1 /*
2 * Copyright (c) 1998, 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/orderAccess.inline.hpp"
36 #include "runtime/osThread.hpp"
37 #include "runtime/stubRoutines.hpp"
38 #include "runtime/thread.inline.hpp"
39 #include "services/threadService.hpp"
40 #include "trace/tracing.hpp"
41 #include "trace/traceMacros.hpp"
42 #include "evtrace/traceEvents.hpp"
43 #include "utilities/dtrace.hpp"
44 #include "utilities/macros.hpp"
45 #include "utilities/preserveException.hpp"
46 #ifdef TARGET_OS_FAMILY_linux
47 # include "os_linux.inline.hpp"
48 #endif
49 #ifdef TARGET_OS_FAMILY_solaris
50 # include "os_solaris.inline.hpp"
51 #endif
52 #ifdef TARGET_OS_FAMILY_windows
53 # include "os_windows.inline.hpp"
54 #endif
55 #ifdef TARGET_OS_FAMILY_bsd
56 # include "os_bsd.inline.hpp"
57 #endif
58
59 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
60 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
61 #define ATTR __attribute__((noinline))
62 #else
63 #define ATTR
64 #endif
65
66
67 #ifdef DTRACE_ENABLED
68
69 // Only bother with this argument setup if dtrace is available
70 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
71
72
73 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
74 char* bytes = NULL; \
75 int len = 0; \
76 jlong jtid = SharedRuntime::get_java_tid(thread); \
77 Symbol* klassname = ((oop)obj)->klass()->name(); \
78 if (klassname != NULL) { \
79 bytes = (char*)klassname->bytes(); \
80 len = klassname->utf8_length(); \
81 }
82
83 #ifndef USDT2
84
85 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
86 jlong, uintptr_t, char*, int);
87 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
88 jlong, uintptr_t, char*, int);
89 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
90 jlong, uintptr_t, char*, int);
91 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
92 jlong, uintptr_t, char*, int);
93 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
94 jlong, uintptr_t, char*, int);
95
96 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
97 { \
98 if (DTraceMonitorProbes) { \
99 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
100 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
101 (monitor), bytes, len, (millis)); \
102 } \
103 }
104
105 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
106 { \
107 if (DTraceMonitorProbes) { \
108 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
109 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
110 (uintptr_t)(monitor), bytes, len); \
111 } \
112 }
113
114 #else /* USDT2 */
115
116 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
117 { \
118 if (DTraceMonitorProbes) { \
119 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
120 HOTSPOT_MONITOR_WAIT(jtid, \
121 (monitor), bytes, len, (millis)); \
122 } \
123 }
124
125 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
126 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
127 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
128 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
129 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
130
131 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
132 { \
133 if (DTraceMonitorProbes) { \
134 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
135 HOTSPOT_MONITOR_##probe(jtid, \
136 (uintptr_t)(monitor), bytes, len); \
137 } \
138 }
139
140 #endif /* USDT2 */
141 #else // ndef DTRACE_ENABLED
142
143 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
144 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
145
146 #endif // ndef DTRACE_ENABLED
147
148 // Tunables ...
149 // The knob* variables are effectively final. Once set they should
150 // never be modified hence. Consider using __read_mostly with GCC.
151
152 int ObjectMonitor::Knob_Verbose = 0 ;
153 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
154 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
155 static int Knob_HandOff = 0 ;
156 static int Knob_ReportSettings = 0 ;
157
158 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
159 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
160 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
161 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
162 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
163 static int Knob_SpinEarly = 1 ;
164 static int Knob_SuccEnabled = 1 ; // futile wake throttling
165 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
166 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
167 static int Knob_Bonus = 100 ; // spin success bonus
168 static int Knob_BonusB = 100 ; // spin success bonus
169 static int Knob_Penalty = 200 ; // spin failure penalty
170 static int Knob_Poverty = 1000 ;
171 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
172 static int Knob_FixedSpin = 0 ;
173 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
174 static int Knob_UsePause = 1 ;
175 static int Knob_ExitPolicy = 0 ;
176 static int Knob_PreSpin = 10 ; // 20-100 likely better
177 static int Knob_ResetEvent = 0 ;
178 static int BackOffMask = 0 ;
179
180 static int Knob_FastHSSEC = 0 ;
181 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
182 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
183 static volatile int InitDone = 0 ;
184
185 #define TrySpin TrySpin_VaryDuration
186
187 // -----------------------------------------------------------------------------
188 // Theory of operations -- Monitors lists, thread residency, etc:
189 //
190 // * A thread acquires ownership of a monitor by successfully
191 // CAS()ing the _owner field from null to non-null.
192 //
193 // * Invariant: A thread appears on at most one monitor list --
194 // cxq, EntryList or WaitSet -- at any one time.
195 //
196 // * Contending threads "push" themselves onto the cxq with CAS
197 // and then spin/park.
198 //
199 // * After a contending thread eventually acquires the lock it must
200 // dequeue itself from either the EntryList or the cxq.
201 //
202 // * The exiting thread identifies and unparks an "heir presumptive"
203 // tentative successor thread on the EntryList. Critically, the
204 // exiting thread doesn't unlink the successor thread from the EntryList.
205 // After having been unparked, the wakee will recontend for ownership of
206 // the monitor. The successor (wakee) will either acquire the lock or
207 // re-park itself.
208 //
209 // Succession is provided for by a policy of competitive handoff.
210 // The exiting thread does _not_ grant or pass ownership to the
211 // successor thread. (This is also referred to as "handoff" succession").
212 // Instead the exiting thread releases ownership and possibly wakes
213 // a successor, so the successor can (re)compete for ownership of the lock.
214 // If the EntryList is empty but the cxq is populated the exiting
215 // thread will drain the cxq into the EntryList. It does so by
216 // by detaching the cxq (installing null with CAS) and folding
217 // the threads from the cxq into the EntryList. The EntryList is
218 // doubly linked, while the cxq is singly linked because of the
219 // CAS-based "push" used to enqueue recently arrived threads (RATs).
220 //
221 // * Concurrency invariants:
222 //
223 // -- only the monitor owner may access or mutate the EntryList.
224 // The mutex property of the monitor itself protects the EntryList
225 // from concurrent interference.
226 // -- Only the monitor owner may detach the cxq.
227 //
228 // * The monitor entry list operations avoid locks, but strictly speaking
229 // they're not lock-free. Enter is lock-free, exit is not.
230 // For a description of 'Methods and apparatus providing non-blocking access
231 // to a resource,' see U.S. Pat. No. 7844973.
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(int after_wait, 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 int trace_flags = 0;
383 { // Change java thread status to indicate blocked on monitor enter.
384 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
385
386 if (EnableEventTracing) {
387 TraceEvents::write_monitor_contended_enter(this, (TraceTypes::monitor_enter_wait) after_wait);
388 }
389
390 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
391 if (JvmtiExport::should_post_monitor_contended_enter()) {
392 JvmtiExport::post_monitor_contended_enter(jt, this);
393
394 // The current thread does not yet own the monitor and does not
395 // yet appear on any queues that would get it made the successor.
396 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
397 // handler cannot accidentally consume an unpark() meant for the
398 // ParkEvent associated with this ObjectMonitor.
399 }
400
401 OSThreadContendState osts(Self->osthread());
402 ThreadBlockInVM tbivm(jt);
403
404 Self->set_current_pending_monitor(this);
405
406 // TODO-FIXME: change the following for(;;) loop to straight-line code.
407 for (;;) {
408 jt->set_suspend_equivalent();
409 // cleared by handle_special_suspend_equivalent_condition()
410 // or java_suspend_self()
411
412 trace_flags |= EnterI (THREAD) ;
413
414 if (!ExitSuspendEquivalent(jt)) break ;
415
416 //
417 // We have acquired the contended monitor, but while we were
418 // waiting another thread suspended us. We don't want to enter
419 // the monitor while suspended because that would surprise the
420 // thread that suspended us.
421 //
422 _recursions = 0 ;
423 _succ = NULL ;
424 exit (false, Self) ;
425
426 jt->java_suspend_self();
427 }
428 Self->set_current_pending_monitor(NULL);
429
430 // We cleared the pending monitor info since we've just gotten past
431 // the enter-check-for-suspend dance and we now own the monitor free
432 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
433 // destructor can go to a safepoint at the end of this block. If we
434 // do a thread dump during that safepoint, then this thread will show
435 // as having "-locked" the monitor, but the OS and java.lang.Thread
436 // states will still report that the thread is blocked trying to
437 // acquire it.
438 }
439
440 Atomic::dec_ptr(&_count);
441 assert (_count >= 0, "invariant") ;
442 Self->_Stalled = 0 ;
443
444 // Must either set _recursions = 0 or ASSERT _recursions == 0.
445 assert (_recursions == 0 , "invariant") ;
446 assert (_owner == Self , "invariant") ;
447 assert (_succ != Self , "invariant") ;
448 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
449
450 // The thread -- now the owner -- is back in vm mode.
451 // Report the glorious news via TI,DTrace and jvmstat.
452 // The probe effect is non-trivial. All the reportage occurs
453 // while we hold the monitor, increasing the length of the critical
454 // section. Amdahl's parallel speedup law comes vividly into play.
455 //
456 // Another option might be to aggregate the events (thread local or
457 // per-monitor aggregation) and defer reporting until a more opportune
458 // time -- such as next time some thread encounters contention but has
459 // yet to acquire the lock. While spinning that thread could
460 // spinning we could increment JVMStat counters, etc.
461
462 if (EnableEventTracing) {
463 TraceEvents::write_monitor_contended_entered(this, (TraceTypes::monitor_entered_flags) trace_flags);
464 }
465
466 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
467 if (JvmtiExport::should_post_monitor_contended_entered()) {
468 JvmtiExport::post_monitor_contended_entered(jt, this);
469
470 // The current thread already owns the monitor and is not going to
471 // call park() for the remainder of the monitor enter protocol. So
472 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
473 // event handler consumed an unpark() issued by the thread that
474 // just exited the monitor.
475 }
476
477 if (event.should_commit()) {
478 event.set_klass(((oop)this->object())->klass());
479 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
480 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
481 event.commit();
482 }
483
484 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
485 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
486 }
487 }
488
489
490 // Caveat: TryLock() is not necessarily serializing if it returns failure.
491 // Callers must compensate as needed.
492
493 int ObjectMonitor::TryLock (Thread * Self) {
494 for (;;) {
495 void * own = _owner ;
496 if (own != NULL) return 0 ;
497 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
498 // Either guarantee _recursions == 0 or set _recursions = 0.
