1 /* 2 * Copyright (c) 1998, 2018, 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 "logging/log.hpp" 28 #include "jfr/jfrEvents.hpp" 29 #include "memory/allocation.inline.hpp" 30 #include "memory/metaspaceShared.hpp" 31 #include "memory/padded.hpp" 32 #include "memory/resourceArea.hpp" 33 #include "oops/markOop.hpp" 34 #include "oops/oop.inline.hpp" 35 #include "runtime/atomic.hpp" 36 #include "runtime/biasedLocking.hpp" 37 #include "runtime/handles.inline.hpp" 38 #include "runtime/interfaceSupport.inline.hpp" 39 #include "runtime/mutexLocker.hpp" 40 #include "runtime/objectMonitor.hpp" 41 #include "runtime/objectMonitor.inline.hpp" 42 #include "runtime/osThread.hpp" 43 #include "runtime/safepointVerifiers.hpp" 44 #include "runtime/sharedRuntime.hpp" 45 #include "runtime/stubRoutines.hpp" 46 #include "runtime/synchronizer.hpp" 47 #include "runtime/thread.inline.hpp" 48 #include "runtime/vframe.hpp" 49 #include "runtime/vmThread.hpp" 50 #include "utilities/align.hpp" 51 #include "utilities/dtrace.hpp" 52 #include "utilities/events.hpp" 53 #include "utilities/preserveException.hpp" 54 55 // The "core" versions of monitor enter and exit reside in this file. 56 // The interpreter and compilers contain specialized transliterated 57 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(), 58 // for instance. If you make changes here, make sure to modify the 59 // interpreter, and both C1 and C2 fast-path inline locking code emission. 60 // 61 // ----------------------------------------------------------------------------- 62 63 #ifdef DTRACE_ENABLED 64 65 // Only bother with this argument setup if dtrace is available 66 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. 67 68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ 69 char* bytes = NULL; \ 70 int len = 0; \ 71 jlong jtid = SharedRuntime::get_java_tid(thread); \ 72 Symbol* klassname = ((oop)(obj))->klass()->name(); \ 73 if (klassname != NULL) { \ 74 bytes = (char*)klassname->bytes(); \ 75 len = klassname->utf8_length(); \ 76 } 77 78 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ 79 { \ 80 if (DTraceMonitorProbes) { \ 81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 82 HOTSPOT_MONITOR_WAIT(jtid, \ 83 (uintptr_t)(monitor), bytes, len, (millis)); \ 84 } \ 85 } 86 87 #define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY 88 #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL 89 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED 90 91 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ 92 { \ 93 if (DTraceMonitorProbes) { \ 94 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 95 HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \ 96 (uintptr_t)(monitor), bytes, len); \ 97 } \ 98 } 99 100 #else // ndef DTRACE_ENABLED 101 102 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} 103 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} 104 105 #endif // ndef DTRACE_ENABLED 106 107 // This exists only as a workaround of dtrace bug 6254741 108 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) { 109 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr); 110 return 0; 111 } 112 113 #define NINFLATIONLOCKS 256 114 static volatile intptr_t gInflationLocks[NINFLATIONLOCKS]; 115 116 // global list of blocks of monitors 117 PaddedEnd<ObjectMonitor> * volatile ObjectSynchronizer::gBlockList = NULL; 118 // global monitor free list 119 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL; 120 // global monitor in-use list, for moribund threads, 121 // monitors they inflated need to be scanned for deflation 122 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL; 123 // count of entries in gOmInUseList 124 int ObjectSynchronizer::gOmInUseCount = 0; 125 126 static volatile intptr_t gListLock = 0; // protects global monitor lists 127 static volatile int gMonitorFreeCount = 0; // # on gFreeList 128 static volatile int gMonitorPopulation = 0; // # Extant -- in circulation 129 130 #define CHAINMARKER (cast_to_oop<intptr_t>(-1)) 131 132 133 // =====================> Quick functions 134 135 // The quick_* forms are special fast-path variants used to improve 136 // performance. In the simplest case, a "quick_*" implementation could 137 // simply return false, in which case the caller will perform the necessary 138 // state transitions and call the slow-path form. 139 // The fast-path is designed to handle frequently arising cases in an efficient 140 // manner and is just a degenerate "optimistic" variant of the slow-path. 141 // returns true -- to indicate the call was satisfied. 142 // returns false -- to indicate the call needs the services of the slow-path. 143 // A no-loitering ordinance is in effect for code in the quick_* family 144 // operators: safepoints or indefinite blocking (blocking that might span a 145 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon 146 // entry. 147 // 148 // Consider: An interesting optimization is to have the JIT recognize the 149 // following common idiom: 150 // synchronized (someobj) { .... ; notify(); } 151 // That is, we find a notify() or notifyAll() call that immediately precedes 152 // the monitorexit operation. In that case the JIT could fuse the operations 153 // into a single notifyAndExit() runtime primitive. 154 155 bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * self, bool all) { 156 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 157 assert(self->is_Java_thread(), "invariant"); 158 assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant"); 159 NoSafepointVerifier nsv; 160 if (obj == NULL) return false; // slow-path for invalid obj 161 const markOop mark = obj->mark(); 162 163 if (mark->has_locker() && self->is_lock_owned((address)mark->locker())) { 164 // Degenerate notify 165 // stack-locked by caller so by definition the implied waitset is empty. 