1 /*
2 * Copyright (c) 1998, 2015, 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 "jfr/jfrEvents.hpp"
28 #include "jfr/support/jfrThreadId.hpp"
29 #include "memory/resourceArea.hpp"
30 #include "oops/markOop.hpp"
31 #include "oops/oop.inline.hpp"
32 #include "runtime/handles.inline.hpp"
33 #include "runtime/interfaceSupport.hpp"
34 #include "runtime/mutexLocker.hpp"
35 #include "runtime/objectMonitor.hpp"
36 #include "runtime/objectMonitor.inline.hpp"
37 #include "runtime/orderAccess.inline.hpp"
38 #include "runtime/osThread.hpp"
39 #include "runtime/stubRoutines.hpp"
40 #include "runtime/thread.inline.hpp"
41 #include "services/threadService.hpp"
42 #include "utilities/dtrace.hpp"
43 #include "utilities/macros.hpp"
44 #include "utilities/preserveException.hpp"
45 #ifdef TARGET_OS_FAMILY_linux
46 # include "os_linux.inline.hpp"
47 #endif
48 #ifdef TARGET_OS_FAMILY_solaris
49 # include "os_solaris.inline.hpp"
50 #endif
51 #ifdef TARGET_OS_FAMILY_windows
52 # include "os_windows.inline.hpp"
53 #endif
54 #ifdef TARGET_OS_FAMILY_bsd
55 # include "os_bsd.inline.hpp"
56 #endif
57 #if INCLUDE_JFR
58 #include "jfr/support/jfrFlush.hpp"
59 #endif
60
61 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
62 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
63 #define ATTR __attribute__((noinline))
64 #else
65 #define ATTR
66 #endif
67
68
69 #ifdef DTRACE_ENABLED
70
71 // Only bother with this argument setup if dtrace is available
72 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
73
74
75 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
76 char* bytes = NULL; \
77 int len = 0; \
78 jlong jtid = SharedRuntime::get_java_tid(thread); \
79 Symbol* klassname = ((oop)obj)->klass()->name(); \
80 if (klassname != NULL) { \
81 bytes = (char*)klassname->bytes(); \
82 len = klassname->utf8_length(); \
83 }
84
85 #ifndef USDT2
86
87 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
88 jlong, uintptr_t, char*, int);
89 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
90 jlong, uintptr_t, char*, int);
91 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
92 jlong, uintptr_t, char*, int);
93 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
94 jlong, uintptr_t, char*, int);
95 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
96 jlong, uintptr_t, char*, int);
97
98 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
99 { \
100 if (DTraceMonitorProbes) { \
101 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
102 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
103 (monitor), bytes, len, (millis)); \
104 } \
105 }
106
107 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
108 { \
109 if (DTraceMonitorProbes) { \
110 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
111 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
112 (uintptr_t)(monitor), bytes, len); \
113 } \
114 }
115
116 #else /* USDT2 */
117
118 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
119 { \
120 if (DTraceMonitorProbes) { \
121 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
122 HOTSPOT_MONITOR_WAIT(jtid, \
123 (monitor), bytes, len, (millis)); \
124 } \
125 }
126
127 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
128 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
129 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
130 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
131 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
132
133 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
134 { \
135 if (DTraceMonitorProbes) { \
136 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
137 HOTSPOT_MONITOR_##probe(jtid, \
138 (uintptr_t)(monitor), bytes, len); \
139 } \
140 }
141
142 #endif /* USDT2 */
143 #else // ndef DTRACE_ENABLED
144
145 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
146 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
147
148 #endif // ndef DTRACE_ENABLED
149
150 // Tunables ...
151 // The knob* variables are effectively final. Once set they should
152 // never be modified hence. Consider using __read_mostly with GCC.
153
154 int ObjectMonitor::Knob_Verbose = 0 ;
155 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
156 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
157 static int Knob_HandOff = 0 ;
158 static int Knob_ReportSettings = 0 ;
159
160 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
161 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
162 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
163 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
164 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
165 static int Knob_SpinEarly = 1 ;
166 static int Knob_SuccEnabled = 1 ; // futile wake throttling
167 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
168 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
169 static int Knob_Bonus = 100 ; // spin success bonus
170 static int Knob_BonusB = 100 ; // spin success bonus
171 static int Knob_Penalty = 200 ; // spin failure penalty
172 static int Knob_Poverty = 1000 ;
173 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
174 static int Knob_FixedSpin = 0 ;
175 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
176 static int Knob_UsePause = 1 ;
177 static int Knob_ExitPolicy = 0 ;
178 static int Knob_PreSpin = 10 ; // 20-100 likely better
179 static int Knob_ResetEvent = 0 ;
180 static int BackOffMask = 0 ;
181
182 static int Knob_FastHSSEC = 0 ;
183 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
184 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
185 static volatile int InitDone = 0 ;
186
187 #define TrySpin TrySpin_VaryDuration
188
189 // -----------------------------------------------------------------------------
190 // Theory of operations -- Monitors lists, thread residency, etc:
191 //
192 // * A thread acquires ownership of a monitor by successfully
193 // CAS()ing the _owner field from null to non-null.
194 //
195 // * Invariant: A thread appears on at most one monitor list --
196 // cxq, EntryList or WaitSet -- at any one time.
197 //
198 // * Contending threads "push" themselves onto the cxq with CAS
199 // and then spin/park.
200 //
201 // * After a contending thread eventually acquires the lock it must
202 // dequeue itself from either the EntryList or the cxq.
203 //
204 // * The exiting thread identifies and unparks an "heir presumptive"
205 // tentative successor thread on the EntryList. Critically, the
206 // exiting thread doesn't unlink the successor thread from the EntryList.
207 // After having been unparked, the wakee will recontend for ownership of
208 // the monitor. The successor (wakee) will either acquire the lock or
209 // re-park itself.
210 //
211 // Succession is provided for by a policy of competitive handoff.
212 // The exiting thread does _not_ grant or pass ownership to the
213 // successor thread. (This is also referred to as "handoff" succession").
214 // Instead the exiting thread releases ownership and possibly wakes
215 // a successor, so the successor can (re)compete for ownership of the lock.
216 // If the EntryList is empty but the cxq is populated the exiting
217 // thread will drain the cxq into the EntryList. It does so by
218 // by detaching the cxq (installing null with CAS) and folding
219 // the threads from the cxq into the EntryList. The EntryList is
220 // doubly linked, while the cxq is singly linked because of the
221 // CAS-based "push" used to enqueue recently arrived threads (RATs).
222 //
223 // * Concurrency invariants:
224 //
225 // -- only the monitor owner may access or mutate the EntryList.
226 // The mutex property of the monitor itself protects the EntryList
227 // from concurrent interference.
228 // -- Only the monitor owner may detach the cxq.
229 //
230 // * The monitor entry list operations avoid locks, but strictly speaking
231 // they're not lock-free. Enter is lock-free, exit is not.
232 // For a description of 'Methods and apparatus providing non-blocking access
233 // to a resource,' see U.S. Pat. No. 7844973.
234 //
235 // * The cxq can have multiple concurrent "pushers" but only one concurrent
236 // detaching thread. This mechanism is immune from the ABA corruption.
237 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
238 //
239 // * Taken together, the cxq and the EntryList constitute or form a
240 // single logical queue of threads stalled trying to acquire the lock.
241 // We use two distinct lists to improve the odds of a constant-time
242 // dequeue operation after acquisition (in the ::enter() epilog) and
243 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
244 // A key desideratum is to minimize queue & monitor metadata manipulation
245 // that occurs while holding the monitor lock -- that is, we want to
246 // minimize monitor lock holds times. Note that even a small amount of
247 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
248 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
249 // locks and monitor metadata.
250 //
251 // Cxq points to the the set of Recently Arrived Threads attempting entry.
252 // Because we push threads onto _cxq with CAS, the RATs must take the form of
253 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
254 // the unlocking thread notices that EntryList is null but _cxq is != null.
255 //
256 // The EntryList is ordered by the prevailing queue discipline and
257 // can be organized in any convenient fashion, such as a doubly-linked list or
258 // a circular doubly-linked list. Critically, we want insert and delete operations
259 // to operate in constant-time. If we need a priority queue then something akin
260 // to Solaris' sleepq would work nicely. Viz.,
261 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
262 // Queue discipline is enforced at ::exit() time, when the unlocking thread
263 // drains the cxq into the EntryList, and orders or reorders the threads on the
264 // EntryList accordingly.
265 //
266 // Barring "lock barging", this mechanism provides fair cyclic ordering,
267 // somewhat similar to an elevator-scan.
268 //
269 // * The monitor synchronization subsystem avoids the use of native
270 // synchronization primitives except for the narrow platform-specific
271 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
272 // the semantics of park-unpark. Put another way, this monitor implementation
273 // depends only on atomic operations and park-unpark. The monitor subsystem
274 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
275 // underlying OS manages the READY<->RUN transitions.
276 //
277 // * Waiting threads reside on the WaitSet list -- wait() puts
278 // the caller onto the WaitSet.
279 //
280 // * notify() or notifyAll() simply transfers threads from the WaitSet to
281 // either the EntryList or cxq. Subsequent exit() operations will
282 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
283 // it's likely the notifyee would simply impale itself on the lock held
284 // by the notifier.
285 //
286 // * An interesting alternative is to encode cxq as (List,LockByte) where
287 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
288 // variable, like _recursions, in the scheme. The threads or Events that form
289 // the list would have to be aligned in 256-byte addresses. A thread would
290 // try to acquire the lock or enqueue itself with CAS, but exiting threads
291 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
292 // Note that is is *not* word-tearing, but it does presume that full-word
293 // CAS operations are coherent with intermix with STB operations. That's true
294 // on most common processors.
295 //
296 // * See also http://blogs.sun.com/dave
297
298
299 // -----------------------------------------------------------------------------
300 // Enter support
301
302 bool ObjectMonitor::try_enter(Thread* THREAD) {
303 if (THREAD != _owner) {
304 if (THREAD->is_lock_owned ((address)_owner)) {
305 assert(_recursions == 0, "internal state error");
306 _owner = THREAD ;
307 _recursions = 1 ;
308 OwnerIsThread = 1 ;
309 return true;
310 }
311 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
312 return false;
313 }
314 return true;
315 } else {
316 _recursions++;
317 return true;
318 }
319 }
320
321 void ATTR ObjectMonitor::enter(TRAPS) {
322 // The following code is ordered to check the most common cases first
323 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
324 Thread * const Self = THREAD ;
325 void * cur ;
326
327 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
328 if (cur == NULL) {
329 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
330 assert (_recursions == 0 , "invariant") ;
331 assert (_owner == Self, "invariant") ;
332 // CONSIDER: set or assert OwnerIsThread == 1
333 return ;
334 }
335
336 if (cur == Self) {
337 // TODO-FIXME: check for integer overflow! BUGID 6557169.
338 _recursions ++ ;
339 return ;
340 }
341
342 if (Self->is_lock_owned ((address)cur)) {
343 assert (_recursions == 0, "internal state error");
344 _recursions = 1 ;
345 // Commute owner from a thread-specific on-stack BasicLockObject address to
346 // a full-fledged "Thread *".
347 _owner = Self ;
348 OwnerIsThread = 1 ;
349 return ;
350 }
351
352 // We've encountered genuine contention.
353 assert (Self->_Stalled == 0, "invariant") ;
354 Self->_Stalled = intptr_t(this) ;
355
356 // Try one round of spinning *before* enqueueing Self
357 // and before going through the awkward and expensive state
358 // transitions. The following spin is strictly optional ...
359 // Note that if we acquire the monitor from an initial spin
360 // we forgo posting JVMTI events and firing DTRACE probes.
