1 /*
2 * Copyright (c) 1998, 2014, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/orderAccess.inline.hpp"
36 #include "runtime/osThread.hpp"
37 #include "runtime/stubRoutines.hpp"
38 #include "runtime/thread.inline.hpp"
39 #include "services/threadService.hpp"
40 #include "trace/tracing.hpp"
41 #include "trace/traceMacros.hpp"
42 #include "utilities/dtrace.hpp"
43 #include "utilities/macros.hpp"
44 #include "utilities/preserveException.hpp"
45 #ifdef TARGET_OS_FAMILY_linux
46 # include "os_linux.inline.hpp"
47 #endif
48 #ifdef TARGET_OS_FAMILY_solaris
49 # include "os_solaris.inline.hpp"
50 #endif
51 #ifdef TARGET_OS_FAMILY_windows
52 # include "os_windows.inline.hpp"
53 #endif
54 #ifdef TARGET_OS_FAMILY_bsd
55 # include "os_bsd.inline.hpp"
56 #endif
57
58 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
59 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
60 #define ATTR __attribute__((noinline))
61 #else
62 #define ATTR
63 #endif
64
65
66 #ifdef DTRACE_ENABLED
67
68 // Only bother with this argument setup if dtrace is available
69 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
70
71
72 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
73 char* bytes = NULL; \
74 int len = 0; \
75 jlong jtid = SharedRuntime::get_java_tid(thread); \
76 Symbol* klassname = ((oop)obj)->klass()->name(); \
77 if (klassname != NULL) { \
78 bytes = (char*)klassname->bytes(); \
79 len = klassname->utf8_length(); \
80 }
81
82 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
83 { \
84 if (DTraceMonitorProbes) { \
85 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
86 HOTSPOT_MONITOR_WAIT(jtid, \
87 (monitor), bytes, len, (millis)); \
88 } \
89 }
90
91 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
92 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
93 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
94 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
95 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
96
97 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
98 { \
99 if (DTraceMonitorProbes) { \
100 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
101 HOTSPOT_MONITOR_##probe(jtid, \
102 (uintptr_t)(monitor), bytes, len); \
103 } \
104 }
105
106 #else // ndef DTRACE_ENABLED
107
108 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
109 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
110
111 #endif // ndef DTRACE_ENABLED
112
113 // Tunables ...
114 // The knob* variables are effectively final. Once set they should
115 // never be modified hence. Consider using __read_mostly with GCC.
116
117 int ObjectMonitor::Knob_Verbose = 0 ;
118 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
119 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
120 static int Knob_HandOff = 0 ;
121 static int Knob_ReportSettings = 0 ;
122
123 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
124 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
125 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
126 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
127 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
128 static int Knob_SpinEarly = 1 ;
129 static int Knob_SuccEnabled = 1 ; // futile wake throttling
130 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
131 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
132 static int Knob_Bonus = 100 ; // spin success bonus
133 static int Knob_BonusB = 100 ; // spin success bonus
134 static int Knob_Penalty = 200 ; // spin failure penalty
135 static int Knob_Poverty = 1000 ;
136 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
137 static int Knob_FixedSpin = 0 ;
138 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
139 static int Knob_UsePause = 1 ;
140 static int Knob_ExitPolicy = 0 ;
141 static int Knob_PreSpin = 10 ; // 20-100 likely better
142 static int Knob_ResetEvent = 0 ;
143 static int BackOffMask = 0 ;
144
145 static int Knob_FastHSSEC = 0 ;
146 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
147 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
148 static volatile int InitDone = 0 ;
149
150 #define TrySpin TrySpin_VaryDuration
151
152 // -----------------------------------------------------------------------------
153 // Theory of operations -- Monitors lists, thread residency, etc:
154 //
155 // * A thread acquires ownership of a monitor by successfully
156 // CAS()ing the _owner field from null to non-null.
157 //
158 // * Invariant: A thread appears on at most one monitor list --
159 // cxq, EntryList or WaitSet -- at any one time.
160 //
161 // * Contending threads "push" themselves onto the cxq with CAS
162 // and then spin/park.
163 //
164 // * After a contending thread eventually acquires the lock it must
165 // dequeue itself from either the EntryList or the cxq.
166 //
167 // * The exiting thread identifies and unparks an "heir presumptive"
168 // tentative successor thread on the EntryList. Critically, the
169 // exiting thread doesn't unlink the successor thread from the EntryList.
170 // After having been unparked, the wakee will recontend for ownership of
171 // the monitor. The successor (wakee) will either acquire the lock or
172 // re-park itself.
173 //
174 // Succession is provided for by a policy of competitive handoff.
175 // The exiting thread does _not_ grant or pass ownership to the
176 // successor thread. (This is also referred to as "handoff" succession").
177 // Instead the exiting thread releases ownership and possibly wakes
178 // a successor, so the successor can (re)compete for ownership of the lock.
179 // If the EntryList is empty but the cxq is populated the exiting
180 // thread will drain the cxq into the EntryList. It does so by
181 // by detaching the cxq (installing null with CAS) and folding
182 // the threads from the cxq into the EntryList. The EntryList is
183 // doubly linked, while the cxq is singly linked because of the
184 // CAS-based "push" used to enqueue recently arrived threads (RATs).
185 //
186 // * Concurrency invariants:
187 //
188 // -- only the monitor owner may access or mutate the EntryList.
189 // The mutex property of the monitor itself protects the EntryList
190 // from concurrent interference.
191 // -- Only the monitor owner may detach the cxq.
192 //
193 // * The monitor entry list operations avoid locks, but strictly speaking
194 // they're not lock-free. Enter is lock-free, exit is not.
195 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
196 //
197 // * The cxq can have multiple concurrent "pushers" but only one concurrent
198 // detaching thread. This mechanism is immune from the ABA corruption.
199 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
200 //
201 // * Taken together, the cxq and the EntryList constitute or form a
202 // single logical queue of threads stalled trying to acquire the lock.
203 // We use two distinct lists to improve the odds of a constant-time
204 // dequeue operation after acquisition (in the ::enter() epilogue) and
205 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
206 // A key desideratum is to minimize queue & monitor metadata manipulation
207 // that occurs while holding the monitor lock -- that is, we want to
208 // minimize monitor lock holds times. Note that even a small amount of
209 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
210 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
211 // locks and monitor metadata.
212 //
213 // Cxq points to the the set of Recently Arrived Threads attempting entry.
214 // Because we push threads onto _cxq with CAS, the RATs must take the form of
215 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
216 // the unlocking thread notices that EntryList is null but _cxq is != null.
217 //
218 // The EntryList is ordered by the prevailing queue discipline and
219 // can be organized in any convenient fashion, such as a doubly-linked list or
220 // a circular doubly-linked list. Critically, we want insert and delete operations
221 // to operate in constant-time. If we need a priority queue then something akin
222 // to Solaris' sleepq would work nicely. Viz.,
223 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
224 // Queue discipline is enforced at ::exit() time, when the unlocking thread
225 // drains the cxq into the EntryList, and orders or reorders the threads on the
226 // EntryList accordingly.
227 //
228 // Barring "lock barging", this mechanism provides fair cyclic ordering,
229 // somewhat similar to an elevator-scan.
230 //
231 // * The monitor synchronization subsystem avoids the use of native
232 // synchronization primitives except for the narrow platform-specific
233 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
234 // the semantics of park-unpark. Put another way, this monitor implementation
235 // depends only on atomic operations and park-unpark. The monitor subsystem
236 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
237 // underlying OS manages the READY<->RUN transitions.
238 //
239 // * Waiting threads reside on the WaitSet list -- wait() puts
240 // the caller onto the WaitSet.
241 //
242 // * notify() or notifyAll() simply transfers threads from the WaitSet to
243 // either the EntryList or cxq. Subsequent exit() operations will
244 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
245 // it's likely the notifyee would simply impale itself on the lock held
246 // by the notifier.
247 //
248 // * An interesting alternative is to encode cxq as (List,LockByte) where
249 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
250 // variable, like _recursions, in the scheme. The threads or Events that form
251 // the list would have to be aligned in 256-byte addresses. A thread would
252 // try to acquire the lock or enqueue itself with CAS, but exiting threads
253 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
254 // Note that is is *not* word-tearing, but it does presume that full-word
255 // CAS operations are coherent with intermix with STB operations. That's true
256 // on most common processors.
257 //
258 // * See also http://blogs.sun.com/dave
259
260
261 // -----------------------------------------------------------------------------
262 // Enter support
263
264 bool ObjectMonitor::try_enter(Thread* THREAD) {
265 if (THREAD != _owner) {
266 if (THREAD->is_lock_owned ((address)_owner)) {
267 assert(_recursions == 0, "internal state error");
268 _owner = THREAD ;
269 _recursions = 1 ;
270 OwnerIsThread = 1 ;
271 return true;
272 }
273 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
274 return false;
275 }
276 return true;
277 } else {
278 _recursions++;
279 return true;
280 }
281 }
282
283 void ATTR ObjectMonitor::enter(TRAPS) {
284 // The following code is ordered to check the most common cases first
285 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
286 Thread * const Self = THREAD ;
287 void * cur ;
288
289 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
290 if (cur == NULL) {
291 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
292 assert (_recursions == 0 , "invariant") ;
293 assert (_owner == Self, "invariant") ;
294 // CONSIDER: set or assert OwnerIsThread == 1
295 return ;
296 }
297
298 if (cur == Self) {
299 // TODO-FIXME: check for integer overflow! BUGID 6557169.
300 _recursions ++ ;
301 return ;
302 }
303
304 if (Self->is_lock_owned ((address)cur)) {
305 assert (_recursions == 0, "internal state error");
306 _recursions = 1 ;
307 // Commute owner from a thread-specific on-stack BasicLockObject address to
308 // a full-fledged "Thread *".
309 _owner = Self ;
310 OwnerIsThread = 1 ;
311 return ;
312 }
313
314 // We've encountered genuine contention.
315 assert (Self->_Stalled == 0, "invariant") ;
316 Self->_Stalled = intptr_t(this) ;
317
318 // Try one round of spinning *before* enqueueing Self
319 // and before going through the awkward and expensive state
320 // transitions. The following spin is strictly optional ...
321 // Note that if we acquire the monitor from an initial spin
322 // we forgo posting JVMTI events and firing DTRACE probes.
323 if (Knob_SpinEarly && TrySpin (Self) > 0) {
324 assert (_owner == Self , "invariant") ;
325 assert (_recursions == 0 , "invariant") ;
326 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
327 Self->_Stalled = 0 ;
328 return ;
329 }
330
331 assert (_owner != Self , "invariant") ;
332 assert (_succ != Self , "invariant") ;
333 assert (Self->is_Java_thread() , "invariant") ;
334 JavaThread * jt = (JavaThread *) Self ;
335 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
336 assert (jt->thread_state() != _thread_blocked , "invariant") ;
337 assert (this->object() != NULL , "invariant") ;
338 assert (_count >= 0, "invariant") ;
339
340 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
341 // Ensure the object-monitor relationship remains stable while there's contention.
342 Atomic::inc_ptr(&_count);
343
344 EventJavaMonitorEnter event;
345
346 { // Change java thread status to indicate blocked on monitor enter.
