objectMonitor.cpp revision 2062:3582bf76420e
1/* 2 * Copyright (c) 1998, 2010, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25#include "precompiled.hpp" 26#include "classfile/vmSymbols.hpp" 27#include "memory/resourceArea.hpp" 28#include "oops/markOop.hpp" 29#include "oops/oop.inline.hpp" 30#include "runtime/handles.inline.hpp" 31#include "runtime/interfaceSupport.hpp" 32#include "runtime/mutexLocker.hpp" 33#include "runtime/objectMonitor.hpp" 34#include "runtime/objectMonitor.inline.hpp" 35#include "runtime/osThread.hpp" 36#include "runtime/stubRoutines.hpp" 37#include "runtime/thread.hpp" 38#include "services/threadService.hpp" 39#include "utilities/dtrace.hpp" 40#include "utilities/preserveException.hpp" 41#ifdef TARGET_OS_FAMILY_linux 42# include "os_linux.inline.hpp" 43# include "thread_linux.inline.hpp" 44#endif 45#ifdef TARGET_OS_FAMILY_solaris 46# include "os_solaris.inline.hpp" 47# include "thread_solaris.inline.hpp" 48#endif 49#ifdef TARGET_OS_FAMILY_windows 50# include "os_windows.inline.hpp" 51# include "thread_windows.inline.hpp" 52#endif 53 54#if defined(__GNUC__) && !defined(IA64) 55 // Need to inhibit inlining for older versions of GCC to avoid build-time failures 56 #define ATTR __attribute__((noinline)) 57#else 58 #define ATTR 59#endif 60 61 62#ifdef DTRACE_ENABLED 63 64// Only bother with this argument setup if dtrace is available 65// TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. 66 67HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify, 68 jlong, uintptr_t, char*, int); 69HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll, 70 jlong, uintptr_t, char*, int); 71HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter, 72 jlong, uintptr_t, char*, int); 73HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered, 74 jlong, uintptr_t, char*, int); 75HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit, 76 jlong, uintptr_t, char*, int); 77 78#define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \ 79 char* bytes = NULL; \ 80 int len = 0; \ 81 jlong jtid = SharedRuntime::get_java_tid(thread); \ 82 Symbol* klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \ 83 if (klassname != NULL) { \ 84 bytes = (char*)klassname->bytes(); \ 85 len = klassname->utf8_length(); \ 86 } 87 88#define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \ 89 { \ 90 if (DTraceMonitorProbes) { \ 91 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \ 92 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \ 93 (monitor), bytes, len, (millis)); \ 94 } \ 95 } 96 97#define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \ 98 { \ 99 if (DTraceMonitorProbes) { \ 100 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \ 101 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \ 102 (uintptr_t)(monitor), bytes, len); \ 103 } \ 104 } 105 106#else // ndef DTRACE_ENABLED 107 108#define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;} 109#define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;} 110 111#endif // ndef DTRACE_ENABLED 112 113// Tunables ... 114// The knob* variables are effectively final. Once set they should 115// never be modified hence. Consider using __read_mostly with GCC. 116 117int ObjectMonitor::Knob_Verbose = 0 ; 118int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool - 119static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins 120static int Knob_HandOff = 0 ; 121static int Knob_ReportSettings = 0 ; 122 123static int Knob_SpinBase = 0 ; // Floor AKA SpinMin 124static int Knob_SpinBackOff = 0 ; // spin-loop backoff 125static int Knob_CASPenalty = -1 ; // Penalty for failed CAS 126static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change 127static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field 128static int Knob_SpinEarly = 1 ; 129static int Knob_SuccEnabled = 1 ; // futile wake throttling 130static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one 131static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs 132static int Knob_Bonus = 100 ; // spin success bonus 133static int Knob_BonusB = 100 ; // spin success bonus 134static int Knob_Penalty = 200 ; // spin failure penalty 135static int Knob_Poverty = 1000 ; 136static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park() 137static int Knob_FixedSpin = 0 ; 138static int Knob_OState = 3 ; // Spinner checks thread state of _owner 139static int Knob_UsePause = 1 ; 140static int Knob_ExitPolicy = 0 ; 141static int Knob_PreSpin = 10 ; // 20-100 likely better 142static int Knob_ResetEvent = 0 ; 143static int BackOffMask = 0 ; 144 145static int Knob_FastHSSEC = 0 ; 146static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee 147static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline 148static volatile int InitDone = 0 ; 149 150#define TrySpin TrySpin_VaryDuration 151 152// ----------------------------------------------------------------------------- 153// Theory of operations -- Monitors lists, thread residency, etc: 154// 155// * A thread acquires ownership of a monitor by successfully 156// CAS()ing the _owner field from null to non-null. 157// 158// * Invariant: A thread appears on at most one monitor list -- 159// cxq, EntryList or WaitSet -- at any one time. 160// 161// * Contending threads "push" themselves onto the cxq with CAS 162// and then spin/park. 163// 164// * After a contending thread eventually acquires the lock it must 165// dequeue itself from either the EntryList or the cxq. 166// 167// * The exiting thread identifies and unparks an "heir presumptive" 168// tentative successor thread on the EntryList. Critically, the 169// exiting thread doesn't unlink the successor thread from the EntryList. 170// After having been unparked, the wakee will recontend for ownership of 171// the monitor. The successor (wakee) will either acquire the lock or 172// re-park itself. 173// 174// Succession is provided for by a policy of competitive handoff. 175// The exiting thread does _not_ grant or pass ownership to the 176// successor thread. (This is also referred to as "handoff" succession"). 177// Instead the exiting thread releases ownership and possibly wakes 178// a successor, so the successor can (re)compete for ownership of the lock. 179// If the EntryList is empty but the cxq is populated the exiting 180// thread will drain the cxq into the EntryList. It does so by 181// by detaching the cxq (installing null with CAS) and folding 182// the threads from the cxq into the EntryList. The EntryList is 183// doubly linked, while the cxq is singly linked because of the 184// CAS-based "push" used to enqueue recently arrived threads (RATs). 185// 186// * Concurrency invariants: 187// 188// -- only the monitor owner may access or mutate the EntryList. 189// The mutex property of the monitor itself protects the EntryList 190// from concurrent interference. 191// -- Only the monitor owner may detach the cxq. 192// 193// * The monitor entry list operations avoid locks, but strictly speaking 194// they're not lock-free. Enter is lock-free, exit is not. 195// See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html 196// 197// * The cxq can have multiple concurrent "pushers" but only one concurrent 198// detaching thread. This mechanism is immune from the ABA corruption. 199// More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 200// 201// * Taken together, the cxq and the EntryList constitute or form a 202// single logical queue of threads stalled trying to acquire the lock. 203// We use two distinct lists to improve the odds of a constant-time 204// dequeue operation after acquisition (in the ::enter() epilog) and 205// to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). 206// A key desideratum is to minimize queue & monitor metadata manipulation 207// that occurs while holding the monitor lock -- that is, we want to 208// minimize monitor lock holds times. Note that even a small amount of 209// fixed spinning will greatly reduce the # of enqueue-dequeue operations 210// on EntryList|cxq. That is, spinning relieves contention on the "inner" 211// locks and monitor metadata. 212// 213// Cxq points to the the set of Recently Arrived Threads attempting entry. 214// Because we push threads onto _cxq with CAS, the RATs must take the form of 215// a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when 216// the unlocking thread notices that EntryList is null but _cxq is != null. 217// 218// The EntryList is ordered by the prevailing queue discipline and 219// can be organized in any convenient fashion, such as a doubly-linked list or 220// a circular doubly-linked list. Critically, we want insert and delete operations 221// to operate in constant-time. If we need a priority queue then something akin 222// to Solaris' sleepq would work nicely. Viz., 223// http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 224// Queue discipline is enforced at ::exit() time, when the unlocking thread 225// drains the cxq into the EntryList, and orders or reorders the threads on the 226// EntryList accordingly. 227// 228// Barring "lock barging", this mechanism provides fair cyclic ordering, 229// somewhat similar to an elevator-scan. 230// 231// * The monitor synchronization subsystem avoids the use of native 232// synchronization primitives except for the narrow platform-specific 233// park-unpark abstraction. See the comments in os_solaris.cpp regarding 234// the semantics of park-unpark. Put another way, this monitor implementation 235// depends only on atomic operations and park-unpark. The monitor subsystem 236// manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 237// underlying OS manages the READY<->RUN transitions. 238// 239// * Waiting threads reside on the WaitSet list -- wait() puts 240// the caller onto the WaitSet. 241// 242// * notify() or notifyAll() simply transfers threads from the WaitSet to 243// either the EntryList or cxq. Subsequent exit() operations will 244// unpark the notifyee. Unparking a notifee in notify() is inefficient - 245// it's likely the notifyee would simply impale itself on the lock held 246// by the notifier. 247// 248// * An interesting alternative is to encode cxq as (List,LockByte) where 249// the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary 250// variable, like _recursions, in the scheme. The threads or Events that form 251// the list would have to be aligned in 256-byte addresses. A thread would 252// try to acquire the lock or enqueue itself with CAS, but exiting threads 253// could use a 1-0 protocol and simply STB to set the LockByte to 0. 254// Note that is is *not* word-tearing, but it does presume that full-word 255// CAS operations are coherent with intermix with STB operations. That's true 256// on most common processors. 257// 258// * See also http://blogs.sun.com/dave 259 260 261// ----------------------------------------------------------------------------- 262// Enter support 263 264bool ObjectMonitor::try_enter(Thread* THREAD) { 265 if (THREAD != _owner) { 266 if (THREAD->is_lock_owned ((address)_owner)) { 267 assert(_recursions == 0, "internal state error"); 268 _owner = THREAD ; 269 _recursions = 1 ; 270 OwnerIsThread = 1 ; 271 return true; 272 } 273 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { 274 return false; 275 } 276 return true; 277 } else { 278 _recursions++; 279 return true; 280 } 281} 282 283void ATTR ObjectMonitor::enter(TRAPS) { 284 // The following code is ordered to check the most common cases first 285 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. 286 Thread * const Self = THREAD ; 287 void * cur ; 288 289 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; 290 if (cur == NULL) { 291 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0. 292 assert (_recursions == 0 , "invariant") ; 293 assert (_owner == Self, "invariant") ; 294 // CONSIDER: set or assert OwnerIsThread == 1 295 return ; 296 } 297 298 if (cur == Self) { 299 // TODO-FIXME: check for integer overflow! BUGID 6557169. 