1 /*
2 * Copyright (c) 1998, 2014, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/handles.inline.hpp"
31 #include "runtime/interfaceSupport.hpp"
32 #include "runtime/mutexLocker.hpp"
33 #include "runtime/objectMonitor.hpp"
34 #include "runtime/objectMonitor.inline.hpp"
35 #include "runtime/orderAccess.inline.hpp"
36 #include "runtime/osThread.hpp"
37 #include "runtime/stubRoutines.hpp"
38 #include "runtime/thread.inline.hpp"
39 #include "services/threadService.hpp"
40 #include "trace/tracing.hpp"
41 #include "trace/traceMacros.hpp"
42 #include "utilities/dtrace.hpp"
43 #include "utilities/macros.hpp"
44 #include "utilities/preserveException.hpp"
45 #ifdef TARGET_OS_FAMILY_linux
46 # include "os_linux.inline.hpp"
47 #endif
48 #ifdef TARGET_OS_FAMILY_solaris
49 # include "os_solaris.inline.hpp"
50 #endif
51 #ifdef TARGET_OS_FAMILY_windows
52 # include "os_windows.inline.hpp"
53 #endif
54 #ifdef TARGET_OS_FAMILY_bsd
55 # include "os_bsd.inline.hpp"
56 #endif
57
58 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
59 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
60 #define ATTR __attribute__((noinline))
61 #else
62 #define ATTR
63 #endif
64
65
66 #ifdef DTRACE_ENABLED
67
68 // Only bother with this argument setup if dtrace is available
69 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
70
71
72 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
73 char* bytes = NULL; \
74 int len = 0; \
75 jlong jtid = SharedRuntime::get_java_tid(thread); \
76 Symbol* klassname = ((oop)obj)->klass()->name(); \
77 if (klassname != NULL) { \
78 bytes = (char*)klassname->bytes(); \
79 len = klassname->utf8_length(); \
80 }
81
82 #ifndef USDT2
83
84 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
85 jlong, uintptr_t, char*, int);
86 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
87 jlong, uintptr_t, char*, int);
88 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
89 jlong, uintptr_t, char*, int);
90 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
91 jlong, uintptr_t, char*, int);
92 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
93 jlong, uintptr_t, char*, int);
94
95 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
96 { \
97 if (DTraceMonitorProbes) { \
98 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
99 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
100 (monitor), bytes, len, (millis)); \
101 } \
102 }
103
104 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
105 { \
106 if (DTraceMonitorProbes) { \
107 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
108 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
109 (uintptr_t)(monitor), bytes, len); \
110 } \
111 }
112
113 #else /* USDT2 */
114
115 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
116 { \
117 if (DTraceMonitorProbes) { \
118 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
119 HOTSPOT_MONITOR_WAIT(jtid, \
120 (monitor), bytes, len, (millis)); \
121 } \
122 }
123
124 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
125 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
126 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
127 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
128 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
129
130 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
131 { \
132 if (DTraceMonitorProbes) { \
133 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
134 HOTSPOT_MONITOR_##probe(jtid, \
135 (uintptr_t)(monitor), bytes, len); \
136 } \
137 }
138
139 #endif /* USDT2 */
140 #else // ndef DTRACE_ENABLED
141
142 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
143 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
144
145 #endif // ndef DTRACE_ENABLED
146
147 // Tunables ...
148 // The knob* variables are effectively final. Once set they should
149 // never be modified hence. Consider using __read_mostly with GCC.
150
151 int ObjectMonitor::Knob_Verbose = 0 ;
152 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool -
153 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
154 static int Knob_HandOff = 0 ;
155 static int Knob_ReportSettings = 0 ;
156
157 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
158 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
159 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
160 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
161 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
162 static int Knob_SpinEarly = 1 ;
163 static int Knob_SuccEnabled = 1 ; // futile wake throttling
164 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
165 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
166 static int Knob_Bonus = 100 ; // spin success bonus
167 static int Knob_BonusB = 100 ; // spin success bonus
168 static int Knob_Penalty = 200 ; // spin failure penalty
169 static int Knob_Poverty = 1000 ;
170 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
171 static int Knob_FixedSpin = 0 ;
172 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
173 static int Knob_UsePause = 1 ;
174 static int Knob_ExitPolicy = 0 ;
175 static int Knob_PreSpin = 10 ; // 20-100 likely better
176 static int Knob_ResetEvent = 0 ;
177 static int BackOffMask = 0 ;
178
179 static int Knob_FastHSSEC = 0 ;
180 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
181 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
182 static volatile int InitDone = 0 ;
183
184 #define TrySpin TrySpin_VaryDuration
185
186 // -----------------------------------------------------------------------------
187 // Theory of operations -- Monitors lists, thread residency, etc:
188 //
189 // * A thread acquires ownership of a monitor by successfully
190 // CAS()ing the _owner field from null to non-null.
191 //
192 // * Invariant: A thread appears on at most one monitor list --
193 // cxq, EntryList or WaitSet -- at any one time.
194 //
195 // * Contending threads "push" themselves onto the cxq with CAS
196 // and then spin/park.
197 //
198 // * After a contending thread eventually acquires the lock it must
199 // dequeue itself from either the EntryList or the cxq.
200 //
201 // * The exiting thread identifies and unparks an "heir presumptive"
202 // tentative successor thread on the EntryList. Critically, the
203 // exiting thread doesn't unlink the successor thread from the EntryList.
204 // After having been unparked, the wakee will recontend for ownership of
205 // the monitor. The successor (wakee) will either acquire the lock or
206 // re-park itself.
207 //
208 // Succession is provided for by a policy of competitive handoff.
209 // The exiting thread does _not_ grant or pass ownership to the
210 // successor thread. (This is also referred to as "handoff" succession").
211 // Instead the exiting thread releases ownership and possibly wakes
212 // a successor, so the successor can (re)compete for ownership of the lock.
213 // If the EntryList is empty but the cxq is populated the exiting
214 // thread will drain the cxq into the EntryList. It does so by
215 // by detaching the cxq (installing null with CAS) and folding
216 // the threads from the cxq into the EntryList. The EntryList is
217 // doubly linked, while the cxq is singly linked because of the
218 // CAS-based "push" used to enqueue recently arrived threads (RATs).
219 //
220 // * Concurrency invariants:
221 //
222 // -- only the monitor owner may access or mutate the EntryList.
223 // The mutex property of the monitor itself protects the EntryList
224 // from concurrent interference.
225 // -- Only the monitor owner may detach the cxq.
226 //
227 // * The monitor entry list operations avoid locks, but strictly speaking
228 // they're not lock-free. Enter is lock-free, exit is not.
229 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
230 //
231 // * The cxq can have multiple concurrent "pushers" but only one concurrent
232 // detaching thread. This mechanism is immune from the ABA corruption.
233 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
234 //
235 // * Taken together, the cxq and the EntryList constitute or form a
236 // single logical queue of threads stalled trying to acquire the lock.
237 // We use two distinct lists to improve the odds of a constant-time
238 // dequeue operation after acquisition (in the ::enter() epilog) and
239 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
240 // A key desideratum is to minimize queue & monitor metadata manipulation
241 // that occurs while holding the monitor lock -- that is, we want to
242 // minimize monitor lock holds times. Note that even a small amount of
243 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
244 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
245 // locks and monitor metadata.
246 //
247 // Cxq points to the the set of Recently Arrived Threads attempting entry.
248 // Because we push threads onto _cxq with CAS, the RATs must take the form of
249 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
250 // the unlocking thread notices that EntryList is null but _cxq is != null.
251 //
252 // The EntryList is ordered by the prevailing queue discipline and
253 // can be organized in any convenient fashion, such as a doubly-linked list or
254 // a circular doubly-linked list. Critically, we want insert and delete operations
255 // to operate in constant-time. If we need a priority queue then something akin
256 // to Solaris' sleepq would work nicely. Viz.,
257 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
258 // Queue discipline is enforced at ::exit() time, when the unlocking thread
259 // drains the cxq into the EntryList, and orders or reorders the threads on the
260 // EntryList accordingly.
261 //
262 // Barring "lock barging", this mechanism provides fair cyclic ordering,
263 // somewhat similar to an elevator-scan.
264 //
265 // * The monitor synchronization subsystem avoids the use of native
266 // synchronization primitives except for the narrow platform-specific
267 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
268 // the semantics of park-unpark. Put another way, this monitor implementation
269 // depends only on atomic operations and park-unpark. The monitor subsystem
270 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
271 // underlying OS manages the READY<->RUN transitions.
272 //
273 // * Waiting threads reside on the WaitSet list -- wait() puts
274 // the caller onto the WaitSet.
275 //
276 // * notify() or notifyAll() simply transfers threads from the WaitSet to
277 // either the EntryList or cxq. Subsequent exit() operations will
278 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
279 // it's likely the notifyee would simply impale itself on the lock held
280 // by the notifier.
281 //
282 // * An interesting alternative is to encode cxq as (List,LockByte) where
283 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
284 // variable, like _recursions, in the scheme. The threads or Events that form
285 // the list would have to be aligned in 256-byte addresses. A thread would
286 // try to acquire the lock or enqueue itself with CAS, but exiting threads
287 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
288 // Note that is is *not* word-tearing, but it does presume that full-word
289 // CAS operations are coherent with intermix with STB operations. That's true
290 // on most common processors.
291 //
292 // * See also http://blogs.sun.com/dave
293
294
295 // -----------------------------------------------------------------------------
296 // Enter support
297
298 bool ObjectMonitor::try_enter(Thread* THREAD) {
299 if (THREAD != _owner) {
300 if (THREAD->is_lock_owned ((address)_owner)) {
301 assert(_recursions == 0, "internal state error");
302 _owner = THREAD ;
303 _recursions = 1 ;
304 OwnerIsThread = 1 ;
305 return true;
306 }
307 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
308 return false;
309 }
310 return true;
311 } else {
312 _recursions++;
313 return true;
314 }
315 }
316
317 void ATTR ObjectMonitor::enter(TRAPS) {
318 // The following code is ordered to check the most common cases first
319 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
320 Thread * const Self = THREAD ;
321 void * cur ;
322
323 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
324 if (cur == NULL) {
325 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
326 assert (_recursions == 0 , "invariant") ;
327 assert (_owner == Self, "invariant") ;
328 // CONSIDER: set or assert OwnerIsThread == 1
329 return ;
330 }
331
332 if (cur == Self) {
333 // TODO-FIXME: check for integer overflow! BUGID 6557169.
334 _recursions ++ ;
335 return ;
336 }
337
338 if (Self->is_lock_owned ((address)cur)) {
339 assert (_recursions == 0, "internal state error");
340 _recursions = 1 ;
341 // Commute owner from a thread-specific on-stack BasicLockObject address to
342 // a full-fledged "Thread *".
343 _owner = Self ;
344 OwnerIsThread = 1 ;
345 return ;
346 }
347
348 // We've encountered genuine contention.
349 assert (Self->_Stalled == 0, "invariant") ;
350 Self->_Stalled = intptr_t(this) ;
351
352 // Try one round of spinning *before* enqueueing Self
353 // and before going through the awkward and expensive state
354 // transitions. The following spin is strictly optional ...
355 // Note that if we acquire the monitor from an initial spin
356 // we forgo posting JVMTI events and firing DTRACE probes.
