Mon, 18 Jun 2012 12:29:21 -0700
7176856: add the JRE name to the error log
Reviewed-by: coleenp, jrose, kvn, twisti
Contributed-by: Krystal Mok <sajia@taobao.com>
1 /*
2 * Copyright (c) 1998, 2011, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/osThread.hpp"
36 #include "runtime/stubRoutines.hpp"
37 #include "runtime/thread.hpp"
38 #include "services/threadService.hpp"
39 #include "utilities/dtrace.hpp"
40 #include "utilities/preserveException.hpp"
41 #ifdef TARGET_OS_FAMILY_linux
42 # include "os_linux.inline.hpp"
43 # include "thread_linux.inline.hpp"
44 #endif
45 #ifdef TARGET_OS_FAMILY_solaris
46 # include "os_solaris.inline.hpp"
47 # include "thread_solaris.inline.hpp"
48 #endif
49 #ifdef TARGET_OS_FAMILY_windows
50 # include "os_windows.inline.hpp"
51 # include "thread_windows.inline.hpp"
52 #endif
53 #ifdef TARGET_OS_FAMILY_bsd
54 # include "os_bsd.inline.hpp"
55 # include "thread_bsd.inline.hpp"
56 #endif
58 #if defined(__GNUC__) && !defined(IA64)
59 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
60 #define ATTR __attribute__((noinline))
61 #else
62 #define ATTR
63 #endif
66 #ifdef DTRACE_ENABLED
68 // Only bother with this argument setup if dtrace is available
69 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
72 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \
73 char* bytes = NULL; \
74 int len = 0; \
75 jlong jtid = SharedRuntime::get_java_tid(thread); \
76 Symbol* klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
77 if (klassname != NULL) { \
78 bytes = (char*)klassname->bytes(); \
79 len = klassname->utf8_length(); \
80 }
82 #ifndef USDT2
84 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
85 jlong, uintptr_t, char*, int);
86 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
87 jlong, uintptr_t, char*, int);
88 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
89 jlong, uintptr_t, char*, int);
90 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
91 jlong, uintptr_t, char*, int);
92 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
93 jlong, uintptr_t, char*, int);
95 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
96 { \
97 if (DTraceMonitorProbes) { \
98 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
99 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
100 (monitor), bytes, len, (millis)); \
101 } \
102 }
104 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
105 { \
106 if (DTraceMonitorProbes) { \
107 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
108 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
109 (uintptr_t)(monitor), bytes, len); \
110 } \
111 }
113 #else /* USDT2 */
115 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
116 { \
117 if (DTraceMonitorProbes) { \
118 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
119 HOTSPOT_MONITOR_WAIT(jtid, \
120 (monitor), bytes, len, (millis)); \
121 } \
122 }
124 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
125 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
126 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
127 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
128 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
130 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
131 { \
132 if (DTraceMonitorProbes) { \
133 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
134 HOTSPOT_MONITOR_##probe(jtid, \
135 (uintptr_t)(monitor), bytes, len); \
136 } \
137 }
139 #endif /* USDT2 */
140 #else // ndef DTRACE_ENABLED
142 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
143 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;}
145 #endif // ndef DTRACE_ENABLED
147 // Tunables ...
148 // The knob* variables are effectively final. Once set they should
149 // never be modified hence. Consider using __read_mostly with GCC.
151 int ObjectMonitor::Knob_Verbose = 0 ;
152 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
153 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
154 static int Knob_HandOff = 0 ;
155 static int Knob_ReportSettings = 0 ;
157 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
158 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
159 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
160 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
161 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
162 static int Knob_SpinEarly = 1 ;
163 static int Knob_SuccEnabled = 1 ; // futile wake throttling
164 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
165 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
166 static int Knob_Bonus = 100 ; // spin success bonus
167 static int Knob_BonusB = 100 ; // spin success bonus
168 static int Knob_Penalty = 200 ; // spin failure penalty
169 static int Knob_Poverty = 1000 ;
170 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
171 static int Knob_FixedSpin = 0 ;
172 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
173 static int Knob_UsePause = 1 ;
174 static int Knob_ExitPolicy = 0 ;
175 static int Knob_PreSpin = 10 ; // 20-100 likely better
176 static int Knob_ResetEvent = 0 ;
177 static int BackOffMask = 0 ;
179 static int Knob_FastHSSEC = 0 ;
180 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
181 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
182 static volatile int InitDone = 0 ;
184 #define TrySpin TrySpin_VaryDuration
186 // -----------------------------------------------------------------------------
187 // Theory of operations -- Monitors lists, thread residency, etc:
188 //
189 // * A thread acquires ownership of a monitor by successfully
190 // CAS()ing the _owner field from null to non-null.
191 //
192 // * Invariant: A thread appears on at most one monitor list --
193 // cxq, EntryList or WaitSet -- at any one time.
194 //
195 // * Contending threads "push" themselves onto the cxq with CAS
196 // and then spin/park.
197 //
198 // * After a contending thread eventually acquires the lock it must
199 // dequeue itself from either the EntryList or the cxq.
200 //
201 // * The exiting thread identifies and unparks an "heir presumptive"
202 // tentative successor thread on the EntryList. Critically, the
203 // exiting thread doesn't unlink the successor thread from the EntryList.
204 // After having been unparked, the wakee will recontend for ownership of
205 // the monitor. The successor (wakee) will either acquire the lock or
206 // re-park itself.
207 //
208 // Succession is provided for by a policy of competitive handoff.
209 // The exiting thread does _not_ grant or pass ownership to the
210 // successor thread. (This is also referred to as "handoff" succession").
211 // Instead the exiting thread releases ownership and possibly wakes
212 // a successor, so the successor can (re)compete for ownership of the lock.
213 // If the EntryList is empty but the cxq is populated the exiting
214 // thread will drain the cxq into the EntryList. It does so by
215 // by detaching the cxq (installing null with CAS) and folding
216 // the threads from the cxq into the EntryList. The EntryList is
217 // doubly linked, while the cxq is singly linked because of the
218 // CAS-based "push" used to enqueue recently arrived threads (RATs).
219 //
220 // * Concurrency invariants:
221 //
222 // -- only the monitor owner may access or mutate the EntryList.
223 // The mutex property of the monitor itself protects the EntryList
224 // from concurrent interference.
225 // -- Only the monitor owner may detach the cxq.
226 //
227 // * The monitor entry list operations avoid locks, but strictly speaking
228 // they're not lock-free. Enter is lock-free, exit is not.
229 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
230 //
231 // * The cxq can have multiple concurrent "pushers" but only one concurrent
232 // detaching thread. This mechanism is immune from the ABA corruption.
233 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
234 //
235 // * Taken together, the cxq and the EntryList constitute or form a
236 // single logical queue of threads stalled trying to acquire the lock.
237 // We use two distinct lists to improve the odds of a constant-time
238 // dequeue operation after acquisition (in the ::enter() epilog) and
239 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
240 // A key desideratum is to minimize queue & monitor metadata manipulation
241 // that occurs while holding the monitor lock -- that is, we want to
242 // minimize monitor lock holds times. Note that even a small amount of
243 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
244 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
245 // locks and monitor metadata.
246 //
247 // Cxq points to the the set of Recently Arrived Threads attempting entry.
248 // Because we push threads onto _cxq with CAS, the RATs must take the form of
249 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
250 // the unlocking thread notices that EntryList is null but _cxq is != null.
251 //
252 // The EntryList is ordered by the prevailing queue discipline and
253 // can be organized in any convenient fashion, such as a doubly-linked list or
254 // a circular doubly-linked list. Critically, we want insert and delete operations
255 // to operate in constant-time. If we need a priority queue then something akin
256 // to Solaris' sleepq would work nicely. Viz.,
257 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
258 // Queue discipline is enforced at ::exit() time, when the unlocking thread
259 // drains the cxq into the EntryList, and orders or reorders the threads on the
260 // EntryList accordingly.
261 //
262 // Barring "lock barging", this mechanism provides fair cyclic ordering,
263 // somewhat similar to an elevator-scan.
264 //
265 // * The monitor synchronization subsystem avoids the use of native
266 // synchronization primitives except for the narrow platform-specific
267 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
268 // the semantics of park-unpark. Put another way, this monitor implementation
269 // depends only on atomic operations and park-unpark. The monitor subsystem
270 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
271 // underlying OS manages the READY<->RUN transitions.
272 //
273 // * Waiting threads reside on the WaitSet list -- wait() puts
274 // the caller onto the WaitSet.
275 //
276 // * notify() or notifyAll() simply transfers threads from the WaitSet to
277 // either the EntryList or cxq. Subsequent exit() operations will
278 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
279 // it's likely the notifyee would simply impale itself on the lock held
280 // by the notifier.
281 //
282 // * An interesting alternative is to encode cxq as (List,LockByte) where
283 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
284 // variable, like _recursions, in the scheme. The threads or Events that form
285 // the list would have to be aligned in 256-byte addresses. A thread would
286 // try to acquire the lock or enqueue itself with CAS, but exiting threads
287 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
288 // Note that is is *not* word-tearing, but it does presume that full-word
289 // CAS operations are coherent with intermix with STB operations. That's true
290 // on most common processors.
291 //
292 // * See also http://blogs.sun.com/dave
295 // -----------------------------------------------------------------------------
296 // Enter support
298 bool ObjectMonitor::try_enter(Thread* THREAD) {
299 if (THREAD != _owner) {
300 if (THREAD->is_lock_owned ((address)_owner)) {
301 assert(_recursions == 0, "internal state error");
302 _owner = THREAD ;
303 _recursions = 1 ;
304 OwnerIsThread = 1 ;
305 return true;
306 }
307 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
308 return false;
309 }
310 return true;
311 } else {
312 _recursions++;
313 return true;
314 }
315 }
317 void ATTR ObjectMonitor::enter(TRAPS) {
318 // The following code is ordered to check the most common cases first
319 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
320 Thread * const Self = THREAD ;
321 void * cur ;
323 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
324 if (cur == NULL) {
325 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
326 assert (_recursions == 0 , "invariant") ;
327 assert (_owner == Self, "invariant") ;
328 // CONSIDER: set or assert OwnerIsThread == 1
329 return ;
330 }
332 if (cur == Self) {
333 // TODO-FIXME: check for integer overflow! BUGID 6557169.
334 _recursions ++ ;
335 return ;
336 }
338 if (Self->is_lock_owned ((address)cur)) {
339 assert (_recursions == 0, "internal state error");
340 _recursions = 1 ;
341 // Commute owner from a thread-specific on-stack BasicLockObject address to
342 // a full-fledged "Thread *".
343 _owner = Self ;
344 OwnerIsThread = 1 ;
345 return ;
346 }
348 // We've encountered genuine contention.