361 if (Knob_SpinEarly && TrySpin (Self) > 0) {
362 assert (_owner == Self , "invariant") ;
363 assert (_recursions == 0 , "invariant") ;
364 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
365 Self->_Stalled = 0 ;
366 return ;
367 }
368
369 assert (_owner != Self , "invariant") ;
370 assert (_succ != Self , "invariant") ;
371 assert (Self->is_Java_thread() , "invariant") ;
372 JavaThread * jt = (JavaThread *) Self ;
373 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
374 assert (jt->thread_state() != _thread_blocked , "invariant") ;
375 assert (this->object() != NULL , "invariant") ;
376 assert (_count >= 0, "invariant") ;
377
378 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
379 // Ensure the object-monitor relationship remains stable while there's contention.
380 Atomic::inc_ptr(&_count);
381
382 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
383 EventJavaMonitorEnter event;
384 if (event.should_commit()) {
385 event.set_monitorClass(((oop)this->object())->klass());
386 event.set_address((uintptr_t)(this->object_addr()));
387 }
388
389 { // Change java thread status to indicate blocked on monitor enter.
390 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
391
392 Self->set_current_pending_monitor(this);
393
394 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
395 if (JvmtiExport::should_post_monitor_contended_enter()) {
396 JvmtiExport::post_monitor_contended_enter(jt, this);
397
398 // The current thread does not yet own the monitor and does not
399 // yet appear on any queues that would get it made the successor.
400 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
401 // handler cannot accidentally consume an unpark() meant for the
402 // ParkEvent associated with this ObjectMonitor.
403 }
404
405 OSThreadContendState osts(Self->osthread());
406 ThreadBlockInVM tbivm(jt);
407
408 // TODO-FIXME: change the following for(;;) loop to straight-line code.
409 for (;;) {
410 jt->set_suspend_equivalent();
411 // cleared by handle_special_suspend_equivalent_condition()
412 // or java_suspend_self()
413
414 EnterI (THREAD) ;
415
416 if (!ExitSuspendEquivalent(jt)) break ;
417
418 //
419 // We have acquired the contended monitor, but while we were
420 // waiting another thread suspended us. We don't want to enter
421 // the monitor while suspended because that would surprise the
422 // thread that suspended us.
423 //
424 _recursions = 0 ;
425 _succ = NULL ;
426 exit (false, Self) ;
427
428 jt->java_suspend_self();
429 }
430 Self->set_current_pending_monitor(NULL);
431
432 // We cleared the pending monitor info since we've just gotten past
433 // the enter-check-for-suspend dance and we now own the monitor free
434 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
435 // destructor can go to a safepoint at the end of this block. If we
436 // do a thread dump during that safepoint, then this thread will show
437 // as having "-locked" the monitor, but the OS and java.lang.Thread
438 // states will still report that the thread is blocked trying to
439 // acquire it.
440 }
441
442 Atomic::dec_ptr(&_count);
443 assert (_count >= 0, "invariant") ;
444 Self->_Stalled = 0 ;
445
446 // Must either set _recursions = 0 or ASSERT _recursions == 0.
447 assert (_recursions == 0 , "invariant") ;
448 assert (_owner == Self , "invariant") ;
449 assert (_succ != Self , "invariant") ;
450 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
451
452 // The thread -- now the owner -- is back in vm mode.
453 // Report the glorious news via TI,DTrace and jvmstat.
454 // The probe effect is non-trivial. All the reportage occurs
455 // while we hold the monitor, increasing the length of the critical
456 // section. Amdahl's parallel speedup law comes vividly into play.
457 //
458 // Another option might be to aggregate the events (thread local or
459 // per-monitor aggregation) and defer reporting until a more opportune
460 // time -- such as next time some thread encounters contention but has
461 // yet to acquire the lock. While spinning that thread could
462 // spinning we could increment JVMStat counters, etc.
463
464 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
465 if (JvmtiExport::should_post_monitor_contended_entered()) {
466 JvmtiExport::post_monitor_contended_entered(jt, this);
467
468 // The current thread already owns the monitor and is not going to
469 // call park() for the remainder of the monitor enter protocol. So
470 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
471 // event handler consumed an unpark() issued by the thread that
472 // just exited the monitor.
473 }
474
475 if (event.should_commit()) {
476 event.set_previousOwner((uintptr_t)_previous_owner_tid);
477 event.commit();
478 }
479
480 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
481 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
482 }
483 }
484
485
486 // Caveat: TryLock() is not necessarily serializing if it returns failure.
487 // Callers must compensate as needed.
488
489 int ObjectMonitor::TryLock (Thread * Self) {
490 for (;;) {
491 void * own = _owner ;
492 if (own != NULL) return 0 ;
493 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
494 // Either guarantee _recursions == 0 or set _recursions = 0.
495 assert (_recursions == 0, "invariant") ;
496 assert (_owner == Self, "invariant") ;
497 // CONSIDER: set or assert that OwnerIsThread == 1
498 return 1 ;
499 }
500 // The lock had been free momentarily, but we lost the race to the lock.
501 // Interference -- the CAS failed.
502 // We can either return -1 or retry.
503 // Retry doesn't make as much sense because the lock was just acquired.
504 if (true) return -1 ;
505 }
506 }
507
508 void ATTR ObjectMonitor::EnterI (TRAPS) {
509 Thread * Self = THREAD ;
510 assert (Self->is_Java_thread(), "invariant") ;
511 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
512
513 // Try the lock - TATAS
514 if (TryLock (Self) > 0) {
515 assert (_succ != Self , "invariant") ;
516 assert (_owner == Self , "invariant") ;
517 assert (_Responsible != Self , "invariant") ;
518 return ;
519 }
520
521 DeferredInitialize () ;
522
523 // We try one round of spinning *before* enqueueing Self.
524 //
525 // If the _owner is ready but OFFPROC we could use a YieldTo()
526 // operation to donate the remainder of this thread's quantum
527 // to the owner. This has subtle but beneficial affinity
528 // effects.
529
530 if (TrySpin (Self) > 0) {
531 assert (_owner == Self , "invariant") ;
532 assert (_succ != Self , "invariant") ;
533 assert (_Responsible != Self , "invariant") ;
534 return ;
535 }
536
537 // The Spin failed -- Enqueue and park the thread ...
538 assert (_succ != Self , "invariant") ;
539 assert (_owner != Self , "invariant") ;
540 assert (_Responsible != Self , "invariant") ;
541
542 // Enqueue "Self" on ObjectMonitor's _cxq.
543 //
544 // Node acts as a proxy for Self.
545 // As an aside, if were to ever rewrite the synchronization code mostly
546 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
547 // Java objects. This would avoid awkward lifecycle and liveness issues,
548 // as well as eliminate a subset of ABA issues.
549 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
550 //
551
552 ObjectWaiter node(Self) ;
553 Self->_ParkEvent->reset() ;
554 node._prev = (ObjectWaiter *) 0xBAD ;
555 node.TState = ObjectWaiter::TS_CXQ ;
556
557 // Push "Self" onto the front of the _cxq.
558 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
559 // Note that spinning tends to reduce the rate at which threads
560 // enqueue and dequeue on EntryList|cxq.
561 ObjectWaiter * nxt ;
562 for (;;) {
563 node._next = nxt = _cxq ;
564 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
565
566 // Interference - the CAS failed because _cxq changed. Just retry.
567 // As an optional optimization we retry the lock.
568 if (TryLock (Self) > 0) {
569 assert (_succ != Self , "invariant") ;
570 assert (_owner == Self , "invariant") ;
571 assert (_Responsible != Self , "invariant") ;
572 return ;
573 }
574 }
575
576 // Check for cxq|EntryList edge transition to non-null. This indicates
577 // the onset of contention. While contention persists exiting threads
578 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
579 // operations revert to the faster 1-0 mode. This enter operation may interleave
580 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
581 // arrange for one of the contending thread to use a timed park() operations
582 // to detect and recover from the race. (Stranding is form of progress failure
583 // where the monitor is unlocked but all the contending threads remain parked).
584 // That is, at least one of the contended threads will periodically poll _owner.
585 // One of the contending threads will become the designated "Responsible" thread.
586 // The Responsible thread uses a timed park instead of a normal indefinite park
587 // operation -- it periodically wakes and checks for and recovers from potential
588 // strandings admitted by 1-0 exit operations. We need at most one Responsible
589 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
590 // be responsible for a monitor.
591 //
592 // Currently, one of the contended threads takes on the added role of "Responsible".
593 // A viable alternative would be to use a dedicated "stranding checker" thread
594 // that periodically iterated over all the threads (or active monitors) and unparked
595 // successors where there was risk of stranding. This would help eliminate the
596 // timer scalability issues we see on some platforms as we'd only have one thread
597 // -- the checker -- parked on a timer.
598
599 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
600 // Try to assume the role of responsible thread for the monitor.
601 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
602 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
603 }
604
605 // The lock have been released while this thread was occupied queueing
606 // itself onto _cxq. To close the race and avoid "stranding" and
607 // progress-liveness failure we must resample-retry _owner before parking.
608 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
609 // In this case the ST-MEMBAR is accomplished with CAS().
610 //
611 // TODO: Defer all thread state transitions until park-time.
612 // Since state transitions are heavy and inefficient we'd like
613 // to defer the state transitions until absolutely necessary,
614 // and in doing so avoid some transitions ...
615
616 TEVENT (Inflated enter - Contention) ;
617 int nWakeups = 0 ;
618 int RecheckInterval = 1 ;
619
620 for (;;) {
621
622 if (TryLock (Self) > 0) break ;
623 assert (_owner != Self, "invariant") ;
624
625 if ((SyncFlags & 2) && _Responsible == NULL) {
626 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
627 }
628
629 // park self
630 if (_Responsible == Self || (SyncFlags & 1)) {
631 TEVENT (Inflated enter - park TIMED) ;
632 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
633 // Increase the RecheckInterval, but clamp the value.
634 RecheckInterval *= 8 ;
635 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
636 } else {
637 TEVENT (Inflated enter - park UNTIMED) ;
638 Self->_ParkEvent->park() ;
639 }
640
641 if (TryLock(Self) > 0) break ;
642
643 // The lock is still contested.
644 // Keep a tally of the # of futile wakeups.
645 // Note that the counter is not protected by a lock or updated by atomics.
646 // That is by design - we trade "lossy" counters which are exposed to
647 // races during updates for a lower probe effect.
648 TEVENT (Inflated enter - Futile wakeup) ;
649 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
650 ObjectMonitor::_sync_FutileWakeups->inc() ;
651 }
652 ++ nWakeups ;
653
654 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
655 // We can defer clearing _succ until after the spin completes
656 // TrySpin() must tolerate being called with _succ == Self.
657 // Try yet another round of adaptive spinning.
658 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
659
660 // We can find that we were unpark()ed and redesignated _succ while
661 // we were spinning. That's harmless. If we iterate and call park(),
662 // park() will consume the event and return immediately and we'll
663 // just spin again. This pattern can repeat, leaving _succ to simply
664 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
665 // Alternately, we can sample fired() here, and if set, forgo spinning
666 // in the next iteration.
667
668 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
669 Self->_ParkEvent->reset() ;
670 OrderAccess::fence() ;
671 }
672 if (_succ == Self) _succ = NULL ;
673
674 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
675 OrderAccess::fence() ;
676 }
677
678 // Egress :
679 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
680 // Normally we'll find Self on the EntryList .
681 // From the perspective of the lock owner (this thread), the
682 // EntryList is stable and cxq is prepend-only.
683 // The head of cxq is volatile but the interior is stable.
684 // In addition, Self.TState is stable.
