1 /*
2 * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/osThread.hpp"
36 #include "runtime/stubRoutines.hpp"
37 #include "runtime/thread.hpp"
38 #include "services/threadService.hpp"
39 #include "trace/tracing.hpp"
40 #include "trace/traceMacros.hpp"
41 #include "utilities/dtrace.hpp"
42 #include "utilities/preserveException.hpp"
43 #ifdef TARGET_OS_FAMILY_linux
44 # include "os_linux.inline.hpp"
45 # include "thread_linux.inline.hpp"
46 #endif
47 #ifdef TARGET_OS_FAMILY_solaris
48 # include "os_solaris.inline.hpp"
49 # include "thread_solaris.inline.hpp"
50 #endif
51 #ifdef TARGET_OS_FAMILY_windows
52 # include "os_windows.inline.hpp"
53 # include "thread_windows.inline.hpp"
54 #endif
55 #ifdef TARGET_OS_FAMILY_bsd
56 # include "os_bsd.inline.hpp"
57 # include "thread_bsd.inline.hpp"
58 #endif
59
60 #if defined(__GNUC__) && !defined(IA64)
61 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
62 #define ATTR __attribute__((noinline))
63 #else
64 #define ATTR
65 #endif
66
67
68 #ifdef DTRACE_ENABLED
69
70 // Only bother with this argument setup if dtrace is available
71 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
72
73
74 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \
75 char* bytes = NULL; \
76 int len = 0; \
77 jlong jtid = SharedRuntime::get_java_tid(thread); \
78 Symbol* klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
79 if (klassname != NULL) { \
80 bytes = (char*)klassname->bytes(); \
81 len = klassname->utf8_length(); \
82 }
83
84 #ifndef USDT2
85
86 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
87 jlong, uintptr_t, char*, int);
88 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
89 jlong, uintptr_t, char*, int);
90 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
91 jlong, uintptr_t, char*, int);
92 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
93 jlong, uintptr_t, char*, int);
94 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
95 jlong, uintptr_t, char*, int);
96
97 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
98 { \
99 if (DTraceMonitorProbes) { \
100 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
101 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
102 (monitor), bytes, len, (millis)); \
103 } \
104 }
105
106 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
107 { \
108 if (DTraceMonitorProbes) { \
109 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
110 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
111 (uintptr_t)(monitor), bytes, len); \
112 } \
113 }
114
115 #else /* USDT2 */
116
117 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
118 { \
119 if (DTraceMonitorProbes) { \
120 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
121 HOTSPOT_MONITOR_WAIT(jtid, \
122 (monitor), bytes, len, (millis)); \
123 } \
124 }
125
126 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
127 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
128 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
129 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
130 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
131
132 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
133 { \
134 if (DTraceMonitorProbes) { \
135 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
136 HOTSPOT_MONITOR_##probe(jtid, \
137 (uintptr_t)(monitor), bytes, len); \
138 } \
139 }
140
141 #endif /* USDT2 */
142 #else // ndef DTRACE_ENABLED
143
144 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
145 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;}
146
147 #endif // ndef DTRACE_ENABLED
148
149 // Tunables ...
150 // The knob* variables are effectively final. Once set they should
151 // never be modified hence. Consider using __read_mostly with GCC.
152
153 int ObjectMonitor::Knob_Verbose = 0 ;
154 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
155 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
156 static int Knob_HandOff = 0 ;
157 static int Knob_ReportSettings = 0 ;
158
159 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
160 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
161 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
162 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
163 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
164 static int Knob_SpinEarly = 1 ;
165 static int Knob_SuccEnabled = 1 ; // futile wake throttling
166 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
167 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
168 static int Knob_Bonus = 100 ; // spin success bonus
169 static int Knob_BonusB = 100 ; // spin success bonus
170 static int Knob_Penalty = 200 ; // spin failure penalty
171 static int Knob_Poverty = 1000 ;
172 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
173 static int Knob_FixedSpin = 0 ;
174 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
175 static int Knob_UsePause = 1 ;
176 static int Knob_ExitPolicy = 0 ;
177 static int Knob_PreSpin = 10 ; // 20-100 likely better
178 static int Knob_ResetEvent = 0 ;
179 static int BackOffMask = 0 ;
180
181 static int Knob_FastHSSEC = 0 ;
182 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
183 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
184 static volatile int InitDone = 0 ;
185
186 #define TrySpin TrySpin_VaryDuration
187
188 // -----------------------------------------------------------------------------
189 // Theory of operations -- Monitors lists, thread residency, etc:
190 //
191 // * A thread acquires ownership of a monitor by successfully
192 // CAS()ing the _owner field from null to non-null.
193 //
194 // * Invariant: A thread appears on at most one monitor list --
195 // cxq, EntryList or WaitSet -- at any one time.
196 //
197 // * Contending threads "push" themselves onto the cxq with CAS
198 // and then spin/park.
199 //
200 // * After a contending thread eventually acquires the lock it must
201 // dequeue itself from either the EntryList or the cxq.
202 //
203 // * The exiting thread identifies and unparks an "heir presumptive"
204 // tentative successor thread on the EntryList. Critically, the
205 // exiting thread doesn't unlink the successor thread from the EntryList.
206 // After having been unparked, the wakee will recontend for ownership of
207 // the monitor. The successor (wakee) will either acquire the lock or
208 // re-park itself.
209 //
210 // Succession is provided for by a policy of competitive handoff.
211 // The exiting thread does _not_ grant or pass ownership to the
212 // successor thread. (This is also referred to as "handoff" succession").
213 // Instead the exiting thread releases ownership and possibly wakes
214 // a successor, so the successor can (re)compete for ownership of the lock.
215 // If the EntryList is empty but the cxq is populated the exiting
216 // thread will drain the cxq into the EntryList. It does so by
217 // by detaching the cxq (installing null with CAS) and folding
218 // the threads from the cxq into the EntryList. The EntryList is
219 // doubly linked, while the cxq is singly linked because of the
220 // CAS-based "push" used to enqueue recently arrived threads (RATs).
221 //
222 // * Concurrency invariants:
223 //
224 // -- only the monitor owner may access or mutate the EntryList.
225 // The mutex property of the monitor itself protects the EntryList
226 // from concurrent interference.
227 // -- Only the monitor owner may detach the cxq.
228 //
229 // * The monitor entry list operations avoid locks, but strictly speaking
230 // they're not lock-free. Enter is lock-free, exit is not.
231 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
232 //
233 // * The cxq can have multiple concurrent "pushers" but only one concurrent
234 // detaching thread. This mechanism is immune from the ABA corruption.
235 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
236 //
237 // * Taken together, the cxq and the EntryList constitute or form a
238 // single logical queue of threads stalled trying to acquire the lock.
239 // We use two distinct lists to improve the odds of a constant-time
240 // dequeue operation after acquisition (in the ::enter() epilog) and
241 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
242 // A key desideratum is to minimize queue & monitor metadata manipulation
243 // that occurs while holding the monitor lock -- that is, we want to
244 // minimize monitor lock holds times. Note that even a small amount of
245 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
246 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
247 // locks and monitor metadata.
248 //
249 // Cxq points to the the set of Recently Arrived Threads attempting entry.
250 // Because we push threads onto _cxq with CAS, the RATs must take the form of
251 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
252 // the unlocking thread notices that EntryList is null but _cxq is != null.
253 //
254 // The EntryList is ordered by the prevailing queue discipline and
255 // can be organized in any convenient fashion, such as a doubly-linked list or
256 // a circular doubly-linked list. Critically, we want insert and delete operations
257 // to operate in constant-time. If we need a priority queue then something akin
258 // to Solaris' sleepq would work nicely. Viz.,
259 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
260 // Queue discipline is enforced at ::exit() time, when the unlocking thread
261 // drains the cxq into the EntryList, and orders or reorders the threads on the
262 // EntryList accordingly.
263 //
264 // Barring "lock barging", this mechanism provides fair cyclic ordering,
265 // somewhat similar to an elevator-scan.
266 //
267 // * The monitor synchronization subsystem avoids the use of native
268 // synchronization primitives except for the narrow platform-specific
269 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
270 // the semantics of park-unpark. Put another way, this monitor implementation
271 // depends only on atomic operations and park-unpark. The monitor subsystem
272 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
273 // underlying OS manages the READY<->RUN transitions.
274 //
275 // * Waiting threads reside on the WaitSet list -- wait() puts
276 // the caller onto the WaitSet.
277 //
278 // * notify() or notifyAll() simply transfers threads from the WaitSet to
279 // either the EntryList or cxq. Subsequent exit() operations will
280 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
281 // it's likely the notifyee would simply impale itself on the lock held
282 // by the notifier.
283 //
284 // * An interesting alternative is to encode cxq as (List,LockByte) where
285 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
286 // variable, like _recursions, in the scheme. The threads or Events that form
287 // the list would have to be aligned in 256-byte addresses. A thread would
288 // try to acquire the lock or enqueue itself with CAS, but exiting threads
289 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
290 // Note that is is *not* word-tearing, but it does presume that full-word
291 // CAS operations are coherent with intermix with STB operations. That's true
292 // on most common processors.
293 //
294 // * See also http://blogs.sun.com/dave
295
296
297 // -----------------------------------------------------------------------------
298 // Enter support
299
300 bool ObjectMonitor::try_enter(Thread* THREAD) {
301 if (THREAD != _owner) {
302 if (THREAD->is_lock_owned ((address)_owner)) {
303 assert(_recursions == 0, "internal state error");
304 _owner = THREAD ;
305 _recursions = 1 ;
306 OwnerIsThread = 1 ;
307 return true;
308 }
309 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
310 return false;
311 }
312 return true;
313 } else {
314 _recursions++;
315 return true;
316 }
317 }
318
319 void ATTR ObjectMonitor::enter(TRAPS) {
320 // The following code is ordered to check the most common cases first
321 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
322 Thread * const Self = THREAD ;
323 void * cur ;
324
325 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
326 if (cur == NULL) {
327 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
328 assert (_recursions == 0 , "invariant") ;
329 assert (_owner == Self, "invariant") ;
330 // CONSIDER: set or assert OwnerIsThread == 1
331 return ;
332 }
333
334 if (cur == Self) {
335 // TODO-FIXME: check for integer overflow! BUGID 6557169.
336 _recursions ++ ;
337 return ;
338 }
339
340 if (Self->is_lock_owned ((address)cur)) {
341 assert (_recursions == 0, "internal state error");
342 _recursions = 1 ;
343 // Commute owner from a thread-specific on-stack BasicLockObject address to
344 // a full-fledged "Thread *".
345 _owner = Self ;
346 OwnerIsThread = 1 ;
347 return ;
348 }
349
350 // We've encountered genuine contention.
351 assert (Self->_Stalled == 0, "invariant") ;
352 Self->_Stalled = intptr_t(this) ;
353
354 // Try one round of spinning *before* enqueueing Self
355 // and before going through the awkward and expensive state
356 // transitions. The following spin is strictly optional ...
357 // Note that if we acquire the monitor from an initial spin
358 // we forgo posting JVMTI events and firing DTRACE probes.