349 assert (Self->_Stalled == 0, "invariant") ;
350 Self->_Stalled = intptr_t(this) ;
352 // Try one round of spinning *before* enqueueing Self
353 // and before going through the awkward and expensive state
354 // transitions. The following spin is strictly optional ...
355 // Note that if we acquire the monitor from an initial spin
356 // we forgo posting JVMTI events and firing DTRACE probes.
357 if (Knob_SpinEarly && TrySpin (Self) > 0) {
358 assert (_owner == Self , "invariant") ;
359 assert (_recursions == 0 , "invariant") ;
360 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
361 Self->_Stalled = 0 ;
362 return ;
363 }
365 assert (_owner != Self , "invariant") ;
366 assert (_succ != Self , "invariant") ;
367 assert (Self->is_Java_thread() , "invariant") ;
368 JavaThread * jt = (JavaThread *) Self ;
369 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
370 assert (jt->thread_state() != _thread_blocked , "invariant") ;
371 assert (this->object() != NULL , "invariant") ;
372 assert (_count >= 0, "invariant") ;
374 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
375 // Ensure the object-monitor relationship remains stable while there's contention.
376 Atomic::inc_ptr(&_count);
378 { // Change java thread status to indicate blocked on monitor enter.
379 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
381 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
382 if (JvmtiExport::should_post_monitor_contended_enter()) {
383 JvmtiExport::post_monitor_contended_enter(jt, this);
384 }
386 OSThreadContendState osts(Self->osthread());
387 ThreadBlockInVM tbivm(jt);
389 Self->set_current_pending_monitor(this);
391 // TODO-FIXME: change the following for(;;) loop to straight-line code.
392 for (;;) {
393 jt->set_suspend_equivalent();
394 // cleared by handle_special_suspend_equivalent_condition()
395 // or java_suspend_self()
397 EnterI (THREAD) ;
399 if (!ExitSuspendEquivalent(jt)) break ;
401 //
402 // We have acquired the contended monitor, but while we were
403 // waiting another thread suspended us. We don't want to enter
404 // the monitor while suspended because that would surprise the
405 // thread that suspended us.
406 //
407 _recursions = 0 ;
408 _succ = NULL ;
409 exit (Self) ;
411 jt->java_suspend_self();
412 }
413 Self->set_current_pending_monitor(NULL);
414 }
416 Atomic::dec_ptr(&_count);
417 assert (_count >= 0, "invariant") ;
418 Self->_Stalled = 0 ;
420 // Must either set _recursions = 0 or ASSERT _recursions == 0.
421 assert (_recursions == 0 , "invariant") ;
422 assert (_owner == Self , "invariant") ;
423 assert (_succ != Self , "invariant") ;
424 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
426 // The thread -- now the owner -- is back in vm mode.
427 // Report the glorious news via TI,DTrace and jvmstat.
428 // The probe effect is non-trivial. All the reportage occurs
429 // while we hold the monitor, increasing the length of the critical
430 // section. Amdahl's parallel speedup law comes vividly into play.
431 //
432 // Another option might be to aggregate the events (thread local or
433 // per-monitor aggregation) and defer reporting until a more opportune
434 // time -- such as next time some thread encounters contention but has
435 // yet to acquire the lock. While spinning that thread could
436 // spinning we could increment JVMStat counters, etc.
438 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
439 if (JvmtiExport::should_post_monitor_contended_entered()) {
440 JvmtiExport::post_monitor_contended_entered(jt, this);
441 }
442 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
443 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
444 }
445 }
448 // Caveat: TryLock() is not necessarily serializing if it returns failure.
449 // Callers must compensate as needed.
451 int ObjectMonitor::TryLock (Thread * Self) {
452 for (;;) {
453 void * own = _owner ;
454 if (own != NULL) return 0 ;
455 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
456 // Either guarantee _recursions == 0 or set _recursions = 0.
457 assert (_recursions == 0, "invariant") ;
458 assert (_owner == Self, "invariant") ;
459 // CONSIDER: set or assert that OwnerIsThread == 1
460 return 1 ;
461 }
462 // The lock had been free momentarily, but we lost the race to the lock.
463 // Interference -- the CAS failed.
464 // We can either return -1 or retry.
465 // Retry doesn't make as much sense because the lock was just acquired.
466 if (true) return -1 ;
467 }
468 }
470 void ATTR ObjectMonitor::EnterI (TRAPS) {
471 Thread * Self = THREAD ;
472 assert (Self->is_Java_thread(), "invariant") ;
473 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
475 // Try the lock - TATAS
476 if (TryLock (Self) > 0) {
477 assert (_succ != Self , "invariant") ;
478 assert (_owner == Self , "invariant") ;
479 assert (_Responsible != Self , "invariant") ;
480 return ;
481 }
483 DeferredInitialize () ;
485 // We try one round of spinning *before* enqueueing Self.
486 //
487 // If the _owner is ready but OFFPROC we could use a YieldTo()
488 // operation to donate the remainder of this thread's quantum
489 // to the owner. This has subtle but beneficial affinity
490 // effects.
492 if (TrySpin (Self) > 0) {
493 assert (_owner == Self , "invariant") ;
494 assert (_succ != Self , "invariant") ;
495 assert (_Responsible != Self , "invariant") ;
496 return ;
497 }
499 // The Spin failed -- Enqueue and park the thread ...
500 assert (_succ != Self , "invariant") ;
501 assert (_owner != Self , "invariant") ;
502 assert (_Responsible != Self , "invariant") ;
504 // Enqueue "Self" on ObjectMonitor's _cxq.
505 //
506 // Node acts as a proxy for Self.
507 // As an aside, if were to ever rewrite the synchronization code mostly
508 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
509 // Java objects. This would avoid awkward lifecycle and liveness issues,
510 // as well as eliminate a subset of ABA issues.
511 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
512 //
514 ObjectWaiter node(Self) ;
515 Self->_ParkEvent->reset() ;
516 node._prev = (ObjectWaiter *) 0xBAD ;
517 node.TState = ObjectWaiter::TS_CXQ ;
519 // Push "Self" onto the front of the _cxq.
520 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
521 // Note that spinning tends to reduce the rate at which threads
522 // enqueue and dequeue on EntryList|cxq.
523 ObjectWaiter * nxt ;
524 for (;;) {
525 node._next = nxt = _cxq ;
526 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
528 // Interference - the CAS failed because _cxq changed. Just retry.
529 // As an optional optimization we retry the lock.
530 if (TryLock (Self) > 0) {
531 assert (_succ != Self , "invariant") ;
532 assert (_owner == Self , "invariant") ;
533 assert (_Responsible != Self , "invariant") ;
534 return ;
535 }
536 }
538 // Check for cxq|EntryList edge transition to non-null. This indicates
539 // the onset of contention. While contention persists exiting threads
540 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
541 // operations revert to the faster 1-0 mode. This enter operation may interleave
542 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
543 // arrange for one of the contending thread to use a timed park() operations
544 // to detect and recover from the race. (Stranding is form of progress failure
545 // where the monitor is unlocked but all the contending threads remain parked).
546 // That is, at least one of the contended threads will periodically poll _owner.
547 // One of the contending threads will become the designated "Responsible" thread.
548 // The Responsible thread uses a timed park instead of a normal indefinite park
549 // operation -- it periodically wakes and checks for and recovers from potential
550 // strandings admitted by 1-0 exit operations. We need at most one Responsible
551 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
552 // be responsible for a monitor.
553 //
554 // Currently, one of the contended threads takes on the added role of "Responsible".
555 // A viable alternative would be to use a dedicated "stranding checker" thread
556 // that periodically iterated over all the threads (or active monitors) and unparked
557 // successors where there was risk of stranding. This would help eliminate the
558 // timer scalability issues we see on some platforms as we'd only have one thread
559 // -- the checker -- parked on a timer.
561 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
562 // Try to assume the role of responsible thread for the monitor.
563 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
564 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
565 }
567 // The lock have been released while this thread was occupied queueing
568 // itself onto _cxq. To close the race and avoid "stranding" and
569 // progress-liveness failure we must resample-retry _owner before parking.
570 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
571 // In this case the ST-MEMBAR is accomplished with CAS().
572 //
573 // TODO: Defer all thread state transitions until park-time.
574 // Since state transitions are heavy and inefficient we'd like
575 // to defer the state transitions until absolutely necessary,
576 // and in doing so avoid some transitions ...
578 TEVENT (Inflated enter - Contention) ;
579 int nWakeups = 0 ;
580 int RecheckInterval = 1 ;
582 for (;;) {
584 if (TryLock (Self) > 0) break ;
585 assert (_owner != Self, "invariant") ;
587 if ((SyncFlags & 2) && _Responsible == NULL) {
588 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
589 }
591 // park self
592 if (_Responsible == Self || (SyncFlags & 1)) {
593 TEVENT (Inflated enter - park TIMED) ;
594 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
595 // Increase the RecheckInterval, but clamp the value.
596 RecheckInterval *= 8 ;
597 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
598 } else {
599 TEVENT (Inflated enter - park UNTIMED) ;
600 Self->_ParkEvent->park() ;
601 }
603 if (TryLock(Self) > 0) break ;
605 // The lock is still contested.
606 // Keep a tally of the # of futile wakeups.
607 // Note that the counter is not protected by a lock or updated by atomics.
608 // That is by design - we trade "lossy" counters which are exposed to
609 // races during updates for a lower probe effect.
610 TEVENT (Inflated enter - Futile wakeup) ;
611 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
612 ObjectMonitor::_sync_FutileWakeups->inc() ;
613 }
614 ++ nWakeups ;
616 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
617 // We can defer clearing _succ until after the spin completes
618 // TrySpin() must tolerate being called with _succ == Self.
619 // Try yet another round of adaptive spinning.
620 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
622 // We can find that we were unpark()ed and redesignated _succ while
623 // we were spinning. That's harmless. If we iterate and call park(),
624 // park() will consume the event and return immediately and we'll
625 // just spin again. This pattern can repeat, leaving _succ to simply
626 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
627 // Alternately, we can sample fired() here, and if set, forgo spinning
628 // in the next iteration.
630 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
631 Self->_ParkEvent->reset() ;
632 OrderAccess::fence() ;
633 }
634 if (_succ == Self) _succ = NULL ;
636 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
637 OrderAccess::fence() ;
638 }
640 // Egress :
641 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
642 // Normally we'll find Self on the EntryList .
643 // From the perspective of the lock owner (this thread), the
644 // EntryList is stable and cxq is prepend-only.
645 // The head of cxq is volatile but the interior is stable.
646 // In addition, Self.TState is stable.
648 assert (_owner == Self , "invariant") ;
649 assert (object() != NULL , "invariant") ;
650 // I'd like to write:
651 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
652 // but as we're at a safepoint that's not safe.
654 UnlinkAfterAcquire (Self, &node) ;
655 if (_succ == Self) _succ = NULL ;
657 assert (_succ != Self, "invariant") ;
658 if (_Responsible == Self) {
659 _Responsible = NULL ;
660 // Dekker pivot-point.