300 _recursions ++ ; 301 return ; 302 } 303 304 if (Self->is_lock_owned ((address)cur)) { 305 assert (_recursions == 0, "internal state error"); 306 _recursions = 1 ; 307 // Commute owner from a thread-specific on-stack BasicLockObject address to 308 // a full-fledged "Thread *". 309 _owner = Self ; 310 OwnerIsThread = 1 ; 311 return ; 312 } 313 314 // We've encountered genuine contention. 315 assert (Self->_Stalled == 0, "invariant") ; 316 Self->_Stalled = intptr_t(this) ; 317 318 // Try one round of spinning *before* enqueueing Self 319 // and before going through the awkward and expensive state 320 // transitions. The following spin is strictly optional ... 321 // Note that if we acquire the monitor from an initial spin 322 // we forgo posting JVMTI events and firing DTRACE probes. 323 if (Knob_SpinEarly && TrySpin (Self) > 0) { 324 assert (_owner == Self , "invariant") ; 325 assert (_recursions == 0 , "invariant") ; 326 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; 327 Self->_Stalled = 0 ; 328 return ; 329 } 330 331 assert (_owner != Self , "invariant") ; 332 assert (_succ != Self , "invariant") ; 333 assert (Self->is_Java_thread() , "invariant") ; 334 JavaThread * jt = (JavaThread *) Self ; 335 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ; 336 assert (jt->thread_state() != _thread_blocked , "invariant") ; 337 assert (this->object() != NULL , "invariant") ; 338 assert (_count >= 0, "invariant") ; 339 340 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy(). 341 // Ensure the object-monitor relationship remains stable while there's contention. 342 Atomic::inc_ptr(&_count); 343 344 { // Change java thread status to indicate blocked on monitor enter. 345 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); 346 347 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); 348 if (JvmtiExport::should_post_monitor_contended_enter()) { 349 JvmtiExport::post_monitor_contended_enter(jt, this); 350 } 351 352 OSThreadContendState osts(Self->osthread()); 353 ThreadBlockInVM tbivm(jt); 354 355 Self->set_current_pending_monitor(this); 356 357 // TODO-FIXME: change the following for(;;) loop to straight-line code. 358 for (;;) { 359 jt->set_suspend_equivalent(); 360 // cleared by handle_special_suspend_equivalent_condition() 361 // or java_suspend_self() 362 363 EnterI (THREAD) ; 364 365 if (!ExitSuspendEquivalent(jt)) break ; 366 367 // 368 // We have acquired the contended monitor, but while we were 369 // waiting another thread suspended us. We don't want to enter 370 // the monitor while suspended because that would surprise the 371 // thread that suspended us. 372 // 373 _recursions = 0 ; 374 _succ = NULL ; 375 exit (Self) ; 376 377 jt->java_suspend_self(); 378 } 379 Self->set_current_pending_monitor(NULL); 380 } 381 382 Atomic::dec_ptr(&_count); 383 assert (_count >= 0, "invariant") ; 384 Self->_Stalled = 0 ; 385 386 // Must either set _recursions = 0 or ASSERT _recursions == 0. 387 assert (_recursions == 0 , "invariant") ; 388 assert (_owner == Self , "invariant") ; 389 assert (_succ != Self , "invariant") ; 390 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; 391 392 // The thread -- now the owner -- is back in vm mode. 393 // Report the glorious news via TI,DTrace and jvmstat. 394 // The probe effect is non-trivial. All the reportage occurs 395 // while we hold the monitor, increasing the length of the critical 396 // section. Amdahl's parallel speedup law comes vividly into play. 397 // 398 // Another option might be to aggregate the events (thread local or 399 // per-monitor aggregation) and defer reporting until a more opportune 400 // time -- such as next time some thread encounters contention but has 401 // yet to acquire the lock. While spinning that thread could 402 // spinning we could increment JVMStat counters, etc. 403 404 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); 405 if (JvmtiExport::should_post_monitor_contended_entered()) { 406 JvmtiExport::post_monitor_contended_entered(jt, this); 407 } 408 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) { 409 ObjectMonitor::_sync_ContendedLockAttempts->inc() ; 410 } 411} 412 413 414// Caveat: TryLock() is not necessarily serializing if it returns failure. 415// Callers must compensate as needed. 416 417int ObjectMonitor::TryLock (Thread * Self) { 418 for (;;) { 419 void * own = _owner ; 420 if (own != NULL) return 0 ; 421 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) { 422 // Either guarantee _recursions == 0 or set _recursions = 0. 423 assert (_recursions == 0, "invariant") ; 424 assert (_owner == Self, "invariant") ; 425 // CONSIDER: set or assert that OwnerIsThread == 1 426 return 1 ; 427 } 428 // The lock had been free momentarily, but we lost the race to the lock. 429 // Interference -- the CAS failed. 430 // We can either return -1 or retry. 431 // Retry doesn't make as much sense because the lock was just acquired. 432 if (true) return -1 ; 433 } 434} 435 436void ATTR ObjectMonitor::EnterI (TRAPS) { 437 Thread * Self = THREAD ; 438 assert (Self->is_Java_thread(), "invariant") ; 439 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ; 440 441 // Try the lock - TATAS 442 if (TryLock (Self) > 0) { 443 assert (_succ != Self , "invariant") ; 444 assert (_owner == Self , "invariant") ; 445 assert (_Responsible != Self , "invariant") ; 446 return ; 447 } 448 449 DeferredInitialize () ; 450 451 // We try one round of spinning *before* enqueueing Self. 452 // 453 // If the _owner is ready but OFFPROC we could use a YieldTo() 454 // operation to donate the remainder of this thread's quantum 455 // to the owner. This has subtle but beneficial affinity 456 // effects. 457 458 if (TrySpin (Self) > 0) { 459 assert (_owner == Self , "invariant") ; 460 assert (_succ != Self , "invariant") ; 461 assert (_Responsible != Self , "invariant") ; 462 return ; 463 } 464 465 // The Spin failed -- Enqueue and park the thread ... 466 assert (_succ != Self , "invariant") ; 467 assert (_owner != Self , "invariant") ; 468 assert (_Responsible != Self , "invariant") ; 469 470 // Enqueue "Self" on ObjectMonitor's _cxq. 471 // 472 // Node acts as a proxy for Self. 473 // As an aside, if were to ever rewrite the synchronization code mostly 474 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class 475 // Java objects. This would avoid awkward lifecycle and liveness issues, 476 // as well as eliminate a subset of ABA issues. 477 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. 478 // 479 480 ObjectWaiter node(Self) ; 481 Self->_ParkEvent->reset() ; 482 node._prev = (ObjectWaiter *) 0xBAD ; 483 node.TState = ObjectWaiter::TS_CXQ ; 484 485 // Push "Self" onto the front of the _cxq. 486 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. 487 // Note that spinning tends to reduce the rate at which threads 488 // enqueue and dequeue on EntryList|cxq. 489 ObjectWaiter * nxt ; 490 for (;;) { 491 node._next = nxt = _cxq ; 492 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ; 493 494 // Interference - the CAS failed because _cxq changed. Just retry. 495 // As an optional optimization we retry the lock. 496 if (TryLock (Self) > 0) { 497 assert (_succ != Self , "invariant") ; 498 assert (_owner == Self , "invariant") ; 499 assert (_Responsible != Self , "invariant") ; 500 return ; 501 } 502 } 503 504 // Check for cxq|EntryList edge transition to non-null. This indicates 505 // the onset of contention. While contention persists exiting threads 506 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit 507 // operations revert to the faster 1-0 mode. This enter operation may interleave 508 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we 509 // arrange for one of the contending thread to use a timed park() operations 510 // to detect and recover from the race. (Stranding is form of progress failure 511 // where the monitor is unlocked but all the contending threads remain parked). 512 // That is, at least one of the contended threads will periodically poll _owner. 513 // One of the contending threads will become the designated "Responsible" thread. 514 // The Responsible thread uses a timed park instead of a normal indefinite park 515 // operation -- it periodically wakes and checks for and recovers from potential 516 // strandings admitted by 1-0 exit operations. We need at most one Responsible 517 // thread per-monitor at any given moment. Only threads on cxq|EntryList may 518 // be responsible for a monitor. 519 // 520 // Currently, one of the contended threads takes on the added role of "Responsible". 521 // A viable alternative would be to use a dedicated "stranding checker" thread 522 // that periodically iterated over all the threads (or active monitors) and unparked 523 // successors where there was risk of stranding. This would help eliminate the 524 // timer scalability issues we see on some platforms as we'd only have one thread 525 // -- the checker -- parked on a timer. 526 527 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) { 528 // Try to assume the role of responsible thread for the monitor. 529 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } 530 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; 531 } 532 533 // The lock have been released while this thread was occupied queueing 534 // itself onto _cxq. To close the race and avoid "stranding" and 535 // progress-liveness failure we must resample-retry _owner before parking. 536 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. 537 // In this case the ST-MEMBAR is accomplished with CAS(). 538 // 539 // TODO: Defer all thread state transitions until park-time. 540 // Since state transitions are heavy and inefficient we'd like 541 // to defer the state transitions until absolutely necessary, 542 // and in doing so avoid some transitions ... 543 544 TEVENT (Inflated enter - Contention) ; 545 int nWakeups = 0 ; 546 int RecheckInterval = 1 ; 547 548 for (;;) { 549 550 if (TryLock (Self) > 0) break ; 551 assert (_owner != Self, "invariant") ; 552 553 if ((SyncFlags & 2) && _Responsible == NULL) { 554 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; 555 } 556 557 // park self 558 if (_Responsible == Self || (SyncFlags & 1)) { 559 TEVENT (Inflated enter - park TIMED) ; 560 Self->_ParkEvent->park ((jlong) RecheckInterval) ; 561 // Increase the RecheckInterval, but clamp the value. 562 RecheckInterval *= 8 ; 563 if (RecheckInterval > 1000) RecheckInterval = 1000 ; 564 } else { 565 TEVENT (Inflated enter - park UNTIMED) ; 566 Self->_ParkEvent->park() ; 567 } 568 569 if (TryLock(Self) > 0) break ; 570 571 // The lock is still contested. 572 // Keep a tally of the # of futile wakeups. 573 // Note that the counter is not protected by a lock or updated by atomics. 574 // That is by design - we trade "lossy" counters which are exposed to 575 // races during updates for a lower probe effect. 576 TEVENT (Inflated enter - Futile wakeup) ; 577 if (ObjectMonitor::_sync_FutileWakeups != NULL) { 578 ObjectMonitor::_sync_FutileWakeups->inc() ; 579 } 580 ++ nWakeups ; 581 582 // Assuming this is not a spurious wakeup we'll normally find _succ == Self. 583 // We can defer clearing _succ until after the spin completes 584 // TrySpin() must tolerate being called with _succ == Self. 585 // Try yet another round of adaptive spinning. 586 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ; 587 588 // We can find that we were unpark()ed and redesignated _succ while 589 // we were spinning. That's harmless. If we iterate and call park(), 590 // park() will consume the event and return immediately and we'll 591 // just spin again. This pattern can repeat, leaving _succ to simply 592 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks(). 593 // Alternately, we can sample fired() here, and if set, forgo spinning 594 // in the next iteration. 