347 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
348
349 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
350 if (JvmtiExport::should_post_monitor_contended_enter()) {
351 JvmtiExport::post_monitor_contended_enter(jt, this);
352
353 // The current thread does not yet own the monitor and does not
354 // yet appear on any queues that would get it made the successor.
355 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
356 // handler cannot accidentally consume an unpark() meant for the
357 // ParkEvent associated with this ObjectMonitor.
358 }
359
360 OSThreadContendState osts(Self->osthread());
361 ThreadBlockInVM tbivm(jt);
362
363 Self->set_current_pending_monitor(this);
364
365 // TODO-FIXME: change the following for(;;) loop to straight-line code.
366 for (;;) {
367 jt->set_suspend_equivalent();
368 // cleared by handle_special_suspend_equivalent_condition()
369 // or java_suspend_self()
370
371 EnterI (THREAD) ;
372
373 if (!ExitSuspendEquivalent(jt)) break ;
374
375 //
376 // We have acquired the contended monitor, but while we were
377 // waiting another thread suspended us. We don't want to enter
378 // the monitor while suspended because that would surprise the
379 // thread that suspended us.
380 //
381 _recursions = 0 ;
382 _succ = NULL ;
383 exit (false, Self) ;
384
385 jt->java_suspend_self();
386 }
387 Self->set_current_pending_monitor(NULL);
388 }
389
390 Atomic::dec_ptr(&_count);
391 assert (_count >= 0, "invariant") ;
392 Self->_Stalled = 0 ;
393
394 // Must either set _recursions = 0 or ASSERT _recursions == 0.
395 assert (_recursions == 0 , "invariant") ;
396 assert (_owner == Self , "invariant") ;
397 assert (_succ != Self , "invariant") ;
398 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
399
400 // The thread -- now the owner -- is back in vm mode.
401 // Report the glorious news via TI,DTrace and jvmstat.
402 // The probe effect is non-trivial. All the reportage occurs
403 // while we hold the monitor, increasing the length of the critical
404 // section. Amdahl's parallel speedup law comes vividly into play.
405 //
406 // Another option might be to aggregate the events (thread local or
407 // per-monitor aggregation) and defer reporting until a more opportune
408 // time -- such as next time some thread encounters contention but has
409 // yet to acquire the lock. While spinning that thread could
410 // spinning we could increment JVMStat counters, etc.
411
412 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
413 if (JvmtiExport::should_post_monitor_contended_entered()) {
414 JvmtiExport::post_monitor_contended_entered(jt, this);
415
416 // The current thread already owns the monitor and is not going to
417 // call park() for the remainder of the monitor enter protocol. So
418 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
419 // event handler consumed an unpark() issued by the thread that
420 // just exited the monitor.
421 }
422
423 if (event.should_commit()) {
424 event.set_klass(((oop)this->object())->klass());
425 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
426 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
427 event.commit();
428 }
429
430 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
431 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
432 }
433 }
434
435
436 // Caveat: TryLock() is not necessarily serializing if it returns failure.
437 // Callers must compensate as needed.
438
439 int ObjectMonitor::TryLock (Thread * Self) {
440 for (;;) {
441 void * own = _owner ;
442 if (own != NULL) return 0 ;
443 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
444 // Either guarantee _recursions == 0 or set _recursions = 0.
445 assert (_recursions == 0, "invariant") ;
446 assert (_owner == Self, "invariant") ;
447 // CONSIDER: set or assert that OwnerIsThread == 1
448 return 1 ;
449 }
450 // The lock had been free momentarily, but we lost the race to the lock.
451 // Interference -- the CAS failed.
452 // We can either return -1 or retry.
453 // Retry doesn't make as much sense because the lock was just acquired.
454 if (true) return -1 ;
455 }
456 }
457
458 void ATTR ObjectMonitor::EnterI (TRAPS) {
459 Thread * Self = THREAD ;
460 assert (Self->is_Java_thread(), "invariant") ;
461 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
462
463 // Try the lock - TATAS
464 if (TryLock (Self) > 0) {
465 assert (_succ != Self , "invariant") ;
466 assert (_owner == Self , "invariant") ;
467 assert (_Responsible != Self , "invariant") ;
468 return ;
469 }
470
471 DeferredInitialize () ;
472
473 // We try one round of spinning *before* enqueueing Self.
474 //
475 // If the _owner is ready but OFFPROC we could use a YieldTo()
476 // operation to donate the remainder of this thread's quantum
477 // to the owner. This has subtle but beneficial affinity
478 // effects.
479
480 if (TrySpin (Self) > 0) {
481 assert (_owner == Self , "invariant") ;
482 assert (_succ != Self , "invariant") ;
483 assert (_Responsible != Self , "invariant") ;
484 return ;
485 }
486
487 // The Spin failed -- Enqueue and park the thread ...
488 assert (_succ != Self , "invariant") ;
489 assert (_owner != Self , "invariant") ;
490 assert (_Responsible != Self , "invariant") ;
491
492 // Enqueue "Self" on ObjectMonitor's _cxq.
493 //
494 // Node acts as a proxy for Self.
495 // As an aside, if were to ever rewrite the synchronization code mostly
496 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
497 // Java objects. This would avoid awkward lifecycle and liveness issues,
498 // as well as eliminate a subset of ABA issues.
499 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
500 //
501
502 ObjectWaiter node(Self) ;
503 Self->_ParkEvent->reset() ;
504 node._prev = (ObjectWaiter *) 0xBAD ;
505 node.TState = ObjectWaiter::TS_CXQ ;
506
507 // Push "Self" onto the front of the _cxq.
508 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
509 // Note that spinning tends to reduce the rate at which threads
510 // enqueue and dequeue on EntryList|cxq.
511 ObjectWaiter * nxt ;
512 for (;;) {
513 node._next = nxt = _cxq ;
514 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
515
516 // Interference - the CAS failed because _cxq changed. Just retry.
517 // As an optional optimization we retry the lock.
518 if (TryLock (Self) > 0) {
519 assert (_succ != Self , "invariant") ;
520 assert (_owner == Self , "invariant") ;
521 assert (_Responsible != Self , "invariant") ;
522 return ;
523 }
524 }
525
526 // Check for cxq|EntryList edge transition to non-null. This indicates
527 // the onset of contention. While contention persists exiting threads
528 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
529 // operations revert to the faster 1-0 mode. This enter operation may interleave
530 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
531 // arrange for one of the contending thread to use a timed park() operations
532 // to detect and recover from the race. (Stranding is form of progress failure
533 // where the monitor is unlocked but all the contending threads remain parked).
534 // That is, at least one of the contended threads will periodically poll _owner.
535 // One of the contending threads will become the designated "Responsible" thread.
536 // The Responsible thread uses a timed park instead of a normal indefinite park
537 // operation -- it periodically wakes and checks for and recovers from potential
538 // strandings admitted by 1-0 exit operations. We need at most one Responsible
539 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
540 // be responsible for a monitor.
541 //
542 // Currently, one of the contended threads takes on the added role of "Responsible".
543 // A viable alternative would be to use a dedicated "stranding checker" thread
544 // that periodically iterated over all the threads (or active monitors) and unparked
545 // successors where there was risk of stranding. This would help eliminate the
546 // timer scalability issues we see on some platforms as we'd only have one thread
547 // -- the checker -- parked on a timer.
548
549 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
550 // Try to assume the role of responsible thread for the monitor.
551 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
552 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
553 }
554
555 // The lock have been released while this thread was occupied queueing
556 // itself onto _cxq. To close the race and avoid "stranding" and
557 // progress-liveness failure we must resample-retry _owner before parking.
558 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
559 // In this case the ST-MEMBAR is accomplished with CAS().
560 //
561 // TODO: Defer all thread state transitions until park-time.
562 // Since state transitions are heavy and inefficient we'd like
563 // to defer the state transitions until absolutely necessary,
564 // and in doing so avoid some transitions ...
565
566 TEVENT (Inflated enter - Contention) ;
567 int nWakeups = 0 ;
568 int RecheckInterval = 1 ;
569
570 for (;;) {
571
572 if (TryLock (Self) > 0) break ;
573 assert (_owner != Self, "invariant") ;
574
575 if ((SyncFlags & 2) && _Responsible == NULL) {
576 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
577 }
578
579 // park self
580 if (_Responsible == Self || (SyncFlags & 1)) {
581 TEVENT (Inflated enter - park TIMED) ;
582 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
583 // Increase the RecheckInterval, but clamp the value.
584 RecheckInterval *= 8 ;
585 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
586 } else {
587 TEVENT (Inflated enter - park UNTIMED) ;
588 Self->_ParkEvent->park() ;
589 }
590
591 if (TryLock(Self) > 0) break ;
592
593 // The lock is still contested.
594 // Keep a tally of the # of futile wakeups.
595 // Note that the counter is not protected by a lock or updated by atomics.
596 // That is by design - we trade "lossy" counters which are exposed to
597 // races during updates for a lower probe effect.
598 TEVENT (Inflated enter - Futile wakeup) ;
599 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
600 ObjectMonitor::_sync_FutileWakeups->inc() ;
601 }
602 ++ nWakeups ;
603
604 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
605 // We can defer clearing _succ until after the spin completes
606 // TrySpin() must tolerate being called with _succ == Self.
607 // Try yet another round of adaptive spinning.
608 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
609
610 // We can find that we were unpark()ed and redesignated _succ while
611 // we were spinning. That's harmless. If we iterate and call park(),
612 // park() will consume the event and return immediately and we'll
613 // just spin again. This pattern can repeat, leaving _succ to simply
614 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
615 // Alternately, we can sample fired() here, and if set, forgo spinning
616 // in the next iteration.
617
618 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
619 Self->_ParkEvent->reset() ;
620 OrderAccess::fence() ;
621 }
622 if (_succ == Self) _succ = NULL ;
623
624 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
625 OrderAccess::fence() ;
626 }
627
628 // Egress :
629 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
630 // Normally we'll find Self on the EntryList .
631 // From the perspective of the lock owner (this thread), the
632 // EntryList is stable and cxq is prepend-only.
633 // The head of cxq is volatile but the interior is stable.
634 // In addition, Self.TState is stable.
635
636 assert (_owner == Self , "invariant") ;
637 assert (object() != NULL , "invariant") ;
638 // I'd like to write:
639 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
640 // but as we're at a safepoint that's not safe.
641
642 UnlinkAfterAcquire (Self, &node) ;
643 if (_succ == Self) _succ = NULL ;
644
645 assert (_succ != Self, "invariant") ;
646 if (_Responsible == Self) {
647 _Responsible = NULL ;
648 OrderAccess::fence(); // Dekker pivot-point
649
650 // We may leave threads on cxq|EntryList without a designated
651 // "Responsible" thread. This is benign. When this thread subsequently
652 // exits the monitor it can "see" such preexisting "old" threads --
653 // threads that arrived on the cxq|EntryList before the fence, above --
654 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
655 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
656 // non-null and elect a new "Responsible" timer thread.
657 //
658 // This thread executes:
659 // ST Responsible=null; MEMBAR (in enter epilogue - here)
660 // LD cxq|EntryList (in subsequent exit)
661 //
662 // Entering threads in the slow/contended path execute:
663 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
664 // The (ST cxq; MEMBAR) is accomplished with CAS().
