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() epilog) 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 // The current thread does not yet own the monitor and does not
387 // yet appear on any queues that would get it made the successor.
388 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
389 // handler cannot accidentally consume an unpark() meant for the
390 // ParkEvent associated with this ObjectMonitor.
391 }
392
393 OSThreadContendState osts(Self->osthread());
394 ThreadBlockInVM tbivm(jt);
395
396 Self->set_current_pending_monitor(this);
397
398 // TODO-FIXME: change the following for(;;) loop to straight-line code.
399 for (;;) {
400 jt->set_suspend_equivalent();
401 // cleared by handle_special_suspend_equivalent_condition()
402 // or java_suspend_self()
403
404 EnterI (THREAD) ;
405
406 if (!ExitSuspendEquivalent(jt)) break ;
407
408 //
409 // We have acquired the contended monitor, but while we were
410 // waiting another thread suspended us. We don't want to enter
411 // the monitor while suspended because that would surprise the
412 // thread that suspended us.
413 //
414 _recursions = 0 ;
415 _succ = NULL ;
416 exit (false, Self) ;
417
418 jt->java_suspend_self();
419 }
420 Self->set_current_pending_monitor(NULL);
421 }
422
423 Atomic::dec_ptr(&_count);
424 assert (_count >= 0, "invariant") ;
425 Self->_Stalled = 0 ;
426
427 // Must either set _recursions = 0 or ASSERT _recursions == 0.
428 assert (_recursions == 0 , "invariant") ;
429 assert (_owner == Self , "invariant") ;
430 assert (_succ != Self , "invariant") ;
431 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
432
433 // The thread -- now the owner -- is back in vm mode.
434 // Report the glorious news via TI,DTrace and jvmstat.
435 // The probe effect is non-trivial. All the reportage occurs
436 // while we hold the monitor, increasing the length of the critical
437 // section. Amdahl's parallel speedup law comes vividly into play.
438 //
439 // Another option might be to aggregate the events (thread local or
440 // per-monitor aggregation) and defer reporting until a more opportune
441 // time -- such as next time some thread encounters contention but has
442 // yet to acquire the lock. While spinning that thread could
443 // spinning we could increment JVMStat counters, etc.
444
445 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
446 if (JvmtiExport::should_post_monitor_contended_entered()) {
447 JvmtiExport::post_monitor_contended_entered(jt, this);
448
449 // The current thread already owns the monitor and is not going to
450 // call park() for the remainder of the monitor enter protocol. So
451 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
452 // event handler consumed an unpark() issued by the thread that
453 // just exited the monitor.
454 }
455
456 if (event.should_commit()) {
457 event.set_klass(((oop)this->object())->klass());
458 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
459 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
460 event.commit();
461 }
462
463 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
464 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
465 }
466 }
467
468
469 // Caveat: TryLock() is not necessarily serializing if it returns failure.
470 // Callers must compensate as needed.
471
472 int ObjectMonitor::TryLock (Thread * Self) {
473 for (;;) {
474 void * own = _owner ;
475 if (own != NULL) return 0 ;
476 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
477 // Either guarantee _recursions == 0 or set _recursions = 0.
478 assert (_recursions == 0, "invariant") ;
479 assert (_owner == Self, "invariant") ;
480 // CONSIDER: set or assert that OwnerIsThread == 1
481 return 1 ;
482 }
483 // The lock had been free momentarily, but we lost the race to the lock.
484 // Interference -- the CAS failed.
485 // We can either return -1 or retry.
486 // Retry doesn't make as much sense because the lock was just acquired.
487 if (true) return -1 ;
488 }
489 }
490
491 void ATTR ObjectMonitor::EnterI (TRAPS) {
492 Thread * Self = THREAD ;
493 assert (Self->is_Java_thread(), "invariant") ;
494 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
495
496 // Try the lock - TATAS
497 if (TryLock (Self) > 0) {
498 assert (_succ != Self , "invariant") ;
499 assert (_owner == Self , "invariant") ;
500 assert (_Responsible != Self , "invariant") ;
501 return ;
502 }
503
504 DeferredInitialize () ;
505
506 // We try one round of spinning *before* enqueueing Self.
507 //
508 // If the _owner is ready but OFFPROC we could use a YieldTo()
509 // operation to donate the remainder of this thread's quantum
510 // to the owner. This has subtle but beneficial affinity
511 // effects.
512
513 if (TrySpin (Self) > 0) {
514 assert (_owner == Self , "invariant") ;
515 assert (_succ != Self , "invariant") ;
516 assert (_Responsible != Self , "invariant") ;
517 return ;
518 }
519
520 // The Spin failed -- Enqueue and park the thread ...
521 assert (_succ != Self , "invariant") ;
522 assert (_owner != Self , "invariant") ;
523 assert (_Responsible != Self , "invariant") ;
524
525 // Enqueue "Self" on ObjectMonitor's _cxq.
526 //
527 // Node acts as a proxy for Self.
528 // As an aside, if were to ever rewrite the synchronization code mostly
529 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
530 // Java objects. This would avoid awkward lifecycle and liveness issues,
531 // as well as eliminate a subset of ABA issues.
532 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
533 //
534
535 ObjectWaiter node(Self) ;
536 Self->_ParkEvent->reset() ;
537 node._prev = (ObjectWaiter *) 0xBAD ;
538 node.TState = ObjectWaiter::TS_CXQ ;
539
540 // Push "Self" onto the front of the _cxq.
541 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
542 // Note that spinning tends to reduce the rate at which threads
543 // enqueue and dequeue on EntryList|cxq.
544 ObjectWaiter * nxt ;
545 for (;;) {
546 node._next = nxt = _cxq ;
547 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
548
549 // Interference - the CAS failed because _cxq changed. Just retry.
550 // As an optional optimization we retry the lock.
551 if (TryLock (Self) > 0) {
552 assert (_succ != Self , "invariant") ;
553 assert (_owner == Self , "invariant") ;
554 assert (_Responsible != Self , "invariant") ;
555 return ;
556 }
557 }
558
559 // Check for cxq|EntryList edge transition to non-null. This indicates
560 // the onset of contention. While contention persists exiting threads
561 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
562 // operations revert to the faster 1-0 mode. This enter operation may interleave
563 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
564 // arrange for one of the contending thread to use a timed park() operations
565 // to detect and recover from the race. (Stranding is form of progress failure
566 // where the monitor is unlocked but all the contending threads remain parked).
567 // That is, at least one of the contended threads will periodically poll _owner.
568 // One of the contending threads will become the designated "Responsible" thread.
569 // The Responsible thread uses a timed park instead of a normal indefinite park
570 // operation -- it periodically wakes and checks for and recovers from potential
571 // strandings admitted by 1-0 exit operations. We need at most one Responsible
572 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
573 // be responsible for a monitor.
574 //
575 // Currently, one of the contended threads takes on the added role of "Responsible".
576 // A viable alternative would be to use a dedicated "stranding checker" thread
577 // that periodically iterated over all the threads (or active monitors) and unparked
578 // successors where there was risk of stranding. This would help eliminate the
579 // timer scalability issues we see on some platforms as we'd only have one thread
580 // -- the checker -- parked on a timer.
581
582 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
583 // Try to assume the role of responsible thread for the monitor.
584 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
585 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
586 }
587
588 // The lock have been released while this thread was occupied queueing
589 // itself onto _cxq. To close the race and avoid "stranding" and
590 // progress-liveness failure we must resample-retry _owner before parking.
591 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
592 // In this case the ST-MEMBAR is accomplished with CAS().
593 //
594 // TODO: Defer all thread state transitions until park-time.
595 // Since state transitions are heavy and inefficient we'd like
596 // to defer the state transitions until absolutely necessary,
597 // and in doing so avoid some transitions ...
598
599 TEVENT (Inflated enter - Contention) ;
600 int nWakeups = 0 ;
601 int RecheckInterval = 1 ;
602
603 for (;;) {
604
605 if (TryLock (Self) > 0) break ;
606 assert (_owner != Self, "invariant") ;
607
608 if ((SyncFlags & 2) && _Responsible == NULL) {
609 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
610 }
611
612 // park self
613 if (_Responsible == Self || (SyncFlags & 1)) {
614 TEVENT (Inflated enter - park TIMED) ;
615 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
616 // Increase the RecheckInterval, but clamp the value.
617 RecheckInterval *= 8 ;
618 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
619 } else {
620 TEVENT (Inflated enter - park UNTIMED) ;
621 Self->_ParkEvent->park() ;
622 }
623
624 if (TryLock(Self) > 0) break ;
625
626 // The lock is still contested.
627 // Keep a tally of the # of futile wakeups.
628 // Note that the counter is not protected by a lock or updated by atomics.
629 // That is by design - we trade "lossy" counters which are exposed to
630 // races during updates for a lower probe effect.
631 TEVENT (Inflated enter - Futile wakeup) ;
632 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
633 ObjectMonitor::_sync_FutileWakeups->inc() ;
634 }
635 ++ nWakeups ;
636
637 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
638 // We can defer clearing _succ until after the spin completes
639 // TrySpin() must tolerate being called with _succ == Self.
640 // Try yet another round of adaptive spinning.
641 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
642
643 // We can find that we were unpark()ed and redesignated _succ while
644 // we were spinning. That's harmless. If we iterate and call park(),
645 // park() will consume the event and return immediately and we'll
646 // just spin again. This pattern can repeat, leaving _succ to simply
647 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
648 // Alternately, we can sample fired() here, and if set, forgo spinning
649 // in the next iteration.
650
651 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
652 Self->_ParkEvent->reset() ;
653 OrderAccess::fence() ;
654 }
655 if (_succ == Self) _succ = NULL ;
656
657 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
658 OrderAccess::fence() ;
659 }
660
661 // Egress :
662 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
663 // Normally we'll find Self on the EntryList .
