1 /* 2 * Copyright (c) 1998, 2019, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #include "precompiled.hpp" 26 #include "classfile/vmSymbols.hpp" 27 #include "jfr/jfrEvents.hpp" 28 #include "jfr/support/jfrThreadId.hpp" 29 #include "memory/allocation.inline.hpp" 30 #include "memory/resourceArea.hpp" 31 #include "oops/markWord.hpp" 32 #include "oops/oop.inline.hpp" 33 #include "runtime/atomic.hpp" 34 #include "runtime/handles.inline.hpp" 35 #include "runtime/interfaceSupport.inline.hpp" 36 #include "runtime/mutexLocker.hpp" 37 #include "runtime/objectMonitor.hpp" 38 #include "runtime/objectMonitor.inline.hpp" 39 #include "runtime/orderAccess.hpp" 40 #include "runtime/osThread.hpp" 41 #include "runtime/safepointMechanism.inline.hpp" 42 #include "runtime/sharedRuntime.hpp" 43 #include "runtime/stubRoutines.hpp" 44 #include "runtime/thread.inline.hpp" 45 #include "services/threadService.hpp" 46 #include "utilities/dtrace.hpp" 47 #include "utilities/macros.hpp" 48 #include "utilities/preserveException.hpp" 49 #if INCLUDE_JFR 50 #include "jfr/support/jfrFlush.hpp" 51 #endif 52 53 #ifdef DTRACE_ENABLED 54 55 // Only bother with this argument setup if dtrace is available 56 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. 57 58 59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ 60 char* bytes = NULL; \ 61 int len = 0; \ 62 jlong jtid = SharedRuntime::get_java_tid(thread); \ 63 Symbol* klassname = ((oop)obj)->klass()->name(); \ 64 if (klassname != NULL) { \ 65 bytes = (char*)klassname->bytes(); \ 66 len = klassname->utf8_length(); \ 67 } 68 69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ 70 { \ 71 if (DTraceMonitorProbes) { \ 72 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 73 HOTSPOT_MONITOR_WAIT(jtid, \ 74 (monitor), bytes, len, (millis)); \ 75 } \ 76 } 77 78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER 79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED 80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT 81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY 82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL 83 84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ 85 { \ 86 if (DTraceMonitorProbes) { \ 87 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 88 HOTSPOT_MONITOR_##probe(jtid, \ 89 (uintptr_t)(monitor), bytes, len); \ 90 } \ 91 } 92 93 #else // ndef DTRACE_ENABLED 94 95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} 96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} 97 98 #endif // ndef DTRACE_ENABLED 99 100 // Tunables ... 101 // The knob* variables are effectively final. Once set they should 102 // never be modified hence. Consider using __read_mostly with GCC. 103 104 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool - 105 106 static int Knob_Bonus = 100; // spin success bonus 107 static int Knob_BonusB = 100; // spin success bonus 108 static int Knob_Penalty = 200; // spin failure penalty 109 static int Knob_Poverty = 1000; 110 static int Knob_FixedSpin = 0; 111 static int Knob_PreSpin = 10; // 20-100 likely better 112 113 DEBUG_ONLY(static volatile bool InitDone = false;) 114 115 // ----------------------------------------------------------------------------- 116 // Theory of operations -- Monitors lists, thread residency, etc: 117 // 118 // * A thread acquires ownership of a monitor by successfully 119 // CAS()ing the _owner field from null to non-null. 120 // 121 // * Invariant: A thread appears on at most one monitor list -- 122 // cxq, EntryList or WaitSet -- at any one time. 123 // 124 // * Contending threads "push" themselves onto the cxq with CAS 125 // and then spin/park. 126 // 127 // * After a contending thread eventually acquires the lock it must 128 // dequeue itself from either the EntryList or the cxq. 129 // 130 // * The exiting thread identifies and unparks an "heir presumptive" 131 // tentative successor thread on the EntryList. Critically, the 132 // exiting thread doesn't unlink the successor thread from the EntryList. 133 // After having been unparked, the wakee will recontend for ownership of 134 // the monitor. The successor (wakee) will either acquire the lock or 135 // re-park itself. 136 // 137 // Succession is provided for by a policy of competitive handoff. 138 // The exiting thread does _not_ grant or pass ownership to the 139 // successor thread. (This is also referred to as "handoff" succession"). 140 // Instead the exiting thread releases ownership and possibly wakes 141 // a successor, so the successor can (re)compete for ownership of the lock. 142 // If the EntryList is empty but the cxq is populated the exiting 143 // thread will drain the cxq into the EntryList. It does so by 144 // by detaching the cxq (installing null with CAS) and folding 145 // the threads from the cxq into the EntryList. The EntryList is 146 // doubly linked, while the cxq is singly linked because of the 147 // CAS-based "push" used to enqueue recently arrived threads (RATs). 148 // 149 // * Concurrency invariants: 150 // 151 // -- only the monitor owner may access or mutate the EntryList. 152 // The mutex property of the monitor itself protects the EntryList 153 // from concurrent interference. 154 // -- Only the monitor owner may detach the cxq. 155 // 156 // * The monitor entry list operations avoid locks, but strictly speaking 157 // they're not lock-free. Enter is lock-free, exit is not. 158 // For a description of 'Methods and apparatus providing non-blocking access 159 // to a resource,' see U.S. Pat. No. 7844973. 160 // 161 // * The cxq can have multiple concurrent "pushers" but only one concurrent 162 // detaching thread. This mechanism is immune from the ABA corruption. 163 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 164 // 165 // * Taken together, the cxq and the EntryList constitute or form a 166 // single logical queue of threads stalled trying to acquire the lock. 167 // We use two distinct lists to improve the odds of a constant-time 168 // dequeue operation after acquisition (in the ::enter() epilogue) and 169 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). 170 // A key desideratum is to minimize queue & monitor metadata manipulation 171 // that occurs while holding the monitor lock -- that is, we want to 172 // minimize monitor lock holds times. Note that even a small amount of 173 // fixed spinning will greatly reduce the # of enqueue-dequeue operations 174 // on EntryList|cxq. That is, spinning relieves contention on the "inner" 175 // locks and monitor metadata. 176 // 177 // Cxq points to the set of Recently Arrived Threads attempting entry. 178 // Because we push threads onto _cxq with CAS, the RATs must take the form of 179 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when 180 // the unlocking thread notices that EntryList is null but _cxq is != null. 181 // 182 // The EntryList is ordered by the prevailing queue discipline and 183 // can be organized in any convenient fashion, such as a doubly-linked list or 184 // a circular doubly-linked list. Critically, we want insert and delete operations 185 // to operate in constant-time. If we need a priority queue then something akin 186 // to Solaris' sleepq would work nicely. Viz., 187 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 188 // Queue discipline is enforced at ::exit() time, when the unlocking thread 189 // drains the cxq into the EntryList, and orders or reorders the threads on the 190 // EntryList accordingly. 191 // 192 // Barring "lock barging", this mechanism provides fair cyclic ordering, 193 // somewhat similar to an elevator-scan. 194 // 195 // * The monitor synchronization subsystem avoids the use of native 196 // synchronization primitives except for the narrow platform-specific 197 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 198 // the semantics of park-unpark. Put another way, this monitor implementation 199 // depends only on atomic operations and park-unpark. The monitor subsystem 200 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 201 // underlying OS manages the READY<->RUN transitions. 202 // 203 // * Waiting threads reside on the WaitSet list -- wait() puts 204 // the caller onto the WaitSet. 205 // 206 // * notify() or notifyAll() simply transfers threads from the WaitSet to 207 // either the EntryList or cxq. Subsequent exit() operations will 208 // unpark the notifyee. Unparking a notifee in notify() is inefficient - 209 // it's likely the notifyee would simply impale itself on the lock held 210 // by the notifier. 211 // 212 // * An interesting alternative is to encode cxq as (List,LockByte) where 213 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary 214 // variable, like _recursions, in the scheme. The threads or Events that form 215 // the list would have to be aligned in 256-byte addresses. A thread would 216 // try to acquire the lock or enqueue itself with CAS, but exiting threads 217 // could use a 1-0 protocol and simply STB to set the LockByte to 0. 218 // Note that is is *not* word-tearing, but it does presume that full-word 219 // CAS operations are coherent with intermix with STB operations. That's true 220 // on most common processors. 221 // 222 // * See also http://blogs.sun.com/dave 223 224 225 void* ObjectMonitor::operator new (size_t size) throw() { 226 return AllocateHeap(size, mtInternal); 227 } 228 void* ObjectMonitor::operator new[] (size_t size) throw() { 229 return operator new (size); 230 } 231 void ObjectMonitor::operator delete(void* p) { 232 FreeHeap(p); 233 } 234 void ObjectMonitor::operator delete[] (void *p) { 235 operator delete(p); 236 } 237 238 // ----------------------------------------------------------------------------- 239 // Enter support 240 241 void ObjectMonitor::enter(TRAPS) { 242 jint l_ref_count = ref_count(); 243 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count()); 244 245 // The following code is ordered to check the most common cases first 246 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. 247 Thread * const Self = THREAD; 248 249 void* cur = try_set_owner_from(Self, NULL); 250 if (cur == NULL) { 251 assert(_recursions == 0, "invariant"); 252 return; 253 } 254 255 if (cur == Self) { 256 // TODO-FIXME: check for integer overflow! BUGID 6557169. 257 _recursions++; 258 return; 259 } 260 261 if (Self->is_lock_owned((address)cur)) { 262 assert(_recursions == 0, "internal state error"); 263 _recursions = 1; 264 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*. 265 return; 266 } 267 268 if (AsyncDeflateIdleMonitors && 269 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) { 270 // The deflation protocol finished the first part (setting owner), 271 // but it failed the second part (making ref_count negative) and 272 // bailed. Or the ObjectMonitor was async deflated and reused. 273 // Acquired the monitor. 274 assert(_recursions == 0, "invariant"); 275 return; 276 } 277 278 // We've encountered genuine contention. 279 assert(Self->_Stalled == 0, "invariant"); 280 Self->_Stalled = intptr_t(this); 281 282 // Try one round of spinning *before* enqueueing Self 283 // and before going through the awkward and expensive state 284 // transitions. The following spin is strictly optional ... 285 // Note that if we acquire the monitor from an initial spin 286 // we forgo posting JVMTI events and firing DTRACE probes. 