665 //
666 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
667 // exit operation from floating above the ST Responsible=null.
668 }
669
670 // We've acquired ownership with CAS().
671 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
672 // But since the CAS() this thread may have also stored into _succ,
673 // EntryList, cxq or Responsible. These meta-data updates must be
674 // visible __before this thread subsequently drops the lock.
675 // Consider what could occur if we didn't enforce this constraint --
676 // STs to monitor meta-data and user-data could reorder with (become
677 // visible after) the ST in exit that drops ownership of the lock.
678 // Some other thread could then acquire the lock, but observe inconsistent
679 // or old monitor meta-data and heap data. That violates the JMM.
680 // To that end, the 1-0 exit() operation must have at least STST|LDST
681 // "release" barrier semantics. Specifically, there must be at least a
682 // STST|LDST barrier in exit() before the ST of null into _owner that drops
683 // the lock. The barrier ensures that changes to monitor meta-data and data
684 // protected by the lock will be visible before we release the lock, and
685 // therefore before some other thread (CPU) has a chance to acquire the lock.
686 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
687 //
688 // Critically, any prior STs to _succ or EntryList must be visible before
689 // the ST of null into _owner in the *subsequent* (following) corresponding
690 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
691 // execute a serializing instruction.
692
693 if (SyncFlags & 8) {
694 OrderAccess::fence() ;
695 }
696 return ;
697 }
698
699 // ReenterI() is a specialized inline form of the latter half of the
700 // contended slow-path from EnterI(). We use ReenterI() only for
701 // monitor reentry in wait().
702 //
703 // In the future we should reconcile EnterI() and ReenterI(), adding
704 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
705 // loop accordingly.
706
707 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
708 assert (Self != NULL , "invariant") ;
709 assert (SelfNode != NULL , "invariant") ;
710 assert (SelfNode->_thread == Self , "invariant") ;
711 assert (_waiters > 0 , "invariant") ;
712 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
713 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
714 JavaThread * jt = (JavaThread *) Self ;
715
716 int nWakeups = 0 ;
717 for (;;) {
718 ObjectWaiter::TStates v = SelfNode->TState ;
719 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
720 assert (_owner != Self, "invariant") ;
721
722 if (TryLock (Self) > 0) break ;
723 if (TrySpin (Self) > 0) break ;
724
725 TEVENT (Wait Reentry - parking) ;
726
727 // State transition wrappers around park() ...
728 // ReenterI() wisely defers state transitions until
729 // it's clear we must park the thread.
730 {
731 OSThreadContendState osts(Self->osthread());
732 ThreadBlockInVM tbivm(jt);
733
734 // cleared by handle_special_suspend_equivalent_condition()
735 // or java_suspend_self()
736 jt->set_suspend_equivalent();
737 if (SyncFlags & 1) {
738 Self->_ParkEvent->park ((jlong)1000) ;
739 } else {
740 Self->_ParkEvent->park () ;
741 }
742
743 // were we externally suspended while we were waiting?
744 for (;;) {
745 if (!ExitSuspendEquivalent (jt)) break ;
746 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
747 jt->java_suspend_self();
748 jt->set_suspend_equivalent();
749 }
750 }
751
752 // Try again, but just so we distinguish between futile wakeups and
753 // successful wakeups. The following test isn't algorithmically
754 // necessary, but it helps us maintain sensible statistics.
755 if (TryLock(Self) > 0) break ;
756
757 // The lock is still contested.
758 // Keep a tally of the # of futile wakeups.
759 // Note that the counter is not protected by a lock or updated by atomics.
760 // That is by design - we trade "lossy" counters which are exposed to
761 // races during updates for a lower probe effect.
762 TEVENT (Wait Reentry - futile wakeup) ;
763 ++ nWakeups ;
764
765 // Assuming this is not a spurious wakeup we'll normally
766 // find that _succ == Self.
767 if (_succ == Self) _succ = NULL ;
768
769 // Invariant: after clearing _succ a contending thread
770 // *must* retry _owner before parking.
771 OrderAccess::fence() ;
772
773 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
774 ObjectMonitor::_sync_FutileWakeups->inc() ;
775 }
776 }
777
778 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
779 // Normally we'll find Self on the EntryList.
780 // Unlinking from the EntryList is constant-time and atomic-free.
781 // From the perspective of the lock owner (this thread), the
782 // EntryList is stable and cxq is prepend-only.
783 // The head of cxq is volatile but the interior is stable.
784 // In addition, Self.TState is stable.
785
786 assert (_owner == Self, "invariant") ;
787 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
788 UnlinkAfterAcquire (Self, SelfNode) ;
789 if (_succ == Self) _succ = NULL ;
790 assert (_succ != Self, "invariant") ;
791 SelfNode->TState = ObjectWaiter::TS_RUN ;
792 OrderAccess::fence() ; // see comments at the end of EnterI()
793 }
794
795 // after the thread acquires the lock in ::enter(). Equally, we could defer
796 // unlinking the thread until ::exit()-time.
797
798 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
799 {
800 assert (_owner == Self, "invariant") ;
801 assert (SelfNode->_thread == Self, "invariant") ;
802
803 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
804 // Normal case: remove Self from the DLL EntryList .
805 // This is a constant-time operation.
806 ObjectWaiter * nxt = SelfNode->_next ;
807 ObjectWaiter * prv = SelfNode->_prev ;
808 if (nxt != NULL) nxt->_prev = prv ;
809 if (prv != NULL) prv->_next = nxt ;
810 if (SelfNode == _EntryList ) _EntryList = nxt ;
811 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
812 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
813 TEVENT (Unlink from EntryList) ;
814 } else {
815 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
816 // Inopportune interleaving -- Self is still on the cxq.
817 // This usually means the enqueue of self raced an exiting thread.
818 // Normally we'll find Self near the front of the cxq, so
819 // dequeueing is typically fast. If needbe we can accelerate
820 // this with some MCS/CHL-like bidirectional list hints and advisory
821 // back-links so dequeueing from the interior will normally operate
822 // in constant-time.
823 // Dequeue Self from either the head (with CAS) or from the interior
824 // with a linear-time scan and normal non-atomic memory operations.
825 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
826 // and then unlink Self from EntryList. We have to drain eventually,
827 // so it might as well be now.
828
829 ObjectWaiter * v = _cxq ;
830 assert (v != NULL, "invariant") ;
831 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
832 // The CAS above can fail from interference IFF a "RAT" arrived.
833 // In that case Self must be in the interior and can no longer be
834 // at the head of cxq.
835 if (v == SelfNode) {
836 assert (_cxq != v, "invariant") ;
837 v = _cxq ; // CAS above failed - start scan at head of list
838 }
839 ObjectWaiter * p ;
840 ObjectWaiter * q = NULL ;
841 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
842 q = p ;
843 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
844 }
845 assert (v != SelfNode, "invariant") ;
846 assert (p == SelfNode, "Node not found on cxq") ;
847 assert (p != _cxq, "invariant") ;
848 assert (q != NULL, "invariant") ;
849 assert (q->_next == p, "invariant") ;
850 q->_next = p->_next ;
851 }
852 TEVENT (Unlink from cxq) ;
853 }
854
855 // Diagnostic hygiene ...
856 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
857 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
858 SelfNode->TState = ObjectWaiter::TS_RUN ;
859 }
860
861 // -----------------------------------------------------------------------------
862 // Exit support
863 //
864 // exit()
865 // ~~~~~~
866 // Note that the collector can't reclaim the objectMonitor or deflate
867 // the object out from underneath the thread calling ::exit() as the
868 // thread calling ::exit() never transitions to a stable state.
869 // This inhibits GC, which in turn inhibits asynchronous (and
870 // inopportune) reclamation of "this".
871 //
872 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
873 // There's one exception to the claim above, however. EnterI() can call
874 // exit() to drop a lock if the acquirer has been externally suspended.
875 // In that case exit() is called with _thread_state as _thread_blocked,
876 // but the monitor's _count field is > 0, which inhibits reclamation.
877 //
878 // 1-0 exit
879 // ~~~~~~~~
880 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
881 // the fast-path operators have been optimized so the common ::exit()
882 // operation is 1-0. See i486.ad fast_unlock(), for instance.
883 // The code emitted by fast_unlock() elides the usual MEMBAR. This
884 // greatly improves latency -- MEMBAR and CAS having considerable local
885 // latency on modern processors -- but at the cost of "stranding". Absent the
886 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
887 // ::enter() path, resulting in the entering thread being stranding
888 // and a progress-liveness failure. Stranding is extremely rare.
889 // We use timers (timed park operations) & periodic polling to detect
890 // and recover from stranding. Potentially stranded threads periodically
891 // wake up and poll the lock. See the usage of the _Responsible variable.
892 //
893 // The CAS() in enter provides for safety and exclusion, while the CAS or
894 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
895 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
896 // We detect and recover from stranding with timers.
897 //
898 // If a thread transiently strands it'll park until (a) another
899 // thread acquires the lock and then drops the lock, at which time the
900 // exiting thread will notice and unpark the stranded thread, or, (b)
901 // the timer expires. If the lock is high traffic then the stranding latency
902 // will be low due to (a). If the lock is low traffic then the odds of
903 // stranding are lower, although the worst-case stranding latency
904 // is longer. Critically, we don't want to put excessive load in the
905 // platform's timer subsystem. We want to minimize both the timer injection
906 // rate (timers created/sec) as well as the number of timers active at
907 // any one time. (more precisely, we want to minimize timer-seconds, which is
908 // the integral of the # of active timers at any instant over time).
909 // Both impinge on OS scalability. Given that, at most one thread parked on
910 // a monitor will use a timer.
911
912 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
913 Thread * Self = THREAD ;
914 if (THREAD != _owner) {
915 if (THREAD->is_lock_owned((address) _owner)) {
916 // Transmute _owner from a BasicLock pointer to a Thread address.
917 // We don't need to hold _mutex for this transition.
918 // Non-null to Non-null is safe as long as all readers can
919 // tolerate either flavor.
920 assert (_recursions == 0, "invariant") ;
921 _owner = THREAD ;
922 _recursions = 0 ;
923 OwnerIsThread = 1 ;
924 } else {
925 // NOTE: we need to handle unbalanced monitor enter/exit
926 // in native code by throwing an exception.
927 // TODO: Throw an IllegalMonitorStateException ?
928 TEVENT (Exit - Throw IMSX) ;
929 assert(false, "Non-balanced monitor enter/exit!");
930 if (false) {
931 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
932 }
933 return;
934 }
935 }
936
937 if (_recursions != 0) {
938 _recursions--; // this is simple recursive enter
939 TEVENT (Inflated exit - recursive) ;
940 return ;
941 }
942
943 // Invariant: after setting Responsible=null an thread must execute
944 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
945 if ((SyncFlags & 4) == 0) {
946 _Responsible = NULL ;
947 }
948
949 #if INCLUDE_TRACE
950 // get the owner's thread id for the MonitorEnter event
951 // if it is enabled and the thread isn't suspended
952 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
953 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
954 }
955 #endif
956
957 for (;;) {
958 assert (THREAD == _owner, "invariant") ;
959
960
961 if (Knob_ExitPolicy == 0) {
962 // release semantics: prior loads and stores from within the critical section
963 // must not float (reorder) past the following store that drops the lock.