661 // Consider OrderAccess::storeload() here
663 // We may leave threads on cxq|EntryList without a designated
664 // "Responsible" thread. This is benign. When this thread subsequently
665 // exits the monitor it can "see" such preexisting "old" threads --
666 // threads that arrived on the cxq|EntryList before the fence, above --
667 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
668 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
669 // non-null and elect a new "Responsible" timer thread.
670 //
671 // This thread executes:
672 // ST Responsible=null; MEMBAR (in enter epilog - here)
673 // LD cxq|EntryList (in subsequent exit)
674 //
675 // Entering threads in the slow/contended path execute:
676 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
677 // The (ST cxq; MEMBAR) is accomplished with CAS().
678 //
679 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
680 // exit operation from floating above the ST Responsible=null.
681 //
682 // In *practice* however, EnterI() is always followed by some atomic
683 // operation such as the decrement of _count in ::enter(). Those atomics
684 // obviate the need for the explicit MEMBAR, above.
685 }
687 // We've acquired ownership with CAS().
688 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
689 // But since the CAS() this thread may have also stored into _succ,
690 // EntryList, cxq or Responsible. These meta-data updates must be
691 // visible __before this thread subsequently drops the lock.
692 // Consider what could occur if we didn't enforce this constraint --
693 // STs to monitor meta-data and user-data could reorder with (become
694 // visible after) the ST in exit that drops ownership of the lock.
695 // Some other thread could then acquire the lock, but observe inconsistent
696 // or old monitor meta-data and heap data. That violates the JMM.
697 // To that end, the 1-0 exit() operation must have at least STST|LDST
698 // "release" barrier semantics. Specifically, there must be at least a
699 // STST|LDST barrier in exit() before the ST of null into _owner that drops
700 // the lock. The barrier ensures that changes to monitor meta-data and data
701 // protected by the lock will be visible before we release the lock, and
702 // therefore before some other thread (CPU) has a chance to acquire the lock.
703 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
704 //
705 // Critically, any prior STs to _succ or EntryList must be visible before
706 // the ST of null into _owner in the *subsequent* (following) corresponding
707 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
708 // execute a serializing instruction.
710 if (SyncFlags & 8) {
711 OrderAccess::fence() ;
712 }
713 return ;
714 }
716 // ReenterI() is a specialized inline form of the latter half of the
717 // contended slow-path from EnterI(). We use ReenterI() only for
718 // monitor reentry in wait().
719 //
720 // In the future we should reconcile EnterI() and ReenterI(), adding
721 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
722 // loop accordingly.
724 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
725 assert (Self != NULL , "invariant") ;
726 assert (SelfNode != NULL , "invariant") ;
727 assert (SelfNode->_thread == Self , "invariant") ;
728 assert (_waiters > 0 , "invariant") ;
729 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
730 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
731 JavaThread * jt = (JavaThread *) Self ;
733 int nWakeups = 0 ;
734 for (;;) {
735 ObjectWaiter::TStates v = SelfNode->TState ;
736 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
737 assert (_owner != Self, "invariant") ;
739 if (TryLock (Self) > 0) break ;
740 if (TrySpin (Self) > 0) break ;
742 TEVENT (Wait Reentry - parking) ;
744 // State transition wrappers around park() ...
745 // ReenterI() wisely defers state transitions until
746 // it's clear we must park the thread.
747 {
748 OSThreadContendState osts(Self->osthread());
749 ThreadBlockInVM tbivm(jt);
751 // cleared by handle_special_suspend_equivalent_condition()
752 // or java_suspend_self()
753 jt->set_suspend_equivalent();
754 if (SyncFlags & 1) {
755 Self->_ParkEvent->park ((jlong)1000) ;
756 } else {
757 Self->_ParkEvent->park () ;
758 }
760 // were we externally suspended while we were waiting?
761 for (;;) {
762 if (!ExitSuspendEquivalent (jt)) break ;
763 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
764 jt->java_suspend_self();
765 jt->set_suspend_equivalent();
766 }
767 }
769 // Try again, but just so we distinguish between futile wakeups and
770 // successful wakeups. The following test isn't algorithmically
771 // necessary, but it helps us maintain sensible statistics.
772 if (TryLock(Self) > 0) break ;
774 // The lock is still contested.
775 // Keep a tally of the # of futile wakeups.
776 // Note that the counter is not protected by a lock or updated by atomics.
777 // That is by design - we trade "lossy" counters which are exposed to
778 // races during updates for a lower probe effect.
779 TEVENT (Wait Reentry - futile wakeup) ;
780 ++ nWakeups ;
782 // Assuming this is not a spurious wakeup we'll normally
783 // find that _succ == Self.
784 if (_succ == Self) _succ = NULL ;
786 // Invariant: after clearing _succ a contending thread
787 // *must* retry _owner before parking.
788 OrderAccess::fence() ;
790 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
791 ObjectMonitor::_sync_FutileWakeups->inc() ;
792 }
793 }
795 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
796 // Normally we'll find Self on the EntryList.
797 // Unlinking from the EntryList is constant-time and atomic-free.
798 // From the perspective of the lock owner (this thread), the
799 // EntryList is stable and cxq is prepend-only.
800 // The head of cxq is volatile but the interior is stable.
801 // In addition, Self.TState is stable.
803 assert (_owner == Self, "invariant") ;
804 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
805 UnlinkAfterAcquire (Self, SelfNode) ;
806 if (_succ == Self) _succ = NULL ;
807 assert (_succ != Self, "invariant") ;
808 SelfNode->TState = ObjectWaiter::TS_RUN ;
809 OrderAccess::fence() ; // see comments at the end of EnterI()
810 }
812 // after the thread acquires the lock in ::enter(). Equally, we could defer
813 // unlinking the thread until ::exit()-time.
815 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
816 {
817 assert (_owner == Self, "invariant") ;
818 assert (SelfNode->_thread == Self, "invariant") ;
820 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
821 // Normal case: remove Self from the DLL EntryList .
822 // This is a constant-time operation.
823 ObjectWaiter * nxt = SelfNode->_next ;
824 ObjectWaiter * prv = SelfNode->_prev ;
825 if (nxt != NULL) nxt->_prev = prv ;
826 if (prv != NULL) prv->_next = nxt ;
827 if (SelfNode == _EntryList ) _EntryList = nxt ;
828 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
829 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
830 TEVENT (Unlink from EntryList) ;
831 } else {
832 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
833 // Inopportune interleaving -- Self is still on the cxq.
834 // This usually means the enqueue of self raced an exiting thread.
835 // Normally we'll find Self near the front of the cxq, so
836 // dequeueing is typically fast. If needbe we can accelerate
837 // this with some MCS/CHL-like bidirectional list hints and advisory
838 // back-links so dequeueing from the interior will normally operate
839 // in constant-time.
840 // Dequeue Self from either the head (with CAS) or from the interior
841 // with a linear-time scan and normal non-atomic memory operations.
842 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
843 // and then unlink Self from EntryList. We have to drain eventually,
844 // so it might as well be now.
846 ObjectWaiter * v = _cxq ;
847 assert (v != NULL, "invariant") ;
848 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
849 // The CAS above can fail from interference IFF a "RAT" arrived.
850 // In that case Self must be in the interior and can no longer be
851 // at the head of cxq.
852 if (v == SelfNode) {
853 assert (_cxq != v, "invariant") ;
854 v = _cxq ; // CAS above failed - start scan at head of list
855 }
856 ObjectWaiter * p ;
857 ObjectWaiter * q = NULL ;
858 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
859 q = p ;
860 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
861 }
862 assert (v != SelfNode, "invariant") ;
863 assert (p == SelfNode, "Node not found on cxq") ;
864 assert (p != _cxq, "invariant") ;
865 assert (q != NULL, "invariant") ;
866 assert (q->_next == p, "invariant") ;
867 q->_next = p->_next ;
868 }
869 TEVENT (Unlink from cxq) ;
870 }
872 // Diagnostic hygiene ...
873 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
874 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
875 SelfNode->TState = ObjectWaiter::TS_RUN ;
876 }
878 // -----------------------------------------------------------------------------
879 // Exit support
880 //
881 // exit()
882 // ~~~~~~
883 // Note that the collector can't reclaim the objectMonitor or deflate
884 // the object out from underneath the thread calling ::exit() as the
885 // thread calling ::exit() never transitions to a stable state.
886 // This inhibits GC, which in turn inhibits asynchronous (and
887 // inopportune) reclamation of "this".
888 //
889 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
890 // There's one exception to the claim above, however. EnterI() can call
891 // exit() to drop a lock if the acquirer has been externally suspended.
892 // In that case exit() is called with _thread_state as _thread_blocked,
893 // but the monitor's _count field is > 0, which inhibits reclamation.
894 //
895 // 1-0 exit
896 // ~~~~~~~~
897 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
898 // the fast-path operators have been optimized so the common ::exit()
899 // operation is 1-0. See i486.ad fast_unlock(), for instance.
900 // The code emitted by fast_unlock() elides the usual MEMBAR. This
901 // greatly improves latency -- MEMBAR and CAS having considerable local
902 // latency on modern processors -- but at the cost of "stranding". Absent the
903 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
904 // ::enter() path, resulting in the entering thread being stranding
905 // and a progress-liveness failure. Stranding is extremely rare.
906 // We use timers (timed park operations) & periodic polling to detect
907 // and recover from stranding. Potentially stranded threads periodically
908 // wake up and poll the lock. See the usage of the _Responsible variable.
909 //
910 // The CAS() in enter provides for safety and exclusion, while the CAS or
911 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
912 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
913 // We detect and recover from stranding with timers.
914 //
915 // If a thread transiently strands it'll park until (a) another
916 // thread acquires the lock and then drops the lock, at which time the
917 // exiting thread will notice and unpark the stranded thread, or, (b)
918 // the timer expires. If the lock is high traffic then the stranding latency
919 // will be low due to (a). If the lock is low traffic then the odds of
920 // stranding are lower, although the worst-case stranding latency
921 // is longer. Critically, we don't want to put excessive load in the
922 // platform's timer subsystem. We want to minimize both the timer injection
923 // rate (timers created/sec) as well as the number of timers active at
924 // any one time. (more precisely, we want to minimize timer-seconds, which is
925 // the integral of the # of active timers at any instant over time).
926 // Both impinge on OS scalability. Given that, at most one thread parked on
927 // a monitor will use a timer.
929 void ATTR ObjectMonitor::exit(TRAPS) {
930 Thread * Self = THREAD ;
931 if (THREAD != _owner) {
932 if (THREAD->is_lock_owned((address) _owner)) {
933 // Transmute _owner from a BasicLock pointer to a Thread address.
934 // We don't need to hold _mutex for this transition.
935 // Non-null to Non-null is safe as long as all readers can
936 // tolerate either flavor.