287 if (TrySpin(Self) > 0) { 288 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner)); 289 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions); 290 assert(((oop)object())->mark() == markWord::encode(this), 291 "object mark must match encoded this: mark=" INTPTR_FORMAT 292 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(), 293 markWord::encode(this).value()); 294 Self->_Stalled = 0; 295 return; 296 } 297 298 assert(_owner != Self, "invariant"); 299 assert(_succ != Self, "invariant"); 300 assert(Self->is_Java_thread(), "invariant"); 301 JavaThread * jt = (JavaThread *) Self; 302 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 303 assert(jt->thread_state() != _thread_blocked, "invariant"); 304 assert(this->object() != NULL, "invariant"); 305 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions); 306 307 // Prevent deflation. See ObjectSynchronizer::deflate_monitor(), 308 // ObjectSynchronizer::deflate_monitor_using_JT() and is_busy(). 309 // Ensure the object <-> monitor relationship remains stable while 310 // there's contention. 311 Atomic::add(1, &_contentions); 312 313 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);) 314 EventJavaMonitorEnter event; 315 if (event.should_commit()) { 316 event.set_monitorClass(((oop)this->object())->klass()); 317 event.set_address((uintptr_t)(this->object_addr())); 318 } 319 320 { // Change java thread status to indicate blocked on monitor enter. 321 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); 322 323 Self->set_current_pending_monitor(this); 324 325 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); 326 if (JvmtiExport::should_post_monitor_contended_enter()) { 327 JvmtiExport::post_monitor_contended_enter(jt, this); 328 329 // The current thread does not yet own the monitor and does not 330 // yet appear on any queues that would get it made the successor. 331 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event 332 // handler cannot accidentally consume an unpark() meant for the 333 // ParkEvent associated with this ObjectMonitor. 334 } 335 336 OSThreadContendState osts(Self->osthread()); 337 ThreadBlockInVM tbivm(jt); 338 339 // TODO-FIXME: change the following for(;;) loop to straight-line code. 340 for (;;) { 341 jt->set_suspend_equivalent(); 342 // cleared by handle_special_suspend_equivalent_condition() 343 // or java_suspend_self() 344 345 EnterI(THREAD); 346 347 if (!ExitSuspendEquivalent(jt)) break; 348 349 // We have acquired the contended monitor, but while we were 350 // waiting another thread suspended us. We don't want to enter 351 // the monitor while suspended because that would surprise the 352 // thread that suspended us. 353 // 354 _recursions = 0; 355 _succ = NULL; 356 exit(false, Self); 357 358 jt->java_suspend_self(); 359 } 360 Self->set_current_pending_monitor(NULL); 361 362 // We cleared the pending monitor info since we've just gotten past 363 // the enter-check-for-suspend dance and we now own the monitor free 364 // and clear, i.e., it is no longer pending. The ThreadBlockInVM 365 // destructor can go to a safepoint at the end of this block. If we 366 // do a thread dump during that safepoint, then this thread will show 367 // as having "-locked" the monitor, but the OS and java.lang.Thread 368 // states will still report that the thread is blocked trying to 369 // acquire it. 370 } 371 372 Atomic::dec(&_contentions); 373 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions); 374 Self->_Stalled = 0; 375 376 // Must either set _recursions = 0 or ASSERT _recursions == 0. 377 assert(_recursions == 0, "invariant"); 378 assert(_owner == Self, "invariant"); 379 assert(_succ != Self, "invariant"); 380 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 381 382 // The thread -- now the owner -- is back in vm mode. 383 // Report the glorious news via TI,DTrace and jvmstat. 384 // The probe effect is non-trivial. All the reportage occurs 385 // while we hold the monitor, increasing the length of the critical 386 // section. Amdahl's parallel speedup law comes vividly into play. 387 // 388 // Another option might be to aggregate the events (thread local or 389 // per-monitor aggregation) and defer reporting until a more opportune 390 // time -- such as next time some thread encounters contention but has 391 // yet to acquire the lock. While spinning that thread could 392 // spinning we could increment JVMStat counters, etc. 393 394 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); 395 if (JvmtiExport::should_post_monitor_contended_entered()) { 396 JvmtiExport::post_monitor_contended_entered(jt, this); 397 398 // The current thread already owns the monitor and is not going to 399 // call park() for the remainder of the monitor enter protocol. So 400 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED 401 // event handler consumed an unpark() issued by the thread that 402 // just exited the monitor. 403 } 404 if (event.should_commit()) { 405 event.set_previousOwner((uintptr_t)_previous_owner_tid); 406 event.commit(); 407 } 408 OM_PERFDATA_OP(ContendedLockAttempts, inc()); 409 } 410 411 // Caveat: TryLock() is not necessarily serializing if it returns failure. 412 // Callers must compensate as needed. 413 414 int ObjectMonitor::TryLock(Thread * Self) { 415 void * own = _owner; 416 if (own != NULL) return 0; 417 if (try_set_owner_from(Self, NULL) == NULL) { 418 assert(_recursions == 0, "invariant"); 419 return 1; 420 } 421 // The lock had been free momentarily, but we lost the race to the lock. 422 // Interference -- the CAS failed. 423 // We can either return -1 or retry. 424 // Retry doesn't make as much sense because the lock was just acquired. 425 return -1; 426 } 427 428 // Install the displaced mark word (dmw) of a deflating ObjectMonitor 429 // into the header of the object associated with the monitor. This 430 // idempotent method is called by a thread that is deflating a 431 // monitor and by other threads that have detected a race with the 432 // deflation process. 433 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) { 434 // This function must only be called when (owner == DEFLATER_MARKER 435 // && ref_count <= 0), but we can't guarantee that here because 436 // those values could change when the ObjectMonitor gets moved from 437 // the global free list to a per-thread free list. 438 439 guarantee(obj != NULL, "must be non-NULL"); 440 if (object() != obj) { 441 // ObjectMonitor's object ref no longer refers to the target object 442 // so the object's header has already been restored. 443 return; 444 } 445 446 markWord dmw = header(); 447 if (dmw.value() == 0) { 448 // ObjectMonitor's header/dmw has been cleared so the ObjectMonitor 449 // has been deflated and taken off the global free list. 450 return; 451 } 452 453 // A non-NULL dmw has to be either neutral (not locked and not marked) 454 // or is already participating in this restoration protocol. 455 assert(dmw.is_neutral() || (dmw.is_marked() && dmw.hash() == 0), 456 "failed precondition: dmw=" INTPTR_FORMAT, dmw.value()); 457 458 markWord marked_dmw = markWord::zero(); 459 if (!dmw.is_marked() && dmw.hash() == 0) { 460 // This dmw has not yet started the restoration protocol so we 461 // mark a copy of the dmw to begin the protocol. 462 // Note: A dmw with a hashcode does not take this code path. 463 marked_dmw = dmw.set_marked(); 464 465 // All of the callers to this function can be racing with each 466 // other trying to update the _header field. 467 dmw = (markWord) Atomic::cmpxchg(marked_dmw, &_header, dmw); 468 if (dmw.value() == 0) { 469 // ObjectMonitor's header/dmw has been cleared so the object's 470 // header has already been restored. 471 return; 472 } 473 // The _header field is now marked. The winner's 'dmw' variable 474 // contains the original, unmarked header/dmw value and any 475 // losers have a marked header/dmw value that will be cleaned 476 // up below. 477 } 478 479 if (dmw.is_marked()) { 480 // Clear the mark from the header/dmw copy in preparation for 481 // possible restoration from this thread. 482 assert(dmw.hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT, 483 dmw.value()); 484 dmw = dmw.set_unmarked(); 485 } 486 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value()); 487 488 // Install displaced mark word if the object's header still points 489 // to this ObjectMonitor. All racing callers to this function will 490 // reach this point, but only one can win. 491 obj->cas_set_mark(dmw, markWord::encode(this)); 492 493 // Note: It does not matter which thread restored the header/dmw 494 // into the object's header. The thread deflating the monitor just 495 // wanted the object's header restored and it is. The threads that 496 // detected a race with the deflation process also wanted the 497 // object's header restored before they retry their operation and 498 // because it is restored they will only retry once. 499 } 500 501 // Convert the fields used by is_busy() to a string that can be 502 // used for diagnostic output. 503 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) { 504 ss->print("is_busy: contentions=%d, waiters=%d, ", _contentions, _waiters); 505 if (!AsyncDeflateIdleMonitors) { 506 ss->print("owner=" INTPTR_FORMAT, p2i(_owner)); 507 } else if (_owner != DEFLATER_MARKER) { 508 ss->print("owner=" INTPTR_FORMAT, p2i(_owner)); 509 } else { 510 // We report NULL instead of DEFLATER_MARKER here because is_busy() 511 // ignores DEFLATER_MARKER values. 512 ss->print("owner=" INTPTR_FORMAT, NULL); 513 } 514 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq), 515 p2i(_EntryList)); 516 return ss->base(); 517 } 518 519 #define MAX_RECHECK_INTERVAL 1000 520 521 void ObjectMonitor::EnterI(TRAPS) { 522 jint l_ref_count = ref_count(); 523 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count()); 524 525 Thread * const Self = THREAD; 526 assert(Self->is_Java_thread(), "invariant"); 527 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant"); 528 529 // Try the lock - TATAS 530 if (TryLock (Self) > 0) { 531 assert(_succ != Self, "invariant"); 532 assert(_owner == Self, "invariant"); 533 assert(_Responsible != Self, "invariant"); 534 return; 535 } 536 537 if (AsyncDeflateIdleMonitors && 538 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) { 539 // The deflation protocol finished the first part (setting owner), 540 // but it failed the second part (making ref_count negative) and 541 // bailed. Or the ObjectMonitor was async deflated and reused. 542 // Acquired the monitor. 543 assert(_succ != Self, "invariant"); 544 assert(_Responsible != Self, "invariant"); 545 return; 546 } 547 548 assert(InitDone, "Unexpectedly not initialized"); 549 550 // We try one round of spinning *before* enqueueing Self. 551 // 552 // If the _owner is ready but OFFPROC we could use a YieldTo() 553 // operation to donate the remainder of this thread's quantum 554 // to the owner. This has subtle but beneficial affinity 555 // effects. 556 557 if (TrySpin(Self) > 0) { 558 assert(_owner == Self, "invariant"); 559 assert(_succ != Self, "invariant"); 560 assert(_Responsible != Self, "invariant"); 561 return; 562 } 563 564 // The Spin failed -- Enqueue and park the thread ... 565 assert(_succ != Self, "invariant"); 566 assert(_owner != Self, "invariant"); 567 assert(_Responsible != Self, "invariant"); 568 569 // Enqueue "Self" on ObjectMonitor's _cxq. 570 // 571 // Node acts as a proxy for Self. 572 // As an aside, if were to ever rewrite the synchronization code mostly 573 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class 574 // Java objects. This would avoid awkward lifecycle and liveness issues, 575 // as well as eliminate a subset of ABA issues. 576 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. 577 578 ObjectWaiter node(Self); 579 Self->_ParkEvent->reset(); 580 node._prev = (ObjectWaiter *) 0xBAD; 581 node.TState = ObjectWaiter::TS_CXQ; 582 583 // Push "Self" onto the front of the _cxq. 584 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. 