359 if (Knob_SpinEarly && TrySpin (Self) > 0) {
360 assert (_owner == Self , "invariant") ;
361 assert (_recursions == 0 , "invariant") ;
362 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
363 Self->_Stalled = 0 ;
364 return ;
365 }
366
367 assert (_owner != Self , "invariant") ;
368 assert (_succ != Self , "invariant") ;
369 assert (Self->is_Java_thread() , "invariant") ;
370 JavaThread * jt = (JavaThread *) Self ;
371 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
372 assert (jt->thread_state() != _thread_blocked , "invariant") ;
373 assert (this->object() != NULL , "invariant") ;
374 assert (_count >= 0, "invariant") ;
375
376 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
377 // Ensure the object-monitor relationship remains stable while there's contention.
378 Atomic::inc_ptr(&_count);
379
380 EventJavaMonitorEnter event;
381
382 { // Change java thread status to indicate blocked on monitor enter.
383 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
384
385 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
386 if (JvmtiExport::should_post_monitor_contended_enter()) {
387 JvmtiExport::post_monitor_contended_enter(jt, this);
388 }
389
390 OSThreadContendState osts(Self->osthread());
391 ThreadBlockInVM tbivm(jt);
392
393 Self->set_current_pending_monitor(this);
394
395 // TODO-FIXME: change the following for(;;) loop to straight-line code.
396 for (;;) {
397 jt->set_suspend_equivalent();
398 // cleared by handle_special_suspend_equivalent_condition()
399 // or java_suspend_self()
400
401 EnterI (THREAD) ;
402
403 if (!ExitSuspendEquivalent(jt)) break ;
404
405 //
406 // We have acquired the contended monitor, but while we were
407 // waiting another thread suspended us. We don't want to enter
408 // the monitor while suspended because that would surprise the
409 // thread that suspended us.
410 //
411 _recursions = 0 ;
412 _succ = NULL ;
413 exit (false, Self) ;
414
415 jt->java_suspend_self();
416 }
417 Self->set_current_pending_monitor(NULL);
418 }
419
420 Atomic::dec_ptr(&_count);
421 assert (_count >= 0, "invariant") ;
422 Self->_Stalled = 0 ;
423
424 // Must either set _recursions = 0 or ASSERT _recursions == 0.
425 assert (_recursions == 0 , "invariant") ;
426 assert (_owner == Self , "invariant") ;
427 assert (_succ != Self , "invariant") ;
428 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
429
430 // The thread -- now the owner -- is back in vm mode.
431 // Report the glorious news via TI,DTrace and jvmstat.
432 // The probe effect is non-trivial. All the reportage occurs
433 // while we hold the monitor, increasing the length of the critical
434 // section. Amdahl's parallel speedup law comes vividly into play.
435 //
436 // Another option might be to aggregate the events (thread local or
437 // per-monitor aggregation) and defer reporting until a more opportune
438 // time -- such as next time some thread encounters contention but has
439 // yet to acquire the lock. While spinning that thread could
440 // spinning we could increment JVMStat counters, etc.
441
442 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
443 if (JvmtiExport::should_post_monitor_contended_entered()) {
444 JvmtiExport::post_monitor_contended_entered(jt, this);
445 }
446
447 if (event.should_commit()) {
448 event.set_klass(((oop)this->object())->klass());
449 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
450 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
451 event.commit();
452 }
453
454 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
455 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
456 }
457 }
458
459
460 // Caveat: TryLock() is not necessarily serializing if it returns failure.
461 // Callers must compensate as needed.
462
463 int ObjectMonitor::TryLock (Thread * Self) {
464 for (;;) {
465 void * own = _owner ;
466 if (own != NULL) return 0 ;
467 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
468 // Either guarantee _recursions == 0 or set _recursions = 0.
469 assert (_recursions == 0, "invariant") ;
470 assert (_owner == Self, "invariant") ;
471 // CONSIDER: set or assert that OwnerIsThread == 1
472 return 1 ;
473 }
474 // The lock had been free momentarily, but we lost the race to the lock.
475 // Interference -- the CAS failed.
476 // We can either return -1 or retry.
477 // Retry doesn't make as much sense because the lock was just acquired.
478 if (true) return -1 ;
479 }
480 }
481
482 void ATTR ObjectMonitor::EnterI (TRAPS) {
483 Thread * Self = THREAD ;
484 assert (Self->is_Java_thread(), "invariant") ;
485 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
486
487 // Try the lock - TATAS
488 if (TryLock (Self) > 0) {
489 assert (_succ != Self , "invariant") ;
490 assert (_owner == Self , "invariant") ;
491 assert (_Responsible != Self , "invariant") ;
492 return ;
493 }
494
495 DeferredInitialize () ;
496
497 // We try one round of spinning *before* enqueueing Self.
498 //
499 // If the _owner is ready but OFFPROC we could use a YieldTo()
500 // operation to donate the remainder of this thread's quantum
501 // to the owner. This has subtle but beneficial affinity
502 // effects.
503
504 if (TrySpin (Self) > 0) {
505 assert (_owner == Self , "invariant") ;
506 assert (_succ != Self , "invariant") ;
507 assert (_Responsible != Self , "invariant") ;
508 return ;
509 }
510
511 // The Spin failed -- Enqueue and park the thread ...
512 assert (_succ != Self , "invariant") ;
513 assert (_owner != Self , "invariant") ;
514 assert (_Responsible != Self , "invariant") ;
515
516 // Enqueue "Self" on ObjectMonitor's _cxq.
517 //
518 // Node acts as a proxy for Self.
519 // As an aside, if were to ever rewrite the synchronization code mostly
520 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
521 // Java objects. This would avoid awkward lifecycle and liveness issues,
522 // as well as eliminate a subset of ABA issues.
523 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
524 //
525
526 ObjectWaiter node(Self) ;
527 Self->_ParkEvent->reset() ;
528 node._prev = (ObjectWaiter *) 0xBAD ;
529 node.TState = ObjectWaiter::TS_CXQ ;
530
531 // Push "Self" onto the front of the _cxq.
532 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
533 // Note that spinning tends to reduce the rate at which threads
534 // enqueue and dequeue on EntryList|cxq.
535 ObjectWaiter * nxt ;
536 for (;;) {
537 node._next = nxt = _cxq ;
538 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
539
540 // Interference - the CAS failed because _cxq changed. Just retry.
541 // As an optional optimization we retry the lock.
542 if (TryLock (Self) > 0) {
543 assert (_succ != Self , "invariant") ;
544 assert (_owner == Self , "invariant") ;
545 assert (_Responsible != Self , "invariant") ;
546 return ;
547 }
548 }
549
550 // Check for cxq|EntryList edge transition to non-null. This indicates
551 // the onset of contention. While contention persists exiting threads
552 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
553 // operations revert to the faster 1-0 mode. This enter operation may interleave
554 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
555 // arrange for one of the contending thread to use a timed park() operations
556 // to detect and recover from the race. (Stranding is form of progress failure
557 // where the monitor is unlocked but all the contending threads remain parked).
558 // That is, at least one of the contended threads will periodically poll _owner.
559 // One of the contending threads will become the designated "Responsible" thread.
560 // The Responsible thread uses a timed park instead of a normal indefinite park
561 // operation -- it periodically wakes and checks for and recovers from potential
562 // strandings admitted by 1-0 exit operations. We need at most one Responsible
563 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
564 // be responsible for a monitor.
565 //
566 // Currently, one of the contended threads takes on the added role of "Responsible".
567 // A viable alternative would be to use a dedicated "stranding checker" thread
568 // that periodically iterated over all the threads (or active monitors) and unparked
569 // successors where there was risk of stranding. This would help eliminate the
570 // timer scalability issues we see on some platforms as we'd only have one thread
571 // -- the checker -- parked on a timer.
572
573 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
574 // Try to assume the role of responsible thread for the monitor.
575 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
576 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
577 }
578
579 // The lock have been released while this thread was occupied queueing
580 // itself onto _cxq. To close the race and avoid "stranding" and
581 // progress-liveness failure we must resample-retry _owner before parking.
582 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
583 // In this case the ST-MEMBAR is accomplished with CAS().
584 //
585 // TODO: Defer all thread state transitions until park-time.
586 // Since state transitions are heavy and inefficient we'd like
587 // to defer the state transitions until absolutely necessary,
588 // and in doing so avoid some transitions ...
589
590 TEVENT (Inflated enter - Contention) ;
591 int nWakeups = 0 ;
592 int RecheckInterval = 1 ;
593
594 for (;;) {
595
596 if (TryLock (Self) > 0) break ;
597 assert (_owner != Self, "invariant") ;
598
599 if ((SyncFlags & 2) && _Responsible == NULL) {
600 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
601 }
602
603 // park self
604 if (_Responsible == Self || (SyncFlags & 1)) {
605 TEVENT (Inflated enter - park TIMED) ;
606 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
607 // Increase the RecheckInterval, but clamp the value.
608 RecheckInterval *= 8 ;
609 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
610 } else {
611 TEVENT (Inflated enter - park UNTIMED) ;
612 Self->_ParkEvent->park() ;
613 }
614
615 if (TryLock(Self) > 0) break ;
616
617 // The lock is still contested.
618 // Keep a tally of the # of futile wakeups.
619 // Note that the counter is not protected by a lock or updated by atomics.
620 // That is by design - we trade "lossy" counters which are exposed to
621 // races during updates for a lower probe effect.
622 TEVENT (Inflated enter - Futile wakeup) ;
623 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
624 ObjectMonitor::_sync_FutileWakeups->inc() ;
625 }
626 ++ nWakeups ;
627
628 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
629 // We can defer clearing _succ until after the spin completes
630 // TrySpin() must tolerate being called with _succ == Self.
631 // Try yet another round of adaptive spinning.
632 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
633
634 // We can find that we were unpark()ed and redesignated _succ while
635 // we were spinning. That's harmless. If we iterate and call park(),
636 // park() will consume the event and return immediately and we'll
637 // just spin again. This pattern can repeat, leaving _succ to simply
638 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
639 // Alternately, we can sample fired() here, and if set, forgo spinning
640 // in the next iteration.
641
642 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
643 Self->_ParkEvent->reset() ;
644 OrderAccess::fence() ;
645 }
646 if (_succ == Self) _succ = NULL ;
647
648 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
649 OrderAccess::fence() ;
650 }
651
652 // Egress :
653 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
654 // Normally we'll find Self on the EntryList .
655 // From the perspective of the lock owner (this thread), the
656 // EntryList is stable and cxq is prepend-only.
657 // The head of cxq is volatile but the interior is stable.
658 // In addition, Self.TState is stable.
659
660 assert (_owner == Self , "invariant") ;
661 assert (object() != NULL , "invariant") ;
662 // I'd like to write:
663 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
664 // but as we're at a safepoint that's not safe.
665
666 UnlinkAfterAcquire (Self, &node) ;
667 if (_succ == Self) _succ = NULL ;
668
669 assert (_succ != Self, "invariant") ;
670 if (_Responsible == Self) {
671 _Responsible = NULL ;
672 OrderAccess::fence(); // Dekker pivot-point
673
674 // We may leave threads on cxq|EntryList without a designated
675 // "Responsible" thread. This is benign. When this thread subsequently
676 // exits the monitor it can "see" such preexisting "old" threads --
677 // threads that arrived on the cxq|EntryList before the fence, above --
678 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
679 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
680 // non-null and elect a new "Responsible" timer thread.
