1 /*
2 * Copyright (c) 1998, 2014, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/osThread.hpp"
36 #include "runtime/stubRoutines.hpp"
37 #include "runtime/thread.hpp"
38 #include "services/threadService.hpp"
39 #include "trace/tracing.hpp"
40 #include "trace/traceMacros.hpp"
41 #include "utilities/dtrace.hpp"
42 #include "utilities/preserveException.hpp"
43 #ifdef TARGET_OS_FAMILY_linux
44 # include "os_linux.inline.hpp"
45 # include "thread_linux.inline.hpp"
46 #endif
47 #ifdef TARGET_OS_FAMILY_solaris
48 # include "os_solaris.inline.hpp"
49 # include "thread_solaris.inline.hpp"
50 #endif
51 #ifdef TARGET_OS_FAMILY_windows
52 # include "os_windows.inline.hpp"
53 # include "thread_windows.inline.hpp"
54 #endif
55 #ifdef TARGET_OS_FAMILY_bsd
56 # include "os_bsd.inline.hpp"
57 # include "thread_bsd.inline.hpp"
58 #endif
59
60 #if defined(__GNUC__) && !defined(IA64)
61 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
62 #define ATTR __attribute__((noinline))
63 #else
64 #define ATTR
65 #endif
66
67
68 #ifdef DTRACE_ENABLED
69
70 // Only bother with this argument setup if dtrace is available
71 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
72
73
74 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \
75 char* bytes = NULL; \
76 int len = 0; \
77 jlong jtid = SharedRuntime::get_java_tid(thread); \
78 Symbol* klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
79 if (klassname != NULL) { \
80 bytes = (char*)klassname->bytes(); \
81 len = klassname->utf8_length(); \
82 }
83
84 #ifndef USDT2
85
86 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
87 jlong, uintptr_t, char*, int);
88 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
89 jlong, uintptr_t, char*, int);
90 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
91 jlong, uintptr_t, char*, int);
92 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
93 jlong, uintptr_t, char*, int);
94 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
95 jlong, uintptr_t, char*, int);
96
97 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
98 { \
99 if (DTraceMonitorProbes) { \
100 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
101 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
102 (monitor), bytes, len, (millis)); \
103 } \
104 }
105
106 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
107 { \
108 if (DTraceMonitorProbes) { \
109 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
110 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
111 (uintptr_t)(monitor), bytes, len); \
112 } \
113 }
114
115 #else /* USDT2 */
116
117 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
118 { \
119 if (DTraceMonitorProbes) { \
120 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
121 HOTSPOT_MONITOR_WAIT(jtid, \
122 (monitor), bytes, len, (millis)); \
123 } \
124 }
125
126 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
127 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
128 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
129 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
130 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
131
132 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
133 { \
134 if (DTraceMonitorProbes) { \
135 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
136 HOTSPOT_MONITOR_##probe(jtid, \
137 (uintptr_t)(monitor), bytes, len); \
138 } \
139 }
140
141 #endif /* USDT2 */
142 #else // ndef DTRACE_ENABLED
143
144 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
145 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;}
146
147 #endif // ndef DTRACE_ENABLED
148
149 // Tunables ...
150 // The knob* variables are effectively final. Once set they should
151 // never be modified hence. Consider using __read_mostly with GCC.
152
153 int ObjectMonitor::Knob_Verbose = 0 ;
154 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
155 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
156 static int Knob_HandOff = 0 ;
157 static int Knob_ReportSettings = 0 ;
158
159 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
160 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
161 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
162 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
163 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
164 static int Knob_SpinEarly = 1 ;
165 static int Knob_SuccEnabled = 1 ; // futile wake throttling
166 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
167 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
168 static int Knob_Bonus = 100 ; // spin success bonus
169 static int Knob_BonusB = 100 ; // spin success bonus
170 static int Knob_Penalty = 200 ; // spin failure penalty
171 static int Knob_Poverty = 1000 ;
172 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
173 static int Knob_FixedSpin = 0 ;
174 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
175 static int Knob_UsePause = 1 ;
176 static int Knob_ExitPolicy = 0 ;
177 static int Knob_PreSpin = 10 ; // 20-100 likely better
178 static int Knob_ResetEvent = 0 ;
179 static int BackOffMask = 0 ;
180
181 static int Knob_FastHSSEC = 0 ;
182 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
183 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
184 static volatile int InitDone = 0 ;
185
186 #define TrySpin TrySpin_VaryDuration
187
188 // -----------------------------------------------------------------------------
189 // Theory of operations -- Monitors lists, thread residency, etc:
190 //
191 // * A thread acquires ownership of a monitor by successfully
192 // CAS()ing the _owner field from null to non-null.
193 //
194 // * Invariant: A thread appears on at most one monitor list --
195 // cxq, EntryList or WaitSet -- at any one time.
196 //
197 // * Contending threads "push" themselves onto the cxq with CAS
198 // and then spin/park.
199 //
200 // * After a contending thread eventually acquires the lock it must
201 // dequeue itself from either the EntryList or the cxq.
202 //
203 // * The exiting thread identifies and unparks an "heir presumptive"
204 // tentative successor thread on the EntryList. Critically, the
205 // exiting thread doesn't unlink the successor thread from the EntryList.
206 // After having been unparked, the wakee will recontend for ownership of
207 // the monitor. The successor (wakee) will either acquire the lock or
208 // re-park itself.
209 //
210 // Succession is provided for by a policy of competitive handoff.
211 // The exiting thread does _not_ grant or pass ownership to the
212 // successor thread. (This is also referred to as "handoff" succession").
213 // Instead the exiting thread releases ownership and possibly wakes
214 // a successor, so the successor can (re)compete for ownership of the lock.
215 // If the EntryList is empty but the cxq is populated the exiting
216 // thread will drain the cxq into the EntryList. It does so by
217 // by detaching the cxq (installing null with CAS) and folding
218 // the threads from the cxq into the EntryList. The EntryList is
219 // doubly linked, while the cxq is singly linked because of the
220 // CAS-based "push" used to enqueue recently arrived threads (RATs).
221 //
222 // * Concurrency invariants:
223 //
224 // -- only the monitor owner may access or mutate the EntryList.
225 // The mutex property of the monitor itself protects the EntryList
226 // from concurrent interference.
227 // -- Only the monitor owner may detach the cxq.
228 //
229 // * The monitor entry list operations avoid locks, but strictly speaking
230 // they're not lock-free. Enter is lock-free, exit is not.
231 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
232 //
233 // * The cxq can have multiple concurrent "pushers" but only one concurrent
234 // detaching thread. This mechanism is immune from the ABA corruption.
235 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
236 //
237 // * Taken together, the cxq and the EntryList constitute or form a
238 // single logical queue of threads stalled trying to acquire the lock.
239 // We use two distinct lists to improve the odds of a constant-time
240 // dequeue operation after acquisition (in the ::enter() epilog) and
241 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
242 // A key desideratum is to minimize queue & monitor metadata manipulation
243 // that occurs while holding the monitor lock -- that is, we want to
244 // minimize monitor lock holds times. Note that even a small amount of
245 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
246 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
247 // locks and monitor metadata.
248 //
249 // Cxq points to the the set of Recently Arrived Threads attempting entry.
250 // Because we push threads onto _cxq with CAS, the RATs must take the form of
251 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
252 // the unlocking thread notices that EntryList is null but _cxq is != null.
253 //
254 // The EntryList is ordered by the prevailing queue discipline and
255 // can be organized in any convenient fashion, such as a doubly-linked list or
256 // a circular doubly-linked list. Critically, we want insert and delete operations
257 // to operate in constant-time. If we need a priority queue then something akin
258 // to Solaris' sleepq would work nicely. Viz.,
259 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
260 // Queue discipline is enforced at ::exit() time, when the unlocking thread
261 // drains the cxq into the EntryList, and orders or reorders the threads on the
262 // EntryList accordingly.
263 //
264 // Barring "lock barging", this mechanism provides fair cyclic ordering,
265 // somewhat similar to an elevator-scan.
266 //
267 // * The monitor synchronization subsystem avoids the use of native
268 // synchronization primitives except for the narrow platform-specific
269 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
270 // the semantics of park-unpark. Put another way, this monitor implementation
271 // depends only on atomic operations and park-unpark. The monitor subsystem
272 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
273 // underlying OS manages the READY<->RUN transitions.
274 //
275 // * Waiting threads reside on the WaitSet list -- wait() puts
276 // the caller onto the WaitSet.
277 //
278 // * notify() or notifyAll() simply transfers threads from the WaitSet to
279 // either the EntryList or cxq. Subsequent exit() operations will
280 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
281 // it's likely the notifyee would simply impale itself on the lock held
282 // by the notifier.
283 //
284 // * An interesting alternative is to encode cxq as (List,LockByte) where
285 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
286 // variable, like _recursions, in the scheme. The threads or Events that form
287 // the list would have to be aligned in 256-byte addresses. A thread would
288 // try to acquire the lock or enqueue itself with CAS, but exiting threads
289 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
290 // Note that is is *not* word-tearing, but it does presume that full-word
291 // CAS operations are coherent with intermix with STB operations. That's true
292 // on most common processors.
293 //
294 // * See also http://blogs.sun.com/dave
295
296
297 // -----------------------------------------------------------------------------
298 // Enter support
299
300 bool ObjectMonitor::try_enter(Thread* THREAD) {
301 if (THREAD != _owner) {
302 if (THREAD->is_lock_owned ((address)_owner)) {
303 assert(_recursions == 0, "internal state error");
304 _owner = THREAD ;
305 _recursions = 1 ;
306 OwnerIsThread = 1 ;
307 return true;
308 }
309 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
310 return false;
311 }
312 return true;
313 } else {
314 _recursions++;
315 return true;
316 }
317 }
318
319 void ATTR ObjectMonitor::enter(TRAPS) {
320 // The following code is ordered to check the most common cases first
321 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
322 Thread * const Self = THREAD ;
323 void * cur ;
324
325 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
326 if (cur == NULL) {
327 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
328 assert (_recursions == 0 , "invariant") ;
329 assert (_owner == Self, "invariant") ;
330 // CONSIDER: set or assert OwnerIsThread == 1
331 return ;
332 }
333
334 if (cur == Self) {
335 // TODO-FIXME: check for integer overflow! BUGID 6557169.
336 _recursions ++ ;
337 return ;
338 }
339
340 if (Self->is_lock_owned ((address)cur)) {
341 assert (_recursions == 0, "internal state error");
342 _recursions = 1 ;
343 // Commute owner from a thread-specific on-stack BasicLockObject address to
344 // a full-fledged "Thread *".
345 _owner = Self ;
346 OwnerIsThread = 1 ;
347 return ;
348 }
349
350 // We've encountered genuine contention.
351 assert (Self->_Stalled == 0, "invariant") ;
352 Self->_Stalled = intptr_t(this) ;
353
354 // Try one round of spinning *before* enqueueing Self
355 // and before going through the awkward and expensive state
356 // transitions. The following spin is strictly optional ...
357 // Note that if we acquire the monitor from an initial spin
358 // we forgo posting JVMTI events and firing DTRACE probes.