937 assert (_recursions == 0, "invariant") ;
938 _owner = THREAD ;
939 _recursions = 0 ;
940 OwnerIsThread = 1 ;
941 } else {
942 // NOTE: we need to handle unbalanced monitor enter/exit
943 // in native code by throwing an exception.
944 // TODO: Throw an IllegalMonitorStateException ?
945 TEVENT (Exit - Throw IMSX) ;
946 assert(false, "Non-balanced monitor enter/exit!");
947 if (false) {
948 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
949 }
950 return;
951 }
952 }
954 if (_recursions != 0) {
955 _recursions--; // this is simple recursive enter
956 TEVENT (Inflated exit - recursive) ;
957 return ;
958 }
960 // Invariant: after setting Responsible=null an thread must execute
961 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
962 if ((SyncFlags & 4) == 0) {
963 _Responsible = NULL ;
964 }
966 for (;;) {
967 assert (THREAD == _owner, "invariant") ;
970 if (Knob_ExitPolicy == 0) {
971 // release semantics: prior loads and stores from within the critical section
972 // must not float (reorder) past the following store that drops the lock.
973 // On SPARC that requires MEMBAR #loadstore|#storestore.
974 // But of course in TSO #loadstore|#storestore is not required.
975 // I'd like to write one of the following:
976 // A. OrderAccess::release() ; _owner = NULL
977 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
978 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
979 // store into a _dummy variable. That store is not needed, but can result
980 // in massive wasteful coherency traffic on classic SMP systems.
981 // Instead, I use release_store(), which is implemented as just a simple
982 // ST on x64, x86 and SPARC.
983 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
984 OrderAccess::storeload() ; // See if we need to wake a successor
985 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
986 TEVENT (Inflated exit - simple egress) ;
987 return ;
988 }
989 TEVENT (Inflated exit - complex egress) ;
991 // Normally the exiting thread is responsible for ensuring succession,
992 // but if other successors are ready or other entering threads are spinning
993 // then this thread can simply store NULL into _owner and exit without
994 // waking a successor. The existence of spinners or ready successors
995 // guarantees proper succession (liveness). Responsibility passes to the
996 // ready or running successors. The exiting thread delegates the duty.
997 // More precisely, if a successor already exists this thread is absolved
998 // of the responsibility of waking (unparking) one.
999 //
1000 // The _succ variable is critical to reducing futile wakeup frequency.
1001 // _succ identifies the "heir presumptive" thread that has been made
1002 // ready (unparked) but that has not yet run. We need only one such
1003 // successor thread to guarantee progress.
1004 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1005 // section 3.3 "Futile Wakeup Throttling" for details.
1006 //
1007 // Note that spinners in Enter() also set _succ non-null.
1008 // In the current implementation spinners opportunistically set
1009 // _succ so that exiting threads might avoid waking a successor.
1010 // Another less appealing alternative would be for the exiting thread
1011 // to drop the lock and then spin briefly to see if a spinner managed
1012 // to acquire the lock. If so, the exiting thread could exit
1013 // immediately without waking a successor, otherwise the exiting
1014 // thread would need to dequeue and wake a successor.
1015 // (Note that we'd need to make the post-drop spin short, but no
1016 // shorter than the worst-case round-trip cache-line migration time.
1017 // The dropped lock needs to become visible to the spinner, and then
1018 // the acquisition of the lock by the spinner must become visible to
1019 // the exiting thread).
1020 //
1022 // It appears that an heir-presumptive (successor) must be made ready.
1023 // Only the current lock owner can manipulate the EntryList or
1024 // drain _cxq, so we need to reacquire the lock. If we fail
1025 // to reacquire the lock the responsibility for ensuring succession
1026 // falls to the new owner.
1027 //
1028 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1029 return ;
1030 }
1031 TEVENT (Exit - Reacquired) ;
1032 } else {
1033 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1034 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1035 OrderAccess::storeload() ;
1036 // Ratify the previously observed values.
1037 if (_cxq == NULL || _succ != NULL) {
1038 TEVENT (Inflated exit - simple egress) ;
1039 return ;
1040 }
1042 // inopportune interleaving -- the exiting thread (this thread)
1043 // in the fast-exit path raced an entering thread in the slow-enter
1044 // path.
1045 // We have two choices:
1046 // A. Try to reacquire the lock.
1047 // If the CAS() fails return immediately, otherwise
1048 // we either restart/rerun the exit operation, or simply
1049 // fall-through into the code below which wakes a successor.
1050 // B. If the elements forming the EntryList|cxq are TSM
1051 // we could simply unpark() the lead thread and return
1052 // without having set _succ.
1053 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1054 TEVENT (Inflated exit - reacquired succeeded) ;
1055 return ;
1056 }
1057 TEVENT (Inflated exit - reacquired failed) ;
1058 } else {
1059 TEVENT (Inflated exit - complex egress) ;
1060 }
1061 }
1063 guarantee (_owner == THREAD, "invariant") ;
1065 ObjectWaiter * w = NULL ;
1066 int QMode = Knob_QMode ;
1068 if (QMode == 2 && _cxq != NULL) {
1069 // QMode == 2 : cxq has precedence over EntryList.
1070 // Try to directly wake a successor from the cxq.
1071 // If successful, the successor will need to unlink itself from cxq.
1072 w = _cxq ;
1073 assert (w != NULL, "invariant") ;
1074 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1075 ExitEpilog (Self, w) ;
1076 return ;
1077 }
1079 if (QMode == 3 && _cxq != NULL) {
1080 // Aggressively drain cxq into EntryList at the first opportunity.
1081 // This policy ensure that recently-run threads live at the head of EntryList.
1082 // Drain _cxq into EntryList - bulk transfer.
1083 // First, detach _cxq.
1084 // The following loop is tantamount to: w = swap (&cxq, NULL)
1085 w = _cxq ;
1086 for (;;) {
1087 assert (w != NULL, "Invariant") ;
1088 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1089 if (u == w) break ;
1090 w = u ;
1091 }
1092 assert (w != NULL , "invariant") ;
1094 ObjectWaiter * q = NULL ;
1095 ObjectWaiter * p ;
1096 for (p = w ; p != NULL ; p = p->_next) {
1097 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1098 p->TState = ObjectWaiter::TS_ENTER ;
1099 p->_prev = q ;
1100 q = p ;
1101 }
1103 // Append the RATs to the EntryList
1104 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1105 ObjectWaiter * Tail ;
1106 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1107 if (Tail == NULL) {
1108 _EntryList = w ;
1109 } else {
1110 Tail->_next = w ;
1111 w->_prev = Tail ;
1112 }
1114 // Fall thru into code that tries to wake a successor from EntryList
1115 }
1117 if (QMode == 4 && _cxq != NULL) {
1118 // Aggressively drain cxq into EntryList at the first opportunity.
1119 // This policy ensure that recently-run threads live at the head of EntryList.
1121 // Drain _cxq into EntryList - bulk transfer.
1122 // First, detach _cxq.
1123 // The following loop is tantamount to: w = swap (&cxq, NULL)
1124 w = _cxq ;
1125 for (;;) {
1126 assert (w != NULL, "Invariant") ;
1127 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1128 if (u == w) break ;
1129 w = u ;
1130 }
1131 assert (w != NULL , "invariant") ;
1133 ObjectWaiter * q = NULL ;
1134 ObjectWaiter * p ;
1135 for (p = w ; p != NULL ; p = p->_next) {
1136 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1137 p->TState = ObjectWaiter::TS_ENTER ;
1138 p->_prev = q ;
1139 q = p ;
1140 }
1142 // Prepend the RATs to the EntryList
1143 if (_EntryList != NULL) {
1144 q->_next = _EntryList ;
1145 _EntryList->_prev = q ;
1146 }
1147 _EntryList = w ;
1149 // Fall thru into code that tries to wake a successor from EntryList
1150 }
1152 w = _EntryList ;
1153 if (w != NULL) {
1154 // I'd like to write: guarantee (w->_thread != Self).
1155 // But in practice an exiting thread may find itself on the EntryList.
1156 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1157 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1158 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1159 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1160 // release the lock "O". T2 resumes immediately after the ST of null into
1161 // _owner, above. T2 notices that the EntryList is populated, so it
1162 // reacquires the lock and then finds itself on the EntryList.
1163 // Given all that, we have to tolerate the circumstance where "w" is
1164 // associated with Self.
1165 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1166 ExitEpilog (Self, w) ;
1167 return ;
1168 }
1170 // If we find that both _cxq and EntryList are null then just
1171 // re-run the exit protocol from the top.
1172 w = _cxq ;
1173 if (w == NULL) continue ;
1175 // Drain _cxq into EntryList - bulk transfer.
1176 // First, detach _cxq.
1177 // The following loop is tantamount to: w = swap (&cxq, NULL)
1178 for (;;) {
1179 assert (w != NULL, "Invariant") ;
1180 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1181 if (u == w) break ;
1182 w = u ;
1183 }
1184 TEVENT (Inflated exit - drain cxq into EntryList) ;
1186 assert (w != NULL , "invariant") ;
1187 assert (_EntryList == NULL , "invariant") ;
1189 // Convert the LIFO SLL anchored by _cxq into a DLL.
1190 // The list reorganization step operates in O(LENGTH(w)) time.
1191 // It's critical that this step operate quickly as
1192 // "Self" still holds the outer-lock, restricting parallelism
1193 // and effectively lengthening the critical section.
1194 // Invariant: s chases t chases u.
1195 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1196 // we have faster access to the tail.
1198 if (QMode == 1) {
1199 // QMode == 1 : drain cxq to EntryList, reversing order
1200 // We also reverse the order of the list.
1201 ObjectWaiter * s = NULL ;
1202 ObjectWaiter * t = w ;
1203 ObjectWaiter * u = NULL ;
1204 while (t != NULL) {
1205 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1206 t->TState = ObjectWaiter::TS_ENTER ;
1207 u = t->_next ;
1208 t->_prev = u ;
1209 t->_next = s ;
1210 s = t;
1211 t = u ;
1212 }
1213 _EntryList = s ;
1214 assert (s != NULL, "invariant") ;
1215 } else {
1216 // QMode == 0 or QMode == 2
1217 _EntryList = w ;
1218 ObjectWaiter * q = NULL ;
1219 ObjectWaiter * p ;
1220 for (p = w ; p != NULL ; p = p->_next) {
1221 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1222 p->TState = ObjectWaiter::TS_ENTER ;
1223 p->_prev = q ;
1224 q = p ;
1225 }
1226 }
1228 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1229 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1231 // See if we can abdicate to a spinner instead of waking a thread.
1232 // A primary goal of the implementation is to reduce the
1233 // context-switch rate.