964 // On SPARC that requires MEMBAR #loadstore|#storestore.
965 // But of course in TSO #loadstore|#storestore is not required.
966 // I'd like to write one of the following:
967 // A. OrderAccess::release() ; _owner = NULL
968 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
969 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
970 // store into a _dummy variable. That store is not needed, but can result
971 // in massive wasteful coherency traffic on classic SMP systems.
972 // Instead, I use release_store(), which is implemented as just a simple
973 // ST on x64, x86 and SPARC.
974 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
975 OrderAccess::storeload() ; // See if we need to wake a successor
976 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
977 TEVENT (Inflated exit - simple egress) ;
978 return ;
979 }
980 TEVENT (Inflated exit - complex egress) ;
981
982 // Normally the exiting thread is responsible for ensuring succession,
983 // but if other successors are ready or other entering threads are spinning
984 // then this thread can simply store NULL into _owner and exit without
985 // waking a successor. The existence of spinners or ready successors
986 // guarantees proper succession (liveness). Responsibility passes to the
987 // ready or running successors. The exiting thread delegates the duty.
988 // More precisely, if a successor already exists this thread is absolved
989 // of the responsibility of waking (unparking) one.
990 //
991 // The _succ variable is critical to reducing futile wakeup frequency.
992 // _succ identifies the "heir presumptive" thread that has been made
993 // ready (unparked) but that has not yet run. We need only one such
994 // successor thread to guarantee progress.
995 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
996 // section 3.3 "Futile Wakeup Throttling" for details.
997 //
998 // Note that spinners in Enter() also set _succ non-null.
999 // In the current implementation spinners opportunistically set
1000 // _succ so that exiting threads might avoid waking a successor.
1001 // Another less appealing alternative would be for the exiting thread
1002 // to drop the lock and then spin briefly to see if a spinner managed
1003 // to acquire the lock. If so, the exiting thread could exit
1004 // immediately without waking a successor, otherwise the exiting
1005 // thread would need to dequeue and wake a successor.
1006 // (Note that we'd need to make the post-drop spin short, but no
1007 // shorter than the worst-case round-trip cache-line migration time.
1008 // The dropped lock needs to become visible to the spinner, and then
1009 // the acquisition of the lock by the spinner must become visible to
1010 // the exiting thread).
1011 //
1012
1013 // It appears that an heir-presumptive (successor) must be made ready.
1014 // Only the current lock owner can manipulate the EntryList or
1015 // drain _cxq, so we need to reacquire the lock. If we fail
1016 // to reacquire the lock the responsibility for ensuring succession
1017 // falls to the new owner.
1018 //
1019 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1020 return ;
1021 }
1022 TEVENT (Exit - Reacquired) ;
1023 } else {
1024 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1025 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1026 OrderAccess::storeload() ;
1027 // Ratify the previously observed values.
1028 if (_cxq == NULL || _succ != NULL) {
1029 TEVENT (Inflated exit - simple egress) ;
1030 return ;
1031 }
1032
1033 // inopportune interleaving -- the exiting thread (this thread)
1034 // in the fast-exit path raced an entering thread in the slow-enter
1035 // path.
1036 // We have two choices:
1037 // A. Try to reacquire the lock.
1038 // If the CAS() fails return immediately, otherwise
1039 // we either restart/rerun the exit operation, or simply
1040 // fall-through into the code below which wakes a successor.
1041 // B. If the elements forming the EntryList|cxq are TSM
1042 // we could simply unpark() the lead thread and return
1043 // without having set _succ.
1044 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1045 TEVENT (Inflated exit - reacquired succeeded) ;
1046 return ;
1047 }
1048 TEVENT (Inflated exit - reacquired failed) ;
1049 } else {
1050 TEVENT (Inflated exit - complex egress) ;
1051 }
1052 }
1053
1054 guarantee (_owner == THREAD, "invariant") ;
1055
1056 ObjectWaiter * w = NULL ;
1057 int QMode = Knob_QMode ;
1058
1059 if (QMode == 2 && _cxq != NULL) {
1060 // QMode == 2 : cxq has precedence over EntryList.
1061 // Try to directly wake a successor from the cxq.
1062 // If successful, the successor will need to unlink itself from cxq.
1063 w = _cxq ;
1064 assert (w != NULL, "invariant") ;
1065 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1066 ExitEpilog (Self, w) ;
1067 return ;
1068 }
1069
1070 if (QMode == 3 && _cxq != NULL) {
1071 // Aggressively drain cxq into EntryList at the first opportunity.
1072 // This policy ensure that recently-run threads live at the head of EntryList.
1073 // Drain _cxq into EntryList - bulk transfer.
1074 // First, detach _cxq.
1075 // The following loop is tantamount to: w = swap (&cxq, NULL)
1076 w = _cxq ;
1077 for (;;) {
1078 assert (w != NULL, "Invariant") ;
1079 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1080 if (u == w) break ;
1081 w = u ;
1082 }
1083 assert (w != NULL , "invariant") ;
1084
1085 ObjectWaiter * q = NULL ;
1086 ObjectWaiter * p ;
1087 for (p = w ; p != NULL ; p = p->_next) {
1088 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1089 p->TState = ObjectWaiter::TS_ENTER ;
1090 p->_prev = q ;
1091 q = p ;
1092 }
1093
1094 // Append the RATs to the EntryList
1095 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1096 ObjectWaiter * Tail ;
1097 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1098 if (Tail == NULL) {
1099 _EntryList = w ;
1100 } else {
1101 Tail->_next = w ;
1102 w->_prev = Tail ;
1103 }
1104
1105 // Fall thru into code that tries to wake a successor from EntryList
1106 }
1107
1108 if (QMode == 4 && _cxq != NULL) {
1109 // Aggressively drain cxq into EntryList at the first opportunity.
1110 // This policy ensure that recently-run threads live at the head of EntryList.
1111
1112 // Drain _cxq into EntryList - bulk transfer.
1113 // First, detach _cxq.
1114 // The following loop is tantamount to: w = swap (&cxq, NULL)
1115 w = _cxq ;
1116 for (;;) {
1117 assert (w != NULL, "Invariant") ;
1118 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1119 if (u == w) break ;
1120 w = u ;
1121 }
1122 assert (w != NULL , "invariant") ;
1123
1124 ObjectWaiter * q = NULL ;
1125 ObjectWaiter * p ;
1126 for (p = w ; p != NULL ; p = p->_next) {
1127 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1128 p->TState = ObjectWaiter::TS_ENTER ;
1129 p->_prev = q ;
1130 q = p ;
1131 }
1132
1133 // Prepend the RATs to the EntryList
1134 if (_EntryList != NULL) {
1135 q->_next = _EntryList ;
1136 _EntryList->_prev = q ;
1137 }
1138 _EntryList = w ;
1139
1140 // Fall thru into code that tries to wake a successor from EntryList
1141 }
1142
1143 w = _EntryList ;
1144 if (w != NULL) {
1145 // I'd like to write: guarantee (w->_thread != Self).
1146 // But in practice an exiting thread may find itself on the EntryList.
1147 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1148 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1149 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1150 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1151 // release the lock "O". T2 resumes immediately after the ST of null into
1152 // _owner, above. T2 notices that the EntryList is populated, so it
1153 // reacquires the lock and then finds itself on the EntryList.
1154 // Given all that, we have to tolerate the circumstance where "w" is
1155 // associated with Self.
1156 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1157 ExitEpilog (Self, w) ;
1158 return ;
1159 }
1160
1161 // If we find that both _cxq and EntryList are null then just
1162 // re-run the exit protocol from the top.
1163 w = _cxq ;
1164 if (w == NULL) continue ;
1165
1166 // Drain _cxq into EntryList - bulk transfer.
1167 // First, detach _cxq.
1168 // The following loop is tantamount to: w = swap (&cxq, NULL)
1169 for (;;) {
1170 assert (w != NULL, "Invariant") ;
1171 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1172 if (u == w) break ;
1173 w = u ;
1174 }
1175 TEVENT (Inflated exit - drain cxq into EntryList) ;
1176
1177 assert (w != NULL , "invariant") ;
1178 assert (_EntryList == NULL , "invariant") ;
1179
1180 // Convert the LIFO SLL anchored by _cxq into a DLL.
1181 // The list reorganization step operates in O(LENGTH(w)) time.
1182 // It's critical that this step operate quickly as
1183 // "Self" still holds the outer-lock, restricting parallelism
1184 // and effectively lengthening the critical section.
1185 // Invariant: s chases t chases u.
1186 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1187 // we have faster access to the tail.
1188
1189 if (QMode == 1) {
1190 // QMode == 1 : drain cxq to EntryList, reversing order
1191 // We also reverse the order of the list.
1192 ObjectWaiter * s = NULL ;
1193 ObjectWaiter * t = w ;
1194 ObjectWaiter * u = NULL ;
1195 while (t != NULL) {
1196 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1197 t->TState = ObjectWaiter::TS_ENTER ;
1198 u = t->_next ;
1199 t->_prev = u ;
1200 t->_next = s ;
1201 s = t;
1202 t = u ;
1203 }
1204 _EntryList = s ;
1205 assert (s != NULL, "invariant") ;
1206 } else {
1207 // QMode == 0 or QMode == 2
1208 _EntryList = w ;
1209 ObjectWaiter * q = NULL ;
1210 ObjectWaiter * p ;
1211 for (p = w ; p != NULL ; p = p->_next) {
1212 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1213 p->TState = ObjectWaiter::TS_ENTER ;
1214 p->_prev = q ;
1215 q = p ;
1216 }
1217 }
1218
1219 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1220 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1221
1222 // See if we can abdicate to a spinner instead of waking a thread.
1223 // A primary goal of the implementation is to reduce the
1224 // context-switch rate.
1225 if (_succ != NULL) continue;
1226
1227 w = _EntryList ;
1228 if (w != NULL) {
1229 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1230 ExitEpilog (Self, w) ;
1231 return ;
1232 }
1233 }
1234 }
1235
1236 // ExitSuspendEquivalent:
1237 // A faster alternate to handle_special_suspend_equivalent_condition()
1238 //
1239 // handle_special_suspend_equivalent_condition() unconditionally
1240 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1241 // operations have high latency. Note that in ::enter() we call HSSEC
1242 // while holding the monitor, so we effectively lengthen the critical sections.
1243 //
1244 // There are a number of possible solutions:
1245 //
1246 // A. To ameliorate the problem we might also defer state transitions
1247 // to as late as possible -- just prior to parking.
1248 // Given that, we'd call HSSEC after having returned from park(),
1249 // but before attempting to acquire the monitor. This is only a
1250 // partial solution. It avoids calling HSSEC while holding the
1251 // monitor (good), but it still increases successor reacquisition latency --
1252 // the interval between unparking a successor and the time the successor
1253 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1254 // If we use this technique we can also avoid EnterI()-exit() loop
1255 // in ::enter() where we iteratively drop the lock and then attempt
1256 // to reacquire it after suspending.