681 //
682 // This thread executes:
683 // ST Responsible=null; MEMBAR (in enter epilog - here)
684 // LD cxq|EntryList (in subsequent exit)
685 //
686 // Entering threads in the slow/contended path execute:
687 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
688 // The (ST cxq; MEMBAR) is accomplished with CAS().
689 //
690 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
691 // exit operation from floating above the ST Responsible=null.
692 }
693
694 // We've acquired ownership with CAS().
695 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
696 // But since the CAS() this thread may have also stored into _succ,
697 // EntryList, cxq or Responsible. These meta-data updates must be
698 // visible __before this thread subsequently drops the lock.
699 // Consider what could occur if we didn't enforce this constraint --
700 // STs to monitor meta-data and user-data could reorder with (become
701 // visible after) the ST in exit that drops ownership of the lock.
702 // Some other thread could then acquire the lock, but observe inconsistent
703 // or old monitor meta-data and heap data. That violates the JMM.
704 // To that end, the 1-0 exit() operation must have at least STST|LDST
705 // "release" barrier semantics. Specifically, there must be at least a
706 // STST|LDST barrier in exit() before the ST of null into _owner that drops
707 // the lock. The barrier ensures that changes to monitor meta-data and data
708 // protected by the lock will be visible before we release the lock, and
709 // therefore before some other thread (CPU) has a chance to acquire the lock.
710 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
711 //
712 // Critically, any prior STs to _succ or EntryList must be visible before
713 // the ST of null into _owner in the *subsequent* (following) corresponding
714 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
715 // execute a serializing instruction.
716
717 if (SyncFlags & 8) {
718 OrderAccess::fence() ;
719 }
720 return ;
721 }
722
723 // ReenterI() is a specialized inline form of the latter half of the
724 // contended slow-path from EnterI(). We use ReenterI() only for
725 // monitor reentry in wait().
726 //
727 // In the future we should reconcile EnterI() and ReenterI(), adding
728 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
729 // loop accordingly.
730
731 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
732 assert (Self != NULL , "invariant") ;
733 assert (SelfNode != NULL , "invariant") ;
734 assert (SelfNode->_thread == Self , "invariant") ;
735 assert (_waiters > 0 , "invariant") ;
736 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
737 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
738 JavaThread * jt = (JavaThread *) Self ;
739
740 int nWakeups = 0 ;
741 for (;;) {
742 ObjectWaiter::TStates v = SelfNode->TState ;
743 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
744 assert (_owner != Self, "invariant") ;
745
746 if (TryLock (Self) > 0) break ;
747 if (TrySpin (Self) > 0) break ;
748
749 TEVENT (Wait Reentry - parking) ;
750
751 // State transition wrappers around park() ...
752 // ReenterI() wisely defers state transitions until
753 // it's clear we must park the thread.
754 {
755 OSThreadContendState osts(Self->osthread());
756 ThreadBlockInVM tbivm(jt);
757
758 // cleared by handle_special_suspend_equivalent_condition()
759 // or java_suspend_self()
760 jt->set_suspend_equivalent();
761 if (SyncFlags & 1) {
762 Self->_ParkEvent->park ((jlong)1000) ;
763 } else {
764 Self->_ParkEvent->park () ;
765 }
766
767 // were we externally suspended while we were waiting?
768 for (;;) {
769 if (!ExitSuspendEquivalent (jt)) break ;
770 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
771 jt->java_suspend_self();
772 jt->set_suspend_equivalent();
773 }
774 }
775
776 // Try again, but just so we distinguish between futile wakeups and
777 // successful wakeups. The following test isn't algorithmically
778 // necessary, but it helps us maintain sensible statistics.
779 if (TryLock(Self) > 0) break ;
780
781 // The lock is still contested.
782 // Keep a tally of the # of futile wakeups.
783 // Note that the counter is not protected by a lock or updated by atomics.
784 // That is by design - we trade "lossy" counters which are exposed to
785 // races during updates for a lower probe effect.
786 TEVENT (Wait Reentry - futile wakeup) ;
787 ++ nWakeups ;
788
789 // Assuming this is not a spurious wakeup we'll normally
790 // find that _succ == Self.
791 if (_succ == Self) _succ = NULL ;
792
793 // Invariant: after clearing _succ a contending thread
794 // *must* retry _owner before parking.
795 OrderAccess::fence() ;
796
797 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
798 ObjectMonitor::_sync_FutileWakeups->inc() ;
799 }
800 }
801
802 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
803 // Normally we'll find Self on the EntryList.
804 // Unlinking from the EntryList is constant-time and atomic-free.
805 // From the perspective of the lock owner (this thread), the
806 // EntryList is stable and cxq is prepend-only.
807 // The head of cxq is volatile but the interior is stable.
808 // In addition, Self.TState is stable.
809
810 assert (_owner == Self, "invariant") ;
811 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
812 UnlinkAfterAcquire (Self, SelfNode) ;
813 if (_succ == Self) _succ = NULL ;
814 assert (_succ != Self, "invariant") ;
815 SelfNode->TState = ObjectWaiter::TS_RUN ;
816 OrderAccess::fence() ; // see comments at the end of EnterI()
817 }
818
819 // after the thread acquires the lock in ::enter(). Equally, we could defer
820 // unlinking the thread until ::exit()-time.
821
822 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
823 {
824 assert (_owner == Self, "invariant") ;
825 assert (SelfNode->_thread == Self, "invariant") ;
826
827 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
828 // Normal case: remove Self from the DLL EntryList .
829 // This is a constant-time operation.
830 ObjectWaiter * nxt = SelfNode->_next ;
831 ObjectWaiter * prv = SelfNode->_prev ;
832 if (nxt != NULL) nxt->_prev = prv ;
833 if (prv != NULL) prv->_next = nxt ;
834 if (SelfNode == _EntryList ) _EntryList = nxt ;
835 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
836 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
837 TEVENT (Unlink from EntryList) ;
838 } else {
839 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
840 // Inopportune interleaving -- Self is still on the cxq.
841 // This usually means the enqueue of self raced an exiting thread.
842 // Normally we'll find Self near the front of the cxq, so
843 // dequeueing is typically fast. If needbe we can accelerate
844 // this with some MCS/CHL-like bidirectional list hints and advisory
845 // back-links so dequeueing from the interior will normally operate
846 // in constant-time.
847 // Dequeue Self from either the head (with CAS) or from the interior
848 // with a linear-time scan and normal non-atomic memory operations.
849 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
850 // and then unlink Self from EntryList. We have to drain eventually,
851 // so it might as well be now.
852
853 ObjectWaiter * v = _cxq ;
854 assert (v != NULL, "invariant") ;
855 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
856 // The CAS above can fail from interference IFF a "RAT" arrived.
857 // In that case Self must be in the interior and can no longer be
858 // at the head of cxq.
859 if (v == SelfNode) {
860 assert (_cxq != v, "invariant") ;
861 v = _cxq ; // CAS above failed - start scan at head of list
862 }
863 ObjectWaiter * p ;
864 ObjectWaiter * q = NULL ;
865 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
866 q = p ;
867 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
868 }
869 assert (v != SelfNode, "invariant") ;
870 assert (p == SelfNode, "Node not found on cxq") ;
871 assert (p != _cxq, "invariant") ;
872 assert (q != NULL, "invariant") ;
873 assert (q->_next == p, "invariant") ;
874 q->_next = p->_next ;
875 }
876 TEVENT (Unlink from cxq) ;
877 }
878
879 // Diagnostic hygiene ...
880 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
881 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
882 SelfNode->TState = ObjectWaiter::TS_RUN ;
883 }
884
885 // -----------------------------------------------------------------------------
886 // Exit support
887 //
888 // exit()
889 // ~~~~~~
890 // Note that the collector can't reclaim the objectMonitor or deflate
891 // the object out from underneath the thread calling ::exit() as the
892 // thread calling ::exit() never transitions to a stable state.
893 // This inhibits GC, which in turn inhibits asynchronous (and
894 // inopportune) reclamation of "this".
895 //
896 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
897 // There's one exception to the claim above, however. EnterI() can call
898 // exit() to drop a lock if the acquirer has been externally suspended.
899 // In that case exit() is called with _thread_state as _thread_blocked,
900 // but the monitor's _count field is > 0, which inhibits reclamation.
901 //
902 // 1-0 exit
903 // ~~~~~~~~
904 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
905 // the fast-path operators have been optimized so the common ::exit()
906 // operation is 1-0. See i486.ad fast_unlock(), for instance.
907 // The code emitted by fast_unlock() elides the usual MEMBAR. This
908 // greatly improves latency -- MEMBAR and CAS having considerable local
909 // latency on modern processors -- but at the cost of "stranding". Absent the
910 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
911 // ::enter() path, resulting in the entering thread being stranding
912 // and a progress-liveness failure. Stranding is extremely rare.
913 // We use timers (timed park operations) & periodic polling to detect
914 // and recover from stranding. Potentially stranded threads periodically
915 // wake up and poll the lock. See the usage of the _Responsible variable.
916 //
917 // The CAS() in enter provides for safety and exclusion, while the CAS or
918 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
919 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
920 // We detect and recover from stranding with timers.
921 //
922 // If a thread transiently strands it'll park until (a) another
923 // thread acquires the lock and then drops the lock, at which time the
924 // exiting thread will notice and unpark the stranded thread, or, (b)
925 // the timer expires. If the lock is high traffic then the stranding latency
926 // will be low due to (a). If the lock is low traffic then the odds of
927 // stranding are lower, although the worst-case stranding latency
928 // is longer. Critically, we don't want to put excessive load in the
929 // platform's timer subsystem. We want to minimize both the timer injection
930 // rate (timers created/sec) as well as the number of timers active at
931 // any one time. (more precisely, we want to minimize timer-seconds, which is
932 // the integral of the # of active timers at any instant over time).
933 // Both impinge on OS scalability. Given that, at most one thread parked on
934 // a monitor will use a timer.
935
936 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
937 Thread * Self = THREAD ;
938 if (THREAD != _owner) {
939 if (THREAD->is_lock_owned((address) _owner)) {
940 // Transmute _owner from a BasicLock pointer to a Thread address.
941 // We don't need to hold _mutex for this transition.
942 // Non-null to Non-null is safe as long as all readers can
943 // tolerate either flavor.
944 assert (_recursions == 0, "invariant") ;
945 _owner = THREAD ;
946 _recursions = 0 ;
947 OwnerIsThread = 1 ;
948 } else {
949 // NOTE: we need to handle unbalanced monitor enter/exit
950 // in native code by throwing an exception.
951 // TODO: Throw an IllegalMonitorStateException ?