359 if (Knob_SpinEarly && TrySpin (Self) > 0) {
360 assert (_owner == Self , "invariant") ;
361 assert (_recursions == 0 , "invariant") ;
362 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
363 Self->_Stalled = 0 ;
364 return ;
365 }
366
367 assert (_owner != Self , "invariant") ;
368 assert (_succ != Self , "invariant") ;
369 assert (Self->is_Java_thread() , "invariant") ;
370 JavaThread * jt = (JavaThread *) Self ;
371 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
372 assert (jt->thread_state() != _thread_blocked , "invariant") ;
373 assert (this->object() != NULL , "invariant") ;
374 assert (_count >= 0, "invariant") ;
375
376 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
377 // Ensure the object-monitor relationship remains stable while there's contention.
378 Atomic::inc_ptr(&_count);
379
380 EventJavaMonitorEnter event;
381
382 { // Change java thread status to indicate blocked on monitor enter.
383 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
384
385 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
386 if (JvmtiExport::should_post_monitor_contended_enter()) {
387 JvmtiExport::post_monitor_contended_enter(jt, this);
388
389 // The current thread does not yet own the monitor and does not
390 // yet appear on any queues that would get it made the successor.
391 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
392 // handler cannot accidentally consume an unpark() meant for the
393 // ParkEvent associated with this ObjectMonitor.
394 }
395
396 OSThreadContendState osts(Self->osthread());
397 ThreadBlockInVM tbivm(jt);
398
399 Self->set_current_pending_monitor(this);
400
401 // TODO-FIXME: change the following for(;;) loop to straight-line code.
402 for (;;) {
403 jt->set_suspend_equivalent();
404 // cleared by handle_special_suspend_equivalent_condition()
405 // or java_suspend_self()
406
407 EnterI (THREAD) ;
408
409 if (!ExitSuspendEquivalent(jt)) break ;
410
411 //
412 // We have acquired the contended monitor, but while we were
413 // waiting another thread suspended us. We don't want to enter
414 // the monitor while suspended because that would surprise the
415 // thread that suspended us.
416 //
417 _recursions = 0 ;
418 _succ = NULL ;
419 exit (false, Self) ;
420
421 jt->java_suspend_self();
422 }
423 Self->set_current_pending_monitor(NULL);
424 }
425
426 Atomic::dec_ptr(&_count);
427 assert (_count >= 0, "invariant") ;
428 Self->_Stalled = 0 ;
429
430 // Must either set _recursions = 0 or ASSERT _recursions == 0.
431 assert (_recursions == 0 , "invariant") ;
432 assert (_owner == Self , "invariant") ;
433 assert (_succ != Self , "invariant") ;
434 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
435
436 // The thread -- now the owner -- is back in vm mode.
437 // Report the glorious news via TI,DTrace and jvmstat.
438 // The probe effect is non-trivial. All the reportage occurs
439 // while we hold the monitor, increasing the length of the critical
440 // section. Amdahl's parallel speedup law comes vividly into play.
441 //
442 // Another option might be to aggregate the events (thread local or
443 // per-monitor aggregation) and defer reporting until a more opportune
444 // time -- such as next time some thread encounters contention but has
445 // yet to acquire the lock. While spinning that thread could
446 // spinning we could increment JVMStat counters, etc.
447
448 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
449 if (JvmtiExport::should_post_monitor_contended_entered()) {
450 JvmtiExport::post_monitor_contended_entered(jt, this);
451
452 // The current thread already owns the monitor and is not going to
453 // call park() for the remainder of the monitor enter protocol. So
454 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
455 // event handler consumed an unpark() issued by the thread that
456 // just exited the monitor.
457 }
458
459 if (event.should_commit()) {
460 event.set_klass(((oop)this->object())->klass());
461 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
462 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
463 event.commit();
464 }
465
466 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
467 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
468 }
469 }
470
471
472 // Caveat: TryLock() is not necessarily serializing if it returns failure.
473 // Callers must compensate as needed.
474
475 int ObjectMonitor::TryLock (Thread * Self) {
476 for (;;) {
477 void * own = _owner ;
478 if (own != NULL) return 0 ;
479 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
480 // Either guarantee _recursions == 0 or set _recursions = 0.
481 assert (_recursions == 0, "invariant") ;
482 assert (_owner == Self, "invariant") ;
483 // CONSIDER: set or assert that OwnerIsThread == 1
484 return 1 ;
485 }
486 // The lock had been free momentarily, but we lost the race to the lock.
487 // Interference -- the CAS failed.
488 // We can either return -1 or retry.
489 // Retry doesn't make as much sense because the lock was just acquired.
490 if (true) return -1 ;
491 }
492 }
493
494 void ATTR ObjectMonitor::EnterI (TRAPS) {
495 Thread * Self = THREAD ;
496 assert (Self->is_Java_thread(), "invariant") ;
497 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
498
499 // Try the lock - TATAS
500 if (TryLock (Self) > 0) {
501 assert (_succ != Self , "invariant") ;
502 assert (_owner == Self , "invariant") ;
503 assert (_Responsible != Self , "invariant") ;
504 return ;
505 }
506
507 DeferredInitialize () ;
508
509 // We try one round of spinning *before* enqueueing Self.
510 //
511 // If the _owner is ready but OFFPROC we could use a YieldTo()
512 // operation to donate the remainder of this thread's quantum
513 // to the owner. This has subtle but beneficial affinity
514 // effects.
515
516 if (TrySpin (Self) > 0) {
517 assert (_owner == Self , "invariant") ;
518 assert (_succ != Self , "invariant") ;
519 assert (_Responsible != Self , "invariant") ;
520 return ;
521 }
522
523 // The Spin failed -- Enqueue and park the thread ...
524 assert (_succ != Self , "invariant") ;
525 assert (_owner != Self , "invariant") ;
526 assert (_Responsible != Self , "invariant") ;
527
528 // Enqueue "Self" on ObjectMonitor's _cxq.
529 //
530 // Node acts as a proxy for Self.
531 // As an aside, if were to ever rewrite the synchronization code mostly
532 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
533 // Java objects. This would avoid awkward lifecycle and liveness issues,
534 // as well as eliminate a subset of ABA issues.
535 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
536 //
537
538 ObjectWaiter node(Self) ;
539 Self->_ParkEvent->reset() ;
540 node._prev = (ObjectWaiter *) 0xBAD ;
541 node.TState = ObjectWaiter::TS_CXQ ;
542
543 // Push "Self" onto the front of the _cxq.
544 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
545 // Note that spinning tends to reduce the rate at which threads
546 // enqueue and dequeue on EntryList|cxq.
547 ObjectWaiter * nxt ;
548 for (;;) {
549 node._next = nxt = _cxq ;
550 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
551
552 // Interference - the CAS failed because _cxq changed. Just retry.
553 // As an optional optimization we retry the lock.
554 if (TryLock (Self) > 0) {
555 assert (_succ != Self , "invariant") ;
556 assert (_owner == Self , "invariant") ;
557 assert (_Responsible != Self , "invariant") ;
558 return ;
559 }
560 }
561
562 // Check for cxq|EntryList edge transition to non-null. This indicates
563 // the onset of contention. While contention persists exiting threads
564 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
565 // operations revert to the faster 1-0 mode. This enter operation may interleave
566 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
567 // arrange for one of the contending thread to use a timed park() operations
568 // to detect and recover from the race. (Stranding is form of progress failure
569 // where the monitor is unlocked but all the contending threads remain parked).
570 // That is, at least one of the contended threads will periodically poll _owner.
571 // One of the contending threads will become the designated "Responsible" thread.
572 // The Responsible thread uses a timed park instead of a normal indefinite park
573 // operation -- it periodically wakes and checks for and recovers from potential
574 // strandings admitted by 1-0 exit operations. We need at most one Responsible
575 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
576 // be responsible for a monitor.
577 //
578 // Currently, one of the contended threads takes on the added role of "Responsible".
579 // A viable alternative would be to use a dedicated "stranding checker" thread
580 // that periodically iterated over all the threads (or active monitors) and unparked
581 // successors where there was risk of stranding. This would help eliminate the
582 // timer scalability issues we see on some platforms as we'd only have one thread
583 // -- the checker -- parked on a timer.
584
585 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
586 // Try to assume the role of responsible thread for the monitor.
587 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
588 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
589 }
590
591 // The lock have been released while this thread was occupied queueing
592 // itself onto _cxq. To close the race and avoid "stranding" and
593 // progress-liveness failure we must resample-retry _owner before parking.
594 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
595 // In this case the ST-MEMBAR is accomplished with CAS().
596 //
597 // TODO: Defer all thread state transitions until park-time.
598 // Since state transitions are heavy and inefficient we'd like
599 // to defer the state transitions until absolutely necessary,
600 // and in doing so avoid some transitions ...
601
602 TEVENT (Inflated enter - Contention) ;
603 int nWakeups = 0 ;
604 int RecheckInterval = 1 ;
605
606 for (;;) {
607
608 if (TryLock (Self) > 0) break ;
609 assert (_owner != Self, "invariant") ;
610
611 if ((SyncFlags & 2) && _Responsible == NULL) {
612 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
613 }
614
615 // park self
616 if (_Responsible == Self || (SyncFlags & 1)) {
617 TEVENT (Inflated enter - park TIMED) ;
618 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
619 // Increase the RecheckInterval, but clamp the value.
620 RecheckInterval *= 8 ;
621 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
622 } else {
623 TEVENT (Inflated enter - park UNTIMED) ;
624 Self->_ParkEvent->park() ;
625 }
626
627 if (TryLock(Self) > 0) break ;
628
629 // The lock is still contested.
630 // Keep a tally of the # of futile wakeups.
631 // Note that the counter is not protected by a lock or updated by atomics.
632 // That is by design - we trade "lossy" counters which are exposed to
633 // races during updates for a lower probe effect.
634 TEVENT (Inflated enter - Futile wakeup) ;
635 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
636 ObjectMonitor::_sync_FutileWakeups->inc() ;
637 }
638 ++ nWakeups ;
639
640 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
641 // We can defer clearing _succ until after the spin completes
642 // TrySpin() must tolerate being called with _succ == Self.
643 // Try yet another round of adaptive spinning.
644 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
645
646 // We can find that we were unpark()ed and redesignated _succ while
647 // we were spinning. That's harmless. If we iterate and call park(),
648 // park() will consume the event and return immediately and we'll
649 // just spin again. This pattern can repeat, leaving _succ to simply
650 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
651 // Alternately, we can sample fired() here, and if set, forgo spinning
652 // in the next iteration.
653
654 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
655 Self->_ParkEvent->reset() ;
656 OrderAccess::fence() ;
657 }
658 if (_succ == Self) _succ = NULL ;
659
660 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
661 OrderAccess::fence() ;
662 }
663
664 // Egress :
665 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
666 // Normally we'll find Self on the EntryList .
667 // From the perspective of the lock owner (this thread), the
668 // EntryList is stable and cxq is prepend-only.
669 // The head of cxq is volatile but the interior is stable.
670 // In addition, Self.TState is stable.
671
672 assert (_owner == Self , "invariant") ;
673 assert (object() != NULL , "invariant") ;
674 // I'd like to write:
675 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
676 // but as we're at a safepoint that's not safe.