595 596 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) { 597 Self->_ParkEvent->reset() ; 598 OrderAccess::fence() ; 599 } 600 if (_succ == Self) _succ = NULL ; 601 602 // Invariant: after clearing _succ a thread *must* retry _owner before parking. 603 OrderAccess::fence() ; 604 } 605 606 // Egress : 607 // Self has acquired the lock -- Unlink Self from the cxq or EntryList. 608 // Normally we'll find Self on the EntryList . 609 // From the perspective of the lock owner (this thread), the 610 // EntryList is stable and cxq is prepend-only. 611 // The head of cxq is volatile but the interior is stable. 612 // In addition, Self.TState is stable. 613 614 assert (_owner == Self , "invariant") ; 615 assert (object() != NULL , "invariant") ; 616 // I'd like to write: 617 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; 618 // but as we're at a safepoint that's not safe. 619 620 UnlinkAfterAcquire (Self, &node) ; 621 if (_succ == Self) _succ = NULL ; 622 623 assert (_succ != Self, "invariant") ; 624 if (_Responsible == Self) { 625 _Responsible = NULL ; 626 // Dekker pivot-point. 627 // Consider OrderAccess::storeload() here 628 629 // We may leave threads on cxq|EntryList without a designated 630 // "Responsible" thread. This is benign. When this thread subsequently 631 // exits the monitor it can "see" such preexisting "old" threads -- 632 // threads that arrived on the cxq|EntryList before the fence, above -- 633 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads 634 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible 635 // non-null and elect a new "Responsible" timer thread. 636 // 637 // This thread executes: 638 // ST Responsible=null; MEMBAR (in enter epilog - here) 639 // LD cxq|EntryList (in subsequent exit) 640 // 641 // Entering threads in the slow/contended path execute: 642 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) 643 // The (ST cxq; MEMBAR) is accomplished with CAS(). 644 // 645 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent 646 // exit operation from floating above the ST Responsible=null. 647 // 648 // In *practice* however, EnterI() is always followed by some atomic 649 // operation such as the decrement of _count in ::enter(). Those atomics 650 // obviate the need for the explicit MEMBAR, above. 651 } 652 653 // We've acquired ownership with CAS(). 654 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. 655 // But since the CAS() this thread may have also stored into _succ, 656 // EntryList, cxq or Responsible. These meta-data updates must be 657 // visible __before this thread subsequently drops the lock. 658 // Consider what could occur if we didn't enforce this constraint -- 659 // STs to monitor meta-data and user-data could reorder with (become 660 // visible after) the ST in exit that drops ownership of the lock. 661 // Some other thread could then acquire the lock, but observe inconsistent 662 // or old monitor meta-data and heap data. That violates the JMM. 663 // To that end, the 1-0 exit() operation must have at least STST|LDST 664 // "release" barrier semantics. Specifically, there must be at least a 665 // STST|LDST barrier in exit() before the ST of null into _owner that drops 666 // the lock. The barrier ensures that changes to monitor meta-data and data 667 // protected by the lock will be visible before we release the lock, and 668 // therefore before some other thread (CPU) has a chance to acquire the lock. 669 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. 670 // 671 // Critically, any prior STs to _succ or EntryList must be visible before 672 // the ST of null into _owner in the *subsequent* (following) corresponding 673 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily 674 // execute a serializing instruction. 675 676 if (SyncFlags & 8) { 677 OrderAccess::fence() ; 678 } 679 return ; 680} 681 682// ReenterI() is a specialized inline form of the latter half of the 683// contended slow-path from EnterI(). We use ReenterI() only for 684// monitor reentry in wait(). 685// 686// In the future we should reconcile EnterI() and ReenterI(), adding 687// Knob_Reset and Knob_SpinAfterFutile support and restructuring the 688// loop accordingly. 689 690void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) { 691 assert (Self != NULL , "invariant") ; 692 assert (SelfNode != NULL , "invariant") ; 693 assert (SelfNode->_thread == Self , "invariant") ; 694 assert (_waiters > 0 , "invariant") ; 695 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ; 696 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ; 697 JavaThread * jt = (JavaThread *) Self ; 698 699 int nWakeups = 0 ; 700 for (;;) { 701 ObjectWaiter::TStates v = SelfNode->TState ; 702 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; 703 assert (_owner != Self, "invariant") ; 704 705 if (TryLock (Self) > 0) break ; 706 if (TrySpin (Self) > 0) break ; 707 708 TEVENT (Wait Reentry - parking) ; 709 710 // State transition wrappers around park() ... 711 // ReenterI() wisely defers state transitions until 712 // it's clear we must park the thread. 713 { 714 OSThreadContendState osts(Self->osthread()); 715 ThreadBlockInVM tbivm(jt); 716 717 // cleared by handle_special_suspend_equivalent_condition() 718 // or java_suspend_self() 719 jt->set_suspend_equivalent(); 720 if (SyncFlags & 1) { 721 Self->_ParkEvent->park ((jlong)1000) ; 722 } else { 723 Self->_ParkEvent->park () ; 724 } 725 726 // were we externally suspended while we were waiting? 727 for (;;) { 728 if (!ExitSuspendEquivalent (jt)) break ; 729 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } 730 jt->java_suspend_self(); 731 jt->set_suspend_equivalent(); 732 } 733 } 734 735 // Try again, but just so we distinguish between futile wakeups and 736 // successful wakeups. The following test isn't algorithmically 737 // necessary, but it helps us maintain sensible statistics. 738 if (TryLock(Self) > 0) break ; 739 740 // The lock is still contested. 741 // Keep a tally of the # of futile wakeups. 742 // Note that the counter is not protected by a lock or updated by atomics. 743 // That is by design - we trade "lossy" counters which are exposed to 744 // races during updates for a lower probe effect. 745 TEVENT (Wait Reentry - futile wakeup) ; 746 ++ nWakeups ; 747 748 // Assuming this is not a spurious wakeup we'll normally 749 // find that _succ == Self. 750 if (_succ == Self) _succ = NULL ; 751 752 // Invariant: after clearing _succ a contending thread 753 // *must* retry _owner before parking. 754 OrderAccess::fence() ; 755 756 if (ObjectMonitor::_sync_FutileWakeups != NULL) { 757 ObjectMonitor::_sync_FutileWakeups->inc() ; 758 } 759 } 760 761 // Self has acquired the lock -- Unlink Self from the cxq or EntryList . 762 // Normally we'll find Self on the EntryList. 763 // Unlinking from the EntryList is constant-time and atomic-free. 764 // From the perspective of the lock owner (this thread), the 765 // EntryList is stable and cxq is prepend-only. 766 // The head of cxq is volatile but the interior is stable. 767 // In addition, Self.TState is stable. 768 769 assert (_owner == Self, "invariant") ; 770 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; 771 UnlinkAfterAcquire (Self, SelfNode) ; 772 if (_succ == Self) _succ = NULL ; 773 assert (_succ != Self, "invariant") ; 774 SelfNode->TState = ObjectWaiter::TS_RUN ; 775 OrderAccess::fence() ; // see comments at the end of EnterI() 776} 777 778// after the thread acquires the lock in ::enter(). Equally, we could defer 779// unlinking the thread until ::exit()-time. 780 781void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode) 782{ 783 assert (_owner == Self, "invariant") ; 784 assert (SelfNode->_thread == Self, "invariant") ; 785 786 if (SelfNode->TState == ObjectWaiter::TS_ENTER) { 787 // Normal case: remove Self from the DLL EntryList . 788 // This is a constant-time operation. 789 ObjectWaiter * nxt = SelfNode->_next ; 790 ObjectWaiter * prv = SelfNode->_prev ; 791 if (nxt != NULL) nxt->_prev = prv ; 792 if (prv != NULL) prv->_next = nxt ; 793 if (SelfNode == _EntryList ) _EntryList = nxt ; 794 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ; 795 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ; 796 TEVENT (Unlink from EntryList) ; 797 } else { 798 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ; 799 // Inopportune interleaving -- Self is still on the cxq. 800 // This usually means the enqueue of self raced an exiting thread. 801 // Normally we'll find Self near the front of the cxq, so 802 // dequeueing is typically fast. If needbe we can accelerate 803 // this with some MCS/CHL-like bidirectional list hints and advisory 804 // back-links so dequeueing from the interior will normally operate 805 // in constant-time. 806 // Dequeue Self from either the head (with CAS) or from the interior 807 // with a linear-time scan and normal non-atomic memory operations. 808 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList 809 // and then unlink Self from EntryList. We have to drain eventually, 810 // so it might as well be now. 811 812 ObjectWaiter * v = _cxq ; 813 assert (v != NULL, "invariant") ; 814 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) { 815 // The CAS above can fail from interference IFF a "RAT" arrived. 816 // In that case Self must be in the interior and can no longer be 817 // at the head of cxq. 818 if (v == SelfNode) { 819 assert (_cxq != v, "invariant") ; 820 v = _cxq ; // CAS above failed - start scan at head of list 821 } 822 ObjectWaiter * p ; 823 ObjectWaiter * q = NULL ; 824 for (p = v ; p != NULL && p != SelfNode; p = p->_next) { 825 q = p ; 826 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ; 827 } 828 assert (v != SelfNode, "invariant") ; 829 assert (p == SelfNode, "Node not found on cxq") ; 830 assert (p != _cxq, "invariant") ; 831 assert (q != NULL, "invariant") ; 832 assert (q->_next == p, "invariant") ; 833 q->_next = p->_next ; 834 } 835 TEVENT (Unlink from cxq) ; 836 } 837 838 // Diagnostic hygiene ... 839 SelfNode->_prev = (ObjectWaiter *) 0xBAD ; 840 SelfNode->_next = (ObjectWaiter *) 0xBAD ; 841 SelfNode->TState = ObjectWaiter::TS_RUN ; 842} 843 844// ----------------------------------------------------------------------------- 845// Exit support 846// 847// exit() 848// ~~~~~~ 849// Note that the collector can't reclaim the objectMonitor or deflate 850// the object out from underneath the thread calling ::exit() as the 851// thread calling ::exit() never transitions to a stable state. 852// This inhibits GC, which in turn inhibits asynchronous (and 853// inopportune) reclamation of "this". 854// 855// We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; 856// There's one exception to the claim above, however. EnterI() can call 857// exit() to drop a lock if the acquirer has been externally suspended. 858// In that case exit() is called with _thread_state as _thread_blocked, 859// but the monitor's _count field is > 0, which inhibits reclamation. 860// 861// 1-0 exit 862// ~~~~~~~~ 863// ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of 864// the fast-path operators have been optimized so the common ::exit() 865// operation is 1-0. See i486.ad fast_unlock(), for instance. 866// The code emitted by fast_unlock() elides the usual MEMBAR. This 867// greatly improves latency -- MEMBAR and CAS having considerable local 868// latency on modern processors -- but at the cost of "stranding". Absent the 869// MEMBAR, a thread in fast_unlock() can race a thread in the slow 870// ::enter() path, resulting in the entering thread being stranding 871// and a progress-liveness failure. Stranding is extremely rare. 872// We use timers (timed park operations) & periodic polling to detect 873// and recover from stranding. Potentially stranded threads periodically 874// wake up and poll the lock. See the usage of the _Responsible variable. 875// 876// The CAS() in enter provides for safety and exclusion, while the CAS or 877// MEMBAR in exit provides for progress and avoids stranding. 