585 // Note that spinning tends to reduce the rate at which threads 586 // enqueue and dequeue on EntryList|cxq. 587 ObjectWaiter * nxt; 588 for (;;) { 589 node._next = nxt = _cxq; 590 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break; 591 592 // Interference - the CAS failed because _cxq changed. Just retry. 593 // As an optional optimization we retry the lock. 594 if (TryLock (Self) > 0) { 595 assert(_succ != Self, "invariant"); 596 assert(_owner == Self, "invariant"); 597 assert(_Responsible != Self, "invariant"); 598 return; 599 } 600 } 601 602 // Check for cxq|EntryList edge transition to non-null. This indicates 603 // the onset of contention. While contention persists exiting threads 604 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit 605 // operations revert to the faster 1-0 mode. This enter operation may interleave 606 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we 607 // arrange for one of the contending thread to use a timed park() operations 608 // to detect and recover from the race. (Stranding is form of progress failure 609 // where the monitor is unlocked but all the contending threads remain parked). 610 // That is, at least one of the contended threads will periodically poll _owner. 611 // One of the contending threads will become the designated "Responsible" thread. 612 // The Responsible thread uses a timed park instead of a normal indefinite park 613 // operation -- it periodically wakes and checks for and recovers from potential 614 // strandings admitted by 1-0 exit operations. We need at most one Responsible 615 // thread per-monitor at any given moment. Only threads on cxq|EntryList may 616 // be responsible for a monitor. 617 // 618 // Currently, one of the contended threads takes on the added role of "Responsible". 619 // A viable alternative would be to use a dedicated "stranding checker" thread 620 // that periodically iterated over all the threads (or active monitors) and unparked 621 // successors where there was risk of stranding. This would help eliminate the 622 // timer scalability issues we see on some platforms as we'd only have one thread 623 // -- the checker -- parked on a timer. 624 625 if (nxt == NULL && _EntryList == NULL) { 626 // Try to assume the role of responsible thread for the monitor. 627 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } 628 Atomic::replace_if_null(Self, &_Responsible); 629 } 630 631 // The lock might have been released while this thread was occupied queueing 632 // itself onto _cxq. To close the race and avoid "stranding" and 633 // progress-liveness failure we must resample-retry _owner before parking. 634 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. 635 // In this case the ST-MEMBAR is accomplished with CAS(). 636 // 637 // TODO: Defer all thread state transitions until park-time. 638 // Since state transitions are heavy and inefficient we'd like 639 // to defer the state transitions until absolutely necessary, 640 // and in doing so avoid some transitions ... 641 642 int nWakeups = 0; 643 int recheckInterval = 1; 644 645 for (;;) { 646 647 if (TryLock(Self) > 0) break; 648 assert(_owner != Self, "invariant"); 649 650 // park self 651 if (_Responsible == Self) { 652 Self->_ParkEvent->park((jlong) recheckInterval); 653 // Increase the recheckInterval, but clamp the value. 654 recheckInterval *= 8; 655 if (recheckInterval > MAX_RECHECK_INTERVAL) { 656 recheckInterval = MAX_RECHECK_INTERVAL; 657 } 658 } else { 659 Self->_ParkEvent->park(); 660 } 661 662 if (TryLock(Self) > 0) break; 663 664 if (AsyncDeflateIdleMonitors && 665 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) { 666 // The deflation protocol finished the first part (setting owner), 667 // but it failed the second part (making ref_count negative) and 668 // bailed. Or the ObjectMonitor was async deflated and reused. 669 // Acquired the monitor. 670 break; 671 } 672 673 // The lock is still contested. 674 // Keep a tally of the # of futile wakeups. 675 // Note that the counter is not protected by a lock or updated by atomics. 676 // That is by design - we trade "lossy" counters which are exposed to 677 // races during updates for a lower probe effect. 678 679 // This PerfData object can be used in parallel with a safepoint. 680 // See the work around in PerfDataManager::destroy(). 681 OM_PERFDATA_OP(FutileWakeups, inc()); 682 ++nWakeups; 683 684 // Assuming this is not a spurious wakeup we'll normally find _succ == Self. 685 // We can defer clearing _succ until after the spin completes 686 // TrySpin() must tolerate being called with _succ == Self. 687 // Try yet another round of adaptive spinning. 688 if (TrySpin(Self) > 0) break; 689 690 // We can find that we were unpark()ed and redesignated _succ while 691 // we were spinning. That's harmless. If we iterate and call park(), 692 // park() will consume the event and return immediately and we'll 693 // just spin again. This pattern can repeat, leaving _succ to simply 694 // spin on a CPU. 695 696 if (_succ == Self) _succ = NULL; 697 698 // Invariant: after clearing _succ a thread *must* retry _owner before parking. 699 OrderAccess::fence(); 700 } 701 702 // Egress : 703 // Self has acquired the lock -- Unlink Self from the cxq or EntryList. 704 // Normally we'll find Self on the EntryList . 705 // From the perspective of the lock owner (this thread), the 706 // EntryList is stable and cxq is prepend-only. 707 // The head of cxq is volatile but the interior is stable. 708 // In addition, Self.TState is stable. 709 710 assert(_owner == Self, "invariant"); 711 assert(object() != NULL, "invariant"); 712 // I'd like to write: 713 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ; 714 // but as we're at a safepoint that's not safe. 715 716 UnlinkAfterAcquire(Self, &node); 717 if (_succ == Self) _succ = NULL; 718 719 assert(_succ != Self, "invariant"); 720 if (_Responsible == Self) { 721 _Responsible = NULL; 722 OrderAccess::fence(); // Dekker pivot-point 723 724 // We may leave threads on cxq|EntryList without a designated 725 // "Responsible" thread. This is benign. When this thread subsequently 726 // exits the monitor it can "see" such preexisting "old" threads -- 727 // threads that arrived on the cxq|EntryList before the fence, above -- 728 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads 729 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible 730 // non-null and elect a new "Responsible" timer thread. 731 // 732 // This thread executes: 733 // ST Responsible=null; MEMBAR (in enter epilogue - here) 734 // LD cxq|EntryList (in subsequent exit) 735 // 736 // Entering threads in the slow/contended path execute: 737 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) 738 // The (ST cxq; MEMBAR) is accomplished with CAS(). 739 // 740 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent 741 // exit operation from floating above the ST Responsible=null. 742 } 743 744 // We've acquired ownership with CAS(). 745 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. 746 // But since the CAS() this thread may have also stored into _succ, 747 // EntryList, cxq or Responsible. These meta-data updates must be 748 // visible __before this thread subsequently drops the lock. 749 // Consider what could occur if we didn't enforce this constraint -- 750 // STs to monitor meta-data and user-data could reorder with (become 751 // visible after) the ST in exit that drops ownership of the lock. 752 // Some other thread could then acquire the lock, but observe inconsistent 753 // or old monitor meta-data and heap data. That violates the JMM. 754 // To that end, the 1-0 exit() operation must have at least STST|LDST 755 // "release" barrier semantics. Specifically, there must be at least a 756 // STST|LDST barrier in exit() before the ST of null into _owner that drops 757 // the lock. The barrier ensures that changes to monitor meta-data and data 758 // protected by the lock will be visible before we release the lock, and 759 // therefore before some other thread (CPU) has a chance to acquire the lock. 760 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. 761 // 762 // Critically, any prior STs to _succ or EntryList must be visible before 763 // the ST of null into _owner in the *subsequent* (following) corresponding 764 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily 765 // execute a serializing instruction. 766 767 return; 768 } 769 770 // ReenterI() is a specialized inline form of the latter half of the 771 // contended slow-path from EnterI(). We use ReenterI() only for 772 // monitor reentry in wait(). 773 // 774 // In the future we should reconcile EnterI() and ReenterI(). 775 776 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) { 777 jint l_ref_count = ref_count(); 778 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count()); 779 780 assert(Self != NULL, "invariant"); 781 assert(SelfNode != NULL, "invariant"); 782 assert(SelfNode->_thread == Self, "invariant"); 783 assert(_waiters > 0, "invariant"); 784 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 785 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant"); 786 JavaThread * jt = (JavaThread *) Self; 787 788 int nWakeups = 0; 789 for (;;) { 790 ObjectWaiter::TStates v = SelfNode->TState; 791 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 792 assert(_owner != Self, "invariant"); 793 794 if (TryLock(Self) > 0) break; 795 if (TrySpin(Self) > 0) break; 796 797 if (AsyncDeflateIdleMonitors && 798 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) { 799 // The deflation protocol finished the first part (setting owner), 800 // but it failed the second part (making ref_count negative) and 801 // bailed. Or the ObjectMonitor was async deflated and reused. 802 // Acquired the monitor. 803 break; 804 } 805 806 // State transition wrappers around park() ... 807 // ReenterI() wisely defers state transitions until 808 // it's clear we must park the thread. 809 { 810 OSThreadContendState osts(Self->osthread()); 811 ThreadBlockInVM tbivm(jt); 812 813 // cleared by handle_special_suspend_equivalent_condition() 814 // or java_suspend_self() 815 jt->set_suspend_equivalent(); 816 Self->_ParkEvent->park(); 817 818 // were we externally suspended while we were waiting? 819 for (;;) { 820 if (!ExitSuspendEquivalent(jt)) break; 821 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } 822 jt->java_suspend_self(); 823 jt->set_suspend_equivalent(); 824 } 825 } 826 827 // Try again, but just so we distinguish between futile wakeups and 828 // successful wakeups. The following test isn't algorithmically 829 // necessary, but it helps us maintain sensible statistics. 830 if (TryLock(Self) > 0) break; 831 832 // The lock is still contested. 833 // Keep a tally of the # of futile wakeups. 834 // Note that the counter is not protected by a lock or updated by atomics. 835 // That is by design - we trade "lossy" counters which are exposed to 836 // races during updates for a lower probe effect. 837 ++nWakeups; 838 839 // Assuming this is not a spurious wakeup we'll normally 840 // find that _succ == Self. 841 if (_succ == Self) _succ = NULL; 842 843 // Invariant: after clearing _succ a contending thread 844 // *must* retry _owner before parking. 845 OrderAccess::fence(); 846 847 // This PerfData object can be used in parallel with a safepoint. 848 // See the work around in PerfDataManager::destroy(). 849 OM_PERFDATA_OP(FutileWakeups, inc()); 850 } 851 852 // Self has acquired the lock -- Unlink Self from the cxq or EntryList . 853 // Normally we'll find Self on the EntryList. 