499 assert (_recursions == 0, "invariant") ;
500 assert (_owner == Self, "invariant") ;
501 // CONSIDER: set or assert that OwnerIsThread == 1
502 return 1 ;
503 }
504 // The lock had been free momentarily, but we lost the race to the lock.
505 // Interference -- the CAS failed.
506 // We can either return -1 or retry.
507 // Retry doesn't make as much sense because the lock was just acquired.
508 if (true) return -1 ;
509 }
510 }
511
512 int ATTR ObjectMonitor::EnterI (TRAPS) {
513 int trace_flags = 0;
514
515 Thread * Self = THREAD ;
516 assert (Self->is_Java_thread(), "invariant") ;
517 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
518
519 // Try the lock - TATAS
520 if (TryLock (Self) > 0) {
521 assert (_succ != Self , "invariant") ;
522 assert (_owner == Self , "invariant") ;
523 assert (_Responsible != Self , "invariant") ;
524 return trace_flags;
525 }
526
527 DeferredInitialize () ;
528
529 // We try one round of spinning *before* enqueueing Self.
530 //
531 // If the _owner is ready but OFFPROC we could use a YieldTo()
532 // operation to donate the remainder of this thread's quantum
533 // to the owner. This has subtle but beneficial affinity
534 // effects.
535
536 if (TrySpin (Self) > 0) {
537 assert (_owner == Self , "invariant") ;
538 assert (_succ != Self , "invariant") ;
539 assert (_Responsible != Self , "invariant") ;
540 return trace_flags;
541 }
542
543 // The Spin failed -- Enqueue and park the thread ...
544 assert (_succ != Self , "invariant") ;
545 assert (_owner != Self , "invariant") ;
546 assert (_Responsible != Self , "invariant") ;
547
548 // Enqueue "Self" on ObjectMonitor's _cxq.
549 //
550 // Node acts as a proxy for Self.
551 // As an aside, if were to ever rewrite the synchronization code mostly
552 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
553 // Java objects. This would avoid awkward lifecycle and liveness issues,
554 // as well as eliminate a subset of ABA issues.
555 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
556 //
557
558 ObjectWaiter node(Self) ;
559 Self->_ParkEvent->reset() ;
560 node._prev = (ObjectWaiter *) 0xBAD ;
561 node.TState = ObjectWaiter::TS_CXQ ;
562
563 // Push "Self" onto the front of the _cxq.
564 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
565 // Note that spinning tends to reduce the rate at which threads
566 // enqueue and dequeue on EntryList|cxq.
567 ObjectWaiter * nxt ;
568 for (;;) {
569 node._next = nxt = _cxq ;
570 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
571
572 // Interference - the CAS failed because _cxq changed. Just retry.
573 // As an optional optimization we retry the lock.
574 if (TryLock (Self) > 0) {
575 assert (_succ != Self , "invariant") ;
576 assert (_owner == Self , "invariant") ;
577 assert (_Responsible != Self , "invariant") ;
578 return trace_flags;
579 }
580 }
581
582 trace_flags |= TraceTypes::entered_queued;
583
584 // Check for cxq|EntryList edge transition to non-null. This indicates
585 // the onset of contention. While contention persists exiting threads
586 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
587 // operations revert to the faster 1-0 mode. This enter operation may interleave
588 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
589 // arrange for one of the contending thread to use a timed park() operations
590 // to detect and recover from the race. (Stranding is form of progress failure
591 // where the monitor is unlocked but all the contending threads remain parked).
592 // That is, at least one of the contended threads will periodically poll _owner.
593 // One of the contending threads will become the designated "Responsible" thread.
594 // The Responsible thread uses a timed park instead of a normal indefinite park
595 // operation -- it periodically wakes and checks for and recovers from potential
596 // strandings admitted by 1-0 exit operations. We need at most one Responsible
597 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
598 // be responsible for a monitor.
599 //
600 // Currently, one of the contended threads takes on the added role of "Responsible".
601 // A viable alternative would be to use a dedicated "stranding checker" thread
602 // that periodically iterated over all the threads (or active monitors) and unparked
603 // successors where there was risk of stranding. This would help eliminate the
604 // timer scalability issues we see on some platforms as we'd only have one thread
605 // -- the checker -- parked on a timer.
606
607 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
608 // Try to assume the role of responsible thread for the monitor.
609 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
610 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
611 }
612
613 // The lock have been released while this thread was occupied queueing
614 // itself onto _cxq. To close the race and avoid "stranding" and
615 // progress-liveness failure we must resample-retry _owner before parking.
616 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
617 // In this case the ST-MEMBAR is accomplished with CAS().
618 //
619 // TODO: Defer all thread state transitions until park-time.
620 // Since state transitions are heavy and inefficient we'd like
621 // to defer the state transitions until absolutely necessary,
622 // and in doing so avoid some transitions ...
623
624 TEVENT (Inflated enter - Contention) ;
625 int nWakeups = 0 ;
626 int RecheckInterval = 1 ;
627
628 for (;;) {
629
630 if (TryLock (Self) > 0) break ;
631 assert (_owner != Self, "invariant") ;
632
633 if ((SyncFlags & 2) && _Responsible == NULL) {
634 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
635 }
636
637 // park self
638 if (_Responsible == Self || (SyncFlags & 1)) {
639 TEVENT (Inflated enter - park TIMED) ;
640 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
641 // Increase the RecheckInterval, but clamp the value.
642 RecheckInterval *= 8 ;
643 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
644 } else {
645 TEVENT (Inflated enter - park UNTIMED) ;
646 Self->_ParkEvent->park() ;
647 }
648
649 trace_flags |= TraceTypes::entered_parked;
650
651 if (TryLock(Self) > 0) break ;
652
653 // The lock is still contested.
654 // Keep a tally of the # of futile wakeups.
655 // Note that the counter is not protected by a lock or updated by atomics.
656 // That is by design - we trade "lossy" counters which are exposed to
657 // races during updates for a lower probe effect.
658 TEVENT (Inflated enter - Futile wakeup) ;
659 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
660 ObjectMonitor::_sync_FutileWakeups->inc() ;
661 }
662 ++ nWakeups ;
663
664 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
665 // We can defer clearing _succ until after the spin completes
666 // TrySpin() must tolerate being called with _succ == Self.
667 // Try yet another round of adaptive spinning.
668 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
669
670 // We can find that we were unpark()ed and redesignated _succ while
671 // we were spinning. That's harmless. If we iterate and call park(),
672 // park() will consume the event and return immediately and we'll
673 // just spin again. This pattern can repeat, leaving _succ to simply
674 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
675 // Alternately, we can sample fired() here, and if set, forgo spinning
676 // in the next iteration.
677
678 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
679 Self->_ParkEvent->reset() ;
680 OrderAccess::fence() ;
681 }
682 if (_succ == Self) _succ = NULL ;
683
684 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
685 OrderAccess::fence() ;
686 }
687
688 // Egress :
689 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
690 // Normally we'll find Self on the EntryList .
691 // From the perspective of the lock owner (this thread), the
692 // EntryList is stable and cxq is prepend-only.
693 // The head of cxq is volatile but the interior is stable.
694 // In addition, Self.TState is stable.
695
696 assert (_owner == Self , "invariant") ;
697 assert (object() != NULL , "invariant") ;
698 // I'd like to write:
699 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
700 // but as we're at a safepoint that's not safe.
701
702 UnlinkAfterAcquire (Self, &node) ;
703 if (_succ == Self) _succ = NULL ;
704
705 assert (_succ != Self, "invariant") ;
706 if (_Responsible == Self) {
707 _Responsible = NULL ;
708 OrderAccess::fence(); // Dekker pivot-point
709
710 // We may leave threads on cxq|EntryList without a designated
711 // "Responsible" thread. This is benign. When this thread subsequently
712 // exits the monitor it can "see" such preexisting "old" threads --
713 // threads that arrived on the cxq|EntryList before the fence, above --
714 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
715 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
716 // non-null and elect a new "Responsible" timer thread.
717 //
718 // This thread executes:
719 // ST Responsible=null; MEMBAR (in enter epilog - here)
720 // LD cxq|EntryList (in subsequent exit)
721 //
722 // Entering threads in the slow/contended path execute:
723 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
724 // The (ST cxq; MEMBAR) is accomplished with CAS().
725 //
726 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
727 // exit operation from floating above the ST Responsible=null.
728 }
729
730 // We've acquired ownership with CAS().
731 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
732 // But since the CAS() this thread may have also stored into _succ,
733 // EntryList, cxq or Responsible. These meta-data updates must be
734 // visible __before this thread subsequently drops the lock.
735 // Consider what could occur if we didn't enforce this constraint --
736 // STs to monitor meta-data and user-data could reorder with (become
737 // visible after) the ST in exit that drops ownership of the lock.
738 // Some other thread could then acquire the lock, but observe inconsistent
739 // or old monitor meta-data and heap data. That violates the JMM.
740 // To that end, the 1-0 exit() operation must have at least STST|LDST
741 // "release" barrier semantics. Specifically, there must be at least a
742 // STST|LDST barrier in exit() before the ST of null into _owner that drops
743 // the lock. The barrier ensures that changes to monitor meta-data and data
744 // protected by the lock will be visible before we release the lock, and
745 // therefore before some other thread (CPU) has a chance to acquire the lock.
746 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
747 //
748 // Critically, any prior STs to _succ or EntryList must be visible before
749 // the ST of null into _owner in the *subsequent* (following) corresponding
750 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
751 // execute a serializing instruction.
752
753 if (SyncFlags & 8) {
754 OrderAccess::fence() ;
755 }
756 return trace_flags;
757 }
758
759 // ReenterI() is a specialized inline form of the latter half of the
760 // contended slow-path from EnterI(). We use ReenterI() only for
761 // monitor reentry in wait().
762 //
763 // In the future we should reconcile EnterI() and ReenterI(), adding
764 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
765 // loop accordingly.
766
767 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
768 assert (Self != NULL , "invariant") ;
769 assert (SelfNode != NULL , "invariant") ;
770 assert (SelfNode->_thread == Self , "invariant") ;
771 assert (_waiters > 0 , "invariant") ;
772 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
773 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
774 JavaThread * jt = (JavaThread *) Self ;
775
776 int nWakeups = 0 ;
777 for (;;) {
778 ObjectWaiter::TStates v = SelfNode->TState ;
779 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
780 assert (_owner != Self, "invariant") ;
781
782 if (TryLock (Self) > 0) break ;
783 if (TrySpin (Self) > 0) break ;
784
785 TEVENT (Wait Reentry - parking) ;
786
787 // State transition wrappers around park() ...