1-0 locking 878// eliminates the CAS/MEMBAR from the exist path, but it admits stranding. 879// We detect and recover from stranding with timers. 880// 881// If a thread transiently strands it'll park until (a) another 882// thread acquires the lock and then drops the lock, at which time the 883// exiting thread will notice and unpark the stranded thread, or, (b) 884// the timer expires. If the lock is high traffic then the stranding latency 885// will be low due to (a). If the lock is low traffic then the odds of 886// stranding are lower, although the worst-case stranding latency 887// is longer. Critically, we don't want to put excessive load in the 888// platform's timer subsystem. We want to minimize both the timer injection 889// rate (timers created/sec) as well as the number of timers active at 890// any one time. (more precisely, we want to minimize timer-seconds, which is 891// the integral of the # of active timers at any instant over time). 892// Both impinge on OS scalability. Given that, at most one thread parked on 893// a monitor will use a timer. 894 895void ATTR ObjectMonitor::exit(TRAPS) { 896 Thread * Self = THREAD ; 897 if (THREAD != _owner) { 898 if (THREAD->is_lock_owned((address) _owner)) { 899 // Transmute _owner from a BasicLock pointer to a Thread address. 900 // We don't need to hold _mutex for this transition. 901 // Non-null to Non-null is safe as long as all readers can 902 // tolerate either flavor. 903 assert (_recursions == 0, "invariant") ; 904 _owner = THREAD ; 905 _recursions = 0 ; 906 OwnerIsThread = 1 ; 907 } else { 908 // NOTE: we need to handle unbalanced monitor enter/exit 909 // in native code by throwing an exception. 910 // TODO: Throw an IllegalMonitorStateException ? 911 TEVENT (Exit - Throw IMSX) ; 912 assert(false, "Non-balanced monitor enter/exit!"); 913 if (false) { 914 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); 915 } 916 return; 917 } 918 } 919 920 if (_recursions != 0) { 921 _recursions--; // this is simple recursive enter 922 TEVENT (Inflated exit - recursive) ; 923 return ; 924 } 925 926 // Invariant: after setting Responsible=null an thread must execute 927 // a MEMBAR or other serializing instruction before fetching EntryList|cxq. 928 if ((SyncFlags & 4) == 0) { 929 _Responsible = NULL ; 930 } 931 932 for (;;) { 933 assert (THREAD == _owner, "invariant") ; 934 935 936 if (Knob_ExitPolicy == 0) { 937 // release semantics: prior loads and stores from within the critical section 938 // must not float (reorder) past the following store that drops the lock. 939 // On SPARC that requires MEMBAR #loadstore|#storestore. 940 // But of course in TSO #loadstore|#storestore is not required. 941 // I'd like to write one of the following: 942 // A. OrderAccess::release() ; _owner = NULL 943 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL; 944 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both 945 // store into a _dummy variable. That store is not needed, but can result 946 // in massive wasteful coherency traffic on classic SMP systems. 947 // Instead, I use release_store(), which is implemented as just a simple 948 // ST on x64, x86 and SPARC. 949 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock 950 OrderAccess::storeload() ; // See if we need to wake a successor 951 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { 952 TEVENT (Inflated exit - simple egress) ; 953 return ; 954 } 955 TEVENT (Inflated exit - complex egress) ; 956 957 // Normally the exiting thread is responsible for ensuring succession, 958 // but if other successors are ready or other entering threads are spinning 959 // then this thread can simply store NULL into _owner and exit without 960 // waking a successor. The existence of spinners or ready successors 961 // guarantees proper succession (liveness). Responsibility passes to the 962 // ready or running successors. The exiting thread delegates the duty. 963 // More precisely, if a successor already exists this thread is absolved 964 // of the responsibility of waking (unparking) one. 965 // 966 // The _succ variable is critical to reducing futile wakeup frequency. 967 // _succ identifies the "heir presumptive" thread that has been made 968 // ready (unparked) but that has not yet run. We need only one such 969 // successor thread to guarantee progress. 970 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 971 // section 3.3 "Futile Wakeup Throttling" for details. 972 // 973 // Note that spinners in Enter() also set _succ non-null. 974 // In the current implementation spinners opportunistically set 975 // _succ so that exiting threads might avoid waking a successor. 976 // Another less appealing alternative would be for the exiting thread 977 // to drop the lock and then spin briefly to see if a spinner managed 978 // to acquire the lock. If so, the exiting thread could exit 979 // immediately without waking a successor, otherwise the exiting 980 // thread would need to dequeue and wake a successor. 981 // (Note that we'd need to make the post-drop spin short, but no 982 // shorter than the worst-case round-trip cache-line migration time. 983 // The dropped lock needs to become visible to the spinner, and then 984 // the acquisition of the lock by the spinner must become visible to 985 // the exiting thread). 986 // 987 988 // It appears that an heir-presumptive (successor) must be made ready. 989 // Only the current lock owner can manipulate the EntryList or 990 // drain _cxq, so we need to reacquire the lock. If we fail 991 // to reacquire the lock the responsibility for ensuring succession 992 // falls to the new owner. 993 // 994 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { 995 return ; 996 } 997 TEVENT (Exit - Reacquired) ; 998 } else { 999 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { 1000 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock 1001 OrderAccess::storeload() ; 1002 // Ratify the previously observed values. 1003 if (_cxq == NULL || _succ != NULL) { 1004 TEVENT (Inflated exit - simple egress) ; 1005 return ; 1006 } 1007 1008 // inopportune interleaving -- the exiting thread (this thread) 1009 // in the fast-exit path raced an entering thread in the slow-enter 1010 // path. 1011 // We have two choices: 1012 // A. Try to reacquire the lock. 1013 // If the CAS() fails return immediately, otherwise 1014 // we either restart/rerun the exit operation, or simply 1015 // fall-through into the code below which wakes a successor. 1016 // B. If the elements forming the EntryList|cxq are TSM 1017 // we could simply unpark() the lead thread and return 1018 // without having set _succ. 1019 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { 1020 TEVENT (Inflated exit - reacquired succeeded) ; 1021 return ; 1022 } 1023 TEVENT (Inflated exit - reacquired failed) ; 1024 } else { 1025 TEVENT (Inflated exit - complex egress) ; 1026 } 1027 } 1028 1029 guarantee (_owner == THREAD, "invariant") ; 1030 1031 ObjectWaiter * w = NULL ; 1032 int QMode = Knob_QMode ; 1033 1034 if (QMode == 2 && _cxq != NULL) { 1035 // QMode == 2 : cxq has precedence over EntryList. 1036 // Try to directly wake a successor from the cxq. 1037 // If successful, the successor will need to unlink itself from cxq. 1038 w = _cxq ; 1039 assert (w != NULL, "invariant") ; 1040 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ; 1041 ExitEpilog (Self, w) ; 1042 return ; 1043 } 1044 1045 if (QMode == 3 && _cxq != NULL) { 1046 // Aggressively drain cxq into EntryList at the first opportunity. 1047 // This policy ensure that recently-run threads live at the head of EntryList. 1048 // Drain _cxq into EntryList - bulk transfer. 1049 // First, detach _cxq. 1050 // The following loop is tantamount to: w = swap (&cxq, NULL) 1051 w = _cxq ; 1052 for (;;) { 1053 assert (w != NULL, "Invariant") ; 1054 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; 1055 if (u == w) break ; 1056 w = u ; 1057 } 1058 assert (w != NULL , "invariant") ; 1059 1060 ObjectWaiter * q = NULL ; 1061 ObjectWaiter * p ; 1062 for (p = w ; p != NULL ; p = p->_next) { 1063 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; 1064 p->TState = ObjectWaiter::TS_ENTER ; 1065 p->_prev = q ; 1066 q = p ; 1067 } 1068 1069 // Append the RATs to the EntryList 1070 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time. 1071 ObjectWaiter * Tail ; 1072 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ; 1073 if (Tail == NULL) { 1074 _EntryList = w ; 1075 } else { 1076 Tail->_next = w ; 1077 w->_prev = Tail ; 1078 } 1079 1080 // Fall thru into code that tries to wake a successor from EntryList 1081 } 1082 1083 if (QMode == 4 && _cxq != NULL) { 1084 // Aggressively drain cxq into EntryList at the first opportunity. 1085 // This policy ensure that recently-run threads live at the head of EntryList. 1086 1087 // Drain _cxq into EntryList - bulk transfer. 1088 // First, detach _cxq. 1089 // The following loop is tantamount to: w = swap (&cxq, NULL) 1090 w = _cxq ; 1091 for (;;) { 1092 assert (w != NULL, "Invariant") ; 1093 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; 1094 if (u == w) break ; 1095 w = u ; 1096 } 1097 assert (w != NULL , "invariant") ; 1098 1099 ObjectWaiter * q = NULL ; 1100 ObjectWaiter * p ; 1101 for (p = w ; p != NULL ; p = p->_next) { 1102 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; 1103 p->TState = ObjectWaiter::TS_ENTER ; 1104 p->_prev = q ; 1105 q = p ; 1106 } 1107 1108 // Prepend the RATs to the EntryList 1109 if (_EntryList != NULL) { 1110 q->_next = _EntryList ; 1111 _EntryList->_prev = q ; 1112 } 1113 _EntryList = w ; 1114 1115 // Fall thru into code that tries to wake a successor from EntryList 1116 } 1117 1118 w = _EntryList ; 1119 if (w != NULL) { 1120 // I'd like to write: guarantee (w->_thread != Self). 1121 // But in practice an exiting thread may find itself on the EntryList. 1122 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and 1123 // then calls exit(). Exit release the lock by setting O._owner to NULL. 1124 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The 1125 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then 1126 // release the lock "O". T2 resumes immediately after the ST of null into 1127 // _owner, above. T2 notices that the EntryList is populated, so it 1128 // reacquires the lock and then finds itself on the EntryList. 1129 // Given all that, we have to tolerate the circumstance where "w" is 1130 // associated with Self. 1131 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; 1132 ExitEpilog (Self, w) ; 1133 return ; 1134 } 1135 1136 // If we find that both _cxq and EntryList are null then just 1137 // re-run the exit protocol from the top. 1138 w = _cxq ; 1139 if (w == NULL) continue ; 1140 1141 // Drain _cxq into EntryList - bulk transfer. 1142 // First, detach _cxq. 1143 // The following loop is tantamount to: w = swap (&cxq, NULL) 1144 for (;;) { 1145 assert (w != NULL, "Invariant") ; 1146 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; 1147 if (u == w) break ; 1148 w = u ; 1149 } 1150 TEVENT (Inflated exit - drain cxq into EntryList) ; 1151 1152 assert (w != NULL , "invariant") ; 1153 assert (_EntryList == NULL , "invariant") ; 1154 1155 // Convert the LIFO SLL anchored by _cxq into a DLL. 1156 // The list reorganization step operates in O(LENGTH(w)) time. 1157 // It's critical that this step operate quickly as 1158 // "Self" still holds the outer-lock, restricting parallelism 1159 // and effectively lengthening the critical section. 