1257 //
1258 // B. In the future we might fold all the suspend bits into a
1259 // composite per-thread suspend flag and then update it with CAS().
1260 // Alternately, a Dekker-like mechanism with multiple variables
1261 // would suffice:
1262 // ST Self->_suspend_equivalent = false
1263 // MEMBAR
1264 // LD Self_>_suspend_flags
1265 //
1266
1267
1268 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1269 int Mode = Knob_FastHSSEC ;
1270 if (Mode && !jSelf->is_external_suspend()) {
1271 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1272 jSelf->clear_suspend_equivalent() ;
1273 if (2 == Mode) OrderAccess::storeload() ;
1274 if (!jSelf->is_external_suspend()) return false ;
1275 // We raced a suspension -- fall thru into the slow path
1276 TEVENT (ExitSuspendEquivalent - raced) ;
1277 jSelf->set_suspend_equivalent() ;
1278 }
1279 return jSelf->handle_special_suspend_equivalent_condition() ;
1280 }
1281
1282
1283 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1284 assert (_owner == Self, "invariant") ;
1285
1286 // Exit protocol:
1287 // 1. ST _succ = wakee
1288 // 2. membar #loadstore|#storestore;
1289 // 2. ST _owner = NULL
1290 // 3. unpark(wakee)
1291
1292 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1293 ParkEvent * Trigger = Wakee->_event ;
1294
1295 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1296 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1297 // out-of-scope (non-extant).
1298 Wakee = NULL ;
1299
1300 // Drop the lock
1301 OrderAccess::release_store_ptr (&_owner, NULL) ;
1302 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1303
1304 if (SafepointSynchronize::do_call_back()) {
1305 TEVENT (unpark before SAFEPOINT) ;
1306 }
1307
1308 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1309 Trigger->unpark() ;
1310
1311 // Maintain stats and report events to JVMTI
1312 if (ObjectMonitor::_sync_Parks != NULL) {
1313 ObjectMonitor::_sync_Parks->inc() ;
1314 }
1315 }
1316
1317
1318 // -----------------------------------------------------------------------------
1319 // Class Loader deadlock handling.
1320 //
1321 // complete_exit exits a lock returning recursion count
1322 // complete_exit/reenter operate as a wait without waiting
1323 // complete_exit requires an inflated monitor
1324 // The _owner field is not always the Thread addr even with an
1325 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1326 // thread due to contention.
1327 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1328 Thread * const Self = THREAD;
1329 assert(Self->is_Java_thread(), "Must be Java thread!");
1330 JavaThread *jt = (JavaThread *)THREAD;
1331
1332 DeferredInitialize();
1333
1334 if (THREAD != _owner) {
1335 if (THREAD->is_lock_owned ((address)_owner)) {
1336 assert(_recursions == 0, "internal state error");
1337 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1338 _recursions = 0 ;
1339 OwnerIsThread = 1 ;
1340 }
1341 }
1342
1343 guarantee(Self == _owner, "complete_exit not owner");
1344 intptr_t save = _recursions; // record the old recursion count
1345 _recursions = 0; // set the recursion level to be 0
1346 exit (true, Self) ; // exit the monitor
1347 guarantee (_owner != Self, "invariant");
1348 return save;
1349 }
1350
1351 // reenter() enters a lock and sets recursion count
1352 // complete_exit/reenter operate as a wait without waiting
1353 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1354 Thread * const Self = THREAD;
1355 assert(Self->is_Java_thread(), "Must be Java thread!");
1356 JavaThread *jt = (JavaThread *)THREAD;
1357
1358 guarantee(_owner != Self, "reenter already owner");
1359 enter (THREAD); // enter the monitor
1360 guarantee (_recursions == 0, "reenter recursion");
1361 _recursions = recursions;
1362 return;
1363 }
1364
1365
1366 // -----------------------------------------------------------------------------
1367 // A macro is used below because there may already be a pending
1368 // exception which should not abort the execution of the routines
1369 // which use this (which is why we don't put this into check_slow and
1370 // call it with a CHECK argument).
1371
1372 #define CHECK_OWNER() \
1373 do { \
1374 if (THREAD != _owner) { \
1375 if (THREAD->is_lock_owned((address) _owner)) { \
1376 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1377 _recursions = 0; \
1378 OwnerIsThread = 1 ; \
1379 } else { \
1380 TEVENT (Throw IMSX) ; \
1381 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1382 } \
1383 } \
1384 } while (false)
1385
1386 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1387 // TODO-FIXME: remove check_slow() -- it's likely dead.
1388
1389 void ObjectMonitor::check_slow(TRAPS) {
1390 TEVENT (check_slow - throw IMSX) ;
1391 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1392 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1393 }
1394
1395 static int Adjust (volatile int * adr, int dx) {
1396 int v ;
1397 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1398 return v ;
1399 }
1400
1401 // helper method for posting a monitor wait event
1402 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1403 jlong notifier_tid,
1404 jlong timeout,
1405 bool timedout) {
1406 event->set_klass(((oop)this->object())->klass());
1407 event->set_timeout((TYPE_ULONG)timeout);
1408 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1409 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1410 event->set_timedOut((TYPE_BOOLEAN)timedout);
1411 event->commit();
1412 }
1413
1414 // -----------------------------------------------------------------------------
1415 // Wait/Notify/NotifyAll
1416 //
1417 // Note: a subset of changes to ObjectMonitor::wait()
1418 // will need to be replicated in complete_exit above
1419 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1420 Thread * const Self = THREAD ;
1421 assert(Self->is_Java_thread(), "Must be Java thread!");
1422 JavaThread *jt = (JavaThread *)THREAD;
1423
1424 DeferredInitialize () ;
1425
1426 // Throw IMSX or IEX.
1427 CHECK_OWNER();
1428
1429 EventJavaMonitorWait event;
1430
1431 // check for a pending interrupt
1432 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1433 // post monitor waited event. Note that this is past-tense, we are done waiting.
1434 if (JvmtiExport::should_post_monitor_waited()) {
1435 // Note: 'false' parameter is passed here because the
1436 // wait was not timed out due to thread interrupt.
1437 JvmtiExport::post_monitor_waited(jt, this, false);
1438
1439 // In this short circuit of the monitor wait protocol, the
1440 // current thread never drops ownership of the monitor and
1441 // never gets added to the wait queue so the current thread
1442 // cannot be made the successor. This means that the
1443 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1444 // consume an unpark() meant for the ParkEvent associated with
1445 // this ObjectMonitor.
1446 }
1447 if (event.should_commit()) {
1448 post_monitor_wait_event(&event, 0, millis, false);
1449 }
1450 TEVENT (Wait - Throw IEX) ;
1451 THROW(vmSymbols::java_lang_InterruptedException());
1452 return ;
1453 }
1454
1455 TEVENT (Wait) ;
1456
1457 assert (Self->_Stalled == 0, "invariant") ;
1458 Self->_Stalled = intptr_t(this) ;
1459 jt->set_current_waiting_monitor(this);
1460
1461 // create a node to be put into the queue
1462 // Critically, after we reset() the event but prior to park(), we must check
1463 // for a pending interrupt.
1464 ObjectWaiter node(Self);
1465 node.TState = ObjectWaiter::TS_WAIT ;
1466 Self->_ParkEvent->reset() ;
1467 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1468
1469 // Enter the waiting queue, which is a circular doubly linked list in this case
1470 // but it could be a priority queue or any data structure.
1471 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1472 // by the the owner of the monitor *except* in the case where park()
1473 // returns because of a timeout of interrupt. Contention is exceptionally rare
1474 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1475
1476 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1477 AddWaiter (&node) ;
1478 Thread::SpinRelease (&_WaitSetLock) ;
1479
1480 if ((SyncFlags & 4) == 0) {
1481 _Responsible = NULL ;
1482 }
1483 intptr_t save = _recursions; // record the old recursion count
1484 _waiters++; // increment the number of waiters
1485 _recursions = 0; // set the recursion level to be 1
1486 exit (true, Self) ; // exit the monitor
1487 guarantee (_owner != Self, "invariant") ;
1488
1489 // The thread is on the WaitSet list - now park() it.
1490 // On MP systems it's conceivable that a brief spin before we park
1491 // could be profitable.
1492 //
1493 // TODO-FIXME: change the following logic to a loop of the form
1494 // while (!timeout && !interrupted && _notified == 0) park()
1495
1496 int ret = OS_OK ;
1497 int WasNotified = 0 ;
1498 { // State transition wrappers
1499 OSThread* osthread = Self->osthread();
1500 OSThreadWaitState osts(osthread, true);
1501 {
1502 ThreadBlockInVM tbivm(jt);
1503 // Thread is in thread_blocked state and oop access is unsafe.
1504 jt->set_suspend_equivalent();
1505
1506 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1507 // Intentionally empty
1508 } else
1509 if (node._notified == 0) {
1510 if (millis <= 0) {
1511 Self->_ParkEvent->park () ;
1512 } else {
1513 ret = Self->_ParkEvent->park (millis) ;
1514 }
1515 }
1516
1517 // were we externally suspended while we were waiting?
1518 if (ExitSuspendEquivalent (jt)) {
1519 // TODO-FIXME: add -- if succ == Self then succ = null.
1520 jt->java_suspend_self();
1521 }
1522
1523 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1524
1525
1526 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1527 // from the WaitSet to the EntryList.
1528 // See if we need to remove Node from the WaitSet.
1529 // We use double-checked locking to avoid grabbing _WaitSetLock
1530 // if the thread is not on the wait queue.
1531 //
1532 // Note that we don't need a fence before the fetch of TState.
1533 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1534 // written by the is thread. (perhaps the fetch might even be satisfied
1535 // by a look-aside into the processor's own store buffer, although given
1536 // the length of the code path between the prior ST and this load that's
1537 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1538 // then we'll acquire the lock and then re-fetch a fresh TState value.
1539 // That is, we fail toward safety.
1540
1541 if (node.TState == ObjectWaiter::TS_WAIT) {
1542 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1543 if (node.TState == ObjectWaiter::TS_WAIT) {
1544 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1545 assert(node._notified == 0, "invariant");
1546 node.TState = ObjectWaiter::TS_RUN ;
1547 }
1548 Thread::SpinRelease (&_WaitSetLock) ;
1549 }
1550
1551 // The thread is now either on off-list (TS_RUN),
1552 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1553 // The Node's TState variable is stable from the perspective of this thread.
1554 // No other threads will asynchronously modify TState.
1555 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1556 OrderAccess::loadload() ;
1557 if (_succ == Self) _succ = NULL ;
1558 WasNotified = node._notified ;
1559
1560 // Reentry phase -- reacquire the monitor.
1561 // re-enter contended monitor after object.wait().
1562 // retain OBJECT_WAIT state until re-enter successfully completes
1563 // Thread state is thread_in_vm and oop access is again safe,
1564 // although the raw address of the object may have changed.
1565 // (Don't cache naked oops over safepoints, of course).
1566
1567 // post monitor waited event. Note that this is past-tense, we are done waiting.