677
678 UnlinkAfterAcquire (Self, &node) ;
679 if (_succ == Self) _succ = NULL ;
680
681 assert (_succ != Self, "invariant") ;
682 if (_Responsible == Self) {
683 _Responsible = NULL ;
684 OrderAccess::fence(); // Dekker pivot-point
685
686 // We may leave threads on cxq|EntryList without a designated
687 // "Responsible" thread. This is benign. When this thread subsequently
688 // exits the monitor it can "see" such preexisting "old" threads --
689 // threads that arrived on the cxq|EntryList before the fence, above --
690 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
691 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
692 // non-null and elect a new "Responsible" timer thread.
693 //
694 // This thread executes:
695 // ST Responsible=null; MEMBAR (in enter epilog - here)
696 // LD cxq|EntryList (in subsequent exit)
697 //
698 // Entering threads in the slow/contended path execute:
699 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
700 // The (ST cxq; MEMBAR) is accomplished with CAS().
701 //
702 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
703 // exit operation from floating above the ST Responsible=null.
704 }
705
706 // We've acquired ownership with CAS().
707 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
708 // But since the CAS() this thread may have also stored into _succ,
709 // EntryList, cxq or Responsible. These meta-data updates must be
710 // visible __before this thread subsequently drops the lock.
711 // Consider what could occur if we didn't enforce this constraint --
712 // STs to monitor meta-data and user-data could reorder with (become
713 // visible after) the ST in exit that drops ownership of the lock.
714 // Some other thread could then acquire the lock, but observe inconsistent
715 // or old monitor meta-data and heap data. That violates the JMM.
716 // To that end, the 1-0 exit() operation must have at least STST|LDST
717 // "release" barrier semantics. Specifically, there must be at least a
718 // STST|LDST barrier in exit() before the ST of null into _owner that drops
719 // the lock. The barrier ensures that changes to monitor meta-data and data
720 // protected by the lock will be visible before we release the lock, and
721 // therefore before some other thread (CPU) has a chance to acquire the lock.
722 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
723 //
724 // Critically, any prior STs to _succ or EntryList must be visible before
725 // the ST of null into _owner in the *subsequent* (following) corresponding
726 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
727 // execute a serializing instruction.
728
729 if (SyncFlags & 8) {
730 OrderAccess::fence() ;
731 }
732 return ;
733 }
734
735 // ReenterI() is a specialized inline form of the latter half of the
736 // contended slow-path from EnterI(). We use ReenterI() only for
737 // monitor reentry in wait().
738 //
739 // In the future we should reconcile EnterI() and ReenterI(), adding
740 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
741 // loop accordingly.
742
743 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
744 assert (Self != NULL , "invariant") ;
745 assert (SelfNode != NULL , "invariant") ;
746 assert (SelfNode->_thread == Self , "invariant") ;
747 assert (_waiters > 0 , "invariant") ;
748 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
749 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
750 JavaThread * jt = (JavaThread *) Self ;
751
752 int nWakeups = 0 ;
753 for (;;) {
754 ObjectWaiter::TStates v = SelfNode->TState ;
755 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
756 assert (_owner != Self, "invariant") ;
757
758 if (TryLock (Self) > 0) break ;
759 if (TrySpin (Self) > 0) break ;
760
761 TEVENT (Wait Reentry - parking) ;
762
763 // State transition wrappers around park() ...
764 // ReenterI() wisely defers state transitions until
765 // it's clear we must park the thread.
766 {
767 OSThreadContendState osts(Self->osthread());
768 ThreadBlockInVM tbivm(jt);
769
770 // cleared by handle_special_suspend_equivalent_condition()
771 // or java_suspend_self()
772 jt->set_suspend_equivalent();
773 if (SyncFlags & 1) {
774 Self->_ParkEvent->park ((jlong)1000) ;
775 } else {
776 Self->_ParkEvent->park () ;
777 }
778
779 // were we externally suspended while we were waiting?
780 for (;;) {
781 if (!ExitSuspendEquivalent (jt)) break ;
782 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
783 jt->java_suspend_self();
784 jt->set_suspend_equivalent();
785 }
786 }
787
788 // Try again, but just so we distinguish between futile wakeups and
789 // successful wakeups. The following test isn't algorithmically
790 // necessary, but it helps us maintain sensible statistics.
791 if (TryLock(Self) > 0) break ;
792
793 // The lock is still contested.
794 // Keep a tally of the # of futile wakeups.
795 // Note that the counter is not protected by a lock or updated by atomics.
796 // That is by design - we trade "lossy" counters which are exposed to
797 // races during updates for a lower probe effect.
798 TEVENT (Wait Reentry - futile wakeup) ;
799 ++ nWakeups ;
800
801 // Assuming this is not a spurious wakeup we'll normally
802 // find that _succ == Self.
803 if (_succ == Self) _succ = NULL ;
804
805 // Invariant: after clearing _succ a contending thread
806 // *must* retry _owner before parking.
807 OrderAccess::fence() ;
808
809 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
810 ObjectMonitor::_sync_FutileWakeups->inc() ;
811 }
812 }
813
814 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
815 // Normally we'll find Self on the EntryList.
816 // Unlinking from the EntryList is constant-time and atomic-free.
817 // From the perspective of the lock owner (this thread), the
818 // EntryList is stable and cxq is prepend-only.
819 // The head of cxq is volatile but the interior is stable.
820 // In addition, Self.TState is stable.
821
822 assert (_owner == Self, "invariant") ;
823 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
824 UnlinkAfterAcquire (Self, SelfNode) ;
825 if (_succ == Self) _succ = NULL ;
826 assert (_succ != Self, "invariant") ;
827 SelfNode->TState = ObjectWaiter::TS_RUN ;
828 OrderAccess::fence() ; // see comments at the end of EnterI()
829 }
830
831 // after the thread acquires the lock in ::enter(). Equally, we could defer
832 // unlinking the thread until ::exit()-time.
833
834 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
835 {
836 assert (_owner == Self, "invariant") ;
837 assert (SelfNode->_thread == Self, "invariant") ;
838
839 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
840 // Normal case: remove Self from the DLL EntryList .
841 // This is a constant-time operation.
842 ObjectWaiter * nxt = SelfNode->_next ;
843 ObjectWaiter * prv = SelfNode->_prev ;
844 if (nxt != NULL) nxt->_prev = prv ;
845 if (prv != NULL) prv->_next = nxt ;
846 if (SelfNode == _EntryList ) _EntryList = nxt ;
847 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
848 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
849 TEVENT (Unlink from EntryList) ;
850 } else {
851 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
852 // Inopportune interleaving -- Self is still on the cxq.
853 // This usually means the enqueue of self raced an exiting thread.
854 // Normally we'll find Self near the front of the cxq, so
855 // dequeueing is typically fast. If needbe we can accelerate
856 // this with some MCS/CHL-like bidirectional list hints and advisory
857 // back-links so dequeueing from the interior will normally operate
858 // in constant-time.
859 // Dequeue Self from either the head (with CAS) or from the interior
860 // with a linear-time scan and normal non-atomic memory operations.
861 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
862 // and then unlink Self from EntryList. We have to drain eventually,
863 // so it might as well be now.
864
865 ObjectWaiter * v = _cxq ;
866 assert (v != NULL, "invariant") ;
867 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
868 // The CAS above can fail from interference IFF a "RAT" arrived.
869 // In that case Self must be in the interior and can no longer be
870 // at the head of cxq.
871 if (v == SelfNode) {
872 assert (_cxq != v, "invariant") ;
873 v = _cxq ; // CAS above failed - start scan at head of list
874 }
875 ObjectWaiter * p ;
876 ObjectWaiter * q = NULL ;
877 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
878 q = p ;
879 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
880 }
881 assert (v != SelfNode, "invariant") ;
882 assert (p == SelfNode, "Node not found on cxq") ;
883 assert (p != _cxq, "invariant") ;
884 assert (q != NULL, "invariant") ;
885 assert (q->_next == p, "invariant") ;
886 q->_next = p->_next ;
887 }
888 TEVENT (Unlink from cxq) ;
889 }
890
891 // Diagnostic hygiene ...
892 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
893 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
894 SelfNode->TState = ObjectWaiter::TS_RUN ;
895 }
896
897 // -----------------------------------------------------------------------------
898 // Exit support
899 //
900 // exit()
901 // ~~~~~~
902 // Note that the collector can't reclaim the objectMonitor or deflate
903 // the object out from underneath the thread calling ::exit() as the
904 // thread calling ::exit() never transitions to a stable state.
905 // This inhibits GC, which in turn inhibits asynchronous (and
906 // inopportune) reclamation of "this".
907 //
908 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
909 // There's one exception to the claim above, however. EnterI() can call
910 // exit() to drop a lock if the acquirer has been externally suspended.
911 // In that case exit() is called with _thread_state as _thread_blocked,
912 // but the monitor's _count field is > 0, which inhibits reclamation.
913 //
914 // 1-0 exit
915 // ~~~~~~~~
916 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
917 // the fast-path operators have been optimized so the common ::exit()
918 // operation is 1-0. See i486.ad fast_unlock(), for instance.
919 // The code emitted by fast_unlock() elides the usual MEMBAR. This
920 // greatly improves latency -- MEMBAR and CAS having considerable local
921 // latency on modern processors -- but at the cost of "stranding". Absent the
922 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
923 // ::enter() path, resulting in the entering thread being stranding
924 // and a progress-liveness failure. Stranding is extremely rare.
925 // We use timers (timed park operations) & periodic polling to detect
926 // and recover from stranding. Potentially stranded threads periodically
927 // wake up and poll the lock. See the usage of the _Responsible variable.
928 //
929 // The CAS() in enter provides for safety and exclusion, while the CAS or
930 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
931 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
932 // We detect and recover from stranding with timers.
933 //
934 // If a thread transiently strands it'll park until (a) another
935 // thread acquires the lock and then drops the lock, at which time the
936 // exiting thread will notice and unpark the stranded thread, or, (b)
937 // the timer expires. If the lock is high traffic then the stranding latency
938 // will be low due to (a). If the lock is low traffic then the odds of
939 // stranding are lower, although the worst-case stranding latency
940 // is longer. Critically, we don't want to put excessive load in the
941 // platform's timer subsystem. We want to minimize both the timer injection
942 // rate (timers created/sec) as well as the number of timers active at
943 // any one time. (more precisely, we want to minimize timer-seconds, which is
944 // the integral of the # of active timers at any instant over time).
945 // Both impinge on OS scalability. Given that, at most one thread parked on
946 // a monitor will use a timer.
947
948 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
949 Thread * Self = THREAD ;
950 if (THREAD != _owner) {
951 if (THREAD->is_lock_owned((address) _owner)) {
952 // Transmute _owner from a BasicLock pointer to a Thread address.
953 // We don't need to hold _mutex for this transition.
954 // Non-null to Non-null is safe as long as all readers can
955 // tolerate either flavor.
956 assert (_recursions == 0, "invariant") ;
957 _owner = THREAD ;
958 _recursions = 0 ;
959 OwnerIsThread = 1 ;
960 } else {
961 // NOTE: we need to handle unbalanced monitor enter/exit
962 // in native code by throwing an exception.
963 // TODO: Throw an IllegalMonitorStateException ?