788 // ReenterI() wisely defers state transitions until
789 // it's clear we must park the thread.
790 {
791 OSThreadContendState osts(Self->osthread());
792 ThreadBlockInVM tbivm(jt);
793
794 // cleared by handle_special_suspend_equivalent_condition()
795 // or java_suspend_self()
796 jt->set_suspend_equivalent();
797 if (SyncFlags & 1) {
798 Self->_ParkEvent->park ((jlong)1000) ;
799 } else {
800 Self->_ParkEvent->park () ;
801 }
802
803 // were we externally suspended while we were waiting?
804 for (;;) {
805 if (!ExitSuspendEquivalent (jt)) break ;
806 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
807 jt->java_suspend_self();
808 jt->set_suspend_equivalent();
809 }
810 }
811
812 // Try again, but just so we distinguish between futile wakeups and
813 // successful wakeups. The following test isn't algorithmically
814 // necessary, but it helps us maintain sensible statistics.
815 if (TryLock(Self) > 0) break ;
816
817 // The lock is still contested.
818 // Keep a tally of the # of futile wakeups.
819 // Note that the counter is not protected by a lock or updated by atomics.
820 // That is by design - we trade "lossy" counters which are exposed to
821 // races during updates for a lower probe effect.
822 TEVENT (Wait Reentry - futile wakeup) ;
823 ++ nWakeups ;
824
825 // Assuming this is not a spurious wakeup we'll normally
826 // find that _succ == Self.
827 if (_succ == Self) _succ = NULL ;
828
829 // Invariant: after clearing _succ a contending thread
830 // *must* retry _owner before parking.
831 OrderAccess::fence() ;
832
833 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
834 ObjectMonitor::_sync_FutileWakeups->inc() ;
835 }
836 }
837
838 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
839 // Normally we'll find Self on the EntryList.
840 // Unlinking from the EntryList is constant-time and atomic-free.
841 // From the perspective of the lock owner (this thread), the
842 // EntryList is stable and cxq is prepend-only.
843 // The head of cxq is volatile but the interior is stable.
844 // In addition, Self.TState is stable.
845
846 assert (_owner == Self, "invariant") ;
847 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
848 UnlinkAfterAcquire (Self, SelfNode) ;
849 if (_succ == Self) _succ = NULL ;
850 assert (_succ != Self, "invariant") ;
851 SelfNode->TState = ObjectWaiter::TS_RUN ;
852 OrderAccess::fence() ; // see comments at the end of EnterI()
853 }
854
855 // after the thread acquires the lock in ::enter(). Equally, we could defer
856 // unlinking the thread until ::exit()-time.
857
858 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
859 {
860 assert (_owner == Self, "invariant") ;
861 assert (SelfNode->_thread == Self, "invariant") ;
862
863 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
864 // Normal case: remove Self from the DLL EntryList .
865 // This is a constant-time operation.
866 ObjectWaiter * nxt = SelfNode->_next ;
867 ObjectWaiter * prv = SelfNode->_prev ;
868 if (nxt != NULL) nxt->_prev = prv ;
869 if (prv != NULL) prv->_next = nxt ;
870 if (SelfNode == _EntryList ) _EntryList = nxt ;
871 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
872 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
873 TEVENT (Unlink from EntryList) ;
874 } else {
875 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
876 // Inopportune interleaving -- Self is still on the cxq.
877 // This usually means the enqueue of self raced an exiting thread.
878 // Normally we'll find Self near the front of the cxq, so
879 // dequeueing is typically fast. If needbe we can accelerate
880 // this with some MCS/CHL-like bidirectional list hints and advisory
881 // back-links so dequeueing from the interior will normally operate
882 // in constant-time.
883 // Dequeue Self from either the head (with CAS) or from the interior
884 // with a linear-time scan and normal non-atomic memory operations.
885 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
886 // and then unlink Self from EntryList. We have to drain eventually,
887 // so it might as well be now.
888
889 ObjectWaiter * v = _cxq ;
890 assert (v != NULL, "invariant") ;
891 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
892 // The CAS above can fail from interference IFF a "RAT" arrived.
893 // In that case Self must be in the interior and can no longer be
894 // at the head of cxq.
895 if (v == SelfNode) {
896 assert (_cxq != v, "invariant") ;
897 v = _cxq ; // CAS above failed - start scan at head of list
898 }
899 ObjectWaiter * p ;
900 ObjectWaiter * q = NULL ;
901 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
902 q = p ;
903 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
904 }
905 assert (v != SelfNode, "invariant") ;
906 assert (p == SelfNode, "Node not found on cxq") ;
907 assert (p != _cxq, "invariant") ;
908 assert (q != NULL, "invariant") ;
909 assert (q->_next == p, "invariant") ;
910 q->_next = p->_next ;
911 }
912 TEVENT (Unlink from cxq) ;
913 }
914
915 // Diagnostic hygiene ...
916 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
917 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
918 SelfNode->TState = ObjectWaiter::TS_RUN ;
919 }
920
921 // -----------------------------------------------------------------------------
922 // Exit support
923 //
924 // exit()
925 // ~~~~~~
926 // Note that the collector can't reclaim the objectMonitor or deflate
927 // the object out from underneath the thread calling ::exit() as the
928 // thread calling ::exit() never transitions to a stable state.
929 // This inhibits GC, which in turn inhibits asynchronous (and
930 // inopportune) reclamation of "this".
931 //
932 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
933 // There's one exception to the claim above, however. EnterI() can call
934 // exit() to drop a lock if the acquirer has been externally suspended.
935 // In that case exit() is called with _thread_state as _thread_blocked,
936 // but the monitor's _count field is > 0, which inhibits reclamation.
937 //
938 // 1-0 exit
939 // ~~~~~~~~
940 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
941 // the fast-path operators have been optimized so the common ::exit()
942 // operation is 1-0. See i486.ad fast_unlock(), for instance.
943 // The code emitted by fast_unlock() elides the usual MEMBAR. This
944 // greatly improves latency -- MEMBAR and CAS having considerable local
945 // latency on modern processors -- but at the cost of "stranding". Absent the
946 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
947 // ::enter() path, resulting in the entering thread being stranding
948 // and a progress-liveness failure. Stranding is extremely rare.
949 // We use timers (timed park operations) & periodic polling to detect
950 // and recover from stranding. Potentially stranded threads periodically
951 // wake up and poll the lock. See the usage of the _Responsible variable.
952 //
953 // The CAS() in enter provides for safety and exclusion, while the CAS or
954 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
955 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
956 // We detect and recover from stranding with timers.
957 //
958 // If a thread transiently strands it'll park until (a) another
959 // thread acquires the lock and then drops the lock, at which time the
960 // exiting thread will notice and unpark the stranded thread, or, (b)
961 // the timer expires. If the lock is high traffic then the stranding latency
962 // will be low due to (a). If the lock is low traffic then the odds of
963 // stranding are lower, although the worst-case stranding latency
964 // is longer. Critically, we don't want to put excessive load in the
965 // platform's timer subsystem. We want to minimize both the timer injection
966 // rate (timers created/sec) as well as the number of timers active at
967 // any one time. (more precisely, we want to minimize timer-seconds, which is
968 // the integral of the # of active timers at any instant over time).
969 // Both impinge on OS scalability. Given that, at most one thread parked on
970 // a monitor will use a timer.
971
972 void ATTR ObjectMonitor::exit(intptr_t *exit_stack_id_for_wait, bool not_suspended, TRAPS) {
973 Thread * Self = THREAD ;
974 if (THREAD != _owner) {
975 if (THREAD->is_lock_owned((address) _owner)) {
976 // Transmute _owner from a BasicLock pointer to a Thread address.
977 // We don't need to hold _mutex for this transition.
978 // Non-null to Non-null is safe as long as all readers can
979 // tolerate either flavor.
980 assert (_recursions == 0, "invariant") ;
981 _owner = THREAD ;
982 _recursions = 0 ;
983 OwnerIsThread = 1 ;
984 } else {
985 // NOTE: we need to handle unbalanced monitor enter/exit
986 // in native code by throwing an exception.
987 // TODO: Throw an IllegalMonitorStateException ?
988 TEVENT (Exit - Throw IMSX) ;
989 assert(false, "Non-balanced monitor enter/exit!");
990 if (false) {
991 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
992 }
993 return;
994 }
995 }
996
997 if (_recursions != 0) {
998 _recursions--; // this is simple recursive enter
999 TEVENT (Inflated exit - recursive) ;
1000 return ;
1001 }
1002
1003 // Invariant: after setting Responsible=null an thread must execute
1004 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1005 if ((SyncFlags & 4) == 0) {
1006 _Responsible = NULL ;
1007 }
1008
1009 #if INCLUDE_TRACE
1010 // get the owner's thread id for the MonitorEnter event
1011 // if it is enabled and the thread isn't suspended
1012 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
1013 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
1014 }
1015 #endif
1016
1017 TraceEventMonitorContendedExited event(this);
1018 if (exit_stack_id_for_wait != NULL) {
1019 // This is a temporary exit for Object.wait().
1020 // We don't want to use the current stack trace as the lock site, so if we
1021 // end up writing the event, we allocate a stack id that we resolve later
1022 // when the monitor is really exited. When there are multiple waits, we
1023 // reuse the first preallocated stack id.
1024 event.set_use_or_preallocate_stack_id_at((TraceTypes::stack_id *) exit_stack_id_for_wait);
1025 event.set_resolve_stack(false);
1026 } else {
1027 // true exit
1028 event.set_resolve_stack(true);
1029 if (_trace_exit_stack != 0) {
1030 event.set_use_stack_id(_trace_exit_stack);
1031 event.enable(); // always write the exit event to resolve the stack
1032 }
1033 }
1034 if ((intptr_t(_EntryList) | intptr_t(_cxq)) != 0) {
1035 // there are queued threads -- we are definitely writing a trace event
1036 event.enable();
1037 }
1038
1039 _trace_exit_stack = 0;
1040
1041 for (;;) {
1042 assert (THREAD == _owner, "invariant") ;
1043
1044 //
1045 // NOTE: we have removed all code paths for ExitPolicy != 0 and QMode != 0
1046 // knob values for simplicity of event tracing.
1047 //
1048
1049 // release semantics: prior loads and stores from within the critical section
1050 // must not float (reorder) past the following store that drops the lock.
1051 // On SPARC that requires MEMBAR #loadstore|#storestore.