357 if (Knob_SpinEarly && TrySpin (Self) > 0) {
358 assert (_owner == Self , "invariant") ;
359 assert (_recursions == 0 , "invariant") ;
360 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
361 Self->_Stalled = 0 ;
362 return ;
363 }
364
365 assert (_owner != Self , "invariant") ;
366 assert (_succ != Self , "invariant") ;
367 assert (Self->is_Java_thread() , "invariant") ;
368 JavaThread * jt = (JavaThread *) Self ;
369 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
370 assert (jt->thread_state() != _thread_blocked , "invariant") ;
371 assert (this->object() != NULL , "invariant") ;
372 assert (_count >= 0, "invariant") ;
373
374 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
375 // Ensure the object-monitor relationship remains stable while there's contention.
376 Atomic::inc_ptr(&_count);
377
378 EventJavaMonitorEnter event;
379
380 { // Change java thread status to indicate blocked on monitor enter.
381 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
382
383 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
384 if (JvmtiExport::should_post_monitor_contended_enter()) {
385 JvmtiExport::post_monitor_contended_enter(jt, this);
386
387 // The current thread does not yet own the monitor and does not
388 // yet appear on any queues that would get it made the successor.
389 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
390 // handler cannot accidentally consume an unpark() meant for the
391 // ParkEvent associated with this ObjectMonitor.
392 }
393
394 OSThreadContendState osts(Self->osthread());
395 ThreadBlockInVM tbivm(jt);
396
397 Self->set_current_pending_monitor(this);
398
399 // TODO-FIXME: change the following for(;;) loop to straight-line code.
400 for (;;) {
401 jt->set_suspend_equivalent();
402 // cleared by handle_special_suspend_equivalent_condition()
403 // or java_suspend_self()
404
405 EnterI (THREAD) ;
406
407 if (!ExitSuspendEquivalent(jt)) break ;
408
409 //
410 // We have acquired the contended monitor, but while we were
411 // waiting another thread suspended us. We don't want to enter
412 // the monitor while suspended because that would surprise the
413 // thread that suspended us.
414 //
415 _recursions = 0 ;
416 _succ = NULL ;
417 exit (false, Self) ;
418
419 jt->java_suspend_self();
420 }
421 Self->set_current_pending_monitor(NULL);
422
423 // We cleared the pending monitor info since we've just gotten past
424 // the enter-check-for-suspend dance and we now own the monitor free
425 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
426 // destructor can go to a safepoint at the end of this block. If we
427 // do a thread dump during that safepoint, then this thread will show
428 // as having "-locked" the monitor, but the OS and java.lang.Thread
429 // states will still report that the thread is blocked trying to
430 // acquire it.
431 }
432
433 Atomic::dec_ptr(&_count);
434 assert (_count >= 0, "invariant") ;
435 Self->_Stalled = 0 ;
436
437 // Must either set _recursions = 0 or ASSERT _recursions == 0.
438 assert (_recursions == 0 , "invariant") ;
439 assert (_owner == Self , "invariant") ;
440 assert (_succ != Self , "invariant") ;
441 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
442
443 // The thread -- now the owner -- is back in vm mode.
444 // Report the glorious news via TI,DTrace and jvmstat.
445 // The probe effect is non-trivial. All the reportage occurs
446 // while we hold the monitor, increasing the length of the critical
447 // section. Amdahl's parallel speedup law comes vividly into play.
448 //
449 // Another option might be to aggregate the events (thread local or
450 // per-monitor aggregation) and defer reporting until a more opportune
451 // time -- such as next time some thread encounters contention but has
452 // yet to acquire the lock. While spinning that thread could
453 // spinning we could increment JVMStat counters, etc.
454
455 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
456 if (JvmtiExport::should_post_monitor_contended_entered()) {
457 JvmtiExport::post_monitor_contended_entered(jt, this);
458
459 // The current thread already owns the monitor and is not going to
460 // call park() for the remainder of the monitor enter protocol. So
461 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
462 // event handler consumed an unpark() issued by the thread that
463 // just exited the monitor.
464 }
465
466 if (event.should_commit()) {
467 event.set_klass(((oop)this->object())->klass());
468 event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
469 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
470 event.commit();
471 }
472
473 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
474 ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
475 }
476 }
477
478
479 // Caveat: TryLock() is not necessarily serializing if it returns failure.
480 // Callers must compensate as needed.
481
482 int ObjectMonitor::TryLock (Thread * Self) {
483 for (;;) {
484 void * own = _owner ;
485 if (own != NULL) return 0 ;
486 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
487 // Either guarantee _recursions == 0 or set _recursions = 0.
488 assert (_recursions == 0, "invariant") ;
489 assert (_owner == Self, "invariant") ;
490 // CONSIDER: set or assert that OwnerIsThread == 1
491 return 1 ;
492 }
493 // The lock had been free momentarily, but we lost the race to the lock.
494 // Interference -- the CAS failed.
495 // We can either return -1 or retry.
496 // Retry doesn't make as much sense because the lock was just acquired.
497 if (true) return -1 ;
498 }
499 }
500
501 void ATTR ObjectMonitor::EnterI (TRAPS) {
502 Thread * Self = THREAD ;
503 assert (Self->is_Java_thread(), "invariant") ;
504 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
505
506 // Try the lock - TATAS
507 if (TryLock (Self) > 0) {
508 assert (_succ != Self , "invariant") ;
509 assert (_owner == Self , "invariant") ;
510 assert (_Responsible != Self , "invariant") ;
511 return ;
512 }
513
514 DeferredInitialize () ;
515
516 // We try one round of spinning *before* enqueueing Self.
517 //
518 // If the _owner is ready but OFFPROC we could use a YieldTo()
519 // operation to donate the remainder of this thread's quantum
520 // to the owner. This has subtle but beneficial affinity
521 // effects.
522
523 if (TrySpin (Self) > 0) {
524 assert (_owner == Self , "invariant") ;
525 assert (_succ != Self , "invariant") ;
526 assert (_Responsible != Self , "invariant") ;
527 return ;
528 }
529
530 // The Spin failed -- Enqueue and park the thread ...
531 assert (_succ != Self , "invariant") ;
532 assert (_owner != Self , "invariant") ;
533 assert (_Responsible != Self , "invariant") ;
534
535 // Enqueue "Self" on ObjectMonitor's _cxq.
536 //
537 // Node acts as a proxy for Self.
538 // As an aside, if were to ever rewrite the synchronization code mostly
539 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
540 // Java objects. This would avoid awkward lifecycle and liveness issues,
541 // as well as eliminate a subset of ABA issues.
542 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
543 //
544
545 ObjectWaiter node(Self) ;
546 Self->_ParkEvent->reset() ;
547 node._prev = (ObjectWaiter *) 0xBAD ;
548 node.TState = ObjectWaiter::TS_CXQ ;
549
550 // Push "Self" onto the front of the _cxq.
551 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
552 // Note that spinning tends to reduce the rate at which threads
553 // enqueue and dequeue on EntryList|cxq.
554 ObjectWaiter * nxt ;
555 for (;;) {
556 node._next = nxt = _cxq ;
557 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
558
559 // Interference - the CAS failed because _cxq changed. Just retry.
560 // As an optional optimization we retry the lock.
561 if (TryLock (Self) > 0) {
562 assert (_succ != Self , "invariant") ;
563 assert (_owner == Self , "invariant") ;
564 assert (_Responsible != Self , "invariant") ;
565 return ;
566 }
567 }
568
569 // Check for cxq|EntryList edge transition to non-null. This indicates
570 // the onset of contention. While contention persists exiting threads
571 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
572 // operations revert to the faster 1-0 mode. This enter operation may interleave
573 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
574 // arrange for one of the contending thread to use a timed park() operations
575 // to detect and recover from the race. (Stranding is form of progress failure
576 // where the monitor is unlocked but all the contending threads remain parked).
577 // That is, at least one of the contended threads will periodically poll _owner.
578 // One of the contending threads will become the designated "Responsible" thread.
579 // The Responsible thread uses a timed park instead of a normal indefinite park
580 // operation -- it periodically wakes and checks for and recovers from potential
581 // strandings admitted by 1-0 exit operations. We need at most one Responsible
582 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
583 // be responsible for a monitor.
584 //
585 // Currently, one of the contended threads takes on the added role of "Responsible".
586 // A viable alternative would be to use a dedicated "stranding checker" thread
587 // that periodically iterated over all the threads (or active monitors) and unparked
588 // successors where there was risk of stranding. This would help eliminate the
589 // timer scalability issues we see on some platforms as we'd only have one thread
590 // -- the checker -- parked on a timer.
591
592 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
593 // Try to assume the role of responsible thread for the monitor.
594 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
595 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
596 }
597
598 // The lock have been released while this thread was occupied queueing
599 // itself onto _cxq. To close the race and avoid "stranding" and
600 // progress-liveness failure we must resample-retry _owner before parking.
601 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
602 // In this case the ST-MEMBAR is accomplished with CAS().
603 //
604 // TODO: Defer all thread state transitions until park-time.
605 // Since state transitions are heavy and inefficient we'd like
606 // to defer the state transitions until absolutely necessary,
607 // and in doing so avoid some transitions ...
608
609 TEVENT (Inflated enter - Contention) ;
610 int nWakeups = 0 ;
611 int RecheckInterval = 1 ;
612
613 for (;;) {
614
615 if (TryLock (Self) > 0) break ;
616 assert (_owner != Self, "invariant") ;
617
618 if ((SyncFlags & 2) && _Responsible == NULL) {
619 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
620 }
621
622 // park self
623 if (_Responsible == Self || (SyncFlags & 1)) {
624 TEVENT (Inflated enter - park TIMED) ;
625 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
626 // Increase the RecheckInterval, but clamp the value.
627 RecheckInterval *= 8 ;
628 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
629 } else {
630 TEVENT (Inflated enter - park UNTIMED) ;
631 Self->_ParkEvent->park() ;
632 }
633
634 if (TryLock(Self) > 0) break ;
635
636 // The lock is still contested.
637 // Keep a tally of the # of futile wakeups.
638 // Note that the counter is not protected by a lock or updated by atomics.
639 // That is by design - we trade "lossy" counters which are exposed to
640 // races during updates for a lower probe effect.
641 TEVENT (Inflated enter - Futile wakeup) ;
642 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
643 ObjectMonitor::_sync_FutileWakeups->inc() ;
644 }
645 ++ nWakeups ;
646
647 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
648 // We can defer clearing _succ until after the spin completes
649 // TrySpin() must tolerate being called with _succ == Self.
650 // Try yet another round of adaptive spinning.
651 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
652
653 // We can find that we were unpark()ed and redesignated _succ while
654 // we were spinning. That's harmless. If we iterate and call park(),
655 // park() will consume the event and return immediately and we'll
656 // just spin again. This pattern can repeat, leaving _succ to simply
657 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
658 // Alternately, we can sample fired() here, and if set, forgo spinning
659 // in the next iteration.
660
661 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
662 Self->_ParkEvent->reset() ;
663 OrderAccess::fence() ;
664 }
665 if (_succ == Self) _succ = NULL ;
666
667 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
668 OrderAccess::fence() ;
669 }
670
671 // Egress :
672 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
673 // Normally we'll find Self on the EntryList .
674 // From the perspective of the lock owner (this thread), the
675 // EntryList is stable and cxq is prepend-only.
676 // The head of cxq is volatile but the interior is stable.
677 // In addition, Self.TState is stable.
678
679 assert (_owner == Self , "invariant") ;
680 assert (object() != NULL , "invariant") ;
681 // I'd like to write:
682 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
683 // but as we're at a safepoint that's not safe.