964 TEVENT (Exit - Throw IMSX) ;
965 assert(false, "Non-balanced monitor enter/exit!");
966 if (false) {
967 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
968 }
969 return;
970 }
971 }
972
973 if (_recursions != 0) {
974 _recursions--; // this is simple recursive enter
975 TEVENT (Inflated exit - recursive) ;
976 return ;
977 }
978
979 // Invariant: after setting Responsible=null an thread must execute
980 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
981 if ((SyncFlags & 4) == 0) {
982 _Responsible = NULL ;
983 }
984
985 #if INCLUDE_TRACE
986 // get the owner's thread id for the MonitorEnter event
987 // if it is enabled and the thread isn't suspended
988 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
989 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
990 }
991 #endif
992
993 for (;;) {
994 assert (THREAD == _owner, "invariant") ;
995
996
997 if (Knob_ExitPolicy == 0) {
998 // release semantics: prior loads and stores from within the critical section
999 // must not float (reorder) past the following store that drops the lock.
1000 // On SPARC that requires MEMBAR #loadstore|#storestore.
1001 // But of course in TSO #loadstore|#storestore is not required.
1002 // I'd like to write one of the following:
1003 // A. OrderAccess::release() ; _owner = NULL
1004 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
1005 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
1006 // store into a _dummy variable. That store is not needed, but can result
1007 // in massive wasteful coherency traffic on classic SMP systems.
1008 // Instead, I use release_store(), which is implemented as just a simple
1009 // ST on x64, x86 and SPARC.
1010 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1011 OrderAccess::storeload() ; // See if we need to wake a successor
1012 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1013 TEVENT (Inflated exit - simple egress) ;
1014 return ;
1015 }
1016 TEVENT (Inflated exit - complex egress) ;
1017
1018 // Normally the exiting thread is responsible for ensuring succession,
1019 // but if other successors are ready or other entering threads are spinning
1020 // then this thread can simply store NULL into _owner and exit without
1021 // waking a successor. The existence of spinners or ready successors
1022 // guarantees proper succession (liveness). Responsibility passes to the
1023 // ready or running successors. The exiting thread delegates the duty.
1024 // More precisely, if a successor already exists this thread is absolved
1025 // of the responsibility of waking (unparking) one.
1026 //
1027 // The _succ variable is critical to reducing futile wakeup frequency.
1028 // _succ identifies the "heir presumptive" thread that has been made
1029 // ready (unparked) but that has not yet run. We need only one such
1030 // successor thread to guarantee progress.
1031 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1032 // section 3.3 "Futile Wakeup Throttling" for details.
1033 //
1034 // Note that spinners in Enter() also set _succ non-null.
1035 // In the current implementation spinners opportunistically set
1036 // _succ so that exiting threads might avoid waking a successor.
1037 // Another less appealing alternative would be for the exiting thread
1038 // to drop the lock and then spin briefly to see if a spinner managed
1039 // to acquire the lock. If so, the exiting thread could exit
1040 // immediately without waking a successor, otherwise the exiting
1041 // thread would need to dequeue and wake a successor.
1042 // (Note that we'd need to make the post-drop spin short, but no
1043 // shorter than the worst-case round-trip cache-line migration time.
1044 // The dropped lock needs to become visible to the spinner, and then
1045 // the acquisition of the lock by the spinner must become visible to
1046 // the exiting thread).
1047 //
1048
1049 // It appears that an heir-presumptive (successor) must be made ready.
1050 // Only the current lock owner can manipulate the EntryList or
1051 // drain _cxq, so we need to reacquire the lock. If we fail
1052 // to reacquire the lock the responsibility for ensuring succession
1053 // falls to the new owner.
1054 //
1055 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1056 return ;
1057 }
1058 TEVENT (Exit - Reacquired) ;
1059 } else {
1060 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1061 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1062 OrderAccess::storeload() ;
1063 // Ratify the previously observed values.
1064 if (_cxq == NULL || _succ != NULL) {
1065 TEVENT (Inflated exit - simple egress) ;
1066 return ;
1067 }
1068
1069 // inopportune interleaving -- the exiting thread (this thread)
1070 // in the fast-exit path raced an entering thread in the slow-enter
1071 // path.
1072 // We have two choices:
1073 // A. Try to reacquire the lock.
1074 // If the CAS() fails return immediately, otherwise
1075 // we either restart/rerun the exit operation, or simply
1076 // fall-through into the code below which wakes a successor.
1077 // B. If the elements forming the EntryList|cxq are TSM
1078 // we could simply unpark() the lead thread and return
1079 // without having set _succ.
1080 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1081 TEVENT (Inflated exit - reacquired succeeded) ;
1082 return ;
1083 }
1084 TEVENT (Inflated exit - reacquired failed) ;
1085 } else {
1086 TEVENT (Inflated exit - complex egress) ;
1087 }
1088 }
1089
1090 guarantee (_owner == THREAD, "invariant") ;
1091
1092 ObjectWaiter * w = NULL ;
1093 int QMode = Knob_QMode ;
1094
1095 if (QMode == 2 && _cxq != NULL) {
1096 // QMode == 2 : cxq has precedence over EntryList.
1097 // Try to directly wake a successor from the cxq.
1098 // If successful, the successor will need to unlink itself from cxq.
1099 w = _cxq ;
1100 assert (w != NULL, "invariant") ;
1101 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1102 ExitEpilog (Self, w) ;
1103 return ;
1104 }
1105
1106 if (QMode == 3 && _cxq != NULL) {
1107 // Aggressively drain cxq into EntryList at the first opportunity.
1108 // This policy ensure that recently-run threads live at the head of EntryList.
1109 // Drain _cxq into EntryList - bulk transfer.
1110 // First, detach _cxq.
1111 // The following loop is tantamount to: w = swap (&cxq, NULL)
1112 w = _cxq ;
1113 for (;;) {
1114 assert (w != NULL, "Invariant") ;
1115 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1116 if (u == w) break ;
1117 w = u ;
1118 }
1119 assert (w != NULL , "invariant") ;
1120
1121 ObjectWaiter * q = NULL ;
1122 ObjectWaiter * p ;
1123 for (p = w ; p != NULL ; p = p->_next) {
1124 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1125 p->TState = ObjectWaiter::TS_ENTER ;
1126 p->_prev = q ;
1127 q = p ;
1128 }
1129
1130 // Append the RATs to the EntryList
1131 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1132 ObjectWaiter * Tail ;
1133 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1134 if (Tail == NULL) {
1135 _EntryList = w ;
1136 } else {
1137 Tail->_next = w ;
1138 w->_prev = Tail ;
1139 }
1140
1141 // Fall thru into code that tries to wake a successor from EntryList
1142 }
1143
1144 if (QMode == 4 && _cxq != NULL) {
1145 // Aggressively drain cxq into EntryList at the first opportunity.
1146 // This policy ensure that recently-run threads live at the head of EntryList.
1147
1148 // Drain _cxq into EntryList - bulk transfer.
1149 // First, detach _cxq.
1150 // The following loop is tantamount to: w = swap (&cxq, NULL)
1151 w = _cxq ;
1152 for (;;) {
1153 assert (w != NULL, "Invariant") ;
1154 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1155 if (u == w) break ;
1156 w = u ;
1157 }
1158 assert (w != NULL , "invariant") ;
1159
1160 ObjectWaiter * q = NULL ;
1161 ObjectWaiter * p ;
1162 for (p = w ; p != NULL ; p = p->_next) {
1163 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1164 p->TState = ObjectWaiter::TS_ENTER ;
1165 p->_prev = q ;
1166 q = p ;
1167 }
1168
1169 // Prepend the RATs to the EntryList
1170 if (_EntryList != NULL) {
1171 q->_next = _EntryList ;
1172 _EntryList->_prev = q ;
1173 }
1174 _EntryList = w ;
1175
1176 // Fall thru into code that tries to wake a successor from EntryList
1177 }
1178
1179 w = _EntryList ;
1180 if (w != NULL) {
1181 // I'd like to write: guarantee (w->_thread != Self).
1182 // But in practice an exiting thread may find itself on the EntryList.
1183 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1184 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1185 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1186 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1187 // release the lock "O". T2 resumes immediately after the ST of null into
1188 // _owner, above. T2 notices that the EntryList is populated, so it
1189 // reacquires the lock and then finds itself on the EntryList.
1190 // Given all that, we have to tolerate the circumstance where "w" is
1191 // associated with Self.
1192 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1193 ExitEpilog (Self, w) ;
1194 return ;
1195 }
1196
1197 // If we find that both _cxq and EntryList are null then just
1198 // re-run the exit protocol from the top.
1199 w = _cxq ;
1200 if (w == NULL) continue ;
1201
1202 // Drain _cxq into EntryList - bulk transfer.
1203 // First, detach _cxq.
1204 // The following loop is tantamount to: w = swap (&cxq, NULL)
1205 for (;;) {
1206 assert (w != NULL, "Invariant") ;
1207 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1208 if (u == w) break ;
1209 w = u ;
1210 }
1211 TEVENT (Inflated exit - drain cxq into EntryList) ;
1212
1213 assert (w != NULL , "invariant") ;
1214 assert (_EntryList == NULL , "invariant") ;
1215
1216 // Convert the LIFO SLL anchored by _cxq into a DLL.
1217 // The list reorganization step operates in O(LENGTH(w)) time.
1218 // It's critical that this step operate quickly as
1219 // "Self" still holds the outer-lock, restricting parallelism
1220 // and effectively lengthening the critical section.
1221 // Invariant: s chases t chases u.
1222 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1223 // we have faster access to the tail.
1224
1225 if (QMode == 1) {
1226 // QMode == 1 : drain cxq to EntryList, reversing order
1227 // We also reverse the order of the list.
1228 ObjectWaiter * s = NULL ;
1229 ObjectWaiter * t = w ;
1230 ObjectWaiter * u = NULL ;
1231 while (t != NULL) {
1232 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1233 t->TState = ObjectWaiter::TS_ENTER ;
1234 u = t->_next ;
1235 t->_prev = u ;
1236 t->_next = s ;
1237 s = t;
1238 t = u ;
1239 }
1240 _EntryList = s ;
1241 assert (s != NULL, "invariant") ;
1242 } else {
1243 // QMode == 0 or QMode == 2
1244 _EntryList = w ;
1245 ObjectWaiter * q = NULL ;
1246 ObjectWaiter * p ;
1247 for (p = w ; p != NULL ; p = p->_next) {
1248 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1249 p->TState = ObjectWaiter::TS_ENTER ;
1250 p->_prev = q ;
1251 q = p ;
1252 }
1253 }
1254
1255 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1256 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1257
1258 // See if we can abdicate to a spinner instead of waking a thread.
1259 // A primary goal of the implementation is to reduce the
1260 // context-switch rate.
1261 if (_succ != NULL) continue;
1262
1263 w = _EntryList ;
1264 if (w != NULL) {
1265 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1266 ExitEpilog (Self, w) ;
1267 return ;
1268 }
1269 }
1270 }
1271
1272 // ExitSuspendEquivalent:
1273 // A faster alternate to handle_special_suspend_equivalent_condition()
1274 //
1275 // handle_special_suspend_equivalent_condition() unconditionally
1276 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1277 // operations have high latency. Note that in ::enter() we call HSSEC
1278 // while holding the monitor, so we effectively lengthen the critical sections.
1279 //
1280 // There are a number of possible solutions:
1281 //
1282 // A. To ameliorate the problem we might also defer state transitions
1283 // to as late as possible -- just prior to parking.