1234 if (_succ != NULL) continue;
1236 w = _EntryList ;
1237 if (w != NULL) {
1238 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1239 ExitEpilog (Self, w) ;
1240 return ;
1241 }
1242 }
1243 }
1245 // ExitSuspendEquivalent:
1246 // A faster alternate to handle_special_suspend_equivalent_condition()
1247 //
1248 // handle_special_suspend_equivalent_condition() unconditionally
1249 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1250 // operations have high latency. Note that in ::enter() we call HSSEC
1251 // while holding the monitor, so we effectively lengthen the critical sections.
1252 //
1253 // There are a number of possible solutions:
1254 //
1255 // A. To ameliorate the problem we might also defer state transitions
1256 // to as late as possible -- just prior to parking.
1257 // Given that, we'd call HSSEC after having returned from park(),
1258 // but before attempting to acquire the monitor. This is only a
1259 // partial solution. It avoids calling HSSEC while holding the
1260 // monitor (good), but it still increases successor reacquisition latency --
1261 // the interval between unparking a successor and the time the successor
1262 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1263 // If we use this technique we can also avoid EnterI()-exit() loop
1264 // in ::enter() where we iteratively drop the lock and then attempt
1265 // to reacquire it after suspending.
1266 //
1267 // B. In the future we might fold all the suspend bits into a
1268 // composite per-thread suspend flag and then update it with CAS().
1269 // Alternately, a Dekker-like mechanism with multiple variables
1270 // would suffice:
1271 // ST Self->_suspend_equivalent = false
1272 // MEMBAR
1273 // LD Self_>_suspend_flags
1274 //
1277 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1278 int Mode = Knob_FastHSSEC ;
1279 if (Mode && !jSelf->is_external_suspend()) {
1280 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1281 jSelf->clear_suspend_equivalent() ;
1282 if (2 == Mode) OrderAccess::storeload() ;
1283 if (!jSelf->is_external_suspend()) return false ;
1284 // We raced a suspension -- fall thru into the slow path
1285 TEVENT (ExitSuspendEquivalent - raced) ;
1286 jSelf->set_suspend_equivalent() ;
1287 }
1288 return jSelf->handle_special_suspend_equivalent_condition() ;
1289 }
1292 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1293 assert (_owner == Self, "invariant") ;
1295 // Exit protocol:
1296 // 1. ST _succ = wakee
1297 // 2. membar #loadstore|#storestore;
1298 // 2. ST _owner = NULL
1299 // 3. unpark(wakee)
1301 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1302 ParkEvent * Trigger = Wakee->_event ;
1304 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1305 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1306 // out-of-scope (non-extant).
1307 Wakee = NULL ;
1309 // Drop the lock
1310 OrderAccess::release_store_ptr (&_owner, NULL) ;
1311 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1313 if (SafepointSynchronize::do_call_back()) {
1314 TEVENT (unpark before SAFEPOINT) ;
1315 }
1317 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1318 Trigger->unpark() ;
1320 // Maintain stats and report events to JVMTI
1321 if (ObjectMonitor::_sync_Parks != NULL) {
1322 ObjectMonitor::_sync_Parks->inc() ;
1323 }
1324 }
1327 // -----------------------------------------------------------------------------
1328 // Class Loader deadlock handling.
1329 //
1330 // complete_exit exits a lock returning recursion count
1331 // complete_exit/reenter operate as a wait without waiting
1332 // complete_exit requires an inflated monitor
1333 // The _owner field is not always the Thread addr even with an
1334 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1335 // thread due to contention.
1336 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1337 Thread * const Self = THREAD;
1338 assert(Self->is_Java_thread(), "Must be Java thread!");
1339 JavaThread *jt = (JavaThread *)THREAD;
1341 DeferredInitialize();
1343 if (THREAD != _owner) {
1344 if (THREAD->is_lock_owned ((address)_owner)) {
1345 assert(_recursions == 0, "internal state error");
1346 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1347 _recursions = 0 ;
1348 OwnerIsThread = 1 ;
1349 }
1350 }
1352 guarantee(Self == _owner, "complete_exit not owner");
1353 intptr_t save = _recursions; // record the old recursion count
1354 _recursions = 0; // set the recursion level to be 0
1355 exit (Self) ; // exit the monitor
1356 guarantee (_owner != Self, "invariant");
1357 return save;
1358 }
1360 // reenter() enters a lock and sets recursion count
1361 // complete_exit/reenter operate as a wait without waiting
1362 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1363 Thread * const Self = THREAD;
1364 assert(Self->is_Java_thread(), "Must be Java thread!");
1365 JavaThread *jt = (JavaThread *)THREAD;
1367 guarantee(_owner != Self, "reenter already owner");
1368 enter (THREAD); // enter the monitor
1369 guarantee (_recursions == 0, "reenter recursion");
1370 _recursions = recursions;
1371 return;
1372 }
1375 // -----------------------------------------------------------------------------
1376 // A macro is used below because there may already be a pending
1377 // exception which should not abort the execution of the routines
1378 // which use this (which is why we don't put this into check_slow and
1379 // call it with a CHECK argument).
1381 #define CHECK_OWNER() \
1382 do { \
1383 if (THREAD != _owner) { \
1384 if (THREAD->is_lock_owned((address) _owner)) { \
1385 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1386 _recursions = 0; \
1387 OwnerIsThread = 1 ; \
1388 } else { \
1389 TEVENT (Throw IMSX) ; \
1390 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1391 } \
1392 } \
1393 } while (false)
1395 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1396 // TODO-FIXME: remove check_slow() -- it's likely dead.
1398 void ObjectMonitor::check_slow(TRAPS) {
1399 TEVENT (check_slow - throw IMSX) ;
1400 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1401 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1402 }
1404 static int Adjust (volatile int * adr, int dx) {
1405 int v ;
1406 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1407 return v ;
1408 }
1409 // -----------------------------------------------------------------------------
1410 // Wait/Notify/NotifyAll
1411 //
1412 // Note: a subset of changes to ObjectMonitor::wait()
1413 // will need to be replicated in complete_exit above
1414 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1415 Thread * const Self = THREAD ;
1416 assert(Self->is_Java_thread(), "Must be Java thread!");
1417 JavaThread *jt = (JavaThread *)THREAD;
1419 DeferredInitialize () ;
1421 // Throw IMSX or IEX.
1422 CHECK_OWNER();
1424 // check for a pending interrupt
1425 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1426 // post monitor waited event. Note that this is past-tense, we are done waiting.
1427 if (JvmtiExport::should_post_monitor_waited()) {
1428 // Note: 'false' parameter is passed here because the
1429 // wait was not timed out due to thread interrupt.
1430 JvmtiExport::post_monitor_waited(jt, this, false);
1431 }
1432 TEVENT (Wait - Throw IEX) ;
1433 THROW(vmSymbols::java_lang_InterruptedException());
1434 return ;
1435 }
1436 TEVENT (Wait) ;
1438 assert (Self->_Stalled == 0, "invariant") ;
1439 Self->_Stalled = intptr_t(this) ;
1440 jt->set_current_waiting_monitor(this);
1442 // create a node to be put into the queue
1443 // Critically, after we reset() the event but prior to park(), we must check
1444 // for a pending interrupt.
1445 ObjectWaiter node(Self);
1446 node.TState = ObjectWaiter::TS_WAIT ;
1447 Self->_ParkEvent->reset() ;
1448 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1450 // Enter the waiting queue, which is a circular doubly linked list in this case
1451 // but it could be a priority queue or any data structure.
1452 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1453 // by the the owner of the monitor *except* in the case where park()
1454 // returns because of a timeout of interrupt. Contention is exceptionally rare
1455 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1457 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1458 AddWaiter (&node) ;
1459 Thread::SpinRelease (&_WaitSetLock) ;
1461 if ((SyncFlags & 4) == 0) {
1462 _Responsible = NULL ;
1463 }
1464 intptr_t save = _recursions; // record the old recursion count
1465 _waiters++; // increment the number of waiters
1466 _recursions = 0; // set the recursion level to be 1
1467 exit (Self) ; // exit the monitor
1468 guarantee (_owner != Self, "invariant") ;
1470 // As soon as the ObjectMonitor's ownership is dropped in the exit()
1471 // call above, another thread can enter() the ObjectMonitor, do the
1472 // notify(), and exit() the ObjectMonitor. If the other thread's
1473 // exit() call chooses this thread as the successor and the unpark()
1474 // call happens to occur while this thread is posting a
1475 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
1476 // handler using RawMonitors and consuming the unpark().
1477 //
1478 // To avoid the problem, we re-post the event. This does no harm
1479 // even if the original unpark() was not consumed because we are the
1480 // chosen successor for this monitor.
1481 if (node._notified != 0 && _succ == Self) {
1482 node._event->unpark();
1483 }
1485 // The thread is on the WaitSet list - now park() it.
1486 // On MP systems it's conceivable that a brief spin before we park
1487 // could be profitable.
1488 //
1489 // TODO-FIXME: change the following logic to a loop of the form
1490 // while (!timeout && !interrupted && _notified == 0) park()
1492 int ret = OS_OK ;
1493 int WasNotified = 0 ;
1494 { // State transition wrappers
1495 OSThread* osthread = Self->osthread();
1496 OSThreadWaitState osts(osthread, true);
1497 {
1498 ThreadBlockInVM tbivm(jt);
1499 // Thread is in thread_blocked state and oop access is unsafe.
1500 jt->set_suspend_equivalent();
1502 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1503 // Intentionally empty
1504 } else
1505 if (node._notified == 0) {
1506 if (millis <= 0) {
1507 Self->_ParkEvent->park () ;
1508 } else {
1509 ret = Self->_ParkEvent->park (millis) ;
1510 }
1511 }
1513 // were we externally suspended while we were waiting?
1514 if (ExitSuspendEquivalent (jt)) {
1515 // TODO-FIXME: add -- if succ == Self then succ = null.
1516 jt->java_suspend_self();
1517 }
1519 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1522 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1523 // from the WaitSet to the EntryList.
1524 // See if we need to remove Node from the WaitSet.
1525 // We use double-checked locking to avoid grabbing _WaitSetLock
1526 // if the thread is not on the wait queue.
1527 //
1528 // Note that we don't need a fence before the fetch of TState.
1529 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1530 // written by the is thread. (perhaps the fetch might even be satisfied
1531 // by a look-aside into the processor's own store buffer, although given
1532 // the length of the code path between the prior ST and this load that's
1533 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1534 // then we'll acquire the lock and then re-fetch a fresh TState value.
1535 // That is, we fail toward safety.
1537 if (node.TState == ObjectWaiter::TS_WAIT) {
1538 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1539 if (node.TState == ObjectWaiter::TS_WAIT) {
1540 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1541 assert(node._notified == 0, "invariant");
1542 node.TState = ObjectWaiter::TS_RUN ;
1543 }
1544 Thread::SpinRelease (&_WaitSetLock) ;
1545 }
1547 // The thread is now either on off-list (TS_RUN),
1548 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1549 // The Node's TState variable is stable from the perspective of this thread.