685
686 assert (_owner == Self , "invariant") ;
687 assert (object() != NULL , "invariant") ;
688 // I'd like to write:
689 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
690 // but as we're at a safepoint that's not safe.
691
692 UnlinkAfterAcquire (Self, &node) ;
693 if (_succ == Self) _succ = NULL ;
694
695 assert (_succ != Self, "invariant") ;
696 if (_Responsible == Self) {
697 _Responsible = NULL ;
698 OrderAccess::fence(); // Dekker pivot-point
699
700 // We may leave threads on cxq|EntryList without a designated
701 // "Responsible" thread. This is benign. When this thread subsequently
702 // exits the monitor it can "see" such preexisting "old" threads --
703 // threads that arrived on the cxq|EntryList before the fence, above --
704 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
705 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
706 // non-null and elect a new "Responsible" timer thread.
707 //
708 // This thread executes:
709 // ST Responsible=null; MEMBAR (in enter epilog - here)
710 // LD cxq|EntryList (in subsequent exit)
711 //
712 // Entering threads in the slow/contended path execute:
713 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
714 // The (ST cxq; MEMBAR) is accomplished with CAS().
715 //
716 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
717 // exit operation from floating above the ST Responsible=null.
718 }
719
720 // We've acquired ownership with CAS().
721 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
722 // But since the CAS() this thread may have also stored into _succ,
723 // EntryList, cxq or Responsible. These meta-data updates must be
724 // visible __before this thread subsequently drops the lock.
725 // Consider what could occur if we didn't enforce this constraint --
726 // STs to monitor meta-data and user-data could reorder with (become
727 // visible after) the ST in exit that drops ownership of the lock.
728 // Some other thread could then acquire the lock, but observe inconsistent
729 // or old monitor meta-data and heap data. That violates the JMM.
730 // To that end, the 1-0 exit() operation must have at least STST|LDST
731 // "release" barrier semantics. Specifically, there must be at least a
732 // STST|LDST barrier in exit() before the ST of null into _owner that drops
733 // the lock. The barrier ensures that changes to monitor meta-data and data
734 // protected by the lock will be visible before we release the lock, and
735 // therefore before some other thread (CPU) has a chance to acquire the lock.
736 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
737 //
738 // Critically, any prior STs to _succ or EntryList must be visible before
739 // the ST of null into _owner in the *subsequent* (following) corresponding
740 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
741 // execute a serializing instruction.
742
743 if (SyncFlags & 8) {
744 OrderAccess::fence() ;
745 }
746 return ;
747 }
748
749 // ReenterI() is a specialized inline form of the latter half of the
750 // contended slow-path from EnterI(). We use ReenterI() only for
751 // monitor reentry in wait().
752 //
753 // In the future we should reconcile EnterI() and ReenterI(), adding
754 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
755 // loop accordingly.
756
757 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
758 assert (Self != NULL , "invariant") ;
759 assert (SelfNode != NULL , "invariant") ;
760 assert (SelfNode->_thread == Self , "invariant") ;
761 assert (_waiters > 0 , "invariant") ;
762 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
763 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
764 JavaThread * jt = (JavaThread *) Self ;
765
766 int nWakeups = 0 ;
767 for (;;) {
768 ObjectWaiter::TStates v = SelfNode->TState ;
769 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
770 assert (_owner != Self, "invariant") ;
771
772 if (TryLock (Self) > 0) break ;
773 if (TrySpin (Self) > 0) break ;
774
775 TEVENT (Wait Reentry - parking) ;
776
777 // State transition wrappers around park() ...
778 // ReenterI() wisely defers state transitions until
779 // it's clear we must park the thread.
780 {
781 OSThreadContendState osts(Self->osthread());
782 ThreadBlockInVM tbivm(jt);
783
784 // cleared by handle_special_suspend_equivalent_condition()
785 // or java_suspend_self()
786 jt->set_suspend_equivalent();
787 if (SyncFlags & 1) {
788 Self->_ParkEvent->park ((jlong)1000) ;
789 } else {
790 Self->_ParkEvent->park () ;
791 }
792
793 // were we externally suspended while we were waiting?
794 for (;;) {
795 if (!ExitSuspendEquivalent (jt)) break ;
796 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
797 jt->java_suspend_self();
798 jt->set_suspend_equivalent();
799 }
800 }
801
802 // Try again, but just so we distinguish between futile wakeups and
803 // successful wakeups. The following test isn't algorithmically
804 // necessary, but it helps us maintain sensible statistics.
805 if (TryLock(Self) > 0) break ;
806
807 // The lock is still contested.
808 // Keep a tally of the # of futile wakeups.
809 // Note that the counter is not protected by a lock or updated by atomics.
810 // That is by design - we trade "lossy" counters which are exposed to
811 // races during updates for a lower probe effect.
812 TEVENT (Wait Reentry - futile wakeup) ;
813 ++ nWakeups ;
814
815 // Assuming this is not a spurious wakeup we'll normally
816 // find that _succ == Self.
817 if (_succ == Self) _succ = NULL ;
818
819 // Invariant: after clearing _succ a contending thread
820 // *must* retry _owner before parking.
821 OrderAccess::fence() ;
822
823 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
824 ObjectMonitor::_sync_FutileWakeups->inc() ;
825 }
826 }
827
828 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
829 // Normally we'll find Self on the EntryList.
830 // Unlinking from the EntryList is constant-time and atomic-free.
831 // From the perspective of the lock owner (this thread), the
832 // EntryList is stable and cxq is prepend-only.
833 // The head of cxq is volatile but the interior is stable.
834 // In addition, Self.TState is stable.
835
836 assert (_owner == Self, "invariant") ;
837 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
838 UnlinkAfterAcquire (Self, SelfNode) ;
839 if (_succ == Self) _succ = NULL ;
840 assert (_succ != Self, "invariant") ;
841 SelfNode->TState = ObjectWaiter::TS_RUN ;
842 OrderAccess::fence() ; // see comments at the end of EnterI()
843 }
844
845 // after the thread acquires the lock in ::enter(). Equally, we could defer
846 // unlinking the thread until ::exit()-time.
847
848 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
849 {
850 assert (_owner == Self, "invariant") ;
851 assert (SelfNode->_thread == Self, "invariant") ;
852
853 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
854 // Normal case: remove Self from the DLL EntryList .
855 // This is a constant-time operation.
856 ObjectWaiter * nxt = SelfNode->_next ;
857 ObjectWaiter * prv = SelfNode->_prev ;
858 if (nxt != NULL) nxt->_prev = prv ;
859 if (prv != NULL) prv->_next = nxt ;
860 if (SelfNode == _EntryList ) _EntryList = nxt ;
861 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
862 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
863 TEVENT (Unlink from EntryList) ;
864 } else {
865 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
866 // Inopportune interleaving -- Self is still on the cxq.
867 // This usually means the enqueue of self raced an exiting thread.
868 // Normally we'll find Self near the front of the cxq, so
869 // dequeueing is typically fast. If needbe we can accelerate
870 // this with some MCS/CHL-like bidirectional list hints and advisory
871 // back-links so dequeueing from the interior will normally operate
872 // in constant-time.
873 // Dequeue Self from either the head (with CAS) or from the interior
874 // with a linear-time scan and normal non-atomic memory operations.
875 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
876 // and then unlink Self from EntryList. We have to drain eventually,
877 // so it might as well be now.
878
879 ObjectWaiter * v = _cxq ;
880 assert (v != NULL, "invariant") ;
881 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
882 // The CAS above can fail from interference IFF a "RAT" arrived.
883 // In that case Self must be in the interior and can no longer be
884 // at the head of cxq.
885 if (v == SelfNode) {
886 assert (_cxq != v, "invariant") ;
887 v = _cxq ; // CAS above failed - start scan at head of list
888 }
889 ObjectWaiter * p ;
890 ObjectWaiter * q = NULL ;
891 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
892 q = p ;
893 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
894 }
895 assert (v != SelfNode, "invariant") ;
896 assert (p == SelfNode, "Node not found on cxq") ;
897 assert (p != _cxq, "invariant") ;
898 assert (q != NULL, "invariant") ;
899 assert (q->_next == p, "invariant") ;
900 q->_next = p->_next ;
901 }
902 TEVENT (Unlink from cxq) ;
903 }
904
905 // Diagnostic hygiene ...
906 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
907 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
908 SelfNode->TState = ObjectWaiter::TS_RUN ;
909 }
910
911 // -----------------------------------------------------------------------------
912 // Exit support
913 //
914 // exit()
915 // ~~~~~~
916 // Note that the collector can't reclaim the objectMonitor or deflate
917 // the object out from underneath the thread calling ::exit() as the
918 // thread calling ::exit() never transitions to a stable state.
919 // This inhibits GC, which in turn inhibits asynchronous (and
920 // inopportune) reclamation of "this".
921 //
922 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
923 // There's one exception to the claim above, however. EnterI() can call
924 // exit() to drop a lock if the acquirer has been externally suspended.
925 // In that case exit() is called with _thread_state as _thread_blocked,
926 // but the monitor's _count field is > 0, which inhibits reclamation.
927 //
928 // 1-0 exit
929 // ~~~~~~~~
930 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
931 // the fast-path operators have been optimized so the common ::exit()
932 // operation is 1-0. See i486.ad fast_unlock(), for instance.
933 // The code emitted by fast_unlock() elides the usual MEMBAR. This
934 // greatly improves latency -- MEMBAR and CAS having considerable local
935 // latency on modern processors -- but at the cost of "stranding". Absent the
936 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
937 // ::enter() path, resulting in the entering thread being stranding
938 // and a progress-liveness failure. Stranding is extremely rare.
939 // We use timers (timed park operations) & periodic polling to detect
940 // and recover from stranding. Potentially stranded threads periodically
941 // wake up and poll the lock. See the usage of the _Responsible variable.
942 //
943 // The CAS() in enter provides for safety and exclusion, while the CAS or
944 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
945 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
946 // We detect and recover from stranding with timers.
947 //
948 // If a thread transiently strands it'll park until (a) another
949 // thread acquires the lock and then drops the lock, at which time the
950 // exiting thread will notice and unpark the stranded thread, or, (b)
951 // the timer expires. If the lock is high traffic then the stranding latency
952 // will be low due to (a). If the lock is low traffic then the odds of
953 // stranding are lower, although the worst-case stranding latency
954 // is longer. Critically, we don't want to put excessive load in the
955 // platform's timer subsystem. We want to minimize both the timer injection
956 // rate (timers created/sec) as well as the number of timers active at
957 // any one time. (more precisely, we want to minimize timer-seconds, which is
958 // the integral of the # of active timers at any instant over time).
959 // Both impinge on OS scalability. Given that, at most one thread parked on
960 // a monitor will use a timer.
961
962 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
963 Thread * Self = THREAD ;
964 if (THREAD != _owner) {
965 if (THREAD->is_lock_owned((address) _owner)) {
966 // Transmute _owner from a BasicLock pointer to a Thread address.
967 // We don't need to hold _mutex for this transition.
968 // Non-null to Non-null is safe as long as all readers can
969 // tolerate either flavor.
970 assert (_recursions == 0, "invariant") ;
971 _owner = THREAD ;
972 _recursions = 0 ;
973 OwnerIsThread = 1 ;
974 } else {
975 // NOTE: we need to handle unbalanced monitor enter/exit
976 // in native code by throwing an exception.
977 // TODO: Throw an IllegalMonitorStateException ?