684
685 UnlinkAfterAcquire (Self, &node) ;
686 if (_succ == Self) _succ = NULL ;
687
688 assert (_succ != Self, "invariant") ;
689 if (_Responsible == Self) {
690 _Responsible = NULL ;
691 OrderAccess::fence(); // Dekker pivot-point
692
693 // We may leave threads on cxq|EntryList without a designated
694 // "Responsible" thread. This is benign. When this thread subsequently
695 // exits the monitor it can "see" such preexisting "old" threads --
696 // threads that arrived on the cxq|EntryList before the fence, above --
697 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
698 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
699 // non-null and elect a new "Responsible" timer thread.
700 //
701 // This thread executes:
702 // ST Responsible=null; MEMBAR (in enter epilog - here)
703 // LD cxq|EntryList (in subsequent exit)
704 //
705 // Entering threads in the slow/contended path execute:
706 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
707 // The (ST cxq; MEMBAR) is accomplished with CAS().
708 //
709 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
710 // exit operation from floating above the ST Responsible=null.
711 }
712
713 // We've acquired ownership with CAS().
714 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
715 // But since the CAS() this thread may have also stored into _succ,
716 // EntryList, cxq or Responsible. These meta-data updates must be
717 // visible __before this thread subsequently drops the lock.
718 // Consider what could occur if we didn't enforce this constraint --
719 // STs to monitor meta-data and user-data could reorder with (become
720 // visible after) the ST in exit that drops ownership of the lock.
721 // Some other thread could then acquire the lock, but observe inconsistent
722 // or old monitor meta-data and heap data. That violates the JMM.
723 // To that end, the 1-0 exit() operation must have at least STST|LDST
724 // "release" barrier semantics. Specifically, there must be at least a
725 // STST|LDST barrier in exit() before the ST of null into _owner that drops
726 // the lock. The barrier ensures that changes to monitor meta-data and data
727 // protected by the lock will be visible before we release the lock, and
728 // therefore before some other thread (CPU) has a chance to acquire the lock.
729 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
730 //
731 // Critically, any prior STs to _succ or EntryList must be visible before
732 // the ST of null into _owner in the *subsequent* (following) corresponding
733 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
734 // execute a serializing instruction.
735
736 if (SyncFlags & 8) {
737 OrderAccess::fence() ;
738 }
739 return ;
740 }
741
742 // ReenterI() is a specialized inline form of the latter half of the
743 // contended slow-path from EnterI(). We use ReenterI() only for
744 // monitor reentry in wait().
745 //
746 // In the future we should reconcile EnterI() and ReenterI(), adding
747 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
748 // loop accordingly.
749
750 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
751 assert (Self != NULL , "invariant") ;
752 assert (SelfNode != NULL , "invariant") ;
753 assert (SelfNode->_thread == Self , "invariant") ;
754 assert (_waiters > 0 , "invariant") ;
755 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
756 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
757 JavaThread * jt = (JavaThread *) Self ;
758
759 int nWakeups = 0 ;
760 for (;;) {
761 ObjectWaiter::TStates v = SelfNode->TState ;
762 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
763 assert (_owner != Self, "invariant") ;
764
765 if (TryLock (Self) > 0) break ;
766 if (TrySpin (Self) > 0) break ;
767
768 TEVENT (Wait Reentry - parking) ;
769
770 // State transition wrappers around park() ...
771 // ReenterI() wisely defers state transitions until
772 // it's clear we must park the thread.
773 {
774 OSThreadContendState osts(Self->osthread());
775 ThreadBlockInVM tbivm(jt);
776
777 // cleared by handle_special_suspend_equivalent_condition()
778 // or java_suspend_self()
779 jt->set_suspend_equivalent();
780 if (SyncFlags & 1) {
781 Self->_ParkEvent->park ((jlong)1000) ;
782 } else {
783 Self->_ParkEvent->park () ;
784 }
785
786 // were we externally suspended while we were waiting?
787 for (;;) {
788 if (!ExitSuspendEquivalent (jt)) break ;
789 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
790 jt->java_suspend_self();
791 jt->set_suspend_equivalent();
792 }
793 }
794
795 // Try again, but just so we distinguish between futile wakeups and
796 // successful wakeups. The following test isn't algorithmically
797 // necessary, but it helps us maintain sensible statistics.
798 if (TryLock(Self) > 0) break ;
799
800 // The lock is still contested.
801 // Keep a tally of the # of futile wakeups.
802 // Note that the counter is not protected by a lock or updated by atomics.
803 // That is by design - we trade "lossy" counters which are exposed to
804 // races during updates for a lower probe effect.
805 TEVENT (Wait Reentry - futile wakeup) ;
806 ++ nWakeups ;
807
808 // Assuming this is not a spurious wakeup we'll normally
809 // find that _succ == Self.
810 if (_succ == Self) _succ = NULL ;
811
812 // Invariant: after clearing _succ a contending thread
813 // *must* retry _owner before parking.
814 OrderAccess::fence() ;
815
816 if (ObjectMonitor::_sync_FutileWakeups != NULL) {
817 ObjectMonitor::_sync_FutileWakeups->inc() ;
818 }
819 }
820
821 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
822 // Normally we'll find Self on the EntryList.
823 // Unlinking from the EntryList is constant-time and atomic-free.
824 // From the perspective of the lock owner (this thread), the
825 // EntryList is stable and cxq is prepend-only.
826 // The head of cxq is volatile but the interior is stable.
827 // In addition, Self.TState is stable.
828
829 assert (_owner == Self, "invariant") ;
830 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
831 UnlinkAfterAcquire (Self, SelfNode) ;
832 if (_succ == Self) _succ = NULL ;
833 assert (_succ != Self, "invariant") ;
834 SelfNode->TState = ObjectWaiter::TS_RUN ;
835 OrderAccess::fence() ; // see comments at the end of EnterI()
836 }
837
838 // after the thread acquires the lock in ::enter(). Equally, we could defer
839 // unlinking the thread until ::exit()-time.
840
841 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
842 {
843 assert (_owner == Self, "invariant") ;
844 assert (SelfNode->_thread == Self, "invariant") ;
845
846 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
847 // Normal case: remove Self from the DLL EntryList .
848 // This is a constant-time operation.
849 ObjectWaiter * nxt = SelfNode->_next ;
850 ObjectWaiter * prv = SelfNode->_prev ;
851 if (nxt != NULL) nxt->_prev = prv ;
852 if (prv != NULL) prv->_next = nxt ;
853 if (SelfNode == _EntryList ) _EntryList = nxt ;
854 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
855 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
856 TEVENT (Unlink from EntryList) ;
857 } else {
858 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
859 // Inopportune interleaving -- Self is still on the cxq.
860 // This usually means the enqueue of self raced an exiting thread.
861 // Normally we'll find Self near the front of the cxq, so
862 // dequeueing is typically fast. If needbe we can accelerate
863 // this with some MCS/CHL-like bidirectional list hints and advisory
864 // back-links so dequeueing from the interior will normally operate
865 // in constant-time.
866 // Dequeue Self from either the head (with CAS) or from the interior
867 // with a linear-time scan and normal non-atomic memory operations.
868 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
869 // and then unlink Self from EntryList. We have to drain eventually,
870 // so it might as well be now.
871
872 ObjectWaiter * v = _cxq ;
873 assert (v != NULL, "invariant") ;
874 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
875 // The CAS above can fail from interference IFF a "RAT" arrived.
876 // In that case Self must be in the interior and can no longer be
877 // at the head of cxq.
878 if (v == SelfNode) {
879 assert (_cxq != v, "invariant") ;
880 v = _cxq ; // CAS above failed - start scan at head of list
881 }
882 ObjectWaiter * p ;
883 ObjectWaiter * q = NULL ;
884 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
885 q = p ;
886 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
887 }
888 assert (v != SelfNode, "invariant") ;
889 assert (p == SelfNode, "Node not found on cxq") ;
890 assert (p != _cxq, "invariant") ;
891 assert (q != NULL, "invariant") ;
892 assert (q->_next == p, "invariant") ;
893 q->_next = p->_next ;
894 }
895 TEVENT (Unlink from cxq) ;
896 }
897
898 // Diagnostic hygiene ...
899 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
900 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
901 SelfNode->TState = ObjectWaiter::TS_RUN ;
902 }
903
904 // -----------------------------------------------------------------------------
905 // Exit support
906 //
907 // exit()
908 // ~~~~~~
909 // Note that the collector can't reclaim the objectMonitor or deflate
910 // the object out from underneath the thread calling ::exit() as the
911 // thread calling ::exit() never transitions to a stable state.
912 // This inhibits GC, which in turn inhibits asynchronous (and
913 // inopportune) reclamation of "this".
914 //
915 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
916 // There's one exception to the claim above, however. EnterI() can call
917 // exit() to drop a lock if the acquirer has been externally suspended.
918 // In that case exit() is called with _thread_state as _thread_blocked,
919 // but the monitor's _count field is > 0, which inhibits reclamation.
920 //
921 // 1-0 exit
922 // ~~~~~~~~
923 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
924 // the fast-path operators have been optimized so the common ::exit()
925 // operation is 1-0. See i486.ad fast_unlock(), for instance.
926 // The code emitted by fast_unlock() elides the usual MEMBAR. This
927 // greatly improves latency -- MEMBAR and CAS having considerable local
928 // latency on modern processors -- but at the cost of "stranding". Absent the
929 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
930 // ::enter() path, resulting in the entering thread being stranding
931 // and a progress-liveness failure. Stranding is extremely rare.
932 // We use timers (timed park operations) & periodic polling to detect
933 // and recover from stranding. Potentially stranded threads periodically
934 // wake up and poll the lock. See the usage of the _Responsible variable.
935 //
936 // The CAS() in enter provides for safety and exclusion, while the CAS or
937 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
938 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
939 // We detect and recover from stranding with timers.
940 //
941 // If a thread transiently strands it'll park until (a) another
942 // thread acquires the lock and then drops the lock, at which time the
943 // exiting thread will notice and unpark the stranded thread, or, (b)
944 // the timer expires. If the lock is high traffic then the stranding latency
945 // will be low due to (a). If the lock is low traffic then the odds of
946 // stranding are lower, although the worst-case stranding latency
947 // is longer. Critically, we don't want to put excessive load in the
948 // platform's timer subsystem. We want to minimize both the timer injection
949 // rate (timers created/sec) as well as the number of timers active at
950 // any one time. (more precisely, we want to minimize timer-seconds, which is
951 // the integral of the # of active timers at any instant over time).
952 // Both impinge on OS scalability. Given that, at most one thread parked on
953 // a monitor will use a timer.
954
955 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
956 Thread * Self = THREAD ;
957 if (THREAD != _owner) {
958 if (THREAD->is_lock_owned((address) _owner)) {
959 // Transmute _owner from a BasicLock pointer to a Thread address.
960 // We don't need to hold _mutex for this transition.
961 // Non-null to Non-null is safe as long as all readers can
962 // tolerate either flavor.
963 assert (_recursions == 0, "invariant") ;
964 _owner = THREAD ;
965 _recursions = 0 ;
966 OwnerIsThread = 1 ;
967 } else {
968 // NOTE: we need to handle unbalanced monitor enter/exit
969 // in native code by throwing an exception.
970 // TODO: Throw an IllegalMonitorStateException ?