952 TEVENT (Exit - Throw IMSX) ;
953 assert(false, "Non-balanced monitor enter/exit!");
954 if (false) {
955 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
956 }
957 return;
958 }
959 }
960
961 if (_recursions != 0) {
962 _recursions--; // this is simple recursive enter
963 TEVENT (Inflated exit - recursive) ;
964 return ;
965 }
966
967 // Invariant: after setting Responsible=null an thread must execute
968 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
969 if ((SyncFlags & 4) == 0) {
970 _Responsible = NULL ;
971 }
972
973 #if INCLUDE_TRACE
974 // get the owner's thread id for the MonitorEnter event
975 // if it is enabled and the thread isn't suspended
976 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
977 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
978 }
979 #endif
980
981 for (;;) {
982 assert (THREAD == _owner, "invariant") ;
983
984
985 if (Knob_ExitPolicy == 0) {
986 // release semantics: prior loads and stores from within the critical section
987 // must not float (reorder) past the following store that drops the lock.
988 // On SPARC that requires MEMBAR #loadstore|#storestore.
989 // But of course in TSO #loadstore|#storestore is not required.
990 // I'd like to write one of the following:
991 // A. OrderAccess::release() ; _owner = NULL
992 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
993 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
994 // store into a _dummy variable. That store is not needed, but can result
995 // in massive wasteful coherency traffic on classic SMP systems.
996 // Instead, I use release_store(), which is implemented as just a simple
997 // ST on x64, x86 and SPARC.
998 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
999 OrderAccess::storeload() ; // See if we need to wake a successor
1000 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1001 TEVENT (Inflated exit - simple egress) ;
1002 return ;
1003 }
1004 TEVENT (Inflated exit - complex egress) ;
1005
1006 // Normally the exiting thread is responsible for ensuring succession,
1007 // but if other successors are ready or other entering threads are spinning
1008 // then this thread can simply store NULL into _owner and exit without
1009 // waking a successor. The existence of spinners or ready successors
1010 // guarantees proper succession (liveness). Responsibility passes to the
1011 // ready or running successors. The exiting thread delegates the duty.
1012 // More precisely, if a successor already exists this thread is absolved
1013 // of the responsibility of waking (unparking) one.
1014 //
1015 // The _succ variable is critical to reducing futile wakeup frequency.
1016 // _succ identifies the "heir presumptive" thread that has been made
1017 // ready (unparked) but that has not yet run. We need only one such
1018 // successor thread to guarantee progress.
1019 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1020 // section 3.3 "Futile Wakeup Throttling" for details.
1021 //
1022 // Note that spinners in Enter() also set _succ non-null.
1023 // In the current implementation spinners opportunistically set
1024 // _succ so that exiting threads might avoid waking a successor.
1025 // Another less appealing alternative would be for the exiting thread
1026 // to drop the lock and then spin briefly to see if a spinner managed
1027 // to acquire the lock. If so, the exiting thread could exit
1028 // immediately without waking a successor, otherwise the exiting
1029 // thread would need to dequeue and wake a successor.
1030 // (Note that we'd need to make the post-drop spin short, but no
1031 // shorter than the worst-case round-trip cache-line migration time.
1032 // The dropped lock needs to become visible to the spinner, and then
1033 // the acquisition of the lock by the spinner must become visible to
1034 // the exiting thread).
1035 //
1036
1037 // It appears that an heir-presumptive (successor) must be made ready.
1038 // Only the current lock owner can manipulate the EntryList or
1039 // drain _cxq, so we need to reacquire the lock. If we fail
1040 // to reacquire the lock the responsibility for ensuring succession
1041 // falls to the new owner.
1042 //
1043 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1044 return ;
1045 }
1046 TEVENT (Exit - Reacquired) ;
1047 } else {
1048 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1049 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1050 OrderAccess::storeload() ;
1051 // Ratify the previously observed values.
1052 if (_cxq == NULL || _succ != NULL) {
1053 TEVENT (Inflated exit - simple egress) ;
1054 return ;
1055 }
1056
1057 // inopportune interleaving -- the exiting thread (this thread)
1058 // in the fast-exit path raced an entering thread in the slow-enter
1059 // path.
1060 // We have two choices:
1061 // A. Try to reacquire the lock.
1062 // If the CAS() fails return immediately, otherwise
1063 // we either restart/rerun the exit operation, or simply
1064 // fall-through into the code below which wakes a successor.
1065 // B. If the elements forming the EntryList|cxq are TSM
1066 // we could simply unpark() the lead thread and return
1067 // without having set _succ.
1068 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1069 TEVENT (Inflated exit - reacquired succeeded) ;
1070 return ;
1071 }
1072 TEVENT (Inflated exit - reacquired failed) ;
1073 } else {
1074 TEVENT (Inflated exit - complex egress) ;
1075 }
1076 }
1077
1078 guarantee (_owner == THREAD, "invariant") ;
1079
1080 ObjectWaiter * w = NULL ;
1081 int QMode = Knob_QMode ;
1082
1083 if (QMode == 2 && _cxq != NULL) {
1084 // QMode == 2 : cxq has precedence over EntryList.
1085 // Try to directly wake a successor from the cxq.
1086 // If successful, the successor will need to unlink itself from cxq.
1087 w = _cxq ;
1088 assert (w != NULL, "invariant") ;
1089 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1090 ExitEpilog (Self, w) ;
1091 return ;
1092 }
1093
1094 if (QMode == 3 && _cxq != NULL) {
1095 // Aggressively drain cxq into EntryList at the first opportunity.
1096 // This policy ensure that recently-run threads live at the head of EntryList.
1097 // Drain _cxq into EntryList - bulk transfer.
1098 // First, detach _cxq.
1099 // The following loop is tantamount to: w = swap (&cxq, NULL)
1100 w = _cxq ;
1101 for (;;) {
1102 assert (w != NULL, "Invariant") ;
1103 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1104 if (u == w) break ;
1105 w = u ;
1106 }
1107 assert (w != NULL , "invariant") ;
1108
1109 ObjectWaiter * q = NULL ;
1110 ObjectWaiter * p ;
1111 for (p = w ; p != NULL ; p = p->_next) {
1112 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1113 p->TState = ObjectWaiter::TS_ENTER ;
1114 p->_prev = q ;
1115 q = p ;
1116 }
1117
1118 // Append the RATs to the EntryList
1119 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1120 ObjectWaiter * Tail ;
1121 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1122 if (Tail == NULL) {
1123 _EntryList = w ;
1124 } else {
1125 Tail->_next = w ;
1126 w->_prev = Tail ;
1127 }
1128
1129 // Fall thru into code that tries to wake a successor from EntryList
1130 }
1131
1132 if (QMode == 4 && _cxq != NULL) {
1133 // Aggressively drain cxq into EntryList at the first opportunity.
1134 // This policy ensure that recently-run threads live at the head of EntryList.
1135
1136 // Drain _cxq into EntryList - bulk transfer.
1137 // First, detach _cxq.
1138 // The following loop is tantamount to: w = swap (&cxq, NULL)
1139 w = _cxq ;
1140 for (;;) {
1141 assert (w != NULL, "Invariant") ;
1142 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1143 if (u == w) break ;
1144 w = u ;
1145 }
1146 assert (w != NULL , "invariant") ;
1147
1148 ObjectWaiter * q = NULL ;
1149 ObjectWaiter * p ;
1150 for (p = w ; p != NULL ; p = p->_next) {
1151 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1152 p->TState = ObjectWaiter::TS_ENTER ;
1153 p->_prev = q ;
1154 q = p ;
1155 }
1156
1157 // Prepend the RATs to the EntryList
1158 if (_EntryList != NULL) {
1159 q->_next = _EntryList ;
1160 _EntryList->_prev = q ;
1161 }
1162 _EntryList = w ;
1163
1164 // Fall thru into code that tries to wake a successor from EntryList
1165 }
1166
1167 w = _EntryList ;
1168 if (w != NULL) {
1169 // I'd like to write: guarantee (w->_thread != Self).
1170 // But in practice an exiting thread may find itself on the EntryList.
1171 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1172 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1173 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1174 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1175 // release the lock "O". T2 resumes immediately after the ST of null into
1176 // _owner, above. T2 notices that the EntryList is populated, so it
1177 // reacquires the lock and then finds itself on the EntryList.
1178 // Given all that, we have to tolerate the circumstance where "w" is
1179 // associated with Self.
1180 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1181 ExitEpilog (Self, w) ;
1182 return ;
1183 }
1184
1185 // If we find that both _cxq and EntryList are null then just
1186 // re-run the exit protocol from the top.
1187 w = _cxq ;
1188 if (w == NULL) continue ;
1189
1190 // Drain _cxq into EntryList - bulk transfer.
1191 // First, detach _cxq.
1192 // The following loop is tantamount to: w = swap (&cxq, NULL)
1193 for (;;) {
1194 assert (w != NULL, "Invariant") ;
1195 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1196 if (u == w) break ;
1197 w = u ;
1198 }
1199 TEVENT (Inflated exit - drain cxq into EntryList) ;
1200
1201 assert (w != NULL , "invariant") ;
1202 assert (_EntryList == NULL , "invariant") ;
1203
1204 // Convert the LIFO SLL anchored by _cxq into a DLL.
1205 // The list reorganization step operates in O(LENGTH(w)) time.
1206 // It's critical that this step operate quickly as
1207 // "Self" still holds the outer-lock, restricting parallelism
1208 // and effectively lengthening the critical section.
1209 // Invariant: s chases t chases u.
1210 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1211 // we have faster access to the tail.
1212
1213 if (QMode == 1) {
1214 // QMode == 1 : drain cxq to EntryList, reversing order
1215 // We also reverse the order of the list.
1216 ObjectWaiter * s = NULL ;
1217 ObjectWaiter * t = w ;
1218 ObjectWaiter * u = NULL ;
1219 while (t != NULL) {
1220 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1221 t->TState = ObjectWaiter::TS_ENTER ;
1222 u = t->_next ;
1223 t->_prev = u ;
1224 t->_next = s ;
1225 s = t;
1226 t = u ;
1227 }
1228 _EntryList = s ;
1229 assert (s != NULL, "invariant") ;
1230 } else {
1231 // QMode == 0 or QMode == 2
1232 _EntryList = w ;
1233 ObjectWaiter * q = NULL ;
1234 ObjectWaiter * p ;
1235 for (p = w ; p != NULL ; p = p->_next) {
1236 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1237 p->TState = ObjectWaiter::TS_ENTER ;
1238 p->_prev = q ;
1239 q = p ;
1240 }
1241 }
1242
1243 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1244 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1245
1246 // See if we can abdicate to a spinner instead of waking a thread.
1247 // A primary goal of the implementation is to reduce the
1248 // context-switch rate.
1249 if (_succ != NULL) continue;
1250
1251 w = _EntryList ;
1252 if (w != NULL) {
1253 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1254 ExitEpilog (Self, w) ;
1255 return ;
1256 }
1257 }
1258 }
1259
1260 // ExitSuspendEquivalent:
1261 // A faster alternate to handle_special_suspend_equivalent_condition()
1262 //
1263 // handle_special_suspend_equivalent_condition() unconditionally
1264 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1265 // operations have high latency. Note that in ::enter() we call HSSEC
1266 // while holding the monitor, so we effectively lengthen the critical sections.
1267 //
1268 // There are a number of possible solutions:
1269 //
1270 // A. To ameliorate the problem we might also defer state transitions
1271 // to as late as possible -- just prior to parking.