854 // Unlinking from the EntryList is constant-time and atomic-free. 855 // From the perspective of the lock owner (this thread), the 856 // EntryList is stable and cxq is prepend-only. 857 // The head of cxq is volatile but the interior is stable. 858 // In addition, Self.TState is stable. 859 860 assert(_owner == Self, "invariant"); 861 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 862 UnlinkAfterAcquire(Self, SelfNode); 863 if (_succ == Self) _succ = NULL; 864 assert(_succ != Self, "invariant"); 865 SelfNode->TState = ObjectWaiter::TS_RUN; 866 OrderAccess::fence(); // see comments at the end of EnterI() 867 } 868 869 // By convention we unlink a contending thread from EntryList|cxq immediately 870 // after the thread acquires the lock in ::enter(). Equally, we could defer 871 // unlinking the thread until ::exit()-time. 872 873 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) { 874 assert(_owner == Self, "invariant"); 875 assert(SelfNode->_thread == Self, "invariant"); 876 877 if (SelfNode->TState == ObjectWaiter::TS_ENTER) { 878 // Normal case: remove Self from the DLL EntryList . 879 // This is a constant-time operation. 880 ObjectWaiter * nxt = SelfNode->_next; 881 ObjectWaiter * prv = SelfNode->_prev; 882 if (nxt != NULL) nxt->_prev = prv; 883 if (prv != NULL) prv->_next = nxt; 884 if (SelfNode == _EntryList) _EntryList = nxt; 885 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant"); 886 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant"); 887 } else { 888 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant"); 889 // Inopportune interleaving -- Self is still on the cxq. 890 // This usually means the enqueue of self raced an exiting thread. 891 // Normally we'll find Self near the front of the cxq, so 892 // dequeueing is typically fast. If needbe we can accelerate 893 // this with some MCS/CHL-like bidirectional list hints and advisory 894 // back-links so dequeueing from the interior will normally operate 895 // in constant-time. 896 // Dequeue Self from either the head (with CAS) or from the interior 897 // with a linear-time scan and normal non-atomic memory operations. 898 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList 899 // and then unlink Self from EntryList. We have to drain eventually, 900 // so it might as well be now. 901 902 ObjectWaiter * v = _cxq; 903 assert(v != NULL, "invariant"); 904 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) { 905 // The CAS above can fail from interference IFF a "RAT" arrived. 906 // In that case Self must be in the interior and can no longer be 907 // at the head of cxq. 908 if (v == SelfNode) { 909 assert(_cxq != v, "invariant"); 910 v = _cxq; // CAS above failed - start scan at head of list 911 } 912 ObjectWaiter * p; 913 ObjectWaiter * q = NULL; 914 for (p = v; p != NULL && p != SelfNode; p = p->_next) { 915 q = p; 916 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant"); 917 } 918 assert(v != SelfNode, "invariant"); 919 assert(p == SelfNode, "Node not found on cxq"); 920 assert(p != _cxq, "invariant"); 921 assert(q != NULL, "invariant"); 922 assert(q->_next == p, "invariant"); 923 q->_next = p->_next; 924 } 925 } 926 927 #ifdef ASSERT 928 // Diagnostic hygiene ... 929 SelfNode->_prev = (ObjectWaiter *) 0xBAD; 930 SelfNode->_next = (ObjectWaiter *) 0xBAD; 931 SelfNode->TState = ObjectWaiter::TS_RUN; 932 #endif 933 } 934 935 // ----------------------------------------------------------------------------- 936 // Exit support 937 // 938 // exit() 939 // ~~~~~~ 940 // Note that the collector can't reclaim the objectMonitor or deflate 941 // the object out from underneath the thread calling ::exit() as the 942 // thread calling ::exit() never transitions to a stable state. 943 // This inhibits GC, which in turn inhibits asynchronous (and 944 // inopportune) reclamation of "this". 945 // 946 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; 947 // There's one exception to the claim above, however. EnterI() can call 948 // exit() to drop a lock if the acquirer has been externally suspended. 949 // In that case exit() is called with _thread_state as _thread_blocked, 950 // but the monitor's _contentions field is > 0, which inhibits reclamation. 951 // 952 // 1-0 exit 953 // ~~~~~~~~ 954 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of 955 // the fast-path operators have been optimized so the common ::exit() 956 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock(). 957 // The code emitted by fast_unlock() elides the usual MEMBAR. This 958 // greatly improves latency -- MEMBAR and CAS having considerable local 959 // latency on modern processors -- but at the cost of "stranding". Absent the 960 // MEMBAR, a thread in fast_unlock() can race a thread in the slow 961 // ::enter() path, resulting in the entering thread being stranding 962 // and a progress-liveness failure. Stranding is extremely rare. 963 // We use timers (timed park operations) & periodic polling to detect 964 // and recover from stranding. Potentially stranded threads periodically 965 // wake up and poll the lock. See the usage of the _Responsible variable. 966 // 967 // The CAS() in enter provides for safety and exclusion, while the CAS or 968 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking 969 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding. 970 // We detect and recover from stranding with timers. 971 // 972 // If a thread transiently strands it'll park until (a) another 973 // thread acquires the lock and then drops the lock, at which time the 974 // exiting thread will notice and unpark the stranded thread, or, (b) 975 // the timer expires. If the lock is high traffic then the stranding latency 976 // will be low due to (a). If the lock is low traffic then the odds of 977 // stranding are lower, although the worst-case stranding latency 978 // is longer. Critically, we don't want to put excessive load in the 979 // platform's timer subsystem. We want to minimize both the timer injection 980 // rate (timers created/sec) as well as the number of timers active at 981 // any one time. (more precisely, we want to minimize timer-seconds, which is 982 // the integral of the # of active timers at any instant over time). 983 // Both impinge on OS scalability. Given that, at most one thread parked on 984 // a monitor will use a timer. 985 // 986 // There is also the risk of a futile wake-up. If we drop the lock 987 // another thread can reacquire the lock immediately, and we can 988 // then wake a thread unnecessarily. This is benign, and we've 989 // structured the code so the windows are short and the frequency 990 // of such futile wakups is low. 991 992 void ObjectMonitor::exit(bool not_suspended, TRAPS) { 993 Thread * const Self = THREAD; 994 if (THREAD != _owner) { 995 void* cur = _owner; 996 if (THREAD->is_lock_owned((address)cur)) { 997 assert(_recursions == 0, "invariant"); 998 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*. 999 _recursions = 0; 1000 } else { 1001 // Apparent unbalanced locking ... 1002 // Naively we'd like to throw IllegalMonitorStateException. 1003 // As a practical matter we can neither allocate nor throw an 1004 // exception as ::exit() can be called from leaf routines. 1005 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties. 1006 // Upon deeper reflection, however, in a properly run JVM the only 1007 // way we should encounter this situation is in the presence of 1008 // unbalanced JNI locking. TODO: CheckJNICalls. 1009 // See also: CR4414101 1010 tty->print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT 1011 " is exiting an ObjectMonitor it does not own.", 1012 p2i(THREAD)); 1013 tty->print_cr("The imbalance is possibly caused by JNI locking."); 1014 print_debug_style_on(tty); 1015 // Changing this from an assert() to ADIM_guarantee() may run 1016 // afoul of any test that is inducing non-balanced JNI locking. 1017 ADIM_guarantee(false, "Non-balanced monitor enter/exit!"); 1018 return; 1019 } 1020 } 1021 1022 if (_recursions != 0) { 1023 _recursions--; // this is simple recursive enter 1024 return; 1025 } 1026 1027 // Invariant: after setting Responsible=null an thread must execute 1028 // a MEMBAR or other serializing instruction before fetching EntryList|cxq. 1029 _Responsible = NULL; 1030 1031 #if INCLUDE_JFR 1032 // get the owner's thread id for the MonitorEnter event 1033 // if it is enabled and the thread isn't suspended 1034 if (not_suspended && EventJavaMonitorEnter::is_enabled()) { 1035 _previous_owner_tid = JFR_THREAD_ID(Self); 1036 } 1037 #endif 1038 1039 for (;;) { 1040 assert(THREAD == _owner, "invariant"); 1041 1042 // release semantics: prior loads and stores from within the critical section 1043 // must not float (reorder) past the following store that drops the lock. 1044 if (AsyncDeflateIdleMonitors) { 1045 set_owner_from(NULL, Self); 1046 } else { 1047 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock 1048 OrderAccess::storeload(); // See if we need to wake a successor 1049 } 1050 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { 1051 return; 1052 } 1053 // Other threads are blocked trying to acquire the lock. 1054 1055 // Normally the exiting thread is responsible for ensuring succession, 1056 // but if other successors are ready or other entering threads are spinning 1057 // then this thread can simply store NULL into _owner and exit without 1058 // waking a successor. The existence of spinners or ready successors 1059 // guarantees proper succession (liveness). Responsibility passes to the 1060 // ready or running successors. The exiting thread delegates the duty. 1061 // More precisely, if a successor already exists this thread is absolved 1062 // of the responsibility of waking (unparking) one. 1063 // 1064 // The _succ variable is critical to reducing futile wakeup frequency. 1065 // _succ identifies the "heir presumptive" thread that has been made 1066 // ready (unparked) but that has not yet run. We need only one such 1067 // successor thread to guarantee progress. 1068 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 1069 // section 3.3 "Futile Wakeup Throttling" for details. 1070 // 1071 // Note that spinners in Enter() also set _succ non-null. 1072 // In the current implementation spinners opportunistically set 1073 // _succ so that exiting threads might avoid waking a successor. 1074 // Another less appealing alternative would be for the exiting thread 1075 // to drop the lock and then spin briefly to see if a spinner managed 1076 // to acquire the lock. If so, the exiting thread could exit 1077 // immediately without waking a successor, otherwise the exiting 1078 // thread would need to dequeue and wake a successor. 1079 // (Note that we'd need to make the post-drop spin short, but no 1080 // shorter than the worst-case round-trip cache-line migration time. 1081 // The dropped lock needs to become visible to the spinner, and then 1082 // the acquisition of the lock by the spinner must become visible to 1083 // the exiting thread). 1084 1085 // It appears that an heir-presumptive (successor) must be made ready. 1086 // Only the current lock owner can manipulate the EntryList or 1087 // drain _cxq, so we need to reacquire the lock. If we fail 1088 // to reacquire the lock the responsibility for ensuring succession 1089 // falls to the new owner. 1090 // 1091 if (try_set_owner_from(Self, NULL) != NULL) { 1092 return; 1093 } 1094 1095 guarantee(_owner == THREAD, "invariant"); 1096 1097 ObjectWaiter * w = NULL; 1098 1099 w = _EntryList; 1100 if (w != NULL) { 1101 // I'd like to write: guarantee (w->_thread != Self). 1102 // But in practice an exiting thread may find itself on the EntryList. 