1550 // No other threads will asynchronously modify TState.
1551 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1552 OrderAccess::loadload() ;
1553 if (_succ == Self) _succ = NULL ;
1554 WasNotified = node._notified ;
1556 // Reentry phase -- reacquire the monitor.
1557 // re-enter contended monitor after object.wait().
1558 // retain OBJECT_WAIT state until re-enter successfully completes
1559 // Thread state is thread_in_vm and oop access is again safe,
1560 // although the raw address of the object may have changed.
1561 // (Don't cache naked oops over safepoints, of course).
1563 // post monitor waited event. Note that this is past-tense, we are done waiting.
1564 if (JvmtiExport::should_post_monitor_waited()) {
1565 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1566 }
1567 OrderAccess::fence() ;
1569 assert (Self->_Stalled != 0, "invariant") ;
1570 Self->_Stalled = 0 ;
1572 assert (_owner != Self, "invariant") ;
1573 ObjectWaiter::TStates v = node.TState ;
1574 if (v == ObjectWaiter::TS_RUN) {
1575 enter (Self) ;
1576 } else {
1577 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1578 ReenterI (Self, &node) ;
1579 node.wait_reenter_end(this);
1580 }
1582 // Self has reacquired the lock.
1583 // Lifecycle - the node representing Self must not appear on any queues.
1584 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1585 // want residual elements associated with this thread left on any lists.
1586 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1587 assert (_owner == Self, "invariant") ;
1588 assert (_succ != Self , "invariant") ;
1589 } // OSThreadWaitState()
1591 jt->set_current_waiting_monitor(NULL);
1593 guarantee (_recursions == 0, "invariant") ;
1594 _recursions = save; // restore the old recursion count
1595 _waiters--; // decrement the number of waiters
1597 // Verify a few postconditions
1598 assert (_owner == Self , "invariant") ;
1599 assert (_succ != Self , "invariant") ;
1600 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1602 if (SyncFlags & 32) {
1603 OrderAccess::fence() ;
1604 }
1606 // check if the notification happened
1607 if (!WasNotified) {
1608 // no, it could be timeout or Thread.interrupt() or both
1609 // check for interrupt event, otherwise it is timeout
1610 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1611 TEVENT (Wait - throw IEX from epilog) ;
1612 THROW(vmSymbols::java_lang_InterruptedException());
1613 }
1614 }
1616 // NOTE: Spurious wake up will be consider as timeout.
1617 // Monitor notify has precedence over thread interrupt.
1618 }
1621 // Consider:
1622 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1623 // then instead of transferring a thread from the WaitSet to the EntryList
1624 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1626 void ObjectMonitor::notify(TRAPS) {
1627 CHECK_OWNER();
1628 if (_WaitSet == NULL) {
1629 TEVENT (Empty-Notify) ;
1630 return ;
1631 }
1632 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1634 int Policy = Knob_MoveNotifyee ;
1636 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1637 ObjectWaiter * iterator = DequeueWaiter() ;
1638 if (iterator != NULL) {
1639 TEVENT (Notify1 - Transfer) ;
1640 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1641 guarantee (iterator->_notified == 0, "invariant") ;
1642 if (Policy != 4) {
1643 iterator->TState = ObjectWaiter::TS_ENTER ;
1644 }
1645 iterator->_notified = 1 ;
1647 ObjectWaiter * List = _EntryList ;
1648 if (List != NULL) {
1649 assert (List->_prev == NULL, "invariant") ;
1650 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1651 assert (List != iterator, "invariant") ;
1652 }
1654 if (Policy == 0) { // prepend to EntryList
1655 if (List == NULL) {
1656 iterator->_next = iterator->_prev = NULL ;
1657 _EntryList = iterator ;
1658 } else {
1659 List->_prev = iterator ;
1660 iterator->_next = List ;
1661 iterator->_prev = NULL ;
1662 _EntryList = iterator ;
1663 }
1664 } else
1665 if (Policy == 1) { // append to EntryList
1666 if (List == NULL) {
1667 iterator->_next = iterator->_prev = NULL ;
1668 _EntryList = iterator ;
1669 } else {
1670 // CONSIDER: finding the tail currently requires a linear-time walk of
1671 // the EntryList. We can make tail access constant-time by converting to
1672 // a CDLL instead of using our current DLL.
1673 ObjectWaiter * Tail ;
1674 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1675 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1676 Tail->_next = iterator ;
1677 iterator->_prev = Tail ;
1678 iterator->_next = NULL ;
1679 }
1680 } else
1681 if (Policy == 2) { // prepend to cxq
1682 // prepend to cxq
1683 if (List == NULL) {
1684 iterator->_next = iterator->_prev = NULL ;
1685 _EntryList = iterator ;
1686 } else {
1687 iterator->TState = ObjectWaiter::TS_CXQ ;
1688 for (;;) {
1689 ObjectWaiter * Front = _cxq ;
1690 iterator->_next = Front ;
1691 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1692 break ;
1693 }
1694 }
1695 }
1696 } else
1697 if (Policy == 3) { // append to cxq
1698 iterator->TState = ObjectWaiter::TS_CXQ ;
1699 for (;;) {
1700 ObjectWaiter * Tail ;
1701 Tail = _cxq ;
1702 if (Tail == NULL) {
1703 iterator->_next = NULL ;
1704 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1705 break ;
1706 }
1707 } else {
1708 while (Tail->_next != NULL) Tail = Tail->_next ;
1709 Tail->_next = iterator ;
1710 iterator->_prev = Tail ;
1711 iterator->_next = NULL ;
1712 break ;
1713 }
1714 }
1715 } else {
1716 ParkEvent * ev = iterator->_event ;
1717 iterator->TState = ObjectWaiter::TS_RUN ;
1718 OrderAccess::fence() ;
1719 ev->unpark() ;
1720 }
1722 if (Policy < 4) {
1723 iterator->wait_reenter_begin(this);
1724 }
1726 // _WaitSetLock protects the wait queue, not the EntryList. We could
1727 // move the add-to-EntryList operation, above, outside the critical section
1728 // protected by _WaitSetLock. In practice that's not useful. With the
1729 // exception of wait() timeouts and interrupts the monitor owner
1730 // is the only thread that grabs _WaitSetLock. There's almost no contention
1731 // on _WaitSetLock so it's not profitable to reduce the length of the
1732 // critical section.
1733 }
1735 Thread::SpinRelease (&_WaitSetLock) ;
1737 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1738 ObjectMonitor::_sync_Notifications->inc() ;
1739 }
1740 }
1743 void ObjectMonitor::notifyAll(TRAPS) {
1744 CHECK_OWNER();
1745 ObjectWaiter* iterator;
1746 if (_WaitSet == NULL) {
1747 TEVENT (Empty-NotifyAll) ;
1748 return ;
1749 }
1750 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1752 int Policy = Knob_MoveNotifyee ;
1753 int Tally = 0 ;
1754 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1756 for (;;) {
1757 iterator = DequeueWaiter () ;
1758 if (iterator == NULL) break ;
1759 TEVENT (NotifyAll - Transfer1) ;
1760 ++Tally ;
1762 // Disposition - what might we do with iterator ?
1763 // a. add it directly to the EntryList - either tail or head.
1764 // b. push it onto the front of the _cxq.
1765 // For now we use (a).
1767 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1768 guarantee (iterator->_notified == 0, "invariant") ;
1769 iterator->_notified = 1 ;
1770 if (Policy != 4) {
1771 iterator->TState = ObjectWaiter::TS_ENTER ;
1772 }
1774 ObjectWaiter * List = _EntryList ;
1775 if (List != NULL) {
1776 assert (List->_prev == NULL, "invariant") ;
1777 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1778 assert (List != iterator, "invariant") ;
1779 }
1781 if (Policy == 0) { // prepend to EntryList
1782 if (List == NULL) {
1783 iterator->_next = iterator->_prev = NULL ;
1784 _EntryList = iterator ;
1785 } else {
1786 List->_prev = iterator ;
1787 iterator->_next = List ;
1788 iterator->_prev = NULL ;
1789 _EntryList = iterator ;
1790 }
1791 } else
1792 if (Policy == 1) { // append to EntryList
1793 if (List == NULL) {
1794 iterator->_next = iterator->_prev = NULL ;
1795 _EntryList = iterator ;
1796 } else {
1797 // CONSIDER: finding the tail currently requires a linear-time walk of
1798 // the EntryList. We can make tail access constant-time by converting to
1799 // a CDLL instead of using our current DLL.
1800 ObjectWaiter * Tail ;
1801 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1802 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1803 Tail->_next = iterator ;
1804 iterator->_prev = Tail ;
1805 iterator->_next = NULL ;
1806 }
1807 } else
1808 if (Policy == 2) { // prepend to cxq
1809 // prepend to cxq
1810 iterator->TState = ObjectWaiter::TS_CXQ ;
1811 for (;;) {
1812 ObjectWaiter * Front = _cxq ;
1813 iterator->_next = Front ;
1814 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1815 break ;
1816 }
1817 }
1818 } else
1819 if (Policy == 3) { // append to cxq
1820 iterator->TState = ObjectWaiter::TS_CXQ ;
1821 for (;;) {
1822 ObjectWaiter * Tail ;
1823 Tail = _cxq ;
1824 if (Tail == NULL) {
1825 iterator->_next = NULL ;
1826 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1827 break ;
1828 }
1829 } else {
1830 while (Tail->_next != NULL) Tail = Tail->_next ;
1831 Tail->_next = iterator ;
1832 iterator->_prev = Tail ;
1833 iterator->_next = NULL ;
1834 break ;
1835 }
1836 }
1837 } else {
1838 ParkEvent * ev = iterator->_event ;
1839 iterator->TState = ObjectWaiter::TS_RUN ;
1840 OrderAccess::fence() ;
1841 ev->unpark() ;
1842 }
1844 if (Policy < 4) {
1845 iterator->wait_reenter_begin(this);
1846 }
1848 // _WaitSetLock protects the wait queue, not the EntryList. We could
1849 // move the add-to-EntryList operation, above, outside the critical section
1850 // protected by _WaitSetLock. In practice that's not useful. With the
1851 // exception of wait() timeouts and interrupts the monitor owner
1852 // is the only thread that grabs _WaitSetLock. There's almost no contention
1853 // on _WaitSetLock so it's not profitable to reduce the length of the
1854 // critical section.
1855 }
1857 Thread::SpinRelease (&_WaitSetLock) ;
1859 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1860 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1861 }
1862 }
1864 // -----------------------------------------------------------------------------
1865 // Adaptive Spinning Support
1866 //
1867 // Adaptive spin-then-block - rational spinning
1868 //
1869 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1870 // algorithm. On high order SMP systems it would be better to start with
1871 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1872 // a contending thread could enqueue itself on the cxq and then spin locally
1873 // on a thread-specific variable such as its ParkEvent._Event flag.
