(this);
1763 }
1764
1765 // _WaitSetLock protects the wait queue, not the EntryList. We could
1766 // move the add-to-EntryList operation, above, outside the critical section
1767 // protected by _WaitSetLock. In practice that's not useful. With the
1768 // exception of wait() timeouts and interrupts the monitor owner
1769 // is the only thread that grabs _WaitSetLock. There's almost no contention
1770 // on _WaitSetLock so it's not profitable to reduce the length of the
1771 // critical section.
1772 }
1773
1774 Thread::SpinRelease (&_WaitSetLock) ;
1775
1776 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1777 ObjectMonitor::_sync_Notifications->inc() ;
1778 }
1779 }
1780
1781
1782 void ObjectMonitor::notifyAll(TRAPS) {
1783 CHECK_OWNER();
1784 ObjectWaiter* iterator;
1785 if (_WaitSet == NULL) {
1786 TEVENT (Empty-NotifyAll) ;
1787 return ;
1788 }
1789 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1790
1791 int Policy = Knob_MoveNotifyee ;
1792 int Tally = 0 ;
1793 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1794
1795 for (;;) {
1796 iterator = DequeueWaiter () ;
1797 if (iterator == NULL) break ;
1798 TEVENT (NotifyAll - Transfer1) ;
1799 ++Tally ;
1800
1801 // Disposition - what might we do with iterator ?
1802 // a. add it directly to the EntryList - either tail or head.
1803 // b. push it onto the front of the _cxq.
1804 // For now we use (a).
1805
1806 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1807 guarantee (iterator->_notified == 0, "invariant") ;
1808 iterator->_notified = 1 ;
1809 Thread * Self = THREAD;
1810 iterator->_notifier_tid = Self->osthread()->thread_id();
1811 if (Policy != 4) {
1812 iterator->TState = ObjectWaiter::TS_ENTER ;
1813 }
1814
1815 ObjectWaiter * List = _EntryList ;
1816 if (List != NULL) {
1817 assert (List->_prev == NULL, "invariant") ;
1818 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1819 assert (List != iterator, "invariant") ;
1820 }
1821
1822 if (Policy == 0) { // prepend to EntryList
1823 if (List == NULL) {
1824 iterator->_next = iterator->_prev = NULL ;
1825 _EntryList = iterator ;
1826 } else {
1827 List->_prev = iterator ;
1828 iterator->_next = List ;
1829 iterator->_prev = NULL ;
1830 _EntryList = iterator ;
1831 }
1832 } else
1833 if (Policy == 1) { // append to EntryList
1834 if (List == NULL) {
1835 iterator->_next = iterator->_prev = NULL ;
1836 _EntryList = iterator ;
1837 } else {
1838 // CONSIDER: finding the tail currently requires a linear-time walk of
1839 // the EntryList. We can make tail access constant-time by converting to
1840 // a CDLL instead of using our current DLL.
1841 ObjectWaiter * Tail ;
1842 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1843 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1844 Tail->_next = iterator ;
1845 iterator->_prev = Tail ;
1846 iterator->_next = NULL ;
1847 }
1848 } else
1849 if (Policy == 2) { // prepend to cxq
1850 // prepend to cxq
1851 iterator->TState = ObjectWaiter::TS_CXQ ;
1852 for (;;) {
1853 ObjectWaiter * Front = _cxq ;
1854 iterator->_next = Front ;
1855 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1856 break ;
1857 }
1858 }
1859 } else
1860 if (Policy == 3) { // append to cxq
1861 iterator->TState = ObjectWaiter::TS_CXQ ;
1862 for (;;) {
1863 ObjectWaiter * Tail ;
1864 Tail = _cxq ;
1865 if (Tail == NULL) {
1866 iterator->_next = NULL ;
1867 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1868 break ;
1869 }
1870 } else {
1871 while (Tail->_next != NULL) Tail = Tail->_next ;
1872 Tail->_next = iterator ;
1873 iterator->_prev = Tail ;
1874 iterator->_next = NULL ;
1875 break ;
1876 }
1877 }
1878 } else {
1879 ParkEvent * ev = iterator->_event ;
1880 iterator->TState = ObjectWaiter::TS_RUN ;
1881 OrderAccess::fence() ;
1882 ev->unpark() ;
1883 }
1884
1885 if (Policy < 4) {
1886 iterator->wait_reenter_begin(this);
1887 }
1888
1889 // _WaitSetLock protects the wait queue, not the EntryList. We could
1890 // move the add-to-EntryList operation, above, outside the critical section
1891 // protected by _WaitSetLock. In practice that's not useful. With the
1892 // exception of wait() timeouts and interrupts the monitor owner
1893 // is the only thread that grabs _WaitSetLock. There's almost no contention
1894 // on _WaitSetLock so it's not profitable to reduce the length of the
1895 // critical section.