1103 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and 1104 // then calls exit(). Exit release the lock by setting O._owner to NULL. 1105 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The 1106 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then 1107 // release the lock "O". T2 resumes immediately after the ST of null into 1108 // _owner, above. T2 notices that the EntryList is populated, so it 1109 // reacquires the lock and then finds itself on the EntryList. 1110 // Given all that, we have to tolerate the circumstance where "w" is 1111 // associated with Self. 1112 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1113 ExitEpilog(Self, w); 1114 return; 1115 } 1116 1117 // If we find that both _cxq and EntryList are null then just 1118 // re-run the exit protocol from the top. 1119 w = _cxq; 1120 if (w == NULL) continue; 1121 1122 // Drain _cxq into EntryList - bulk transfer. 1123 // First, detach _cxq. 1124 // The following loop is tantamount to: w = swap(&cxq, NULL) 1125 for (;;) { 1126 assert(w != NULL, "Invariant"); 1127 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w); 1128 if (u == w) break; 1129 w = u; 1130 } 1131 1132 assert(w != NULL, "invariant"); 1133 assert(_EntryList == NULL, "invariant"); 1134 1135 // Convert the LIFO SLL anchored by _cxq into a DLL. 1136 // The list reorganization step operates in O(LENGTH(w)) time. 1137 // It's critical that this step operate quickly as 1138 // "Self" still holds the outer-lock, restricting parallelism 1139 // and effectively lengthening the critical section. 1140 // Invariant: s chases t chases u. 1141 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so 1142 // we have faster access to the tail. 1143 1144 _EntryList = w; 1145 ObjectWaiter * q = NULL; 1146 ObjectWaiter * p; 1147 for (p = w; p != NULL; p = p->_next) { 1148 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1149 p->TState = ObjectWaiter::TS_ENTER; 1150 p->_prev = q; 1151 q = p; 1152 } 1153 1154 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL 1155 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). 1156 1157 // See if we can abdicate to a spinner instead of waking a thread. 1158 // A primary goal of the implementation is to reduce the 1159 // context-switch rate. 1160 if (_succ != NULL) continue; 1161 1162 w = _EntryList; 1163 if (w != NULL) { 1164 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1165 ExitEpilog(Self, w); 1166 return; 1167 } 1168 } 1169 } 1170 1171 // ExitSuspendEquivalent: 1172 // A faster alternate to handle_special_suspend_equivalent_condition() 1173 // 1174 // handle_special_suspend_equivalent_condition() unconditionally 1175 // acquires the SR_lock. On some platforms uncontended MutexLocker() 1176 // operations have high latency. Note that in ::enter() we call HSSEC 1177 // while holding the monitor, so we effectively lengthen the critical sections. 1178 // 1179 // There are a number of possible solutions: 1180 // 1181 // A. To ameliorate the problem we might also defer state transitions 1182 // to as late as possible -- just prior to parking. 1183 // Given that, we'd call HSSEC after having returned from park(), 1184 // but before attempting to acquire the monitor. This is only a 1185 // partial solution. It avoids calling HSSEC while holding the 1186 // monitor (good), but it still increases successor reacquisition latency -- 1187 // the interval between unparking a successor and the time the successor 1188 // resumes and retries the lock. See ReenterI(), which defers state transitions. 1189 // If we use this technique we can also avoid EnterI()-exit() loop 1190 // in ::enter() where we iteratively drop the lock and then attempt 1191 // to reacquire it after suspending. 1192 // 1193 // B. In the future we might fold all the suspend bits into a 1194 // composite per-thread suspend flag and then update it with CAS(). 1195 // Alternately, a Dekker-like mechanism with multiple variables 1196 // would suffice: 1197 // ST Self->_suspend_equivalent = false 1198 // MEMBAR 1199 // LD Self_>_suspend_flags 1200 1201 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) { 1202 return jSelf->handle_special_suspend_equivalent_condition(); 1203 } 1204 1205 1206 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) { 1207 assert(_owner == Self, "invariant"); 1208 1209 // Exit protocol: 1210 // 1. ST _succ = wakee 1211 // 2. membar #loadstore|#storestore; 1212 // 2. ST _owner = NULL 1213 // 3. unpark(wakee) 1214 1215 _succ = Wakee->_thread; 1216 ParkEvent * Trigger = Wakee->_event; 1217 1218 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. 1219 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be 1220 // out-of-scope (non-extant). 1221 Wakee = NULL; 1222 1223 // Drop the lock 1224 if (AsyncDeflateIdleMonitors) { 1225 set_owner_from(NULL, Self); 1226 } else { 1227 OrderAccess::release_store(&_owner, (void*)NULL); 1228 OrderAccess::fence(); // ST _owner vs LD in unpark() 1229 } 1230 1231 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); 1232 Trigger->unpark(); 1233 1234 // Maintain stats and report events to JVMTI 1235 OM_PERFDATA_OP(Parks, inc()); 1236 } 1237 1238 1239 // ----------------------------------------------------------------------------- 1240 // Class Loader deadlock handling. 1241 // 1242 // complete_exit exits a lock returning recursion count 1243 // complete_exit/reenter operate as a wait without waiting 1244 // complete_exit requires an inflated monitor 1245 // The _owner field is not always the Thread addr even with an 1246 // inflated monitor, e.g. the monitor can be inflated by a non-owning 1247 // thread due to contention. 1248 intptr_t ObjectMonitor::complete_exit(TRAPS) { 1249 Thread * const Self = THREAD; 1250 assert(Self->is_Java_thread(), "Must be Java thread!"); 1251 JavaThread *jt = (JavaThread *)THREAD; 1252 1253 assert(InitDone, "Unexpectedly not initialized"); 1254 1255 if (THREAD != _owner) { 1256 void* cur = _owner; 1257 if (THREAD->is_lock_owned((address)cur)) { 1258 assert(_recursions == 0, "internal state error"); 1259 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*. 1260 _recursions = 0; 1261 } 1262 } 1263 1264 guarantee(Self == _owner, "complete_exit not owner"); 1265 intptr_t save = _recursions; // record the old recursion count 1266 _recursions = 0; // set the recursion level to be 0 1267 exit(true, Self); // exit the monitor 1268 guarantee(_owner != Self, "invariant"); 1269 return save; 1270 } 1271 1272 // reenter() enters a lock and sets recursion count 1273 // complete_exit/reenter operate as a wait without waiting 1274 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) { 1275 Thread * const Self = THREAD; 1276 assert(Self->is_Java_thread(), "Must be Java thread!"); 1277 JavaThread *jt = (JavaThread *)THREAD; 1278 1279 guarantee(_owner != Self, "reenter already owner"); 1280 enter(THREAD); 1281 // Entered the monitor. 1282 guarantee(_recursions == 0, "reenter recursion"); 1283 _recursions = recursions; 1284 } 1285 1286 // Checks that the current THREAD owns this monitor and causes an 1287 // immediate return if it doesn't. We don't use the CHECK macro 1288 // because we want the IMSE to be the only exception that is thrown 1289 // from the call site when false is returned. Any other pending 1290 // exception is ignored. 1291 #define CHECK_OWNER() \ 1292 do { \ 1293 if (!check_owner(THREAD)) { \ 1294 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \ 1295 return; \ 1296 } \ 1297 } while (false) 1298 1299 // Returns true if the specified thread owns the ObjectMonitor. 1300 // Otherwise returns false and throws IllegalMonitorStateException 1301 // (IMSE). If there is a pending exception and the specified thread 1302 // is not the owner, that exception will be replaced by the IMSE. 1303 bool ObjectMonitor::check_owner(Thread* THREAD) { 1304 if (_owner == THREAD) { 1305 return true; 1306 } 1307 void* cur = _owner; 1308 if (THREAD->is_lock_owned((address)cur)) { 1309 simply_set_owner_from_BasicLock(THREAD, cur); // Convert from BasicLock* to Thread*. 1310 _recursions = 0; 1311 return true; 1312 } 1313 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(), 1314 "current thread is not owner", false); 1315 } 1316 1317 static void post_monitor_wait_event(EventJavaMonitorWait* event, 1318 ObjectMonitor* monitor, 1319 jlong notifier_tid, 1320 jlong timeout, 1321 bool timedout) { 1322 assert(event != NULL, "invariant"); 1323 assert(monitor != NULL, "invariant"); 1324 event->set_monitorClass(((oop)monitor->object())->klass()); 1325 event->set_timeout(timeout); 1326 event->set_address((uintptr_t)monitor->object_addr()); 1327 event->set_notifier(notifier_tid); 1328 event->set_timedOut(timedout); 1329 event->commit(); 1330 } 1331 1332 // ----------------------------------------------------------------------------- 1333 // Wait/Notify/NotifyAll 1334 // 1335 // Note: a subset of changes to ObjectMonitor::wait() 1336 // will need to be replicated in complete_exit 1337 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1338 Thread * const Self = THREAD; 1339 assert(Self->is_Java_thread(), "Must be Java thread!"); 1340 JavaThread *jt = (JavaThread *)THREAD; 1341 1342 assert(InitDone, "Unexpectedly not initialized"); 1343 1344 CHECK_OWNER(); // Throws IMSE if not owner. 1345 1346 EventJavaMonitorWait event; 1347 1348 // check for a pending interrupt 1349 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) { 1350 // post monitor waited event. Note that this is past-tense, we are done waiting. 1351 if (JvmtiExport::should_post_monitor_waited()) { 1352 // Note: 'false' parameter is passed here because the 1353 // wait was not timed out due to thread interrupt. 1354 JvmtiExport::post_monitor_waited(jt, this, false); 1355 1356 // In this short circuit of the monitor wait protocol, the 1357 // current thread never drops ownership of the monitor and 1358 // never gets added to the wait queue so the current thread 1359 // cannot be made the successor. This means that the 1360 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally 1361 // consume an unpark() meant for the ParkEvent associated with 1362 // this ObjectMonitor. 1363 } 1364 if (event.should_commit()) { 1365 post_monitor_wait_event(&event, this, 0, millis, false); 1366 } 1367 THROW(vmSymbols::java_lang_InterruptedException()); 1368 return; 1369 } 1370 1371 assert(Self->_Stalled == 0, "invariant"); 1372 Self->_Stalled = intptr_t(this); 1373 jt->set_current_waiting_monitor(this); 1374 1375 // create a node to be put into the queue 1376 // Critically, after we reset() the event but prior to park(), we must check 1377 // for a pending interrupt. 1378 ObjectWaiter node(Self); 1379 node.TState = ObjectWaiter::TS_WAIT; 1380 Self->_ParkEvent->reset(); 1381 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag 1382 1383 // Enter the waiting queue, which is a circular doubly linked list in this case 1384 // but it could be a priority queue or any data structure. 1385 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only 1386 // by the the owner of the monitor *except* in the case where park() 1387 // returns because of a timeout of interrupt. Contention is exceptionally rare 1388 // so we use a simple spin-lock instead of a heavier-weight blocking lock. 1389 1390 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add"); 1391 AddWaiter(&node); 1392 Thread::SpinRelease(&_WaitSetLock); 1393 1394 _Responsible = NULL; 1395 1396 intptr_t save = _recursions; // record the old recursion count 1397 _waiters++; // increment the number of waiters 1398 _recursions = 0; // set the recursion level to be 1 1399 exit(true, Self); // exit the monitor 1400 guarantee(_owner != Self, "invariant"); 1401 1402 // The thread is on the WaitSet list - now park() it. 1403 // On MP systems it's conceivable that a brief spin before we park 1404 // could be profitable. 