971 TEVENT (Exit - Throw IMSX) ;
972 assert(false, "Non-balanced monitor enter/exit!");
973 if (false) {
974 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
975 }
976 return;
977 }
978 }
979
980 if (_recursions != 0) {
981 _recursions--; // this is simple recursive enter
982 TEVENT (Inflated exit - recursive) ;
983 return ;
984 }
985
986 // Invariant: after setting Responsible=null an thread must execute
987 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
988 if ((SyncFlags & 4) == 0) {
989 _Responsible = NULL ;
990 }
991
992 #if INCLUDE_TRACE
993 // get the owner's thread id for the MonitorEnter event
994 // if it is enabled and the thread isn't suspended
995 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
996 _previous_owner_tid = SharedRuntime::get_java_tid(Self);
997 }
998 #endif
999
1000 for (;;) {
1001 assert (THREAD == _owner, "invariant") ;
1002
1003
1004 if (Knob_ExitPolicy == 0) {
1005 // release semantics: prior loads and stores from within the critical section
1006 // must not float (reorder) past the following store that drops the lock.
1007 // On SPARC that requires MEMBAR #loadstore|#storestore.
1008 // But of course in TSO #loadstore|#storestore is not required.
1009 // I'd like to write one of the following:
1010 // A. OrderAccess::release() ; _owner = NULL
1011 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
1012 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
1013 // store into a _dummy variable. That store is not needed, but can result
1014 // in massive wasteful coherency traffic on classic SMP systems.
1015 // Instead, I use release_store(), which is implemented as just a simple
1016 // ST on x64, x86 and SPARC.
1017 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1018 OrderAccess::storeload() ; // See if we need to wake a successor
1019 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1020 TEVENT (Inflated exit - simple egress) ;
1021 return ;
1022 }
1023 TEVENT (Inflated exit - complex egress) ;
1024
1025 // Normally the exiting thread is responsible for ensuring succession,
1026 // but if other successors are ready or other entering threads are spinning
1027 // then this thread can simply store NULL into _owner and exit without
1028 // waking a successor. The existence of spinners or ready successors
1029 // guarantees proper succession (liveness). Responsibility passes to the
1030 // ready or running successors. The exiting thread delegates the duty.
1031 // More precisely, if a successor already exists this thread is absolved
1032 // of the responsibility of waking (unparking) one.
1033 //
1034 // The _succ variable is critical to reducing futile wakeup frequency.
1035 // _succ identifies the "heir presumptive" thread that has been made
1036 // ready (unparked) but that has not yet run. We need only one such
1037 // successor thread to guarantee progress.
1038 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1039 // section 3.3 "Futile Wakeup Throttling" for details.
1040 //
1041 // Note that spinners in Enter() also set _succ non-null.
1042 // In the current implementation spinners opportunistically set
1043 // _succ so that exiting threads might avoid waking a successor.
1044 // Another less appealing alternative would be for the exiting thread
1045 // to drop the lock and then spin briefly to see if a spinner managed
1046 // to acquire the lock. If so, the exiting thread could exit
1047 // immediately without waking a successor, otherwise the exiting
1048 // thread would need to dequeue and wake a successor.
1049 // (Note that we'd need to make the post-drop spin short, but no
1050 // shorter than the worst-case round-trip cache-line migration time.
1051 // The dropped lock needs to become visible to the spinner, and then
1052 // the acquisition of the lock by the spinner must become visible to
1053 // the exiting thread).
1054 //
1055
1056 // It appears that an heir-presumptive (successor) must be made ready.
1057 // Only the current lock owner can manipulate the EntryList or
1058 // drain _cxq, so we need to reacquire the lock. If we fail
1059 // to reacquire the lock the responsibility for ensuring succession
1060 // falls to the new owner.
1061 //
1062 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1063 return ;
1064 }
1065 TEVENT (Exit - Reacquired) ;
1066 } else {
1067 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1068 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
1069 OrderAccess::storeload() ;
1070 // Ratify the previously observed values.
1071 if (_cxq == NULL || _succ != NULL) {
1072 TEVENT (Inflated exit - simple egress) ;
1073 return ;
1074 }
1075
1076 // inopportune interleaving -- the exiting thread (this thread)
1077 // in the fast-exit path raced an entering thread in the slow-enter
1078 // path.
1079 // We have two choices:
1080 // A. Try to reacquire the lock.
1081 // If the CAS() fails return immediately, otherwise
1082 // we either restart/rerun the exit operation, or simply
1083 // fall-through into the code below which wakes a successor.
1084 // B. If the elements forming the EntryList|cxq are TSM
1085 // we could simply unpark() the lead thread and return
1086 // without having set _succ.
1087 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1088 TEVENT (Inflated exit - reacquired succeeded) ;
1089 return ;
1090 }
1091 TEVENT (Inflated exit - reacquired failed) ;
1092 } else {
1093 TEVENT (Inflated exit - complex egress) ;
1094 }
1095 }
1096
1097 guarantee (_owner == THREAD, "invariant") ;
1098
1099 ObjectWaiter * w = NULL ;
1100 int QMode = Knob_QMode ;
1101
1102 if (QMode == 2 && _cxq != NULL) {
1103 // QMode == 2 : cxq has precedence over EntryList.
1104 // Try to directly wake a successor from the cxq.
1105 // If successful, the successor will need to unlink itself from cxq.
1106 w = _cxq ;
1107 assert (w != NULL, "invariant") ;
1108 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1109 ExitEpilog (Self, w) ;
1110 return ;
1111 }
1112
1113 if (QMode == 3 && _cxq != NULL) {
1114 // Aggressively drain cxq into EntryList at the first opportunity.
1115 // This policy ensure that recently-run threads live at the head of EntryList.
1116 // Drain _cxq into EntryList - bulk transfer.
1117 // First, detach _cxq.
1118 // The following loop is tantamount to: w = swap (&cxq, NULL)
1119 w = _cxq ;
1120 for (;;) {
1121 assert (w != NULL, "Invariant") ;
1122 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1123 if (u == w) break ;
1124 w = u ;
1125 }
1126 assert (w != NULL , "invariant") ;
1127
1128 ObjectWaiter * q = NULL ;
1129 ObjectWaiter * p ;
1130 for (p = w ; p != NULL ; p = p->_next) {
1131 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1132 p->TState = ObjectWaiter::TS_ENTER ;
1133 p->_prev = q ;
1134 q = p ;
1135 }
1136
1137 // Append the RATs to the EntryList
1138 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1139 ObjectWaiter * Tail ;
1140 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
1141 if (Tail == NULL) {
1142 _EntryList = w ;
1143 } else {
1144 Tail->_next = w ;
1145 w->_prev = Tail ;
1146 }
1147
1148 // Fall thru into code that tries to wake a successor from EntryList
1149 }
1150
1151 if (QMode == 4 && _cxq != NULL) {
1152 // Aggressively drain cxq into EntryList at the first opportunity.
1153 // This policy ensure that recently-run threads live at the head of EntryList.
1154
1155 // Drain _cxq into EntryList - bulk transfer.
1156 // First, detach _cxq.
1157 // The following loop is tantamount to: w = swap (&cxq, NULL)
1158 w = _cxq ;
1159 for (;;) {
1160 assert (w != NULL, "Invariant") ;
1161 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1162 if (u == w) break ;
1163 w = u ;
1164 }
1165 assert (w != NULL , "invariant") ;
1166
1167 ObjectWaiter * q = NULL ;
1168 ObjectWaiter * p ;
1169 for (p = w ; p != NULL ; p = p->_next) {
1170 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1171 p->TState = ObjectWaiter::TS_ENTER ;
1172 p->_prev = q ;
1173 q = p ;
1174 }
1175
1176 // Prepend the RATs to the EntryList
1177 if (_EntryList != NULL) {
1178 q->_next = _EntryList ;
1179 _EntryList->_prev = q ;
1180 }
1181 _EntryList = w ;
1182
1183 // Fall thru into code that tries to wake a successor from EntryList
1184 }
1185
1186 w = _EntryList ;
1187 if (w != NULL) {
1188 // I'd like to write: guarantee (w->_thread != Self).
1189 // But in practice an exiting thread may find itself on the EntryList.
1190 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1191 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1192 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
1193 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1194 // release the lock "O". T2 resumes immediately after the ST of null into
1195 // _owner, above. T2 notices that the EntryList is populated, so it
1196 // reacquires the lock and then finds itself on the EntryList.
1197 // Given all that, we have to tolerate the circumstance where "w" is
1198 // associated with Self.
1199 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1200 ExitEpilog (Self, w) ;
1201 return ;
1202 }
1203
1204 // If we find that both _cxq and EntryList are null then just
1205 // re-run the exit protocol from the top.
1206 w = _cxq ;
1207 if (w == NULL) continue ;
1208
1209 // Drain _cxq into EntryList - bulk transfer.
1210 // First, detach _cxq.
1211 // The following loop is tantamount to: w = swap (&cxq, NULL)
1212 for (;;) {
1213 assert (w != NULL, "Invariant") ;
1214 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
1215 if (u == w) break ;
1216 w = u ;
1217 }
1218 TEVENT (Inflated exit - drain cxq into EntryList) ;
1219
1220 assert (w != NULL , "invariant") ;
1221 assert (_EntryList == NULL , "invariant") ;
1222
1223 // Convert the LIFO SLL anchored by _cxq into a DLL.
1224 // The list reorganization step operates in O(LENGTH(w)) time.
1225 // It's critical that this step operate quickly as
1226 // "Self" still holds the outer-lock, restricting parallelism
1227 // and effectively lengthening the critical section.
1228 // Invariant: s chases t chases u.
1229 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1230 // we have faster access to the tail.
1231
1232 if (QMode == 1) {
1233 // QMode == 1 : drain cxq to EntryList, reversing order
1234 // We also reverse the order of the list.
1235 ObjectWaiter * s = NULL ;
1236 ObjectWaiter * t = w ;
1237 ObjectWaiter * u = NULL ;
1238 while (t != NULL) {
1239 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
1240 t->TState = ObjectWaiter::TS_ENTER ;
1241 u = t->_next ;
1242 t->_prev = u ;
1243 t->_next = s ;
1244 s = t;
1245 t = u ;
1246 }
1247 _EntryList = s ;
1248 assert (s != NULL, "invariant") ;
1249 } else {
1250 // QMode == 0 or QMode == 2
1251 _EntryList = w ;
1252 ObjectWaiter * q = NULL ;
1253 ObjectWaiter * p ;
1254 for (p = w ; p != NULL ; p = p->_next) {
1255 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
1256 p->TState = ObjectWaiter::TS_ENTER ;
1257 p->_prev = q ;
1258 q = p ;
1259 }
1260 }
1261
1262 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1263 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1264
1265 // See if we can abdicate to a spinner instead of waking a thread.
1266 // A primary goal of the implementation is to reduce the
1267 // context-switch rate.
1268 if (_succ != NULL) continue;
1269
1270 w = _EntryList ;
1271 if (w != NULL) {
1272 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1273 ExitEpilog (Self, w) ;
1274 return ;
1275 }
1276 }
1277 }
1278
1279 // ExitSuspendEquivalent:
1280 // A faster alternate to handle_special_suspend_equivalent_condition()
1281 //
1282 // handle_special_suspend_equivalent_condition() unconditionally
1283 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1284 // operations have high latency. Note that in ::enter() we call HSSEC
1285 // while holding the monitor, so we effectively lengthen the critical sections.
1286 //
1287 // There are a number of possible solutions:
1288 //
1289 // A. To ameliorate the problem we might also defer state transitions
1290 // to as late as possible -- just prior to parking.