1052 // But of course in TSO #loadstore|#storestore is not required.
1053 // I'd like to write one of the following:
1054 // A. OrderAccess::release() ; _owner = NULL
1055 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
1056 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
1057 // store into a _dummy variable. That store is not needed, but can result
1058 // in massive wasteful coherency traffic on classic SMP systems.
1059 // Instead, I use release_store(), which is implemented as just a simple
1060 // ST on x64, x86 and SPARC.
1061 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1062 OrderAccess::storeload() ; // See if we need to wake a successor
1063 bool queues_empty = ((intptr_t(_EntryList) | intptr_t(_cxq)) == 0);
1064 bool have_succ = (_succ != NULL);
1065 if (!queues_empty) {
1066 // some thread might have entered itself on _cxq in the meantime
1067 event.enable();
1068 }
1069 if (queues_empty || have_succ) {
1070 TEVENT (Inflated exit - simple egress) ;
1071 return ;
1072 }
1073 TEVENT (Inflated exit - complex egress) ;
1074
1075 // Normally the exiting thread is responsible for ensuring succession,
1076 // but if other successors are ready or other entering threads are spinning
1077 // then this thread can simply store NULL into _owner and exit without
1078 // waking a successor. The existence of spinners or ready successors
1079 // guarantees proper succession (liveness). Responsibility passes to the
1080 // ready or running successors. The exiting thread delegates the duty.
1081 // More precisely, if a successor already exists this thread is absolved
1082 // of the responsibility of waking (unparking) one.
1083 //
1084 // The _succ variable is critical to reducing futile wakeup frequency.
1085 // _succ identifies the "heir presumptive" thread that has been made
1086 // ready (unparked) but that has not yet run. We need only one such
1087 // successor thread to guarantee progress.
1088 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1089 // section 3.3 "Futile Wakeup Throttling" for details.
1090 //
1091 // Note that spinners in Enter() also set _succ non-null.
1092 // In the current implementation spinners opportunistically set
1093 // _succ so that exiting threads might avoid waking a successor.
1094 // Another less appealing alternative would be for the exiting thread
1095 // to drop the lock and then spin briefly to see if a spinner managed
1096 // to acquire the lock. If so, the exiting thread could exit
1097 // immediately without waking a successor, otherwise the exiting
1098 // thread would need to dequeue and wake a successor.
1099 // (Note that we'd need to make the post-drop spin short, but no
1100 // shorter than the worst-case round-trip cache-line migration time.
1101 // The dropped lock needs to become visible to the spinner, and then
1102 // the acquisition of the lock by the spinner must become visible to
1103 // the exiting thread).
1104 //
1105
1106 // It appears that an heir-presumptive (successor) must be made ready.
1107 // Only the current lock owner can manipulate the EntryList or
1108 // drain _cxq, so we need to reacquire the lock. If we fail
1109 // to reacquire the lock the responsibility for ensuring succession
1110 // falls to the new owner.
1111 //
1112 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1113 return ;
1114 }
1115 TEVENT (Exit - Reacquired) ;
1116
1117 guarantee (_owner == THREAD, "invariant") ;
1118
1119 ObjectWaiter * w = NULL ;
1120
1121 w = _EntryList ;
1122 if (w != NULL) {
1123 // I'd like to write: guarantee (w->_thread != Self).
1124 // But in practice an exiting thread may find itself on the EntryList.
1125 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1126 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1127 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1128 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1129 // release the lock "O". T2 resumes immediately after the ST of null into
1130 // _owner, above. T2 notices that the EntryList is populated, so it
1131 // reacquires the lock and then finds itself on the EntryList.
1132 // Given all that, we have to tolerate the circumstance where "w" is
1133 // associated with Self.
1134 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1135 ExitEpilog (Self, w) ;
1136 return ;
1137 }
1138
1139 // If we find that both _cxq and EntryList are null then just
1140 // re-run the exit protocol from the top.
1141 w = _cxq ;
1142 if (w == NULL) continue ;
1143
1144 // Drain _cxq into EntryList - bulk transfer.
1145 // First, detach _cxq.
1146 // The following loop is tantamount to: w = swap (&cxq, NULL)
1147 for (;;) {
1148 assert (w != NULL, "Invariant") ;
1149 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1150 if (u == w) break ;
1151 w = u ;
1152 }
1153 TEVENT (Inflated exit - drain cxq into EntryList) ;
1154
1155 assert (w != NULL , "invariant") ;
1156 assert (_EntryList == NULL , "invariant") ;
1157
1158 // Convert the LIFO SLL anchored by _cxq into a DLL.
1159 // The list reorganization step operates in O(LENGTH(w)) time.
1160 // It's critical that this step operate quickly as
1161 // "Self" still holds the outer-lock, restricting parallelism
1162 // and effectively lengthening the critical section.
1163 // Invariant: s chases t chases u.
1164 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1165 // we have faster access to the tail.
1166
1167 _EntryList = w ;
1168 ObjectWaiter * q = NULL ;
1169 ObjectWaiter * p ;
1170 for (p = w ; p != NULL ; p = p->_next) {
1171 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1172 p->TState = ObjectWaiter::TS_ENTER ;
1173 p->_prev = q ;
1174 q = p ;
1175 }
1176
1177 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1178 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1179
1180 // See if we can abdicate to a spinner instead of waking a thread.
1181 // A primary goal of the implementation is to reduce the
1182 // context-switch rate.
1183 if (_succ != NULL) continue;
1184
1185 w = _EntryList ;
1186 if (w != NULL) {
1187 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1188 ExitEpilog (Self, w) ;
1189 return ;
1190 }
1191 }
1192 }
1193
1194 // ExitSuspendEquivalent:
1195 // A faster alternate to handle_special_suspend_equivalent_condition()
1196 //
1197 // handle_special_suspend_equivalent_condition() unconditionally
1198 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1199 // operations have high latency. Note that in ::enter() we call HSSEC
1200 // while holding the monitor, so we effectively lengthen the critical sections.
1201 //
1202 // There are a number of possible solutions:
1203 //
1204 // A. To ameliorate the problem we might also defer state transitions
1205 // to as late as possible -- just prior to parking.
1206 // Given that, we'd call HSSEC after having returned from park(),
1207 // but before attempting to acquire the monitor. This is only a
1208 // partial solution. It avoids calling HSSEC while holding the
1209 // monitor (good), but it still increases successor reacquisition latency --
1210 // the interval between unparking a successor and the time the successor
1211 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1212 // If we use this technique we can also avoid EnterI()-exit() loop
1213 // in ::enter() where we iteratively drop the lock and then attempt
1214 // to reacquire it after suspending.
1215 //
1216 // B. In the future we might fold all the suspend bits into a
1217 // composite per-thread suspend flag and then update it with CAS().
1218 // Alternately, a Dekker-like mechanism with multiple variables
1219 // would suffice:
1220 // ST Self->_suspend_equivalent = false
1221 // MEMBAR
1222 // LD Self_>_suspend_flags
1223 //
1224
1225
1226 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1227 int Mode = Knob_FastHSSEC ;
1228 if (Mode && !jSelf->is_external_suspend()) {
1229 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1230 jSelf->clear_suspend_equivalent() ;
1231 if (2 == Mode) OrderAccess::storeload() ;
1232 if (!jSelf->is_external_suspend()) return false ;
1233 // We raced a suspension -- fall thru into the slow path
1234 TEVENT (ExitSuspendEquivalent - raced) ;
1235 jSelf->set_suspend_equivalent() ;
1236 }
1237 return jSelf->handle_special_suspend_equivalent_condition() ;
1238 }
1239
1240
1241 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1242 assert (_owner == Self, "invariant") ;
1243
1244 // Exit protocol:
1245 // 1. ST _succ = wakee
1246 // 2. membar #loadstore|#storestore;
1247 // 2. ST _owner = NULL
1248 // 3. unpark(wakee)
1249
1250 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1251 ParkEvent * Trigger = Wakee->_event ;
1252
1253 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1254 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1255 // out-of-scope (non-extant).
1256 Wakee = NULL ;
1257
1258 // Drop the lock
1259 OrderAccess::release_store_ptr (&_owner, NULL) ;
1260 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1261
1262 if (SafepointSynchronize::do_call_back()) {
1263 TEVENT (unpark before SAFEPOINT) ;
1264 }
1265
1266 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1267 Trigger->unpark() ;
1268
1269 // Maintain stats and report events to JVMTI
1270 if (ObjectMonitor::_sync_Parks != NULL) {
1271 ObjectMonitor::_sync_Parks->inc() ;
1272 }
1273 }
1274
1275
1276 // -----------------------------------------------------------------------------
1277 // Class Loader deadlock handling.
1278 //
1279 // complete_exit exits a lock returning recursion count
1280 // complete_exit/reenter operate as a wait without waiting
1281 // complete_exit requires an inflated monitor
1282 // The _owner field is not always the Thread addr even with an
1283 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1284 // thread due to contention.
1285 void ObjectMonitor::complete_exit(intptr_t *saved_recursions, intptr_t *saved_trace_exit_stack, TRAPS) {
1286 Thread * const Self = THREAD;
1287 assert(Self->is_Java_thread(), "Must be Java thread!");
1288 JavaThread *jt = (JavaThread *)THREAD;
1289
1290 DeferredInitialize();
1291
1292 if (THREAD != _owner) {
1293 if (THREAD->is_lock_owned ((address)_owner)) {
1294 assert(_recursions == 0, "internal state error");
1295 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1296 _recursions = 0 ;
1297 OwnerIsThread = 1 ;
1298 }
1299 }
1300
1301 guarantee(Self == _owner, "complete_exit not owner");
1302 // record old recursion level and exit stack
1303 if (saved_recursions != NULL) *saved_recursions = _recursions;
1304 if (saved_recursions != NULL) *saved_trace_exit_stack = _trace_exit_stack;
1305 _recursions = 0;
1306 exit(saved_trace_exit_stack, true, Self);
1307 guarantee (_owner != Self, "invariant");
1308 }
1309
1310 // reenter() enters a lock and sets recursion count
1311 // complete_exit/reenter operate as a wait without waiting
1312 void ObjectMonitor::reenter(intptr_t saved_recursions, intptr_t saved_trace_exit_stack, TRAPS) {
1313 Thread * const Self = THREAD;
1314 assert(Self->is_Java_thread(), "Must be Java thread!");
1315 JavaThread *jt = (JavaThread *)THREAD;
1316
1317 guarantee(_owner != Self, "reenter already owner");
1318 enter (THREAD); // enter the monitor
1319 guarantee (_recursions == 0, "reenter recursion");
1320 _recursions = saved_recursions;
1321 _trace_exit_stack = saved_trace_exit_stack;
1322 }
1323
1324
1325 // -----------------------------------------------------------------------------
1326 // A macro is used below because there may already be a pending
1327 // exception which should not abort the execution of the routines
1328 // which use this (which is why we don't put this into check_slow and
1329 // call it with a CHECK argument).