664 // From the perspective of the lock owner (this thread), the
665 // EntryList is stable and cxq is prepend-only.
666 // The head of cxq is volatile but the interior is stable.
667 // In addition, Self.TState is stable.
668
669 assert (_owner == Self , "invariant") ;
670 assert (object() != NULL , "invariant") ;
671 // I'd like to write:
672 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
673 // but as we're at a safepoint that's not safe.
674
675 UnlinkAfterAcquire (Self, &node) ;
676 if (_succ == Self) _succ = NULL ;
677
678 assert (_succ != Self, "invariant") ;
679 if (_Responsible == Self) {
680 _Responsible = NULL ;
681 OrderAccess::fence(); // Dekker pivot-point
682
683 // We may leave threads on cxq|EntryList without a designated
684 // "Responsible" thread. This is benign. When this thread subsequently
685 // exits the monitor it can "see" such preexisting "old" threads --
686 // threads that arrived on the cxq|EntryList before the fence, above --
687 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
688 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
689 // non-null and elect a new "Responsible" timer thread.
690 //
691 // This thread executes:
692 // ST Responsible=null; MEMBAR (in enter epilog - here)
693 // LD cxq|EntryList (in subsequent exit)
694 //
695 // Entering threads in the slow/contended path execute:
696 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
697 // The (ST cxq; MEMBAR) is accomplished with CAS().
698 //
699 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
700 // exit operation from floating above the ST Responsible=null.
701 }
702
703 // We've acquired ownership with CAS().
704 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
705 // But since the CAS() this thread may have also stored into _succ,
706 // EntryList, cxq or Responsible. These meta-data updates must be
707 // visible __before this thread subsequently drops the lock.
708 // Consider what could occur if we didn't enforce this constraint --
709 // STs to monitor meta-data and user-data could reorder with (become
710 // visible after) the ST in exit that drops ownership of the lock.
711 // Some other thread could then acquire the lock, but observe inconsistent
712 // or old monitor meta-data and heap data. That violates the JMM.
713 // To that end, the 1-0 exit() operation must have at least STST|LDST
714 // "release" barrier semantics. Specifically, there must be at least a
715 // STST|LDST barrier in exit() before the ST of null into _owner that drops
716 // the lock. The barrier ensures that changes to monitor meta-data and data
717 // protected by the lock will be visible before we release the lock, and
718 // therefore before some other thread (CPU) has a chance to acquire the lock.
719 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
720 //
721 // Critically, any prior STs to _succ or EntryList must be visible before
722 // the ST of null into _owner in the *subsequent* (following) corresponding
723 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
724 // execute a serializing instruction.
725
726 if (SyncFlags & 8) {
727 OrderAccess::fence() ;
728 }
729 return ;
730 }
731
732 // ReenterI() is a specialized inline form of the latter half of the
733 // contended slow-path from EnterI(). We use ReenterI() only for
734 // monitor reentry in wait().
735 //
736 // In the future we should reconcile EnterI() and ReenterI(), adding
737 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
738 // loop accordingly.
739
740 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
741 assert (Self != NULL , "invariant") ;
742 assert (SelfNode != NULL , "invariant") ;
743 assert (SelfNode->_thread == Self , "invariant") ;
744 assert (_waiters > 0 , "invariant") ;
745 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
746 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
747 JavaThread * jt = (JavaThread *) Self ;
748
749 int nWakeups = 0 ;
750 for (;;) {
751 ObjectWaiter::TStates v = SelfNode->TState ;
752 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
753 assert (_owner != Self, "invariant") ;
754
755 if (TryLock (Self) > 0) break ;
756 if (TrySpin (Self) > 0) break ;
757
758 TEVENT (Wait Reentry - parking) ;
759
760 // State transition wrappers around park() ...
761 // ReenterI() wisely defers state transitions until
762 // it's clear we must park the thread.
763 {
764 OSThreadContendState osts(Self->osthread());
765 ThreadBlockInVM tbivm(jt);
766
767 // cleared by handle_special_suspend_equivalent_condition()
768 // or java_suspend_self()
769 jt->set_suspend_equivalent();
770 if (SyncFlags & 1) {
771 Self->_ParkEvent->park ((jlong)1000) ;
772 } else {
773 Self->_ParkEvent->park () ;
774 }
775
776 // were we externally suspended while we were waiting?
777 for (;;) {
778 if (!ExitSuspendEquivalent (jt)) break ;
779 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
780 jt->java_suspend_self();
781 jt->set_suspend_equivalent();
782 }
783 }
784
785 // Try again, but just so we distinguish between futile wakeups and
786 // successful wakeups. The following test isn't algorithmically
787 // necessary, but it helps us maintain sensible statistics.
788 if (TryLock(Self) > 0) break ;
789
790 // The lock is still contested.
791 // Keep a tally of the # of futile wakeups.
792 // Note that the counter is not protected by a lock or updated by atomics.
793 // That is by design - we trade "lossy" counters which are exposed to
794 // races during updates for a lower probe effect.
795 TEVENT (Wait Reentry - futile wakeup) ;
796 ++ nWakeups ;
797
798 // Assuming this is not a spurious wakeup we'll normally
799 // find that _succ == Self.
800 if (_succ == Self) _succ = NULL ;
801
802 // Invariant: after clearing _succ a contending thread
803 // *must* retry _owner before parking.
804 OrderAccess::fence() ;
805
806 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
807 ObjectMonitor::_sync_FutileWakeups->inc() ;
808 }
809 }
810
811 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
812 // Normally we'll find Self on the EntryList.
813 // Unlinking from the EntryList is constant-time and atomic-free.
814 // From the perspective of the lock owner (this thread), the
815 // EntryList is stable and cxq is prepend-only.
816 // The head of cxq is volatile but the interior is stable.
817 // In addition, Self.TState is stable.
818
819 assert (_owner == Self, "invariant") ;
820 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
821 UnlinkAfterAcquire (Self, SelfNode) ;
822 if (_succ == Self) _succ = NULL ;
823 assert (_succ != Self, "invariant") ;
824 SelfNode->TState = ObjectWaiter::TS_RUN ;
825 OrderAccess::fence() ; // see comments at the end of EnterI()
826 }
827
828 // after the thread acquires the lock in ::enter(). Equally, we could defer
829 // unlinking the thread until ::exit()-time.
830
831 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
832 {
833 assert (_owner == Self, "invariant") ;
834 assert (SelfNode->_thread == Self, "invariant") ;
835
836 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
837 // Normal case: remove Self from the DLL EntryList .
838 // This is a constant-time operation.
839 ObjectWaiter * nxt = SelfNode->_next ;
840 ObjectWaiter * prv = SelfNode->_prev ;
841 if (nxt != NULL) nxt->_prev = prv ;
842 if (prv != NULL) prv->_next = nxt ;
843 if (SelfNode == _EntryList ) _EntryList = nxt ;
844 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
845 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
846 TEVENT (Unlink from EntryList) ;
847 } else {
848 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
849 // Inopportune interleaving -- Self is still on the cxq.
850 // This usually means the enqueue of self raced an exiting thread.
851 // Normally we'll find Self near the front of the cxq, so
852 // dequeueing is typically fast. If needbe we can accelerate
853 // this with some MCS/CHL-like bidirectional list hints and advisory
854 // back-links so dequeueing from the interior will normally operate
855 // in constant-time.
856 // Dequeue Self from either the head (with CAS) or from the interior
857 // with a linear-time scan and normal non-atomic memory operations.
858 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
859 // and then unlink Self from EntryList. We have to drain eventually,
860 // so it might as well be now.
861
862 ObjectWaiter * v = _cxq ;
863 assert (v != NULL, "invariant") ;
864 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
865 // The CAS above can fail from interference IFF a "RAT" arrived.
866 // In that case Self must be in the interior and can no longer be
867 // at the head of cxq.
868 if (v == SelfNode) {
869 assert (_cxq != v, "invariant") ;
870 v = _cxq ; // CAS above failed - start scan at head of list
871 }
872 ObjectWaiter * p ;
873 ObjectWaiter * q = NULL ;
874 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
875 q = p ;
876 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
877 }
878 assert (v != SelfNode, "invariant") ;
879 assert (p == SelfNode, "Node not found on cxq") ;
880 assert (p != _cxq, "invariant") ;
881 assert (q != NULL, "invariant") ;
882 assert (q->_next == p, "invariant") ;
883 q->_next = p->_next ;
884 }
885 TEVENT (Unlink from cxq) ;
886 }
887
888 // Diagnostic hygiene ...
889 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
890 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
891 SelfNode->TState = ObjectWaiter::TS_RUN ;
892 }
893
894 // -----------------------------------------------------------------------------
895 // Exit support
896 //
897 // exit()
898 // ~~~~~~
899 // Note that the collector can't reclaim the objectMonitor or deflate
900 // the object out from underneath the thread calling ::exit() as the
901 // thread calling ::exit() never transitions to a stable state.
902 // This inhibits GC, which in turn inhibits asynchronous (and
903 // inopportune) reclamation of "this".
904 //
905 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
906 // There's one exception to the claim above, however. EnterI() can call
907 // exit() to drop a lock if the acquirer has been externally suspended.
908 // In that case exit() is called with _thread_state as _thread_blocked,
909 // but the monitor's _count field is > 0, which inhibits reclamation.
910 //
911 // 1-0 exit
912 // ~~~~~~~~
913 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
914 // the fast-path operators have been optimized so the common ::exit()
915 // operation is 1-0. See i486.ad fast_unlock(), for instance.
916 // The code emitted by fast_unlock() elides the usual MEMBAR. This
917 // greatly improves latency -- MEMBAR and CAS having considerable local
918 // latency on modern processors -- but at the cost of "stranding". Absent the
919 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
920 // ::enter() path, resulting in the entering thread being stranding
921 // and a progress-liveness failure. Stranding is extremely rare.