1405 // 1406 // TODO-FIXME: change the following logic to a loop of the form 1407 // while (!timeout && !interrupted && _notified == 0) park() 1408 1409 int ret = OS_OK; 1410 int WasNotified = 0; 1411 { // State transition wrappers 1412 OSThread* osthread = Self->osthread(); 1413 OSThreadWaitState osts(osthread, true); 1414 { 1415 ThreadBlockInVM tbivm(jt); 1416 // Thread is in thread_blocked state and oop access is unsafe. 1417 jt->set_suspend_equivalent(); 1418 1419 if (interruptible && (jt->is_interrupted(false) || HAS_PENDING_EXCEPTION)) { 1420 // Intentionally empty 1421 } else if (node._notified == 0) { 1422 if (millis <= 0) { 1423 Self->_ParkEvent->park(); 1424 } else { 1425 ret = Self->_ParkEvent->park(millis); 1426 } 1427 } 1428 1429 // were we externally suspended while we were waiting? 1430 if (ExitSuspendEquivalent (jt)) { 1431 // TODO-FIXME: add -- if succ == Self then succ = null. 1432 jt->java_suspend_self(); 1433 } 1434 1435 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm 1436 1437 // Node may be on the WaitSet, the EntryList (or cxq), or in transition 1438 // from the WaitSet to the EntryList. 1439 // See if we need to remove Node from the WaitSet. 1440 // We use double-checked locking to avoid grabbing _WaitSetLock 1441 // if the thread is not on the wait queue. 1442 // 1443 // Note that we don't need a fence before the fetch of TState. 1444 // In the worst case we'll fetch a old-stale value of TS_WAIT previously 1445 // written by the is thread. (perhaps the fetch might even be satisfied 1446 // by a look-aside into the processor's own store buffer, although given 1447 // the length of the code path between the prior ST and this load that's 1448 // highly unlikely). If the following LD fetches a stale TS_WAIT value 1449 // then we'll acquire the lock and then re-fetch a fresh TState value. 1450 // That is, we fail toward safety. 1451 1452 if (node.TState == ObjectWaiter::TS_WAIT) { 1453 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink"); 1454 if (node.TState == ObjectWaiter::TS_WAIT) { 1455 DequeueSpecificWaiter(&node); // unlink from WaitSet 1456 assert(node._notified == 0, "invariant"); 1457 node.TState = ObjectWaiter::TS_RUN; 1458 } 1459 Thread::SpinRelease(&_WaitSetLock); 1460 } 1461 1462 // The thread is now either on off-list (TS_RUN), 1463 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). 1464 // The Node's TState variable is stable from the perspective of this thread. 1465 // No other threads will asynchronously modify TState. 1466 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant"); 1467 OrderAccess::loadload(); 1468 if (_succ == Self) _succ = NULL; 1469 WasNotified = node._notified; 1470 1471 // Reentry phase -- reacquire the monitor. 1472 // re-enter contended monitor after object.wait(). 1473 // retain OBJECT_WAIT state until re-enter successfully completes 1474 // Thread state is thread_in_vm and oop access is again safe, 1475 // although the raw address of the object may have changed. 1476 // (Don't cache naked oops over safepoints, of course). 1477 1478 // post monitor waited event. Note that this is past-tense, we are done waiting. 1479 if (JvmtiExport::should_post_monitor_waited()) { 1480 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); 1481 1482 if (node._notified != 0 && _succ == Self) { 1483 // In this part of the monitor wait-notify-reenter protocol it 1484 // is possible (and normal) for another thread to do a fastpath 1485 // monitor enter-exit while this thread is still trying to get 1486 // to the reenter portion of the protocol. 1487 // 1488 // The ObjectMonitor was notified and the current thread is 1489 // the successor which also means that an unpark() has already 1490 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can 1491 // consume the unpark() that was done when the successor was 1492 // set because the same ParkEvent is shared between Java 1493 // monitors and JVM/TI RawMonitors (for now). 1494 // 1495 // We redo the unpark() to ensure forward progress, i.e., we 1496 // don't want all pending threads hanging (parked) with none 1497 // entering the unlocked monitor. 1498 node._event->unpark(); 1499 } 1500 } 1501 1502 if (event.should_commit()) { 1503 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT); 1504 } 1505 1506 OrderAccess::fence(); 1507 1508 assert(Self->_Stalled != 0, "invariant"); 1509 Self->_Stalled = 0; 1510 1511 assert(_owner != Self, "invariant"); 1512 ObjectWaiter::TStates v = node.TState; 1513 if (v == ObjectWaiter::TS_RUN) { 1514 enter(Self); 1515 } else { 1516 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 1517 ReenterI(Self, &node); 1518 node.wait_reenter_end(this); 1519 } 1520 1521 // Self has reacquired the lock. 1522 // Lifecycle - the node representing Self must not appear on any queues. 1523 // Node is about to go out-of-scope, but even if it were immortal we wouldn't 1524 // want residual elements associated with this thread left on any lists. 1525 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant"); 1526 assert(_owner == Self, "invariant"); 1527 assert(_succ != Self, "invariant"); 1528 } // OSThreadWaitState() 1529 1530 jt->set_current_waiting_monitor(NULL); 1531 1532 guarantee(_recursions == 0, "invariant"); 1533 _recursions = save; // restore the old recursion count 1534 _waiters--; // decrement the number of waiters 1535 1536 // Verify a few postconditions 1537 assert(_owner == Self, "invariant"); 1538 assert(_succ != Self, "invariant"); 1539 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 1540 1541 // check if the notification happened 1542 if (!WasNotified) { 1543 // no, it could be timeout or Thread.interrupt() or both 1544 // check for interrupt event, otherwise it is timeout 1545 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) { 1546 THROW(vmSymbols::java_lang_InterruptedException()); 1547 } 1548 } 1549 1550 // NOTE: Spurious wake up will be consider as timeout. 1551 // Monitor notify has precedence over thread interrupt. 1552 } 1553 1554 1555 // Consider: 1556 // If the lock is cool (cxq == null && succ == null) and we're on an MP system 1557 // then instead of transferring a thread from the WaitSet to the EntryList 1558 // we might just dequeue a thread from the WaitSet and directly unpark() it. 1559 1560 void ObjectMonitor::INotify(Thread * Self) { 1561 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify"); 1562 ObjectWaiter * iterator = DequeueWaiter(); 1563 if (iterator != NULL) { 1564 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant"); 1565 guarantee(iterator->_notified == 0, "invariant"); 1566 // Disposition - what might we do with iterator ? 1567 // a. add it directly to the EntryList - either tail (policy == 1) 1568 // or head (policy == 0). 1569 // b. push it onto the front of the _cxq (policy == 2). 1570 // For now we use (b). 1571 1572 iterator->TState = ObjectWaiter::TS_ENTER; 1573 1574 iterator->_notified = 1; 1575 iterator->_notifier_tid = JFR_THREAD_ID(Self); 1576 1577 ObjectWaiter * list = _EntryList; 1578 if (list != NULL) { 1579 assert(list->_prev == NULL, "invariant"); 1580 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant"); 1581 assert(list != iterator, "invariant"); 1582 } 1583 1584 // prepend to cxq 1585 if (list == NULL) { 1586 iterator->_next = iterator->_prev = NULL; 1587 _EntryList = iterator; 1588 } else { 1589 iterator->TState = ObjectWaiter::TS_CXQ; 1590 for (;;) { 1591 ObjectWaiter * front = _cxq; 1592 iterator->_next = front; 1593 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) { 1594 break; 1595 } 1596 } 1597 } 1598 1599 // _WaitSetLock protects the wait queue, not the EntryList. We could 1600 // move the add-to-EntryList operation, above, outside the critical section 1601 // protected by _WaitSetLock. In practice that's not useful. With the 1602 // exception of wait() timeouts and interrupts the monitor owner 1603 // is the only thread that grabs _WaitSetLock. There's almost no contention 1604 // on _WaitSetLock so it's not profitable to reduce the length of the 1605 // critical section. 1606 1607 iterator->wait_reenter_begin(this); 1608 } 1609 Thread::SpinRelease(&_WaitSetLock); 1610 } 1611 1612 // Consider: a not-uncommon synchronization bug is to use notify() when 1613 // notifyAll() is more appropriate, potentially resulting in stranded 1614 // threads; this is one example of a lost wakeup. A useful diagnostic 1615 // option is to force all notify() operations to behave as notifyAll(). 1616 // 1617 // Note: We can also detect many such problems with a "minimum wait". 1618 // When the "minimum wait" is set to a small non-zero timeout value 1619 // and the program does not hang whereas it did absent "minimum wait", 1620 // that suggests a lost wakeup bug. 1621 1622 void ObjectMonitor::notify(TRAPS) { 1623 CHECK_OWNER(); // Throws IMSE if not owner. 1624 if (_WaitSet == NULL) { 1625 return; 1626 } 1627 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); 1628 INotify(THREAD); 1629 OM_PERFDATA_OP(Notifications, inc(1)); 1630 } 1631 1632 1633 // The current implementation of notifyAll() transfers the waiters one-at-a-time 1634 // from the waitset to the EntryList. This could be done more efficiently with a 1635 // single bulk transfer but in practice it's not time-critical. Beware too, 1636 // that in prepend-mode we invert the order of the waiters. Let's say that the 1637 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend 1638 // mode the waitset will be empty and the EntryList will be "DCBAXYZ". 1639 1640 void ObjectMonitor::notifyAll(TRAPS) { 1641 CHECK_OWNER(); // Throws IMSE if not owner. 1642 if (_WaitSet == NULL) { 1643 return; 1644 } 1645 1646 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); 1647 int tally = 0; 1648 while (_WaitSet != NULL) { 1649 tally++; 1650 INotify(THREAD); 1651 } 1652 1653 OM_PERFDATA_OP(Notifications, inc(tally)); 1654 } 1655 1656 // ----------------------------------------------------------------------------- 1657 // Adaptive Spinning Support 1658 // 1659 // Adaptive spin-then-block - rational spinning 1660 // 1661 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS 1662 // algorithm. On high order SMP systems it would be better to start with 1663 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, 1664 // a contending thread could enqueue itself on the cxq and then spin locally 1665 // on a thread-specific variable such as its ParkEvent._Event flag. 1666 // That's left as an exercise for the reader. Note that global spinning is 1667 // not problematic on Niagara, as the L2 cache serves the interconnect and 1668 // has both low latency and massive bandwidth. 1669 // 1670 // Broadly, we can fix the spin frequency -- that is, the % of contended lock 1671 // acquisition attempts where we opt to spin -- at 100% and vary the spin count 1672 // (duration) or we can fix the count at approximately the duration of 1673 // a context switch and vary the frequency. Of course we could also 1674 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. 1675 // For a description of 'Adaptive spin-then-block mutual exclusion in 1676 // multi-threaded processing,' see U.S. Pat. No. 8046758. 1677 // 1678 // This implementation varies the duration "D", where D varies with 1679 // the success rate of recent spin attempts. (D is capped at approximately 1680 // length of a round-trip context switch). The success rate for recent 1681 // spin attempts is a good predictor of the success rate of future spin 1682 // attempts. The mechanism adapts automatically to varying critical 1683 // section length (lock modality), system load and degree of parallelism. 