1874 // That's left as an exercise for the reader. Note that global spinning is
1875 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1876 // low latency and massive bandwidth.
1877 //
1878 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1879 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1880 // (duration) or we can fix the count at approximately the duration of
1881 // a context switch and vary the frequency. Of course we could also
1882 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1883 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1884 //
1885 // This implementation varies the duration "D", where D varies with
1886 // the success rate of recent spin attempts. (D is capped at approximately
1887 // length of a round-trip context switch). The success rate for recent
1888 // spin attempts is a good predictor of the success rate of future spin
1889 // attempts. The mechanism adapts automatically to varying critical
1890 // section length (lock modality), system load and degree of parallelism.
1891 // D is maintained per-monitor in _SpinDuration and is initialized
1892 // optimistically. Spin frequency is fixed at 100%.
1893 //
1894 // Note that _SpinDuration is volatile, but we update it without locks
1895 // or atomics. The code is designed so that _SpinDuration stays within
1896 // a reasonable range even in the presence of races. The arithmetic
1897 // operations on _SpinDuration are closed over the domain of legal values,
1898 // so at worst a race will install and older but still legal value.
1899 // At the very worst this introduces some apparent non-determinism.
1900 // We might spin when we shouldn't or vice-versa, but since the spin
1901 // count are relatively short, even in the worst case, the effect is harmless.
1902 //
1903 // Care must be taken that a low "D" value does not become an
1904 // an absorbing state. Transient spinning failures -- when spinning
1905 // is overall profitable -- should not cause the system to converge
1906 // on low "D" values. We want spinning to be stable and predictable
1907 // and fairly responsive to change and at the same time we don't want
1908 // it to oscillate, become metastable, be "too" non-deterministic,
1909 // or converge on or enter undesirable stable absorbing states.
1910 //
1911 // We implement a feedback-based control system -- using past behavior
1912 // to predict future behavior. We face two issues: (a) if the
1913 // input signal is random then the spin predictor won't provide optimal
1914 // results, and (b) if the signal frequency is too high then the control
1915 // system, which has some natural response lag, will "chase" the signal.
1916 // (b) can arise from multimodal lock hold times. Transient preemption
1917 // can also result in apparent bimodal lock hold times.
1918 // Although sub-optimal, neither condition is particularly harmful, as
1919 // in the worst-case we'll spin when we shouldn't or vice-versa.
1920 // The maximum spin duration is rather short so the failure modes aren't bad.
1921 // To be conservative, I've tuned the gain in system to bias toward
1922 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1923 // "rings" or oscillates between spinning and not spinning. This happens
1924 // when spinning is just on the cusp of profitability, however, so the
1925 // situation is not dire. The state is benign -- there's no need to add
1926 // hysteresis control to damp the transition rate between spinning and
1927 // not spinning.
1928 //
1930 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1931 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1933 // Spinning: Fixed frequency (100%), vary duration
1936 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1938 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1939 int ctr = Knob_FixedSpin ;
1940 if (ctr != 0) {
1941 while (--ctr >= 0) {
1942 if (TryLock (Self) > 0) return 1 ;
1943 SpinPause () ;
1944 }
1945 return 0 ;
1946 }
1948 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1949 if (TryLock(Self) > 0) {
1950 // Increase _SpinDuration ...
1951 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1952 // Raising _SpurDuration to the poverty line is key.
1953 int x = _SpinDuration ;
1954 if (x < Knob_SpinLimit) {
1955 if (x < Knob_Poverty) x = Knob_Poverty ;
1956 _SpinDuration = x + Knob_BonusB ;
1957 }
1958 return 1 ;
1959 }
1960 SpinPause () ;
1961 }
1963 // Admission control - verify preconditions for spinning
1964 //
1965 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1966 // becoming an absorbing state. Put another way, we spin briefly to
1967 // sample, just in case the system load, parallelism, contention, or lock
1968 // modality changed.
1969 //
1970 // Consider the following alternative:
1971 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1972 // spin attempt. "Periodically" might mean after a tally of
1973 // the # of failed spin attempts (or iterations) reaches some threshold.
1974 // This takes us into the realm of 1-out-of-N spinning, where we
1975 // hold the duration constant but vary the frequency.
1977 ctr = _SpinDuration ;
1978 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
1979 if (ctr <= 0) return 0 ;
1981 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
1982 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1983 TEVENT (Spin abort - notrunnable [TOP]);
1984 return 0 ;
1985 }
1987 int MaxSpin = Knob_MaxSpinners ;
1988 if (MaxSpin >= 0) {
1989 if (_Spinner > MaxSpin) {
1990 TEVENT (Spin abort -- too many spinners) ;
1991 return 0 ;
1992 }
1993 // Slighty racy, but benign ...
1994 Adjust (&_Spinner, 1) ;
1995 }
1997 // We're good to spin ... spin ingress.
1998 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1999 // when preparing to LD...CAS _owner, etc and the CAS is likely
2000 // to succeed.
2001 int hits = 0 ;
2002 int msk = 0 ;
2003 int caspty = Knob_CASPenalty ;
2004 int oxpty = Knob_OXPenalty ;
2005 int sss = Knob_SpinSetSucc ;
2006 if (sss && _succ == NULL ) _succ = Self ;
2007 Thread * prv = NULL ;
2009 // There are three ways to exit the following loop:
2010 // 1. A successful spin where this thread has acquired the lock.
2011 // 2. Spin failure with prejudice
2012 // 3. Spin failure without prejudice
2014 while (--ctr >= 0) {
2016 // Periodic polling -- Check for pending GC
2017 // Threads may spin while they're unsafe.
2018 // We don't want spinning threads to delay the JVM from reaching
2019 // a stop-the-world safepoint or to steal cycles from GC.
2020 // If we detect a pending safepoint we abort in order that
2021 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2022 // this thread, if safe, doesn't steal cycles from GC.
2023 // This is in keeping with the "no loitering in runtime" rule.
2024 // We periodically check to see if there's a safepoint pending.
2025 if ((ctr & 0xFF) == 0) {
2026 if (SafepointSynchronize::do_call_back()) {
2027 TEVENT (Spin: safepoint) ;
2028 goto Abort ; // abrupt spin egress
2029 }
2030 if (Knob_UsePause & 1) SpinPause () ;
2032 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2033 if (hits > 50 && scb != NULL) {
2034 int abend = (*scb)(SpinCallbackArgument, 0) ;
2035 }
2036 }
2038 if (Knob_UsePause & 2) SpinPause() ;
2040 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2041 // This is useful on classic SMP systems, but is of less utility on
2042 // N1-style CMT platforms.
2043 //
2044 // Trade-off: lock acquisition latency vs coherency bandwidth.
2045 // Lock hold times are typically short. A histogram
2046 // of successful spin attempts shows that we usually acquire
2047 // the lock early in the spin. That suggests we want to
2048 // sample _owner frequently in the early phase of the spin,
2049 // but then back-off and sample less frequently as the spin
2050 // progresses. The back-off makes a good citizen on SMP big
2051 // SMP systems. Oversampling _owner can consume excessive
2052 // coherency bandwidth. Relatedly, if we _oversample _owner we
2053 // can inadvertently interfere with the the ST m->owner=null.
2054 // executed by the lock owner.
2055 if (ctr & msk) continue ;
2056 ++hits ;
2057 if ((hits & 0xF) == 0) {
2058 // The 0xF, above, corresponds to the exponent.
2059 // Consider: (msk+1)|msk
2060 msk = ((msk << 2)|3) & BackOffMask ;
2061 }
2063 // Probe _owner with TATAS
2064 // If this thread observes the monitor transition or flicker
2065 // from locked to unlocked to locked, then the odds that this
2066 // thread will acquire the lock in this spin attempt go down
2067 // considerably. The same argument applies if the CAS fails
2068 // or if we observe _owner change from one non-null value to
2069 // another non-null value. In such cases we might abort
2070 // the spin without prejudice or apply a "penalty" to the
2071 // spin count-down variable "ctr", reducing it by 100, say.
2073 Thread * ox = (Thread *) _owner ;
2074 if (ox == NULL) {
2075 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2076 if (ox == NULL) {
2077 // The CAS succeeded -- this thread acquired ownership
2078 // Take care of some bookkeeping to exit spin state.
2079 if (sss && _succ == Self) {
2080 _succ = NULL ;
2081 }
2082 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2084 // Increase _SpinDuration :
2085 // The spin was successful (profitable) so we tend toward
2086 // longer spin attempts in the future.
2087 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2088 // If we acquired the lock early in the spin cycle it
2089 // makes sense to increase _SpinDuration proportionally.
2090 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2091 int x = _SpinDuration ;
2092 if (x < Knob_SpinLimit) {
2093 if (x < Knob_Poverty) x = Knob_Poverty ;
2094 _SpinDuration = x + Knob_Bonus ;
2095 }
2096 return 1 ;
2097 }
2099 // The CAS failed ... we can take any of the following actions:
2100 // * penalize: ctr -= Knob_CASPenalty
2101 // * exit spin with prejudice -- goto Abort;
2102 // * exit spin without prejudice.
2103 // * Since CAS is high-latency, retry again immediately.
2104 prv = ox ;
2105 TEVENT (Spin: cas failed) ;
2106 if (caspty == -2) break ;
2107 if (caspty == -1) goto Abort ;
2108 ctr -= caspty ;
2109 continue ;
2110 }
2112 // Did lock ownership change hands ?
2113 if (ox != prv && prv != NULL ) {
2114 TEVENT (spin: Owner changed)
2115 if (oxpty == -2) break ;
2116 if (oxpty == -1) goto Abort ;
2117 ctr -= oxpty ;
2118 }
2119 prv = ox ;
2121 // Abort the spin if the owner is not executing.
2122 // The owner must be executing in order to drop the lock.
2123 // Spinning while the owner is OFFPROC is idiocy.
2124 // Consider: ctr -= RunnablePenalty ;
2125 if (Knob_OState && NotRunnable (Self, ox)) {
2126 TEVENT (Spin abort - notrunnable);
2127 goto Abort ;
2128 }
2129 if (sss && _succ == NULL ) _succ = Self ;
2130 }
2132 // Spin failed with prejudice -- reduce _SpinDuration.
2133 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2134 // AIMD is globally stable.
2135 TEVENT (Spin failure) ;
2136 {
2137 int x = _SpinDuration ;
2138 if (x > 0) {
2139 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2140 // This is globally sample and tends to damp the response.
2141 x -= Knob_Penalty ;
2142 if (x < 0) x = 0 ;
2143 _SpinDuration = x ;
2144 }
2145 }
2147 Abort:
2148 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2149 if (sss && _succ == Self) {
2150 _succ = NULL ;
2151 // Invariant: after setting succ=null a contending thread
2152 // must recheck-retry _owner before parking. This usually happens
2153 // in the normal usage of TrySpin(), but it's safest
2154 // to make TrySpin() as foolproof as possible.