922 // We use timers (timed park operations) & periodic polling to detect
923 // and recover from stranding. Potentially stranded threads periodically
924 // wake up and poll the lock. See the usage of the _Responsible variable.
925 //
926 // The CAS() in enter provides for safety and exclusion, while the CAS or
927 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
928 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
929 // We detect and recover from stranding with timers.
930 //
931 // If a thread transiently strands it'll park until (a) another
932 // thread acquires the lock and then drops the lock, at which time the
933 // exiting thread will notice and unpark the stranded thread, or, (b)
934 // the timer expires. If the lock is high traffic then the stranding latency
935 // will be low due to (a). If the lock is low traffic then the odds of
936 // stranding are lower, although the worst-case stranding latency
937 // is longer. Critically, we don't want to put excessive load in the
938 // platform's timer subsystem. We want to minimize both the timer injection
939 // rate (timers created/sec) as well as the number of timers active at
940 // any one time. (more precisely, we want to minimize timer-seconds, which is
941 // the integral of the # of active timers at any instant over time).
942 // Both impinge on OS scalability. Given that, at most one thread parked on
943 // a monitor will use a timer.
944
945 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
946 Thread * Self = THREAD ;
947 if (THREAD != _owner) {
948 if (THREAD->is_lock_owned((address) _owner)) {
949 // Transmute _owner from a BasicLock pointer to a Thread address.
950 // We don't need to hold _mutex for this transition.
951 // Non-null to Non-null is safe as long as all readers can
952 // tolerate either flavor.
953 assert (_recursions == 0, "invariant") ;
954 _owner = THREAD ;
955 _recursions = 0 ;
956 OwnerIsThread = 1 ;
957 } else {
958 // NOTE: we need to handle unbalanced monitor enter/exit
959 // in native code by throwing an exception.
960 // TODO: Throw an IllegalMonitorStateException ?
961 TEVENT (Exit - Throw IMSX) ;
962 assert(false, "Non-balanced monitor enter/exit!");
963 if (false) {
964 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
965 }
966 return;
967 }
968 }
969
970 if (_recursions != 0) {
971 _recursions--; // this is simple recursive enter
972 TEVENT (Inflated exit - recursive) ;
973 return ;
974 }
975
976 // Invariant: after setting Responsible=null an thread must execute
977 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
978 if ((SyncFlags & 4) == 0) {
979 _Responsible = NULL ;
980 }
981
982 #if INCLUDE_TRACE
983 // get the owner's thread id for the MonitorEnter event
984 // if it is enabled and the thread isn't suspended
985 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
986 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
987 }
988 #endif
989
990 for (;;) {
991 assert (THREAD == _owner, "invariant") ;
992
993
994 if (Knob_ExitPolicy == 0) {
995 // release semantics: prior loads and stores from within the critical section
996 // must not float (reorder) past the following store that drops the lock.
997 // On SPARC that requires MEMBAR #loadstore|#storestore.
998 // But of course in TSO #loadstore|#storestore is not required.
999 // I'd like to write one of the following:
1000 // A. OrderAccess::release() ; _owner = NULL
1001 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
1002 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
1003 // store into a _dummy variable. That store is not needed, but can result
1004 // in massive wasteful coherency traffic on classic SMP systems.
1005 // Instead, I use release_store(), which is implemented as just a simple
1006 // ST on x64, x86 and SPARC.
1007 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1008 OrderAccess::storeload() ; // See if we need to wake a successor
1009 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1010 TEVENT (Inflated exit - simple egress) ;
1011 return ;
1012 }
1013 TEVENT (Inflated exit - complex egress) ;
1014
1015 // Normally the exiting thread is responsible for ensuring succession,
1016 // but if other successors are ready or other entering threads are spinning
1017 // then this thread can simply store NULL into _owner and exit without
1018 // waking a successor. The existence of spinners or ready successors
1019 // guarantees proper succession (liveness). Responsibility passes to the
1020 // ready or running successors. The exiting thread delegates the duty.
1021 // More precisely, if a successor already exists this thread is absolved
1022 // of the responsibility of waking (unparking) one.
1023 //
1024 // The _succ variable is critical to reducing futile wakeup frequency.
1025 // _succ identifies the "heir presumptive" thread that has been made
1026 // ready (unparked) but that has not yet run. We need only one such
1027 // successor thread to guarantee progress.
1028 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1029 // section 3.3 "Futile Wakeup Throttling" for details.
1030 //
1031 // Note that spinners in Enter() also set _succ non-null.
1032 // In the current implementation spinners opportunistically set
1033 // _succ so that exiting threads might avoid waking a successor.
1034 // Another less appealing alternative would be for the exiting thread
1035 // to drop the lock and then spin briefly to see if a spinner managed
1036 // to acquire the lock. If so, the exiting thread could exit
1037 // immediately without waking a successor, otherwise the exiting
1038 // thread would need to dequeue and wake a successor.
1039 // (Note that we'd need to make the post-drop spin short, but no
1040 // shorter than the worst-case round-trip cache-line migration time.
1041 // The dropped lock needs to become visible to the spinner, and then
1042 // the acquisition of the lock by the spinner must become visible to
1043 // the exiting thread).
1044 //
1045
1046 // It appears that an heir-presumptive (successor) must be made ready.
1047 // Only the current lock owner can manipulate the EntryList or
1048 // drain _cxq, so we need to reacquire the lock. If we fail
1049 // to reacquire the lock the responsibility for ensuring succession
1050 // falls to the new owner.
1051 //
1052 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1053 return ;
1054 }
1055 TEVENT (Exit - Reacquired) ;
1056 } else {
1057 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1058 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1059 OrderAccess::storeload() ;
1060 // Ratify the previously observed values.
1061 if (_cxq == NULL || _succ != NULL) {
1062 TEVENT (Inflated exit - simple egress) ;
1063 return ;
1064 }
1065
1066 // inopportune interleaving -- the exiting thread (this thread)
1067 // in the fast-exit path raced an entering thread in the slow-enter
1068 // path.
1069 // We have two choices:
1070 // A. Try to reacquire the lock.
1071 // If the CAS() fails return immediately, otherwise
1072 // we either restart/rerun the exit operation, or simply
1073 // fall-through into the code below which wakes a successor.
1074 // B. If the elements forming the EntryList|cxq are TSM
1075 // we could simply unpark() the lead thread and return
1076 // without having set _succ.
1077 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1078 TEVENT (Inflated exit - reacquired succeeded) ;
1079 return ;
1080 }
1081 TEVENT (Inflated exit - reacquired failed) ;
1082 } else {
1083 TEVENT (Inflated exit - complex egress) ;
1084 }
1085 }
1086
1087 guarantee (_owner == THREAD, "invariant") ;
1088
1089 ObjectWaiter * w = NULL ;
1090 int QMode = Knob_QMode ;
1091
1092 if (QMode == 2 && _cxq != NULL) {
1093 // QMode == 2 : cxq has precedence over EntryList.
1094 // Try to directly wake a successor from the cxq.
1095 // If successful, the successor will need to unlink itself from cxq.
1096 w = _cxq ;
1097 assert (w != NULL, "invariant") ;
1098 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1099 ExitEpilog (Self, w) ;
1100 return ;
1101 }
1102
1103 if (QMode == 3 && _cxq != NULL) {
1104 // Aggressively drain cxq into EntryList at the first opportunity.
1105 // This policy ensure that recently-run threads live at the head of EntryList.
1106 // Drain _cxq into EntryList - bulk transfer.
1107 // First, detach _cxq.
1108 // The following loop is tantamount to: w = swap (&cxq, NULL)
1109 w = _cxq ;
1110 for (;;) {
1111 assert (w != NULL, "Invariant") ;
1112 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1113 if (u == w) break ;
1114 w = u ;
1115 }
1116 assert (w != NULL , "invariant") ;
1117
1118 ObjectWaiter * q = NULL ;
1119 ObjectWaiter * p ;
1120 for (p = w ; p != NULL ; p = p->_next) {
1121 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1122 p->TState = ObjectWaiter::TS_ENTER ;
1123 p->_prev = q ;
1124 q = p ;
1125 }
1126
1127 // Append the RATs to the EntryList
1128 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1129 ObjectWaiter * Tail ;
1130 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1131 if (Tail == NULL) {
1132 _EntryList = w ;
1133 } else {
1134 Tail->_next = w ;
1135 w->_prev = Tail ;
1136 }
1137
1138 // Fall thru into code that tries to wake a successor from EntryList
1139 }
1140
1141 if (QMode == 4 && _cxq != NULL) {
1142 // Aggressively drain cxq into EntryList at the first opportunity.
1143 // This policy ensure that recently-run threads live at the head of EntryList.
1144
1145 // Drain _cxq into EntryList - bulk transfer.
1146 // First, detach _cxq.
1147 // The following loop is tantamount to: w = swap (&cxq, NULL)
1148 w = _cxq ;
1149 for (;;) {
1150 assert (w != NULL, "Invariant") ;
1151 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1152 if (u == w) break ;
1153 w = u ;
1154 }
1155 assert (w != NULL , "invariant") ;
1156
1157 ObjectWaiter * q = NULL ;
1158 ObjectWaiter * p ;
1159 for (p = w ; p != NULL ; p = p->_next) {
1160 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1161 p->TState = ObjectWaiter::TS_ENTER ;
1162 p->_prev = q ;
1163 q = p ;
1164 }
1165
1166 // Prepend the RATs to the EntryList
1167 if (_EntryList != NULL) {
1168 q->_next = _EntryList ;
1169 _EntryList->_prev = q ;
1170 }
1171 _EntryList = w ;
1172
1173 // Fall thru into code that tries to wake a successor from EntryList
1174 }
1175
1176 w = _EntryList ;
1177 if (w != NULL) {
1178 // I'd like to write: guarantee (w->_thread != Self).