1896 }
1897
1898 Thread::SpinRelease (&_WaitSetLock) ;
1899
1900 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1901 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1902 }
1903 }
1904
1905 // -----------------------------------------------------------------------------
1906 // Adaptive Spinning Support
1907 //
1908 // Adaptive spin-then-block - rational spinning
1909 //
1910 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1911 // algorithm. On high order SMP systems it would be better to start with
1912 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1913 // a contending thread could enqueue itself on the cxq and then spin locally
1914 // on a thread-specific variable such as its ParkEvent._Event flag.
1915 // That's left as an exercise for the reader. Note that global spinning is
1916 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1917 // low latency and massive bandwidth.
1918 //
1919 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1920 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1921 // (duration) or we can fix the count at approximately the duration of
1922 // a context switch and vary the frequency. Of course we could also
1923 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1924 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1925 //
1926 // This implementation varies the duration "D", where D varies with
1927 // the success rate of recent spin attempts. (D is capped at approximately
1928 // length of a round-trip context switch). The success rate for recent
1929 // spin attempts is a good predictor of the success rate of future spin
1930 // attempts. The mechanism adapts automatically to varying critical
1931 // section length (lock modality), system load and degree of parallelism.
1932 // D is maintained per-monitor in _SpinDuration and is initialized
1933 // optimistically. Spin frequency is fixed at 100%.
1934 //
1935 // Note that _SpinDuration is volatile, but we update it without locks
1936 // or atomics. The code is designed so that _SpinDuration stays within
1937 // a reasonable range even in the presence of races. The arithmetic
1938 // operations on _SpinDuration are closed over the domain of legal values,
1939 // so at worst a race will install and older but still legal value.
1940 // At the very worst this introduces some apparent non-determinism.
1941 // We might spin when we shouldn't or vice-versa, but since the spin
1942 // count are relatively short, even in the worst case, the effect is harmless.
1943 //
1944 // Care must be taken that a low "D" value does not become an
1945 // an absorbing state. Transient spinning failures -- when spinning
1946 // is overall profitable -- should not cause the system to converge
1947 // on low "D" values. We want spinning to be stable and predictable
1948 // and fairly responsive to change and at the same time we don't want
1949 // it to oscillate, become metastable, be "too" non-deterministic,
1950 // or converge on or enter undesirable stable absorbing states.
1951 //
1952 // We implement a feedback-based control system -- using past behavior
1953 // to predict future behavior. We face two issues: (a) if the
1954 // input signal is random then the spin predictor won't provide optimal
1955 // results, and (b) if the signal frequency is too high then the control
1956 // system, which has some natural response lag, will "chase" the signal.
1957 // (b) can arise from multimodal lock hold times. Transient preemption
1958 // can also result in apparent bimodal lock hold times.
1959 // Although sub-optimal, neither condition is particularly harmful, as
1960 // in the worst-case we'll spin when we shouldn't or vice-versa.
1961 // The maximum spin duration is rather short so the failure modes aren't bad.