1160 // Invariant: s chases t chases u. 1161 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so 1162 // we have faster access to the tail. 1163 1164 if (QMode == 1) { 1165 // QMode == 1 : drain cxq to EntryList, reversing order 1166 // We also reverse the order of the list. 1167 ObjectWaiter * s = NULL ; 1168 ObjectWaiter * t = w ; 1169 ObjectWaiter * u = NULL ; 1170 while (t != NULL) { 1171 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ; 1172 t->TState = ObjectWaiter::TS_ENTER ; 1173 u = t->_next ; 1174 t->_prev = u ; 1175 t->_next = s ; 1176 s = t; 1177 t = u ; 1178 } 1179 _EntryList = s ; 1180 assert (s != NULL, "invariant") ; 1181 } else { 1182 // QMode == 0 or QMode == 2 1183 _EntryList = w ; 1184 ObjectWaiter * q = NULL ; 1185 ObjectWaiter * p ; 1186 for (p = w ; p != NULL ; p = p->_next) { 1187 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; 1188 p->TState = ObjectWaiter::TS_ENTER ; 1189 p->_prev = q ; 1190 q = p ; 1191 } 1192 } 1193 1194 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL 1195 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). 1196 1197 // See if we can abdicate to a spinner instead of waking a thread. 1198 // A primary goal of the implementation is to reduce the 1199 // context-switch rate. 1200 if (_succ != NULL) continue; 1201 1202 w = _EntryList ; 1203 if (w != NULL) { 1204 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; 1205 ExitEpilog (Self, w) ; 1206 return ; 1207 } 1208 } 1209} 1210 1211// ExitSuspendEquivalent: 1212// A faster alternate to handle_special_suspend_equivalent_condition() 1213// 1214// handle_special_suspend_equivalent_condition() unconditionally 1215// acquires the SR_lock. On some platforms uncontended MutexLocker() 1216// operations have high latency. Note that in ::enter() we call HSSEC 1217// while holding the monitor, so we effectively lengthen the critical sections. 1218// 1219// There are a number of possible solutions: 1220// 1221// A. To ameliorate the problem we might also defer state transitions 1222// to as late as possible -- just prior to parking. 1223// Given that, we'd call HSSEC after having returned from park(), 1224// but before attempting to acquire the monitor. This is only a 1225// partial solution. It avoids calling HSSEC while holding the 1226// monitor (good), but it still increases successor reacquisition latency -- 1227// the interval between unparking a successor and the time the successor 1228// resumes and retries the lock. See ReenterI(), which defers state transitions. 1229// If we use this technique we can also avoid EnterI()-exit() loop 1230// in ::enter() where we iteratively drop the lock and then attempt 1231// to reacquire it after suspending. 1232// 1233// B. In the future we might fold all the suspend bits into a 1234// composite per-thread suspend flag and then update it with CAS(). 1235// Alternately, a Dekker-like mechanism with multiple variables 1236// would suffice: 1237// ST Self->_suspend_equivalent = false 1238// MEMBAR 1239// LD Self_>_suspend_flags 1240// 1241 1242 1243bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) { 1244 int Mode = Knob_FastHSSEC ; 1245 if (Mode && !jSelf->is_external_suspend()) { 1246 assert (jSelf->is_suspend_equivalent(), "invariant") ; 1247 jSelf->clear_suspend_equivalent() ; 1248 if (2 == Mode) OrderAccess::storeload() ; 1249 if (!jSelf->is_external_suspend()) return false ; 1250 // We raced a suspension -- fall thru into the slow path 1251 TEVENT (ExitSuspendEquivalent - raced) ; 1252 jSelf->set_suspend_equivalent() ; 1253 } 1254 return jSelf->handle_special_suspend_equivalent_condition() ; 1255} 1256 1257 1258void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) { 1259 assert (_owner == Self, "invariant") ; 1260 1261 // Exit protocol: 1262 // 1. ST _succ = wakee 1263 // 2. membar #loadstore|#storestore; 1264 // 2. ST _owner = NULL 1265 // 3. unpark(wakee) 1266 1267 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ; 1268 ParkEvent * Trigger = Wakee->_event ; 1269 1270 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. 1271 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be 1272 // out-of-scope (non-extant). 1273 Wakee = NULL ; 1274 1275 // Drop the lock 1276 OrderAccess::release_store_ptr (&_owner, NULL) ; 1277 OrderAccess::fence() ; // ST _owner vs LD in unpark() 1278 1279 if (SafepointSynchronize::do_call_back()) { 1280 TEVENT (unpark before SAFEPOINT) ; 1281 } 1282 1283 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); 1284 Trigger->unpark() ; 1285 1286 // Maintain stats and report events to JVMTI 1287 if (ObjectMonitor::_sync_Parks != NULL) { 1288 ObjectMonitor::_sync_Parks->inc() ; 1289 } 1290} 1291 1292 1293// ----------------------------------------------------------------------------- 1294// Class Loader deadlock handling. 1295// 1296// complete_exit exits a lock returning recursion count 1297// complete_exit/reenter operate as a wait without waiting 1298// complete_exit requires an inflated monitor 1299// The _owner field is not always the Thread addr even with an 1300// inflated monitor, e.g. the monitor can be inflated by a non-owning 1301// thread due to contention. 1302intptr_t ObjectMonitor::complete_exit(TRAPS) { 1303 Thread * const Self = THREAD; 1304 assert(Self->is_Java_thread(), "Must be Java thread!"); 1305 JavaThread *jt = (JavaThread *)THREAD; 1306 1307 DeferredInitialize(); 1308 1309 if (THREAD != _owner) { 1310 if (THREAD->is_lock_owned ((address)_owner)) { 1311 assert(_recursions == 0, "internal state error"); 1312 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ 1313 _recursions = 0 ; 1314 OwnerIsThread = 1 ; 1315 } 1316 } 1317 1318 guarantee(Self == _owner, "complete_exit not owner"); 1319 intptr_t save = _recursions; // record the old recursion count 1320 _recursions = 0; // set the recursion level to be 0 1321 exit (Self) ; // exit the monitor 1322 guarantee (_owner != Self, "invariant"); 1323 return save; 1324} 1325 1326// reenter() enters a lock and sets recursion count 1327// complete_exit/reenter operate as a wait without waiting 1328void ObjectMonitor::reenter(intptr_t recursions, TRAPS) { 1329 Thread * const Self = THREAD; 1330 assert(Self->is_Java_thread(), "Must be Java thread!"); 1331 JavaThread *jt = (JavaThread *)THREAD; 1332 1333 guarantee(_owner != Self, "reenter already owner"); 1334 enter (THREAD); // enter the monitor 1335 guarantee (_recursions == 0, "reenter recursion"); 1336 _recursions = recursions; 1337 return; 1338} 1339 1340 1341// ----------------------------------------------------------------------------- 1342// A macro is used below because there may already be a pending 1343// exception which should not abort the execution of the routines 1344// which use this (which is why we don't put this into check_slow and 1345// call it with a CHECK argument). 1346 1347#define CHECK_OWNER() \ 1348 do { \ 1349 if (THREAD != _owner) { \ 1350 if (THREAD->is_lock_owned((address) _owner)) { \ 1351 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \ 1352 _recursions = 0; \ 1353 OwnerIsThread = 1 ; \ 1354 } else { \ 1355 TEVENT (Throw IMSX) ; \ 1356 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \ 1357 } \ 1358 } \ 1359 } while (false) 1360 1361// check_slow() is a misnomer. It's called to simply to throw an IMSX exception. 1362// TODO-FIXME: remove check_slow() -- it's likely dead. 1363 1364void ObjectMonitor::check_slow(TRAPS) { 1365 TEVENT (check_slow - throw IMSX) ; 1366 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner"); 1367 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner"); 1368} 1369 1370static int Adjust (volatile int * adr, int dx) { 1371 int v ; 1372 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ; 1373 return v ; 1374} 1375// ----------------------------------------------------------------------------- 1376// Wait/Notify/NotifyAll 1377// 1378// Note: a subset of changes to ObjectMonitor::wait() 1379// will need to be replicated in complete_exit above 1380void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1381 Thread * const Self = THREAD ; 1382 assert(Self->is_Java_thread(), "Must be Java thread!"); 1383 JavaThread *jt = (JavaThread *)THREAD; 1384 1385 DeferredInitialize () ; 1386 1387 // Throw IMSX or IEX. 1388 CHECK_OWNER(); 1389 1390 // check for a pending interrupt 1391 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { 1392 // post monitor waited event. Note that this is past-tense, we are done waiting. 1393 if (JvmtiExport::should_post_monitor_waited()) { 1394 // Note: 'false' parameter is passed here because the 1395 // wait was not timed out due to thread interrupt. 1396 JvmtiExport::post_monitor_waited(jt, this, false); 1397 } 1398 TEVENT (Wait - Throw IEX) ; 1399 THROW(vmSymbols::java_lang_InterruptedException()); 1400 return ; 1401 } 1402 TEVENT (Wait) ; 1403 1404 assert (Self->_Stalled == 0, "invariant") ; 1405 Self->_Stalled = intptr_t(this) ; 1406 jt->set_current_waiting_monitor(this); 1407 1408 // create a node to be put into the queue 1409 // Critically, after we reset() the event but prior to park(), we must check 1410 // for a pending interrupt. 1411 ObjectWaiter node(Self); 1412 node.TState = ObjectWaiter::TS_WAIT ; 1413 Self->_ParkEvent->reset() ; 1414 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag 1415 1416 // Enter the waiting queue, which is a circular doubly linked list in this case 1417 // but it could be a priority queue or any data structure. 1418 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only 1419 // by the the owner of the monitor *except* in the case where park() 1420 // returns because of a timeout of interrupt. Contention is exceptionally rare 1421 // so we use a simple spin-lock instead of a heavier-weight blocking lock. 1422 1423 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ; 1424 AddWaiter (&node) ; 1425 Thread::SpinRelease (&_WaitSetLock) ; 1426 1427 if ((SyncFlags & 4) == 0) { 1428 _Responsible = NULL ; 1429 } 1430 intptr_t save = _recursions; // record the old recursion count 1431 _waiters++; // increment the number of waiters 1432 _recursions = 0; // set the recursion level to be 1 1433 exit (Self) ; // exit the monitor 1434 guarantee (_owner != Self, "invariant") ; 1435 1436 // As soon as the ObjectMonitor's ownership is dropped in the exit() 1437 // call above, another thread can enter() the ObjectMonitor, do the 1438 // notify(), and exit() the ObjectMonitor. If the other thread's 1439 // exit() call chooses this thread as the successor and the unpark() 1440 // call happens to occur while this thread is posting a 1441 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event 1442 // handler using RawMonitors and consuming the unpark(). 1443 // 1444 // To avoid the problem, we re-post the event. This does no harm 1445 // even if the original unpark() was not consumed because we are the 1446 // chosen successor for this monitor. 1447 if (node._notified != 0 && _succ == Self) { 1448 node._event->unpark(); 1449 } 1450 1451 // The thread is on the WaitSet list - now park() it. 1452 // On MP systems it's conceivable that a brief spin before we park 1453 // could be profitable. 1454 // 1455 // TODO-FIXME: change the following logic to a loop of the form 1456 // while (!timeout && !interrupted && _notified == 0) park() 1457 1458 int ret = OS_OK ; 1459 int WasNotified = 0 ; 1460 { // State transition wrappers 1461 OSThread* osthread = Self->osthread(); 1462 OSThreadWaitState osts(osthread, true); 1463 { 1464 ThreadBlockInVM tbivm(jt); 1465 // Thread is in thread_blocked state and oop access is unsafe. 1466 jt->set_suspend_equivalent(); 1467 1468 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) { 1469 // Intentionally empty 1470 } else 1471 if (node._notified == 0) { 1472 if (millis <= 0) { 1473 Self->_ParkEvent->park () ; 1474 } else { 1475 ret = Self->_ParkEvent->park (millis) ; 1476 } 1477 } 1478 1479 // were we externally suspended while we were waiting? 1480 if (ExitSuspendEquivalent (jt)) { 1481 // TODO-FIXME: add -- if succ == Self then succ = null. 1482 jt->java_suspend_self(); 1483 } 1484 1485 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm 1486 1487 1488 // Node may be on the WaitSet, the EntryList (or cxq), or in transition 1489 // from the WaitSet to the EntryList. 1490 // See if we need to remove Node from the WaitSet. 