1291 // Given that, we'd call HSSEC after having returned from park(),
1292 // but before attempting to acquire the monitor. This is only a
1293 // partial solution. It avoids calling HSSEC while holding the
1294 // monitor (good), but it still increases successor reacquisition latency --
1295 // the interval between unparking a successor and the time the successor
1296 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1297 // If we use this technique we can also avoid EnterI()-exit() loop
1298 // in ::enter() where we iteratively drop the lock and then attempt
1299 // to reacquire it after suspending.
1300 //
1301 // B. In the future we might fold all the suspend bits into a
1302 // composite per-thread suspend flag and then update it with CAS().
1303 // Alternately, a Dekker-like mechanism with multiple variables
1304 // would suffice:
1305 // ST Self->_suspend_equivalent = false
1306 // MEMBAR
1307 // LD Self_>_suspend_flags
1308 //
1309
1310
1311 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
1312 int Mode = Knob_FastHSSEC ;
1313 if (Mode && !jSelf->is_external_suspend()) {
1314 assert (jSelf->is_suspend_equivalent(), "invariant") ;
1315 jSelf->clear_suspend_equivalent() ;
1316 if (2 == Mode) OrderAccess::storeload() ;
1317 if (!jSelf->is_external_suspend()) return false ;
1318 // We raced a suspension -- fall thru into the slow path
1319 TEVENT (ExitSuspendEquivalent - raced) ;
1320 jSelf->set_suspend_equivalent() ;
1321 }
1322 return jSelf->handle_special_suspend_equivalent_condition() ;
1323 }
1324
1325
1326 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
1327 assert (_owner == Self, "invariant") ;
1328
1329 // Exit protocol:
1330 // 1. ST _succ = wakee
1331 // 2. membar #loadstore|#storestore;
1332 // 2. ST _owner = NULL
1333 // 3. unpark(wakee)
1334
1335 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
1336 ParkEvent * Trigger = Wakee->_event ;
1337
1338 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1339 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1340 // out-of-scope (non-extant).
1341 Wakee = NULL ;
1342
1343 // Drop the lock
1344 OrderAccess::release_store_ptr (&_owner, NULL) ;
1345 OrderAccess::fence() ; // ST _owner vs LD in unpark()
1346
1347 if (SafepointSynchronize::do_call_back()) {
1348 TEVENT (unpark before SAFEPOINT) ;
1349 }
1350
1351 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1352 Trigger->unpark() ;
1353
1354 // Maintain stats and report events to JVMTI
1355 if (ObjectMonitor::_sync_Parks != NULL) {
1356 ObjectMonitor::_sync_Parks->inc() ;
1357 }
1358 }
1359
1360
1361 // -----------------------------------------------------------------------------
1362 // Class Loader deadlock handling.
1363 //
1364 // complete_exit exits a lock returning recursion count
1365 // complete_exit/reenter operate as a wait without waiting
1366 // complete_exit requires an inflated monitor
1367 // The _owner field is not always the Thread addr even with an
1368 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1369 // thread due to contention.
1370 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1371 Thread * const Self = THREAD;
1372 assert(Self->is_Java_thread(), "Must be Java thread!");
1373 JavaThread *jt = (JavaThread *)THREAD;
1374
1375 DeferredInitialize();
1376
1377 if (THREAD != _owner) {
1378 if (THREAD->is_lock_owned ((address)_owner)) {
1379 assert(_recursions == 0, "internal state error");
1380 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
1381 _recursions = 0 ;
1382 OwnerIsThread = 1 ;
1383 }
1384 }
1385
1386 guarantee(Self == _owner, "complete_exit not owner");
1387 intptr_t save = _recursions; // record the old recursion count
1388 _recursions = 0; // set the recursion level to be 0
1389 exit (true, Self) ; // exit the monitor
1390 guarantee (_owner != Self, "invariant");
1391 return save;
1392 }
1393
1394 // reenter() enters a lock and sets recursion count
1395 // complete_exit/reenter operate as a wait without waiting
1396 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1397 Thread * const Self = THREAD;
1398 assert(Self->is_Java_thread(), "Must be Java thread!");
1399 JavaThread *jt = (JavaThread *)THREAD;
1400
1401 guarantee(_owner != Self, "reenter already owner");
1402 enter (THREAD); // enter the monitor
1403 guarantee (_recursions == 0, "reenter recursion");
1404 _recursions = recursions;
1405 return;
1406 }
1407
1408
1409 // -----------------------------------------------------------------------------
1410 // A macro is used below because there may already be a pending
1411 // exception which should not abort the execution of the routines
1412 // which use this (which is why we don't put this into check_slow and
1413 // call it with a CHECK argument).
1414
1415 #define CHECK_OWNER() \
1416 do { \
1417 if (THREAD != _owner) { \
1418 if (THREAD->is_lock_owned((address) _owner)) { \
1419 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
1420 _recursions = 0; \
1421 OwnerIsThread = 1 ; \
1422 } else { \
1423 TEVENT (Throw IMSX) ; \
1424 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1425 } \
1426 } \
1427 } while (false)
1428
1429 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1430 // TODO-FIXME: remove check_slow() -- it's likely dead.
1431
1432 void ObjectMonitor::check_slow(TRAPS) {
1433 TEVENT (check_slow - throw IMSX) ;
1434 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1435 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1436 }
1437
1438 static int Adjust (volatile int * adr, int dx) {
1439 int v ;
1440 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
1441 return v ;
1442 }
1443
1444 // helper method for posting a monitor wait event
1445 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1446 jlong notifier_tid,
1447 jlong timeout,
1448 bool timedout) {
1449 event->set_klass(((oop)this->object())->klass());
1450 event->set_timeout((TYPE_ULONG)timeout);
1451 event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1452 event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1453 event->set_timedOut((TYPE_BOOLEAN)timedout);
1454 event->commit();
1455 }
1456
1457 // -----------------------------------------------------------------------------
1458 // Wait/Notify/NotifyAll
1459 //
1460 // Note: a subset of changes to ObjectMonitor::wait()
1461 // will need to be replicated in complete_exit above
1462 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1463 Thread * const Self = THREAD ;
1464 assert(Self->is_Java_thread(), "Must be Java thread!");
1465 JavaThread *jt = (JavaThread *)THREAD;
1466
1467 DeferredInitialize () ;
1468
1469 // Throw IMSX or IEX.
1470 CHECK_OWNER();
1471
1472 EventJavaMonitorWait event;
1473
1474 // check for a pending interrupt
1475 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1476 // post monitor waited event. Note that this is past-tense, we are done waiting.
1477 if (JvmtiExport::should_post_monitor_waited()) {
1478 // Note: 'false' parameter is passed here because the
1479 // wait was not timed out due to thread interrupt.
1480 JvmtiExport::post_monitor_waited(jt, this, false);
1481
1482 // In this short circuit of the monitor wait protocol, the
1483 // current thread never drops ownership of the monitor and
1484 // never gets added to the wait queue so the current thread
1485 // cannot be made the successor. This means that the
1486 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1487 // consume an unpark() meant for the ParkEvent associated with
1488 // this ObjectMonitor.
1489 }
1490 if (event.should_commit()) {
1491 post_monitor_wait_event(&event, 0, millis, false);
1492 }
1493 TEVENT (Wait - Throw IEX) ;
1494 THROW(vmSymbols::java_lang_InterruptedException());
1495 return ;
1496 }
1497
1498 TEVENT (Wait) ;
1499
1500 assert (Self->_Stalled == 0, "invariant") ;
1501 Self->_Stalled = intptr_t(this) ;
1502 jt->set_current_waiting_monitor(this);
1503
1504 // create a node to be put into the queue
1505 // Critically, after we reset() the event but prior to park(), we must check
1506 // for a pending interrupt.
1507 ObjectWaiter node(Self);
1508 node.TState = ObjectWaiter::TS_WAIT ;
1509 Self->_ParkEvent->reset() ;
1510 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1511
1512 // Enter the waiting queue, which is a circular doubly linked list in this case
1513 // but it could be a priority queue or any data structure.
1514 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1515 // by the the owner of the monitor *except* in the case where park()
1516 // returns because of a timeout of interrupt. Contention is exceptionally rare
1517 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1518
1519 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
1520 AddWaiter (&node) ;
1521 Thread::SpinRelease (&_WaitSetLock) ;
1522
1523 if ((SyncFlags & 4) == 0) {
1524 _Responsible = NULL ;
1525 }
1526 intptr_t save = _recursions; // record the old recursion count
1527 _waiters++; // increment the number of waiters
1528 _recursions = 0; // set the recursion level to be 1
1529 exit (true, Self) ; // exit the monitor
1530 guarantee (_owner != Self, "invariant") ;
1531
1532 // The thread is on the WaitSet list - now park() it.
1533 // On MP systems it's conceivable that a brief spin before we park
1534 // could be profitable.
1535 //
1536 // TODO-FIXME: change the following logic to a loop of the form
1537 // while (!timeout && !interrupted && _notified == 0) park()
1538
1539 int ret = OS_OK ;
1540 int WasNotified = 0 ;
1541 { // State transition wrappers
1542 OSThread* osthread = Self->osthread();
1543 OSThreadWaitState osts(osthread, true);
1544 {
1545 ThreadBlockInVM tbivm(jt);
1546 // Thread is in thread_blocked state and oop access is unsafe.
1547 jt->set_suspend_equivalent();
1548
1549 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1550 // Intentionally empty
1551 } else
1552 if (node._notified == 0) {
1553 if (millis <= 0) {
1554 Self->_ParkEvent->park () ;
1555 } else {
1556 ret = Self->_ParkEvent->park (millis) ;
1557 }
1558 }
1559
1560 // were we externally suspended while we were waiting?
1561 if (ExitSuspendEquivalent (jt)) {
1562 // TODO-FIXME: add -- if succ == Self then succ = null.
1563 jt->java_suspend_self();
1564 }
1565
1566 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1567
1568
1569 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1570 // from the WaitSet to the EntryList.
1571 // See if we need to remove Node from the WaitSet.
1572 // We use double-checked locking to avoid grabbing _WaitSetLock
1573 // if the thread is not on the wait queue.
1574 //
1575 // Note that we don't need a fence before the fetch of TState.
1576 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1577 // written by the is thread. (perhaps the fetch might even be satisfied
1578 // by a look-aside into the processor's own store buffer, although given
1579 // the length of the code path between the prior ST and this load that's
1580 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1581 // then we'll acquire the lock and then re-fetch a fresh TState value.
1582 // That is, we fail toward safety.
1583
1584 if (node.TState == ObjectWaiter::TS_WAIT) {
1585 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1586 if (node.TState == ObjectWaiter::TS_WAIT) {
1587 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
1588 assert(node._notified == 0, "invariant");
1589 node.TState = ObjectWaiter::TS_RUN ;
1590 }
1591 Thread::SpinRelease (&_WaitSetLock) ;
1592 }
1593
1594 // The thread is now either on off-list (TS_RUN),
1595 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1596 // The Node's TState variable is stable from the perspective of this thread.
1597 // No other threads will asynchronously modify TState.
1598 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
1599 OrderAccess::loadload() ;
1600 if (_succ == Self) _succ = NULL ;
1601 WasNotified = node._notified ;
1602
1603 // Reentry phase -- reacquire the monitor.
1604 // re-enter contended monitor after object.wait().