1568 if (JvmtiExport::should_post_monitor_waited()) {
1569 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1570
1571 if (node._notified != 0 && _succ == Self) {
1572 // In this part of the monitor wait-notify-reenter protocol it
1573 // is possible (and normal) for another thread to do a fastpath
1574 // monitor enter-exit while this thread is still trying to get
1575 // to the reenter portion of the protocol.
1576 //
1577 // The ObjectMonitor was notified and the current thread is
1578 // the successor which also means that an unpark() has already
1579 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1580 // consume the unpark() that was done when the successor was
1581 // set because the same ParkEvent is shared between Java
1582 // monitors and JVM/TI RawMonitors (for now).
1583 //
1584 // We redo the unpark() to ensure forward progress, i.e., we
1585 // don't want all pending threads hanging (parked) with none
1586 // entering the unlocked monitor.
1587 node._event->unpark();
1588 }
1589 }
1590
1591 if (event.should_commit()) {
1592 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1593 }
1594
1595 OrderAccess::fence() ;
1596
1597 assert (Self->_Stalled != 0, "invariant") ;
1598 Self->_Stalled = 0 ;
1599
1600 assert (_owner != Self, "invariant") ;
1601 ObjectWaiter::TStates v = node.TState ;
1602 if (v == ObjectWaiter::TS_RUN) {
1603 enter (Self) ;
1604 } else {
1605 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1606 ReenterI (Self, &node) ;
1607 node.wait_reenter_end(this);
1608 }
1609
1610 // Self has reacquired the lock.
1611 // Lifecycle - the node representing Self must not appear on any queues.
1612 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1613 // want residual elements associated with this thread left on any lists.
1614 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1615 assert (_owner == Self, "invariant") ;
1616 assert (_succ != Self , "invariant") ;
1617 } // OSThreadWaitState()
1618
1619 jt->set_current_waiting_monitor(NULL);
1620
1621 guarantee (_recursions == 0, "invariant") ;
1622 _recursions = save; // restore the old recursion count
1623 _waiters--; // decrement the number of waiters
1624
1625 // Verify a few postconditions
1626 assert (_owner == Self , "invariant") ;
1627 assert (_succ != Self , "invariant") ;
1628 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1629
1630 if (SyncFlags & 32) {
1631 OrderAccess::fence() ;
1632 }
1633
1634 // check if the notification happened
1635 if (!WasNotified) {
1636 // no, it could be timeout or Thread.interrupt() or both
1637 // check for interrupt event, otherwise it is timeout
1638 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1639 TEVENT (Wait - throw IEX from epilog) ;
1640 THROW(vmSymbols::java_lang_InterruptedException());
1641 }
1642 }
1643
1644 // NOTE: Spurious wake up will be consider as timeout.
1645 // Monitor notify has precedence over thread interrupt.
1646 }
1647
1648
1649 // Consider:
1650 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1651 // then instead of transferring a thread from the WaitSet to the EntryList
1652 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1653
1654 void ObjectMonitor::notify(TRAPS) {
1655 CHECK_OWNER();
1656 if (_WaitSet == NULL) {
1657 TEVENT (Empty-Notify) ;
1658 return ;
1659 }
1660 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1661
1662 int Policy = Knob_MoveNotifyee ;
1663
1664 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1665 ObjectWaiter * iterator = DequeueWaiter() ;
1666 if (iterator != NULL) {
1667 TEVENT (Notify1 - Transfer) ;
1668 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1669 guarantee (iterator->_notified == 0, "invariant") ;
1670 if (Policy != 4) {
1671 iterator->TState = ObjectWaiter::TS_ENTER ;
1672 }
1673 iterator->_notified = 1 ;
1674 Thread * Self = THREAD;
1675 iterator->_notifier_tid = Self->osthread()->thread_id();
1676
1677 ObjectWaiter * List = _EntryList ;
1678 if (List != NULL) {
1679 assert (List->_prev == NULL, "invariant") ;
1680 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1681 assert (List != iterator, "invariant") ;
1682 }
1683
1684 if (Policy == 0) { // prepend to EntryList
1685 if (List == NULL) {
1686 iterator->_next = iterator->_prev = NULL ;
1687 _EntryList = iterator ;
1688 } else {
1689 List->_prev = iterator ;
1690 iterator->_next = List ;
1691 iterator->_prev = NULL ;
1692 _EntryList = iterator ;
1693 }
1694 } else
1695 if (Policy == 1) { // append to EntryList
1696 if (List == NULL) {
1697 iterator->_next = iterator->_prev = NULL ;
1698 _EntryList = iterator ;
1699 } else {
1700 // CONSIDER: finding the tail currently requires a linear-time walk of
1701 // the EntryList. We can make tail access constant-time by converting to
1702 // a CDLL instead of using our current DLL.
1703 ObjectWaiter * Tail ;
1704 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1705 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1706 Tail->_next = iterator ;
1707 iterator->_prev = Tail ;
1708 iterator->_next = NULL ;
1709 }
1710 } else
1711 if (Policy == 2) { // prepend to cxq
1712 // prepend to cxq
1713 if (List == NULL) {
1714 iterator->_next = iterator->_prev = NULL ;
1715 _EntryList = iterator ;
1716 } else {
1717 iterator->TState = ObjectWaiter::TS_CXQ ;
1718 for (;;) {
1719 ObjectWaiter * Front = _cxq ;
1720 iterator->_next = Front ;
1721 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1722 break ;
1723 }
1724 }
1725 }
1726 } else
1727 if (Policy == 3) { // append to cxq
1728 iterator->TState = ObjectWaiter::TS_CXQ ;
1729 for (;;) {
1730 ObjectWaiter * Tail ;
1731 Tail = _cxq ;
1732 if (Tail == NULL) {
1733 iterator->_next = NULL ;
1734 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1735 break ;
1736 }
1737 } else {
1738 while (Tail->_next != NULL) Tail = Tail->_next ;
1739 Tail->_next = iterator ;
1740 iterator->_prev = Tail ;
1741 iterator->_next = NULL ;
1742 break ;
1743 }
1744 }
1745 } else {
1746 ParkEvent * ev = iterator->_event ;
1747 iterator->TState = ObjectWaiter::TS_RUN ;
1748 OrderAccess::fence() ;
1749 ev->unpark() ;
1750 }
1751
1752 if (Policy < 4) {
1753 iterator->wait_reenter_begin(this);
1754 }
1755
1756 // _WaitSetLock protects the wait queue, not the EntryList. We could
1757 // move the add-to-EntryList operation, above, outside the critical section
1758 // protected by _WaitSetLock. In practice that's not useful. With the
1759 // exception of wait() timeouts and interrupts the monitor owner
1760 // is the only thread that grabs _WaitSetLock. There's almost no contention
1761 // on _WaitSetLock so it's not profitable to reduce the length of the
1762 // critical section.
1763 }
1764
1765 Thread::SpinRelease (&_WaitSetLock) ;
1766
1767 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1768 ObjectMonitor::_sync_Notifications->inc() ;
1769 }
1770 }
1771
1772
1773 void ObjectMonitor::notifyAll(TRAPS) {
1774 CHECK_OWNER();
1775 ObjectWaiter* iterator;
1776 if (_WaitSet == NULL) {
1777 TEVENT (Empty-NotifyAll) ;
1778 return ;
1779 }
1780 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1781
1782 int Policy = Knob_MoveNotifyee ;
1783 int Tally = 0 ;
1784 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1785
1786 for (;;) {
1787 iterator = DequeueWaiter () ;
1788 if (iterator == NULL) break ;
1789 TEVENT (NotifyAll - Transfer1) ;
1790 ++Tally ;
1791
1792 // Disposition - what might we do with iterator ?
1793 // a. add it directly to the EntryList - either tail or head.
1794 // b. push it onto the front of the _cxq.
1795 // For now we use (a).
1796
1797 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1798 guarantee (iterator->_notified == 0, "invariant") ;
1799 iterator->_notified = 1 ;
1800 Thread * Self = THREAD;
1801 iterator->_notifier_tid = Self->osthread()->thread_id();
1802 if (Policy != 4) {
1803 iterator->TState = ObjectWaiter::TS_ENTER ;
1804 }
1805
1806 ObjectWaiter * List = _EntryList ;
1807 if (List != NULL) {
1808 assert (List->_prev == NULL, "invariant") ;
1809 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1810 assert (List != iterator, "invariant") ;
1811 }
1812
1813 if (Policy == 0) { // prepend to EntryList
1814 if (List == NULL) {
1815 iterator->_next = iterator->_prev = NULL ;
1816 _EntryList = iterator ;
1817 } else {
1818 List->_prev = iterator ;
1819 iterator->_next = List ;
1820 iterator->_prev = NULL ;
1821 _EntryList = iterator ;
1822 }
1823 } else
1824 if (Policy == 1) { // append to EntryList
1825 if (List == NULL) {
1826 iterator->_next = iterator->_prev = NULL ;
1827 _EntryList = iterator ;
1828 } else {
1829 // CONSIDER: finding the tail currently requires a linear-time walk of
1830 // the EntryList. We can make tail access constant-time by converting to
1831 // a CDLL instead of using our current DLL.
1832 ObjectWaiter * Tail ;
1833 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1834 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1835 Tail->_next = iterator ;
1836 iterator->_prev = Tail ;
1837 iterator->_next = NULL ;
1838 }
1839 } else
1840 if (Policy == 2) { // prepend to cxq
1841 // prepend to cxq
1842 iterator->TState = ObjectWaiter::TS_CXQ ;
1843 for (;;) {
1844 ObjectWaiter * Front = _cxq ;
1845 iterator->_next = Front ;
1846 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1847 break ;
1848 }
1849 }
1850 } else
1851 if (Policy == 3) { // append to cxq
1852 iterator->TState = ObjectWaiter::TS_CXQ ;
1853 for (;;) {
1854 ObjectWaiter * Tail ;
1855 Tail = _cxq ;
1856 if (Tail == NULL) {
1857 iterator->_next = NULL ;
1858 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1859 break ;
1860 }
1861 } else {
1862 while (Tail->_next != NULL) Tail = Tail->_next ;
1863 Tail->_next = iterator ;
1864 iterator->_prev = Tail ;
1865 iterator->_next = NULL ;
1866 break ;
1867 }
1868 }
1869 } else {
1870 ParkEvent * ev = iterator->_event ;
1871 iterator->TState = ObjectWaiter::TS_RUN ;
1872 OrderAccess::fence() ;
1873 ev->unpark() ;
1874 }
1875
1876 if (Policy < 4) {
1877 iterator->wait_reenter_begin(this);
1878 }
1879
1880 // _WaitSetLock protects the wait queue, not the EntryList. We could
1881 // move the add-to-EntryList operation, above, outside the critical section
1882 // protected by _WaitSetLock. In practice that's not useful. With the
1883 // exception of wait() timeouts and interrupts the monitor owner
1884 // is the only thread that grabs _WaitSetLock. There's almost no contention
1885 // on _WaitSetLock so it's not profitable to reduce the length of the
1886 // critical section.
1887 }
1888
1889 Thread::SpinRelease (&_WaitSetLock) ;
1890
1891 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1892 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1893 }
1894 }
1895
1896 // -----------------------------------------------------------------------------
1897 // Adaptive Spinning Support
1898 //
1899 // Adaptive spin-then-block - rational spinning
1900 //
1901 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1902 // algorithm. On high order SMP systems it would be better to start with
1903 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1904 // a contending thread could enqueue itself on the cxq and then spin locally
1905 // on a thread-specific variable such as its ParkEvent._Event flag.