1330
1331 #define CHECK_OWNER() \
1332 do { \
1333 if (THREAD != _owner) { \
1334 if (THREAD->is_lock_owned((address) _owner)) { \
1335 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1336 _recursions = 0; \
1337 OwnerIsThread = 1 ; \
1338 } else { \
1339 TEVENT (Throw IMSX) ; \
1340 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1341 } \
1342 } \
1343 } while (false)
1344
1345 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1346 // TODO-FIXME: remove check_slow() -- it's likely dead.
1347
1348 void ObjectMonitor::check_slow(TRAPS) {
1349 TEVENT (check_slow - throw IMSX) ;
1350 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1351 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1352 }
1353
1354 static int Adjust (volatile int * adr, int dx) {
1355 int v ;
1356 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1357 return v ;
1358 }
1359
1360 // helper method for posting a monitor wait event
1361 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1362 jlong notifier_tid,
1363 jlong timeout,
1364 bool timedout) {
1365 event->set_klass(((oop)this->object())->klass());
1366 event->set_timeout((TYPE_ULONG)timeout);
1367 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1368 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1369 event->set_timedOut((TYPE_BOOLEAN)timedout);
1370 event->commit();
1371 }
1372
1373 // -----------------------------------------------------------------------------
1374 // Wait/Notify/NotifyAll
1375 //
1376 // Note: a subset of changes to ObjectMonitor::wait()
1377 // will need to be replicated in complete_exit above
1378 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1379 Thread * const Self = THREAD ;
1380 assert(Self->is_Java_thread(), "Must be Java thread!");
1381 JavaThread *jt = (JavaThread *)THREAD;
1382
1383 DeferredInitialize () ;
1384
1385 // Throw IMSX or IEX.
1386 CHECK_OWNER();
1387
1388 EventJavaMonitorWait event;
1389
1390 // check for a pending interrupt
1391 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1392 // post monitor waited event. Note that this is past-tense, we are done waiting.
1393 if (JvmtiExport::should_post_monitor_waited()) {
1394 // Note: 'false' parameter is passed here because the
1395 // wait was not timed out due to thread interrupt.
1396 JvmtiExport::post_monitor_waited(jt, this, false);
1397
1398 // In this short circuit of the monitor wait protocol, the
1399 // current thread never drops ownership of the monitor and
1400 // never gets added to the wait queue so the current thread
1401 // cannot be made the successor. This means that the
1402 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1403 // consume an unpark() meant for the ParkEvent associated with
1404 // this ObjectMonitor.
1405 }
1406 if (event.should_commit()) {
1407 post_monitor_wait_event(&event, 0, millis, false);
1408 }
1409 TEVENT (Wait - Throw IEX) ;
1410 THROW(vmSymbols::java_lang_InterruptedException());
1411 return ;
1412 }
1413
1414 TEVENT (Wait) ;
1415
1416 assert (Self->_Stalled == 0, "invariant") ;
1417 Self->_Stalled = intptr_t(this) ;
1418 jt->set_current_waiting_monitor(this);
1419
1420 // create a node to be put into the queue
1421 // Critically, after we reset() the event but prior to park(), we must check
1422 // for a pending interrupt.
1423 ObjectWaiter node(Self);
1424 node.TState = ObjectWaiter::TS_WAIT ;
1425 Self->_ParkEvent->reset() ;
1426 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1427
1428 // Enter the waiting queue, which is a circular doubly linked list in this case
1429 // but it could be a priority queue or any data structure.
1430 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1431 // by the the owner of the monitor *except* in the case where park()
1432 // returns because of a timeout of interrupt. Contention is exceptionally rare
1433 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1434
1435 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1436 AddWaiter (&node) ;
1437 Thread::SpinRelease (&_WaitSetLock) ;
1438
1439 if ((SyncFlags & 4) == 0) {
1440 _Responsible = NULL ;
1441 }
1442 intptr_t saved_recursions = _recursions; // record the old recursion count
1443 intptr_t saved_trace_exit_stack = _trace_exit_stack;
1444 _waiters++; // increment the number of waiters
1445 _recursions = 0; // set the recursion level to be 1
1446 exit(&saved_trace_exit_stack, true, Self); // exit, knows how to handle exit stack
1447 guarantee (_owner != Self, "invariant") ;
1448
1449 // The thread is on the WaitSet list - now park() it.
1450 // On MP systems it's conceivable that a brief spin before we park
1451 // could be profitable.
1452 //
1453 // TODO-FIXME: change the following logic to a loop of the form
1454 // while (!timeout && !interrupted && _notified == 0) park()
1455
1456 int ret = OS_OK ;
1457 int WasNotified = 0 ;
1458 { // State transition wrappers
1459 OSThread* osthread = Self->osthread();
1460 OSThreadWaitState osts(osthread, true);
1461 {
1462 ThreadBlockInVM tbivm(jt);
1463 // Thread is in thread_blocked state and oop access is unsafe.
1464 jt->set_suspend_equivalent();
1465
1466 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1467 // Intentionally empty
1468 } else
1469 if (node._notified == 0) {
1470 if (millis <= 0) {
1471 Self->_ParkEvent->park () ;
1472 } else {
1473 ret = Self->_ParkEvent->park (millis) ;
1474 }
1475 }
1476
1477 // were we externally suspended while we were waiting?
1478 if (ExitSuspendEquivalent (jt)) {
1479 // TODO-FIXME: add -- if succ == Self then succ = null.
1480 jt->java_suspend_self();
1481 }
1482
1483 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1484
1485
1486 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1487 // from the WaitSet to the EntryList.
1488 // See if we need to remove Node from the WaitSet.
1489 // We use double-checked locking to avoid grabbing _WaitSetLock
1490 // if the thread is not on the wait queue.
1491 //
1492 // Note that we don't need a fence before the fetch of TState.
1493 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1494 // written by the is thread. (perhaps the fetch might even be satisfied
1495 // by a look-aside into the processor's own store buffer, although given
1496 // the length of the code path between the prior ST and this load that's
1497 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1498 // then we'll acquire the lock and then re-fetch a fresh TState value.
1499 // That is, we fail toward safety.
1500
1501 if (node.TState == ObjectWaiter::TS_WAIT) {
1502 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1503 if (node.TState == ObjectWaiter::TS_WAIT) {
1504 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1505 assert(node._notified == 0, "invariant");
1506 node.TState = ObjectWaiter::TS_RUN ;
1507 }
1508 Thread::SpinRelease (&_WaitSetLock) ;
1509 }
1510
1511 // The thread is now either on off-list (TS_RUN),
1512 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1513 // The Node's TState variable is stable from the perspective of this thread.
1514 // No other threads will asynchronously modify TState.
1515 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1516 OrderAccess::loadload() ;
1517 if (_succ == Self) _succ = NULL ;
1518 WasNotified = node._notified ;
1519
1520 // Reentry phase -- reacquire the monitor.
1521 // re-enter contended monitor after object.wait().
1522 // retain OBJECT_WAIT state until re-enter successfully completes
1523 // Thread state is thread_in_vm and oop access is again safe,
1524 // although the raw address of the object may have changed.
1525 // (Don't cache naked oops over safepoints, of course).
1526
1527 // post monitor waited event. Note that this is past-tense, we are done waiting.
1528 if (JvmtiExport::should_post_monitor_waited()) {
1529 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1530
1531 if (node._notified != 0 && _succ == Self) {
1532 // In this part of the monitor wait-notify-reenter protocol it
1533 // is possible (and normal) for another thread to do a fastpath
1534 // monitor enter-exit while this thread is still trying to get
1535 // to the reenter portion of the protocol.
1536 //
1537 // The ObjectMonitor was notified and the current thread is
1538 // the successor which also means that an unpark() has already
1539 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1540 // consume the unpark() that was done when the successor was
1541 // set because the same ParkEvent is shared between Java
1542 // monitors and JVM/TI RawMonitors (for now).
1543 //
1544 // We redo the unpark() to ensure forward progress, i.e., we
1545 // don't want all pending threads hanging (parked) with none
1546 // entering the unlocked monitor.
1547 node._event->unpark();
1548 }
1549 }
1550
1551 if (event.should_commit()) {
1552 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1553 }
1554
1555 OrderAccess::fence() ;
1556
1557 assert (Self->_Stalled != 0, "invariant") ;
1558 Self->_Stalled = 0 ;
1559
1560 assert (_owner != Self, "invariant") ;
1561 ObjectWaiter::TStates v = node.TState ;
1562 if (v == ObjectWaiter::TS_RUN) {
1563 int after_wait = TraceTypes::enter_after_wait_other;
1564 if (node._notified) {
1565 after_wait = TraceTypes::enter_after_wait_notify;
1566 } else if (ret == OS_TIMEOUT) {
1567 after_wait = TraceTypes::enter_after_wait_timeout;
1568 }
1569 enter (after_wait, Self) ;
1570 } else {
1571 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1572 ReenterI (Self, &node) ;
1573 node.wait_reenter_end(this);
1574 }
1575
1576 // Self has reacquired the lock.
1577 // Lifecycle - the node representing Self must not appear on any queues.
1578 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1579 // want residual elements associated with this thread left on any lists.
1580 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1581 assert (_owner == Self, "invariant") ;
1582 assert (_succ != Self , "invariant") ;
1583 } // OSThreadWaitState()
1584
1585 jt->set_current_waiting_monitor(NULL);
1586
1587 guarantee (_recursions == 0, "invariant") ;
1588 // restore the saved recursion count and exit stack
1589 _recursions = saved_recursions;
1590 _trace_exit_stack = saved_trace_exit_stack;
1591 _waiters--; // decrement the number of waiters
1592
1593 // Verify a few postconditions
1594 assert (_owner == Self , "invariant") ;
1595 assert (_succ != Self , "invariant") ;
1596 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1597
1598 if (SyncFlags & 32) {
1599 OrderAccess::fence() ;
1600 }
1601
1602 // check if the notification happened
1603 if (!WasNotified) {
1604 // no, it could be timeout or Thread.interrupt() or both
1605 // check for interrupt event, otherwise it is timeout
1606 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1607 TEVENT (Wait - throw IEX from epilog) ;
1608 THROW(vmSymbols::java_lang_InterruptedException());
1609 }
1610 }
1611
1612 // NOTE: Spurious wake up will be consider as timeout.