1684 // D is maintained per-monitor in _SpinDuration and is initialized 1685 // optimistically. Spin frequency is fixed at 100%. 1686 // 1687 // Note that _SpinDuration is volatile, but we update it without locks 1688 // or atomics. The code is designed so that _SpinDuration stays within 1689 // a reasonable range even in the presence of races. The arithmetic 1690 // operations on _SpinDuration are closed over the domain of legal values, 1691 // so at worst a race will install and older but still legal value. 1692 // At the very worst this introduces some apparent non-determinism. 1693 // We might spin when we shouldn't or vice-versa, but since the spin 1694 // count are relatively short, even in the worst case, the effect is harmless. 1695 // 1696 // Care must be taken that a low "D" value does not become an 1697 // an absorbing state. Transient spinning failures -- when spinning 1698 // is overall profitable -- should not cause the system to converge 1699 // on low "D" values. We want spinning to be stable and predictable 1700 // and fairly responsive to change and at the same time we don't want 1701 // it to oscillate, become metastable, be "too" non-deterministic, 1702 // or converge on or enter undesirable stable absorbing states. 1703 // 1704 // We implement a feedback-based control system -- using past behavior 1705 // to predict future behavior. We face two issues: (a) if the 1706 // input signal is random then the spin predictor won't provide optimal 1707 // results, and (b) if the signal frequency is too high then the control 1708 // system, which has some natural response lag, will "chase" the signal. 1709 // (b) can arise from multimodal lock hold times. Transient preemption 1710 // can also result in apparent bimodal lock hold times. 1711 // Although sub-optimal, neither condition is particularly harmful, as 1712 // in the worst-case we'll spin when we shouldn't or vice-versa. 1713 // The maximum spin duration is rather short so the failure modes aren't bad. 1714 // To be conservative, I've tuned the gain in system to bias toward 1715 // _not spinning. Relatedly, the system can sometimes enter a mode where it 1716 // "rings" or oscillates between spinning and not spinning. This happens 1717 // when spinning is just on the cusp of profitability, however, so the 1718 // situation is not dire. The state is benign -- there's no need to add 1719 // hysteresis control to damp the transition rate between spinning and 1720 // not spinning. 1721 1722 // Spinning: Fixed frequency (100%), vary duration 1723 int ObjectMonitor::TrySpin(Thread * Self) { 1724 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. 1725 int ctr = Knob_FixedSpin; 1726 if (ctr != 0) { 1727 while (--ctr >= 0) { 1728 if (TryLock(Self) > 0) return 1; 1729 SpinPause(); 1730 } 1731 return 0; 1732 } 1733 1734 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) { 1735 if (TryLock(Self) > 0) { 1736 // Increase _SpinDuration ... 1737 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1738 // Raising _SpurDuration to the poverty line is key. 1739 int x = _SpinDuration; 1740 if (x < Knob_SpinLimit) { 1741 if (x < Knob_Poverty) x = Knob_Poverty; 1742 _SpinDuration = x + Knob_BonusB; 1743 } 1744 return 1; 1745 } 1746 SpinPause(); 1747 } 1748 1749 // Admission control - verify preconditions for spinning 1750 // 1751 // We always spin a little bit, just to prevent _SpinDuration == 0 from 1752 // becoming an absorbing state. Put another way, we spin briefly to 1753 // sample, just in case the system load, parallelism, contention, or lock 1754 // modality changed. 1755 // 1756 // Consider the following alternative: 1757 // Periodically set _SpinDuration = _SpinLimit and try a long/full 1758 // spin attempt. "Periodically" might mean after a tally of 1759 // the # of failed spin attempts (or iterations) reaches some threshold. 1760 // This takes us into the realm of 1-out-of-N spinning, where we 1761 // hold the duration constant but vary the frequency. 1762 1763 ctr = _SpinDuration; 1764 if (ctr <= 0) return 0; 1765 1766 if (NotRunnable(Self, (Thread *) _owner)) { 1767 return 0; 1768 } 1769 1770 // We're good to spin ... spin ingress. 1771 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades 1772 // when preparing to LD...CAS _owner, etc and the CAS is likely 1773 // to succeed. 1774 if (_succ == NULL) { 1775 _succ = Self; 1776 } 1777 Thread * prv = NULL; 1778 1779 // There are three ways to exit the following loop: 1780 // 1. A successful spin where this thread has acquired the lock. 1781 // 2. Spin failure with prejudice 1782 // 3. Spin failure without prejudice 1783 1784 while (--ctr >= 0) { 1785 1786 // Periodic polling -- Check for pending GC 1787 // Threads may spin while they're unsafe. 1788 // We don't want spinning threads to delay the JVM from reaching 1789 // a stop-the-world safepoint or to steal cycles from GC. 1790 // If we detect a pending safepoint we abort in order that 1791 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) 1792 // this thread, if safe, doesn't steal cycles from GC. 1793 // This is in keeping with the "no loitering in runtime" rule. 1794 // We periodically check to see if there's a safepoint pending. 1795 if ((ctr & 0xFF) == 0) { 1796 if (SafepointMechanism::should_block(Self)) { 1797 goto Abort; // abrupt spin egress 1798 } 1799 SpinPause(); 1800 } 1801 1802 // Probe _owner with TATAS 1803 // If this thread observes the monitor transition or flicker 1804 // from locked to unlocked to locked, then the odds that this 1805 // thread will acquire the lock in this spin attempt go down 1806 // considerably. The same argument applies if the CAS fails 1807 // or if we observe _owner change from one non-null value to 1808 // another non-null value. In such cases we might abort 1809 // the spin without prejudice or apply a "penalty" to the 1810 // spin count-down variable "ctr", reducing it by 100, say. 1811 1812 Thread * ox = (Thread *) _owner; 1813 if (ox == NULL) { 1814 ox = (Thread*)try_set_owner_from(Self, NULL); 1815 if (ox == NULL) { 1816 // The CAS succeeded -- this thread acquired ownership 1817 // Take care of some bookkeeping to exit spin state. 1818 if (_succ == Self) { 1819 _succ = NULL; 1820 } 1821 1822 // Increase _SpinDuration : 1823 // The spin was successful (profitable) so we tend toward 1824 // longer spin attempts in the future. 1825 // CONSIDER: factor "ctr" into the _SpinDuration adjustment. 1826 // If we acquired the lock early in the spin cycle it 1827 // makes sense to increase _SpinDuration proportionally. 1828 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1829 int x = _SpinDuration; 1830 if (x < Knob_SpinLimit) { 1831 if (x < Knob_Poverty) x = Knob_Poverty; 1832 _SpinDuration = x + Knob_Bonus; 1833 } 1834 return 1; 1835 } 1836 1837 // The CAS failed ... we can take any of the following actions: 1838 // * penalize: ctr -= CASPenalty 1839 // * exit spin with prejudice -- goto Abort; 1840 // * exit spin without prejudice. 1841 // * Since CAS is high-latency, retry again immediately. 1842 prv = ox; 1843 goto Abort; 1844 } 1845 1846 // Did lock ownership change hands ? 1847 if (ox != prv && prv != NULL) { 1848 goto Abort; 1849 } 1850 prv = ox; 1851 1852 // Abort the spin if the owner is not executing. 1853 // The owner must be executing in order to drop the lock. 1854 // Spinning while the owner is OFFPROC is idiocy. 1855 // Consider: ctr -= RunnablePenalty ; 1856 if (NotRunnable(Self, ox)) { 1857 goto Abort; 1858 } 1859 if (_succ == NULL) { 1860 _succ = Self; 1861 } 1862 } 1863 1864 // Spin failed with prejudice -- reduce _SpinDuration. 1865 // TODO: Use an AIMD-like policy to adjust _SpinDuration. 1866 // AIMD is globally stable. 1867 { 1868 int x = _SpinDuration; 1869 if (x > 0) { 1870 // Consider an AIMD scheme like: x -= (x >> 3) + 100 1871 // This is globally sample and tends to damp the response. 1872 x -= Knob_Penalty; 1873 if (x < 0) x = 0; 1874 _SpinDuration = x; 1875 } 1876 } 1877 1878 Abort: 1879 if (_succ == Self) { 1880 _succ = NULL; 1881 // Invariant: after setting succ=null a contending thread 1882 // must recheck-retry _owner before parking. This usually happens 1883 // in the normal usage of TrySpin(), but it's safest 1884 // to make TrySpin() as foolproof as possible. 1885 OrderAccess::fence(); 1886 if (TryLock(Self) > 0) return 1; 1887 } 1888 return 0; 1889 } 1890 1891 // NotRunnable() -- informed spinning 1892 // 1893 // Don't bother spinning if the owner is not eligible to drop the lock. 1894 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm. 1895 // The thread must be runnable in order to drop the lock in timely fashion. 1896 // If the _owner is not runnable then spinning will not likely be 1897 // successful (profitable). 1898 // 1899 // Beware -- the thread referenced by _owner could have died 1900 // so a simply fetch from _owner->_thread_state might trap. 1901 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. 1902 // Because of the lifecycle issues, the _thread_state values 1903 // observed by NotRunnable() might be garbage. NotRunnable must 1904 // tolerate this and consider the observed _thread_state value 1905 // as advisory. 1906 // 1907 // Beware too, that _owner is sometimes a BasicLock address and sometimes 1908 // a thread pointer. 1909 // Alternately, we might tag the type (thread pointer vs basiclock pointer) 1910 // with the LSB of _owner. Another option would be to probabilistically probe 1911 // the putative _owner->TypeTag value. 1912 // 1913 // Checking _thread_state isn't perfect. Even if the thread is 1914 // in_java it might be blocked on a page-fault or have been preempted 1915 // and sitting on a ready/dispatch queue. 1916 // 1917 // The return value from NotRunnable() is *advisory* -- the 1918 // result is based on sampling and is not necessarily coherent. 1919 // The caller must tolerate false-negative and false-positive errors. 1920 // Spinning, in general, is probabilistic anyway. 1921 1922 1923 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) { 1924 // Check ox->TypeTag == 2BAD. 1925 if (ox == NULL) return 0; 1926 1927 // Avoid transitive spinning ... 1928 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. 1929 // Immediately after T1 acquires L it's possible that T2, also 1930 // spinning on L, will see L.Owner=T1 and T1._Stalled=L. 1931 // This occurs transiently after T1 acquired L but before 1932 // T1 managed to clear T1.Stalled. T2 does not need to abort 1933 // its spin in this circumstance. 1934 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1)); 1935 1936 if (BlockedOn == 1) return 1; 1937 if (BlockedOn != 0) { 1938 return BlockedOn != intptr_t(this) && _owner == ox; 1939 } 1940 1941 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant"); 1942 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);; 1943 // consider also: jst != _thread_in_Java -- but that's overspecific. 1944 return jst == _thread_blocked || jst == _thread_in_native; 1945 } 1946 1947 1948 // ----------------------------------------------------------------------------- 1949 // WaitSet management ... 