1272 // Given that, we'd call HSSEC after having returned from park(),
1273 // but before attempting to acquire the monitor. This is only a
1274 // partial solution. It avoids calling HSSEC while holding the
1275 // monitor (good), but it still increases successor reacquisition latency --
1276 // the interval between unparking a successor and the time the successor
1277 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1278 // If we use this technique we can also avoid EnterI()-exit() loop
1279 // in ::enter() where we iteratively drop the lock and then attempt
1280 // to reacquire it after suspending.
1281 //
1282 // B. In the future we might fold all the suspend bits into a
1283 // composite per-thread suspend flag and then update it with CAS().
1284 // Alternately, a Dekker-like mechanism with multiple variables
1285 // would suffice:
1286 // ST Self->_suspend_equivalent = false
1287 // MEMBAR
1288 // LD Self_>_suspend_flags
1289 //
1290
1291
1292 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1293 int Mode = Knob_FastHSSEC ;
1294 if (Mode && !jSelf->is_external_suspend()) {
1295 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1296 jSelf->clear_suspend_equivalent() ;
1297 if (2 == Mode) OrderAccess::storeload() ;
1298 if (!jSelf->is_external_suspend()) return false ;
1299 // We raced a suspension -- fall thru into the slow path
1300 TEVENT (ExitSuspendEquivalent - raced) ;
1301 jSelf->set_suspend_equivalent() ;
1302 }
1303 return jSelf->handle_special_suspend_equivalent_condition() ;
1304 }
1305
1306
1307 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1308 assert (_owner == Self, "invariant") ;
1309
1310 // Exit protocol:
1311 // 1. ST _succ = wakee
1312 // 2. membar #loadstore|#storestore;
1313 // 2. ST _owner = NULL
1314 // 3. unpark(wakee)
1315
1316 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1317 ParkEvent * Trigger = Wakee->_event ;
1318
1319 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1320 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1321 // out-of-scope (non-extant).
1322 Wakee = NULL ;
1323
1324 // Drop the lock
1325 OrderAccess::release_store_ptr (&_owner, NULL) ;
1326 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1327
1328 if (SafepointSynchronize::do_call_back()) {
1329 TEVENT (unpark before SAFEPOINT) ;
1330 }
1331
1332 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1333 Trigger->unpark() ;
1334
1335 // Maintain stats and report events to JVMTI
1336 if (ObjectMonitor::_sync_Parks != NULL) {
1337 ObjectMonitor::_sync_Parks->inc() ;
1338 }
1339 }
1340
1341
1342 // -----------------------------------------------------------------------------
1343 // Class Loader deadlock handling.
1344 //
1345 // complete_exit exits a lock returning recursion count
1346 // complete_exit/reenter operate as a wait without waiting
1347 // complete_exit requires an inflated monitor
1348 // The _owner field is not always the Thread addr even with an
1349 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1350 // thread due to contention.
1351 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1352 Thread * const Self = THREAD;
1353 assert(Self->is_Java_thread(), "Must be Java thread!");
1354 JavaThread *jt = (JavaThread *)THREAD;
1355
1356 DeferredInitialize();
1357
1358 if (THREAD != _owner) {
1359 if (THREAD->is_lock_owned ((address)_owner)) {
1360 assert(_recursions == 0, "internal state error");
1361 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1362 _recursions = 0 ;
1363 OwnerIsThread = 1 ;
1364 }
1365 }
1366
1367 guarantee(Self == _owner, "complete_exit not owner");
1368 intptr_t save = _recursions; // record the old recursion count
1369 _recursions = 0; // set the recursion level to be 0
1370 exit (true, Self) ; // exit the monitor
1371 guarantee (_owner != Self, "invariant");
1372 return save;
1373 }
1374
1375 // reenter() enters a lock and sets recursion count
1376 // complete_exit/reenter operate as a wait without waiting
1377 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1378 Thread * const Self = THREAD;
1379 assert(Self->is_Java_thread(), "Must be Java thread!");
1380 JavaThread *jt = (JavaThread *)THREAD;
1381
1382 guarantee(_owner != Self, "reenter already owner");
1383 enter (THREAD); // enter the monitor
1384 guarantee (_recursions == 0, "reenter recursion");
1385 _recursions = recursions;
1386 return;
1387 }
1388
1389
1390 // -----------------------------------------------------------------------------
1391 // A macro is used below because there may already be a pending
1392 // exception which should not abort the execution of the routines
1393 // which use this (which is why we don't put this into check_slow and
1394 // call it with a CHECK argument).
1395
1396 #define CHECK_OWNER() \
1397 do { \
1398 if (THREAD != _owner) { \
1399 if (THREAD->is_lock_owned((address) _owner)) { \
1400 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1401 _recursions = 0; \
1402 OwnerIsThread = 1 ; \
1403 } else { \
1404 TEVENT (Throw IMSX) ; \
1405 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1406 } \
1407 } \
1408 } while (false)
1409
1410 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1411 // TODO-FIXME: remove check_slow() -- it's likely dead.
1412
1413 void ObjectMonitor::check_slow(TRAPS) {
1414 TEVENT (check_slow - throw IMSX) ;
1415 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1416 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1417 }
1418
1419 static int Adjust (volatile int * adr, int dx) {
1420 int v ;
1421 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1422 return v ;
1423 }
1424
1425 // helper method for posting a monitor wait event
1426 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1427 jlong notifier_tid,
1428 jlong timeout,
1429 bool timedout) {
1430 event->set_klass(((oop)this->object())->klass());
1431 event->set_timeout((TYPE_ULONG)timeout);
1432 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1433 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1434 event->set_timedOut((TYPE_BOOLEAN)timedout);
1435 event->commit();
1436 }
1437
1438 // -----------------------------------------------------------------------------
1439 // Wait/Notify/NotifyAll
1440 //
1441 // Note: a subset of changes to ObjectMonitor::wait()
1442 // will need to be replicated in complete_exit above
1443 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1444 Thread * const Self = THREAD ;
1445 assert(Self->is_Java_thread(), "Must be Java thread!");
1446 JavaThread *jt = (JavaThread *)THREAD;
1447
1448 DeferredInitialize () ;
1449
1450 // Throw IMSX or IEX.
1451 CHECK_OWNER();
1452
1453 EventJavaMonitorWait event;
1454
1455 // check for a pending interrupt
1456 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1457 // post monitor waited event. Note that this is past-tense, we are done waiting.
1458 if (JvmtiExport::should_post_monitor_waited()) {
1459 // Note: 'false' parameter is passed here because the
1460 // wait was not timed out due to thread interrupt.
1461 JvmtiExport::post_monitor_waited(jt, this, false);
1462 }
1463 if (event.should_commit()) {
1464 post_monitor_wait_event(&event, 0, millis, false);
1465 }
1466 TEVENT (Wait - Throw IEX) ;
1467 THROW(vmSymbols::java_lang_InterruptedException());
1468 return ;
1469 }
1470
1471 TEVENT (Wait) ;
1472
1473 assert (Self->_Stalled == 0, "invariant") ;
1474 Self->_Stalled = intptr_t(this) ;
1475 jt->set_current_waiting_monitor(this);
1476
1477 // create a node to be put into the queue
1478 // Critically, after we reset() the event but prior to park(), we must check
1479 // for a pending interrupt.
1480 ObjectWaiter node(Self);
1481 node.TState = ObjectWaiter::TS_WAIT ;
1482 Self->_ParkEvent->reset() ;
1483 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1484
1485 // Enter the waiting queue, which is a circular doubly linked list in this case
1486 // but it could be a priority queue or any data structure.
1487 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1488 // by the the owner of the monitor *except* in the case where park()
1489 // returns because of a timeout of interrupt. Contention is exceptionally rare
1490 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1491
1492 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1493 AddWaiter (&node) ;
1494 Thread::SpinRelease (&_WaitSetLock) ;
1495
1496 if ((SyncFlags & 4) == 0) {
1497 _Responsible = NULL ;
1498 }
1499 intptr_t save = _recursions; // record the old recursion count
1500 _waiters++; // increment the number of waiters
1501 _recursions = 0; // set the recursion level to be 1
1502 exit (true, Self) ; // exit the monitor
1503 guarantee (_owner != Self, "invariant") ;
1504
1505 // As soon as the ObjectMonitor's ownership is dropped in the exit()
1506 // call above, another thread can enter() the ObjectMonitor, do the
1507 // notify(), and exit() the ObjectMonitor. If the other thread's
1508 // exit() call chooses this thread as the successor and the unpark()
1509 // call happens to occur while this thread is posting a
1510 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
1511 // handler using RawMonitors and consuming the unpark().
1512 //
1513 // To avoid the problem, we re-post the event. This does no harm
1514 // even if the original unpark() was not consumed because we are the
1515 // chosen successor for this monitor.
1516 if (node._notified != 0 && _succ == Self) {
1517 node._event->unpark();
1518 }
1519
1520 // The thread is on the WaitSet list - now park() it.
1521 // On MP systems it's conceivable that a brief spin before we park
1522 // could be profitable.
1523 //
1524 // TODO-FIXME: change the following logic to a loop of the form
1525 // while (!timeout && !interrupted && _notified == 0) park()
1526
1527 int ret = OS_OK ;
1528 int WasNotified = 0 ;
1529 { // State transition wrappers
1530 OSThread* osthread = Self->osthread();
1531 OSThreadWaitState osts(osthread, true);
1532 {
1533 ThreadBlockInVM tbivm(jt);
1534 // Thread is in thread_blocked state and oop access is unsafe.
1535 jt->set_suspend_equivalent();
1536
1537 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1538 // Intentionally empty
1539 } else
1540 if (node._notified == 0) {
1541 if (millis <= 0) {
1542 Self->_ParkEvent->park () ;
1543 } else {
1544 ret = Self->_ParkEvent->park (millis) ;
1545 }
1546 }
1547
1548 // were we externally suspended while we were waiting?
1549 if (ExitSuspendEquivalent (jt)) {
1550 // TODO-FIXME: add -- if succ == Self then succ = null.
1551 jt->java_suspend_self();
1552 }
1553
1554 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1555
1556
1557 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1558 // from the WaitSet to the EntryList.
1559 // See if we need to remove Node from the WaitSet.
1560 // We use double-checked locking to avoid grabbing _WaitSetLock
1561 // if the thread is not on the wait queue.
1562 //
1563 // Note that we don't need a fence before the fetch of TState.
1564 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1565 // written by the is thread. (perhaps the fetch might even be satisfied
1566 // by a look-aside into the processor's own store buffer, although given
1567 // the length of the code path between the prior ST and this load that's
1568 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1569 // then we'll acquire the lock and then re-fetch a fresh TState value.
1570 // That is, we fail toward safety.
1571
1572 if (node.TState == ObjectWaiter::TS_WAIT) {
1573 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1574 if (node.TState == ObjectWaiter::TS_WAIT) {
1575 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1576 assert(node._notified == 0, "invariant");
1577 node.TState = ObjectWaiter::TS_RUN ;
1578 }
1579 Thread::SpinRelease (&_WaitSetLock) ;
1580 }
1581
1582 // The thread is now either on off-list (TS_RUN),
1583 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1584 // The Node's TState variable is stable from the perspective of this thread.