1179 // But in practice an exiting thread may find itself on the EntryList.
1180 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1181 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1182 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1183 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1184 // release the lock "O". T2 resumes immediately after the ST of null into
1185 // _owner, above. T2 notices that the EntryList is populated, so it
1186 // reacquires the lock and then finds itself on the EntryList.
1187 // Given all that, we have to tolerate the circumstance where "w" is
1188 // associated with Self.
1189 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1190 ExitEpilog (Self, w) ;
1191 return ;
1192 }
1193
1194 // If we find that both _cxq and EntryList are null then just
1195 // re-run the exit protocol from the top.
1196 w = _cxq ;
1197 if (w == NULL) continue ;
1198
1199 // Drain _cxq into EntryList - bulk transfer.
1200 // First, detach _cxq.
1201 // The following loop is tantamount to: w = swap (&cxq, NULL)
1202 for (;;) {
1203 assert (w != NULL, "Invariant") ;
1204 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1205 if (u == w) break ;
1206 w = u ;
1207 }
1208 TEVENT (Inflated exit - drain cxq into EntryList) ;
1209
1210 assert (w != NULL , "invariant") ;
1211 assert (_EntryList == NULL , "invariant") ;
1212
1213 // Convert the LIFO SLL anchored by _cxq into a DLL.
1214 // The list reorganization step operates in O(LENGTH(w)) time.
1215 // It's critical that this step operate quickly as
1216 // "Self" still holds the outer-lock, restricting parallelism
1217 // and effectively lengthening the critical section.
1218 // Invariant: s chases t chases u.
1219 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1220 // we have faster access to the tail.
1221
1222 if (QMode == 1) {
1223 // QMode == 1 : drain cxq to EntryList, reversing order
1224 // We also reverse the order of the list.
1225 ObjectWaiter * s = NULL ;
1226 ObjectWaiter * t = w ;
1227 ObjectWaiter * u = NULL ;
1228 while (t != NULL) {
1229 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1230 t->TState = ObjectWaiter::TS_ENTER ;
1231 u = t->_next ;
1232 t->_prev = u ;
1233 t->_next = s ;
1234 s = t;
1235 t = u ;
1236 }
1237 _EntryList = s ;
1238 assert (s != NULL, "invariant") ;
1239 } else {
1240 // QMode == 0 or QMode == 2
1241 _EntryList = w ;
1242 ObjectWaiter * q = NULL ;
1243 ObjectWaiter * p ;
1244 for (p = w ; p != NULL ; p = p->_next) {
1245 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1246 p->TState = ObjectWaiter::TS_ENTER ;
1247 p->_prev = q ;
1248 q = p ;
1249 }
1250 }
1251
1252 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1253 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1254
1255 // See if we can abdicate to a spinner instead of waking a thread.
1256 // A primary goal of the implementation is to reduce the
1257 // context-switch rate.
1258 if (_succ != NULL) continue;
1259
1260 w = _EntryList ;
1261 if (w != NULL) {
1262 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1263 ExitEpilog (Self, w) ;
1264 return ;
1265 }
1266 }
1267 }
1268
1269 // ExitSuspendEquivalent:
1270 // A faster alternate to handle_special_suspend_equivalent_condition()
1271 //
1272 // handle_special_suspend_equivalent_condition() unconditionally
1273 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1274 // operations have high latency. Note that in ::enter() we call HSSEC
1275 // while holding the monitor, so we effectively lengthen the critical sections.
1276 //
1277 // There are a number of possible solutions:
1278 //
1279 // A. To ameliorate the problem we might also defer state transitions
1280 // to as late as possible -- just prior to parking.
1281 // Given that, we'd call HSSEC after having returned from park(),
1282 // but before attempting to acquire the monitor. This is only a
1283 // partial solution. It avoids calling HSSEC while holding the
1284 // monitor (good), but it still increases successor reacquisition latency --
1285 // the interval between unparking a successor and the time the successor
1286 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1287 // If we use this technique we can also avoid EnterI()-exit() loop
1288 // in ::enter() where we iteratively drop the lock and then attempt
1289 // to reacquire it after suspending.
1290 //
1291 // B. In the future we might fold all the suspend bits into a
1292 // composite per-thread suspend flag and then update it with CAS().
1293 // Alternately, a Dekker-like mechanism with multiple variables
1294 // would suffice:
1295 // ST Self->_suspend_equivalent = false
1296 // MEMBAR
1297 // LD Self_>_suspend_flags
1298 //
1299
1300
1301 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1302 int Mode = Knob_FastHSSEC ;
1303 if (Mode && !jSelf->is_external_suspend()) {
1304 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1305 jSelf->clear_suspend_equivalent() ;
1306 if (2 == Mode) OrderAccess::storeload() ;
1307 if (!jSelf->is_external_suspend()) return false ;
1308 // We raced a suspension -- fall thru into the slow path
1309 TEVENT (ExitSuspendEquivalent - raced) ;
1310 jSelf->set_suspend_equivalent() ;
1311 }
1312 return jSelf->handle_special_suspend_equivalent_condition() ;
1313 }
1314
1315
1316 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1317 assert (_owner == Self, "invariant") ;
1318
1319 // Exit protocol:
1320 // 1. ST _succ = wakee
1321 // 2. membar #loadstore|#storestore;
1322 // 2. ST _owner = NULL
1323 // 3. unpark(wakee)
1324
1325 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1326 ParkEvent * Trigger = Wakee->_event ;
1327
1328 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1329 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1330 // out-of-scope (non-extant).
1331 Wakee = NULL ;
1332
1333 // Drop the lock
1334 OrderAccess::release_store_ptr (&_owner, NULL) ;
1335 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1336
1337 if (SafepointSynchronize::do_call_back()) {
1338 TEVENT (unpark before SAFEPOINT) ;
1339 }
1340
1341 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1342 Trigger->unpark() ;
1343
1344 // Maintain stats and report events to JVMTI
1345 if (ObjectMonitor::_sync_Parks != NULL) {
1346 ObjectMonitor::_sync_Parks->inc() ;
1347 }
1348 }
1349
1350
1351 // -----------------------------------------------------------------------------
1352 // Class Loader deadlock handling.
1353 //
1354 // complete_exit exits a lock returning recursion count
1355 // complete_exit/reenter operate as a wait without waiting
1356 // complete_exit requires an inflated monitor
1357 // The _owner field is not always the Thread addr even with an
1358 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1359 // thread due to contention.
1360 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1361 Thread * const Self = THREAD;
1362 assert(Self->is_Java_thread(), "Must be Java thread!");
1363 JavaThread *jt = (JavaThread *)THREAD;
1364
1365 DeferredInitialize();
1366
1367 if (THREAD != _owner) {
1368 if (THREAD->is_lock_owned ((address)_owner)) {
1369 assert(_recursions == 0, "internal state error");
1370 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1371 _recursions = 0 ;
1372 OwnerIsThread = 1 ;
1373 }
1374 }
1375
1376 guarantee(Self == _owner, "complete_exit not owner");
1377 intptr_t save = _recursions; // record the old recursion count
1378 _recursions = 0; // set the recursion level to be 0
1379 exit (true, Self) ; // exit the monitor
1380 guarantee (_owner != Self, "invariant");
1381 return save;
1382 }
1383
1384 // reenter() enters a lock and sets recursion count
1385 // complete_exit/reenter operate as a wait without waiting
1386 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1387 Thread * const Self = THREAD;
1388 assert(Self->is_Java_thread(), "Must be Java thread!");
1389 JavaThread *jt = (JavaThread *)THREAD;
1390
1391 guarantee(_owner != Self, "reenter already owner");
1392 enter (THREAD); // enter the monitor
1393 guarantee (_recursions == 0, "reenter recursion");
1394 _recursions = recursions;
1395 return;
1396 }
1397
1398
1399 // -----------------------------------------------------------------------------
1400 // A macro is used below because there may already be a pending
1401 // exception which should not abort the execution of the routines
1402 // which use this (which is why we don't put this into check_slow and
1403 // call it with a CHECK argument).
1404
1405 #define CHECK_OWNER() \
1406 do { \
1407 if (THREAD != _owner) { \
1408 if (THREAD->is_lock_owned((address) _owner)) { \
1409 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1410 _recursions = 0; \
1411 OwnerIsThread = 1 ; \
1412 } else { \
1413 TEVENT (Throw IMSX) ; \
1414 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1415 } \
1416 } \
1417 } while (false)
1418
1419 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1420 // TODO-FIXME: remove check_slow() -- it's likely dead.
1421
1422 void ObjectMonitor::check_slow(TRAPS) {
1423 TEVENT (check_slow - throw IMSX) ;
1424 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1425 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1426 }
1427
1428 static int Adjust (volatile int * adr, int dx) {
1429 int v ;
1430 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1431 return v ;
1432 }
1433
1434 // helper method for posting a monitor wait event
1435 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1436 jlong notifier_tid,
1437 jlong timeout,
1438 bool timedout) {
1439 event->set_klass(((oop)this->object())->klass());
1440 event->set_timeout((TYPE_ULONG)timeout);
1441 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1442 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1443 event->set_timedOut((TYPE_BOOLEAN)timedout);
1444 event->commit();
1445 }
1446
1447 // -----------------------------------------------------------------------------
1448 // Wait/Notify/NotifyAll
1449 //
1450 // Note: a subset of changes to ObjectMonitor::wait()
1451 // will need to be replicated in complete_exit above
1452 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1453 Thread * const Self = THREAD ;
1454 assert(Self->is_Java_thread(), "Must be Java thread!");
1455 JavaThread *jt = (JavaThread *)THREAD;
1456
1457 DeferredInitialize () ;
1458
1459 // Throw IMSX or IEX.