1613 // Monitor notify has precedence over thread interrupt.
1614 }
1615
1616
1617 // Consider:
1618 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1619 // then instead of transferring a thread from the WaitSet to the EntryList
1620 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1621
1622 void ObjectMonitor::notify(TRAPS) {
1623 CHECK_OWNER();
1624 if (_WaitSet == NULL) {
1625 TEVENT (Empty-Notify) ;
1626 return ;
1627 }
1628 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1629
1630 int Policy = Knob_MoveNotifyee ;
1631
1632 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1633 ObjectWaiter * iterator = DequeueWaiter() ;
1634 if (iterator != NULL) {
1635 TEVENT (Notify1 - Transfer) ;
1636 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1637 guarantee (iterator->_notified == 0, "invariant") ;
1638 if (Policy != 4) {
1639 iterator->TState = ObjectWaiter::TS_ENTER ;
1640 }
1641 iterator->_notified = 1 ;
1642 Thread * Self = THREAD;
1643 iterator->_notifier_tid = Self->osthread()->thread_id();
1644
1645 ObjectWaiter * List = _EntryList ;
1646 if (List != NULL) {
1647 assert (List->_prev == NULL, "invariant") ;
1648 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1649 assert (List != iterator, "invariant") ;
1650 }
1651
1652 if (Policy == 0) { // prepend to EntryList
1653 if (List == NULL) {
1654 iterator->_next = iterator->_prev = NULL ;
1655 _EntryList = iterator ;
1656 } else {
1657 List->_prev = iterator ;
1658 iterator->_next = List ;
1659 iterator->_prev = NULL ;
1660 _EntryList = iterator ;
1661 }
1662 } else
1663 if (Policy == 1) { // append to EntryList
1664 if (List == NULL) {
1665 iterator->_next = iterator->_prev = NULL ;
1666 _EntryList = iterator ;
1667 } else {
1668 // CONSIDER: finding the tail currently requires a linear-time walk of
1669 // the EntryList. We can make tail access constant-time by converting to
1670 // a CDLL instead of using our current DLL.
1671 ObjectWaiter * Tail ;
1672 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1673 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1674 Tail->_next = iterator ;
1675 iterator->_prev = Tail ;
1676 iterator->_next = NULL ;
1677 }
1678 } else
1679 if (Policy == 2) { // prepend to cxq
1680 // prepend to cxq
1681 if (List == NULL) {
1682 iterator->_next = iterator->_prev = NULL ;
1683 _EntryList = iterator ;
1684 } else {
1685 iterator->TState = ObjectWaiter::TS_CXQ ;
1686 for (;;) {
1687 ObjectWaiter * Front = _cxq ;
1688 iterator->_next = Front ;
1689 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1690 break ;
1691 }
1692 }
1693 }
1694 } else
1695 if (Policy == 3) { // append to cxq
1696 iterator->TState = ObjectWaiter::TS_CXQ ;
1697 for (;;) {
1698 ObjectWaiter * Tail ;
1699 Tail = _cxq ;
1700 if (Tail == NULL) {
1701 iterator->_next = NULL ;
1702 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1703 break ;
1704 }
1705 } else {
1706 while (Tail->_next != NULL) Tail = Tail->_next ;
1707 Tail->_next = iterator ;
1708 iterator->_prev = Tail ;
1709 iterator->_next = NULL ;
1710 break ;
1711 }
1712 }
1713 } else {
1714 ParkEvent * ev = iterator->_event ;
1715 iterator->TState = ObjectWaiter::TS_RUN ;
1716 OrderAccess::fence() ;
1717 ev->unpark() ;
1718 }
1719
1720 if (Policy < 4) {
1721 iterator->wait_reenter_begin(this);
1722 }
1723
1724 // _WaitSetLock protects the wait queue, not the EntryList. We could
1725 // move the add-to-EntryList operation, above, outside the critical section
1726 // protected by _WaitSetLock. In practice that's not useful. With the
1727 // exception of wait() timeouts and interrupts the monitor owner
1728 // is the only thread that grabs _WaitSetLock. There's almost no contention
1729 // on _WaitSetLock so it's not profitable to reduce the length of the
1730 // critical section.
1731 }
1732
1733 Thread::SpinRelease (&_WaitSetLock) ;
1734
1735 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1736 ObjectMonitor::_sync_Notifications->inc() ;
1737 }
1738 }
1739
1740
1741 void ObjectMonitor::notifyAll(TRAPS) {
1742 CHECK_OWNER();
1743 ObjectWaiter* iterator;
1744 if (_WaitSet == NULL) {
1745 TEVENT (Empty-NotifyAll) ;
1746 return ;
1747 }
1748 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1749
1750 int Policy = Knob_MoveNotifyee ;
1751 int Tally = 0 ;
1752 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1753
1754 for (;;) {
1755 iterator = DequeueWaiter () ;
1756 if (iterator == NULL) break ;
1757 TEVENT (NotifyAll - Transfer1) ;
1758 ++Tally ;
1759
1760 // Disposition - what might we do with iterator ?
1761 // a. add it directly to the EntryList - either tail or head.
1762 // b. push it onto the front of the _cxq.
1763 // For now we use (a).
1764
1765 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1766 guarantee (iterator->_notified == 0, "invariant") ;
1767 iterator->_notified = 1 ;
1768 Thread * Self = THREAD;
1769 iterator->_notifier_tid = Self->osthread()->thread_id();
1770 if (Policy != 4) {
1771 iterator->TState = ObjectWaiter::TS_ENTER ;
1772 }
1773
1774 ObjectWaiter * List = _EntryList ;
1775 if (List != NULL) {
1776 assert (List->_prev == NULL, "invariant") ;
1777 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1778 assert (List != iterator, "invariant") ;
1779 }
1780
1781 if (Policy == 0) { // prepend to EntryList
1782 if (List == NULL) {
1783 iterator->_next = iterator->_prev = NULL ;
1784 _EntryList = iterator ;
1785 } else {
1786 List->_prev = iterator ;
1787 iterator->_next = List ;
1788 iterator->_prev = NULL ;
1789 _EntryList = iterator ;
1790 }
1791 } else
1792 if (Policy == 1) { // append to EntryList
1793 if (List == NULL) {
1794 iterator->_next = iterator->_prev = NULL ;
1795 _EntryList = iterator ;
1796 } else {
1797 // CONSIDER: finding the tail currently requires a linear-time walk of
1798 // the EntryList. We can make tail access constant-time by converting to
1799 // a CDLL instead of using our current DLL.
1800 ObjectWaiter * Tail ;
1801 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1802 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1803 Tail->_next = iterator ;
1804 iterator->_prev = Tail ;
1805 iterator->_next = NULL ;
1806 }
1807 } else
1808 if (Policy == 2) { // prepend to cxq
1809 // prepend to cxq
1810 iterator->TState = ObjectWaiter::TS_CXQ ;
1811 for (;;) {
1812 ObjectWaiter * Front = _cxq ;
1813 iterator->_next = Front ;
1814 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1815 break ;
1816 }
1817 }
1818 } else
1819 if (Policy == 3) { // append to cxq
1820 iterator->TState = ObjectWaiter::TS_CXQ ;
1821 for (;;) {
1822 ObjectWaiter * Tail ;
1823 Tail = _cxq ;
1824 if (Tail == NULL) {
1825 iterator->_next = NULL ;
1826 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1827 break ;
1828 }
1829 } else {
1830 while (Tail->_next != NULL) Tail = Tail->_next ;
1831 Tail->_next = iterator ;
1832 iterator->_prev = Tail ;
1833 iterator->_next = NULL ;
1834 break ;
1835 }
1836 }
1837 } else {
1838 ParkEvent * ev = iterator->_event ;
1839 iterator->TState = ObjectWaiter::TS_RUN ;
1840 OrderAccess::fence() ;
1841 ev->unpark() ;
1842 }
1843
1844 if (Policy < 4) {
1845 iterator->wait_reenter_begin(this);
1846 }
1847
1848 // _WaitSetLock protects the wait queue, not the EntryList. We could
1849 // move the add-to-EntryList operation, above, outside the critical section
1850 // protected by _WaitSetLock. In practice that's not useful. With the
1851 // exception of wait() timeouts and interrupts the monitor owner
1852 // is the only thread that grabs _WaitSetLock. There's almost no contention
1853 // on _WaitSetLock so it's not profitable to reduce the length of the
1854 // critical section.
1855 }
1856
1857 Thread::SpinRelease (&_WaitSetLock) ;
1858
1859 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1860 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1861 }
1862 }
1863
1864 // -----------------------------------------------------------------------------
1865 // Adaptive Spinning Support
1866 //
1867 // Adaptive spin-then-block - rational spinning
1868 //
1869 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1870 // algorithm. On high order SMP systems it would be better to start with
1871 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1872 // a contending thread could enqueue itself on the cxq and then spin locally
1873 // on a thread-specific variable such as its ParkEvent._Event flag.
1874 // That's left as an exercise for the reader. Note that global spinning is
1875 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1876 // low latency and massive bandwidth.
1877 //
1878 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1879 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1880 // (duration) or we can fix the count at approximately the duration of
1881 // a context switch and vary the frequency. Of course we could also
1882 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1883 // For a description of 'Adaptive spin-then-block mutual exclusion in
1884 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1885 //
1886 // This implementation varies the duration "D", where D varies with
1887 // the success rate of recent spin attempts. (D is capped at approximately
1888 // length of a round-trip context switch). The success rate for recent
1889 // spin attempts is a good predictor of the success rate of future spin
1890 // attempts. The mechanism adapts automatically to varying critical
1891 // section length (lock modality), system load and degree of parallelism.
1892 // D is maintained per-monitor in _SpinDuration and is initialized
1893 // optimistically. Spin frequency is fixed at 100%.
1894 //
1895 // Note that _SpinDuration is volatile, but we update it without locks
1896 // or atomics. The code is designed so that _SpinDuration stays within
1897 // a reasonable range even in the presence of races. The arithmetic
1898 // operations on _SpinDuration are closed over the domain of legal values,
1899 // so at worst a race will install and older but still legal value.
1900 // At the very worst this introduces some apparent non-determinism.
1901 // We might spin when we shouldn't or vice-versa, but since the spin
1902 // count are relatively short, even in the worst case, the effect is harmless.