1906 // That's left as an exercise for the reader. Note that global spinning is
1907 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1908 // low latency and massive bandwidth.
1909 //
1910 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1911 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1912 // (duration) or we can fix the count at approximately the duration of
1913 // a context switch and vary the frequency. Of course we could also
1914 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1915 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1916 //
1917 // This implementation varies the duration "D", where D varies with
1918 // the success rate of recent spin attempts. (D is capped at approximately
1919 // length of a round-trip context switch). The success rate for recent
1920 // spin attempts is a good predictor of the success rate of future spin
1921 // attempts. The mechanism adapts automatically to varying critical
1922 // section length (lock modality), system load and degree of parallelism.
1923 // D is maintained per-monitor in _SpinDuration and is initialized
1924 // optimistically. Spin frequency is fixed at 100%.
1925 //
1926 // Note that _SpinDuration is volatile, but we update it without locks
1927 // or atomics. The code is designed so that _SpinDuration stays within
1928 // a reasonable range even in the presence of races. The arithmetic
1929 // operations on _SpinDuration are closed over the domain of legal values,
1930 // so at worst a race will install and older but still legal value.
1931 // At the very worst this introduces some apparent non-determinism.
1932 // We might spin when we shouldn't or vice-versa, but since the spin
1933 // count are relatively short, even in the worst case, the effect is harmless.
1934 //
1935 // Care must be taken that a low "D" value does not become an
1936 // an absorbing state. Transient spinning failures -- when spinning
1937 // is overall profitable -- should not cause the system to converge
1938 // on low "D" values. We want spinning to be stable and predictable
1939 // and fairly responsive to change and at the same time we don't want
1940 // it to oscillate, become metastable, be "too" non-deterministic,
1941 // or converge on or enter undesirable stable absorbing states.
1942 //
1943 // We implement a feedback-based control system -- using past behavior
1944 // to predict future behavior. We face two issues: (a) if the
1945 // input signal is random then the spin predictor won't provide optimal
1946 // results, and (b) if the signal frequency is too high then the control
1947 // system, which has some natural response lag, will "chase" the signal.
1948 // (b) can arise from multimodal lock hold times. Transient preemption
1949 // can also result in apparent bimodal lock hold times.
1950 // Although sub-optimal, neither condition is particularly harmful, as
1951 // in the worst-case we'll spin when we shouldn't or vice-versa.
1952 // The maximum spin duration is rather short so the failure modes aren't bad.
1953 // To be conservative, I've tuned the gain in system to bias toward
1954 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1955 // "rings" or oscillates between spinning and not spinning. This happens
1956 // when spinning is just on the cusp of profitability, however, so the
1957 // situation is not dire. The state is benign -- there's no need to add
1958 // hysteresis control to damp the transition rate between spinning and
1959 // not spinning.
1960 //
1961
1962 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1963 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1964
1965 // Spinning: Fixed frequency (100%), vary duration
1966
1967
1968 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1969
1970 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1971 int ctr = Knob_FixedSpin ;
1972 if (ctr != 0) {
1973 while (--ctr >= 0) {
1974 if (TryLock (Self) > 0) return 1 ;
1975 SpinPause () ;
1976 }
1977 return 0 ;
1978 }
1979
1980 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1981 if (TryLock(Self) > 0) {
1982 // Increase _SpinDuration ...
1983 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1984 // Raising _SpurDuration to the poverty line is key.
1985 int x = _SpinDuration ;
1986 if (x < Knob_SpinLimit) {
1987 if (x < Knob_Poverty) x = Knob_Poverty ;
1988 _SpinDuration = x + Knob_BonusB ;
1989 }
1990 return 1 ;
1991 }
1992 SpinPause () ;
1993 }
1994
1995 // Admission control - verify preconditions for spinning
1996 //
1997 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1998 // becoming an absorbing state. Put another way, we spin briefly to
1999 // sample, just in case the system load, parallelism, contention, or lock
2000 // modality changed.
2001 //
2002 // Consider the following alternative:
2003 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2004 // spin attempt. "Periodically" might mean after a tally of
2005 // the # of failed spin attempts (or iterations) reaches some threshold.
2006 // This takes us into the realm of 1-out-of-N spinning, where we
2007 // hold the duration constant but vary the frequency.
2008
2009 ctr = _SpinDuration ;
2010 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2011 if (ctr <= 0) return 0 ;
2012
2013 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2014 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2015 TEVENT (Spin abort - notrunnable [TOP]);
2016 return 0 ;
2017 }
2018
2019 int MaxSpin = Knob_MaxSpinners ;
2020 if (MaxSpin >= 0) {
2021 if (_Spinner > MaxSpin) {
2022 TEVENT (Spin abort -- too many spinners) ;
2023 return 0 ;
2024 }
2025 // Slightly racy, but benign ...
2026 Adjust (&_Spinner, 1) ;
2027 }
2028
2029 // We're good to spin ... spin ingress.
2030 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2031 // when preparing to LD...CAS _owner, etc and the CAS is likely
2032 // to succeed.
2033 int hits = 0 ;
2034 int msk = 0 ;
2035 int caspty = Knob_CASPenalty ;
2036 int oxpty = Knob_OXPenalty ;
2037 int sss = Knob_SpinSetSucc ;
2038 if (sss && _succ == NULL ) _succ = Self ;
2039 Thread * prv = NULL ;
2040
2041 // There are three ways to exit the following loop:
2042 // 1. A successful spin where this thread has acquired the lock.
2043 // 2. Spin failure with prejudice
2044 // 3. Spin failure without prejudice
2045
2046 while (--ctr >= 0) {
2047
2048 // Periodic polling -- Check for pending GC
2049 // Threads may spin while they're unsafe.
2050 // We don't want spinning threads to delay the JVM from reaching
2051 // a stop-the-world safepoint or to steal cycles from GC.
2052 // If we detect a pending safepoint we abort in order that
2053 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2054 // this thread, if safe, doesn't steal cycles from GC.
2055 // This is in keeping with the "no loitering in runtime" rule.
2056 // We periodically check to see if there's a safepoint pending.
2057 if ((ctr & 0xFF) == 0) {
2058 if (SafepointSynchronize::do_call_back()) {
2059 TEVENT (Spin: safepoint) ;
2060 goto Abort ; // abrupt spin egress
2061 }
2062 if (Knob_UsePause & 1) SpinPause () ;
2063
2064 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2065 if (hits > 50 && scb != NULL) {
2066 int abend = (*scb)(SpinCallbackArgument, 0) ;
2067 }
2068 }
2069
2070 if (Knob_UsePause & 2) SpinPause() ;
2071
2072 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2073 // This is useful on classic SMP systems, but is of less utility on
2074 // N1-style CMT platforms.
2075 //
2076 // Trade-off: lock acquisition latency vs coherency bandwidth.
2077 // Lock hold times are typically short. A histogram
2078 // of successful spin attempts shows that we usually acquire
2079 // the lock early in the spin. That suggests we want to
2080 // sample _owner frequently in the early phase of the spin,
2081 // but then back-off and sample less frequently as the spin
2082 // progresses. The back-off makes a good citizen on SMP big
2083 // SMP systems. Oversampling _owner can consume excessive
2084 // coherency bandwidth. Relatedly, if we _oversample _owner we
2085 // can inadvertently interfere with the the ST m->owner=null.
2086 // executed by the lock owner.
2087 if (ctr & msk) continue ;
2088 ++hits ;
2089 if ((hits & 0xF) == 0) {
2090 // The 0xF, above, corresponds to the exponent.
2091 // Consider: (msk+1)|msk
2092 msk = ((msk << 2)|3) & BackOffMask ;
2093 }
2094
2095 // Probe _owner with TATAS
2096 // If this thread observes the monitor transition or flicker
2097 // from locked to unlocked to locked, then the odds that this
2098 // thread will acquire the lock in this spin attempt go down
2099 // considerably. The same argument applies if the CAS fails
2100 // or if we observe _owner change from one non-null value to
2101 // another non-null value. In such cases we might abort
2102 // the spin without prejudice or apply a "penalty" to the
2103 // spin count-down variable "ctr", reducing it by 100, say.
2104
2105 Thread * ox = (Thread *) _owner ;
2106 if (ox == NULL) {
2107 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2108 if (ox == NULL) {
2109 // The CAS succeeded -- this thread acquired ownership
2110 // Take care of some bookkeeping to exit spin state.
2111 if (sss && _succ == Self) {
2112 _succ = NULL ;
2113 }
2114 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2115
2116 // Increase _SpinDuration :
2117 // The spin was successful (profitable) so we tend toward
2118 // longer spin attempts in the future.
2119 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2120 // If we acquired the lock early in the spin cycle it
2121 // makes sense to increase _SpinDuration proportionally.
2122 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2123 int x = _SpinDuration ;
2124 if (x < Knob_SpinLimit) {
2125 if (x < Knob_Poverty) x = Knob_Poverty ;
2126 _SpinDuration = x + Knob_Bonus ;
2127 }
2128 return 1 ;
2129 }
2130
2131 // The CAS failed ... we can take any of the following actions:
2132 // * penalize: ctr -= Knob_CASPenalty
2133 // * exit spin with prejudice -- goto Abort;
2134 // * exit spin without prejudice.
2135 // * Since CAS is high-latency, retry again immediately.
2136 prv = ox ;
2137 TEVENT (Spin: cas failed) ;
2138 if (caspty == -2) break ;
2139 if (caspty == -1) goto Abort ;
2140 ctr -= caspty ;
2141 continue ;
2142 }
2143
2144 // Did lock ownership change hands ?
2145 if (ox != prv && prv != NULL ) {
2146 TEVENT (spin: Owner changed)
2147 if (oxpty == -2) break ;
2148 if (oxpty == -1) goto Abort ;
2149 ctr -= oxpty ;
2150 }
2151 prv = ox ;
2152
2153 // Abort the spin if the owner is not executing.
2154 // The owner must be executing in order to drop the lock.
2155 // Spinning while the owner is OFFPROC is idiocy.
2156 // Consider: ctr -= RunnablePenalty ;
2157 if (Knob_OState && NotRunnable (Self, ox)) {
2158 TEVENT (Spin abort - notrunnable);
2159 goto Abort ;
2160 }
2161 if (sss && _succ == NULL ) _succ = Self ;
2162 }
2163
2164 // Spin failed with prejudice -- reduce _SpinDuration.
2165 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2166 // AIMD is globally stable.
2167 TEVENT (Spin failure) ;
2168 {
2169 int x = _SpinDuration ;
2170 if (x > 0) {
2171 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2172 // This is globally sample and tends to damp the response.
2173 x -= Knob_Penalty ;
2174 if (x < 0) x = 0 ;
2175 _SpinDuration = x ;
2176 }
2177 }
2178
2179 Abort:
2180 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2181 if (sss && _succ == Self) {
2182 _succ = NULL ;
2183 // Invariant: after setting succ=null a contending thread
2184 // must recheck-retry _owner before parking. This usually happens
2185 // in the normal usage of TrySpin(), but it's safest
2186 // to make TrySpin() as foolproof as possible.