1460 CHECK_OWNER();
1461
1462 EventJavaMonitorWait event;
1463
1464 // check for a pending interrupt
1465 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1466 // post monitor waited event. Note that this is past-tense, we are done waiting.
1467 if (JvmtiExport::should_post_monitor_waited()) {
1468 // Note: 'false' parameter is passed here because the
1469 // wait was not timed out due to thread interrupt.
1470 JvmtiExport::post_monitor_waited(jt, this, false);
1471
1472 // In this short circuit of the monitor wait protocol, the
1473 // current thread never drops ownership of the monitor and
1474 // never gets added to the wait queue so the current thread
1475 // cannot be made the successor. This means that the
1476 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1477 // consume an unpark() meant for the ParkEvent associated with
1478 // this ObjectMonitor.
1479 }
1480 if (event.should_commit()) {
1481 post_monitor_wait_event(&event, 0, millis, false);
1482 }
1483 TEVENT (Wait - Throw IEX) ;
1484 THROW(vmSymbols::java_lang_InterruptedException());
1485 return ;
1486 }
1487
1488 TEVENT (Wait) ;
1489
1490 assert (Self->_Stalled == 0, "invariant") ;
1491 Self->_Stalled = intptr_t(this) ;
1492 jt->set_current_waiting_monitor(this);
1493
1494 // create a node to be put into the queue
1495 // Critically, after we reset() the event but prior to park(), we must check
1496 // for a pending interrupt.
1497 ObjectWaiter node(Self);
1498 node.TState = ObjectWaiter::TS_WAIT ;
1499 Self->_ParkEvent->reset() ;
1500 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1501
1502 // Enter the waiting queue, which is a circular doubly linked list in this case
1503 // but it could be a priority queue or any data structure.
1504 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1505 // by the the owner of the monitor *except* in the case where park()
1506 // returns because of a timeout of interrupt. Contention is exceptionally rare
1507 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1508
1509 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1510 AddWaiter (&node) ;
1511 Thread::SpinRelease (&_WaitSetLock) ;
1512
1513 if ((SyncFlags & 4) == 0) {
1514 _Responsible = NULL ;
1515 }
1516 intptr_t save = _recursions; // record the old recursion count
1517 _waiters++; // increment the number of waiters
1518 _recursions = 0; // set the recursion level to be 1
1519 exit (true, Self) ; // exit the monitor
1520 guarantee (_owner != Self, "invariant") ;
1521
1522 // The thread is on the WaitSet list - now park() it.
1523 // On MP systems it's conceivable that a brief spin before we park
1524 // could be profitable.
1525 //
1526 // TODO-FIXME: change the following logic to a loop of the form
1527 // while (!timeout && !interrupted && _notified == 0) park()
1528
1529 int ret = OS_OK ;
1530 int WasNotified = 0 ;
1531 { // State transition wrappers
1532 OSThread* osthread = Self->osthread();
1533 OSThreadWaitState osts(osthread, true);
1534 {
1535 ThreadBlockInVM tbivm(jt);
1536 // Thread is in thread_blocked state and oop access is unsafe.
1537 jt->set_suspend_equivalent();
1538
1539 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1540 // Intentionally empty
1541 } else
1542 if (node._notified == 0) {
1543 if (millis <= 0) {
1544 Self->_ParkEvent->park () ;
1545 } else {
1546 ret = Self->_ParkEvent->park (millis) ;
1547 }
1548 }
1549
1550 // were we externally suspended while we were waiting?
1551 if (ExitSuspendEquivalent (jt)) {
1552 // TODO-FIXME: add -- if succ == Self then succ = null.
1553 jt->java_suspend_self();
1554 }
1555
1556 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1557
1558
1559 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1560 // from the WaitSet to the EntryList.
1561 // See if we need to remove Node from the WaitSet.
1562 // We use double-checked locking to avoid grabbing _WaitSetLock
1563 // if the thread is not on the wait queue.
1564 //
1565 // Note that we don't need a fence before the fetch of TState.
1566 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1567 // written by the is thread. (perhaps the fetch might even be satisfied
1568 // by a look-aside into the processor's own store buffer, although given
1569 // the length of the code path between the prior ST and this load that's
1570 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1571 // then we'll acquire the lock and then re-fetch a fresh TState value.
1572 // That is, we fail toward safety.
1573
1574 if (node.TState == ObjectWaiter::TS_WAIT) {
1575 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1576 if (node.TState == ObjectWaiter::TS_WAIT) {
1577 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1578 assert(node._notified == 0, "invariant");
1579 node.TState = ObjectWaiter::TS_RUN ;
1580 }
1581 Thread::SpinRelease (&_WaitSetLock) ;
1582 }
1583
1584 // The thread is now either on off-list (TS_RUN),
1585 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1586 // The Node's TState variable is stable from the perspective of this thread.
1587 // No other threads will asynchronously modify TState.
1588 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1589 OrderAccess::loadload() ;
1590 if (_succ == Self) _succ = NULL ;
1591 WasNotified = node._notified ;
1592
1593 // Reentry phase -- reacquire the monitor.
1594 // re-enter contended monitor after object.wait().
1595 // retain OBJECT_WAIT state until re-enter successfully completes
1596 // Thread state is thread_in_vm and oop access is again safe,
1597 // although the raw address of the object may have changed.
1598 // (Don't cache naked oops over safepoints, of course).
1599
1600 // post monitor waited event. Note that this is past-tense, we are done waiting.
1601 if (JvmtiExport::should_post_monitor_waited()) {
1602 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1603
1604 if (node._notified != 0 && _succ == Self) {
1605 // In this part of the monitor wait-notify-reenter protocol it
1606 // is possible (and normal) for another thread to do a fastpath
1607 // monitor enter-exit while this thread is still trying to get
1608 // to the reenter portion of the protocol.
1609 //
1610 // The ObjectMonitor was notified and the current thread is
1611 // the successor which also means that an unpark() has already
1612 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1613 // consume the unpark() that was done when the successor was
1614 // set because the same ParkEvent is shared between Java
1615 // monitors and JVM/TI RawMonitors (for now).
1616 //
1617 // We redo the unpark() to ensure forward progress, i.e., we
1618 // don't want all pending threads hanging (parked) with none
1619 // entering the unlocked monitor.
1620 node._event->unpark();
1621 }
1622 }
1623
1624 if (event.should_commit()) {
1625 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1626 }
1627
1628 OrderAccess::fence() ;
1629
1630 assert (Self->_Stalled != 0, "invariant") ;
1631 Self->_Stalled = 0 ;
1632
1633 assert (_owner != Self, "invariant") ;
1634 ObjectWaiter::TStates v = node.TState ;
1635 if (v == ObjectWaiter::TS_RUN) {
1636 enter (Self) ;
1637 } else {
1638 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1639 ReenterI (Self, &node) ;
1640 node.wait_reenter_end(this);
1641 }
1642
1643 // Self has reacquired the lock.
1644 // Lifecycle - the node representing Self must not appear on any queues.
1645 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1646 // want residual elements associated with this thread left on any lists.
1647 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1648 assert (_owner == Self, "invariant") ;
1649 assert (_succ != Self , "invariant") ;
1650 } // OSThreadWaitState()
1651
1652 jt->set_current_waiting_monitor(NULL);
1653
1654 guarantee (_recursions == 0, "invariant") ;
1655 _recursions = save; // restore the old recursion count
1656 _waiters--; // decrement the number of waiters
1657
1658 // Verify a few postconditions
1659 assert (_owner == Self , "invariant") ;
1660 assert (_succ != Self , "invariant") ;
1661 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1662
1663 if (SyncFlags & 32) {
1664 OrderAccess::fence() ;
1665 }
1666
1667 // check if the notification happened
1668 if (!WasNotified) {
1669 // no, it could be timeout or Thread.interrupt() or both
1670 // check for interrupt event, otherwise it is timeout
1671 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1672 TEVENT (Wait - throw IEX from epilog) ;
1673 THROW(vmSymbols::java_lang_InterruptedException());
1674 }
1675 }
1676
1677 // NOTE: Spurious wake up will be consider as timeout.
1678 // Monitor notify has precedence over thread interrupt.
1679 }
1680
1681
1682 // Consider:
1683 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1684 // then instead of transferring a thread from the WaitSet to the EntryList
1685 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1686
1687 void ObjectMonitor::notify(TRAPS) {
1688 CHECK_OWNER();
1689 if (_WaitSet == NULL) {
1690 TEVENT (Empty-Notify) ;
1691 return ;
1692 }
1693 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1694
1695 int Policy = Knob_MoveNotifyee ;
1696
1697 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1698 ObjectWaiter * iterator = DequeueWaiter() ;
1699 if (iterator != NULL) {
1700 TEVENT (Notify1 - Transfer) ;
1701 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1702 guarantee (iterator->_notified == 0, "invariant") ;
1703 if (Policy != 4) {
1704 iterator->TState = ObjectWaiter::TS_ENTER ;
1705 }
1706 iterator->_notified = 1 ;
1707 Thread * Self = THREAD;
1708 iterator->_notifier_tid = Self->osthread()->thread_id();
1709
1710 ObjectWaiter * List = _EntryList ;
1711 if (List != NULL) {
1712 assert (List->_prev == NULL, "invariant") ;
1713 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1714 assert (List != iterator, "invariant") ;
1715 }
1716
1717 if (Policy == 0) { // prepend to EntryList
1718 if (List == NULL) {
1719 iterator->_next = iterator->_prev = NULL ;
1720 _EntryList = iterator ;
1721 } else {
1722 List->_prev = iterator ;
1723 iterator->_next = List ;
1724 iterator->_prev = NULL ;
1725 _EntryList = iterator ;
1726 }
1727 } else
1728 if (Policy == 1) { // append to EntryList
1729 if (List == NULL) {
1730 iterator->_next = iterator->_prev = NULL ;
1731 _EntryList = iterator ;
1732 } else {
1733 // CONSIDER: finding the tail currently requires a linear-time walk of
1734 // the EntryList. We can make tail access constant-time by converting to
1735 // a CDLL instead of using our current DLL.