1491 // We use double-checked locking to avoid grabbing _WaitSetLock 1492 // if the thread is not on the wait queue. 1493 // 1494 // Note that we don't need a fence before the fetch of TState. 1495 // In the worst case we'll fetch a old-stale value of TS_WAIT previously 1496 // written by the is thread. (perhaps the fetch might even be satisfied 1497 // by a look-aside into the processor's own store buffer, although given 1498 // the length of the code path between the prior ST and this load that's 1499 // highly unlikely). If the following LD fetches a stale TS_WAIT value 1500 // then we'll acquire the lock and then re-fetch a fresh TState value. 1501 // That is, we fail toward safety. 1502 1503 if (node.TState == ObjectWaiter::TS_WAIT) { 1504 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ; 1505 if (node.TState == ObjectWaiter::TS_WAIT) { 1506 DequeueSpecificWaiter (&node) ; // unlink from WaitSet 1507 assert(node._notified == 0, "invariant"); 1508 node.TState = ObjectWaiter::TS_RUN ; 1509 } 1510 Thread::SpinRelease (&_WaitSetLock) ; 1511 } 1512 1513 // The thread is now either on off-list (TS_RUN), 1514 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). 1515 // The Node's TState variable is stable from the perspective of this thread. 1516 // No other threads will asynchronously modify TState. 1517 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ; 1518 OrderAccess::loadload() ; 1519 if (_succ == Self) _succ = NULL ; 1520 WasNotified = node._notified ; 1521 1522 // Reentry phase -- reacquire the monitor. 1523 // re-enter contended monitor after object.wait(). 1524 // retain OBJECT_WAIT state until re-enter successfully completes 1525 // Thread state is thread_in_vm and oop access is again safe, 1526 // although the raw address of the object may have changed. 1527 // (Don't cache naked oops over safepoints, of course). 1528 1529 // post monitor waited event. Note that this is past-tense, we are done waiting. 1530 if (JvmtiExport::should_post_monitor_waited()) { 1531 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); 1532 } 1533 OrderAccess::fence() ; 1534 1535 assert (Self->_Stalled != 0, "invariant") ; 1536 Self->_Stalled = 0 ; 1537 1538 assert (_owner != Self, "invariant") ; 1539 ObjectWaiter::TStates v = node.TState ; 1540 if (v == ObjectWaiter::TS_RUN) { 1541 enter (Self) ; 1542 } else { 1543 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; 1544 ReenterI (Self, &node) ; 1545 node.wait_reenter_end(this); 1546 } 1547 1548 // Self has reacquired the lock. 1549 // Lifecycle - the node representing Self must not appear on any queues. 1550 // Node is about to go out-of-scope, but even if it were immortal we wouldn't 1551 // want residual elements associated with this thread left on any lists. 1552 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ; 1553 assert (_owner == Self, "invariant") ; 1554 assert (_succ != Self , "invariant") ; 1555 } // OSThreadWaitState() 1556 1557 jt->set_current_waiting_monitor(NULL); 1558 1559 guarantee (_recursions == 0, "invariant") ; 1560 _recursions = save; // restore the old recursion count 1561 _waiters--; // decrement the number of waiters 1562 1563 // Verify a few postconditions 1564 assert (_owner == Self , "invariant") ; 1565 assert (_succ != Self , "invariant") ; 1566 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; 1567 1568 if (SyncFlags & 32) { 1569 OrderAccess::fence() ; 1570 } 1571 1572 // check if the notification happened 1573 if (!WasNotified) { 1574 // no, it could be timeout or Thread.interrupt() or both 1575 // check for interrupt event, otherwise it is timeout 1576 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { 1577 TEVENT (Wait - throw IEX from epilog) ; 1578 THROW(vmSymbols::java_lang_InterruptedException()); 1579 } 1580 } 1581 1582 // NOTE: Spurious wake up will be consider as timeout. 1583 // Monitor notify has precedence over thread interrupt. 1584} 1585 1586 1587// Consider: 1588// If the lock is cool (cxq == null && succ == null) and we're on an MP system 1589// then instead of transferring a thread from the WaitSet to the EntryList 1590// we might just dequeue a thread from the WaitSet and directly unpark() it. 1591 1592void ObjectMonitor::notify(TRAPS) { 1593 CHECK_OWNER(); 1594 if (_WaitSet == NULL) { 1595 TEVENT (Empty-Notify) ; 1596 return ; 1597 } 1598 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); 1599 1600 int Policy = Knob_MoveNotifyee ; 1601 1602 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ; 1603 ObjectWaiter * iterator = DequeueWaiter() ; 1604 if (iterator != NULL) { 1605 TEVENT (Notify1 - Transfer) ; 1606 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; 1607 guarantee (iterator->_notified == 0, "invariant") ; 1608 if (Policy != 4) { 1609 iterator->TState = ObjectWaiter::TS_ENTER ; 1610 } 1611 iterator->_notified = 1 ; 1612 1613 ObjectWaiter * List = _EntryList ; 1614 if (List != NULL) { 1615 assert (List->_prev == NULL, "invariant") ; 1616 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; 1617 assert (List != iterator, "invariant") ; 1618 } 1619 1620 if (Policy == 0) { // prepend to EntryList 1621 if (List == NULL) { 1622 iterator->_next = iterator->_prev = NULL ; 1623 _EntryList = iterator ; 1624 } else { 1625 List->_prev = iterator ; 1626 iterator->_next = List ; 1627 iterator->_prev = NULL ; 1628 _EntryList = iterator ; 1629 } 1630 } else 1631 if (Policy == 1) { // append to EntryList 1632 if (List == NULL) { 1633 iterator->_next = iterator->_prev = NULL ; 1634 _EntryList = iterator ; 1635 } else { 1636 // CONSIDER: finding the tail currently requires a linear-time walk of 1637 // the EntryList. We can make tail access constant-time by converting to 1638 // a CDLL instead of using our current DLL. 1639 ObjectWaiter * Tail ; 1640 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; 1641 assert (Tail != NULL && Tail->_next == NULL, "invariant") ; 1642 Tail->_next = iterator ; 1643 iterator->_prev = Tail ; 1644 iterator->_next = NULL ; 1645 } 1646 } else 1647 if (Policy == 2) { // prepend to cxq 1648 // prepend to cxq 1649 if (List == NULL) { 1650 iterator->_next = iterator->_prev = NULL ; 1651 _EntryList = iterator ; 1652 } else { 1653 iterator->TState = ObjectWaiter::TS_CXQ ; 1654 for (;;) { 1655 ObjectWaiter * Front = _cxq ; 1656 iterator->_next = Front ; 1657 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { 1658 break ; 1659 } 1660 } 1661 } 1662 } else 1663 if (Policy == 3) { // append to cxq 1664 iterator->TState = ObjectWaiter::TS_CXQ ; 1665 for (;;) { 1666 ObjectWaiter * Tail ; 1667 Tail = _cxq ; 1668 if (Tail == NULL) { 1669 iterator->_next = NULL ; 1670 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { 1671 break ; 1672 } 1673 } else { 1674 while (Tail->_next != NULL) Tail = Tail->_next ; 1675 Tail->_next = iterator ; 1676 iterator->_prev = Tail ; 1677 iterator->_next = NULL ; 1678 break ; 1679 } 1680 } 1681 } else { 1682 ParkEvent * ev = iterator->_event ; 1683 iterator->TState = ObjectWaiter::TS_RUN ; 1684 OrderAccess::fence() ; 1685 ev->unpark() ; 1686 } 1687 1688 if (Policy < 4) { 1689 iterator->wait_reenter_begin(this); 1690 } 1691 1692 // _WaitSetLock protects the wait queue, not the EntryList. We could 1693 // move the add-to-EntryList operation, above, outside the critical section 1694 // protected by _WaitSetLock. In practice that's not useful. With the 1695 // exception of wait() timeouts and interrupts the monitor owner 1696 // is the only thread that grabs _WaitSetLock. There's almost no contention 1697 // on _WaitSetLock so it's not profitable to reduce the length of the 1698 // critical section. 1699 } 1700 1701 Thread::SpinRelease (&_WaitSetLock) ; 1702 1703 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) { 1704 ObjectMonitor::_sync_Notifications->inc() ; 1705 } 1706} 1707 1708 1709void ObjectMonitor::notifyAll(TRAPS) { 1710 CHECK_OWNER(); 1711 ObjectWaiter* iterator; 1712 if (_WaitSet == NULL) { 1713 TEVENT (Empty-NotifyAll) ; 1714 return ; 1715 } 1716 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); 1717 1718 int Policy = Knob_MoveNotifyee ; 1719 int Tally = 0 ; 1720 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ; 1721 1722 for (;;) { 1723 iterator = DequeueWaiter () ; 1724 if (iterator == NULL) break ; 1725 TEVENT (NotifyAll - Transfer1) ; 1726 ++Tally ; 1727 1728 // Disposition - what might we do with iterator ? 1729 // a. add it directly to the EntryList - either tail or head. 1730 // b. push it onto the front of the _cxq. 1731 // For now we use (a). 1732 1733 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; 1734 guarantee (iterator->_notified == 0, "invariant") ; 1735 iterator->_notified = 1 ; 1736 if (Policy != 4) { 1737 iterator->TState = ObjectWaiter::TS_ENTER ; 1738 } 1739 1740 ObjectWaiter * List = _EntryList ; 1741 if (List != NULL) { 1742 assert (List->_prev == NULL, "invariant") ; 1743 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; 1744 assert (List != iterator, "invariant") ; 1745 } 1746 1747 if (Policy == 0) { // prepend to EntryList 1748 if (List == NULL) { 1749 iterator->_next = iterator->_prev = NULL ; 1750 _EntryList = iterator ; 1751 } else { 1752 List->_prev = iterator ; 1753 iterator->_next = List ; 1754 iterator->_prev = NULL ; 1755 _EntryList = iterator ; 1756 } 1757 } else 1758 if (Policy == 1) { // append to EntryList 1759 if (List == NULL) { 1760 iterator->_next = iterator->_prev = NULL ; 1761 _EntryList = iterator ; 1762 } else { 1763 // CONSIDER: finding the tail currently requires a linear-time walk of 1764 // the EntryList. We can make tail access constant-time by converting to 1765 // a CDLL instead of using our current DLL. 1766 ObjectWaiter * Tail ; 1767 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; 1768 assert (Tail != NULL && Tail->_next == NULL, "invariant") ; 1769 Tail->_next = iterator ; 1770 iterator->_prev = Tail ; 1771 iterator->_next = NULL ; 1772 } 1773 } else 1774 if (Policy == 2) { // prepend to cxq 1775 // prepend to cxq 1776 iterator->TState = ObjectWaiter::TS_CXQ ; 1777 for (;;) { 1778 ObjectWaiter * Front = _cxq ; 1779 iterator->_next = Front ; 1780 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { 1781 break ; 1782 } 1783 } 1784 } else 1785 if (Policy == 3) { // append to cxq 1786 iterator->TState = ObjectWaiter::TS_CXQ ; 1787 for (;;) { 1788 ObjectWaiter * Tail ; 1789 Tail = _cxq ; 1790 if (Tail == NULL) { 1791 iterator->_next = NULL ; 1792 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { 1793 break ; 1794 } 1795 } else { 1796 while (Tail->_next != NULL) Tail = Tail->_next ; 1797 Tail->_next = iterator ; 1798 iterator->_prev = Tail ; 1799 iterator->_next = NULL ; 1800 break ; 1801 } 1802 } 1803 } else { 1804 ParkEvent * ev = iterator->_event ; 1805 iterator->TState = ObjectWaiter::TS_RUN ; 1806 OrderAccess::fence() ; 1807 ev->unpark() ; 1808 } 1809 1810 if (Policy < 4) { 1811 iterator->wait_reenter_begin(this); 1812 } 1813 1814 // _WaitSetLock protects the wait queue, not the EntryList. We could 1815 // move the add-to-EntryList operation, above, outside the critical section 1816 // protected by _WaitSetLock. In practice that's not useful. With the 1817 // exception of wait() timeouts and interrupts the monitor owner 1818 // is the only thread that grabs _WaitSetLock. There's almost no contention 1819 // on _WaitSetLock so it's not profitable to reduce the length of the 1820 // critical section. 1821 } 1822 1823 Thread::SpinRelease (&_WaitSetLock) ; 1824 1825 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) { 1826 ObjectMonitor::_sync_Notifications->inc(Tally) ; 1827 } 1828} 1829 1830// ----------------------------------------------------------------------------- 1831// Adaptive Spinning Support 1832// 1833// Adaptive spin-then-block - rational spinning 1834// 1835// Note that we spin "globally" on _owner with a classic SMP-polite TATAS 1836// algorithm. On high order SMP systems it would be better to start with 1837// a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, 1838// a contending thread could enqueue itself on the cxq and then spin locally 1839// on a thread-specific variable such as its ParkEvent._Event flag. 1840// That's left as an exercise for the reader. Note that global spinning is 1841// not problematic on Niagara, as the L2$ serves the interconnect and has both 1842// low latency and massive bandwidth. 