1903 //
1904 // Care must be taken that a low "D" value does not become an
1905 // an absorbing state. Transient spinning failures -- when spinning
1906 // is overall profitable -- should not cause the system to converge
1907 // on low "D" values. We want spinning to be stable and predictable
1908 // and fairly responsive to change and at the same time we don't want
1909 // it to oscillate, become metastable, be "too" non-deterministic,
1910 // or converge on or enter undesirable stable absorbing states.
1911 //
1912 // We implement a feedback-based control system -- using past behavior
1913 // to predict future behavior. We face two issues: (a) if the
1914 // input signal is random then the spin predictor won't provide optimal
1915 // results, and (b) if the signal frequency is too high then the control
1916 // system, which has some natural response lag, will "chase" the signal.
1917 // (b) can arise from multimodal lock hold times. Transient preemption
1918 // can also result in apparent bimodal lock hold times.
1919 // Although sub-optimal, neither condition is particularly harmful, as
1920 // in the worst-case we'll spin when we shouldn't or vice-versa.
1921 // The maximum spin duration is rather short so the failure modes aren't bad.
1922 // To be conservative, I've tuned the gain in system to bias toward
1923 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1924 // "rings" or oscillates between spinning and not spinning. This happens
1925 // when spinning is just on the cusp of profitability, however, so the
1926 // situation is not dire. The state is benign -- there's no need to add
1927 // hysteresis control to damp the transition rate between spinning and
1928 // not spinning.
1929 //
1930
1931 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
1932 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
1933
1934 // Spinning: Fixed frequency (100%), vary duration
1935
1936
1937 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
1938
1939 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1940 int ctr = Knob_FixedSpin ;
1941 if (ctr != 0) {
1942 while (--ctr >= 0) {
1943 if (TryLock (Self) > 0) return 1 ;
1944 SpinPause () ;
1945 }
1946 return 0 ;
1947 }
1948
1949 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
1950 if (TryLock(Self) > 0) {
1951 // Increase _SpinDuration ...
1952 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1953 // Raising _SpurDuration to the poverty line is key.
1954 int x = _SpinDuration ;
1955 if (x < Knob_SpinLimit) {
1956 if (x < Knob_Poverty) x = Knob_Poverty ;
1957 _SpinDuration = x + Knob_BonusB ;
1958 }
1959 return 1 ;
1960 }
1961 SpinPause () ;
1962 }
1963
1964 // Admission control - verify preconditions for spinning
1965 //
1966 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1967 // becoming an absorbing state. Put another way, we spin briefly to
1968 // sample, just in case the system load, parallelism, contention, or lock
1969 // modality changed.
1970 //
1971 // Consider the following alternative:
1972 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1973 // spin attempt. "Periodically" might mean after a tally of
1974 // the # of failed spin attempts (or iterations) reaches some threshold.
1975 // This takes us into the realm of 1-out-of-N spinning, where we
1976 // hold the duration constant but vary the frequency.
1977
1978 ctr = _SpinDuration ;
1979 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
1980 if (ctr <= 0) return 0 ;
1981
1982 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
1983 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1984 TEVENT (Spin abort - notrunnable [TOP]);
1985 return 0 ;
1986 }
1987
1988 int MaxSpin = Knob_MaxSpinners ;
1989 if (MaxSpin >= 0) {
1990 if (_Spinner > MaxSpin) {
1991 TEVENT (Spin abort -- too many spinners) ;
1992 return 0 ;
1993 }
1994 // Slighty racy, but benign ...
1995 Adjust (&_Spinner, 1) ;
1996 }
1997
1998 // We're good to spin ... spin ingress.
1999 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2000 // when preparing to LD...CAS _owner, etc and the CAS is likely
2001 // to succeed.
2002 int hits = 0 ;
2003 int msk = 0 ;
2004 int caspty = Knob_CASPenalty ;
2005 int oxpty = Knob_OXPenalty ;
2006 int sss = Knob_SpinSetSucc ;
2007 if (sss && _succ == NULL ) _succ = Self ;
2008 Thread * prv = NULL ;
2009
2010 // There are three ways to exit the following loop:
2011 // 1. A successful spin where this thread has acquired the lock.
2012 // 2. Spin failure with prejudice
2013 // 3. Spin failure without prejudice
2014
2015 while (--ctr >= 0) {
2016
2017 // Periodic polling -- Check for pending GC
2018 // Threads may spin while they're unsafe.
2019 // We don't want spinning threads to delay the JVM from reaching
2020 // a stop-the-world safepoint or to steal cycles from GC.
2021 // If we detect a pending safepoint we abort in order that
2022 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2023 // this thread, if safe, doesn't steal cycles from GC.
2024 // This is in keeping with the "no loitering in runtime" rule.
2025 // We periodically check to see if there's a safepoint pending.
2026 if ((ctr & 0xFF) == 0) {
2027 if (SafepointSynchronize::do_call_back()) {
2028 TEVENT (Spin: safepoint) ;
2029 goto Abort ; // abrupt spin egress
2030 }
2031 if (Knob_UsePause & 1) SpinPause () ;
2032
2033 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2034 if (hits > 50 && scb != NULL) {
2035 int abend = (*scb)(SpinCallbackArgument, 0) ;
2036 }
2037 }
2038
2039 if (Knob_UsePause & 2) SpinPause() ;
2040
2041 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2042 // This is useful on classic SMP systems, but is of less utility on
2043 // N1-style CMT platforms.
2044 //
2045 // Trade-off: lock acquisition latency vs coherency bandwidth.
2046 // Lock hold times are typically short. A histogram
2047 // of successful spin attempts shows that we usually acquire
2048 // the lock early in the spin. That suggests we want to
2049 // sample _owner frequently in the early phase of the spin,
2050 // but then back-off and sample less frequently as the spin
2051 // progresses. The back-off makes a good citizen on SMP big
2052 // SMP systems. Oversampling _owner can consume excessive
2053 // coherency bandwidth. Relatedly, if we _oversample _owner we
2054 // can inadvertently interfere with the the ST m->owner=null.
2055 // executed by the lock owner.
2056 if (ctr & msk) continue ;
2057 ++hits ;
2058 if ((hits & 0xF) == 0) {
2059 // The 0xF, above, corresponds to the exponent.
2060 // Consider: (msk+1)|msk
2061 msk = ((msk << 2)|3) & BackOffMask ;
2062 }
2063
2064 // Probe _owner with TATAS
2065 // If this thread observes the monitor transition or flicker
2066 // from locked to unlocked to locked, then the odds that this
2067 // thread will acquire the lock in this spin attempt go down
2068 // considerably. The same argument applies if the CAS fails
2069 // or if we observe _owner change from one non-null value to
2070 // another non-null value. In such cases we might abort
2071 // the spin without prejudice or apply a "penalty" to the
2072 // spin count-down variable "ctr", reducing it by 100, say.
2073
2074 Thread * ox = (Thread *) _owner ;
2075 if (ox == NULL) {
2076 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2077 if (ox == NULL) {
2078 // The CAS succeeded -- this thread acquired ownership
2079 // Take care of some bookkeeping to exit spin state.
2080 if (sss && _succ == Self) {
2081 _succ = NULL ;
2082 }
2083 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2084
2085 // Increase _SpinDuration :
2086 // The spin was successful (profitable) so we tend toward
2087 // longer spin attempts in the future.
2088 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2089 // If we acquired the lock early in the spin cycle it
2090 // makes sense to increase _SpinDuration proportionally.
2091 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2092 int x = _SpinDuration ;
2093 if (x < Knob_SpinLimit) {
2094 if (x < Knob_Poverty) x = Knob_Poverty ;
2095 _SpinDuration = x + Knob_Bonus ;
2096 }
2097 return 1 ;
2098 }
2099
2100 // The CAS failed ... we can take any of the following actions:
2101 // * penalize: ctr -= Knob_CASPenalty
2102 // * exit spin with prejudice -- goto Abort;
2103 // * exit spin without prejudice.
2104 // * Since CAS is high-latency, retry again immediately.
2105 prv = ox ;
2106 TEVENT (Spin: cas failed) ;
2107 if (caspty == -2) break ;
2108 if (caspty == -1) goto Abort ;
2109 ctr -= caspty ;
2110 continue ;
2111 }
2112
2113 // Did lock ownership change hands ?
2114 if (ox != prv && prv != NULL ) {
2115 TEVENT (spin: Owner changed)
2116 if (oxpty == -2) break ;
2117 if (oxpty == -1) goto Abort ;
2118 ctr -= oxpty ;
2119 }
2120 prv = ox ;
2121
2122 // Abort the spin if the owner is not executing.
2123 // The owner must be executing in order to drop the lock.
2124 // Spinning while the owner is OFFPROC is idiocy.
2125 // Consider: ctr -= RunnablePenalty ;
2126 if (Knob_OState && NotRunnable (Self, ox)) {
2127 TEVENT (Spin abort - notrunnable);
2128 goto Abort ;
2129 }
2130 if (sss && _succ == NULL ) _succ = Self ;
2131 }
2132
2133 // Spin failed with prejudice -- reduce _SpinDuration.
2134 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2135 // AIMD is globally stable.
2136 TEVENT (Spin failure) ;
2137 {
2138 int x = _SpinDuration ;
2139 if (x > 0) {
2140 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2141 // This is globally sample and tends to damp the response.
2142 x -= Knob_Penalty ;
2143 if (x < 0) x = 0 ;
2144 _SpinDuration = x ;
2145 }
2146 }
2147
2148 Abort:
2149 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2150 if (sss && _succ == Self) {
2151 _succ = NULL ;
2152 // Invariant: after setting succ=null a contending thread
2153 // must recheck-retry _owner before parking. This usually happens
2154 // in the normal usage of TrySpin(), but it's safest
2155 // to make TrySpin() as foolproof as possible.
2156 OrderAccess::fence() ;
2157 if (TryLock(Self) > 0) return 1 ;
2158 }
2159 return 0 ;
2160 }
2161
2162 // NotRunnable() -- informed spinning
2163 //
2164 // Don't bother spinning if the owner is not eligible to drop the lock.
2165 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2166 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2167 // The thread must be runnable in order to drop the lock in timely fashion.
2168 // If the _owner is not runnable then spinning will not likely be
2169 // successful (profitable).
2170 //
2171 // Beware -- the thread referenced by _owner could have died
2172 // so a simply fetch from _owner->_thread_state might trap.
2173 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2174 // Because of the lifecycle issues the schedctl and _thread_state values
2175 // observed by NotRunnable() might be garbage. NotRunnable must
2176 // tolerate this and consider the observed _thread_state value
2177 // as advisory.