1736 ObjectWaiter * Tail ;
1737 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1738 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1739 Tail->_next = iterator ;
1740 iterator->_prev = Tail ;
1741 iterator->_next = NULL ;
1742 }
1743 } else
1744 if (Policy == 2) { // prepend to cxq
1745 // prepend to cxq
1746 if (List == NULL) {
1747 iterator->_next = iterator->_prev = NULL ;
1748 _EntryList = iterator ;
1749 } else {
1750 iterator->TState = ObjectWaiter::TS_CXQ ;
1751 for (;;) {
1752 ObjectWaiter * Front = _cxq ;
1753 iterator->_next = Front ;
1754 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1755 break ;
1756 }
1757 }
1758 }
1759 } else
1760 if (Policy == 3) { // append to cxq
1761 iterator->TState = ObjectWaiter::TS_CXQ ;
1762 for (;;) {
1763 ObjectWaiter * Tail ;
1764 Tail = _cxq ;
1765 if (Tail == NULL) {
1766 iterator->_next = NULL ;
1767 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1768 break ;
1769 }
1770 } else {
1771 while (Tail->_next != NULL) Tail = Tail->_next ;
1772 Tail->_next = iterator ;
1773 iterator->_prev = Tail ;
1774 iterator->_next = NULL ;
1775 break ;
1776 }
1777 }
1778 } else {
1779 ParkEvent * ev = iterator->_event ;
1780 iterator->TState = ObjectWaiter::TS_RUN ;
1781 OrderAccess::fence() ;
1782 ev->unpark() ;
1783 }
1784
1785 if (Policy < 4) {
1786 iterator->wait_reenter_begin(this);
1787 }
1788
1789 // _WaitSetLock protects the wait queue, not the EntryList. We could
1790 // move the add-to-EntryList operation, above, outside the critical section
1791 // protected by _WaitSetLock. In practice that's not useful. With the
1792 // exception of wait() timeouts and interrupts the monitor owner
1793 // is the only thread that grabs _WaitSetLock. There's almost no contention
1794 // on _WaitSetLock so it's not profitable to reduce the length of the
1795 // critical section.
1796 }
1797
1798 Thread::SpinRelease (&_WaitSetLock) ;
1799
1800 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1801 ObjectMonitor::_sync_Notifications->inc() ;
1802 }
1803 }
1804
1805
1806 void ObjectMonitor::notifyAll(TRAPS) {
1807 CHECK_OWNER();
1808 ObjectWaiter* iterator;
1809 if (_WaitSet == NULL) {
1810 TEVENT (Empty-NotifyAll) ;
1811 return ;
1812 }
1813 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1814
1815 int Policy = Knob_MoveNotifyee ;
1816 int Tally = 0 ;
1817 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1818
1819 for (;;) {
1820 iterator = DequeueWaiter () ;
1821 if (iterator == NULL) break ;
1822 TEVENT (NotifyAll - Transfer1) ;
1823 ++Tally ;
1824
1825 // Disposition - what might we do with iterator ?
1826 // a. add it directly to the EntryList - either tail or head.
1827 // b. push it onto the front of the _cxq.
1828 // For now we use (a).
1829
1830 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1831 guarantee (iterator->_notified == 0, "invariant") ;
1832 iterator->_notified = 1 ;
1833 Thread * Self = THREAD;
1834 iterator->_notifier_tid = Self->osthread()->thread_id();
1835 if (Policy != 4) {
1836 iterator->TState = ObjectWaiter::TS_ENTER ;
1837 }
1838
1839 ObjectWaiter * List = _EntryList ;
1840 if (List != NULL) {
1841 assert (List->_prev == NULL, "invariant") ;
1842 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1843 assert (List != iterator, "invariant") ;
1844 }
1845
1846 if (Policy == 0) { // prepend to EntryList
1847 if (List == NULL) {
1848 iterator->_next = iterator->_prev = NULL ;
1849 _EntryList = iterator ;
1850 } else {
1851 List->_prev = iterator ;
1852 iterator->_next = List ;
1853 iterator->_prev = NULL ;
1854 _EntryList = iterator ;
1855 }
1856 } else
1857 if (Policy == 1) { // append to EntryList
1858 if (List == NULL) {
1859 iterator->_next = iterator->_prev = NULL ;
1860 _EntryList = iterator ;
1861 } else {
1862 // CONSIDER: finding the tail currently requires a linear-time walk of
1863 // the EntryList. We can make tail access constant-time by converting to
1864 // a CDLL instead of using our current DLL.
1865 ObjectWaiter * Tail ;
1866 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1867 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1868 Tail->_next = iterator ;
1869 iterator->_prev = Tail ;
1870 iterator->_next = NULL ;
1871 }
1872 } else
1873 if (Policy == 2) { // prepend to cxq
1874 // prepend to cxq
1875 iterator->TState = ObjectWaiter::TS_CXQ ;
1876 for (;;) {
1877 ObjectWaiter * Front = _cxq ;
1878 iterator->_next = Front ;
1879 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1880 break ;
1881 }
1882 }
1883 } else
1884 if (Policy == 3) { // append to cxq
1885 iterator->TState = ObjectWaiter::TS_CXQ ;
1886 for (;;) {
1887 ObjectWaiter * Tail ;
1888 Tail = _cxq ;
1889 if (Tail == NULL) {
1890 iterator->_next = NULL ;
1891 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1892 break ;
1893 }
1894 } else {
1895 while (Tail->_next != NULL) Tail = Tail->_next ;
1896 Tail->_next = iterator ;
1897 iterator->_prev = Tail ;
1898 iterator->_next = NULL ;
1899 break ;
1900 }
1901 }
1902 } else {
1903 ParkEvent * ev = iterator->_event ;
1904 iterator->TState = ObjectWaiter::TS_RUN ;
1905 OrderAccess::fence() ;
1906 ev->unpark() ;
1907 }
1908
1909 if (Policy < 4) {
1910 iterator->wait_reenter_begin(this);
1911 }
1912
1913 // _WaitSetLock protects the wait queue, not the EntryList. We could
1914 // move the add-to-EntryList operation, above, outside the critical section
1915 // protected by _WaitSetLock. In practice that's not useful. With the
1916 // exception of wait() timeouts and interrupts the monitor owner
1917 // is the only thread that grabs _WaitSetLock. There's almost no contention
1918 // on _WaitSetLock so it's not profitable to reduce the length of the
1919 // critical section.
1920 }
1921
1922 Thread::SpinRelease (&_WaitSetLock) ;
1923
1924 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1925 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1926 }
1927 }
1928
1929 // -----------------------------------------------------------------------------
1930 // Adaptive Spinning Support
1931 //
1932 // Adaptive spin-then-block - rational spinning
1933 //
1934 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1935 // algorithm. On high order SMP systems it would be better to start with
1936 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1937 // a contending thread could enqueue itself on the cxq and then spin locally
1938 // on a thread-specific variable such as its ParkEvent._Event flag.
1939 // That's left as an exercise for the reader. Note that global spinning is
1940 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1941 // low latency and massive bandwidth.
1942 //
1943 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1944 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1945 // (duration) or we can fix the count at approximately the duration of
1946 // a context switch and vary the frequency. Of course we could also
1947 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1948 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1949 //
1950 // This implementation varies the duration "D", where D varies with
1951 // the success rate of recent spin attempts. (D is capped at approximately
1952 // length of a round-trip context switch). The success rate for recent
1953 // spin attempts is a good predictor of the success rate of future spin
1954 // attempts. The mechanism adapts automatically to varying critical
1955 // section length (lock modality), system load and degree of parallelism.
1956 // D is maintained per-monitor in _SpinDuration and is initialized
1957 // optimistically. Spin frequency is fixed at 100%.
1958 //
1959 // Note that _SpinDuration is volatile, but we update it without locks
1960 // or atomics. The code is designed so that _SpinDuration stays within
1961 // a reasonable range even in the presence of races. The arithmetic
1962 // operations on _SpinDuration are closed over the domain of legal values,
1963 // so at worst a race will install and older but still legal value.
1964 // At the very worst this introduces some apparent non-determinism.
1965 // We might spin when we shouldn't or vice-versa, but since the spin
1966 // count are relatively short, even in the worst case, the effect is harmless.
1967 //
1968 // Care must be taken that a low "D" value does not become an
1969 // an absorbing state. Transient spinning failures -- when spinning
1970 // is overall profitable -- should not cause the system to converge
1971 // on low "D" values. We want spinning to be stable and predictable
1972 // and fairly responsive to change and at the same time we don't want
1973 // it to oscillate, become metastable, be "too" non-deterministic,
1974 // or converge on or enter undesirable stable absorbing states.
1975 //
1976 // We implement a feedback-based control system -- using past behavior
1977 // to predict future behavior. We face two issues: (a) if the
1978 // input signal is random then the spin predictor won't provide optimal
1979 // results, and (b) if the signal frequency is too high then the control
1980 // system, which has some natural response lag, will "chase" the signal.
1981 // (b) can arise from multimodal lock hold times. Transient preemption
1982 // can also result in apparent bimodal lock hold times.
1983 // Although sub-optimal, neither condition is particularly harmful, as
1984 // in the worst-case we'll spin when we shouldn't or vice-versa.
1985 // The maximum spin duration is rather short so the failure modes aren't bad.
1986 // To be conservative, I've tuned the gain in system to bias toward
1987 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1988 // "rings" or oscillates between spinning and not spinning. This happens
1989 // when spinning is just on the cusp of profitability, however, so the
1990 // situation is not dire. The state is benign -- there's no need to add
1991 // hysteresis control to damp the transition rate between spinning and
1992 // not spinning.