1950 1951 ObjectWaiter::ObjectWaiter(Thread* thread) { 1952 _next = NULL; 1953 _prev = NULL; 1954 _notified = 0; 1955 _notifier_tid = 0; 1956 TState = TS_RUN; 1957 _thread = thread; 1958 _event = thread->_ParkEvent; 1959 _active = false; 1960 assert(_event != NULL, "invariant"); 1961 } 1962 1963 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) { 1964 JavaThread *jt = (JavaThread *)this->_thread; 1965 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); 1966 } 1967 1968 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) { 1969 JavaThread *jt = (JavaThread *)this->_thread; 1970 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); 1971 } 1972 1973 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { 1974 assert(node != NULL, "should not add NULL node"); 1975 assert(node->_prev == NULL, "node already in list"); 1976 assert(node->_next == NULL, "node already in list"); 1977 // put node at end of queue (circular doubly linked list) 1978 if (_WaitSet == NULL) { 1979 _WaitSet = node; 1980 node->_prev = node; 1981 node->_next = node; 1982 } else { 1983 ObjectWaiter* head = _WaitSet; 1984 ObjectWaiter* tail = head->_prev; 1985 assert(tail->_next == head, "invariant check"); 1986 tail->_next = node; 1987 head->_prev = node; 1988 node->_next = head; 1989 node->_prev = tail; 1990 } 1991 } 1992 1993 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { 1994 // dequeue the very first waiter 1995 ObjectWaiter* waiter = _WaitSet; 1996 if (waiter) { 1997 DequeueSpecificWaiter(waiter); 1998 } 1999 return waiter; 2000 } 2001 2002 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { 2003 assert(node != NULL, "should not dequeue NULL node"); 2004 assert(node->_prev != NULL, "node already removed from list"); 2005 assert(node->_next != NULL, "node already removed from list"); 2006 // when the waiter has woken up because of interrupt, 2007 // timeout or other spurious wake-up, dequeue the 2008 // waiter from waiting list 2009 ObjectWaiter* next = node->_next; 2010 if (next == node) { 2011 assert(node->_prev == node, "invariant check"); 2012 _WaitSet = NULL; 2013 } else { 2014 ObjectWaiter* prev = node->_prev; 2015 assert(prev->_next == node, "invariant check"); 2016 assert(next->_prev == node, "invariant check"); 2017 next->_prev = prev; 2018 prev->_next = next; 2019 if (_WaitSet == node) { 2020 _WaitSet = next; 2021 } 2022 } 2023 node->_next = NULL; 2024 node->_prev = NULL; 2025 } 2026 2027 // ----------------------------------------------------------------------------- 2028 // PerfData support 2029 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL; 2030 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL; 2031 PerfCounter * ObjectMonitor::_sync_Parks = NULL; 2032 PerfCounter * ObjectMonitor::_sync_Notifications = NULL; 2033 PerfCounter * ObjectMonitor::_sync_Inflations = NULL; 2034 PerfCounter * ObjectMonitor::_sync_Deflations = NULL; 2035 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL; 2036 2037 // One-shot global initialization for the sync subsystem. 2038 // We could also defer initialization and initialize on-demand 2039 // the first time we call ObjectSynchronizer::inflate(). 2040 // Initialization would be protected - like so many things - by 2041 // the MonitorCache_lock. 2042 2043 void ObjectMonitor::Initialize() { 2044 assert(!InitDone, "invariant"); 2045 2046 if (!os::is_MP()) { 2047 Knob_SpinLimit = 0; 2048 Knob_PreSpin = 0; 2049 Knob_FixedSpin = -1; 2050 } 2051 2052 if (UsePerfData) { 2053 EXCEPTION_MARK; 2054 #define NEWPERFCOUNTER(n) \ 2055 { \ 2056 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \ 2057 CHECK); \ 2058 } 2059 #define NEWPERFVARIABLE(n) \ 2060 { \ 2061 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \ 2062 CHECK); \ 2063 } 2064 NEWPERFCOUNTER(_sync_Inflations); 2065 NEWPERFCOUNTER(_sync_Deflations); 2066 NEWPERFCOUNTER(_sync_ContendedLockAttempts); 2067 NEWPERFCOUNTER(_sync_FutileWakeups); 2068 NEWPERFCOUNTER(_sync_Parks); 2069 NEWPERFCOUNTER(_sync_Notifications); 2070 NEWPERFVARIABLE(_sync_MonExtant); 2071 #undef NEWPERFCOUNTER 2072 #undef NEWPERFVARIABLE 2073 } 2074 2075 DEBUG_ONLY(InitDone = true;) 2076 } 2077 2078 ObjectMonitorHandle::~ObjectMonitorHandle() { 2079 if (_om_ptr != NULL) { 2080 _om_ptr->dec_ref_count(); 2081 _om_ptr = NULL; 2082 } 2083 } 2084 2085 // Save the ObjectMonitor* associated with the specified markWord and 2086 // increment the ref_count. This function should only be called if 2087 // the caller has verified mark.has_monitor() == true. The object 2088 // parameter is needed to verify that ObjectMonitor* has not been 2089 // deflated and reused for another object. 2090 // 2091 // This function returns true if the ObjectMonitor* has been safely 2092 // saved. This function returns false if we have lost a race with 2093 // async deflation; the caller should retry as appropriate. 2094 // 2095 bool ObjectMonitorHandle::save_om_ptr(oop object, markWord mark) { 2096 guarantee(mark.has_monitor(), "sanity check: mark=" INTPTR_FORMAT, 2097 mark.value()); 2098 2099 ObjectMonitor* om_ptr = mark.monitor(); 2100 om_ptr->inc_ref_count(); 2101 2102 if (AsyncDeflateIdleMonitors) { 2103 // Race here if monitor is not owned! The above ref_count bump 2104 // will cause subsequent async deflation to skip it. However, 2105 // previous or concurrent async deflation is a race. 2106 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) { 2107 // Async deflation is in progress and our ref_count increment 2108 // above lost the race to async deflation. Attempt to restore 2109 // the header/dmw to the object's header so that we only retry 2110 // once if the deflater thread happens to be slow. 2111 om_ptr->install_displaced_markword_in_object(object); 2112 om_ptr->dec_ref_count(); 2113 return false; 2114 } 2115 if (om_ptr->ref_count() <= 0) { 2116 // Async deflation is in the process of bailing out, but has not 2117 // yet restored the ref_count field so we return false to force 2118 // a retry. We want a positive ref_count value for a true return. 2119 om_ptr->dec_ref_count(); 2120 return false; 2121 } 2122 // The ObjectMonitor could have been deflated and reused for 2123 // another object before we bumped the ref_count so make sure 2124 // our object still refers to this ObjectMonitor. 2125 const markWord tmp = object->mark(); 2126 if (!tmp.has_monitor() || tmp.monitor() != om_ptr) { 2127 // Async deflation and reuse won the race so we have to retry. 2128 // Skip object header restoration since that's already done. 2129 om_ptr->dec_ref_count(); 2130 return false; 2131 } 2132 } 2133 2134 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT, 2135 p2i(_om_ptr)); 2136 _om_ptr = om_ptr; 2137 return true; 2138 } 2139 2140 // For internal use by ObjectSynchronizer::inflate(). 2141 // This function is only used when we don't have to worry about async 2142 // deflation of the specified ObjectMonitor*. 2143 void ObjectMonitorHandle::set_om_ptr(ObjectMonitor* om_ptr) { 2144 if (_om_ptr == NULL) { 2145 ADIM_guarantee(om_ptr != NULL, "cannot clear an unset om_ptr"); 2146 om_ptr->inc_ref_count(); 2147 _om_ptr = om_ptr; 2148 } else { 2149 ADIM_guarantee(om_ptr == NULL, "can only clear a set om_ptr"); 2150 _om_ptr->dec_ref_count(); 2151 _om_ptr = NULL; 2152 } 2153 } 2154 2155 // Save the specified ObjectMonitor* if it is safe, i.e., not being 2156 // async deflated. 2157 // 2158 // This function returns true if the ObjectMonitor* has been safely 2159 // saved. This function returns false if the specified ObjectMonitor* 2160 // is NULL or if we have lost a race with async deflation; the caller 2161 // can retry as appropriate. 2162 bool ObjectMonitorHandle::set_om_ptr_if_safe(ObjectMonitor* om_ptr) { 2163 if (om_ptr == NULL) { 2164 return false; // Nothing to save if input is NULL 2165 } 2166 2167 om_ptr->inc_ref_count(); 2168 2169 if (AsyncDeflateIdleMonitors) { 2170 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) { 2171 // Async deflation is in progress and our ref_count increment 2172 // above lost the race to async deflation. 2173 om_ptr->dec_ref_count(); 2174 return false; 2175 } 2176 if (om_ptr->ref_count() <= 0) { 2177 // Async deflation is in the process of bailing out, but has not 2178 // yet restored the ref_count field so we return false to force 2179 // a retry. We want a positive ref_count value for a true return. 2180 om_ptr->dec_ref_count(); 2181 return false; 2182 } 2183 // Unlike save_om_ptr(), we don't have context to determine if 2184 // the ObjectMonitor has been deflated and reused for another 2185 // object. 2186 } 2187 2188 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT, 2189 p2i(_om_ptr)); 2190 _om_ptr = om_ptr; 2191 return true; 2192 } 2193 2194 // Unset the _om_ptr field and decrement the ref_count field. 2195 void ObjectMonitorHandle::unset_om_ptr() { 2196 ADIM_guarantee(_om_ptr != NULL, "_om_ptr must not be NULL"); 2197 _om_ptr->dec_ref_count(); 2198 _om_ptr = NULL; 2199 } 2200 2201 void ObjectMonitor::print_on(outputStream* st) const { 2202 // The minimal things to print for markWord printing, more can be added for debugging and logging. 2203 st->print("{contentions=0x%08x,waiters=0x%08x" 2204 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}", 2205 contentions(), waiters(), recursions(), 2206 p2i(owner())); 2207 } 2208 void ObjectMonitor::print() const { print_on(tty); } 2209 2210 // Print the ObjectMonitor like a debugger would: 2211 // 2212 // (ObjectMonitor) 0x00007fdfb6012e40 = { 2213 // _header = (_value = 1) 2214 // _object = 0x000000070ff45fd0 2215 // _allocation_state = Old 2216 // _pad_buf0 = { 2217 // [0] = '\0' 2218 // ... 2219 // [43] = '\0' 2220 // } 2221 // _owner = 0x0000000000000000 2222 // _previous_owner_tid = 0 2223 // _pad_buf1 = { 2224 // [0] = '\0' 2225 // ... 2226 // [47] = '\0' 2227 // } 2228 // _ref_count = 1 2229 // _pad_buf2 = { 2230 // [0] = '\0' 2231 // ... 2232 // [59] = '\0' 2233 // } 2234 // _next_om = 0x0000000000000000 2235 // _recursions = 0 2236 // _EntryList = 0x0000000000000000 2237 // _cxq = 0x0000000000000000 2238 // _succ = 0x0000000000000000 2239 // _Responsible = 0x0000000000000000 2240 // _Spinner = 0 2241 // _SpinDuration = 5000 2242 // _contentions = 0 2243 // _WaitSet = 0x0000700009756248 2244 // _waiters = 1 2245 // _WaitSetLock = 0 2246 // } 2247 // 2248 void ObjectMonitor::print_debug_style_on(outputStream* st) const { 2249 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this)); 2250 st->print_cr(" _header = " INTPTR_FORMAT, header().value()); 2251 st->print_cr(" _object = " INTPTR_FORMAT, p2i(_object)); 2252 st->print(" _allocation_state = "); 2253 if (is_free()) { 2254 st->print("Free"); 2255 } else if (is_old()) { 2256 st->print("Old"); 2257 } else if (is_new()) { 2258 st->print("New"); 2259 } else { 2260 st->print("unknown=%d", _allocation_state); 2261 } 2262 st->cr(); 2263 st->print_cr(" _pad_buf0 = {"); 2264 st->print_cr(" [0] = '\\0'"); 2265 st->print_cr(" ..."); 2266 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1); 2267 st->print_cr(" }"); 2268 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(_owner)); 2269 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid); 2270 st->print_cr(" _pad_buf1 = {"); 2271 st->print_cr(" [0] = '\\0'"); 2272 st->print_cr(" ..."); 2273 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1); 2274 st->print_cr(" }"); 2275 st->print_cr(" _ref_count = %d", ref_count()); 2276 st->print_cr(" _pad_buf2 = {"); 2277 st->print_cr(" [0] = '\\0'"); 2278 st->print_cr(" ..."); 2279 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1); 2280 st->print_cr(" }"); 2281 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(_next_om)); 2282 st->print_cr(" _recursions = " INTX_FORMAT, _recursions); 2283 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList)); 2284 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq)); 2285 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ)); 2286 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible)); 2287 st->print_cr(" _Spinner = %d", _Spinner); 2288 st->print_cr(" _SpinDuration = %d", _SpinDuration); 2289 st->print_cr(" _contentions = %d", _contentions); 2290 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet)); 2291 st->print_cr(" _waiters = %d", _waiters); 2292 st->print_cr(" _WaitSetLock = %d", _WaitSetLock); 2293 st->print_cr("}"); 2294 }