978 TEVENT (Exit - Throw IMSX) ;
979 assert(false, "Non-balanced monitor enter/exit!");
980 if (false) {
981 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
982 }
983 return;
984 }
985 }
986
987 if (_recursions != 0) {
988 _recursions--; // this is simple recursive enter
989 TEVENT (Inflated exit - recursive) ;
990 return ;
991 }
992
993 // Invariant: after setting Responsible=null an thread must execute
994 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
995 if ((SyncFlags & 4) == 0) {
996 _Responsible = NULL ;
997 }
998
999 #if INCLUDE_JFR
1000 // get the owner's thread id for the MonitorEnter event
1001 // if it is enabled and the thread isn't suspended
1002 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1003 _previous_owner_tid = JFR_THREAD_ID(Self);
1004 }
1005 #endif
1006
1007 for (;;) {
1008 assert (THREAD == _owner, "invariant") ;
1009
1010
1011 if (Knob_ExitPolicy == 0) {
1012 // release semantics: prior loads and stores from within the critical section
1013 // must not float (reorder) past the following store that drops the lock.
1014 // On SPARC that requires MEMBAR #loadstore|#storestore.
1015 // But of course in TSO #loadstore|#storestore is not required.
1016 // I'd like to write one of the following:
1017 // A. OrderAccess::release() ; _owner = NULL
1018 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
1019 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
1020 // store into a _dummy variable. That store is not needed, but can result
1021 // in massive wasteful coherency traffic on classic SMP systems.
1022 // Instead, I use release_store(), which is implemented as just a simple
1023 // ST on x64, x86 and SPARC.
1024 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1025 OrderAccess::storeload() ; // See if we need to wake a successor
1026 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1027 TEVENT (Inflated exit - simple egress) ;
1028 return ;
1029 }
1030 TEVENT (Inflated exit - complex egress) ;
1031
1032 // Normally the exiting thread is responsible for ensuring succession,
1033 // but if other successors are ready or other entering threads are spinning
1034 // then this thread can simply store NULL into _owner and exit without
1035 // waking a successor. The existence of spinners or ready successors
1036 // guarantees proper succession (liveness). Responsibility passes to the
1037 // ready or running successors. The exiting thread delegates the duty.
1038 // More precisely, if a successor already exists this thread is absolved
1039 // of the responsibility of waking (unparking) one.
1040 //
1041 // The _succ variable is critical to reducing futile wakeup frequency.
1042 // _succ identifies the "heir presumptive" thread that has been made
1043 // ready (unparked) but that has not yet run. We need only one such
1044 // successor thread to guarantee progress.
1045 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1046 // section 3.3 "Futile Wakeup Throttling" for details.
1047 //
1048 // Note that spinners in Enter() also set _succ non-null.
1049 // In the current implementation spinners opportunistically set
1050 // _succ so that exiting threads might avoid waking a successor.
1051 // Another less appealing alternative would be for the exiting thread
1052 // to drop the lock and then spin briefly to see if a spinner managed
1053 // to acquire the lock. If so, the exiting thread could exit
1054 // immediately without waking a successor, otherwise the exiting
1055 // thread would need to dequeue and wake a successor.
1056 // (Note that we'd need to make the post-drop spin short, but no
1057 // shorter than the worst-case round-trip cache-line migration time.
1058 // The dropped lock needs to become visible to the spinner, and then
1059 // the acquisition of the lock by the spinner must become visible to
1060 // the exiting thread).
1061 //
1062
1063 // It appears that an heir-presumptive (successor) must be made ready.
1064 // Only the current lock owner can manipulate the EntryList or
1065 // drain _cxq, so we need to reacquire the lock. If we fail
1066 // to reacquire the lock the responsibility for ensuring succession
1067 // falls to the new owner.
1068 //
1069 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1070 return ;
1071 }
1072 TEVENT (Exit - Reacquired) ;
1073 } else {
1074 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1075 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1076 OrderAccess::storeload() ;
1077 // Ratify the previously observed values.
1078 if (_cxq == NULL || _succ != NULL) {
1079 TEVENT (Inflated exit - simple egress) ;
1080 return ;
1081 }
1082
1083 // inopportune interleaving -- the exiting thread (this thread)
1084 // in the fast-exit path raced an entering thread in the slow-enter
1085 // path.
1086 // We have two choices:
1087 // A. Try to reacquire the lock.
1088 // If the CAS() fails return immediately, otherwise
1089 // we either restart/rerun the exit operation, or simply
1090 // fall-through into the code below which wakes a successor.
1091 // B. If the elements forming the EntryList|cxq are TSM
1092 // we could simply unpark() the lead thread and return
1093 // without having set _succ.
1094 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1095 TEVENT (Inflated exit - reacquired succeeded) ;
1096 return ;
1097 }
1098 TEVENT (Inflated exit - reacquired failed) ;
1099 } else {
1100 TEVENT (Inflated exit - complex egress) ;
1101 }
1102 }
1103
1104 guarantee (_owner == THREAD, "invariant") ;
1105
1106 ObjectWaiter * w = NULL ;
1107 int QMode = Knob_QMode ;
1108
1109 if (QMode == 2 && _cxq != NULL) {
1110 // QMode == 2 : cxq has precedence over EntryList.
1111 // Try to directly wake a successor from the cxq.
1112 // If successful, the successor will need to unlink itself from cxq.
1113 w = _cxq ;
1114 assert (w != NULL, "invariant") ;
1115 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1116 ExitEpilog (Self, w) ;
1117 return ;
1118 }
1119
1120 if (QMode == 3 && _cxq != NULL) {
1121 // Aggressively drain cxq into EntryList at the first opportunity.
1122 // This policy ensure that recently-run threads live at the head of EntryList.
1123 // Drain _cxq into EntryList - bulk transfer.
1124 // First, detach _cxq.
1125 // The following loop is tantamount to: w = swap (&cxq, NULL)
1126 w = _cxq ;
1127 for (;;) {
1128 assert (w != NULL, "Invariant") ;
1129 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1130 if (u == w) break ;
1131 w = u ;
1132 }
1133 assert (w != NULL , "invariant") ;
1134
1135 ObjectWaiter * q = NULL ;
1136 ObjectWaiter * p ;
1137 for (p = w ; p != NULL ; p = p->_next) {
1138 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1139 p->TState = ObjectWaiter::TS_ENTER ;
1140 p->_prev = q ;
1141 q = p ;
1142 }
1143
1144 // Append the RATs to the EntryList
1145 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1146 ObjectWaiter * Tail ;
1147 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1148 if (Tail == NULL) {
1149 _EntryList = w ;
1150 } else {
1151 Tail->_next = w ;
1152 w->_prev = Tail ;
1153 }
1154
1155 // Fall thru into code that tries to wake a successor from EntryList
1156 }
1157
1158 if (QMode == 4 && _cxq != NULL) {
1159 // Aggressively drain cxq into EntryList at the first opportunity.
1160 // This policy ensure that recently-run threads live at the head of EntryList.
1161
1162 // Drain _cxq into EntryList - bulk transfer.
1163 // First, detach _cxq.
1164 // The following loop is tantamount to: w = swap (&cxq, NULL)
1165 w = _cxq ;
1166 for (;;) {
1167 assert (w != NULL, "Invariant") ;
1168 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1169 if (u == w) break ;
1170 w = u ;
1171 }
1172 assert (w != NULL , "invariant") ;
1173
1174 ObjectWaiter * q = NULL ;
1175 ObjectWaiter * p ;
1176 for (p = w ; p != NULL ; p = p->_next) {
1177 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1178 p->TState = ObjectWaiter::TS_ENTER ;
1179 p->_prev = q ;
1180 q = p ;
1181 }
1182
1183 // Prepend the RATs to the EntryList
1184 if (_EntryList != NULL) {
1185 q->_next = _EntryList ;
1186 _EntryList->_prev = q ;
1187 }
1188 _EntryList = w ;
1189
1190 // Fall thru into code that tries to wake a successor from EntryList
1191 }
1192
1193 w = _EntryList ;
1194 if (w != NULL) {
1195 // I'd like to write: guarantee (w->_thread != Self).
1196 // But in practice an exiting thread may find itself on the EntryList.
1197 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1198 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1199 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1200 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1201 // release the lock "O". T2 resumes immediately after the ST of null into
1202 // _owner, above. T2 notices that the EntryList is populated, so it
1203 // reacquires the lock and then finds itself on the EntryList.
1204 // Given all that, we have to tolerate the circumstance where "w" is
1205 // associated with Self.
1206 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1207 ExitEpilog (Self, w) ;
1208 return ;
1209 }
1210
1211 // If we find that both _cxq and EntryList are null then just
1212 // re-run the exit protocol from the top.
1213 w = _cxq ;
1214 if (w == NULL) continue ;
1215
1216 // Drain _cxq into EntryList - bulk transfer.
1217 // First, detach _cxq.
1218 // The following loop is tantamount to: w = swap (&cxq, NULL)
1219 for (;;) {
1220 assert (w != NULL, "Invariant") ;
1221 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1222 if (u == w) break ;
1223 w = u ;
1224 }
1225 TEVENT (Inflated exit - drain cxq into EntryList) ;
1226
1227 assert (w != NULL , "invariant") ;
1228 assert (_EntryList == NULL , "invariant") ;
1229
1230 // Convert the LIFO SLL anchored by _cxq into a DLL.
1231 // The list reorganization step operates in O(LENGTH(w)) time.
1232 // It's critical that this step operate quickly as
1233 // "Self" still holds the outer-lock, restricting parallelism
1234 // and effectively lengthening the critical section.
1235 // Invariant: s chases t chases u.
1236 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1237 // we have faster access to the tail.
1238
1239 if (QMode == 1) {
1240 // QMode == 1 : drain cxq to EntryList, reversing order
1241 // We also reverse the order of the list.
1242 ObjectWaiter * s = NULL ;
1243 ObjectWaiter * t = w ;
1244 ObjectWaiter * u = NULL ;
1245 while (t != NULL) {
1246 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1247 t->TState = ObjectWaiter::TS_ENTER ;
1248 u = t->_next ;
1249 t->_prev = u ;
1250 t->_next = s ;
1251 s = t;
1252 t = u ;
1253 }
1254 _EntryList = s ;
1255 assert (s != NULL, "invariant") ;
1256 } else {
1257 // QMode == 0 or QMode == 2
1258 _EntryList = w ;
1259 ObjectWaiter * q = NULL ;
1260 ObjectWaiter * p ;
1261 for (p = w ; p != NULL ; p = p->_next) {
1262 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1263 p->TState = ObjectWaiter::TS_ENTER ;
1264 p->_prev = q ;
1265 q = p ;
1266 }
1267 }
1268
1269 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1270 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1271
1272 // See if we can abdicate to a spinner instead of waking a thread.
1273 // A primary goal of the implementation is to reduce the
1274 // context-switch rate.
1275 if (_succ != NULL) continue;
1276
1277 w = _EntryList ;
1278 if (w != NULL) {
1279 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1280 ExitEpilog (Self, w) ;
1281 return ;
1282 }
1283 }
1284 }
1285
1286 // ExitSuspendEquivalent:
1287 // A faster alternate to handle_special_suspend_equivalent_condition()
1288 //
1289 // handle_special_suspend_equivalent_condition() unconditionally
1290 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1291 // operations have high latency. Note that in ::enter() we call HSSEC
1292 // while holding the monitor, so we effectively lengthen the critical sections.
1293 //
1294 // There are a number of possible solutions:
1295 //
1296 // A. To ameliorate the problem we might also defer state transitions
1297 // to as late as possible -- just prior to parking.
1298 // Given that, we'd call HSSEC after having returned from park(),
1299 // but before attempting to acquire the monitor. This is only a
1300 // partial solution. It avoids calling HSSEC while holding the
1301 // monitor (good), but it still increases successor reacquisition latency --
1302 // the interval between unparking a successor and the time the successor
1303 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1304 // If we use this technique we can also avoid EnterI()-exit() loop
1305 // in ::enter() where we iteratively drop the lock and then attempt
1306 // to reacquire it after suspending.