1962 // To be conservative, I've tuned the gain in system to bias toward
1963 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1964 // "rings" or oscillates between spinning and not spinning. This happens
1965 // when spinning is just on the cusp of profitability, however, so the
1966 // situation is not dire. The state is benign -- there's no need to add
1967 // hysteresis control to damp the transition rate between spinning and
1968 // not spinning.
1969 //
1970
1971 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1972 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1973
1974 // Spinning: Fixed frequency (100%), vary duration
1975
1976
1977 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1978
1979 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1980 int ctr = Knob_FixedSpin ;
1981 if (ctr != 0) {
1982 while (--ctr >= 0) {
1983 if (TryLock (Self) > 0) return 1 ;
1984 SpinPause () ;
1985 }
1986 return 0 ;
1987 }
1988
1989 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1990 if (TryLock(Self) > 0) {
1991 // Increase _SpinDuration ...
1992 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1993 // Raising _SpurDuration to the poverty line is key.
1994 int x = _SpinDuration ;
1995 if (x < Knob_SpinLimit) {
1996 if (x < Knob_Poverty) x = Knob_Poverty ;
1997 _SpinDuration = x + Knob_BonusB ;
1998 }
1999 return 1 ;
2000 }
2001 SpinPause () ;
2002 }
2003
2004 // Admission control - verify preconditions for spinning
2005 //
2006 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2007 // becoming an absorbing state. Put another way, we spin briefly to
2008 // sample, just in case the system load, parallelism, contention, or lock
2009 // modality changed.
2010 //
2011 // Consider the following alternative:
2012 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2013 // spin attempt. "Periodically" might mean after a tally of
2014 // the # of failed spin attempts (or iterations) reaches some threshold.
2015 // This takes us into the realm of 1-out-of-N spinning, where we
2016 // hold the duration constant but vary the frequency.
2017
2018 ctr = _SpinDuration ;
2019 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2020 if (ctr <= 0) return 0 ;
2021
2022 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2023 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2024 TEVENT (Spin abort - notrunnable [TOP]);
2025 return 0 ;
2026 }
2027
2028 int MaxSpin = Knob_MaxSpinners ;
2029 if (MaxSpin >= 0) {
2030 if (_Spinner > MaxSpin) {
2031 TEVENT (Spin abort -- too many spinners) ;
2032 return 0 ;
2033 }
2034 // Slightly racy, but benign ...
2035 Adjust (&_Spinner, 1) ;
2036 }
2037
2038 // We're good to spin ... spin ingress.
2039 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2040 // when preparing to LD...CAS _owner, etc and the CAS is likely
2041 // to succeed.
2042 int hits = 0 ;
2043 int msk = 0 ;
2044 int caspty = Knob_CASPenalty ;
2045 int oxpty = Knob_OXPenalty ;
2046 int sss = Knob_SpinSetSucc ;
2047 if (sss && _succ == NULL ) _succ = Self ;
2048 Thread * prv = NULL ;
2049
2050 // There are three ways to exit the following loop:
2051 // 1. A successful spin where this thread has acquired the lock.
2052 // 2. Spin failure with prejudice
2053 // 3. Spin failure without prejudice
2054
2055 while (--ctr >= 0) {
2056
2057 // Periodic polling -- Check for pending GC
2058 // Threads may spin while they're unsafe.
2059 // We don't want spinning threads to delay the JVM from reaching
2060 // a stop-the-world safepoint or to steal cycles from GC.
2061 // If we detect a pending safepoint we abort in order that
2062 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2063 // this thread, if safe, doesn't steal cycles from GC.
2064 // This is in keeping with the "no loitering in runtime" rule.
2065 // We periodically check to see if there's a safepoint pending.
2066 if ((ctr & 0xFF) == 0) {
2067 if (SafepointSynchronize::do_call_back()) {
2068 TEVENT (Spin: safepoint) ;
2069 goto Abort ; // abrupt spin egress
2070 }
2071 if (Knob_UsePause & 1) SpinPause () ;
2072
2073 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2074 if (hits > 50 && scb != NULL) {
2075 int abend = (*scb)(SpinCallbackArgument, 0) ;
2076 }
2077 }
2078
2079 if (Knob_UsePause & 2) SpinPause() ;
2080
2081 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2082 // This is useful on classic SMP systems, but is of less utility on
2083 // N1-style CMT platforms.