1605 // retain OBJECT_WAIT state until re-enter successfully completes
1606 // Thread state is thread_in_vm and oop access is again safe,
1607 // although the raw address of the object may have changed.
1608 // (Don't cache naked oops over safepoints, of course).
1609
1610 // post monitor waited event. Note that this is past-tense, we are done waiting.
1611 if (JvmtiExport::should_post_monitor_waited()) {
1612 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1613
1614 if (node._notified != 0 && _succ == Self) {
1615 // In this part of the monitor wait-notify-reenter protocol it
1616 // is possible (and normal) for another thread to do a fastpath
1617 // monitor enter-exit while this thread is still trying to get
1618 // to the reenter portion of the protocol.
1619 //
1620 // The ObjectMonitor was notified and the current thread is
1621 // the successor which also means that an unpark() has already
1622 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1623 // consume the unpark() that was done when the successor was
1624 // set because the same ParkEvent is shared between Java
1625 // monitors and JVM/TI RawMonitors (for now).
1626 //
1627 // We redo the unpark() to ensure forward progress, i.e., we
1628 // don't want all pending threads hanging (parked) with none
1629 // entering the unlocked monitor.
1630 node._event->unpark();
1631 }
1632 }
1633
1634 if (event.should_commit()) {
1635 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1636 }
1637
1638 OrderAccess::fence() ;
1639
1640 assert (Self->_Stalled != 0, "invariant") ;
1641 Self->_Stalled = 0 ;
1642
1643 assert (_owner != Self, "invariant") ;
1644 ObjectWaiter::TStates v = node.TState ;
1645 if (v == ObjectWaiter::TS_RUN) {
1646 enter (Self) ;
1647 } else {
1648 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
1649 ReenterI (Self, &node) ;
1650 node.wait_reenter_end(this);
1651 }
1652
1653 // Self has reacquired the lock.
1654 // Lifecycle - the node representing Self must not appear on any queues.
1655 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1656 // want residual elements associated with this thread left on any lists.
1657 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
1658 assert (_owner == Self, "invariant") ;
1659 assert (_succ != Self , "invariant") ;
1660 } // OSThreadWaitState()
1661
1662 jt->set_current_waiting_monitor(NULL);
1663
1664 guarantee (_recursions == 0, "invariant") ;
1665 _recursions = save; // restore the old recursion count
1666 _waiters--; // decrement the number of waiters
1667
1668 // Verify a few postconditions
1669 assert (_owner == Self , "invariant") ;
1670 assert (_succ != Self , "invariant") ;
1671 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
1672
1673 if (SyncFlags & 32) {
1674 OrderAccess::fence() ;
1675 }
1676
1677 // check if the notification happened
1678 if (!WasNotified) {
1679 // no, it could be timeout or Thread.interrupt() or both
1680 // check for interrupt event, otherwise it is timeout
1681 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1682 TEVENT (Wait - throw IEX from epilog) ;
1683 THROW(vmSymbols::java_lang_InterruptedException());
1684 }
1685 }
1686
1687 // NOTE: Spurious wake up will be consider as timeout.
1688 // Monitor notify has precedence over thread interrupt.
1689 }
1690
1691
1692 // Consider:
1693 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1694 // then instead of transferring a thread from the WaitSet to the EntryList
1695 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1696
1697 void ObjectMonitor::notify(TRAPS) {
1698 CHECK_OWNER();
1699 if (_WaitSet == NULL) {
1700 TEVENT (Empty-Notify) ;
1701 return ;
1702 }
1703 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1704
1705 int Policy = Knob_MoveNotifyee ;
1706
1707 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
1708 ObjectWaiter * iterator = DequeueWaiter() ;
1709 if (iterator != NULL) {
1710 TEVENT (Notify1 - Transfer) ;
1711 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1712 guarantee (iterator->_notified == 0, "invariant") ;
1713 if (Policy != 4) {
1714 iterator->TState = ObjectWaiter::TS_ENTER ;
1715 }
1716 iterator->_notified = 1 ;
1717 Thread * Self = THREAD;
1718 iterator->_notifier_tid = Self->osthread()->thread_id();
1719
1720 ObjectWaiter * List = _EntryList ;
1721 if (List != NULL) {
1722 assert (List->_prev == NULL, "invariant") ;
1723 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1724 assert (List != iterator, "invariant") ;
1725 }
1726
1727 if (Policy == 0) { // prepend to EntryList
1728 if (List == NULL) {
1729 iterator->_next = iterator->_prev = NULL ;
1730 _EntryList = iterator ;
1731 } else {
1732 List->_prev = iterator ;
1733 iterator->_next = List ;
1734 iterator->_prev = NULL ;
1735 _EntryList = iterator ;
1736 }
1737 } else
1738 if (Policy == 1) { // append to EntryList
1739 if (List == NULL) {
1740 iterator->_next = iterator->_prev = NULL ;
1741 _EntryList = iterator ;
1742 } else {
1743 // CONSIDER: finding the tail currently requires a linear-time walk of
1744 // the EntryList. We can make tail access constant-time by converting to
1745 // a CDLL instead of using our current DLL.
1746 ObjectWaiter * Tail ;
1747 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1748 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1749 Tail->_next = iterator ;
1750 iterator->_prev = Tail ;
1751 iterator->_next = NULL ;
1752 }
1753 } else
1754 if (Policy == 2) { // prepend to cxq
1755 // prepend to cxq
1756 if (List == NULL) {
1757 iterator->_next = iterator->_prev = NULL ;
1758 _EntryList = iterator ;
1759 } else {
1760 iterator->TState = ObjectWaiter::TS_CXQ ;
1761 for (;;) {
1762 ObjectWaiter * Front = _cxq ;
1763 iterator->_next = Front ;
1764 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1765 break ;
1766 }
1767 }
1768 }
1769 } else
1770 if (Policy == 3) { // append to cxq
1771 iterator->TState = ObjectWaiter::TS_CXQ ;
1772 for (;;) {
1773 ObjectWaiter * Tail ;
1774 Tail = _cxq ;
1775 if (Tail == NULL) {
1776 iterator->_next = NULL ;
1777 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1778 break ;
1779 }
1780 } else {
1781 while (Tail->_next != NULL) Tail = Tail->_next ;
1782 Tail->_next = iterator ;
1783 iterator->_prev = Tail ;
1784 iterator->_next = NULL ;
1785 break ;
1786 }
1787 }
1788 } else {
1789 ParkEvent * ev = iterator->_event ;
1790 iterator->TState = ObjectWaiter::TS_RUN ;
1791 OrderAccess::fence() ;
1792 ev->unpark() ;
1793 }
1794
1795 if (Policy < 4) {
1796 iterator->wait_reenter_begin(this);
1797 }
1798
1799 // _WaitSetLock protects the wait queue, not the EntryList. We could
1800 // move the add-to-EntryList operation, above, outside the critical section
1801 // protected by _WaitSetLock. In practice that's not useful. With the
1802 // exception of wait() timeouts and interrupts the monitor owner
1803 // is the only thread that grabs _WaitSetLock. There's almost no contention
1804 // on _WaitSetLock so it's not profitable to reduce the length of the
1805 // critical section.
1806 }
1807
1808 Thread::SpinRelease (&_WaitSetLock) ;
1809
1810 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
1811 ObjectMonitor::_sync_Notifications->inc() ;
1812 }
1813 }
1814
1815
1816 void ObjectMonitor::notifyAll(TRAPS) {
1817 CHECK_OWNER();
1818 ObjectWaiter* iterator;
1819 if (_WaitSet == NULL) {
1820 TEVENT (Empty-NotifyAll) ;
1821 return ;
1822 }
1823 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1824
1825 int Policy = Knob_MoveNotifyee ;
1826 int Tally = 0 ;
1827 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
1828
1829 for (;;) {
1830 iterator = DequeueWaiter () ;
1831 if (iterator == NULL) break ;
1832 TEVENT (NotifyAll - Transfer1) ;
1833 ++Tally ;
1834
1835 // Disposition - what might we do with iterator ?
1836 // a. add it directly to the EntryList - either tail or head.
1837 // b. push it onto the front of the _cxq.
1838 // For now we use (a).
1839
1840 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
1841 guarantee (iterator->_notified == 0, "invariant") ;
1842 iterator->_notified = 1 ;
1843 Thread * Self = THREAD;
1844 iterator->_notifier_tid = Self->osthread()->thread_id();
1845 if (Policy != 4) {
1846 iterator->TState = ObjectWaiter::TS_ENTER ;
1847 }
1848
1849 ObjectWaiter * List = _EntryList ;
1850 if (List != NULL) {
1851 assert (List->_prev == NULL, "invariant") ;
1852 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
1853 assert (List != iterator, "invariant") ;
1854 }
1855
1856 if (Policy == 0) { // prepend to EntryList
1857 if (List == NULL) {
1858 iterator->_next = iterator->_prev = NULL ;
1859 _EntryList = iterator ;
1860 } else {
1861 List->_prev = iterator ;
1862 iterator->_next = List ;
1863 iterator->_prev = NULL ;
1864 _EntryList = iterator ;
1865 }
1866 } else
1867 if (Policy == 1) { // append to EntryList
1868 if (List == NULL) {
1869 iterator->_next = iterator->_prev = NULL ;
1870 _EntryList = iterator ;
1871 } else {
1872 // CONSIDER: finding the tail currently requires a linear-time walk of
1873 // the EntryList. We can make tail access constant-time by converting to
1874 // a CDLL instead of using our current DLL.
1875 ObjectWaiter * Tail ;
1876 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
1877 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
1878 Tail->_next = iterator ;
1879 iterator->_prev = Tail ;
1880 iterator->_next = NULL ;
1881 }
1882 } else
1883 if (Policy == 2) { // prepend to cxq
1884 // prepend to cxq
1885 iterator->TState = ObjectWaiter::TS_CXQ ;
1886 for (;;) {
1887 ObjectWaiter * Front = _cxq ;
1888 iterator->_next = Front ;
1889 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
1890 break ;
1891 }
1892 }
1893 } else
1894 if (Policy == 3) { // append to cxq
1895 iterator->TState = ObjectWaiter::TS_CXQ ;
1896 for (;;) {
1897 ObjectWaiter * Tail ;
1898 Tail = _cxq ;
1899 if (Tail == NULL) {
1900 iterator->_next = NULL ;
1901 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
1902 break ;
1903 }
1904 } else {
1905 while (Tail->_next != NULL) Tail = Tail->_next ;
1906 Tail->_next = iterator ;
1907 iterator->_prev = Tail ;
1908 iterator->_next = NULL ;
1909 break ;
1910 }
1911 }
1912 } else {
1913 ParkEvent * ev = iterator->_event ;
1914 iterator->TState = ObjectWaiter::TS_RUN ;
1915 OrderAccess::fence() ;
1916 ev->unpark() ;
1917 }
1918
1919 if (Policy < 4) {
1920 iterator->wait_reenter_begin(this);
1921 }
1922
1923 // _WaitSetLock protects the wait queue, not the EntryList. We could
1924 // move the add-to-EntryList operation, above, outside the critical section
1925 // protected by _WaitSetLock. In practice that's not useful. With the
1926 // exception of wait() timeouts and interrupts the monitor owner
1927 // is the only thread that grabs _WaitSetLock. There's almost no contention
1928 // on _WaitSetLock so it's not profitable to reduce the length of the
1929 // critical section.