1307 //
1308 // B. In the future we might fold all the suspend bits into a
1309 // composite per-thread suspend flag and then update it with CAS().
1310 // Alternately, a Dekker-like mechanism with multiple variables
1311 // would suffice:
1312 // ST Self->_suspend_equivalent = false
1313 // MEMBAR
1314 // LD Self_>_suspend_flags
1315 //
1316
1317
1318 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1319 int Mode = Knob_FastHSSEC ;
1320 if (Mode && !jSelf->is_external_suspend()) {
1321 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1322 jSelf->clear_suspend_equivalent() ;
1323 if (2 == Mode) OrderAccess::storeload() ;
1324 if (!jSelf->is_external_suspend()) return false ;
1325 // We raced a suspension -- fall thru into the slow path
1326 TEVENT (ExitSuspendEquivalent - raced) ;
1327 jSelf->set_suspend_equivalent() ;
1328 }
1329 return jSelf->handle_special_suspend_equivalent_condition() ;
1330 }
1331
1332
1333 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1334 assert (_owner == Self, "invariant") ;
1335
1336 // Exit protocol:
1337 // 1. ST _succ = wakee
1338 // 2. membar #loadstore|#storestore;
1339 // 2. ST _owner = NULL
1340 // 3. unpark(wakee)
1341
1342 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1343 ParkEvent * Trigger = Wakee->_event ;
1344
1345 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1346 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1347 // out-of-scope (non-extant).
1348 Wakee = NULL ;
1349
1350 // Drop the lock
1351 OrderAccess::release_store_ptr (&_owner, NULL) ;
1352 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1353
1354 if (SafepointSynchronize::do_call_back()) {
1355 TEVENT (unpark before SAFEPOINT) ;
1356 }
1357
1358 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1359 Trigger->unpark() ;
1360
1361 // Maintain stats and report events to JVMTI
1362 if (ObjectMonitor::_sync_Parks != NULL) {
1363 ObjectMonitor::_sync_Parks->inc() ;
1364 }
1365 }
1366
1367
1368 // -----------------------------------------------------------------------------
1369 // Class Loader deadlock handling.
1370 //
1371 // complete_exit exits a lock returning recursion count
1372 // complete_exit/reenter operate as a wait without waiting
1373 // complete_exit requires an inflated monitor
1374 // The _owner field is not always the Thread addr even with an
1375 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1376 // thread due to contention.
1377 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1378 Thread * const Self = THREAD;
1379 assert(Self->is_Java_thread(), "Must be Java thread!");
1380 JavaThread *jt = (JavaThread *)THREAD;
1381
1382 DeferredInitialize();
1383
1384 if (THREAD != _owner) {
1385 if (THREAD->is_lock_owned ((address)_owner)) {
1386 assert(_recursions == 0, "internal state error");
1387 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1388 _recursions = 0 ;
1389 OwnerIsThread = 1 ;
1390 }
1391 }
1392
1393 guarantee(Self == _owner, "complete_exit not owner");
1394 intptr_t save = _recursions; // record the old recursion count
1395 _recursions = 0; // set the recursion level to be 0
1396 exit (true, Self) ; // exit the monitor
1397 guarantee (_owner != Self, "invariant");
1398 return save;
1399 }
1400
1401 // reenter() enters a lock and sets recursion count
1402 // complete_exit/reenter operate as a wait without waiting
1403 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1404 Thread * const Self = THREAD;
1405 assert(Self->is_Java_thread(), "Must be Java thread!");
1406 JavaThread *jt = (JavaThread *)THREAD;
1407
1408 guarantee(_owner != Self, "reenter already owner");
1409 enter (THREAD); // enter the monitor
1410 guarantee (_recursions == 0, "reenter recursion");
1411 _recursions = recursions;
1412 return;
1413 }
1414
1415
1416 // -----------------------------------------------------------------------------
1417 // A macro is used below because there may already be a pending
1418 // exception which should not abort the execution of the routines
1419 // which use this (which is why we don't put this into check_slow and
1420 // call it with a CHECK argument).
1421
1422 #define CHECK_OWNER() \
1423 do { \
1424 if (THREAD != _owner) { \
1425 if (THREAD->is_lock_owned((address) _owner)) { \
1426 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1427 _recursions = 0; \
1428 OwnerIsThread = 1 ; \
1429 } else { \
1430 TEVENT (Throw IMSX) ; \
1431 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1432 } \
1433 } \
1434 } while (false)
1435
1436 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1437 // TODO-FIXME: remove check_slow() -- it's likely dead.
1438
1439 void ObjectMonitor::check_slow(TRAPS) {
1440 TEVENT (check_slow - throw IMSX) ;
1441 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1442 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1443 }
1444
1445 static int Adjust (volatile int * adr, int dx) {
1446 int v ;
1447 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1448 return v ;
1449 }
1450
1451 // helper method for posting a monitor wait event
1452 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1453 ObjectMonitor* monitor,
1454 jlong notifier_tid,
1455 jlong timeout,
1456 bool timedout) {
1457 assert(monitor != NULL, "invariant");
1458 event->set_monitorClass(((oop)monitor->object())->klass());
1459 event->set_timeout(timeout);
1460 event->set_address((uintptr_t)monitor->object_addr());
1461 event->set_notifier((u8)notifier_tid);
1462 event->set_timedOut(timedout);
1463 event->commit();
1464 }
1465
1466 // -----------------------------------------------------------------------------
1467 // Wait/Notify/NotifyAll
1468 //
1469 // Note: a subset of changes to ObjectMonitor::wait()
1470 // will need to be replicated in complete_exit above
1471 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1472 Thread * const Self = THREAD ;
1473 assert(Self->is_Java_thread(), "Must be Java thread!");
1474 JavaThread *jt = (JavaThread *)THREAD;
1475
1476 DeferredInitialize () ;
1477
1478 // Throw IMSX or IEX.
1479 CHECK_OWNER();
1480
1481 EventJavaMonitorWait event;
1482
1483 // check for a pending interrupt
1484 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1485 // post monitor waited event. Note that this is past-tense, we are done waiting.
1486 if (JvmtiExport::should_post_monitor_waited()) {
1487 // Note: 'false' parameter is passed here because the
1488 // wait was not timed out due to thread interrupt.
1489 JvmtiExport::post_monitor_waited(jt, this, false);
1490
1491 // In this short circuit of the monitor wait protocol, the
1492 // current thread never drops ownership of the monitor and
1493 // never gets added to the wait queue so the current thread
1494 // cannot be made the successor. This means that the
1495 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1496 // consume an unpark() meant for the ParkEvent associated with
1497 // this ObjectMonitor.
1498 }
1499 if (event.should_commit()) {
1500 post_monitor_wait_event(&event, this, 0, millis, false);
1501 }
1502 TEVENT (Wait - Throw IEX) ;
1503 THROW(vmSymbols::java_lang_InterruptedException());
1504 return ;
1505 }
1506
1507 TEVENT (Wait) ;
1508
1509 assert (Self->_Stalled == 0, "invariant") ;
1510 Self->_Stalled = intptr_t(this) ;
1511 jt->set_current_waiting_monitor(this);
1512
1513 // create a node to be put into the queue
1514 // Critically, after we reset() the event but prior to park(), we must check
1515 // for a pending interrupt.
1516 ObjectWaiter node(Self);
1517 node.TState = ObjectWaiter::TS_WAIT ;
1518 Self->_ParkEvent->reset() ;
1519 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1520
1521 // Enter the waiting queue, which is a circular doubly linked list in this case
1522 // but it could be a priority queue or any data structure.
1523 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1524 // by the the owner of the monitor *except* in the case where park()
1525 // returns because of a timeout of interrupt. Contention is exceptionally rare
1526 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1527
1528 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1529 AddWaiter (&node) ;
1530 Thread::SpinRelease (&_WaitSetLock) ;
1531
1532 if ((SyncFlags & 4) == 0) {
1533 _Responsible = NULL ;
1534 }
1535 intptr_t save = _recursions; // record the old recursion count
1536 _waiters++; // increment the number of waiters
1537 _recursions = 0; // set the recursion level to be 1
1538 exit (true, Self) ; // exit the monitor
1539 guarantee (_owner != Self, "invariant") ;
1540
1541 // The thread is on the WaitSet list - now park() it.
1542 // On MP systems it's conceivable that a brief spin before we park
1543 // could be profitable.
1544 //
1545 // TODO-FIXME: change the following logic to a loop of the form
1546 // while (!timeout && !interrupted && _notified == 0) park()
1547
1548 int ret = OS_OK ;
1549 int WasNotified = 0 ;
1550 { // State transition wrappers
1551 OSThread* osthread = Self->osthread();
1552 OSThreadWaitState osts(osthread, true);
1553 {
1554 ThreadBlockInVM tbivm(jt);
1555 // Thread is in thread_blocked state and oop access is unsafe.
1556 jt->set_suspend_equivalent();
1557
1558 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1559 // Intentionally empty
1560 } else
1561 if (node._notified == 0) {
1562 if (millis <= 0) {
1563 Self->_ParkEvent->park () ;
1564 } else {
1565 ret = Self->_ParkEvent->park (millis) ;
1566 }
1567 }
1568
1569 // were we externally suspended while we were waiting?
1570 if (ExitSuspendEquivalent (jt)) {
1571 // TODO-FIXME: add -- if succ == Self then succ = null.
1572 jt->java_suspend_self();
1573 }
1574
1575 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1576
1577
1578 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1579 // from the WaitSet to the EntryList.
1580 // See if we need to remove Node from the WaitSet.
1581 // We use double-checked locking to avoid grabbing _WaitSetLock
1582 // if the thread is not on the wait queue.
1583 //
1584 // Note that we don't need a fence before the fetch of TState.
1585 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1586 // written by the is thread. (perhaps the fetch might even be satisfied
1587 // by a look-aside into the processor's own store buffer, although given
1588 // the length of the code path between the prior ST and this load that's
1589 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1590 // then we'll acquire the lock and then re-fetch a fresh TState value.
1591 // That is, we fail toward safety.
1592
1593 if (node.TState == ObjectWaiter::TS_WAIT) {
1594 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1595 if (node.TState == ObjectWaiter::TS_WAIT) {
1596 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1597 assert(node._notified == 0, "invariant");
1598 node.TState = ObjectWaiter::TS_RUN ;
1599 }
1600 Thread::SpinRelease (&_WaitSetLock) ;
1601 }
1602
1603 // The thread is now either on off-list (TS_RUN),
1604 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1605 // The Node's TState variable is stable from the perspective of this thread.
1606 // No other threads will asynchronously modify TState.
1607 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1608 OrderAccess::loadload() ;
1609 if (_succ == Self) _succ = NULL ;
1610 WasNotified = node._notified ;
1611
1612 // Reentry phase -- reacquire the monitor.
1613 // re-enter contended monitor after object.wait().
1614 // retain OBJECT_WAIT state until re-enter successfully completes
1615 // Thread state is thread_in_vm and oop access is again safe,
1616 // although the raw address of the object may have changed.
1617 // (Don't cache naked oops over safepoints, of course).
1618
1619 // post monitor waited event. Note that this is past-tense, we are done waiting.
1620 if (JvmtiExport::should_post_monitor_waited()) {
1621 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1622
1623 if (node._notified != 0 && _succ == Self) {
1624 // In this part of the monitor wait-notify-reenter protocol it
1625 // is possible (and normal) for another thread to do a fastpath
1626 // monitor enter-exit while this thread is still trying to get
1627 // to the reenter portion of the protocol.