1843// 1844// Broadly, we can fix the spin frequency -- that is, the % of contended lock 1845// acquisition attempts where we opt to spin -- at 100% and vary the spin count 1846// (duration) or we can fix the count at approximately the duration of 1847// a context switch and vary the frequency. Of course we could also 1848// vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. 1849// See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html. 1850// 1851// This implementation varies the duration "D", where D varies with 1852// the success rate of recent spin attempts. (D is capped at approximately 1853// length of a round-trip context switch). The success rate for recent 1854// spin attempts is a good predictor of the success rate of future spin 1855// attempts. The mechanism adapts automatically to varying critical 1856// section length (lock modality), system load and degree of parallelism. 1857// D is maintained per-monitor in _SpinDuration and is initialized 1858// optimistically. Spin frequency is fixed at 100%. 1859// 1860// Note that _SpinDuration is volatile, but we update it without locks 1861// or atomics. The code is designed so that _SpinDuration stays within 1862// a reasonable range even in the presence of races. The arithmetic 1863// operations on _SpinDuration are closed over the domain of legal values, 1864// so at worst a race will install and older but still legal value. 1865// At the very worst this introduces some apparent non-determinism. 1866// We might spin when we shouldn't or vice-versa, but since the spin 1867// count are relatively short, even in the worst case, the effect is harmless. 1868// 1869// Care must be taken that a low "D" value does not become an 1870// an absorbing state. Transient spinning failures -- when spinning 1871// is overall profitable -- should not cause the system to converge 1872// on low "D" values. We want spinning to be stable and predictable 1873// and fairly responsive to change and at the same time we don't want 1874// it to oscillate, become metastable, be "too" non-deterministic, 1875// or converge on or enter undesirable stable absorbing states. 1876// 1877// We implement a feedback-based control system -- using past behavior 1878// to predict future behavior. We face two issues: (a) if the 1879// input signal is random then the spin predictor won't provide optimal 1880// results, and (b) if the signal frequency is too high then the control 1881// system, which has some natural response lag, will "chase" the signal. 1882// (b) can arise from multimodal lock hold times. Transient preemption 1883// can also result in apparent bimodal lock hold times. 1884// Although sub-optimal, neither condition is particularly harmful, as 1885// in the worst-case we'll spin when we shouldn't or vice-versa. 1886// The maximum spin duration is rather short so the failure modes aren't bad. 1887// To be conservative, I've tuned the gain in system to bias toward 1888// _not spinning. Relatedly, the system can sometimes enter a mode where it 1889// "rings" or oscillates between spinning and not spinning. This happens 1890// when spinning is just on the cusp of profitability, however, so the 1891// situation is not dire. The state is benign -- there's no need to add 1892// hysteresis control to damp the transition rate between spinning and 1893// not spinning. 1894// 1895 1896intptr_t ObjectMonitor::SpinCallbackArgument = 0 ; 1897int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ; 1898 1899// Spinning: Fixed frequency (100%), vary duration 1900 1901 1902int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) { 1903 1904 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. 1905 int ctr = Knob_FixedSpin ; 1906 if (ctr != 0) { 1907 while (--ctr >= 0) { 1908 if (TryLock (Self) > 0) return 1 ; 1909 SpinPause () ; 1910 } 1911 return 0 ; 1912 } 1913 1914 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) { 1915 if (TryLock(Self) > 0) { 1916 // Increase _SpinDuration ... 1917 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1918 // Raising _SpurDuration to the poverty line is key. 1919 int x = _SpinDuration ; 1920 if (x < Knob_SpinLimit) { 1921 if (x < Knob_Poverty) x = Knob_Poverty ; 1922 _SpinDuration = x + Knob_BonusB ; 1923 } 1924 return 1 ; 1925 } 1926 SpinPause () ; 1927 } 1928 1929 // Admission control - verify preconditions for spinning 1930 // 1931 // We always spin a little bit, just to prevent _SpinDuration == 0 from 1932 // becoming an absorbing state. Put another way, we spin briefly to 1933 // sample, just in case the system load, parallelism, contention, or lock 1934 // modality changed. 1935 // 1936 // Consider the following alternative: 1937 // Periodically set _SpinDuration = _SpinLimit and try a long/full 1938 // spin attempt. "Periodically" might mean after a tally of 1939 // the # of failed spin attempts (or iterations) reaches some threshold. 1940 // This takes us into the realm of 1-out-of-N spinning, where we 1941 // hold the duration constant but vary the frequency. 1942 1943 ctr = _SpinDuration ; 1944 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ; 1945 if (ctr <= 0) return 0 ; 1946 1947 if (Knob_SuccRestrict && _succ != NULL) return 0 ; 1948 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) { 1949 TEVENT (Spin abort - notrunnable [TOP]); 1950 return 0 ; 1951 } 1952 1953 int MaxSpin = Knob_MaxSpinners ; 1954 if (MaxSpin >= 0) { 1955 if (_Spinner > MaxSpin) { 1956 TEVENT (Spin abort -- too many spinners) ; 1957 return 0 ; 1958 } 1959 // Slighty racy, but benign ... 1960 Adjust (&_Spinner, 1) ; 1961 } 1962 1963 // We're good to spin ... spin ingress. 1964 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades 1965 // when preparing to LD...CAS _owner, etc and the CAS is likely 1966 // to succeed. 1967 int hits = 0 ; 1968 int msk = 0 ; 1969 int caspty = Knob_CASPenalty ; 1970 int oxpty = Knob_OXPenalty ; 1971 int sss = Knob_SpinSetSucc ; 1972 if (sss && _succ == NULL ) _succ = Self ; 1973 Thread * prv = NULL ; 1974 1975 // There are three ways to exit the following loop: 1976 // 1. A successful spin where this thread has acquired the lock. 1977 // 2. Spin failure with prejudice 1978 // 3. Spin failure without prejudice 1979 1980 while (--ctr >= 0) { 1981 1982 // Periodic polling -- Check for pending GC 1983 // Threads may spin while they're unsafe. 1984 // We don't want spinning threads to delay the JVM from reaching 1985 // a stop-the-world safepoint or to steal cycles from GC. 1986 // If we detect a pending safepoint we abort in order that 1987 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) 1988 // this thread, if safe, doesn't steal cycles from GC. 1989 // This is in keeping with the "no loitering in runtime" rule. 1990 // We periodically check to see if there's a safepoint pending. 1991 if ((ctr & 0xFF) == 0) { 1992 if (SafepointSynchronize::do_call_back()) { 1993 TEVENT (Spin: safepoint) ; 1994 goto Abort ; // abrupt spin egress 1995 } 1996 if (Knob_UsePause & 1) SpinPause () ; 1997 1998 int (*scb)(intptr_t,int) = SpinCallbackFunction ; 1999 if (hits > 50 && scb != NULL) { 2000 int abend = (*scb)(SpinCallbackArgument, 0) ; 2001 } 2002 } 2003 2004 if (Knob_UsePause & 2) SpinPause() ; 2005 2006 // Exponential back-off ... Stay off the bus to reduce coherency traffic. 2007 // This is useful on classic SMP systems, but is of less utility on 2008 // N1-style CMT platforms. 2009 // 2010 // Trade-off: lock acquisition latency vs coherency bandwidth. 2011 // Lock hold times are typically short. A histogram 2012 // of successful spin attempts shows that we usually acquire 2013 // the lock early in the spin. That suggests we want to 2014 // sample _owner frequently in the early phase of the spin, 2015 // but then back-off and sample less frequently as the spin 2016 // progresses. The back-off makes a good citizen on SMP big 2017 // SMP systems. Oversampling _owner can consume excessive 2018 // coherency bandwidth. Relatedly, if we _oversample _owner we 2019 // can inadvertently interfere with the the ST m->owner=null. 2020 // executed by the lock owner. 2021 if (ctr & msk) continue ; 2022 ++hits ; 2023 if ((hits & 0xF) == 0) { 2024 // The 0xF, above, corresponds to the exponent. 2025 // Consider: (msk+1)|msk 2026 msk = ((msk << 2)|3) & BackOffMask ; 2027 } 2028 2029 // Probe _owner with TATAS 2030 // If this thread observes the monitor transition or flicker 2031 // from locked to unlocked to locked, then the odds that this 2032 // thread will acquire the lock in this spin attempt go down 2033 // considerably. The same argument applies if the CAS fails 2034 // or if we observe _owner change from one non-null value to 2035 // another non-null value. In such cases we might abort 2036 // the spin without prejudice or apply a "penalty" to the 2037 // spin count-down variable "ctr", reducing it by 100, say. 2038 2039 Thread * ox = (Thread *) _owner ; 2040 if (ox == NULL) { 2041 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; 2042 if (ox == NULL) { 2043 // The CAS succeeded -- this thread acquired ownership 2044 // Take care of some bookkeeping to exit spin state. 2045 if (sss && _succ == Self) { 2046 _succ = NULL ; 2047 } 2048 if (MaxSpin > 0) Adjust (&_Spinner, -1) ; 2049 2050 // Increase _SpinDuration : 2051 // The spin was successful (profitable) so we tend toward 2052 // longer spin attempts in the future. 2053 // CONSIDER: factor "ctr" into the _SpinDuration adjustment. 2054 // If we acquired the lock early in the spin cycle it 2055 // makes sense to increase _SpinDuration proportionally. 2056 // Note that we don't clamp SpinDuration precisely at SpinLimit. 2057 int x = _SpinDuration ; 2058 if (x < Knob_SpinLimit) { 2059 if (x < Knob_Poverty) x = Knob_Poverty ; 2060 _SpinDuration = x + Knob_Bonus ; 2061 } 2062 return 1 ; 2063 } 2064 2065 // The CAS failed ... we can take any of the following actions: 2066 // * penalize: ctr -= Knob_CASPenalty 2067 // * exit spin with prejudice -- goto Abort; 2068 // * exit spin without prejudice. 2069 // * Since CAS is high-latency, retry again immediately. 2070 prv = ox ; 2071 TEVENT (Spin: cas failed) ; 2072 if (caspty == -2) break ; 2073 if (caspty == -1) goto Abort ; 2074 ctr -= caspty ; 2075 continue ; 2076 } 2077 2078 // Did lock ownership change hands ? 2079 if (ox != prv && prv != NULL ) { 2080 TEVENT (spin: Owner changed) 2081 if (oxpty == -2) break ; 2082 if (oxpty == -1) goto Abort ; 2083 ctr -= oxpty ; 2084 } 2085 prv = ox ; 2086 2087 // Abort the spin if the owner is not executing. 2088 // The owner must be executing in order to drop the lock. 2089 // Spinning while the owner is OFFPROC is idiocy. 2090 // Consider: ctr -= RunnablePenalty ; 2091 if (Knob_OState && NotRunnable (Self, ox)) { 2092 TEVENT (Spin abort - notrunnable); 2093 goto Abort ; 2094 } 2095 if (sss && _succ == NULL ) _succ = Self ; 2096 } 2097 2098 // Spin failed with prejudice -- reduce _SpinDuration. 2099 // TODO: Use an AIMD-like policy to adjust _SpinDuration. 2100 // AIMD is globally stable. 2101 TEVENT (Spin failure) ; 2102 { 2103 int x = _SpinDuration ; 2104 if (x > 0) { 2105 // Consider an AIMD scheme like: x -= (x >> 3) + 100 2106 // This is globally sample and tends to damp the response. 2107 x -= Knob_Penalty ; 2108 if (x < 0) x = 0 ; 2109 _SpinDuration = x ; 2110 } 2111 } 2112 2113 Abort: 2114 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ; 2115 if (sss && _succ == Self) { 2116 _succ = NULL ; 2117 // Invariant: after setting succ=null a contending thread 2118 // must recheck-retry _owner before parking. This usually happens 2119 // in the normal usage of TrySpin(), but it's safest 2120 // to make TrySpin() as foolproof as possible. 