1930 }
1931
1932 Thread::SpinRelease (&_WaitSetLock) ;
1933
1934 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1935 ObjectMonitor::_sync_Notifications->inc(Tally) ;
1936 }
1937 }
1938
1939 // -----------------------------------------------------------------------------
1940 // Adaptive Spinning Support
1941 //
1942 // Adaptive spin-then-block - rational spinning
1943 //
1944 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1945 // algorithm. On high order SMP systems it would be better to start with
1946 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1947 // a contending thread could enqueue itself on the cxq and then spin locally
1948 // on a thread-specific variable such as its ParkEvent._Event flag.
1949 // That's left as an exercise for the reader. Note that global spinning is
1950 // not problematic on Niagara, as the L2$ serves the interconnect and has both
1951 // low latency and massive bandwidth.
1952 //
1953 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1954 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1955 // (duration) or we can fix the count at approximately the duration of
1956 // a context switch and vary the frequency. Of course we could also
1957 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1958 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1959 //
1960 // This implementation varies the duration "D", where D varies with
1961 // the success rate of recent spin attempts. (D is capped at approximately
1962 // length of a round-trip context switch). The success rate for recent
1963 // spin attempts is a good predictor of the success rate of future spin
1964 // attempts. The mechanism adapts automatically to varying critical
1965 // section length (lock modality), system load and degree of parallelism.
1966 // D is maintained per-monitor in _SpinDuration and is initialized
1967 // optimistically. Spin frequency is fixed at 100%.
1968 //
1969 // Note that _SpinDuration is volatile, but we update it without locks
1970 // or atomics. The code is designed so that _SpinDuration stays within
1971 // a reasonable range even in the presence of races. The arithmetic
1972 // operations on _SpinDuration are closed over the domain of legal values,
1973 // so at worst a race will install and older but still legal value.
1974 // At the very worst this introduces some apparent non-determinism.
1975 // We might spin when we shouldn't or vice-versa, but since the spin
1976 // count are relatively short, even in the worst case, the effect is harmless.
1977 //
1978 // Care must be taken that a low "D" value does not become an
1979 // an absorbing state. Transient spinning failures -- when spinning
1980 // is overall profitable -- should not cause the system to converge
1981 // on low "D" values. We want spinning to be stable and predictable
1982 // and fairly responsive to change and at the same time we don't want
1983 // it to oscillate, become metastable, be "too" non-deterministic,
1984 // or converge on or enter undesirable stable absorbing states.
1985 //
1986 // We implement a feedback-based control system -- using past behavior
1987 // to predict future behavior. We face two issues: (a) if the
1988 // input signal is random then the spin predictor won't provide optimal
1989 // results, and (b) if the signal frequency is too high then the control
1990 // system, which has some natural response lag, will "chase" the signal.
1991 // (b) can arise from multimodal lock hold times. Transient preemption
1992 // can also result in apparent bimodal lock hold times.
1993 // Although sub-optimal, neither condition is particularly harmful, as
1994 // in the worst-case we'll spin when we shouldn't or vice-versa.
1995 // The maximum spin duration is rather short so the failure modes aren't bad.
1996 // To be conservative, I've tuned the gain in system to bias toward
1997 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1998 // "rings" or oscillates between spinning and not spinning. This happens
1999 // when spinning is just on the cusp of profitability, however, so the
2000 // situation is not dire. The state is benign -- there's no need to add
2001 // hysteresis control to damp the transition rate between spinning and
2002 // not spinning.
2003 //
2004
2005 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
2006 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
2007
2008 // Spinning: Fixed frequency (100%), vary duration
2009
2010
2011 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
2012
2013 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2014 int ctr = Knob_FixedSpin ;
2015 if (ctr != 0) {
2016 while (--ctr >= 0) {
2017 if (TryLock (Self) > 0) return 1 ;
2018 SpinPause () ;
2019 }
2020 return 0 ;
2021 }
2022
2023 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
2024 if (TryLock(Self) > 0) {
2025 // Increase _SpinDuration ...
2026 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2027 // Raising _SpurDuration to the poverty line is key.
2028 int x = _SpinDuration ;
2029 if (x < Knob_SpinLimit) {
2030 if (x < Knob_Poverty) x = Knob_Poverty ;
2031 _SpinDuration = x + Knob_BonusB ;
2032 }
2033 return 1 ;
2034 }
2035 SpinPause () ;
2036 }
2037
2038 // Admission control - verify preconditions for spinning
2039 //
2040 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2041 // becoming an absorbing state. Put another way, we spin briefly to
2042 // sample, just in case the system load, parallelism, contention, or lock
2043 // modality changed.
2044 //
2045 // Consider the following alternative:
2046 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2047 // spin attempt. "Periodically" might mean after a tally of
2048 // the # of failed spin attempts (or iterations) reaches some threshold.
2049 // This takes us into the realm of 1-out-of-N spinning, where we
2050 // hold the duration constant but vary the frequency.
2051
2052 ctr = _SpinDuration ;
2053 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2054 if (ctr <= 0) return 0 ;
2055
2056 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2057 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2058 TEVENT (Spin abort - notrunnable [TOP]);
2059 return 0 ;
2060 }
2061
2062 int MaxSpin = Knob_MaxSpinners ;
2063 if (MaxSpin >= 0) {
2064 if (_Spinner > MaxSpin) {
2065 TEVENT (Spin abort -- too many spinners) ;
2066 return 0 ;
2067 }
2068 // Slighty racy, but benign ...
2069 Adjust (&_Spinner, 1) ;
2070 }
2071
2072 // We're good to spin ... spin ingress.
2073 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2074 // when preparing to LD...CAS _owner, etc and the CAS is likely
2075 // to succeed.
2076 int hits = 0 ;
2077 int msk = 0 ;
2078 int caspty = Knob_CASPenalty ;
2079 int oxpty = Knob_OXPenalty ;
2080 int sss = Knob_SpinSetSucc ;
2081 if (sss && _succ == NULL ) _succ = Self ;
2082 Thread * prv = NULL ;
2083
2084 // There are three ways to exit the following loop:
2085 // 1. A successful spin where this thread has acquired the lock.
2086 // 2. Spin failure with prejudice
2087 // 3. Spin failure without prejudice
2088
2089 while (--ctr >= 0) {
2090
2091 // Periodic polling -- Check for pending GC
2092 // Threads may spin while they're unsafe.
2093 // We don't want spinning threads to delay the JVM from reaching
2094 // a stop-the-world safepoint or to steal cycles from GC.
2095 // If we detect a pending safepoint we abort in order that
2096 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2097 // this thread, if safe, doesn't steal cycles from GC.
2098 // This is in keeping with the "no loitering in runtime" rule.
2099 // We periodically check to see if there's a safepoint pending.
2100 if ((ctr & 0xFF) == 0) {
2101 if (SafepointSynchronize::do_call_back()) {
2102 TEVENT (Spin: safepoint) ;
2103 goto Abort ; // abrupt spin egress
2104 }
2105 if (Knob_UsePause & 1) SpinPause () ;
2106
2107 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2108 if (hits > 50 && scb != NULL) {
2109 int abend = (*scb)(SpinCallbackArgument, 0) ;
2110 }
2111 }
2112
2113 if (Knob_UsePause & 2) SpinPause() ;
2114
2115 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2116 // This is useful on classic SMP systems, but is of less utility on
2117 // N1-style CMT platforms.
2118 //
2119 // Trade-off: lock acquisition latency vs coherency bandwidth.
2120 // Lock hold times are typically short. A histogram
2121 // of successful spin attempts shows that we usually acquire
2122 // the lock early in the spin. That suggests we want to
2123 // sample _owner frequently in the early phase of the spin,
2124 // but then back-off and sample less frequently as the spin
2125 // progresses. The back-off makes a good citizen on SMP big
2126 // SMP systems. Oversampling _owner can consume excessive
2127 // coherency bandwidth. Relatedly, if we _oversample _owner we
2128 // can inadvertently interfere with the the ST m->owner=null.
2129 // executed by the lock owner.
2130 if (ctr & msk) continue ;
2131 ++hits ;
2132 if ((hits & 0xF) == 0) {
2133 // The 0xF, above, corresponds to the exponent.
2134 // Consider: (msk+1)|msk
2135 msk = ((msk << 2)|3) & BackOffMask ;
2136 }
2137
2138 // Probe _owner with TATAS
2139 // If this thread observes the monitor transition or flicker
2140 // from locked to unlocked to locked, then the odds that this
2141 // thread will acquire the lock in this spin attempt go down
2142 // considerably. The same argument applies if the CAS fails
2143 // or if we observe _owner change from one non-null value to
2144 // another non-null value. In such cases we might abort
2145 // the spin without prejudice or apply a "penalty" to the
2146 // spin count-down variable "ctr", reducing it by 100, say.
2147
2148 Thread * ox = (Thread *) _owner ;
2149 if (ox == NULL) {
2150 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2151 if (ox == NULL) {
2152 // The CAS succeeded -- this thread acquired ownership
2153 // Take care of some bookkeeping to exit spin state.
2154 if (sss && _succ == Self) {
2155 _succ = NULL ;
2156 }
2157 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2158
2159 // Increase _SpinDuration :
2160 // The spin was successful (profitable) so we tend toward
2161 // longer spin attempts in the future.
2162 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2163 // If we acquired the lock early in the spin cycle it
2164 // makes sense to increase _SpinDuration proportionally.
2165 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2166 int x = _SpinDuration ;
2167 if (x < Knob_SpinLimit) {
2168 if (x < Knob_Poverty) x = Knob_Poverty ;
2169 _SpinDuration = x + Knob_Bonus ;
2170 }
2171 return 1 ;
2172 }
2173
2174 // The CAS failed ... we can take any of the following actions:
2175 // * penalize: ctr -= Knob_CASPenalty
2176 // * exit spin with prejudice -- goto Abort;
2177 // * exit spin without prejudice.
2178 // * Since CAS is high-latency, retry again immediately.
2179 prv = ox ;
2180 TEVENT (Spin: cas failed) ;
2181 if (caspty == -2) break ;
2182 if (caspty == -1) goto Abort ;
2183 ctr -= caspty ;
2184 continue ;
2185 }
2186
2187 // Did lock ownership change hands ?
2188 if (ox != prv && prv != NULL ) {
2189 TEVENT (spin: Owner changed)
2190 if (oxpty == -2) break ;
2191 if (oxpty == -1) goto Abort ;
2192 ctr -= oxpty ;
2193 }
2194 prv = ox ;
2195
2196 // Abort the spin if the owner is not executing.
2197 // The owner must be executing in order to drop the lock.
2198 // Spinning while the owner is OFFPROC is idiocy.
2199 // Consider: ctr -= RunnablePenalty ;
2200 if (Knob_OState && NotRunnable (Self, ox)) {
2201 TEVENT (Spin abort - notrunnable);
2202 goto Abort ;
2203 }
2204 if (sss && _succ == NULL ) _succ = Self ;
2205 }
2206
2207 // Spin failed with prejudice -- reduce _SpinDuration.
2208 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2209 // AIMD is globally stable.
2210 TEVENT (Spin failure) ;
2211 {
2212 int x = _SpinDuration ;
2213 if (x > 0) {
2214 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2215 // This is globally sample and tends to damp the response.
2216 x -= Knob_Penalty ;
2217 if (x < 0) x = 0 ;
2218 _SpinDuration = x ;
2219 }
2220 }
2221
2222 Abort:
2223 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2224 if (sss && _succ == Self) {
2225 _succ = NULL ;
2226 // Invariant: after setting succ=null a contending thread
2227 // must recheck-retry _owner before parking. This usually happens
2228 // in the normal usage of TrySpin(), but it's safest
2229 // to make TrySpin() as foolproof as possible.