1628 //
1629 // The ObjectMonitor was notified and the current thread is
1630 // the successor which also means that an unpark() has already
1631 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1632 // consume the unpark() that was done when the successor was
1633 // set because the same ParkEvent is shared between Java
1634 // monitors and JVM/TI RawMonitors (for now).
1635 //
1636 // We redo the unpark() to ensure forward progress, i.e., we
1637 // don't want all pending threads hanging (parked) with none
1638 // entering the unlocked monitor.
1639 node._event->unpark();
1640 }
1641 }
1642
1643 if (event.should_commit()) {
1644 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1645 }
1646
1647 OrderAccess::fence() ;
1648
1649 assert (Self->_Stalled != 0, "invariant") ;
1650 Self->_Stalled = 0 ;
1651
1652 assert (_owner != Self, "invariant") ;
1653 ObjectWaiter::TStates v = node.TState ;
1654 if (v == ObjectWaiter::TS_RUN) {
1655 enter (Self) ;
1656 } else {
1657 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1658 ReenterI (Self, &node) ;
1659 node.wait_reenter_end(this);
1660 }
1661
1662 // Self has reacquired the lock.
1663 // Lifecycle - the node representing Self must not appear on any queues.
1664 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1665 // want residual elements associated with this thread left on any lists.
1666 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1667 assert (_owner == Self, "invariant") ;
1668 assert (_succ != Self , "invariant") ;
1669 } // OSThreadWaitState()
1670
1671 jt->set_current_waiting_monitor(NULL);
1672
1673 guarantee (_recursions == 0, "invariant") ;
1674 _recursions = save; // restore the old recursion count
1675 _waiters--; // decrement the number of waiters
1676
1677 // Verify a few postconditions
1678 assert (_owner == Self , "invariant") ;
1679 assert (_succ != Self , "invariant") ;
1680 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1681
1682 if (SyncFlags & 32) {
1683 OrderAccess::fence() ;
1684 }
1685
1686 // check if the notification happened
1687 if (!WasNotified) {
1688 // no, it could be timeout or Thread.interrupt() or both
1689 // check for interrupt event, otherwise it is timeout
1690 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1691 TEVENT (Wait - throw IEX from epilog) ;
1692 THROW(vmSymbols::java_lang_InterruptedException());
1693 }
1694 }
1695
1696 // NOTE: Spurious wake up will be consider as timeout.
1697 // Monitor notify has precedence over thread interrupt.
1698 }
1699
1700
1701 // Consider:
1702 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1703 // then instead of transferring a thread from the WaitSet to the EntryList
1704 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1705
1706 void ObjectMonitor::notify(TRAPS) {
1707 CHECK_OWNER();
1708 if (_WaitSet == NULL) {
1709 TEVENT (Empty-Notify) ;
1710 return ;
1711 }
1712 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1713
1714 int Policy = Knob_MoveNotifyee ;
1715
1716 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1717 ObjectWaiter * iterator = DequeueWaiter() ;
1718 if (iterator != NULL) {
1719 TEVENT (Notify1 - Transfer) ;
1720 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1721 guarantee (iterator->_notified == 0, "invariant") ;
1722 if (Policy != 4) {
1723 iterator->TState = ObjectWaiter::TS_ENTER ;
1724 }
1725 iterator->_notified = 1 ;
1726 Thread * Self = THREAD;
1727 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1728
1729 ObjectWaiter * List = _EntryList ;
1730 if (List != NULL) {
1731 assert (List->_prev == NULL, "invariant") ;
1732 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1733 assert (List != iterator, "invariant") ;
1734 }
1735
1736 if (Policy == 0) { // prepend to EntryList
1737 if (List == NULL) {
1738 iterator->_next = iterator->_prev = NULL ;
1739 _EntryList = iterator ;
1740 } else {
1741 List->_prev = iterator ;
1742 iterator->_next = List ;
1743 iterator->_prev = NULL ;
1744 _EntryList = iterator ;
1745 }
1746 } else
1747 if (Policy == 1) { // append to EntryList
1748 if (List == NULL) {
1749 iterator->_next = iterator->_prev = NULL ;
1750 _EntryList = iterator ;
1751 } else {
1752 // CONSIDER: finding the tail currently requires a linear-time walk of
1753 // the EntryList. We can make tail access constant-time by converting to
1754 // a CDLL instead of using our current DLL.
1755 ObjectWaiter * Tail ;
1756 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1757 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1758 Tail->_next = iterator ;
1759 iterator->_prev = Tail ;
1760 iterator->_next = NULL ;
1761 }
1762 } else
1763 if (Policy == 2) { // prepend to cxq
1764 // prepend to cxq
1765 if (List == NULL) {
1766 iterator->_next = iterator->_prev = NULL ;
1767 _EntryList = iterator ;
1768 } else {
1769 iterator->TState = ObjectWaiter::TS_CXQ ;
1770 for (;;) {
1771 ObjectWaiter * Front = _cxq ;
1772 iterator->_next = Front ;
1773 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1774 break ;
1775 }
1776 }
1777 }
1778 } else
1779 if (Policy == 3) { // append to cxq
1780 iterator->TState = ObjectWaiter::TS_CXQ ;
1781 for (;;) {
1782 ObjectWaiter * Tail ;
1783 Tail = _cxq ;
1784 if (Tail == NULL) {
1785 iterator->_next = NULL ;
1786 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1787 break ;
1788 }
1789 } else {
1790 while (Tail->_next != NULL) Tail = Tail->_next ;
1791 Tail->_next = iterator ;
1792 iterator->_prev = Tail ;
1793 iterator->_next = NULL ;
1794 break ;
1795 }
1796 }
1797 } else {
1798 ParkEvent * ev = iterator->_event ;
1799 iterator->TState = ObjectWaiter::TS_RUN ;
1800 OrderAccess::fence() ;
1801 ev->unpark() ;
1802 }
1803
1804 if (Policy < 4) {
1805 iterator->wait_reenter_begin(this);
1806 }
1807
1808 // _WaitSetLock protects the wait queue, not the EntryList. We could
1809 // move the add-to-EntryList operation, above, outside the critical section
1810 // protected by _WaitSetLock. In practice that's not useful. With the
1811 // exception of wait() timeouts and interrupts the monitor owner
1812 // is the only thread that grabs _WaitSetLock. There's almost no contention
1813 // on _WaitSetLock so it's not profitable to reduce the length of the
1814 // critical section.
1815 }
1816
1817 Thread::SpinRelease (&_WaitSetLock) ;
1818
1819 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1820 ObjectMonitor::_sync_Notifications->inc() ;
1821 }
1822 }
1823
1824
1825 void ObjectMonitor::notifyAll(TRAPS) {
1826 CHECK_OWNER();
1827 ObjectWaiter* iterator;
1828 if (_WaitSet == NULL) {
1829 TEVENT (Empty-NotifyAll) ;
1830 return ;
1831 }
1832 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1833
1834 int Policy = Knob_MoveNotifyee ;
1835 int Tally = 0 ;
1836 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1837
1838 for (;;) {
1839 iterator = DequeueWaiter () ;
1840 if (iterator == NULL) break ;
1841 TEVENT (NotifyAll - Transfer1) ;
1842 ++Tally ;
1843
1844 // Disposition - what might we do with iterator ?
1845 // a. add it directly to the EntryList - either tail or head.
1846 // b. push it onto the front of the _cxq.
1847 // For now we use (a).
1848
1849 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1850 guarantee (iterator->_notified == 0, "invariant") ;
1851 iterator->_notified = 1 ;
1852 Thread * Self = THREAD;
1853 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1854 if (Policy != 4) {
1855 iterator->TState = ObjectWaiter::TS_ENTER ;
1856 }
1857
1858 ObjectWaiter * List = _EntryList ;
1859 if (List != NULL) {
1860 assert (List->_prev == NULL, "invariant") ;
1861 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1862 assert (List != iterator, "invariant") ;
1863 }
1864
1865 if (Policy == 0) { // prepend to EntryList
1866 if (List == NULL) {
1867 iterator->_next = iterator->_prev = NULL ;
1868 _EntryList = iterator ;
1869 } else {
1870 List->_prev = iterator ;
1871 iterator->_next = List ;
1872 iterator->_prev = NULL ;
1873 _EntryList = iterator ;
1874 }
1875 } else
1876 if (Policy == 1) { // append to EntryList
1877 if (List == NULL) {
1878 iterator->_next = iterator->_prev = NULL ;
1879 _EntryList = iterator ;
1880 } else {
1881 // CONSIDER: finding the tail currently requires a linear-time walk of
1882 // the EntryList. We can make tail access constant-time by converting to
1883 // a CDLL instead of using our current DLL.
1884 ObjectWaiter * Tail ;
1885 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1886 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1887 Tail->_next = iterator ;
1888 iterator->_prev = Tail ;
1889 iterator->_next = NULL ;
1890 }
1891 } else
1892 if (Policy == 2) { // prepend to cxq
1893 // prepend to cxq
1894 iterator->TState = ObjectWaiter::TS_CXQ ;
1895 for (;;) {
1896 ObjectWaiter * Front = _cxq ;
1897 iterator->_next = Front ;
1898 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1899 break ;
1900 }
1901 }
1902 } else
1903 if (Policy == 3) { // append to cxq
1904 iterator->TState = ObjectWaiter::TS_CXQ ;
1905 for (;;) {
1906 ObjectWaiter * Tail ;
1907 Tail = _cxq ;
1908 if (Tail == NULL) {
1909 iterator->_next = NULL ;
1910 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1911 break ;
1912 }
1913 } else {
1914 while (Tail->_next != NULL) Tail = Tail->_next ;
1915 Tail->_next = iterator ;
1916 iterator->_prev = Tail ;
1917 iterator->_next = NULL ;
1918 break ;
1919 }
1920 }
1921 } else {
1922 ParkEvent * ev = iterator->_event ;
1923 iterator->TState = ObjectWaiter::TS_RUN ;
1924 OrderAccess::fence() ;
1925 ev->unpark() ;
1926 }
1927
1928 if (Policy < 4) {
1929 iterator->wait_reenter_begin(this);
1930 }
1931
1932 // _WaitSetLock protects the wait queue, not the EntryList. We could
1933 // move the add-to-EntryList operation, above, outside the critical section
1934 // protected by _WaitSetLock. In practice that's not useful. With the
1935 // exception of wait() timeouts and interrupts the monitor owner
1936 // is the only thread that grabs _WaitSetLock. There's almost no contention
1937 // on _WaitSetLock so it's not profitable to reduce the length of the
1938 // critical section.
1939 }
1940
1941 Thread::SpinRelease (&_WaitSetLock) ;
1942
1943 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1944 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1945 }
1946 }
1947
1948 // -----------------------------------------------------------------------------
1949 // Adaptive Spinning Support
1950 //
1951 // Adaptive spin-then-block - rational spinning
1952 //
1953 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1954 // algorithm. On high order SMP systems it would be better to start with
1955 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1956 // a contending thread could enqueue itself on the cxq and then spin locally
1957 // on a thread-specific variable such as its ParkEvent._Event flag.
1958 // That's left as an exercise for the reader. Note that global spinning is
1959 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1960 // low latency and massive bandwidth.
1961 //
1962 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1963 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1964 // (duration) or we can fix the count at approximately the duration of
1965 // a context switch and vary the frequency. Of course we could also
1966 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1967 // For a description of 'Adaptive spin-then-block mutual exclusion in
1968 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1969 //
1970 // This implementation varies the duration "D", where D varies with
1971 // the success rate of recent spin attempts. (D is capped at approximately
1972 // length of a round-trip context switch). The success rate for recent
1973 // spin attempts is a good predictor of the success rate of future spin
1974 // attempts. The mechanism adapts automatically to varying critical
1975 // section length (lock modality), system load and degree of parallelism.