2187 OrderAccess::fence() ;
2188 if (TryLock(Self) > 0) return 1 ;
2189 }
2190 return 0 ;
2191 }
2192
2193 // NotRunnable() -- informed spinning
2194 //
2195 // Don't bother spinning if the owner is not eligible to drop the lock.
2196 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2197 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2198 // The thread must be runnable in order to drop the lock in timely fashion.
2199 // If the _owner is not runnable then spinning will not likely be
2200 // successful (profitable).
2201 //
2202 // Beware -- the thread referenced by _owner could have died
2203 // so a simply fetch from _owner->_thread_state might trap.
2204 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2205 // Because of the lifecycle issues the schedctl and _thread_state values
2206 // observed by NotRunnable() might be garbage. NotRunnable must
2207 // tolerate this and consider the observed _thread_state value
2208 // as advisory.
2209 //
2210 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2211 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2212 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2213 // with the LSB of _owner. Another option would be to probablistically probe
2214 // the putative _owner->TypeTag value.
2215 //
2216 // Checking _thread_state isn't perfect. Even if the thread is
2217 // in_java it might be blocked on a page-fault or have been preempted
2218 // and sitting on a ready/dispatch queue. _thread state in conjunction
2219 // with schedctl.sc_state gives us a good picture of what the
2220 // thread is doing, however.
2221 //
2222 // TODO: check schedctl.sc_state.
2223 // We'll need to use SafeFetch32() to read from the schedctl block.
2224 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2225 //
2226 // The return value from NotRunnable() is *advisory* -- the
2227 // result is based on sampling and is not necessarily coherent.
2228 // The caller must tolerate false-negative and false-positive errors.
2229 // Spinning, in general, is probabilistic anyway.
2230
2231
2232 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2233 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2234 if (!OwnerIsThread) return 0 ;
2235
2236 if (ox == NULL) return 0 ;
2237
2238 // Avoid transitive spinning ...
2239 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2240 // Immediately after T1 acquires L it's possible that T2, also
2241 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2242 // This occurs transiently after T1 acquired L but before
2243 // T1 managed to clear T1.Stalled. T2 does not need to abort
2244 // its spin in this circumstance.
2245 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2246
2247 if (BlockedOn == 1) return 1 ;
2248 if (BlockedOn != 0) {
2249 return BlockedOn != intptr_t(this) && _owner == ox ;
2250 }
2251
2252 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2253 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2254 // consider also: jst != _thread_in_Java -- but that's overspecific.
2255 return jst == _thread_blocked || jst == _thread_in_native ;
2256 }
2257
2258
2259 // -----------------------------------------------------------------------------
2260 // WaitSet management ...
2261
2262 ObjectWaiter::ObjectWaiter(Thread* thread) {
2263 _next = NULL;
2264 _prev = NULL;
2265 _notified = 0;
2266 TState = TS_RUN ;
2267 _thread = thread;
2268 _event = thread->_ParkEvent ;
2269 _active = false;
2270 assert (_event != NULL, "invariant") ;
2271 }
2272
2273 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2274 JavaThread *jt = (JavaThread *)this->_thread;
2275 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2276 }
2277
2278 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2279 JavaThread *jt = (JavaThread *)this->_thread;
2280 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2281 }
2282
2283 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2284 assert(node != NULL, "should not dequeue NULL node");
2285 assert(node->_prev == NULL, "node already in list");
2286 assert(node->_next == NULL, "node already in list");
2287 // put node at end of queue (circular doubly linked list)
2288 if (_WaitSet == NULL) {
2289 _WaitSet = node;
2290 node->_prev = node;
2291 node->_next = node;
2292 } else {
2293 ObjectWaiter* head = _WaitSet ;
2294 ObjectWaiter* tail = head->_prev;
2295 assert(tail->_next == head, "invariant check");
2296 tail->_next = node;
2297 head->_prev = node;
2298 node->_next = head;
2299 node->_prev = tail;
2300 }
2301 }
2302
2303 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2304 // dequeue the very first waiter
2305 ObjectWaiter* waiter = _WaitSet;
2306 if (waiter) {
2307 DequeueSpecificWaiter(waiter);
2308 }
2309 return waiter;
2310 }
2311
2312 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2313 assert(node != NULL, "should not dequeue NULL node");
2314 assert(node->_prev != NULL, "node already removed from list");
2315 assert(node->_next != NULL, "node already removed from list");
2316 // when the waiter has woken up because of interrupt,
2317 // timeout or other spurious wake-up, dequeue the
2318 // waiter from waiting list
2319 ObjectWaiter* next = node->_next;
2320 if (next == node) {
2321 assert(node->_prev == node, "invariant check");
2322 _WaitSet = NULL;
2323 } else {
2324 ObjectWaiter* prev = node->_prev;
2325 assert(prev->_next == node, "invariant check");
2326 assert(next->_prev == node, "invariant check");
2327 next->_prev = prev;
2328 prev->_next = next;
2329 if (_WaitSet == node) {
2330 _WaitSet = next;
2331 }
2332 }
2333 node->_next = NULL;
2334 node->_prev = NULL;
2335 }
2336
2337 // -----------------------------------------------------------------------------
2338 // PerfData support
2339 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2340 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2341 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2342 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2343 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2344 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2345 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2346 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2347 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2348 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2349 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2350 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2351 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2352 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2353 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2354 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2355 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2356 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2357
2358 // One-shot global initialization for the sync subsystem.
2359 // We could also defer initialization and initialize on-demand
2360 // the first time we call inflate(). Initialization would
2361 // be protected - like so many things - by the MonitorCache_lock.
2362
2363 void ObjectMonitor::Initialize () {
2364 static int InitializationCompleted = 0 ;
2365 assert (InitializationCompleted == 0, "invariant") ;
2366 InitializationCompleted = 1 ;
2367 if (UsePerfData) {
2368 EXCEPTION_MARK ;
2369 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2370 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2371 NEWPERFCOUNTER(_sync_Inflations) ;
2372 NEWPERFCOUNTER(_sync_Deflations) ;
2373 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2374 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2375 NEWPERFCOUNTER(_sync_Parks) ;
2376 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2377 NEWPERFCOUNTER(_sync_Notifications) ;
2378 NEWPERFCOUNTER(_sync_SlowEnter) ;
2379 NEWPERFCOUNTER(_sync_SlowExit) ;
2380 NEWPERFCOUNTER(_sync_SlowNotify) ;
2381 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2382 NEWPERFCOUNTER(_sync_FailedSpins) ;
2383 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2384 NEWPERFCOUNTER(_sync_PrivateA) ;
2385 NEWPERFCOUNTER(_sync_PrivateB) ;
2386 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2387 NEWPERFCOUNTER(_sync_MonScavenged) ;
2388 NEWPERFVARIABLE(_sync_MonExtant) ;
2389 #undef NEWPERFCOUNTER
2390 }
2391 }
2392
2393
2394 // Compile-time asserts
2395 // When possible, it's better to catch errors deterministically at
2396 // compile-time than at runtime. The down-side to using compile-time
2397 // asserts is that error message -- often something about negative array
2398 // indices -- is opaque.
2399
2400 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2401
2402 void ObjectMonitor::ctAsserts() {
2403 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2404 }
2405
2406
2407 static char * kvGet (char * kvList, const char * Key) {
2408 if (kvList == NULL) return NULL ;
2409 size_t n = strlen (Key) ;
2410 char * Search ;
2411 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2412 if (strncmp (Search, Key, n) == 0) {
2413 if (Search[n] == '=') return Search + n + 1 ;
2414 if (Search[n] == 0) return (char *) "1" ;
2415 }
2416 }
2417 return NULL ;
2418 }
2419
2420 static int kvGetInt (char * kvList, const char * Key, int Default) {
2421 char * v = kvGet (kvList, Key) ;
2422 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2423 if (Knob_ReportSettings && v != NULL) {
2424 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2425 ::fflush (stdout) ;
2426 }
2427 return rslt ;
2428 }
2429
2430 void ObjectMonitor::DeferredInitialize () {
2431 if (InitDone > 0) return ;
2432 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2433 while (InitDone != 1) ;
2434 return ;
2435 }
2436
2437 // One-shot global initialization ...
2438 // The initialization is idempotent, so we don't need locks.
2439 // In the future consider doing this via os::init_2().
2440 // SyncKnobs consist of <Key>=<Value> pairs in the style
2441 // of environment variables. Start by converting ':' to NUL.
2442
2443 if (SyncKnobs == NULL) SyncKnobs = "" ;
2444
2445 size_t sz = strlen (SyncKnobs) ;
2446 char * knobs = (char *) malloc (sz + 2) ;
2447 if (knobs == NULL) {
2448 vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
2449 guarantee (0, "invariant") ;
2450 }
2451 strcpy (knobs, SyncKnobs) ;
2452 knobs[sz+1] = 0 ;
2453 for (char * p = knobs ; *p ; p++) {
2454 if (*p == ':') *p = 0 ;
2455 }
2456
2457 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2458 SETKNOB(ReportSettings) ;
2459 SETKNOB(Verbose) ;
2460 SETKNOB(FixedSpin) ;
2461 SETKNOB(SpinLimit) ;
2462 SETKNOB(SpinBase) ;
2463 SETKNOB(SpinBackOff);
2464 SETKNOB(CASPenalty) ;
2465 SETKNOB(OXPenalty) ;
2466 SETKNOB(LogSpins) ;
2467 SETKNOB(SpinSetSucc) ;
2468 SETKNOB(SuccEnabled) ;
2469 SETKNOB(SuccRestrict) ;
2470 SETKNOB(Penalty) ;
2471 SETKNOB(Bonus) ;
2472 SETKNOB(BonusB) ;
2473 SETKNOB(Poverty) ;
2474 SETKNOB(SpinAfterFutile) ;
2475 SETKNOB(UsePause) ;
2476 SETKNOB(SpinEarly) ;
2477 SETKNOB(OState) ;
2478 SETKNOB(MaxSpinners) ;
2479 SETKNOB(PreSpin) ;
2480 SETKNOB(ExitPolicy) ;
2481 SETKNOB(QMode);
2482 SETKNOB(ResetEvent) ;
2483 SETKNOB(MoveNotifyee) ;
2484 SETKNOB(FastHSSEC) ;
2485 #undef SETKNOB
2486
2487 if (os::is_MP()) {
2488 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2489 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2490 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2491 } else {
2492 Knob_SpinLimit = 0 ;
2493 Knob_SpinBase = 0 ;
2494 Knob_PreSpin = 0 ;
2495 Knob_FixedSpin = -1 ;
2496 }
2497
2498 if (Knob_LogSpins == 0) {
2499 ObjectMonitor::_sync_FailedSpins = NULL ;
2500 }
2501
2502 free (knobs) ;
2503 OrderAccess::fence() ;
2504 InitDone = 1 ;
2505 }
2506
2507 #ifndef PRODUCT
2508 void ObjectMonitor::verify() {
2509 }
2510
2511 void ObjectMonitor::print() {
2512 }
2513 #endif