2178 //
2179 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2180 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2181 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2182 // with the LSB of _owner. Another option would be to probablistically probe
2183 // the putative _owner->TypeTag value.
2184 //
2185 // Checking _thread_state isn't perfect. Even if the thread is
2186 // in_java it might be blocked on a page-fault or have been preempted
2187 // and sitting on a ready/dispatch queue. _thread state in conjunction
2188 // with schedctl.sc_state gives us a good picture of what the
2189 // thread is doing, however.
2190 //
2191 // TODO: check schedctl.sc_state.
2192 // We'll need to use SafeFetch32() to read from the schedctl block.
2193 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2194 //
2195 // The return value from NotRunnable() is *advisory* -- the
2196 // result is based on sampling and is not necessarily coherent.
2197 // The caller must tolerate false-negative and false-positive errors.
2198 // Spinning, in general, is probabilistic anyway.
2199
2200
2201 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2202 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2203 if (!OwnerIsThread) return 0 ;
2204
2205 if (ox == NULL) return 0 ;
2206
2207 // Avoid transitive spinning ...
2208 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2209 // Immediately after T1 acquires L it's possible that T2, also
2210 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2211 // This occurs transiently after T1 acquired L but before
2212 // T1 managed to clear T1.Stalled. T2 does not need to abort
2213 // its spin in this circumstance.
2214 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2215
2216 if (BlockedOn == 1) return 1 ;
2217 if (BlockedOn != 0) {
2218 return BlockedOn != intptr_t(this) && _owner == ox ;
2219 }
2220
2221 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2222 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2223 // consider also: jst != _thread_in_Java -- but that's overspecific.
2224 return jst == _thread_blocked || jst == _thread_in_native ;
2225 }
2226
2227
2228 // -----------------------------------------------------------------------------
2229 // WaitSet management ...
2230
2231 ObjectWaiter::ObjectWaiter(Thread* thread) {
2232 _next = NULL;
2233 _prev = NULL;
2234 _notified = 0;
2235 TState = TS_RUN ;
2236 _thread = thread;
2237 _event = thread->_ParkEvent ;
2238 _active = false;
2239 assert (_event != NULL, "invariant") ;
2240 }
2241
2242 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2243 JavaThread *jt = (JavaThread *)this->_thread;
2244 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2245 }
2246
2247 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2248 JavaThread *jt = (JavaThread *)this->_thread;
2249 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2250 }
2251
2252 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2253 assert(node != NULL, "should not dequeue NULL node");
2254 assert(node->_prev == NULL, "node already in list");
2255 assert(node->_next == NULL, "node already in list");
2256 // put node at end of queue (circular doubly linked list)
2257 if (_WaitSet == NULL) {
2258 _WaitSet = node;
2259 node->_prev = node;
2260 node->_next = node;
2261 } else {
2262 ObjectWaiter* head = _WaitSet ;
2263 ObjectWaiter* tail = head->_prev;
2264 assert(tail->_next == head, "invariant check");
2265 tail->_next = node;
2266 head->_prev = node;
2267 node->_next = head;
2268 node->_prev = tail;
2269 }
2270 }
2271
2272 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2273 // dequeue the very first waiter
2274 ObjectWaiter* waiter = _WaitSet;
2275 if (waiter) {
2276 DequeueSpecificWaiter(waiter);
2277 }
2278 return waiter;
2279 }
2280
2281 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2282 assert(node != NULL, "should not dequeue NULL node");
2283 assert(node->_prev != NULL, "node already removed from list");
2284 assert(node->_next != NULL, "node already removed from list");
2285 // when the waiter has woken up because of interrupt,
2286 // timeout or other spurious wake-up, dequeue the
2287 // waiter from waiting list
2288 ObjectWaiter* next = node->_next;
2289 if (next == node) {
2290 assert(node->_prev == node, "invariant check");
2291 _WaitSet = NULL;
2292 } else {
2293 ObjectWaiter* prev = node->_prev;
2294 assert(prev->_next == node, "invariant check");
2295 assert(next->_prev == node, "invariant check");
2296 next->_prev = prev;
2297 prev->_next = next;
2298 if (_WaitSet == node) {
2299 _WaitSet = next;
2300 }
2301 }
2302 node->_next = NULL;
2303 node->_prev = NULL;
2304 }
2305
2306 // -----------------------------------------------------------------------------
2307 // PerfData support
2308 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2309 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2310 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2311 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2312 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2313 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2314 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2315 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2316 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2317 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2318 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2319 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2320 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2321 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2322 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2323 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2324 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2325 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2326
2327 // One-shot global initialization for the sync subsystem.
2328 // We could also defer initialization and initialize on-demand
2329 // the first time we call inflate(). Initialization would
2330 // be protected - like so many things - by the MonitorCache_lock.
2331
2332 void ObjectMonitor::Initialize () {
2333 static int InitializationCompleted = 0 ;
2334 assert (InitializationCompleted == 0, "invariant") ;
2335 InitializationCompleted = 1 ;
2336 if (UsePerfData) {
2337 EXCEPTION_MARK ;
2338 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2339 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2340 NEWPERFCOUNTER(_sync_Inflations) ;
2341 NEWPERFCOUNTER(_sync_Deflations) ;
2342 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2343 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2344 NEWPERFCOUNTER(_sync_Parks) ;
2345 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2346 NEWPERFCOUNTER(_sync_Notifications) ;
2347 NEWPERFCOUNTER(_sync_SlowEnter) ;
2348 NEWPERFCOUNTER(_sync_SlowExit) ;
2349 NEWPERFCOUNTER(_sync_SlowNotify) ;
2350 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2351 NEWPERFCOUNTER(_sync_FailedSpins) ;
2352 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2353 NEWPERFCOUNTER(_sync_PrivateA) ;
2354 NEWPERFCOUNTER(_sync_PrivateB) ;
2355 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2356 NEWPERFCOUNTER(_sync_MonScavenged) ;
2357 NEWPERFVARIABLE(_sync_MonExtant) ;
2358 #undef NEWPERFCOUNTER
2359 }
2360 }
2361
2362
2363 // Compile-time asserts
2364 // When possible, it's better to catch errors deterministically at
2365 // compile-time than at runtime. The down-side to using compile-time
2366 // asserts is that error message -- often something about negative array
2367 // indices -- is opaque.
2368
2369 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2370
2371 void ObjectMonitor::ctAsserts() {
2372 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2373 }
2374
2375
2376 static char * kvGet (char * kvList, const char * Key) {
2377 if (kvList == NULL) return NULL ;
2378 size_t n = strlen (Key) ;
2379 char * Search ;
2380 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2381 if (strncmp (Search, Key, n) == 0) {
2382 if (Search[n] == '=') return Search + n + 1 ;
2383 if (Search[n] == 0) return (char *) "1" ;
2384 }
2385 }
2386 return NULL ;
2387 }
2388
2389 static int kvGetInt (char * kvList, const char * Key, int Default) {
2390 char * v = kvGet (kvList, Key) ;
2391 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2392 if (Knob_ReportSettings && v != NULL) {
2393 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2394 ::fflush (stdout) ;
2395 }
2396 return rslt ;
2397 }
2398
2399 void ObjectMonitor::DeferredInitialize () {
2400 if (InitDone > 0) return ;
2401 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2402 while (InitDone != 1) ;
2403 return ;
2404 }
2405
2406 // One-shot global initialization ...
2407 // The initialization is idempotent, so we don't need locks.
2408 // In the future consider doing this via os::init_2().
2409 // SyncKnobs consist of <Key>=<Value> pairs in the style
2410 // of environment variables. Start by converting ':' to NUL.
2411
2412 if (SyncKnobs == NULL) SyncKnobs = "" ;
2413
2414 size_t sz = strlen (SyncKnobs) ;
2415 char * knobs = (char *) malloc (sz + 2) ;
2416 if (knobs == NULL) {
2417 vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
2418 guarantee (0, "invariant") ;
2419 }
2420 strcpy (knobs, SyncKnobs) ;
2421 knobs[sz+1] = 0 ;
2422 for (char * p = knobs ; *p ; p++) {
2423 if (*p == ':') *p = 0 ;
2424 }
2425
2426 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2427 SETKNOB(ReportSettings) ;
2428 SETKNOB(Verbose) ;
2429 SETKNOB(FixedSpin) ;
2430 SETKNOB(SpinLimit) ;
2431 SETKNOB(SpinBase) ;
2432 SETKNOB(SpinBackOff);
2433 SETKNOB(CASPenalty) ;
2434 SETKNOB(OXPenalty) ;
2435 SETKNOB(LogSpins) ;
2436 SETKNOB(SpinSetSucc) ;
2437 SETKNOB(SuccEnabled) ;
2438 SETKNOB(SuccRestrict) ;
2439 SETKNOB(Penalty) ;
2440 SETKNOB(Bonus) ;
2441 SETKNOB(BonusB) ;
2442 SETKNOB(Poverty) ;
2443 SETKNOB(SpinAfterFutile) ;
2444 SETKNOB(UsePause) ;
2445 SETKNOB(SpinEarly) ;
2446 SETKNOB(OState) ;
2447 SETKNOB(MaxSpinners) ;
2448 SETKNOB(PreSpin) ;
2449 SETKNOB(ExitPolicy) ;
2450 SETKNOB(QMode);
2451 SETKNOB(ResetEvent) ;
2452 SETKNOB(MoveNotifyee) ;
2453 SETKNOB(FastHSSEC) ;
2454 #undef SETKNOB
2455
2456 guarantee(Knob_ExitPolicy == 0, "Sorry, event tracing does not support non-default ExitPolicy");
2457 guarantee(Knob_QMode == 0, "Sorry, event tracing does not support non-default QMode");
2458
2459 if (os::is_MP()) {
2460 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2461 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2462 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2463 } else {
2464 Knob_SpinLimit = 0 ;
2465 Knob_SpinBase = 0 ;
2466 Knob_PreSpin = 0 ;
2467 Knob_FixedSpin = -1 ;
2468 }
2469
2470 if (Knob_LogSpins == 0) {
2471 ObjectMonitor::_sync_FailedSpins = NULL ;
2472 }
2473
2474 free (knobs) ;
2475 OrderAccess::fence() ;
2476 InitDone = 1 ;
2477 }
2478
2479 #ifndef PRODUCT
2480 void ObjectMonitor::verify() {
2481 }
2482
2483 void ObjectMonitor::print() {
2484 }
2485 #endif