1284 // Given that, we'd call HSSEC after having returned from park(),
1285 // but before attempting to acquire the monitor. This is only a
1286 // partial solution. It avoids calling HSSEC while holding the
1287 // monitor (good), but it still increases successor reacquisition latency --
1288 // the interval between unparking a successor and the time the successor
1289 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1290 // If we use this technique we can also avoid EnterI()-exit() loop
1291 // in ::enter() where we iteratively drop the lock and then attempt
1292 // to reacquire it after suspending.
1293 //
1294 // B. In the future we might fold all the suspend bits into a
1295 // composite per-thread suspend flag and then update it with CAS().
1296 // Alternately, a Dekker-like mechanism with multiple variables
1297 // would suffice:
1298 // ST Self->_suspend_equivalent = false
1299 // MEMBAR
1300 // LD Self_>_suspend_flags
1301 //
1302
1303
1304 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1305 int Mode = Knob_FastHSSEC ;
1306 if (Mode && !jSelf->is_external_suspend()) {
1307 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1308 jSelf->clear_suspend_equivalent() ;
1309 if (2 == Mode) OrderAccess::storeload() ;
1310 if (!jSelf->is_external_suspend()) return false ;
1311 // We raced a suspension -- fall thru into the slow path
1312 TEVENT (ExitSuspendEquivalent - raced) ;
1313 jSelf->set_suspend_equivalent() ;
1314 }
1315 return jSelf->handle_special_suspend_equivalent_condition() ;
1316 }
1317
1318
1319 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1320 assert (_owner == Self, "invariant") ;
1321
1322 // Exit protocol:
1323 // 1. ST _succ = wakee
1324 // 2. membar #loadstore|#storestore;
1325 // 2. ST _owner = NULL
1326 // 3. unpark(wakee)
1327
1328 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1329 ParkEvent * Trigger = Wakee->_event ;
1330
1331 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1332 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1333 // out-of-scope (non-extant).
1334 Wakee = NULL ;
1335
1336 // Drop the lock
1337 OrderAccess::release_store_ptr (&_owner, NULL) ;
1338 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1339
1340 if (SafepointSynchronize::do_call_back()) {
1341 TEVENT (unpark before SAFEPOINT) ;
1342 }
1343
1344 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1345 Trigger->unpark() ;
1346
1347 // Maintain stats and report events to JVMTI
1348 if (ObjectMonitor::_sync_Parks != NULL) {
1349 ObjectMonitor::_sync_Parks->inc() ;
1350 }
1351 }
1352
1353
1354 // -----------------------------------------------------------------------------
1355 // Class Loader deadlock handling.
1356 //
1357 // complete_exit exits a lock returning recursion count
1358 // complete_exit/reenter operate as a wait without waiting
1359 // complete_exit requires an inflated monitor
1360 // The _owner field is not always the Thread addr even with an
1361 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1362 // thread due to contention.
1363 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1364 Thread * const Self = THREAD;
1365 assert(Self->is_Java_thread(), "Must be Java thread!");
1366 JavaThread *jt = (JavaThread *)THREAD;
1367
1368 DeferredInitialize();
1369
1370 if (THREAD != _owner) {
1371 if (THREAD->is_lock_owned ((address)_owner)) {
1372 assert(_recursions == 0, "internal state error");
1373 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1374 _recursions = 0 ;
1375 OwnerIsThread = 1 ;
1376 }
1377 }
1378
1379 guarantee(Self == _owner, "complete_exit not owner");
1380 intptr_t save = _recursions; // record the old recursion count
1381 _recursions = 0; // set the recursion level to be 0
1382 exit (true, Self) ; // exit the monitor
1383 guarantee (_owner != Self, "invariant");
1384 return save;
1385 }
1386
1387 // reenter() enters a lock and sets recursion count
1388 // complete_exit/reenter operate as a wait without waiting
1389 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1390 Thread * const Self = THREAD;
1391 assert(Self->is_Java_thread(), "Must be Java thread!");
1392 JavaThread *jt = (JavaThread *)THREAD;
1393
1394 guarantee(_owner != Self, "reenter already owner");
1395 enter (THREAD); // enter the monitor
1396 guarantee (_recursions == 0, "reenter recursion");
1397 _recursions = recursions;
1398 return;
1399 }
1400
1401
1402 // -----------------------------------------------------------------------------
1403 // A macro is used below because there may already be a pending
1404 // exception which should not abort the execution of the routines
1405 // which use this (which is why we don't put this into check_slow and
1406 // call it with a CHECK argument).
1407
1408 #define CHECK_OWNER() \
1409 do { \
1410 if (THREAD != _owner) { \
1411 if (THREAD->is_lock_owned((address) _owner)) { \
1412 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1413 _recursions = 0; \
1414 OwnerIsThread = 1 ; \
1415 } else { \
1416 TEVENT (Throw IMSX) ; \
1417 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1418 } \
1419 } \
1420 } while (false)
1421
1422 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1423 // TODO-FIXME: remove check_slow() -- it's likely dead.
1424
1425 void ObjectMonitor::check_slow(TRAPS) {
1426 TEVENT (check_slow - throw IMSX) ;
1427 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1428 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1429 }
1430
1431 static int Adjust (volatile int * adr, int dx) {
1432 int v ;
1433 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1434 return v ;
1435 }
1436
1437 // helper method for posting a monitor wait event
1438 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1439 jlong notifier_tid,
1440 jlong timeout,
1441 bool timedout) {
1442 event->set_klass(((oop)this->object())->klass());
1443 event->set_timeout((TYPE_ULONG)timeout);
1444 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1445 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1446 event->set_timedOut((TYPE_BOOLEAN)timedout);
1447 event->commit();
1448 }
1449
1450 // -----------------------------------------------------------------------------
1451 // Wait/Notify/NotifyAll
1452 //
1453 // Note: a subset of changes to ObjectMonitor::wait()
1454 // will need to be replicated in complete_exit above
1455 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1456 Thread * const Self = THREAD ;
1457 assert(Self->is_Java_thread(), "Must be Java thread!");
1458 JavaThread *jt = (JavaThread *)THREAD;
1459
1460 DeferredInitialize () ;
1461
1462 // Throw IMSX or IEX.
1463 CHECK_OWNER();
1464
1465 EventJavaMonitorWait event;
1466
1467 // check for a pending interrupt
1468 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1469 // post monitor waited event. Note that this is past-tense, we are done waiting.
1470 if (JvmtiExport::should_post_monitor_waited()) {
1471 // Note: 'false' parameter is passed here because the
1472 // wait was not timed out due to thread interrupt.
1473 JvmtiExport::post_monitor_waited(jt, this, false);
1474
1475 // In this short circuit of the monitor wait protocol, the
1476 // current thread never drops ownership of the monitor and
1477 // never gets added to the wait queue so the current thread
1478 // cannot be made the successor. This means that the
1479 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1480 // consume an unpark() meant for the ParkEvent associated with
1481 // this ObjectMonitor.
1482 }
1483 if (event.should_commit()) {
1484 post_monitor_wait_event(&event, 0, millis, false);
1485 }
1486 TEVENT (Wait - Throw IEX) ;
1487 THROW(vmSymbols::java_lang_InterruptedException());
1488 return ;
1489 }
1490
1491 TEVENT (Wait) ;
1492
1493 assert (Self->_Stalled == 0, "invariant") ;
1494 Self->_Stalled = intptr_t(this) ;
1495 jt->set_current_waiting_monitor(this);
1496
1497 // create a node to be put into the queue
1498 // Critically, after we reset() the event but prior to park(), we must check
1499 // for a pending interrupt.
1500 ObjectWaiter node(Self);
1501 node.TState = ObjectWaiter::TS_WAIT ;
1502 Self->_ParkEvent->reset() ;
1503 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1504
1505 // Enter the waiting queue, which is a circular doubly linked list in this case
1506 // but it could be a priority queue or any data structure.
1507 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1508 // by the the owner of the monitor *except* in the case where park()
1509 // returns because of a timeout of interrupt. Contention is exceptionally rare
1510 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1511
1512 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1513 AddWaiter (&node) ;
1514 Thread::SpinRelease (&_WaitSetLock) ;
1515
1516 if ((SyncFlags & 4) == 0) {
1517 _Responsible = NULL ;
1518 }
1519 intptr_t save = _recursions; // record the old recursion count
1520 _waiters++; // increment the number of waiters
1521 _recursions = 0; // set the recursion level to be 1
1522 exit (true, Self) ; // exit the monitor
1523 guarantee (_owner != Self, "invariant") ;
1524
1525 // The thread is on the WaitSet list - now park() it.
1526 // On MP systems it's conceivable that a brief spin before we park
1527 // could be profitable.
1528 //
1529 // TODO-FIXME: change the following logic to a loop of the form
1530 // while (!timeout && !interrupted && _notified == 0) park()
1531
1532 int ret = OS_OK ;
1533 int WasNotified = 0 ;
1534 { // State transition wrappers
1535 OSThread* osthread = Self->osthread();
1536 OSThreadWaitState osts(osthread, true);
1537 {
1538 ThreadBlockInVM tbivm(jt);
1539 // Thread is in thread_blocked state and oop access is unsafe.
1540 jt->set_suspend_equivalent();
1541
1542 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1543 // Intentionally empty
1544 } else
1545 if (node._notified == 0) {
1546 if (millis <= 0) {
1547 Self->_ParkEvent->park () ;
1548 } else {
1549 ret = Self->_ParkEvent->park (millis) ;
1550 }
1551 }
1552
1553 // were we externally suspended while we were waiting?
1554 if (ExitSuspendEquivalent (jt)) {
1555 // TODO-FIXME: add -- if succ == Self then succ = null.
1556 jt->java_suspend_self();
1557 }
1558
1559 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1560
1561
1562 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1563 // from the WaitSet to the EntryList.
1564 // See if we need to remove Node from the WaitSet.
1565 // We use double-checked locking to avoid grabbing _WaitSetLock
1566 // if the thread is not on the wait queue.
1567 //
1568 // Note that we don't need a fence before the fetch of TState.
1569 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1570 // written by the is thread. (perhaps the fetch might even be satisfied
1571 // by a look-aside into the processor's own store buffer, although given
1572 // the length of the code path between the prior ST and this load that's
1573 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1574 // then we'll acquire the lock and then re-fetch a fresh TState value.
1575 // That is, we fail toward safety.
1576
1577 if (node.TState == ObjectWaiter::TS_WAIT) {
1578 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1579 if (node.TState == ObjectWaiter::TS_WAIT) {
1580 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1581 assert(node._notified == 0, "invariant");
1582 node.TState = ObjectWaiter::TS_RUN ;
1583 }
1584 Thread::SpinRelease (&_WaitSetLock) ;
1585 }
1586
1587 // The thread is now either on off-list (TS_RUN),
1588 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1589 // The Node's TState variable is stable from the perspective of this thread.
1590 // No other threads will asynchronously modify TState.
1591 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1592 OrderAccess::loadload() ;
1593 if (_succ == Self) _succ = NULL ;
1594 WasNotified = node._notified ;
1595
1596 // Reentry phase -- reacquire the monitor.
1597 // re-enter contended monitor after object.wait().
1598 // retain OBJECT_WAIT state until re-enter successfully completes
1599 // Thread state is thread_in_vm and oop access is again safe,
1600 // although the raw address of the object may have changed.
1601 // (Don't cache naked oops over safepoints, of course).
1602
1603 // post monitor waited event. Note that this is past-tense, we are done waiting.