2155 OrderAccess::fence() ;
2156 if (TryLock(Self) > 0) return 1 ;
2157 }
2158 return 0 ;
2159 }
2161 // NotRunnable() -- informed spinning
2162 //
2163 // Don't bother spinning if the owner is not eligible to drop the lock.
2164 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2165 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2166 // The thread must be runnable in order to drop the lock in timely fashion.
2167 // If the _owner is not runnable then spinning will not likely be
2168 // successful (profitable).
2169 //
2170 // Beware -- the thread referenced by _owner could have died
2171 // so a simply fetch from _owner->_thread_state might trap.
2172 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2173 // Because of the lifecycle issues the schedctl and _thread_state values
2174 // observed by NotRunnable() might be garbage. NotRunnable must
2175 // tolerate this and consider the observed _thread_state value
2176 // as advisory.
2177 //
2178 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2179 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2180 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2181 // with the LSB of _owner. Another option would be to probablistically probe
2182 // the putative _owner->TypeTag value.
2183 //
2184 // Checking _thread_state isn't perfect. Even if the thread is
2185 // in_java it might be blocked on a page-fault or have been preempted
2186 // and sitting on a ready/dispatch queue. _thread state in conjunction
2187 // with schedctl.sc_state gives us a good picture of what the
2188 // thread is doing, however.
2189 //
2190 // TODO: check schedctl.sc_state.
2191 // We'll need to use SafeFetch32() to read from the schedctl block.
2192 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2193 //
2194 // The return value from NotRunnable() is *advisory* -- the
2195 // result is based on sampling and is not necessarily coherent.
2196 // The caller must tolerate false-negative and false-positive errors.
2197 // Spinning, in general, is probabilistic anyway.
2200 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2201 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2202 if (!OwnerIsThread) return 0 ;
2204 if (ox == NULL) return 0 ;
2206 // Avoid transitive spinning ...
2207 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2208 // Immediately after T1 acquires L it's possible that T2, also
2209 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2210 // This occurs transiently after T1 acquired L but before
2211 // T1 managed to clear T1.Stalled. T2 does not need to abort
2212 // its spin in this circumstance.
2213 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2215 if (BlockedOn == 1) return 1 ;
2216 if (BlockedOn != 0) {
2217 return BlockedOn != intptr_t(this) && _owner == ox ;
2218 }
2220 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2221 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2222 // consider also: jst != _thread_in_Java -- but that's overspecific.
2223 return jst == _thread_blocked || jst == _thread_in_native ;
2224 }
2227 // -----------------------------------------------------------------------------
2228 // WaitSet management ...
2230 ObjectWaiter::ObjectWaiter(Thread* thread) {
2231 _next = NULL;
2232 _prev = NULL;
2233 _notified = 0;
2234 TState = TS_RUN ;
2235 _thread = thread;
2236 _event = thread->_ParkEvent ;
2237 _active = false;
2238 assert (_event != NULL, "invariant") ;
2239 }
2241 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2242 JavaThread *jt = (JavaThread *)this->_thread;
2243 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2244 }
2246 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2247 JavaThread *jt = (JavaThread *)this->_thread;
2248 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2249 }
2251 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2252 assert(node != NULL, "should not dequeue NULL node");
2253 assert(node->_prev == NULL, "node already in list");
2254 assert(node->_next == NULL, "node already in list");
2255 // put node at end of queue (circular doubly linked list)
2256 if (_WaitSet == NULL) {
2257 _WaitSet = node;
2258 node->_prev = node;
2259 node->_next = node;
2260 } else {
2261 ObjectWaiter* head = _WaitSet ;
2262 ObjectWaiter* tail = head->_prev;
2263 assert(tail->_next == head, "invariant check");
2264 tail->_next = node;
2265 head->_prev = node;
2266 node->_next = head;
2267 node->_prev = tail;
2268 }
2269 }
2271 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2272 // dequeue the very first waiter
2273 ObjectWaiter* waiter = _WaitSet;
2274 if (waiter) {
2275 DequeueSpecificWaiter(waiter);
2276 }
2277 return waiter;
2278 }
2280 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2281 assert(node != NULL, "should not dequeue NULL node");
2282 assert(node->_prev != NULL, "node already removed from list");
2283 assert(node->_next != NULL, "node already removed from list");
2284 // when the waiter has woken up because of interrupt,
2285 // timeout or other spurious wake-up, dequeue the
2286 // waiter from waiting list
2287 ObjectWaiter* next = node->_next;
2288 if (next == node) {
2289 assert(node->_prev == node, "invariant check");
2290 _WaitSet = NULL;
2291 } else {
2292 ObjectWaiter* prev = node->_prev;
2293 assert(prev->_next == node, "invariant check");
2294 assert(next->_prev == node, "invariant check");
2295 next->_prev = prev;
2296 prev->_next = next;
2297 if (_WaitSet == node) {
2298 _WaitSet = next;
2299 }
2300 }
2301 node->_next = NULL;
2302 node->_prev = NULL;
2303 }
2305 // -----------------------------------------------------------------------------
2306 // PerfData support
2307 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2308 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2309 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2310 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2311 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2312 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2313 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2314 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2315 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2316 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2317 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2318 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2319 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2320 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2321 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2322 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2323 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2324 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2326 // One-shot global initialization for the sync subsystem.
2327 // We could also defer initialization and initialize on-demand
2328 // the first time we call inflate(). Initialization would
2329 // be protected - like so many things - by the MonitorCache_lock.
2331 void ObjectMonitor::Initialize () {
2332 static int InitializationCompleted = 0 ;
2333 assert (InitializationCompleted == 0, "invariant") ;
2334 InitializationCompleted = 1 ;
2335 if (UsePerfData) {
2336 EXCEPTION_MARK ;
2337 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2338 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2339 NEWPERFCOUNTER(_sync_Inflations) ;
2340 NEWPERFCOUNTER(_sync_Deflations) ;
2341 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2342 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2343 NEWPERFCOUNTER(_sync_Parks) ;
2344 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2345 NEWPERFCOUNTER(_sync_Notifications) ;
2346 NEWPERFCOUNTER(_sync_SlowEnter) ;
2347 NEWPERFCOUNTER(_sync_SlowExit) ;
2348 NEWPERFCOUNTER(_sync_SlowNotify) ;
2349 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2350 NEWPERFCOUNTER(_sync_FailedSpins) ;
2351 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2352 NEWPERFCOUNTER(_sync_PrivateA) ;
2353 NEWPERFCOUNTER(_sync_PrivateB) ;
2354 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2355 NEWPERFCOUNTER(_sync_MonScavenged) ;
2356 NEWPERFVARIABLE(_sync_MonExtant) ;
2357 #undef NEWPERFCOUNTER
2358 }
2359 }
2362 // Compile-time asserts
2363 // When possible, it's better to catch errors deterministically at
2364 // compile-time than at runtime. The down-side to using compile-time
2365 // asserts is that error message -- often something about negative array
2366 // indices -- is opaque.
2368 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2370 void ObjectMonitor::ctAsserts() {
2371 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2372 }
2375 static char * kvGet (char * kvList, const char * Key) {
2376 if (kvList == NULL) return NULL ;
2377 size_t n = strlen (Key) ;
2378 char * Search ;
2379 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2380 if (strncmp (Search, Key, n) == 0) {
2381 if (Search[n] == '=') return Search + n + 1 ;
2382 if (Search[n] == 0) return (char *) "1" ;
2383 }
2384 }
2385 return NULL ;
2386 }
2388 static int kvGetInt (char * kvList, const char * Key, int Default) {
2389 char * v = kvGet (kvList, Key) ;
2390 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2391 if (Knob_ReportSettings && v != NULL) {
2392 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2393 ::fflush (stdout) ;
2394 }
2395 return rslt ;
2396 }
2398 void ObjectMonitor::DeferredInitialize () {
2399 if (InitDone > 0) return ;
2400 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2401 while (InitDone != 1) ;
2402 return ;
2403 }
2405 // One-shot global initialization ...
2406 // The initialization is idempotent, so we don't need locks.
2407 // In the future consider doing this via os::init_2().
2408 // SyncKnobs consist of <Key>=<Value> pairs in the style
2409 // of environment variables. Start by converting ':' to NUL.
2411 if (SyncKnobs == NULL) SyncKnobs = "" ;
2413 size_t sz = strlen (SyncKnobs) ;
2414 char * knobs = (char *) malloc (sz + 2) ;
2415 if (knobs == NULL) {
2416 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2417 guarantee (0, "invariant") ;
2418 }
2419 strcpy (knobs, SyncKnobs) ;
2420 knobs[sz+1] = 0 ;
2421 for (char * p = knobs ; *p ; p++) {
2422 if (*p == ':') *p = 0 ;
2423 }
2425 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2426 SETKNOB(ReportSettings) ;
2427 SETKNOB(Verbose) ;
2428 SETKNOB(FixedSpin) ;
2429 SETKNOB(SpinLimit) ;
2430 SETKNOB(SpinBase) ;
2431 SETKNOB(SpinBackOff);
2432 SETKNOB(CASPenalty) ;
2433 SETKNOB(OXPenalty) ;
2434 SETKNOB(LogSpins) ;
2435 SETKNOB(SpinSetSucc) ;
2436 SETKNOB(SuccEnabled) ;
2437 SETKNOB(SuccRestrict) ;
2438 SETKNOB(Penalty) ;
2439 SETKNOB(Bonus) ;
2440 SETKNOB(BonusB) ;
2441 SETKNOB(Poverty) ;
2442 SETKNOB(SpinAfterFutile) ;
2443 SETKNOB(UsePause) ;
2444 SETKNOB(SpinEarly) ;
2445 SETKNOB(OState) ;
2446 SETKNOB(MaxSpinners) ;
2447 SETKNOB(PreSpin) ;
2448 SETKNOB(ExitPolicy) ;
2449 SETKNOB(QMode);
2450 SETKNOB(ResetEvent) ;
2451 SETKNOB(MoveNotifyee) ;
2452 SETKNOB(FastHSSEC) ;
2453 #undef SETKNOB
2455 if (os::is_MP()) {
2456 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2457 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2458 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2459 } else {
2460 Knob_SpinLimit = 0 ;
2461 Knob_SpinBase = 0 ;
2462 Knob_PreSpin = 0 ;
2463 Knob_FixedSpin = -1 ;
2464 }
2466 if (Knob_LogSpins == 0) {
2467 ObjectMonitor::_sync_FailedSpins = NULL ;
2468 }
2470 free (knobs) ;
2471 OrderAccess::fence() ;
2472 InitDone = 1 ;
2473 }
2475 #ifndef PRODUCT
2476 void ObjectMonitor::verify() {
2477 }
2479 void ObjectMonitor::print() {
2480 }
2481 #endif