1976 // D is maintained per-monitor in _SpinDuration and is initialized
1977 // optimistically. Spin frequency is fixed at 100%.
1978 //
1979 // Note that _SpinDuration is volatile, but we update it without locks
1980 // or atomics. The code is designed so that _SpinDuration stays within
1981 // a reasonable range even in the presence of races. The arithmetic
1982 // operations on _SpinDuration are closed over the domain of legal values,
1983 // so at worst a race will install and older but still legal value.
1984 // At the very worst this introduces some apparent non-determinism.
1985 // We might spin when we shouldn't or vice-versa, but since the spin
1986 // count are relatively short, even in the worst case, the effect is harmless.
1987 //
1988 // Care must be taken that a low "D" value does not become an
1989 // an absorbing state. Transient spinning failures -- when spinning
1990 // is overall profitable -- should not cause the system to converge
1991 // on low "D" values. We want spinning to be stable and predictable
1992 // and fairly responsive to change and at the same time we don't want
1993 // it to oscillate, become metastable, be "too" non-deterministic,
1994 // or converge on or enter undesirable stable absorbing states.
1995 //
1996 // We implement a feedback-based control system -- using past behavior
1997 // to predict future behavior. We face two issues: (a) if the
1998 // input signal is random then the spin predictor won't provide optimal
1999 // results, and (b) if the signal frequency is too high then the control
2000 // system, which has some natural response lag, will "chase" the signal.
2001 // (b) can arise from multimodal lock hold times. Transient preemption
2002 // can also result in apparent bimodal lock hold times.
2003 // Although sub-optimal, neither condition is particularly harmful, as
2004 // in the worst-case we'll spin when we shouldn't or vice-versa.
2005 // The maximum spin duration is rather short so the failure modes aren't bad.
2006 // To be conservative, I've tuned the gain in system to bias toward
2007 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2008 // "rings" or oscillates between spinning and not spinning. This happens
2009 // when spinning is just on the cusp of profitability, however, so the
2010 // situation is not dire. The state is benign -- there's no need to add
2011 // hysteresis control to damp the transition rate between spinning and
2012 // not spinning.
2013 //
2014
2015 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
2016 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
2017
2018 // Spinning: Fixed frequency (100%), vary duration
2019
2020
2021 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
2022
2023 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2024 int ctr = Knob_FixedSpin ;
2025 if (ctr != 0) {
2026 while (--ctr >= 0) {
2027 if (TryLock (Self) > 0) return 1 ;
2028 SpinPause () ;
2029 }
2030 return 0 ;
2031 }
2032
2033 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
2034 if (TryLock(Self) > 0) {
2035 // Increase _SpinDuration ...
2036 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2037 // Raising _SpurDuration to the poverty line is key.
2038 int x = _SpinDuration ;
2039 if (x < Knob_SpinLimit) {
2040 if (x < Knob_Poverty) x = Knob_Poverty ;
2041 _SpinDuration = x + Knob_BonusB ;
2042 }
2043 return 1 ;
2044 }
2045 SpinPause () ;
2046 }
2047
2048 // Admission control - verify preconditions for spinning
2049 //
2050 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2051 // becoming an absorbing state. Put another way, we spin briefly to
2052 // sample, just in case the system load, parallelism, contention, or lock
2053 // modality changed.
2054 //
2055 // Consider the following alternative:
2056 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2057 // spin attempt. "Periodically" might mean after a tally of
2058 // the # of failed spin attempts (or iterations) reaches some threshold.
2059 // This takes us into the realm of 1-out-of-N spinning, where we
2060 // hold the duration constant but vary the frequency.
2061
2062 ctr = _SpinDuration ;
2063 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2064 if (ctr <= 0) return 0 ;
2065
2066 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2067 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2068 TEVENT (Spin abort - notrunnable [TOP]);
2069 return 0 ;
2070 }
2071
2072 int MaxSpin = Knob_MaxSpinners ;
2073 if (MaxSpin >= 0) {
2074 if (_Spinner > MaxSpin) {
2075 TEVENT (Spin abort -- too many spinners) ;
2076 return 0 ;
2077 }
2078 // Slighty racy, but benign ...
2079 Adjust (&_Spinner, 1) ;
2080 }
2081
2082 // We're good to spin ... spin ingress.
2083 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2084 // when preparing to LD...CAS _owner, etc and the CAS is likely
2085 // to succeed.
2086 int hits = 0 ;
2087 int msk = 0 ;
2088 int caspty = Knob_CASPenalty ;
2089 int oxpty = Knob_OXPenalty ;
2090 int sss = Knob_SpinSetSucc ;
2091 if (sss && _succ == NULL ) _succ = Self ;
2092 Thread * prv = NULL ;
2093
2094 // There are three ways to exit the following loop:
2095 // 1. A successful spin where this thread has acquired the lock.
2096 // 2. Spin failure with prejudice
2097 // 3. Spin failure without prejudice
2098
2099 while (--ctr >= 0) {
2100
2101 // Periodic polling -- Check for pending GC
2102 // Threads may spin while they're unsafe.
2103 // We don't want spinning threads to delay the JVM from reaching
2104 // a stop-the-world safepoint or to steal cycles from GC.
2105 // If we detect a pending safepoint we abort in order that
2106 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2107 // this thread, if safe, doesn't steal cycles from GC.
2108 // This is in keeping with the "no loitering in runtime" rule.
2109 // We periodically check to see if there's a safepoint pending.
2110 if ((ctr & 0xFF) == 0) {
2111 if (SafepointSynchronize::do_call_back()) {
2112 TEVENT (Spin: safepoint) ;
2113 goto Abort ; // abrupt spin egress
2114 }
2115 if (Knob_UsePause & 1) SpinPause () ;
2116
2117 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2118 if (hits > 50 && scb != NULL) {
2119 int abend = (*scb)(SpinCallbackArgument, 0) ;
2120 }
2121 }
2122
2123 if (Knob_UsePause & 2) SpinPause() ;
2124
2125 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2126 // This is useful on classic SMP systems, but is of less utility on
2127 // N1-style CMT platforms.
2128 //
2129 // Trade-off: lock acquisition latency vs coherency bandwidth.
2130 // Lock hold times are typically short. A histogram
2131 // of successful spin attempts shows that we usually acquire
2132 // the lock early in the spin. That suggests we want to
2133 // sample _owner frequently in the early phase of the spin,
2134 // but then back-off and sample less frequently as the spin
2135 // progresses. The back-off makes a good citizen on SMP big
2136 // SMP systems. Oversampling _owner can consume excessive
2137 // coherency bandwidth. Relatedly, if we _oversample _owner we
2138 // can inadvertently interfere with the the ST m->owner=null.
2139 // executed by the lock owner.
2140 if (ctr & msk) continue ;
2141 ++hits ;
2142 if ((hits & 0xF) == 0) {
2143 // The 0xF, above, corresponds to the exponent.
2144 // Consider: (msk+1)|msk
2145 msk = ((msk << 2)|3) & BackOffMask ;
2146 }
2147
2148 // Probe _owner with TATAS
2149 // If this thread observes the monitor transition or flicker
2150 // from locked to unlocked to locked, then the odds that this
2151 // thread will acquire the lock in this spin attempt go down
2152 // considerably. The same argument applies if the CAS fails
2153 // or if we observe _owner change from one non-null value to
2154 // another non-null value. In such cases we might abort
2155 // the spin without prejudice or apply a "penalty" to the
2156 // spin count-down variable "ctr", reducing it by 100, say.
2157
2158 Thread * ox = (Thread *) _owner ;
2159 if (ox == NULL) {
2160 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2161 if (ox == NULL) {
2162 // The CAS succeeded -- this thread acquired ownership
2163 // Take care of some bookkeeping to exit spin state.
2164 if (sss && _succ == Self) {
2165 _succ = NULL ;
2166 }
2167 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2168
2169 // Increase _SpinDuration :
2170 // The spin was successful (profitable) so we tend toward
2171 // longer spin attempts in the future.
2172 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2173 // If we acquired the lock early in the spin cycle it
2174 // makes sense to increase _SpinDuration proportionally.
2175 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2176 int x = _SpinDuration ;
2177 if (x < Knob_SpinLimit) {
2178 if (x < Knob_Poverty) x = Knob_Poverty ;
2179 _SpinDuration = x + Knob_Bonus ;
2180 }
2181 return 1 ;
2182 }
2183
2184 // The CAS failed ... we can take any of the following actions:
2185 // * penalize: ctr -= Knob_CASPenalty
2186 // * exit spin with prejudice -- goto Abort;
2187 // * exit spin without prejudice.
2188 // * Since CAS is high-latency, retry again immediately.
2189 prv = ox ;
2190 TEVENT (Spin: cas failed) ;
2191 if (caspty == -2) break ;
2192 if (caspty == -1) goto Abort ;
2193 ctr -= caspty ;
2194 continue ;
2195 }
2196
2197 // Did lock ownership change hands ?
2198 if (ox != prv && prv != NULL ) {
2199 TEVENT (spin: Owner changed)
2200 if (oxpty == -2) break ;
2201 if (oxpty == -1) goto Abort ;
2202 ctr -= oxpty ;
2203 }
2204 prv = ox ;
2205
2206 // Abort the spin if the owner is not executing.
2207 // The owner must be executing in order to drop the lock.
2208 // Spinning while the owner is OFFPROC is idiocy.
2209 // Consider: ctr -= RunnablePenalty ;
2210 if (Knob_OState && NotRunnable (Self, ox)) {
2211 TEVENT (Spin abort - notrunnable);
2212 goto Abort ;
2213 }
2214 if (sss && _succ == NULL ) _succ = Self ;
2215 }
2216
2217 // Spin failed with prejudice -- reduce _SpinDuration.
2218 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2219 // AIMD is globally stable.
2220 TEVENT (Spin failure) ;
2221 {
2222 int x = _SpinDuration ;
2223 if (x > 0) {
2224 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2225 // This is globally sample and tends to damp the response.
2226 x -= Knob_Penalty ;
2227 if (x < 0) x = 0 ;
2228 _SpinDuration = x ;
2229 }
2230 }
2231
2232 Abort:
2233 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2234 if (sss && _succ == Self) {
2235 _succ = NULL ;
2236 // Invariant: after setting succ=null a contending thread
2237 // must recheck-retry _owner before parking. This usually happens
2238 // in the normal usage of TrySpin(), but it's safest
2239 // to make TrySpin() as foolproof as possible.
2240 OrderAccess::fence() ;
2241 if (TryLock(Self) > 0) return 1 ;
2242 }
2243 return 0 ;
2244 }
2245
2246 // NotRunnable() -- informed spinning
2247 //
2248 // Don't bother spinning if the owner is not eligible to drop the lock.
2249 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2250 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2251 // The thread must be runnable in order to drop the lock in timely fashion.
2252 // If the _owner is not runnable then spinning will not likely be
2253 // successful (profitable).
2254 //
2255 // Beware -- the thread referenced by _owner could have died
2256 // so a simply fetch from _owner->_thread_state might trap.
2257 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2258 // Because of the lifecycle issues the schedctl and _thread_state values
2259 // observed by NotRunnable() might be garbage. NotRunnable must
2260 // tolerate this and consider the observed _thread_state value
2261 // as advisory.
2262 //
2263 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2264 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2265 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2266 // with the LSB of _owner. Another option would be to probablistically probe
2267 // the putative _owner->TypeTag value.