2230 OrderAccess::fence() ;
2231 if (TryLock(Self) > 0) return 1 ;
2232 }
2233 return 0 ;
2234 }
2235
2236 // NotRunnable() -- informed spinning
2237 //
2238 // Don't bother spinning if the owner is not eligible to drop the lock.
2239 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2240 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2241 // The thread must be runnable in order to drop the lock in timely fashion.
2242 // If the _owner is not runnable then spinning will not likely be
2243 // successful (profitable).
2244 //
2245 // Beware -- the thread referenced by _owner could have died
2246 // so a simply fetch from _owner->_thread_state might trap.
2247 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2248 // Because of the lifecycle issues the schedctl and _thread_state values
2249 // observed by NotRunnable() might be garbage. NotRunnable must
2250 // tolerate this and consider the observed _thread_state value
2251 // as advisory.
2252 //
2253 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2254 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2255 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2256 // with the LSB of _owner. Another option would be to probablistically probe
2257 // the putative _owner->TypeTag value.
2258 //
2259 // Checking _thread_state isn't perfect. Even if the thread is
2260 // in_java it might be blocked on a page-fault or have been preempted
2261 // and sitting on a ready/dispatch queue. _thread state in conjunction
2262 // with schedctl.sc_state gives us a good picture of what the
2263 // thread is doing, however.
2264 //
2265 // TODO: check schedctl.sc_state.
2266 // We'll need to use SafeFetch32() to read from the schedctl block.
2267 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2268 //
2269 // The return value from NotRunnable() is *advisory* -- the
2270 // result is based on sampling and is not necessarily coherent.
2271 // The caller must tolerate false-negative and false-positive errors.
2272 // Spinning, in general, is probabilistic anyway.
2273
2274
2275 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2276 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2277 if (!OwnerIsThread) return 0 ;
2278
2279 if (ox == NULL) return 0 ;
2280
2281 // Avoid transitive spinning ...
2282 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2283 // Immediately after T1 acquires L it's possible that T2, also
2284 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2285 // This occurs transiently after T1 acquired L but before
2286 // T1 managed to clear T1.Stalled. T2 does not need to abort
2287 // its spin in this circumstance.
2288 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2289
2290 if (BlockedOn == 1) return 1 ;
2291 if (BlockedOn != 0) {
2292 return BlockedOn != intptr_t(this) && _owner == ox ;
2293 }
2294
2295 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2296 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2297 // consider also: jst != _thread_in_Java -- but that's overspecific.
2298 return jst == _thread_blocked || jst == _thread_in_native ;
2299 }
2300
2301
2302 // -----------------------------------------------------------------------------
2303 // WaitSet management ...
2304
2305 ObjectWaiter::ObjectWaiter(Thread* thread) {
2306 _next = NULL;
2307 _prev = NULL;
2308 _notified = 0;
2309 TState = TS_RUN ;
2310 _thread = thread;
2311 _event = thread->_ParkEvent ;
2312 _active = false;
2313 assert (_event != NULL, "invariant") ;
2314 }
2315
2316 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
2317 JavaThread *jt = (JavaThread *)this->_thread;
2318 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2319 }
2320
2321 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
2322 JavaThread *jt = (JavaThread *)this->_thread;
2323 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2324 }
2325
2326 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2327 assert(node != NULL, "should not dequeue NULL node");
2328 assert(node->_prev == NULL, "node already in list");
2329 assert(node->_next == NULL, "node already in list");
2330 // put node at end of queue (circular doubly linked list)
2331 if (_WaitSet == NULL) {
2332 _WaitSet = node;
2333 node->_prev = node;
2334 node->_next = node;
2335 } else {
2336 ObjectWaiter* head = _WaitSet ;
2337 ObjectWaiter* tail = head->_prev;
2338 assert(tail->_next == head, "invariant check");
2339 tail->_next = node;
2340 head->_prev = node;
2341 node->_next = head;
2342 node->_prev = tail;
2343 }
2344 }
2345
2346 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2347 // dequeue the very first waiter
2348 ObjectWaiter* waiter = _WaitSet;
2349 if (waiter) {
2350 DequeueSpecificWaiter(waiter);
2351 }
2352 return waiter;
2353 }
2354
2355 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2356 assert(node != NULL, "should not dequeue NULL node");
2357 assert(node->_prev != NULL, "node already removed from list");
2358 assert(node->_next != NULL, "node already removed from list");
2359 // when the waiter has woken up because of interrupt,
2360 // timeout or other spurious wake-up, dequeue the
2361 // waiter from waiting list
2362 ObjectWaiter* next = node->_next;
2363 if (next == node) {
2364 assert(node->_prev == node, "invariant check");
2365 _WaitSet = NULL;
2366 } else {
2367 ObjectWaiter* prev = node->_prev;
2368 assert(prev->_next == node, "invariant check");
2369 assert(next->_prev == node, "invariant check");
2370 next->_prev = prev;
2371 prev->_next = next;
2372 if (_WaitSet == node) {
2373 _WaitSet = next;
2374 }
2375 }
2376 node->_next = NULL;
2377 node->_prev = NULL;
2378 }
2379
2380 // -----------------------------------------------------------------------------
2381 // PerfData support
2382 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ;
2383 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ;
2384 PerfCounter * ObjectMonitor::_sync_Parks = NULL ;
2385 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ;
2386 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ;
2387 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ;
2388 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ;
2389 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ;
2390 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ;
2391 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ;
2392 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ;
2393 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ;
2394 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ;
2395 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ;
2396 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ;
2397 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ;
2398 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ;
2399 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ;
2400
2401 // One-shot global initialization for the sync subsystem.
2402 // We could also defer initialization and initialize on-demand
2403 // the first time we call inflate(). Initialization would
2404 // be protected - like so many things - by the MonitorCache_lock.
2405
2406 void ObjectMonitor::Initialize () {
2407 static int InitializationCompleted = 0 ;
2408 assert (InitializationCompleted == 0, "invariant") ;
2409 InitializationCompleted = 1 ;
2410 if (UsePerfData) {
2411 EXCEPTION_MARK ;
2412 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
2413 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
2414 NEWPERFCOUNTER(_sync_Inflations) ;
2415 NEWPERFCOUNTER(_sync_Deflations) ;
2416 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
2417 NEWPERFCOUNTER(_sync_FutileWakeups) ;
2418 NEWPERFCOUNTER(_sync_Parks) ;
2419 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
2420 NEWPERFCOUNTER(_sync_Notifications) ;
2421 NEWPERFCOUNTER(_sync_SlowEnter) ;
2422 NEWPERFCOUNTER(_sync_SlowExit) ;
2423 NEWPERFCOUNTER(_sync_SlowNotify) ;
2424 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
2425 NEWPERFCOUNTER(_sync_FailedSpins) ;
2426 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
2427 NEWPERFCOUNTER(_sync_PrivateA) ;
2428 NEWPERFCOUNTER(_sync_PrivateB) ;
2429 NEWPERFCOUNTER(_sync_MonInCirculation) ;
2430 NEWPERFCOUNTER(_sync_MonScavenged) ;
2431 NEWPERFVARIABLE(_sync_MonExtant) ;
2432 #undef NEWPERFCOUNTER
2433 }
2434 }
2435
2436
2437 // Compile-time asserts
2438 // When possible, it's better to catch errors deterministically at
2439 // compile-time than at runtime. The down-side to using compile-time
2440 // asserts is that error message -- often something about negative array
2441 // indices -- is opaque.
2442
2443 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
2444
2445 void ObjectMonitor::ctAsserts() {
2446 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
2447 }
2448
2449
2450 static char * kvGet (char * kvList, const char * Key) {
2451 if (kvList == NULL) return NULL ;
2452 size_t n = strlen (Key) ;
2453 char * Search ;
2454 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2455 if (strncmp (Search, Key, n) == 0) {
2456 if (Search[n] == '=') return Search + n + 1 ;
2457 if (Search[n] == 0) return (char *) "1" ;
2458 }
2459 }
2460 return NULL ;
2461 }
2462
2463 static int kvGetInt (char * kvList, const char * Key, int Default) {
2464 char * v = kvGet (kvList, Key) ;
2465 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2466 if (Knob_ReportSettings && v != NULL) {
2467 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2468 ::fflush (stdout) ;
2469 }
2470 return rslt ;
2471 }
2472
2473 void ObjectMonitor::DeferredInitialize () {
2474 if (InitDone > 0) return ;
2475 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2476 while (InitDone != 1) ;
2477 return ;
2478 }
2479
2480 // One-shot global initialization ...
2481 // The initialization is idempotent, so we don't need locks.
2482 // In the future consider doing this via os::init_2().
2483 // SyncKnobs consist of <Key>=<Value> pairs in the style
2484 // of environment variables. Start by converting ':' to NUL.
2485
2486 if (SyncKnobs == NULL) SyncKnobs = "" ;
2487
2488 size_t sz = strlen (SyncKnobs) ;
2489 char * knobs = (char *) malloc (sz + 2) ;
2490 if (knobs == NULL) {
2491 vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
2492 guarantee (0, "invariant") ;
2493 }
2494 strcpy (knobs, SyncKnobs) ;
2495 knobs[sz+1] = 0 ;
2496 for (char * p = knobs ; *p ; p++) {
2497 if (*p == ':') *p = 0 ;
2498 }
2499
2500 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2501 SETKNOB(ReportSettings) ;
2502 SETKNOB(Verbose) ;
2503 SETKNOB(FixedSpin) ;
2504 SETKNOB(SpinLimit) ;
2505 SETKNOB(SpinBase) ;
2506 SETKNOB(SpinBackOff);
2507 SETKNOB(CASPenalty) ;
2508 SETKNOB(OXPenalty) ;
2509 SETKNOB(LogSpins) ;
2510 SETKNOB(SpinSetSucc) ;
2511 SETKNOB(SuccEnabled) ;
2512 SETKNOB(SuccRestrict) ;
2513 SETKNOB(Penalty) ;
2514 SETKNOB(Bonus) ;
2515 SETKNOB(BonusB) ;
2516 SETKNOB(Poverty) ;
2517 SETKNOB(SpinAfterFutile) ;
2518 SETKNOB(UsePause) ;
2519 SETKNOB(SpinEarly) ;
2520 SETKNOB(OState) ;
2521 SETKNOB(MaxSpinners) ;
2522 SETKNOB(PreSpin) ;
2523 SETKNOB(ExitPolicy) ;
2524 SETKNOB(QMode);
2525 SETKNOB(ResetEvent) ;
2526 SETKNOB(MoveNotifyee) ;
2527 SETKNOB(FastHSSEC) ;
2528 #undef SETKNOB
2529
2530 if (os::is_MP()) {
2531 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2532 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2533 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2534 } else {
2535 Knob_SpinLimit = 0 ;
2536 Knob_SpinBase = 0 ;
2537 Knob_PreSpin = 0 ;
2538 Knob_FixedSpin = -1 ;
2539 }
2540
2541 if (Knob_LogSpins == 0) {
2542 ObjectMonitor::_sync_FailedSpins = NULL ;
2543 }
2544
2545 free (knobs) ;
2546 OrderAccess::fence() ;
2547 InitDone = 1 ;
2548 }
2549
2550 #ifndef PRODUCT
2551 void ObjectMonitor::verify() {
2552 }
2553
2554 void ObjectMonitor::print() {
2555 }
2556 #endif