1585 // No other threads will asynchronously modify TState.
1586 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1587 OrderAccess::loadload() ;
1588 if (_succ == Self) _succ = NULL ;
1589 WasNotified = node._notified ;
1590
1591 // Reentry phase -- reacquire the monitor.
1592 // re-enter contended monitor after object.wait().
1593 // retain OBJECT_WAIT state until re-enter successfully completes
1594 // Thread state is thread_in_vm and oop access is again safe,
1595 // although the raw address of the object may have changed.
1596 // (Don't cache naked oops over safepoints, of course).
1597
1598 // post monitor waited event. Note that this is past-tense, we are done waiting.
1599 if (JvmtiExport::should_post_monitor_waited()) {
1600 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1601 }
1602
1603 if (event.should_commit()) {
1604 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1605 }
1606
1607 OrderAccess::fence() ;
1608
1609 assert (Self->_Stalled != 0, "invariant") ;
1610 Self->_Stalled = 0 ;
1611
1612 assert (_owner != Self, "invariant") ;
1613 ObjectWaiter::TStates v = node.TState ;
1614 if (v == ObjectWaiter::TS_RUN) {
1615 enter (Self) ;
1616 } else {
1617 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1618 ReenterI (Self, &node) ;
1619 node.wait_reenter_end(this);
1620 }
1621
1622 // Self has reacquired the lock.
1623 // Lifecycle - the node representing Self must not appear on any queues.
1624 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1625 // want residual elements associated with this thread left on any lists.
1626 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1627 assert (_owner == Self, "invariant") ;
1628 assert (_succ != Self , "invariant") ;
1629 } // OSThreadWaitState()
1630
1631 jt->set_current_waiting_monitor(NULL);
1632
1633 guarantee (_recursions == 0, "invariant") ;
1634 _recursions = save; // restore the old recursion count
1635 _waiters--; // decrement the number of waiters
1636
1637 // Verify a few postconditions
1638 assert (_owner == Self , "invariant") ;
1639 assert (_succ != Self , "invariant") ;
1640 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1641
1642 if (SyncFlags & 32) {
1643 OrderAccess::fence() ;
1644 }
1645
1646 // check if the notification happened
1647 if (!WasNotified) {
1648 // no, it could be timeout or Thread.interrupt() or both
1649 // check for interrupt event, otherwise it is timeout
1650 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1651 TEVENT (Wait - throw IEX from epilog) ;
1652 THROW(vmSymbols::java_lang_InterruptedException());
1653 }
1654 }
1655
1656 // NOTE: Spurious wake up will be consider as timeout.
1657 // Monitor notify has precedence over thread interrupt.
1658 }
1659
1660
1661 // Consider:
1662 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1663 // then instead of transferring a thread from the WaitSet to the EntryList
1664 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1665
1666 void ObjectMonitor::notify(TRAPS) {
1667 CHECK_OWNER();
1668 if (_WaitSet == NULL) {
1669 TEVENT (Empty-Notify) ;
1670 return ;
1671 }
1672 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1673
1674 int Policy = Knob_MoveNotifyee ;
1675
1676 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1677 ObjectWaiter * iterator = DequeueWaiter() ;
1678 if (iterator != NULL) {
1679 TEVENT (Notify1 - Transfer) ;
1680 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1681 guarantee (iterator->_notified == 0, "invariant") ;
1682 if (Policy != 4) {
1683 iterator->TState = ObjectWaiter::TS_ENTER ;
1684 }
1685 iterator->_notified = 1 ;
1686 Thread * Self = THREAD;
1687 iterator->_notifier_tid = Self->osthread()->thread_id();
1688
1689 ObjectWaiter * List = _EntryList ;
1690 if (List != NULL) {
1691 assert (List->_prev == NULL, "invariant") ;
1692 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1693 assert (List != iterator, "invariant") ;
1694 }
1695
1696 if (Policy == 0) { // prepend to EntryList
1697 if (List == NULL) {
1698 iterator->_next = iterator->_prev = NULL ;
1699 _EntryList = iterator ;
1700 } else {
1701 List->_prev = iterator ;
1702 iterator->_next = List ;
1703 iterator->_prev = NULL ;
1704 _EntryList = iterator ;
1705 }
1706 } else
1707 if (Policy == 1) { // append to EntryList
1708 if (List == NULL) {
1709 iterator->_next = iterator->_prev = NULL ;
1710 _EntryList = iterator ;
1711 } else {
1712 // CONSIDER: finding the tail currently requires a linear-time walk of
1713 // the EntryList. We can make tail access constant-time by converting to
1714 // a CDLL instead of using our current DLL.
1715 ObjectWaiter * Tail ;
1716 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1717 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1718 Tail->_next = iterator ;
1719 iterator->_prev = Tail ;
1720 iterator->_next = NULL ;
1721 }
1722 } else
1723 if (Policy == 2) { // prepend to cxq
1724 // prepend to cxq
1725 if (List == NULL) {
1726 iterator->_next = iterator->_prev = NULL ;
1727 _EntryList = iterator ;
1728 } else {
1729 iterator->TState = ObjectWaiter::TS_CXQ ;
1730 for (;;) {
1731 ObjectWaiter * Front = _cxq ;
1732 iterator->_next = Front ;
1733 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1734 break ;
1735 }
1736 }
1737 }
1738 } else
1739 if (Policy == 3) { // append to cxq
1740 iterator->TState = ObjectWaiter::TS_CXQ ;
1741 for (;;) {
1742 ObjectWaiter * Tail ;
1743 Tail = _cxq ;
1744 if (Tail == NULL) {
1745 iterator->_next = NULL ;
1746 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1747 break ;
1748 }
1749 } else {
1750 while (Tail->_next != NULL) Tail = Tail->_next ;
1751 Tail->_next = iterator ;
1752 iterator->_prev = Tail ;
1753 iterator->_next = NULL ;
1754 break ;
1755 }
1756 }
1757 } else {
1758 ParkEvent * ev = iterator->_event ;
1759 iterator->TState = ObjectWaiter::TS_RUN ;
1760 OrderAccess::fence() ;
1761 ev->unpark() ;
1762 }
1763
1764 if (Policy < 4) {
1765 iterator->wait_reenter_begin(this);
1766 }
1767
1768 // _WaitSetLock protects the wait queue, not the EntryList. We could
1769 // move the add-to-EntryList operation, above, outside the critical section
1770 // protected by _WaitSetLock. In practice that's not useful. With the
1771 // exception of wait() timeouts and interrupts the monitor owner
1772 // is the only thread that grabs _WaitSetLock. There's almost no contention
1773 // on _WaitSetLock so it's not profitable to reduce the length of the
1774 // critical section.
1775 }
1776
1777 Thread::SpinRelease (&_WaitSetLock) ;
1778
1779 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1780 ObjectMonitor::_sync_Notifications->inc() ;
1781 }
1782 }
1783
1784
1785 void ObjectMonitor::notifyAll(TRAPS) {
1786 CHECK_OWNER();
1787 ObjectWaiter* iterator;
1788 if (_WaitSet == NULL) {
1789 TEVENT (Empty-NotifyAll) ;
1790 return ;
1791 }
1792 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1793
1794 int Policy = Knob_MoveNotifyee ;
1795 int Tally = 0 ;
1796 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1797
1798 for (;;) {
1799 iterator = DequeueWaiter () ;
1800 if (iterator == NULL) break ;
1801 TEVENT (NotifyAll - Transfer1) ;
1802 ++Tally ;
1803
1804 // Disposition - what might we do with iterator ?
1805 // a. add it directly to the EntryList - either tail or head.
1806 // b. push it onto the front of the _cxq.
1807 // For now we use (a).
1808
1809 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1810 guarantee (iterator->_notified == 0, "invariant") ;
1811 iterator->_notified = 1 ;
1812 Thread * Self = THREAD;
1813 iterator->_notifier_tid = Self->osthread()->thread_id();
1814 if (Policy != 4) {
1815 iterator->TState = ObjectWaiter::TS_ENTER ;
1816 }
1817
1818 ObjectWaiter * List = _EntryList ;
1819 if (List != NULL) {
1820 assert (List->_prev == NULL, "invariant") ;
1821 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1822 assert (List != iterator, "invariant") ;
1823 }
1824
1825 if (Policy == 0) { // prepend to EntryList
1826 if (List == NULL) {
1827 iterator->_next = iterator->_prev = NULL ;
1828 _EntryList = iterator ;
1829 } else {
1830 List->_prev = iterator ;
1831 iterator->_next = List ;
1832 iterator->_prev = NULL ;
1833 _EntryList = iterator ;
1834 }
1835 } else
1836 if (Policy == 1) { // append to EntryList
1837 if (List == NULL) {
1838 iterator->_next = iterator->_prev = NULL ;
1839 _EntryList = iterator ;
1840 } else {
1841 // CONSIDER: finding the tail currently requires a linear-time walk of
1842 // the EntryList. We can make tail access constant-time by converting to
1843 // a CDLL instead of using our current DLL.
1844 ObjectWaiter * Tail ;
1845 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1846 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1847 Tail->_next = iterator ;
1848 iterator->_prev = Tail ;
1849 iterator->_next = NULL ;
1850 }
1851 } else
1852 if (Policy == 2) { // prepend to cxq
1853 // prepend to cxq
1854 iterator->TState = ObjectWaiter::TS_CXQ ;
1855 for (;;) {
1856 ObjectWaiter * Front = _cxq ;
1857 iterator->_next = Front ;
1858 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1859 break ;
1860 }
1861 }
1862 } else
1863 if (Policy == 3) { // append to cxq
1864 iterator->TState = ObjectWaiter::TS_CXQ ;
1865 for (;;) {
1866 ObjectWaiter * Tail ;
1867 Tail = _cxq ;
1868 if (Tail == NULL) {
1869 iterator->_next = NULL ;
1870 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1871 break ;
1872 }
1873 } else {
1874 while (Tail->_next != NULL) Tail = Tail->_next ;
1875 Tail->_next = iterator ;
1876 iterator->_prev = Tail ;
1877 iterator->_next = NULL ;
1878 break ;
1879 }
1880 }
1881 } else {
1882 ParkEvent * ev = iterator->_event ;
1883 iterator->TState = ObjectWaiter::TS_RUN ;
1884 OrderAccess::fence() ;
1885 ev->unpark() ;
1886 }
1887
1888 if (Policy < 4) {
1889 iterator->wait_reenter_begin(this);
1890 }
1891
1892 // _WaitSetLock protects the wait queue, not the EntryList. We could
1893 // move the add-to-EntryList operation, above, outside the critical section
1894 // protected by _WaitSetLock. In practice that's not useful. With the
1895 // exception of wait() timeouts and interrupts the monitor owner
1896 // is the only thread that grabs _WaitSetLock. There's almost no contention
1897 // on _WaitSetLock so it's not profitable to reduce the length of the
1898 // critical section.
1899 }
1900
1901 Thread::SpinRelease (&_WaitSetLock) ;
1902
1903 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1904 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1905 }
1906 }
1907
1908 // -----------------------------------------------------------------------------
1909 // Adaptive Spinning Support
1910 //
1911 // Adaptive spin-then-block - rational spinning
1912 //
1913 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1914 // algorithm. On high order SMP systems it would be better to start with
1915 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1916 // a contending thread could enqueue itself on the cxq and then spin locally
1917 // on a thread-specific variable such as its ParkEvent._Event flag.