2084 //
2085 // Trade-off: lock acquisition latency vs coherency bandwidth.
2086 // Lock hold times are typically short. A histogram
2087 // of successful spin attempts shows that we usually acquire
2088 // the lock early in the spin. That suggests we want to
2089 // sample _owner frequently in the early phase of the spin,
2090 // but then back-off and sample less frequently as the spin
2091 // progresses. The back-off makes a good citizen on SMP big
2092 // SMP systems. Oversampling _owner can consume excessive
2093 // coherency bandwidth. Relatedly, if we _oversample _owner we
2094 // can inadvertently interfere with the the ST m->owner=null.
2095 // executed by the lock owner.
2096 if (ctr & msk) continue ;
2097 ++hits ;
2098 if ((hits & 0xF) == 0) {
2099 // The 0xF, above, corresponds to the exponent.
2100 // Consider: (msk+1)|msk
2101 msk = ((msk << 2)|3) & BackOffMask ;
2102 }
2103
2104 // Probe _owner with TATAS
2105 // If this thread observes the monitor transition or flicker
2106 // from locked to unlocked to locked, then the odds that this
2107 // thread will acquire the lock in this spin attempt go down
2108 // considerably. The same argument applies if the CAS fails
2109 // or if we observe _owner change from one non-null value to
2110 // another non-null value. In such cases we might abort
2111 // the spin without prejudice or apply a "penalty" to the
2112 // spin count-down variable "ctr", reducing it by 100, say.
2113
2114 Thread * ox = (Thread *) _owner ;
2115 if (ox == NULL) {
2116 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2117 if (ox == NULL) {
2118 // The CAS succeeded -- this thread acquired ownership
2119 // Take care of some bookkeeping to exit spin state.
2120 if (sss && _succ == Self) {
2121 _succ = NULL ;
2122 }
2123 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2124
2125 // Increase _SpinDuration :
2126 // The spin was successful (profitable) so we tend toward
2127 // longer spin attempts in the future.
2128 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2129 // If we acquired the lock early in the spin cycle it
2130 // makes sense to increase _SpinDuration proportionally.
2131 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2132 int x = _SpinDuration ;
2133 if (x < Knob_SpinLimit) {
2134 if (x < Knob_Poverty) x = Knob_Poverty ;
2135 _SpinDuration = x + Knob_Bonus ;
2136 }
2137 return 1 ;
2138 }
2139
2140 // The CAS failed ... we can take any of the following actions:
2141 // * penalize: ctr -= Knob_CASPenalty
2142 // * exit spin with prejudice -- goto Abort;
2143 // * exit spin without prejudice.
2144 // * Since CAS is high-latency, retry again immediately.
2145 prv = ox ;
2146 TEVENT (Spin: cas failed) ;
2147 if (caspty == -2) break ;
2148 if (caspty == -1) goto Abort ;
2149 ctr -= caspty ;
2150 continue ;
2151 }
2152
2153 // Did lock ownership change hands ?
2154 if (ox != prv && prv != NULL ) {
2155 TEVENT (spin: Owner changed)
2156 if (oxpty == -2) break ;
2157 if (oxpty == -1) goto Abort ;
2158 ctr -= oxpty ;
2159 }
2160 prv = ox ;
2161
2162 // Abort the spin if the owner is not executing.
2163 // The owner must be executing in order to drop the lock.
2164 // Spinning while the owner is OFFPROC is idiocy.
2165 // Consider: ctr -= RunnablePenalty ;
2166 if (Knob_OState && NotRunnable (Self, ox)) {
2167 TEVENT (Spin abort - notrunnable);
2168 goto Abort ;
2169 }
2170 if (sss && _succ == NULL ) _succ = Self ;
2171 }
2172
2173 // Spin failed with prejudice -- reduce _SpinDuration.