2121 OrderAccess::fence() ; 2122 if (TryLock(Self) > 0) return 1 ; 2123 } 2124 return 0 ; 2125} 2126 2127// NotRunnable() -- informed spinning 2128// 2129// Don't bother spinning if the owner is not eligible to drop the lock. 2130// Peek at the owner's schedctl.sc_state and Thread._thread_values and 2131// spin only if the owner thread is _thread_in_Java or _thread_in_vm. 2132// The thread must be runnable in order to drop the lock in timely fashion. 2133// If the _owner is not runnable then spinning will not likely be 2134// successful (profitable). 2135// 2136// Beware -- the thread referenced by _owner could have died 2137// so a simply fetch from _owner->_thread_state might trap. 2138// Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. 2139// Because of the lifecycle issues the schedctl and _thread_state values 2140// observed by NotRunnable() might be garbage. NotRunnable must 2141// tolerate this and consider the observed _thread_state value 2142// as advisory. 2143// 2144// Beware too, that _owner is sometimes a BasicLock address and sometimes 2145// a thread pointer. We differentiate the two cases with OwnerIsThread. 2146// Alternately, we might tag the type (thread pointer vs basiclock pointer) 2147// with the LSB of _owner. Another option would be to probablistically probe 2148// the putative _owner->TypeTag value. 2149// 2150// Checking _thread_state isn't perfect. Even if the thread is 2151// in_java it might be blocked on a page-fault or have been preempted 2152// and sitting on a ready/dispatch queue. _thread state in conjunction 2153// with schedctl.sc_state gives us a good picture of what the 2154// thread is doing, however. 2155// 2156// TODO: check schedctl.sc_state. 2157// We'll need to use SafeFetch32() to read from the schedctl block. 2158// See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/ 2159// 2160// The return value from NotRunnable() is *advisory* -- the 2161// result is based on sampling and is not necessarily coherent. 2162// The caller must tolerate false-negative and false-positive errors. 2163// Spinning, in general, is probabilistic anyway. 2164 2165 2166int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) { 2167 // Check either OwnerIsThread or ox->TypeTag == 2BAD. 2168 if (!OwnerIsThread) return 0 ; 2169 2170 if (ox == NULL) return 0 ; 2171 2172 // Avoid transitive spinning ... 2173 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. 2174 // Immediately after T1 acquires L it's possible that T2, also 2175 // spinning on L, will see L.Owner=T1 and T1._Stalled=L. 2176 // This occurs transiently after T1 acquired L but before 2177 // T1 managed to clear T1.Stalled. T2 does not need to abort 2178 // its spin in this circumstance. 2179 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ; 2180 2181 if (BlockedOn == 1) return 1 ; 2182 if (BlockedOn != 0) { 2183 return BlockedOn != intptr_t(this) && _owner == ox ; 2184 } 2185 2186 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ; 2187 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ; 2188 // consider also: jst != _thread_in_Java -- but that's overspecific. 2189 return jst == _thread_blocked || jst == _thread_in_native ; 2190} 2191 2192 2193// ----------------------------------------------------------------------------- 2194// WaitSet management ... 2195 2196ObjectWaiter::ObjectWaiter(Thread* thread) { 2197 _next = NULL; 2198 _prev = NULL; 2199 _notified = 0; 2200 TState = TS_RUN ; 2201 _thread = thread; 2202 _event = thread->_ParkEvent ; 2203 _active = false; 2204 assert (_event != NULL, "invariant") ; 2205} 2206 2207void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) { 2208 JavaThread *jt = (JavaThread *)this->_thread; 2209 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); 2210} 2211 2212void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) { 2213 JavaThread *jt = (JavaThread *)this->_thread; 2214 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); 2215} 2216 2217inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { 2218 assert(node != NULL, "should not dequeue NULL node"); 2219 assert(node->_prev == NULL, "node already in list"); 2220 assert(node->_next == NULL, "node already in list"); 2221 // put node at end of queue (circular doubly linked list) 2222 if (_WaitSet == NULL) { 2223 _WaitSet = node; 2224 node->_prev = node; 2225 node->_next = node; 2226 } else { 2227 ObjectWaiter* head = _WaitSet ; 2228 ObjectWaiter* tail = head->_prev; 2229 assert(tail->_next == head, "invariant check"); 2230 tail->_next = node; 2231 head->_prev = node; 2232 node->_next = head; 2233 node->_prev = tail; 2234 } 2235} 2236 2237inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { 2238 // dequeue the very first waiter 2239 ObjectWaiter* waiter = _WaitSet; 2240 if (waiter) { 2241 DequeueSpecificWaiter(waiter); 2242 } 2243 return waiter; 2244} 2245 2246inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { 2247 assert(node != NULL, "should not dequeue NULL node"); 2248 assert(node->_prev != NULL, "node already removed from list"); 2249 assert(node->_next != NULL, "node already removed from list"); 2250 // when the waiter has woken up because of interrupt, 2251 // timeout or other spurious wake-up, dequeue the 2252 // waiter from waiting list 2253 ObjectWaiter* next = node->_next; 2254 if (next == node) { 2255 assert(node->_prev == node, "invariant check"); 2256 _WaitSet = NULL; 2257 } else { 2258 ObjectWaiter* prev = node->_prev; 2259 assert(prev->_next == node, "invariant check"); 2260 assert(next->_prev == node, "invariant check"); 2261 next->_prev = prev; 2262 prev->_next = next; 2263 if (_WaitSet == node) { 2264 _WaitSet = next; 2265 } 2266 } 2267 node->_next = NULL; 2268 node->_prev = NULL; 2269} 2270 2271// ----------------------------------------------------------------------------- 2272// PerfData support 2273PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ; 2274PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ; 2275PerfCounter * ObjectMonitor::_sync_Parks = NULL ; 2276PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ; 2277PerfCounter * ObjectMonitor::_sync_Notifications = NULL ; 2278PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ; 2279PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ; 2280PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ; 2281PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ; 2282PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ; 2283PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ; 2284PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ; 2285PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ; 2286PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ; 2287PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ; 2288PerfCounter * ObjectMonitor::_sync_Inflations = NULL ; 2289PerfCounter * ObjectMonitor::_sync_Deflations = NULL ; 2290PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ; 2291 2292// One-shot global initialization for the sync subsystem. 2293// We could also defer initialization and initialize on-demand 2294// the first time we call inflate(). Initialization would 2295// be protected - like so many things - by the MonitorCache_lock. 2296 2297void ObjectMonitor::Initialize () { 2298 static int InitializationCompleted = 0 ; 2299 assert (InitializationCompleted == 0, "invariant") ; 2300 InitializationCompleted = 1 ; 2301 if (UsePerfData) { 2302 EXCEPTION_MARK ; 2303 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); } 2304 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); } 2305 NEWPERFCOUNTER(_sync_Inflations) ; 2306 NEWPERFCOUNTER(_sync_Deflations) ; 2307 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ; 2308 NEWPERFCOUNTER(_sync_FutileWakeups) ; 2309 NEWPERFCOUNTER(_sync_Parks) ; 2310 NEWPERFCOUNTER(_sync_EmptyNotifications) ; 2311 NEWPERFCOUNTER(_sync_Notifications) ; 2312 NEWPERFCOUNTER(_sync_SlowEnter) ; 2313 NEWPERFCOUNTER(_sync_SlowExit) ; 2314 NEWPERFCOUNTER(_sync_SlowNotify) ; 2315 NEWPERFCOUNTER(_sync_SlowNotifyAll) ; 2316 NEWPERFCOUNTER(_sync_FailedSpins) ; 2317 NEWPERFCOUNTER(_sync_SuccessfulSpins) ; 2318 NEWPERFCOUNTER(_sync_PrivateA) ; 2319 NEWPERFCOUNTER(_sync_PrivateB) ; 2320 NEWPERFCOUNTER(_sync_MonInCirculation) ; 2321 NEWPERFCOUNTER(_sync_MonScavenged) ; 2322 NEWPERFVARIABLE(_sync_MonExtant) ; 2323 #undef NEWPERFCOUNTER 2324 } 2325} 2326 2327 2328// Compile-time asserts 2329// When possible, it's better to catch errors deterministically at 2330// compile-time than at runtime. The down-side to using compile-time 2331// asserts is that error message -- often something about negative array 2332// indices -- is opaque. 2333 2334#define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); } 2335 2336void ObjectMonitor::ctAsserts() { 2337 CTASSERT(offset_of (ObjectMonitor, _header) == 0); 2338} 2339 2340 2341static char * kvGet (char * kvList, const char * Key) { 2342 if (kvList == NULL) return NULL ; 2343 size_t n = strlen (Key) ; 2344 char * Search ; 2345 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) { 2346 if (strncmp (Search, Key, n) == 0) { 2347 if (Search[n] == '=') return Search + n + 1 ; 2348 if (Search[n] == 0) return (char *) "1" ; 2349 } 2350 } 2351 return NULL ; 2352} 2353 2354static int kvGetInt (char * kvList, const char * Key, int Default) { 2355 char * v = kvGet (kvList, Key) ; 2356 int rslt = v ? ::strtol (v, NULL, 0) : Default ; 2357 if (Knob_ReportSettings && v != NULL) { 2358 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ; 2359 ::fflush (stdout) ; 2360 } 2361 return rslt ; 2362} 2363 2364void ObjectMonitor::DeferredInitialize () { 2365 if (InitDone > 0) return ; 2366 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) { 2367 while (InitDone != 1) ; 2368 return ; 2369 } 2370 2371 // One-shot global initialization ... 2372 // The initialization is idempotent, so we don't need locks. 2373 // In the future consider doing this via os::init_2(). 2374 // SyncKnobs consist of <Key>=<Value> pairs in the style 2375 // of environment variables. Start by converting ':' to NUL. 2376 2377 if (SyncKnobs == NULL) SyncKnobs = "" ; 2378 2379 size_t sz = strlen (SyncKnobs) ; 2380 char * knobs = (char *) malloc (sz + 2) ; 2381 if (knobs == NULL) { 2382 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ; 2383 guarantee (0, "invariant") ; 2384 } 2385 strcpy (knobs, SyncKnobs) ; 2386 knobs[sz+1] = 0 ; 2387 for (char * p = knobs ; *p ; p++) { 2388 if (*p == ':') *p = 0 ; 2389 } 2390 2391 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); } 2392 SETKNOB(ReportSettings) ; 2393 SETKNOB(Verbose) ; 2394 SETKNOB(FixedSpin) ; 2395 SETKNOB(SpinLimit) ; 2396 SETKNOB(SpinBase) ; 2397 SETKNOB(SpinBackOff); 2398 SETKNOB(CASPenalty) ; 2399 SETKNOB(OXPenalty) ; 2400 SETKNOB(LogSpins) ; 2401 SETKNOB(SpinSetSucc) ; 2402 SETKNOB(SuccEnabled) ; 2403 SETKNOB(SuccRestrict) ; 2404 SETKNOB(Penalty) ; 2405 SETKNOB(Bonus) ; 2406 SETKNOB(BonusB) ; 2407 SETKNOB(Poverty) ; 2408 SETKNOB(SpinAfterFutile) ; 2409 SETKNOB(UsePause) ; 2410 SETKNOB(SpinEarly) ; 2411 SETKNOB(OState) ; 2412 SETKNOB(MaxSpinners) ; 2413 SETKNOB(PreSpin) ; 2414 SETKNOB(ExitPolicy) ; 2415 SETKNOB(QMode); 2416 SETKNOB(ResetEvent) ; 2417 SETKNOB(MoveNotifyee) ; 2418 SETKNOB(FastHSSEC) ; 2419 #undef SETKNOB 2420 2421 if (os::is_MP()) { 2422 BackOffMask = (1 << Knob_SpinBackOff) - 1 ; 2423 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ; 2424 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1) 2425 } else { 2426 Knob_SpinLimit = 0 ; 2427 Knob_SpinBase = 0 ; 2428 Knob_PreSpin = 0 ; 2429 Knob_FixedSpin = -1 ; 2430 } 2431 2432 if (Knob_LogSpins == 0) { 2433 ObjectMonitor::_sync_FailedSpins = NULL ; 2434 } 2435 2436 free (knobs) ; 2437 OrderAccess::fence() ; 2438 InitDone = 1 ; 2439} 2440 2441#ifndef PRODUCT 2442void ObjectMonitor::verify() { 2443} 2444 2445void ObjectMonitor::print() { 2446} 2447#endif 2448