1918 // That's left as an exercise for the reader. Note that global spinning is
1919 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1920 // low latency and massive bandwidth.
1921 //
1922 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1923 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1924 // (duration) or we can fix the count at approximately the duration of
1925 // a context switch and vary the frequency. Of course we could also
1926 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1927 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1928 //
1929 // This implementation varies the duration "D", where D varies with
1930 // the success rate of recent spin attempts. (D is capped at approximately
1931 // length of a round-trip context switch). The success rate for recent
1932 // spin attempts is a good predictor of the success rate of future spin
1933 // attempts. The mechanism adapts automatically to varying critical
1934 // section length (lock modality), system load and degree of parallelism.
1935 // D is maintained per-monitor in _SpinDuration and is initialized
1936 // optimistically. Spin frequency is fixed at 100%.
1937 //
1938 // Note that _SpinDuration is volatile, but we update it without locks
1939 // or atomics. The code is designed so that _SpinDuration stays within
1940 // a reasonable range even in the presence of races. The arithmetic
1941 // operations on _SpinDuration are closed over the domain of legal values,
1942 // so at worst a race will install and older but still legal value.
1943 // At the very worst this introduces some apparent non-determinism.
1944 // We might spin when we shouldn't or vice-versa, but since the spin
1945 // count are relatively short, even in the worst case, the effect is harmless.
1946 //
1947 // Care must be taken that a low "D" value does not become an
1948 // an absorbing state. Transient spinning failures -- when spinning
1949 // is overall profitable -- should not cause the system to converge
1950 // on low "D" values. We want spinning to be stable and predictable
1951 // and fairly responsive to change and at the same time we don't want
1952 // it to oscillate, become metastable, be "too" non-deterministic,
1953 // or converge on or enter undesirable stable absorbing states.
1954 //
1955 // We implement a feedback-based control system -- using past behavior
1956 // to predict future behavior. We face two issues: (a) if the
1957 // input signal is random then the spin predictor won't provide optimal
1958 // results, and (b) if the signal frequency is too high then the control
1959 // system, which has some natural response lag, will "chase" the signal.
1960 // (b) can arise from multimodal lock hold times. Transient preemption
1961 // can also result in apparent bimodal lock hold times.
1962 // Although sub-optimal, neither condition is particularly harmful, as
1963 // in the worst-case we'll spin when we shouldn't or vice-versa.
1964 // The maximum spin duration is rather short so the failure modes aren't bad.
1965 // To be conservative, I've tuned the gain in system to bias toward
1966 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1967 // "rings" or oscillates between spinning and not spinning. This happens
1968 // when spinning is just on the cusp of profitability, however, so the
1969 // situation is not dire. The state is benign -- there's no need to add
1970 // hysteresis control to damp the transition rate between spinning and
1971 // not spinning.
1972 //
1973
1974 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1975 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1976
1977 // Spinning: Fixed frequency (100%), vary duration
1978
1979
1980 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1981
1982 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1983 int ctr = Knob_FixedSpin ;
1984 if (ctr != 0) {
1985 while (--ctr >= 0) {
1986 if (TryLock (Self) > 0) return 1 ;
1987 SpinPause () ;
1988 }
1989 return 0 ;
1990 }
1991
1992 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1993 if (TryLock(Self) > 0) {
1994 // Increase _SpinDuration ...
1995 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1996 // Raising _SpurDuration to the poverty line is key.
1997 int x = _SpinDuration ;
1998 if (x < Knob_SpinLimit) {
1999 if (x < Knob_Poverty) x = Knob_Poverty ;
2000 _SpinDuration = x + Knob_BonusB ;
2001 }
2002 return 1 ;
2003 }
2004 SpinPause () ;
2005 }
2006
2007 // Admission control - verify preconditions for spinning
2008 //
2009 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2010 // becoming an absorbing state. Put another way, we spin briefly to
2011 // sample, just in case the system load, parallelism, contention, or lock
2012 // modality changed.
2013 //
2014 // Consider the following alternative:
2015 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2016 // spin attempt. "Periodically" might mean after a tally of
2017 // the # of failed spin attempts (or iterations) reaches some threshold.
2018 // This takes us into the realm of 1-out-of-N spinning, where we
2019 // hold the duration constant but vary the frequency.
2020
2021 ctr = _SpinDuration ;
2022 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2023 if (ctr <= 0) return 0 ;
2024
2025 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2026 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2027 TEVENT (Spin abort - notrunnable [TOP]);
2028 return 0 ;
2029 }
2030
2031 int MaxSpin = Knob_MaxSpinners ;
2032 if (MaxSpin >= 0) {
2033 if (_Spinner > MaxSpin) {
2034 TEVENT (Spin abort -- too many spinners) ;
2035 return 0 ;
2036 }
2037 // Slighty racy, but benign ...
2038 Adjust (&_Spinner, 1) ;
2039 }
2040
2041 // We're good to spin ... spin ingress.
2042 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2043 // when preparing to LD...CAS _owner, etc and the CAS is likely
2044 // to succeed.
2045 int hits = 0 ;
2046 int msk = 0 ;
2047 int caspty = Knob_CASPenalty ;
2048 int oxpty = Knob_OXPenalty ;
2049 int sss = Knob_SpinSetSucc ;
2050 if (sss && _succ == NULL ) _succ = Self ;
2051 Thread * prv = NULL ;
2052
2053 // There are three ways to exit the following loop:
2054 // 1. A successful spin where this thread has acquired the lock.
2055 // 2. Spin failure with prejudice
2056 // 3. Spin failure without prejudice
2057
2058 while (--ctr >= 0) {
2059
2060 // Periodic polling -- Check for pending GC
2061 // Threads may spin while they're unsafe.
2062 // We don't want spinning threads to delay the JVM from reaching
2063 // a stop-the-world safepoint or to steal cycles from GC.
2064 // If we detect a pending safepoint we abort in order that
2065 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2066 // this thread, if safe, doesn't steal cycles from GC.
2067 // This is in keeping with the "no loitering in runtime" rule.
2068 // We periodically check to see if there's a safepoint pending.
2069 if ((ctr & 0xFF) == 0) {
2070 if (SafepointSynchronize::do_call_back()) {
2071 TEVENT (Spin: safepoint) ;
2072 goto Abort ; // abrupt spin egress
2073 }
2074 if (Knob_UsePause & 1) SpinPause () ;
2075
2076 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2077 if (hits > 50 && scb != NULL) {
2078 int abend = (*scb)(SpinCallbackArgument, 0) ;
2079 }
2080 }
2081
2082 if (Knob_UsePause & 2) SpinPause() ;
2083
2084 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2085 // This is useful on classic SMP systems, but is of less utility on
2086 // N1-style CMT platforms.
2087 //
2088 // Trade-off: lock acquisition latency vs coherency bandwidth.
2089 // Lock hold times are typically short. A histogram
2090 // of successful spin attempts shows that we usually acquire
2091 // the lock early in the spin. That suggests we want to
2092 // sample _owner frequently in the early phase of the spin,
2093 // but then back-off and sample less frequently as the spin
2094 // progresses. The back-off makes a good citizen on SMP big
2095 // SMP systems. Oversampling _owner can consume excessive
2096 // coherency bandwidth. Relatedly, if we _oversample _owner we
2097 // can inadvertently interfere with the the ST m->owner=null.
2098 // executed by the lock owner.
2099 if (ctr & msk) continue ;
2100 ++hits ;
2101 if ((hits & 0xF) == 0) {
2102 // The 0xF, above, corresponds to the exponent.
2103 // Consider: (msk+1)|msk
2104 msk = ((msk << 2)|3) & BackOffMask ;
2105 }
2106
2107 // Probe _owner with TATAS
2108 // If this thread observes the monitor transition or flicker
2109 // from locked to unlocked to locked, then the odds that this
2110 // thread will acquire the lock in this spin attempt go down
2111 // considerably. The same argument applies if the CAS fails
2112 // or if we observe _owner change from one non-null value to
2113 // another non-null value. In such cases we might abort
2114 // the spin without prejudice or apply a "penalty" to the
2115 // spin count-down variable "ctr", reducing it by 100, say.
2116
2117 Thread * ox = (Thread *) _owner ;
2118 if (ox == NULL) {
2119 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2120 if (ox == NULL) {
2121 // The CAS succeeded -- this thread acquired ownership
2122 // Take care of some bookkeeping to exit spin state.
2123 if (sss && _succ == Self) {
2124 _succ = NULL ;
2125 }
2126 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2127
2128 // Increase _SpinDuration :
2129 // The spin was successful (profitable) so we tend toward
2130 // longer spin attempts in the future.
2131 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2132 // If we acquired the lock early in the spin cycle it
2133 // makes sense to increase _SpinDuration proportionally.
2134 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2135 int x = _SpinDuration ;
2136 if (x < Knob_SpinLimit) {
2137 if (x < Knob_Poverty) x = Knob_Poverty ;
2138 _SpinDuration = x + Knob_Bonus ;
2139 }
2140 return 1 ;
2141 }
2142
2143 // The CAS failed ... we can take any of the following actions:
2144 // * penalize: ctr -= Knob_CASPenalty
2145 // * exit spin with prejudice -- goto Abort;
2146 // * exit spin without prejudice.
2147 // * Since CAS is high-latency, retry again immediately.
2148 prv = ox ;
2149 TEVENT (Spin: cas failed) ;
2150 if (caspty == -2) break ;
2151 if (caspty == -1) goto Abort ;
2152 ctr -= caspty ;
2153 continue ;
2154 }
2155
2156 // Did lock ownership change hands ?
2157 if (ox != prv && prv != NULL ) {
2158 TEVENT (spin: Owner changed)
2159 if (oxpty == -2) break ;
2160 if (oxpty == -1) goto Abort ;
2161 ctr -= oxpty ;
2162 }
2163 prv = ox ;
2164
2165 // Abort the spin if the owner is not executing.
2166 // The owner must be executing in order to drop the lock.
2167 // Spinning while the owner is OFFPROC is idiocy.
2168 // Consider: ctr -= RunnablePenalty ;
2169 if (Knob_OState && NotRunnable (Self, ox)) {
2170 TEVENT (Spin abort - notrunnable);
2171 goto Abort ;
2172 }
2173 if (sss && _succ == NULL ) _succ = Self ;
2174 }
2175
2176 // Spin failed with prejudice -- reduce _SpinDuration.
2177 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2178 // AIMD is globally stable.
2179 TEVENT (Spin failure) ;
2180 {
2181 int x = _SpinDuration ;
2182 if (x > 0) {
2183 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2184 // This is globally sample and tends to damp the response.
2185 x -= Knob_Penalty ;
2186 if (x < 0) x = 0 ;
2187 _SpinDuration = x ;
2188 }
2189 }
2190
2191 Abort:
2192 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2193 if (sss && _succ == Self) {
2194 _succ = NULL ;
2195 // Invariant: after setting succ=null a contending thread
2196 // must recheck-retry _owner before parking. This usually happens
2197 // in the normal usage of TrySpin(), but it's safest
2198 // to make TrySpin() as foolproof as possible.