1604 if (JvmtiExport::should_post_monitor_waited()) {
1605 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1606 }
1607
1608 // Without the fix for 8028280, it is possible for the above call:
1609 //
1610 // Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1611 //
1612 // to consume the unpark() that was done when the successor was set.
1613 // The solution for this very rare possibility is to redo the unpark()
1614 // outside of the JvmtiExport::should_post_monitor_waited() check.
1615 //
1616 if (node._notified != 0 && _succ == Self) {
1617 // In this part of the monitor wait-notify-reenter protocol it
1618 // is possible (and normal) for another thread to do a fastpath
1619 // monitor enter-exit while this thread is still trying to get
1620 // to the reenter portion of the protocol.
1621 //
1622 // The ObjectMonitor was notified and the current thread is
1623 // the successor which also means that an unpark() has already
1624 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1625 // consume the unpark() that was done when the successor was
1626 // set because the same ParkEvent is shared between Java
1627 // monitors and JVM/TI RawMonitors (for now).
1628 //
1629 // We redo the unpark() to ensure forward progress, i.e., we
1630 // don't want all pending threads hanging (parked) with none
1631 // entering the unlocked monitor.
1632 node._event->unpark();
1633 }
1634
1635 if (event.should_commit()) {
1636 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1637 }
1638
1639 OrderAccess::fence() ;
1640
1641 assert (Self->_Stalled != 0, "invariant") ;
1642 Self->_Stalled = 0 ;
1643
1644 assert (_owner != Self, "invariant") ;
1645 ObjectWaiter::TStates v = node.TState ;
1646 if (v == ObjectWaiter::TS_RUN) {
1647 enter (Self) ;
1648 } else {
1649 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1650 ReenterI (Self, &node) ;
1651 node.wait_reenter_end(this);
1652 }
1653
1654 // Self has reacquired the lock.
1655 // Lifecycle - the node representing Self must not appear on any queues.
1656 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1657 // want residual elements associated with this thread left on any lists.
1658 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1659 assert (_owner == Self, "invariant") ;
1660 assert (_succ != Self , "invariant") ;
1661 } // OSThreadWaitState()
1662
1663 jt->set_current_waiting_monitor(NULL);
1664
1665 guarantee (_recursions == 0, "invariant") ;
1666 _recursions = save; // restore the old recursion count
1667 _waiters--; // decrement the number of waiters
1668
1669 // Verify a few postconditions
1670 assert (_owner == Self , "invariant") ;
1671 assert (_succ != Self , "invariant") ;
1672 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1673
1674 if (SyncFlags & 32) {
1675 OrderAccess::fence() ;
1676 }
1677
1678 // check if the notification happened
1679 if (!WasNotified) {
1680 // no, it could be timeout or Thread.interrupt() or both
1681 // check for interrupt event, otherwise it is timeout
1682 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1683 TEVENT (Wait - throw IEX from epilog) ;
1684 THROW(vmSymbols::java_lang_InterruptedException());
1685 }
1686 }
1687
1688 // NOTE: Spurious wake up will be consider as timeout.
1689 // Monitor notify has precedence over thread interrupt.
1690 }
1691
1692
1693 // Consider:
1694 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1695 // then instead of transferring a thread from the WaitSet to the EntryList
1696 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1697
1698 void ObjectMonitor::notify(TRAPS) {
1699 CHECK_OWNER();
1700 if (_WaitSet == NULL) {
1701 TEVENT (Empty-Notify) ;
1702 return ;
1703 }
1704 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1705
1706 int Policy = Knob_MoveNotifyee ;
1707
1708 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1709 ObjectWaiter * iterator = DequeueWaiter() ;
1710 if (iterator != NULL) {
1711 TEVENT (Notify1 - Transfer) ;
1712 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1713 guarantee (iterator->_notified == 0, "invariant") ;
1714 if (Policy != 4) {
1715 iterator->TState = ObjectWaiter::TS_ENTER ;
1716 }
1717 iterator->_notified = 1 ;
1718 Thread * Self = THREAD;
1719 iterator->_notifier_tid = Self->osthread()->thread_id();
1720
1721 ObjectWaiter * List = _EntryList ;
1722 if (List != NULL) {
1723 assert (List->_prev == NULL, "invariant") ;
1724 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1725 assert (List != iterator, "invariant") ;
1726 }
1727
1728 if (Policy == 0) { // prepend to EntryList
1729 if (List == NULL) {
1730 iterator->_next = iterator->_prev = NULL ;
1731 _EntryList = iterator ;
1732 } else {
1733 List->_prev = iterator ;
1734 iterator->_next = List ;
1735 iterator->_prev = NULL ;
1736 _EntryList = iterator ;
1737 }
1738 } else
1739 if (Policy == 1) { // append to EntryList
1740 if (List == NULL) {
1741 iterator->_next = iterator->_prev = NULL ;
1742 _EntryList = iterator ;
1743 } else {
1744 // CONSIDER: finding the tail currently requires a linear-time walk of
1745 // the EntryList. We can make tail access constant-time by converting to
1746 // a CDLL instead of using our current DLL.
1747 ObjectWaiter * Tail ;
1748 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1749 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1750 Tail->_next = iterator ;
1751 iterator->_prev = Tail ;
1752 iterator->_next = NULL ;
1753 }
1754 } else
1755 if (Policy == 2) { // prepend to cxq
1756 // prepend to cxq
1757 if (List == NULL) {
1758 iterator->_next = iterator->_prev = NULL ;
1759 _EntryList = iterator ;
1760 } else {
1761 iterator->TState = ObjectWaiter::TS_CXQ ;
1762 for (;;) {
1763 ObjectWaiter * Front = _cxq ;
1764 iterator->_next = Front ;
1765 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1766 break ;
1767 }
1768 }
1769 }
1770 } else
1771 if (Policy == 3) { // append to cxq
1772 iterator->TState = ObjectWaiter::TS_CXQ ;
1773 for (;;) {
1774 ObjectWaiter * Tail ;
1775 Tail = _cxq ;
1776 if (Tail == NULL) {
1777 iterator->_next = NULL ;
1778 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1779 break ;
1780 }
1781 } else {
1782 while (Tail->_next != NULL) Tail = Tail->_next ;
1783 Tail->_next = iterator ;
1784 iterator->_prev = Tail ;
1785 iterator->_next = NULL ;
1786 break ;
1787 }
1788 }
1789 } else {
1790 ParkEvent * ev = iterator->_event ;
1791 iterator->TState = ObjectWaiter::TS_RUN ;
1792 OrderAccess::fence() ;
1793 ev->unpark() ;
1794 }
1795
1796 if (Policy < 4) {
1797 iterator->wait_reenter_begin(this);
1798 }
1799
1800 // _WaitSetLock protects the wait queue, not the EntryList. We could
1801 // move the add-to-EntryList operation, above, outside the critical section
1802 // protected by _WaitSetLock. In practice that's not useful. With the
1803 // exception of wait() timeouts and interrupts the monitor owner
1804 // is the only thread that grabs _WaitSetLock. There's almost no contention
1805 // on _WaitSetLock so it's not profitable to reduce the length of the
1806 // critical section.
1807 }
1808
1809 Thread::SpinRelease (&_WaitSetLock) ;
1810
1811 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1812 ObjectMonitor::_sync_Notifications->inc() ;
1813 }
1814 }
1815
1816
1817 void ObjectMonitor::notifyAll(TRAPS) {
1818 CHECK_OWNER();
1819 ObjectWaiter* iterator;
1820 if (_WaitSet == NULL) {
1821 TEVENT (Empty-NotifyAll) ;
1822 return ;
1823 }
1824 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1825
1826 int Policy = Knob_MoveNotifyee ;
1827 int Tally = 0 ;
1828 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1829
1830 for (;;) {
1831 iterator = DequeueWaiter () ;
1832 if (iterator == NULL) break ;
1833 TEVENT (NotifyAll - Transfer1) ;
1834 ++Tally ;
1835
1836 // Disposition - what might we do with iterator ?
1837 // a. add it directly to the EntryList - either tail or head.
1838 // b. push it onto the front of the _cxq.
1839 // For now we use (a).
1840
1841 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1842 guarantee (iterator->_notified == 0, "invariant") ;
1843 iterator->_notified = 1 ;
1844 Thread * Self = THREAD;
1845 iterator->_notifier_tid = Self->osthread()->thread_id();
1846 if (Policy != 4) {
1847 iterator->TState = ObjectWaiter::TS_ENTER ;
1848 }
1849
1850 ObjectWaiter * List = _EntryList ;
1851 if (List != NULL) {
1852 assert (List->_prev == NULL, "invariant") ;
1853 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1854 assert (List != iterator, "invariant") ;
1855 }
1856
1857 if (Policy == 0) { // prepend to EntryList
1858 if (List == NULL) {
1859 iterator->_next = iterator->_prev = NULL ;
1860 _EntryList = iterator ;
1861 } else {
1862 List->_prev = iterator ;
1863 iterator->_next = List ;
1864 iterator->_prev = NULL ;
1865 _EntryList = iterator ;
1866 }
1867 } else
1868 if (Policy == 1) { // append to EntryList
1869 if (List == NULL) {
1870 iterator->_next = iterator->_prev = NULL ;
1871 _EntryList = iterator ;
1872 } else {
1873 // CONSIDER: finding the tail currently requires a linear-time walk of
1874 // the EntryList. We can make tail access constant-time by converting to
1875 // a CDLL instead of using our current DLL.
1876 ObjectWaiter * Tail ;
1877 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1878 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1879 Tail->_next = iterator ;
1880 iterator->_prev = Tail ;
1881 iterator->_next = NULL ;
1882 }
1883 } else
1884 if (Policy == 2) { // prepend to cxq
1885 // prepend to cxq
1886 iterator->TState = ObjectWaiter::TS_CXQ ;
1887 for (;;) {
1888 ObjectWaiter * Front = _cxq ;
1889 iterator->_next = Front ;
1890 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1891 break ;
1892 }
1893 }
1894 } else
1895 if (Policy == 3) { // append to cxq
1896 iterator->TState = ObjectWaiter::TS_CXQ ;
1897 for (;;) {
1898 ObjectWaiter * Tail ;
1899 Tail = _cxq ;
1900 if (Tail == NULL) {
1901 iterator->_next = NULL ;
1902 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1903 break ;
1904 }
1905 } else {
1906 while (Tail->_next != NULL) Tail = Tail->_next ;
1907 Tail->_next = iterator ;
1908 iterator->_prev = Tail ;
1909 iterator->_next = NULL ;
1910 break ;
1911 }
1912 }
1913 } else {
1914 ParkEvent * ev = iterator->_event ;
1915 iterator->TState = ObjectWaiter::TS_RUN ;
1916 OrderAccess::fence() ;
1917 ev->unpark() ;
1918 }
1919
1920 if (Policy < 4) {
1921 iterator->wait_reenter_begin(this);
1922 }
1923
1924 // _WaitSetLock protects the wait queue, not the EntryList. We could
1925 // move the add-to-EntryList operation, above, outside the critical section
1926 // protected by _WaitSetLock. In practice that's not useful. With the
1927 // exception of wait() timeouts and interrupts the monitor owner
1928 // is the only thread that grabs _WaitSetLock. There's almost no contention
1929 // on _WaitSetLock so it's not profitable to reduce the length of the
1930 // critical section.