1993 //
1994
1995 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1996 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1997
1998 // Spinning: Fixed frequency (100%), vary duration
1999
2000
2001 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
2002
2003 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2004 int ctr = Knob_FixedSpin ;
2005 if (ctr != 0) {
2006 while (--ctr >= 0) {
2007 if (TryLock (Self) > 0) return 1 ;
2008 SpinPause () ;
2009 }
2010 return 0 ;
2011 }
2012
2013 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
2014 if (TryLock(Self) > 0) {
2015 // Increase _SpinDuration ...
2016 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2017 // Raising _SpurDuration to the poverty line is key.
2018 int x = _SpinDuration ;
2019 if (x < Knob_SpinLimit) {
2020 if (x < Knob_Poverty) x = Knob_Poverty ;
2021 _SpinDuration = x + Knob_BonusB ;
2022 }
2023 return 1 ;
2024 }
2025 SpinPause () ;
2026 }
2027
2028 // Admission control - verify preconditions for spinning
2029 //
2030 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2031 // becoming an absorbing state. Put another way, we spin briefly to
2032 // sample, just in case the system load, parallelism, contention, or lock
2033 // modality changed.
2034 //
2035 // Consider the following alternative:
2036 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2037 // spin attempt. "Periodically" might mean after a tally of
2038 // the # of failed spin attempts (or iterations) reaches some threshold.
2039 // This takes us into the realm of 1-out-of-N spinning, where we
2040 // hold the duration constant but vary the frequency.
2041
2042 ctr = _SpinDuration ;
2043 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2044 if (ctr <= 0) return 0 ;
2045
2046 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2047 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2048 TEVENT (Spin abort - notrunnable [TOP]);
2049 return 0 ;
2050 }
2051
2052 int MaxSpin = Knob_MaxSpinners ;
2053 if (MaxSpin >= 0) {
2054 if (_Spinner > MaxSpin) {
2055 TEVENT (Spin abort -- too many spinners) ;
2056 return 0 ;
2057 }
2058 // Slighty racy, but benign ...
2059 Adjust (&_Spinner, 1) ;
2060 }
2061
2062 // We're good to spin ... spin ingress.
2063 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2064 // when preparing to LD...CAS _owner, etc and the CAS is likely
2065 // to succeed.
2066 int hits = 0 ;
2067 int msk = 0 ;
2068 int caspty = Knob_CASPenalty ;
2069 int oxpty = Knob_OXPenalty ;
2070 int sss = Knob_SpinSetSucc ;
2071 if (sss && _succ == NULL ) _succ = Self ;
2072 Thread * prv = NULL ;
2073
2074 // There are three ways to exit the following loop:
2075 // 1. A successful spin where this thread has acquired the lock.
2076 // 2. Spin failure with prejudice
2077 // 3. Spin failure without prejudice
2078
2079 while (--ctr >= 0) {
2080
2081 // Periodic polling -- Check for pending GC
2082 // Threads may spin while they're unsafe.
2083 // We don't want spinning threads to delay the JVM from reaching
2084 // a stop-the-world safepoint or to steal cycles from GC.
2085 // If we detect a pending safepoint we abort in order that
2086 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2087 // this thread, if safe, doesn't steal cycles from GC.
2088 // This is in keeping with the "no loitering in runtime" rule.
2089 // We periodically check to see if there's a safepoint pending.
2090 if ((ctr & 0xFF) == 0) {
2091 if (SafepointSynchronize::do_call_back()) {
2092 TEVENT (Spin: safepoint) ;
2093 goto Abort ; // abrupt spin egress
2094 }
2095 if (Knob_UsePause & 1) SpinPause () ;
2096
2097 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2098 if (hits > 50 && scb != NULL) {
2099 int abend = (*scb)(SpinCallbackArgument, 0) ;
2100 }
2101 }
2102
2103 if (Knob_UsePause & 2) SpinPause() ;
2104
2105 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2106 // This is useful on classic SMP systems, but is of less utility on
2107 // N1-style CMT platforms.
2108 //
2109 // Trade-off: lock acquisition latency vs coherency bandwidth.
2110 // Lock hold times are typically short. A histogram
2111 // of successful spin attempts shows that we usually acquire
2112 // the lock early in the spin. That suggests we want to
2113 // sample _owner frequently in the early phase of the spin,
2114 // but then back-off and sample less frequently as the spin
2115 // progresses. The back-off makes a good citizen on SMP big
2116 // SMP systems. Oversampling _owner can consume excessive
2117 // coherency bandwidth. Relatedly, if we _oversample _owner we
2118 // can inadvertently interfere with the the ST m->owner=null.
2119 // executed by the lock owner.
2120 if (ctr & msk) continue ;
2121 ++hits ;
2122 if ((hits & 0xF) == 0) {
2123 // The 0xF, above, corresponds to the exponent.
2124 // Consider: (msk+1)|msk
2125 msk = ((msk << 2)|3) & BackOffMask ;
2126 }
2127
2128 // Probe _owner with TATAS
2129 // If this thread observes the monitor transition or flicker
2130 // from locked to unlocked to locked, then the odds that this
2131 // thread will acquire the lock in this spin attempt go down
2132 // considerably. The same argument applies if the CAS fails
2133 // or if we observe _owner change from one non-null value to
2134 // another non-null value. In such cases we might abort
2135 // the spin without prejudice or apply a "penalty" to the
2136 // spin count-down variable "ctr", reducing it by 100, say.
2137
2138 Thread * ox = (Thread *) _owner ;
2139 if (ox == NULL) {
2140 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2141 if (ox == NULL) {
2142 // The CAS succeeded -- this thread acquired ownership
2143 // Take care of some bookkeeping to exit spin state.
2144 if (sss && _succ == Self) {
2145 _succ = NULL ;
2146 }
2147 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2148
2149 // Increase _SpinDuration :
2150 // The spin was successful (profitable) so we tend toward
2151 // longer spin attempts in the future.
2152 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2153 // If we acquired the lock early in the spin cycle it
2154 // makes sense to increase _SpinDuration proportionally.
2155 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2156 int x = _SpinDuration ;
2157 if (x < Knob_SpinLimit) {
2158 if (x < Knob_Poverty) x = Knob_Poverty ;
2159 _SpinDuration = x + Knob_Bonus ;
2160 }
2161 return 1 ;
2162 }
2163
2164 // The CAS failed ... we can take any of the following actions:
2165 // * penalize: ctr -= Knob_CASPenalty
2166 // * exit spin with prejudice -- goto Abort;
2167 // * exit spin without prejudice.
2168 // * Since CAS is high-latency, retry again immediately.
2169 prv = ox ;
2170 TEVENT (Spin: cas failed) ;
2171 if (caspty == -2) break ;
2172 if (caspty == -1) goto Abort ;
2173 ctr -= caspty ;
2174 continue ;
2175 }
2176
2177 // Did lock ownership change hands ?
2178 if (ox != prv && prv != NULL ) {
2179 TEVENT (spin: Owner changed)
2180 if (oxpty == -2) break ;
2181 if (oxpty == -1) goto Abort ;
2182 ctr -= oxpty ;
2183 }
2184 prv = ox ;
2185
2186 // Abort the spin if the owner is not executing.
2187 // The owner must be executing in order to drop the lock.
2188 // Spinning while the owner is OFFPROC is idiocy.
2189 // Consider: ctr -= RunnablePenalty ;
2190 if (Knob_OState && NotRunnable (Self, ox)) {
2191 TEVENT (Spin abort - notrunnable);
2192 goto Abort ;
2193 }
2194 if (sss && _succ == NULL ) _succ = Self ;
2195 }
2196
2197 // Spin failed with prejudice -- reduce _SpinDuration.
2198 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2199 // AIMD is globally stable.
2200 TEVENT (Spin failure) ;
2201 {
2202 int x = _SpinDuration ;
2203 if (x > 0) {
2204 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2205 // This is globally sample and tends to damp the response.
2206 x -= Knob_Penalty ;
2207 if (x < 0) x = 0 ;
2208 _SpinDuration = x ;
2209 }
2210 }
2211
2212 Abort:
2213 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2214 if (sss && _succ == Self) {
2215 _succ = NULL ;
2216 // Invariant: after setting succ=null a contending thread
2217 // must recheck-retry _owner before parking. This usually happens
2218 // in the normal usage of TrySpin(), but it's safest
2219 // to make TrySpin() as foolproof as possible.
2220 OrderAccess::fence() ;
2221 if (TryLock(Self) > 0) return 1 ;
2222 }
2223 return 0 ;
2224 }
2225
2226 // NotRunnable() -- informed spinning
2227 //
2228 // Don't bother spinning if the owner is not eligible to drop the lock.
2229 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2230 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2231 // The thread must be runnable in order to drop the lock in timely fashion.
2232 // If the _owner is not runnable then spinning will not likely be
2233 // successful (profitable).
2234 //
2235 // Beware -- the thread referenced by _owner could have died
2236 // so a simply fetch from _owner->_thread_state might trap.
2237 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2238 // Because of the lifecycle issues the schedctl and _thread_state values
2239 // observed by NotRunnable() might be garbage. NotRunnable must
2240 // tolerate this and consider the observed _thread_state value
2241 // as advisory.
2242 //
2243 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2244 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2245 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2246 // with the LSB of _owner. Another option would be to probablistically probe
2247 // the putative _owner->TypeTag value.
2248 //
2249 // Checking _thread_state isn't perfect. Even if the thread is
2250 // in_java it might be blocked on a page-fault or have been preempted
2251 // and sitting on a ready/dispatch queue. _thread state in conjunction
2252 // with schedctl.sc_state gives us a good picture of what the
2253 // thread is doing, however.