2199 OrderAccess::fence() ;
2200 if (TryLock(Self) > 0) return 1 ;
2201 }
2202 return 0 ;
2203 }
2204
2205 // NotRunnable() -- informed spinning
2206 //
2207 // Don't bother spinning if the owner is not eligible to drop the lock.
2208 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2209 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2210 // The thread must be runnable in order to drop the lock in timely fashion.
2211 // If the _owner is not runnable then spinning will not likely be
2212 // successful (profitable).
2213 //
2214 // Beware -- the thread referenced by _owner could have died
2215 // so a simply fetch from _owner->_thread_state might trap.
2216 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2217 // Because of the lifecycle issues the schedctl and _thread_state values
2218 // observed by NotRunnable() might be garbage. NotRunnable must
2219 // tolerate this and consider the observed _thread_state value
2220 // as advisory.
2221 //
2222 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2223 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2224 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2225 // with the LSB of _owner. Another option would be to probablistically probe
2226 // the putative _owner->TypeTag value.
2227 //
2228 // Checking _thread_state isn't perfect. Even if the thread is
2229 // in_java it might be blocked on a page-fault or have been preempted
2230 // and sitting on a ready/dispatch queue. _thread state in conjunction
2231 // with schedctl.sc_state gives us a good picture of what the
2232 // thread is doing, however.
2233 //
2234 // TODO: check schedctl.sc_state.
2235 // We'll need to use SafeFetch32() to read from the schedctl block.
2236 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2237 //
2238 // The return value from NotRunnable() is *advisory* -- the
2239 // result is based on sampling and is not necessarily coherent.
2240 // The caller must tolerate false-negative and false-positive errors.
2241 // Spinning, in general, is probabilistic anyway.
2242
2243
2244 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2245 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2246 if (!OwnerIsThread) return 0 ;
2247
2248 if (ox == NULL) return 0 ;
2249
2250 // Avoid transitive spinning ...
2251 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2252 // Immediately after T1 acquires L it's possible that T2, also
2253 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2254 // This occurs transiently after T1 acquired L but before
2255 // T1 managed to clear T1.Stalled. T2 does not need to abort
2256 // its spin in this circumstance.
2257 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2258
2259 if (BlockedOn == 1) return 1 ;
2260 if (BlockedOn != 0) {
2261 return BlockedOn != intptr_t(this) && _owner == ox ;
2262 }
2263
2264 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2265 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2266 // consider also: jst != _thread_in_Java -- but that's overspecific.
2267 return jst == _thread_blocked || jst == _thread_in_native ;
2268 }
2269
2270
2271 // -----------------------------------------------------------------------------
2272 // WaitSet management ...
2273
2274 ObjectWaiter::ObjectWaiter(Thread* thread) {
2275 _next = NULL;
2276 _prev = NULL;
2277 _notified = 0;
2278 TState = TS_RUN ;
2279 _thread = thread;
2280 _event = thread->_ParkEvent ;
2281 _active = false;
2282 assert (_event != NULL, "invariant") ;
2283 }
2284
2285 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2286 JavaThread *jt = (JavaThread *)this->_thread;
2287 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2288 }
2289
2290 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2291 JavaThread *jt = (JavaThread *)this->_thread;
2292 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2293 }
2294
2295 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2296 assert(node != NULL, "should not dequeue NULL node");
2297 assert(node->_prev == NULL, "node already in list");
2298 assert(node->_next == NULL, "node already in list");
2299 // put node at end of queue (circular doubly linked list)
2300 if (_WaitSet == NULL) {
2301 _WaitSet = node;
2302 node->_prev = node;
2303 node->_next = node;
2304 } else {
2305 ObjectWaiter* head = _WaitSet ;
2306 ObjectWaiter* tail = head->_prev;
2307 assert(tail->_next == head, "invariant check");
2308 tail->_next = node;
2309 head->_prev = node;
2310 node->_next = head;
2311 node->_prev = tail;
2312 }
2313 }
2314
2315 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2316 // dequeue the very first waiter
2317 ObjectWaiter* waiter = _WaitSet;
2318 if (waiter) {
2319 DequeueSpecificWaiter(waiter);
2320 }
2321 return waiter;
2322 }
2323
2324 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2325 assert(node != NULL, "should not dequeue NULL node");
2326 assert(node->_prev != NULL, "node already removed from list");
2327 assert(node->_next != NULL, "node already removed from list");
2328 // when the waiter has woken up because of interrupt,
2329 // timeout or other spurious wake-up, dequeue the
2330 // waiter from waiting list
2331 ObjectWaiter* next = node->_next;
2332 if (next == node) {
2333 assert(node->_prev == node, "invariant check");
2334 _WaitSet = NULL;
2335 } else {
2336 ObjectWaiter* prev = node->_prev;
2337 assert(prev->_next == node, "invariant check");
2338 assert(next->_prev == node, "invariant check");
2339 next->_prev = prev;
2340 prev->_next = next;
2341 if (_WaitSet == node) {
2342 _WaitSet = next;
2343 }
2344 }
2345 node->_next = NULL;
2346 node->_prev = NULL;
2347 }
2348
2349 // -----------------------------------------------------------------------------
2350 // PerfData support
2351 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2352 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2353 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2354 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2355 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2356 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2357 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2358 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2359 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2360 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2361 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2362 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2363 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2364 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2365 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2366 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2367 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2368 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2369
2370 // One-shot global initialization for the sync subsystem.
2371 // We could also defer initialization and initialize on-demand
2372 // the first time we call inflate(). Initialization would
2373 // be protected - like so many things - by the MonitorCache_lock.
2374
2375 void ObjectMonitor::Initialize () {
2376 static int InitializationCompleted = 0 ;
2377 assert (InitializationCompleted == 0, "invariant") ;
2378 InitializationCompleted = 1 ;
2379 if (UsePerfData) {
2380 EXCEPTION_MARK ;
2381 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2382 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2383 NEWPERFCOUNTER(_sync_Inflations) ;
2384 NEWPERFCOUNTER(_sync_Deflations) ;
2385 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2386 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2387 NEWPERFCOUNTER(_sync_Parks) ;
2388 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2389 NEWPERFCOUNTER(_sync_Notifications) ;
2390 NEWPERFCOUNTER(_sync_SlowEnter) ;
2391 NEWPERFCOUNTER(_sync_SlowExit) ;
2392 NEWPERFCOUNTER(_sync_SlowNotify) ;
2393 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2394 NEWPERFCOUNTER(_sync_FailedSpins) ;
2395 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2396 NEWPERFCOUNTER(_sync_PrivateA) ;
2397 NEWPERFCOUNTER(_sync_PrivateB) ;
2398 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2399 NEWPERFCOUNTER(_sync_MonScavenged) ;
2400 NEWPERFVARIABLE(_sync_MonExtant) ;
2401 #undef NEWPERFCOUNTER
2402 }
2403 }
2404
2405
2406 // Compile-time asserts
2407 // When possible, it's better to catch errors deterministically at
2408 // compile-time than at runtime. The down-side to using compile-time
2409 // asserts is that error message -- often something about negative array
2410 // indices -- is opaque.
2411
2412 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2413
2414 void ObjectMonitor::ctAsserts() {
2415 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2416 }
2417
2418
2419 static char * kvGet (char * kvList, const char * Key) {
2420 if (kvList == NULL) return NULL ;
2421 size_t n = strlen (Key) ;
2422 char * Search ;
2423 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2424 if (strncmp (Search, Key, n) == 0) {
2425 if (Search[n] == '=') return Search + n + 1 ;
2426 if (Search[n] == 0) return (char *) "1" ;
2427 }
2428 }
2429 return NULL ;
2430 }
2431
2432 static int kvGetInt (char * kvList, const char * Key, int Default) {
2433 char * v = kvGet (kvList, Key) ;
2434 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2435 if (Knob_ReportSettings && v != NULL) {
2436 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2437 ::fflush (stdout) ;
2438 }
2439 return rslt ;
2440 }
2441
2442 void ObjectMonitor::DeferredInitialize () {
2443 if (InitDone > 0) return ;
2444 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2445 while (InitDone != 1) ;
2446 return ;
2447 }
2448
2449 // One-shot global initialization ...
2450 // The initialization is idempotent, so we don't need locks.
2451 // In the future consider doing this via os::init_2().
2452 // SyncKnobs consist of <Key>=<Value> pairs in the style
2453 // of environment variables. Start by converting ':' to NUL.
2454
2455 if (SyncKnobs == NULL) SyncKnobs = "" ;
2456
2457 size_t sz = strlen (SyncKnobs) ;
2458 char * knobs = (char *) malloc (sz + 2) ;
2459 if (knobs == NULL) {
2460 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2461 guarantee (0, "invariant") ;
2462 }
2463 strcpy (knobs, SyncKnobs) ;
2464 knobs[sz+1] = 0 ;
2465 for (char * p = knobs ; *p ; p++) {
2466 if (*p == ':') *p = 0 ;
2467 }
2468
2469 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2470 SETKNOB(ReportSettings) ;
2471 SETKNOB(Verbose) ;
2472 SETKNOB(FixedSpin) ;
2473 SETKNOB(SpinLimit) ;
2474 SETKNOB(SpinBase) ;
2475 SETKNOB(SpinBackOff);
2476 SETKNOB(CASPenalty) ;
2477 SETKNOB(OXPenalty) ;
2478 SETKNOB(LogSpins) ;
2479 SETKNOB(SpinSetSucc) ;
2480 SETKNOB(SuccEnabled) ;
2481 SETKNOB(SuccRestrict) ;
2482 SETKNOB(Penalty) ;
2483 SETKNOB(Bonus) ;
2484 SETKNOB(BonusB) ;
2485 SETKNOB(Poverty) ;
2486 SETKNOB(SpinAfterFutile) ;
2487 SETKNOB(UsePause) ;
2488 SETKNOB(SpinEarly) ;
2489 SETKNOB(OState) ;
2490 SETKNOB(MaxSpinners) ;
2491 SETKNOB(PreSpin) ;
2492 SETKNOB(ExitPolicy) ;
2493 SETKNOB(QMode);
2494 SETKNOB(ResetEvent) ;
2495 SETKNOB(MoveNotifyee) ;
2496 SETKNOB(FastHSSEC) ;
2497 #undef SETKNOB
2498
2499 if (os::is_MP()) {
2500 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2501 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2502 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2503 } else {
2504 Knob_SpinLimit = 0 ;
2505 Knob_SpinBase = 0 ;
2506 Knob_PreSpin = 0 ;
2507 Knob_FixedSpin = -1 ;
2508 }
2509
2510 if (Knob_LogSpins == 0) {
2511 ObjectMonitor::_sync_FailedSpins = NULL ;
2512 }
2513
2514 free (knobs) ;
2515 OrderAccess::fence() ;
2516 InitDone = 1 ;
2517 }
2518
2519 #ifndef PRODUCT
2520 void ObjectMonitor::verify() {
2521 }
2522
2523 void ObjectMonitor::print() {
2524 }
2525 #endif