1931 }
1932
1933 Thread::SpinRelease (&_WaitSetLock) ;
1934
1935 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1936 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1937 }
1938 }
1939
1940 // -----------------------------------------------------------------------------
1941 // Adaptive Spinning Support
1942 //
1943 // Adaptive spin-then-block - rational spinning
1944 //
1945 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1946 // algorithm. On high order SMP systems it would be better to start with
1947 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1948 // a contending thread could enqueue itself on the cxq and then spin locally
1949 // on a thread-specific variable such as its ParkEvent._Event flag.
1950 // That's left as an exercise for the reader. Note that global spinning is
1951 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1952 // low latency and massive bandwidth.
1953 //
1954 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1955 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1956 // (duration) or we can fix the count at approximately the duration of
1957 // a context switch and vary the frequency. Of course we could also
1958 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1959 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1960 //
1961 // This implementation varies the duration "D", where D varies with
1962 // the success rate of recent spin attempts. (D is capped at approximately
1963 // length of a round-trip context switch). The success rate for recent
1964 // spin attempts is a good predictor of the success rate of future spin
1965 // attempts. The mechanism adapts automatically to varying critical
1966 // section length (lock modality), system load and degree of parallelism.
1967 // D is maintained per-monitor in _SpinDuration and is initialized
1968 // optimistically. Spin frequency is fixed at 100%.
1969 //
1970 // Note that _SpinDuration is volatile, but we update it without locks
1971 // or atomics. The code is designed so that _SpinDuration stays within
1972 // a reasonable range even in the presence of races. The arithmetic
1973 // operations on _SpinDuration are closed over the domain of legal values,
1974 // so at worst a race will install and older but still legal value.
1975 // At the very worst this introduces some apparent non-determinism.
1976 // We might spin when we shouldn't or vice-versa, but since the spin
1977 // count are relatively short, even in the worst case, the effect is harmless.
1978 //
1979 // Care must be taken that a low "D" value does not become an
1980 // an absorbing state. Transient spinning failures -- when spinning
1981 // is overall profitable -- should not cause the system to converge
1982 // on low "D" values. We want spinning to be stable and predictable
1983 // and fairly responsive to change and at the same time we don't want
1984 // it to oscillate, become metastable, be "too" non-deterministic,
1985 // or converge on or enter undesirable stable absorbing states.
1986 //
1987 // We implement a feedback-based control system -- using past behavior
1988 // to predict future behavior. We face two issues: (a) if the
1989 // input signal is random then the spin predictor won't provide optimal
1990 // results, and (b) if the signal frequency is too high then the control
1991 // system, which has some natural response lag, will "chase" the signal.
1992 // (b) can arise from multimodal lock hold times. Transient preemption
1993 // can also result in apparent bimodal lock hold times.
1994 // Although sub-optimal, neither condition is particularly harmful, as
1995 // in the worst-case we'll spin when we shouldn't or vice-versa.
1996 // The maximum spin duration is rather short so the failure modes aren't bad.
1997 // To be conservative, I've tuned the gain in system to bias toward
1998 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1999 // "rings" or oscillates between spinning and not spinning. This happens
2000 // when spinning is just on the cusp of profitability, however, so the
2001 // situation is not dire. The state is benign -- there's no need to add
2002 // hysteresis control to damp the transition rate between spinning and
2003 // not spinning.
2004 //
2005
2006 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
2007 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
2008
2009 // Spinning: Fixed frequency (100%), vary duration
2010
2011
2012 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
2013
2014 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2015 int ctr = Knob_FixedSpin ;
2016 if (ctr != 0) {
2017 while (--ctr >= 0) {
2018 if (TryLock (Self) > 0) return 1 ;
2019 SpinPause () ;
2020 }
2021 return 0 ;
2022 }
2023
2024 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
2025 if (TryLock(Self) > 0) {
2026 // Increase _SpinDuration ...
2027 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2028 // Raising _SpurDuration to the poverty line is key.
2029 int x = _SpinDuration ;
2030 if (x < Knob_SpinLimit) {
2031 if (x < Knob_Poverty) x = Knob_Poverty ;
2032 _SpinDuration = x + Knob_BonusB ;
2033 }
2034 return 1 ;
2035 }
2036 SpinPause () ;
2037 }
2038
2039 // Admission control - verify preconditions for spinning
2040 //
2041 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2042 // becoming an absorbing state. Put another way, we spin briefly to
2043 // sample, just in case the system load, parallelism, contention, or lock
2044 // modality changed.
2045 //
2046 // Consider the following alternative:
2047 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2048 // spin attempt. "Periodically" might mean after a tally of
2049 // the # of failed spin attempts (or iterations) reaches some threshold.
2050 // This takes us into the realm of 1-out-of-N spinning, where we
2051 // hold the duration constant but vary the frequency.
2052
2053 ctr = _SpinDuration ;
2054 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2055 if (ctr <= 0) return 0 ;
2056
2057 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2058 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2059 TEVENT (Spin abort - notrunnable [TOP]);
2060 return 0 ;
2061 }
2062
2063 int MaxSpin = Knob_MaxSpinners ;
2064 if (MaxSpin >= 0) {
2065 if (_Spinner > MaxSpin) {
2066 TEVENT (Spin abort -- too many spinners) ;
2067 return 0 ;
2068 }
2069 // Slighty racy, but benign ...
2070 Adjust (&_Spinner, 1) ;
2071 }
2072
2073 // We're good to spin ... spin ingress.
2074 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2075 // when preparing to LD...CAS _owner, etc and the CAS is likely
2076 // to succeed.
2077 int hits = 0 ;
2078 int msk = 0 ;
2079 int caspty = Knob_CASPenalty ;
2080 int oxpty = Knob_OXPenalty ;
2081 int sss = Knob_SpinSetSucc ;
2082 if (sss && _succ == NULL ) _succ = Self ;
2083 Thread * prv = NULL ;
2084
2085 // There are three ways to exit the following loop:
2086 // 1. A successful spin where this thread has acquired the lock.
2087 // 2. Spin failure with prejudice
2088 // 3. Spin failure without prejudice
2089
2090 while (--ctr >= 0) {
2091
2092 // Periodic polling -- Check for pending GC
2093 // Threads may spin while they're unsafe.
2094 // We don't want spinning threads to delay the JVM from reaching
2095 // a stop-the-world safepoint or to steal cycles from GC.
2096 // If we detect a pending safepoint we abort in order that
2097 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2098 // this thread, if safe, doesn't steal cycles from GC.
2099 // This is in keeping with the "no loitering in runtime" rule.
2100 // We periodically check to see if there's a safepoint pending.
2101 if ((ctr & 0xFF) == 0) {
2102 if (SafepointSynchronize::do_call_back()) {
2103 TEVENT (Spin: safepoint) ;
2104 goto Abort ; // abrupt spin egress
2105 }
2106 if (Knob_UsePause & 1) SpinPause () ;
2107
2108 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2109 if (hits > 50 && scb != NULL) {
2110 int abend = (*scb)(SpinCallbackArgument, 0) ;
2111 }
2112 }
2113
2114 if (Knob_UsePause & 2) SpinPause() ;
2115
2116 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2117 // This is useful on classic SMP systems, but is of less utility on
2118 // N1-style CMT platforms.
2119 //
2120 // Trade-off: lock acquisition latency vs coherency bandwidth.
2121 // Lock hold times are typically short. A histogram
2122 // of successful spin attempts shows that we usually acquire
2123 // the lock early in the spin. That suggests we want to
2124 // sample _owner frequently in the early phase of the spin,
2125 // but then back-off and sample less frequently as the spin
2126 // progresses. The back-off makes a good citizen on SMP big
2127 // SMP systems. Oversampling _owner can consume excessive
2128 // coherency bandwidth. Relatedly, if we _oversample _owner we
2129 // can inadvertently interfere with the the ST m->owner=null.
2130 // executed by the lock owner.
2131 if (ctr & msk) continue ;
2132 ++hits ;
2133 if ((hits & 0xF) == 0) {
2134 // The 0xF, above, corresponds to the exponent.
2135 // Consider: (msk+1)|msk
2136 msk = ((msk << 2)|3) & BackOffMask ;
2137 }
2138
2139 // Probe _owner with TATAS
2140 // If this thread observes the monitor transition or flicker
2141 // from locked to unlocked to locked, then the odds that this
2142 // thread will acquire the lock in this spin attempt go down
2143 // considerably. The same argument applies if the CAS fails
2144 // or if we observe _owner change from one non-null value to
2145 // another non-null value. In such cases we might abort
2146 // the spin without prejudice or apply a "penalty" to the
2147 // spin count-down variable "ctr", reducing it by 100, say.
2148
2149 Thread * ox = (Thread *) _owner ;
2150 if (ox == NULL) {
2151 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2152 if (ox == NULL) {
2153 // The CAS succeeded -- this thread acquired ownership
2154 // Take care of some bookkeeping to exit spin state.
2155 if (sss && _succ == Self) {
2156 _succ = NULL ;
2157 }
2158 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2159
2160 // Increase _SpinDuration :
2161 // The spin was successful (profitable) so we tend toward
2162 // longer spin attempts in the future.
2163 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2164 // If we acquired the lock early in the spin cycle it
2165 // makes sense to increase _SpinDuration proportionally.
2166 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2167 int x = _SpinDuration ;
2168 if (x < Knob_SpinLimit) {
2169 if (x < Knob_Poverty) x = Knob_Poverty ;
2170 _SpinDuration = x + Knob_Bonus ;
2171 }
2172 return 1 ;
2173 }
2174
2175 // The CAS failed ... we can take any of the following actions:
2176 // * penalize: ctr -= Knob_CASPenalty
2177 // * exit spin with prejudice -- goto Abort;
2178 // * exit spin without prejudice.
2179 // * Since CAS is high-latency, retry again immediately.
2180 prv = ox ;
2181 TEVENT (Spin: cas failed) ;
2182 if (caspty == -2) break ;
2183 if (caspty == -1) goto Abort ;
2184 ctr -= caspty ;
2185 continue ;
2186 }
2187
2188 // Did lock ownership change hands ?
2189 if (ox != prv && prv != NULL ) {
2190 TEVENT (spin: Owner changed)
2191 if (oxpty == -2) break ;
2192 if (oxpty == -1) goto Abort ;
2193 ctr -= oxpty ;
2194 }
2195 prv = ox ;
2196
2197 // Abort the spin if the owner is not executing.
2198 // The owner must be executing in order to drop the lock.
2199 // Spinning while the owner is OFFPROC is idiocy.
2200 // Consider: ctr -= RunnablePenalty ;
2201 if (Knob_OState && NotRunnable (Self, ox)) {
2202 TEVENT (Spin abort - notrunnable);
2203 goto Abort ;
2204 }
2205 if (sss && _succ == NULL ) _succ = Self ;
2206 }
2207
2208 // Spin failed with prejudice -- reduce _SpinDuration.
2209 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2210 // AIMD is globally stable.
2211 TEVENT (Spin failure) ;
2212 {
2213 int x = _SpinDuration ;
2214 if (x > 0) {
2215 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2216 // This is globally sample and tends to damp the response.