2254 //
2255 // TODO: check schedctl.sc_state.
2256 // We'll need to use SafeFetch32() to read from the schedctl block.
2257 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2258 //
2259 // The return value from NotRunnable() is *advisory* -- the
2260 // result is based on sampling and is not necessarily coherent.
2261 // The caller must tolerate false-negative and false-positive errors.
2262 // Spinning, in general, is probabilistic anyway.
2263
2264
2265 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2266 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2267 if (!OwnerIsThread) return 0 ;
2268
2269 if (ox == NULL) return 0 ;
2270
2271 // Avoid transitive spinning ...
2272 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2273 // Immediately after T1 acquires L it's possible that T2, also
2274 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2275 // This occurs transiently after T1 acquired L but before
2276 // T1 managed to clear T1.Stalled. T2 does not need to abort
2277 // its spin in this circumstance.
2278 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2279
2280 if (BlockedOn == 1) return 1 ;
2281 if (BlockedOn != 0) {
2282 return BlockedOn != intptr_t(this) && _owner == ox ;
2283 }
2284
2285 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2286 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2287 // consider also: jst != _thread_in_Java -- but that's overspecific.
2288 return jst == _thread_blocked || jst == _thread_in_native ;
2289 }
2290
2291
2292 // -----------------------------------------------------------------------------
2293 // WaitSet management ...
2294
2295 ObjectWaiter::ObjectWaiter(Thread* thread) {
2296 _next = NULL;
2297 _prev = NULL;
2298 _notified = 0;
2299 TState = TS_RUN ;
2300 _thread = thread;
2301 _event = thread->_ParkEvent ;
2302 _active = false;
2303 assert (_event != NULL, "invariant") ;
2304 }
2305
2306 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2307 JavaThread *jt = (JavaThread *)this->_thread;
2308 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2309 }
2310
2311 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2312 JavaThread *jt = (JavaThread *)this->_thread;
2313 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2314 }
2315
2316 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2317 assert(node != NULL, "should not dequeue NULL node");
2318 assert(node->_prev == NULL, "node already in list");
2319 assert(node->_next == NULL, "node already in list");
2320 // put node at end of queue (circular doubly linked list)
2321 if (_WaitSet == NULL) {
2322 _WaitSet = node;
2323 node->_prev = node;
2324 node->_next = node;
2325 } else {
2326 ObjectWaiter* head = _WaitSet ;
2327 ObjectWaiter* tail = head->_prev;
2328 assert(tail->_next == head, "invariant check");
2329 tail->_next = node;
2330 head->_prev = node;
2331 node->_next = head;
2332 node->_prev = tail;
2333 }
2334 }
2335
2336 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2337 // dequeue the very first waiter
2338 ObjectWaiter* waiter = _WaitSet;
2339 if (waiter) {
2340 DequeueSpecificWaiter(waiter);
2341 }
2342 return waiter;
2343 }
2344
2345 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2346 assert(node != NULL, "should not dequeue NULL node");
2347 assert(node->_prev != NULL, "node already removed from list");
2348 assert(node->_next != NULL, "node already removed from list");
2349 // when the waiter has woken up because of interrupt,
2350 // timeout or other spurious wake-up, dequeue the
2351 // waiter from waiting list
2352 ObjectWaiter* next = node->_next;
2353 if (next == node) {
2354 assert(node->_prev == node, "invariant check");
2355 _WaitSet = NULL;
2356 } else {
2357 ObjectWaiter* prev = node->_prev;
2358 assert(prev->_next == node, "invariant check");
2359 assert(next->_prev == node, "invariant check");
2360 next->_prev = prev;
2361 prev->_next = next;
2362 if (_WaitSet == node) {
2363 _WaitSet = next;
2364 }
2365 }
2366 node->_next = NULL;
2367 node->_prev = NULL;
2368 }
2369
2370 // -----------------------------------------------------------------------------
2371 // PerfData support
2372 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2373 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2374 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2375 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2376 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2377 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2378 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2379 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2380 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2381 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2382 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2383 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2384 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2385 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2386 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2387 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2388 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2389 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2390
2391 // One-shot global initialization for the sync subsystem.
2392 // We could also defer initialization and initialize on-demand
2393 // the first time we call inflate(). Initialization would
2394 // be protected - like so many things - by the MonitorCache_lock.
2395
2396 void ObjectMonitor::Initialize () {
2397 static int InitializationCompleted = 0 ;
2398 assert (InitializationCompleted == 0, "invariant") ;
2399 InitializationCompleted = 1 ;
2400 if (UsePerfData) {
2401 EXCEPTION_MARK ;
2402 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2403 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2404 NEWPERFCOUNTER(_sync_Inflations) ;
2405 NEWPERFCOUNTER(_sync_Deflations) ;
2406 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2407 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2408 NEWPERFCOUNTER(_sync_Parks) ;
2409 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2410 NEWPERFCOUNTER(_sync_Notifications) ;
2411 NEWPERFCOUNTER(_sync_SlowEnter) ;
2412 NEWPERFCOUNTER(_sync_SlowExit) ;
2413 NEWPERFCOUNTER(_sync_SlowNotify) ;
2414 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2415 NEWPERFCOUNTER(_sync_FailedSpins) ;
2416 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2417 NEWPERFCOUNTER(_sync_PrivateA) ;
2418 NEWPERFCOUNTER(_sync_PrivateB) ;
2419 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2420 NEWPERFCOUNTER(_sync_MonScavenged) ;
2421 NEWPERFVARIABLE(_sync_MonExtant) ;
2422 #undef NEWPERFCOUNTER
2423 }
2424 }
2425
2426
2427 // Compile-time asserts
2428 // When possible, it's better to catch errors deterministically at
2429 // compile-time than at runtime. The down-side to using compile-time
2430 // asserts is that error message -- often something about negative array
2431 // indices -- is opaque.
2432
2433 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2434
2435 void ObjectMonitor::ctAsserts() {
2436 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2437 }
2438
2439
2440 static char * kvGet (char * kvList, const char * Key) {
2441 if (kvList == NULL) return NULL ;
2442 size_t n = strlen (Key) ;
2443 char * Search ;
2444 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2445 if (strncmp (Search, Key, n) == 0) {
2446 if (Search[n] == '=') return Search + n + 1 ;
2447 if (Search[n] == 0) return (char *) "1" ;
2448 }
2449 }
2450 return NULL ;
2451 }
2452
2453 static int kvGetInt (char * kvList, const char * Key, int Default) {
2454 char * v = kvGet (kvList, Key) ;
2455 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2456 if (Knob_ReportSettings && v != NULL) {
2457 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2458 ::fflush (stdout) ;
2459 }
2460 return rslt ;
2461 }
2462
2463 void ObjectMonitor::DeferredInitialize () {
2464 if (InitDone > 0) return ;
2465 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2466 while (InitDone != 1) ;
2467 return ;
2468 }
2469
2470 // One-shot global initialization ...
2471 // The initialization is idempotent, so we don't need locks.
2472 // In the future consider doing this via os::init_2().
2473 // SyncKnobs consist of <Key>=<Value> pairs in the style
2474 // of environment variables. Start by converting ':' to NUL.
2475
2476 if (SyncKnobs == NULL) SyncKnobs = "" ;
2477
2478 size_t sz = strlen (SyncKnobs) ;
2479 char * knobs = (char *) malloc (sz + 2) ;
2480 if (knobs == NULL) {
2481 vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
2482 guarantee (0, "invariant") ;
2483 }
2484 strcpy (knobs, SyncKnobs) ;
2485 knobs[sz+1] = 0 ;
2486 for (char * p = knobs ; *p ; p++) {
2487 if (*p == ':') *p = 0 ;
2488 }
2489
2490 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2491 SETKNOB(ReportSettings) ;
2492 SETKNOB(Verbose) ;
2493 SETKNOB(FixedSpin) ;
2494 SETKNOB(SpinLimit) ;
2495 SETKNOB(SpinBase) ;
2496 SETKNOB(SpinBackOff);
2497 SETKNOB(CASPenalty) ;
2498 SETKNOB(OXPenalty) ;
2499 SETKNOB(LogSpins) ;
2500 SETKNOB(SpinSetSucc) ;
2501 SETKNOB(SuccEnabled) ;
2502 SETKNOB(SuccRestrict) ;
2503 SETKNOB(Penalty) ;
2504 SETKNOB(Bonus) ;
2505 SETKNOB(BonusB) ;
2506 SETKNOB(Poverty) ;
2507 SETKNOB(SpinAfterFutile) ;
2508 SETKNOB(UsePause) ;
2509 SETKNOB(SpinEarly) ;
2510 SETKNOB(OState) ;
2511 SETKNOB(MaxSpinners) ;
2512 SETKNOB(PreSpin) ;
2513 SETKNOB(ExitPolicy) ;
2514 SETKNOB(QMode);
2515 SETKNOB(ResetEvent) ;
2516 SETKNOB(MoveNotifyee) ;
2517 SETKNOB(FastHSSEC) ;
2518 #undef SETKNOB
2519
2520 if (os::is_MP()) {
2521 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2522 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2523 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2524 } else {
2525 Knob_SpinLimit = 0 ;
2526 Knob_SpinBase = 0 ;
2527 Knob_PreSpin = 0 ;
2528 Knob_FixedSpin = -1 ;
2529 }
2530
2531 if (Knob_LogSpins == 0) {
2532 ObjectMonitor::_sync_FailedSpins = NULL ;
2533 }
2534
2535 free (knobs) ;
2536 OrderAccess::fence() ;
2537 InitDone = 1 ;
2538 }
2539
2540 #ifndef PRODUCT
2541 void ObjectMonitor::verify() {
2542 }
2543
2544 void ObjectMonitor::print() {
2545 }
2546 #endif