2174 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2175 // AIMD is globally stable.
2176 TEVENT (Spin failure) ;
2177 {
2178 int x = _SpinDuration ;
2179 if (x > 0) {
2180 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2181 // This is globally sample and tends to damp the response.
2182 x -= Knob_Penalty ;
2183 if (x < 0) x = 0 ;
2184 _SpinDuration = x ;
2185 }
2186 }
2187
2188 Abort:
2189 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2190 if (sss && _succ == Self) {
2191 _succ = NULL ;
2192 // Invariant: after setting succ=null a contending thread
2193 // must recheck-retry _owner before parking. This usually happens
2194 // in the normal usage of TrySpin(), but it's safest
2195 // to make TrySpin() as foolproof as possible.
2196 OrderAccess::fence() ;
2197 if (TryLock(Self) > 0) return 1 ;
2198 }
2199 return 0 ;
2200 }
2201
2202 // NotRunnable() -- informed spinning
2203 //
2204 // Don't bother spinning if the owner is not eligible to drop the lock.
2205 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2206 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2207 // The thread must be runnable in order to drop the lock in timely fashion.
2208 // If the _owner is not runnable then spinning will not likely be
2209 // successful (profitable).
2210 //
2211 // Beware -- the thread referenced by _owner could have died
2212 // so a simply fetch from _owner->_thread_state might trap.
2213 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2214 // Because of the lifecycle issues the schedctl and _thread_state values
2215 // observed by NotRunnable() might be garbage. NotRunnable must
2216 // tolerate this and consider the observed _thread_state value
2217 // as advisory.
2218 //
2219 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2220 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2221 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2222 // with the LSB of _owner. Another option would be to probablistically probe
2223 // the putative _owner->TypeTag value.
2224 //
2225 // Checking _thread_state isn't perfect. Even if the thread is
2226 // in_java it might be blocked on a page-fault or have been preempted
2227 // and sitting on a ready/dispatch queue. _thread state in conjunction
2228 // with schedctl.sc_state gives us a good picture of what the
2229 // thread is doing, however.
2230 //
2231 // TODO: check schedctl.sc_state.
2232 // We'll need to use SafeFetch32() to read from the schedctl block.
2233 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2234 //
2235 // The return value from NotRunnable() is *advisory* -- the
2236 // result is based on sampling and is not necessarily coherent.
2237 // The caller must tolerate false-negative and false-positive errors.
2238 // Spinning, in general, is probabilistic anyway.
2239
2240
2241 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2242 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2243 if (!OwnerIsThread) return 0 ;
2244
2245 if (ox == NULL) return 0 ;
2246
2247 // Avoid transitive spinning ...
2248 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2249 // Immediately after T1 acquires L it's possible that T2, also
2250 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2251 // This occurs transiently after T1 acquired L but before
2252 // T1 managed to clear T1.Stalled. T2 does not need to abort
2253 // its spin in this circumstance.
2254 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2255
2256 if (BlockedOn == 1) return 1 ;
2257 if (BlockedOn != 0) {
2258 return BlockedOn != intptr_t(this) && _owner == ox ;
2259 }
2260
2261 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2262 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2263 // consider also: jst != _thread_in_Java -- but that's overspecific.
2264 return jst == _thread_blocked || jst == _thread_in_native ;
2265 }
2266
2267
2268 // -----------------------------------------------------------------------------
2269 // WaitSet management ...