2217 x -= Knob_Penalty ;
2218 if (x < 0) x = 0 ;
2219 _SpinDuration = x ;
2220 }
2221 }
2222
2223 Abort:
2224 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2225 if (sss && _succ == Self) {
2226 _succ = NULL ;
2227 // Invariant: after setting succ=null a contending thread
2228 // must recheck-retry _owner before parking. This usually happens
2229 // in the normal usage of TrySpin(), but it's safest
2230 // to make TrySpin() as foolproof as possible.
2231 OrderAccess::fence() ;
2232 if (TryLock(Self) > 0) return 1 ;
2233 }
2234 return 0 ;
2235 }
2236
2237 // NotRunnable() -- informed spinning
2238 //
2239 // Don't bother spinning if the owner is not eligible to drop the lock.
2240 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2241 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2242 // The thread must be runnable in order to drop the lock in timely fashion.
2243 // If the _owner is not runnable then spinning will not likely be
2244 // successful (profitable).
2245 //
2246 // Beware -- the thread referenced by _owner could have died
2247 // so a simply fetch from _owner->_thread_state might trap.
2248 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2249 // Because of the lifecycle issues the schedctl and _thread_state values
2250 // observed by NotRunnable() might be garbage. NotRunnable must
2251 // tolerate this and consider the observed _thread_state value
2252 // as advisory.
2253 //
2254 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2255 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2256 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2257 // with the LSB of _owner. Another option would be to probablistically probe
2258 // the putative _owner->TypeTag value.
2259 //
2260 // Checking _thread_state isn't perfect. Even if the thread is
2261 // in_java it might be blocked on a page-fault or have been preempted
2262 // and sitting on a ready/dispatch queue. _thread state in conjunction
2263 // with schedctl.sc_state gives us a good picture of what the
2264 // thread is doing, however.
2265 //
2266 // TODO: check schedctl.sc_state.
2267 // We'll need to use SafeFetch32() to read from the schedctl block.
2268 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2269 //
2270 // The return value from NotRunnable() is *advisory* -- the
2271 // result is based on sampling and is not necessarily coherent.
2272 // The caller must tolerate false-negative and false-positive errors.
2273 // Spinning, in general, is probabilistic anyway.
2274
2275
2276 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2277 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2278 if (!OwnerIsThread) return 0 ;
2279
2280 if (ox == NULL) return 0 ;
2281
2282 // Avoid transitive spinning ...
2283 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2284 // Immediately after T1 acquires L it's possible that T2, also
2285 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2286 // This occurs transiently after T1 acquired L but before
2287 // T1 managed to clear T1.Stalled. T2 does not need to abort
2288 // its spin in this circumstance.
2289 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2290
2291 if (BlockedOn == 1) return 1 ;
2292 if (BlockedOn != 0) {
2293 return BlockedOn != intptr_t(this) && _owner == ox ;
2294 }
2295
2296 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2297 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2298 // consider also: jst != _thread_in_Java -- but that's overspecific.
2299 return jst == _thread_blocked || jst == _thread_in_native ;
2300 }
2301
2302
2303 // -----------------------------------------------------------------------------
2304 // WaitSet management ...
2305
2306 ObjectWaiter::ObjectWaiter(Thread* thread) {
2307 _next = NULL;
2308 _prev = NULL;
2309 _notified = 0;
2310 TState = TS_RUN ;
2311 _thread = thread;
2312 _event = thread->_ParkEvent ;
2313 _active = false;
2314 assert (_event != NULL, "invariant") ;
2315 }
2316
2317 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2318 JavaThread *jt = (JavaThread *)this->_thread;
2319 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2320 }
2321
2322 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2323 JavaThread *jt = (JavaThread *)this->_thread;
2324 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2325 }
2326
2327 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2328 assert(node != NULL, "should not dequeue NULL node");
2329 assert(node->_prev == NULL, "node already in list");
2330 assert(node->_next == NULL, "node already in list");
2331 // put node at end of queue (circular doubly linked list)
2332 if (_WaitSet == NULL) {
2333 _WaitSet = node;
2334 node->_prev = node;
2335 node->_next = node;
2336 } else {
2337 ObjectWaiter* head = _WaitSet ;
2338 ObjectWaiter* tail = head->_prev;
2339 assert(tail->_next == head, "invariant check");
2340 tail->_next = node;
2341 head->_prev = node;
2342 node->_next = head;
2343 node->_prev = tail;
2344 }
2345 }
2346
2347 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2348 // dequeue the very first waiter
2349 ObjectWaiter* waiter = _WaitSet;
2350 if (waiter) {
2351 DequeueSpecificWaiter(waiter);
2352 }
2353 return waiter;
2354 }
2355
2356 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2357 assert(node != NULL, "should not dequeue NULL node");
2358 assert(node->_prev != NULL, "node already removed from list");
2359 assert(node->_next != NULL, "node already removed from list");
2360 // when the waiter has woken up because of interrupt,
2361 // timeout or other spurious wake-up, dequeue the
2362 // waiter from waiting list
2363 ObjectWaiter* next = node->_next;
2364 if (next == node) {
2365 assert(node->_prev == node, "invariant check");
2366 _WaitSet = NULL;
2367 } else {
2368 ObjectWaiter* prev = node->_prev;
2369 assert(prev->_next == node, "invariant check");
2370 assert(next->_prev == node, "invariant check");
2371 next->_prev = prev;
2372 prev->_next = next;
2373 if (_WaitSet == node) {
2374 _WaitSet = next;
2375 }
2376 }
2377 node->_next = NULL;
2378 node->_prev = NULL;
2379 }
2380
2381 // -----------------------------------------------------------------------------
2382 // PerfData support
2383 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2384 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2385 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2386 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2387 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2388 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2389 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2390 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2391 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2392 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2393 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2394 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2395 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2396 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2397 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2398 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2399 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2400 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2401
2402 // One-shot global initialization for the sync subsystem.
2403 // We could also defer initialization and initialize on-demand
2404 // the first time we call inflate(). Initialization would
2405 // be protected - like so many things - by the MonitorCache_lock.
2406
2407 void ObjectMonitor::Initialize () {
2408 static int InitializationCompleted = 0 ;
2409 assert (InitializationCompleted == 0, "invariant") ;
2410 InitializationCompleted = 1 ;
2411 if (UsePerfData) {
2412 EXCEPTION_MARK ;
2413 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2414 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2415 NEWPERFCOUNTER(_sync_Inflations) ;
2416 NEWPERFCOUNTER(_sync_Deflations) ;
2417 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2418 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2419 NEWPERFCOUNTER(_sync_Parks) ;
2420 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2421 NEWPERFCOUNTER(_sync_Notifications) ;
2422 NEWPERFCOUNTER(_sync_SlowEnter) ;
2423 NEWPERFCOUNTER(_sync_SlowExit) ;
2424 NEWPERFCOUNTER(_sync_SlowNotify) ;
2425 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2426 NEWPERFCOUNTER(_sync_FailedSpins) ;
2427 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2428 NEWPERFCOUNTER(_sync_PrivateA) ;
2429 NEWPERFCOUNTER(_sync_PrivateB) ;
2430 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2431 NEWPERFCOUNTER(_sync_MonScavenged) ;
2432 NEWPERFVARIABLE(_sync_MonExtant) ;
2433 #undef NEWPERFCOUNTER
2434 }
2435 }
2436
2437
2438 // Compile-time asserts
2439 // When possible, it's better to catch errors deterministically at
2440 // compile-time than at runtime. The down-side to using compile-time
2441 // asserts is that error message -- often something about negative array
2442 // indices -- is opaque.
2443
2444 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2445
2446 void ObjectMonitor::ctAsserts() {
2447 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2448 }
2449
2450
2451 static char * kvGet (char * kvList, const char * Key) {
2452 if (kvList == NULL) return NULL ;
2453 size_t n = strlen (Key) ;
2454 char * Search ;
2455 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2456 if (strncmp (Search, Key, n) == 0) {
2457 if (Search[n] == '=') return Search + n + 1 ;
2458 if (Search[n] == 0) return (char *) "1" ;
2459 }
2460 }
2461 return NULL ;
2462 }
2463
2464 static int kvGetInt (char * kvList, const char * Key, int Default) {
2465 char * v = kvGet (kvList, Key) ;
2466 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2467 if (Knob_ReportSettings && v != NULL) {
2468 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2469 ::fflush (stdout) ;
2470 }
2471 return rslt ;
2472 }
2473
2474 void ObjectMonitor::DeferredInitialize () {
2475 if (InitDone > 0) return ;
2476 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2477 while (InitDone != 1) ;
2478 return ;
2479 }
2480
2481 // One-shot global initialization ...
2482 // The initialization is idempotent, so we don't need locks.
2483 // In the future consider doing this via os::init_2().
2484 // SyncKnobs consist of <Key>=<Value> pairs in the style
2485 // of environment variables. Start by converting ':' to NUL.
2486
2487 if (SyncKnobs == NULL) SyncKnobs = "" ;
2488
2489 size_t sz = strlen (SyncKnobs) ;
2490 char * knobs = (char *) malloc (sz + 2) ;
2491 if (knobs == NULL) {
2492 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2493 guarantee (0, "invariant") ;
2494 }
2495 strcpy (knobs, SyncKnobs) ;
2496 knobs[sz+1] = 0 ;
2497 for (char * p = knobs ; *p ; p++) {
2498 if (*p == ':') *p = 0 ;
2499 }
2500
2501 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2502 SETKNOB(ReportSettings) ;
2503 SETKNOB(Verbose) ;
2504 SETKNOB(FixedSpin) ;
2505 SETKNOB(SpinLimit) ;
2506 SETKNOB(SpinBase) ;
2507 SETKNOB(SpinBackOff);
2508 SETKNOB(CASPenalty) ;
2509 SETKNOB(OXPenalty) ;
2510 SETKNOB(LogSpins) ;
2511 SETKNOB(SpinSetSucc) ;
2512 SETKNOB(SuccEnabled) ;
2513 SETKNOB(SuccRestrict) ;
2514 SETKNOB(Penalty) ;
2515 SETKNOB(Bonus) ;
2516 SETKNOB(BonusB) ;
2517 SETKNOB(Poverty) ;
2518 SETKNOB(SpinAfterFutile) ;
2519 SETKNOB(UsePause) ;
2520 SETKNOB(SpinEarly) ;
2521 SETKNOB(OState) ;
2522 SETKNOB(MaxSpinners) ;
2523 SETKNOB(PreSpin) ;
2524 SETKNOB(ExitPolicy) ;
2525 SETKNOB(QMode);
2526 SETKNOB(ResetEvent) ;
2527 SETKNOB(MoveNotifyee) ;
2528 SETKNOB(FastHSSEC) ;
2529 #undef SETKNOB
2530
2531 if (os::is_MP()) {
2532 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2533 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2534 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2535 } else {
2536 Knob_SpinLimit = 0 ;
2537 Knob_SpinBase = 0 ;
2538 Knob_PreSpin = 0 ;
2539 Knob_FixedSpin = -1 ;
2540 }
2541
2542 if (Knob_LogSpins == 0) {
2543 ObjectMonitor::_sync_FailedSpins = NULL ;
2544 }
2545
2546 free (knobs) ;
2547 OrderAccess::fence() ;
2548 InitDone = 1 ;
2549 }
2550
2551 #ifndef PRODUCT
2552 void ObjectMonitor::verify() {
2553 }
2554
2555 void ObjectMonitor::print() {
2556 }
2557 #endif