2268 //
2269 // Checking _thread_state isn't perfect. Even if the thread is
2270 // in_java it might be blocked on a page-fault or have been preempted
2271 // and sitting on a ready/dispatch queue. _thread state in conjunction
2272 // with schedctl.sc_state gives us a good picture of what the
2273 // thread is doing, however.
2274 //
2275 // TODO: check schedctl.sc_state.
2276 // We'll need to use SafeFetch32() to read from the schedctl block.
2277 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2278 //
2279 // The return value from NotRunnable() is *advisory* -- the
2280 // result is based on sampling and is not necessarily coherent.
2281 // The caller must tolerate false-negative and false-positive errors.
2282 // Spinning, in general, is probabilistic anyway.
2283
2284
2285 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2286 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2287 if (!OwnerIsThread) return 0 ;
2288
2289 if (ox == NULL) return 0 ;
2290
2291 // Avoid transitive spinning ...
2292 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2293 // Immediately after T1 acquires L it's possible that T2, also
2294 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2295 // This occurs transiently after T1 acquired L but before
2296 // T1 managed to clear T1.Stalled. T2 does not need to abort
2297 // its spin in this circumstance.
2298 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2299
2300 if (BlockedOn == 1) return 1 ;
2301 if (BlockedOn != 0) {
2302 return BlockedOn != intptr_t(this) && _owner == ox ;
2303 }
2304
2305 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2306 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2307 // consider also: jst != _thread_in_Java -- but that's overspecific.
2308 return jst == _thread_blocked || jst == _thread_in_native ;
2309 }
2310
2311
2312 // -----------------------------------------------------------------------------
2313 // WaitSet management ...
2314
2315 ObjectWaiter::ObjectWaiter(Thread* thread) {
2316 _next = NULL;
2317 _prev = NULL;
2318 _notified = 0;
2319 TState = TS_RUN ;
2320 _thread = thread;
2321 _event = thread->_ParkEvent ;
2322 _active = false;
2323 assert (_event != NULL, "invariant") ;
2324 }
2325
2326 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2327 JavaThread *jt = (JavaThread *)this->_thread;
2328 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2329 }
2330
2331 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2332 JavaThread *jt = (JavaThread *)this->_thread;
2333 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2334 }
2335
2336 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2337 assert(node != NULL, "should not dequeue NULL node");
2338 assert(node->_prev == NULL, "node already in list");
2339 assert(node->_next == NULL, "node already in list");
2340 // put node at end of queue (circular doubly linked list)
2341 if (_WaitSet == NULL) {
2342 _WaitSet = node;
2343 node->_prev = node;
2344 node->_next = node;
2345 } else {
2346 ObjectWaiter* head = _WaitSet ;
2347 ObjectWaiter* tail = head->_prev;
2348 assert(tail->_next == head, "invariant check");
2349 tail->_next = node;
2350 head->_prev = node;
2351 node->_next = head;
2352 node->_prev = tail;
2353 }
2354 }
2355
2356 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2357 // dequeue the very first waiter
2358 ObjectWaiter* waiter = _WaitSet;
2359 if (waiter) {
2360 DequeueSpecificWaiter(waiter);
2361 }
2362 return waiter;
2363 }
2364
2365 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2366 assert(node != NULL, "should not dequeue NULL node");
2367 assert(node->_prev != NULL, "node already removed from list");
2368 assert(node->_next != NULL, "node already removed from list");
2369 // when the waiter has woken up because of interrupt,
2370 // timeout or other spurious wake-up, dequeue the
2371 // waiter from waiting list
2372 ObjectWaiter* next = node->_next;
2373 if (next == node) {
2374 assert(node->_prev == node, "invariant check");
2375 _WaitSet = NULL;
2376 } else {
2377 ObjectWaiter* prev = node->_prev;
2378 assert(prev->_next == node, "invariant check");
2379 assert(next->_prev == node, "invariant check");
2380 next->_prev = prev;
2381 prev->_next = next;
2382 if (_WaitSet == node) {
2383 _WaitSet = next;
2384 }
2385 }
2386 node->_next = NULL;
2387 node->_prev = NULL;
2388 }
2389
2390 // -----------------------------------------------------------------------------
2391 // PerfData support
2392 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2393 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2394 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2395 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2396 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2397 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2398 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2399 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2400 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2401 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2402 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2403 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2404 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2405 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2406 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2407 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2408 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2409 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2410
2411 // One-shot global initialization for the sync subsystem.
2412 // We could also defer initialization and initialize on-demand
2413 // the first time we call inflate(). Initialization would
2414 // be protected - like so many things - by the MonitorCache_lock.
2415
2416 void ObjectMonitor::Initialize () {
2417 static int InitializationCompleted = 0 ;
2418 assert (InitializationCompleted == 0, "invariant") ;
2419 InitializationCompleted = 1 ;
2420 if (UsePerfData) {
2421 EXCEPTION_MARK ;
2422 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2423 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2424 NEWPERFCOUNTER(_sync_Inflations) ;
2425 NEWPERFCOUNTER(_sync_Deflations) ;
2426 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2427 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2428 NEWPERFCOUNTER(_sync_Parks) ;
2429 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2430 NEWPERFCOUNTER(_sync_Notifications) ;
2431 NEWPERFCOUNTER(_sync_SlowEnter) ;
2432 NEWPERFCOUNTER(_sync_SlowExit) ;
2433 NEWPERFCOUNTER(_sync_SlowNotify) ;
2434 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2435 NEWPERFCOUNTER(_sync_FailedSpins) ;
2436 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2437 NEWPERFCOUNTER(_sync_PrivateA) ;
2438 NEWPERFCOUNTER(_sync_PrivateB) ;
2439 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2440 NEWPERFCOUNTER(_sync_MonScavenged) ;
2441 NEWPERFVARIABLE(_sync_MonExtant) ;
2442 #undef NEWPERFCOUNTER
2443 }
2444 }
2445
2446
2447 // Compile-time asserts
2448 // When possible, it's better to catch errors deterministically at
2449 // compile-time than at runtime. The down-side to using compile-time
2450 // asserts is that error message -- often something about negative array
2451 // indices -- is opaque.
2452
2453 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2454
2455 void ObjectMonitor::ctAsserts() {
2456 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2457 }
2458
2459
2460 static char * kvGet (char * kvList, const char * Key) {
2461 if (kvList == NULL) return NULL ;
2462 size_t n = strlen (Key) ;
2463 char * Search ;
2464 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2465 if (strncmp (Search, Key, n) == 0) {
2466 if (Search[n] == '=') return Search + n + 1 ;
2467 if (Search[n] == 0) return (char *) "1" ;
2468 }
2469 }
2470 return NULL ;
2471 }
2472
2473 static int kvGetInt (char * kvList, const char * Key, int Default) {
2474 char * v = kvGet (kvList, Key) ;
2475 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2476 if (Knob_ReportSettings && v != NULL) {
2477 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2478 ::fflush (stdout) ;
2479 }
2480 return rslt ;
2481 }
2482
2483 void ObjectMonitor::DeferredInitialize () {
2484 if (InitDone > 0) return ;
2485 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2486 while (InitDone != 1) ;
2487 return ;
2488 }
2489
2490 // One-shot global initialization ...
2491 // The initialization is idempotent, so we don't need locks.
2492 // In the future consider doing this via os::init_2().
2493 // SyncKnobs consist of <Key>=<Value> pairs in the style
2494 // of environment variables. Start by converting ':' to NUL.
2495
2496 if (SyncKnobs == NULL) SyncKnobs = "" ;
2497
2498 size_t sz = strlen (SyncKnobs) ;
2499 char * knobs = (char *) malloc (sz + 2) ;
2500 if (knobs == NULL) {
2501 vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
2502 guarantee (0, "invariant") ;
2503 }
2504 strcpy (knobs, SyncKnobs) ;
2505 knobs[sz+1] = 0 ;
2506 for (char * p = knobs ; *p ; p++) {
2507 if (*p == ':') *p = 0 ;
2508 }
2509
2510 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2511 SETKNOB(ReportSettings) ;
2512 SETKNOB(Verbose) ;
2513 SETKNOB(FixedSpin) ;
2514 SETKNOB(SpinLimit) ;
2515 SETKNOB(SpinBase) ;
2516 SETKNOB(SpinBackOff);
2517 SETKNOB(CASPenalty) ;
2518 SETKNOB(OXPenalty) ;
2519 SETKNOB(LogSpins) ;
2520 SETKNOB(SpinSetSucc) ;
2521 SETKNOB(SuccEnabled) ;
2522 SETKNOB(SuccRestrict) ;
2523 SETKNOB(Penalty) ;
2524 SETKNOB(Bonus) ;
2525 SETKNOB(BonusB) ;
2526 SETKNOB(Poverty) ;
2527 SETKNOB(SpinAfterFutile) ;
2528 SETKNOB(UsePause) ;
2529 SETKNOB(SpinEarly) ;
2530 SETKNOB(OState) ;
2531 SETKNOB(MaxSpinners) ;
2532 SETKNOB(PreSpin) ;
2533 SETKNOB(ExitPolicy) ;
2534 SETKNOB(QMode);
2535 SETKNOB(ResetEvent) ;
2536 SETKNOB(MoveNotifyee) ;
2537 SETKNOB(FastHSSEC) ;
2538 #undef SETKNOB
2539
2540 if (Knob_Verbose) {
2541 sanity_checks();
2542 }
2543
2544 if (os::is_MP()) {
2545 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2546 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2547 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2548 } else {
2549 Knob_SpinLimit = 0 ;
2550 Knob_SpinBase = 0 ;
2551 Knob_PreSpin = 0 ;
2552 Knob_FixedSpin = -1 ;
2553 }
2554
2555 if (Knob_LogSpins == 0) {
2556 ObjectMonitor::_sync_FailedSpins = NULL ;
2557 }
2558
2559 free (knobs) ;
2560 OrderAccess::fence() ;
2561 InitDone = 1 ;
2562 }
2563
2564 void ObjectMonitor::sanity_checks() {
2565 int error_cnt = 0;
2566 int warning_cnt = 0;
2567 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2568
2569 if (verbose) {
2570 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2571 sizeof(ObjectMonitor));
2572 }
2573
2574 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2575 if (verbose) {
2576 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2577 }
2578
2579 ObjectMonitor dummy;
2580 u_char *addr_begin = (u_char*)&dummy;
2581 u_char *addr_header = (u_char*)&dummy._header;
2582 u_char *addr_owner = (u_char*)&dummy._owner;
2583
2584 uint offset_header = (uint)(addr_header - addr_begin);
2585 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2586
2587 uint offset_owner = (uint)(addr_owner - addr_begin);
2588 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2589
2590 if ((uint)(addr_header - addr_begin) != 0) {
2591 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2592 error_cnt++;
2593 }
2594
2595 if (cache_line_size != 0) {
2596 // We were able to determine the L1 data cache line size so
2597 // do some cache line specific sanity checks
2598
2599 if ((offset_owner - offset_header) < cache_line_size) {
2600 tty->print_cr("WARNING: the _header and _owner fields are closer "
2601 "than a cache line which permits false sharing.");
2602 warning_cnt++;
2603 }
2604
2605 if ((sizeof(ObjectMonitor) % cache_line_size) != 0) {
2606 tty->print_cr("WARNING: ObjectMonitor size is not a multiple of "
2607 "a cache line which permits false sharing.");
2608 warning_cnt++;
2609 }
2610 }
2611
2612 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2613 &warning_cnt);
2614
2615 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2616 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2617 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2618 }
2619
2620 guarantee(error_cnt == 0,
2621 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2622 }
2623
2624 #ifndef PRODUCT
2625 void ObjectMonitor::verify() {
2626 }
2627
2628 void ObjectMonitor::print() {
2629 }
2630 #endif