2270
2271 ObjectWaiter::ObjectWaiter(Thread* thread) {
2272 _next = NULL;
2273 _prev = NULL;
2274 _notified = 0;
2275 TState = TS_RUN ;
2276 _thread = thread;
2277 _event = thread->_ParkEvent ;
2278 _active = false;
2279 assert (_event != NULL, "invariant") ;
2280 }
2281
2282 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2283 JavaThread *jt = (JavaThread *)this->_thread;
2284 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2285 }
2286
2287 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2288 JavaThread *jt = (JavaThread *)this->_thread;
2289 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2290 }
2291
2292 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2293 assert(node != NULL, "should not dequeue NULL node");
2294 assert(node->_prev == NULL, "node already in list");
2295 assert(node->_next == NULL, "node already in list");
2296 // put node at end of queue (circular doubly linked list)
2297 if (_WaitSet == NULL) {
2298 _WaitSet = node;
2299 node->_prev = node;
2300 node->_next = node;
2301 } else {
2302 ObjectWaiter* head = _WaitSet ;
2303 ObjectWaiter* tail = head->_prev;
2304 assert(tail->_next == head, "invariant check");
2305 tail->_next = node;
2306 head->_prev = node;
2307 node->_next = head;
2308 node->_prev = tail;
2309 }
2310 }
2311
2312 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2313 // dequeue the very first waiter
2314 ObjectWaiter* waiter = _WaitSet;
2315 if (waiter) {
2316 DequeueSpecificWaiter(waiter);
2317 }
2318 return waiter;
2319 }
2320
2321 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2322 assert(node != NULL, "should not dequeue NULL node");
2323 assert(node->_prev != NULL, "node already removed from list");
2324 assert(node->_next != NULL, "node already removed from list");
2325 // when the waiter has woken up because of interrupt,
2326 // timeout or other spurious wake-up, dequeue the
2327 // waiter from waiting list
2328 ObjectWaiter* next = node->_next;
2329 if (next == node) {
2330 assert(node->_prev == node, "invariant check");
2331 _WaitSet = NULL;
2332 } else {
2333 ObjectWaiter* prev = node->_prev;
2334 assert(prev->_next == node, "invariant check");
2335 assert(next->_prev == node, "invariant check");
2336 next->_prev = prev;
2337 prev->_next = next;
2338 if (_WaitSet == node) {
2339 _WaitSet = next;
2340 }
2341 }
2342 node->_next = NULL;
2343 node->_prev = NULL;
2344 }
2345
2346 // -----------------------------------------------------------------------------
2347 // PerfData support
2348 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2349 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2350 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2351 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2352 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2353 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2354 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2355 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2356 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2357 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2358 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2359 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2360 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2361 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2362 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2363 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2364 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2365 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2366
2367 // One-shot global initialization for the sync subsystem.
2368 // We could also defer initialization and initialize on-demand
2369 // the first time we call inflate(). Initialization would
2370 // be protected - like so many things - by the MonitorCache_lock.
2371
2372 void ObjectMonitor::Initialize () {
2373 static int InitializationCompleted = 0 ;
2374 assert (InitializationCompleted == 0, "invariant") ;
2375 InitializationCompleted = 1 ;
2376 if (UsePerfData) {
2377 EXCEPTION_MARK ;
2378 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2379 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2380 NEWPERFCOUNTER(_sync_Inflations) ;
2381 NEWPERFCOUNTER(_sync_Deflations) ;
2382 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2383 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2384 NEWPERFCOUNTER(_sync_Parks) ;
2385 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2386 NEWPERFCOUNTER(_sync_Notifications) ;
2387 NEWPERFCOUNTER(_sync_SlowEnter) ;
2388 NEWPERFCOUNTER(_sync_SlowExit) ;
2389 NEWPERFCOUNTER(_sync_SlowNotify) ;
2390 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2391 NEWPERFCOUNTER(_sync_FailedSpins) ;
2392 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2393 NEWPERFCOUNTER(_sync_PrivateA) ;
2394 NEWPERFCOUNTER(_sync_PrivateB) ;
2395 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2396 NEWPERFCOUNTER(_sync_MonScavenged) ;
2397 NEWPERFVARIABLE(_sync_MonExtant) ;
2398 #undef NEWPERFCOUNTER
2399 }
2400 }
2401
2402
2403 // Compile-time asserts
2404 // When possible, it's better to catch errors deterministically at
2405 // compile-time than at runtime. The down-side to using compile-time
2406 // asserts is that error message -- often something about negative array
2407 // indices -- is opaque.
2408
2409 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2410
2411 void ObjectMonitor::ctAsserts() {
2412 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2413 }
2414
2415
2416 static char * kvGet (char * kvList, const char * Key) {
2417 if (kvList == NULL) return NULL ;
2418 size_t n = strlen (Key) ;
2419 char * Search ;
2420 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2421 if (strncmp (Search, Key, n) == 0) {
2422 if (Search[n] == '=') return Search + n + 1 ;
2423 if (Search[n] == 0) return (char *) "1" ;
2424 }
2425 }
2426 return NULL ;
2427 }
2428
2429 static int kvGetInt (char * kvList, const char * Key, int Default) {
2430 char * v = kvGet (kvList, Key) ;
2431 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2432 if (Knob_ReportSettings && v != NULL) {
2433 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2434 ::fflush (stdout) ;
2435 }
2436 return rslt ;
2437 }
2438
2439 void ObjectMonitor::DeferredInitialize () {
2440 if (InitDone > 0) return ;
2441 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2442 while (InitDone != 1) ;
2443 return ;
2444 }
2445
2446 // One-shot global initialization ...
2447 // The initialization is idempotent, so we don't need locks.
2448 // In the future consider doing this via os::init_2().
2449 // SyncKnobs consist of <Key>=<Value> pairs in the style
2450 // of environment variables. Start by converting ':' to NUL.
2451
2452 if (SyncKnobs == NULL) SyncKnobs = "" ;
2453
2454 size_t sz = strlen (SyncKnobs) ;
2455 char * knobs = (char *) malloc (sz + 2) ;
2456 if (knobs == NULL) {
2457 vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
2458 guarantee (0, "invariant") ;
2459 }
2460 strcpy (knobs, SyncKnobs) ;
2461 knobs[sz+1] = 0 ;
2462 for (char * p = knobs ; *p ; p++) {
2463 if (*p == ':') *p = 0 ;
2464 }
2465
2466 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2467 SETKNOB(ReportSettings) ;
2468 SETKNOB(Verbose) ;
2469 SETKNOB(FixedSpin) ;
2470 SETKNOB(SpinLimit) ;
2471 SETKNOB(SpinBase) ;
2472 SETKNOB(SpinBackOff);
2473 SETKNOB(CASPenalty) ;
2474 SETKNOB(OXPenalty) ;
2475 SETKNOB(LogSpins) ;
2476 SETKNOB(SpinSetSucc) ;
2477 SETKNOB(SuccEnabled) ;
2478 SETKNOB(SuccRestrict) ;
2479 SETKNOB(Penalty) ;
2480 SETKNOB(Bonus) ;
2481 SETKNOB(BonusB) ;
2482 SETKNOB(Poverty) ;
2483 SETKNOB(SpinAfterFutile) ;
2484 SETKNOB(UsePause) ;
2485 SETKNOB(SpinEarly) ;
2486 SETKNOB(OState) ;
2487 SETKNOB(MaxSpinners) ;
2488 SETKNOB(PreSpin) ;
2489 SETKNOB(ExitPolicy) ;
2490 SETKNOB(QMode);
2491 SETKNOB(ResetEvent) ;
2492 SETKNOB(MoveNotifyee) ;
2493 SETKNOB(FastHSSEC) ;
2494 #undef SETKNOB
2495
2496 if (os::is_MP()) {
2497 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2498 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2499 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2500 } else {
2501 Knob_SpinLimit = 0 ;
2502 Knob_SpinBase = 0 ;
2503 Knob_PreSpin = 0 ;
2504 Knob_FixedSpin = -1 ;
2505 }
2506
2507 if (Knob_LogSpins == 0) {
2508 ObjectMonitor::_sync_FailedSpins = NULL ;
2509 }
2510
2511 free (knobs) ;
2512 OrderAccess::fence() ;
2513 InitDone = 1 ;
2514 }
2515
2516 #ifndef PRODUCT
2517 void ObjectMonitor::verify() {
2518 }
2519
2520 void ObjectMonitor::print() {
2521 }
2522 #endif