166 return true; 167 } 168 169 if (mark->has_monitor()) { 170 ObjectMonitor * const mon = mark->monitor(); 171 assert(oopDesc::equals((oop) mon->object(), obj), "invariant"); 172 if (mon->owner() != self) return false; // slow-path for IMS exception 173 174 if (mon->first_waiter() != NULL) { 175 // We have one or more waiters. Since this is an inflated monitor 176 // that we own, we can transfer one or more threads from the waitset 177 // to the entrylist here and now, avoiding the slow-path. 178 if (all) { 179 DTRACE_MONITOR_PROBE(notifyAll, mon, obj, self); 180 } else { 181 DTRACE_MONITOR_PROBE(notify, mon, obj, self); 182 } 183 int tally = 0; 184 do { 185 mon->INotify(self); 186 ++tally; 187 } while (mon->first_waiter() != NULL && all); 188 OM_PERFDATA_OP(Notifications, inc(tally)); 189 } 190 return true; 191 } 192 193 // biased locking and any other IMS exception states take the slow-path 194 return false; 195 } 196 197 198 // The LockNode emitted directly at the synchronization site would have 199 // been too big if it were to have included support for the cases of inflated 200 // recursive enter and exit, so they go here instead. 201 // Note that we can't safely call AsyncPrintJavaStack() from within 202 // quick_enter() as our thread state remains _in_Java. 203 204 bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self, 205 BasicLock * lock) { 206 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 207 assert(Self->is_Java_thread(), "invariant"); 208 assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant"); 209 NoSafepointVerifier nsv; 210 if (obj == NULL) return false; // Need to throw NPE 211 const markOop mark = obj->mark(); 212 213 if (mark->has_monitor()) { 214 ObjectMonitor * const m = mark->monitor(); 215 assert(oopDesc::equals((oop) m->object(), obj), "invariant"); 216 Thread * const owner = (Thread *) m->_owner; 217 218 // Lock contention and Transactional Lock Elision (TLE) diagnostics 219 // and observability 220 // Case: light contention possibly amenable to TLE 221 // Case: TLE inimical operations such as nested/recursive synchronization 222 223 if (owner == Self) { 224 m->_recursions++; 225 return true; 226 } 227 228 // This Java Monitor is inflated so obj's header will never be 229 // displaced to this thread's BasicLock. Make the displaced header 230 // non-NULL so this BasicLock is not seen as recursive nor as 231 // being locked. We do this unconditionally so that this thread's 232 // BasicLock cannot be mis-interpreted by any stack walkers. For 233 // performance reasons, stack walkers generally first check for 234 // Biased Locking in the object's header, the second check is for 235 // stack-locking in the object's header, the third check is for 236 // recursive stack-locking in the displaced header in the BasicLock, 237 // and last are the inflated Java Monitor (ObjectMonitor) checks. 238 lock->set_displaced_header(markOopDesc::unused_mark()); 239 240 if (owner == NULL && Atomic::replace_if_null(Self, &(m->_owner))) { 241 assert(m->_recursions == 0, "invariant"); 242 assert(m->_owner == Self, "invariant"); 243 return true; 244 } 245 } 246 247 // Note that we could inflate in quick_enter. 248 // This is likely a useful optimization 249 // Critically, in quick_enter() we must not: 250 // -- perform bias revocation, or 251 // -- block indefinitely, or 252 // -- reach a safepoint 253 254 return false; // revert to slow-path 255 } 256 257 // ----------------------------------------------------------------------------- 258 // Fast Monitor Enter/Exit 259 // This the fast monitor enter. The interpreter and compiler use 260 // some assembly copies of this code. Make sure update those code 261 // if the following function is changed. The implementation is 262 // extremely sensitive to race condition. Be careful. 263 264 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, 265 bool attempt_rebias, TRAPS) { 266 if (UseBiasedLocking) { 267 if (!SafepointSynchronize::is_at_safepoint()) { 268 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD); 269 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) { 270 return; 271 } 272 } else { 273 assert(!attempt_rebias, "can not rebias toward VM thread"); 274 BiasedLocking::revoke_at_safepoint(obj); 275 } 276 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 277 } 278 279 slow_enter(obj, lock, THREAD); 280 } 281 282 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) { 283 markOop mark = object->mark(); 284 // We cannot check for Biased Locking if we are racing an inflation. 285 assert(mark == markOopDesc::INFLATING() || 286 !mark->has_bias_pattern(), "should not see bias pattern here"); 287 288 markOop dhw = lock->displaced_header(); 289 if (dhw == NULL) { 290 // If the displaced header is NULL, then this exit matches up with 291 // a recursive enter. No real work to do here except for diagnostics. 292 #ifndef PRODUCT 293 if (mark != markOopDesc::INFLATING()) { 294 // Only do diagnostics if we are not racing an inflation. Simply 295 // exiting a recursive enter of a Java Monitor that is being 296 // inflated is safe; see the has_monitor() comment below. 297 assert(!mark->is_neutral(), "invariant"); 298 assert(!mark->has_locker() || 299 THREAD->is_lock_owned((address)mark->locker()), "invariant"); 300 if (mark->has_monitor()) { 301 // The BasicLock's displaced_header is marked as a recursive 302 // enter and we have an inflated Java Monitor (ObjectMonitor). 303 // This is a special case where the Java Monitor was inflated 304 // after this thread entered the stack-lock recursively. When a 305 // Java Monitor is inflated, we cannot safely walk the Java 306 // Monitor owner's stack and update the BasicLocks because a 307 // Java Monitor can be asynchronously inflated by a thread that 308 // does not own the Java Monitor. 309 ObjectMonitor * m = mark->monitor(); 310 assert(((oop)(m->object()))->mark() == mark, "invariant"); 311 assert(m->is_entered(THREAD), "invariant"); 312 } 313 } 314 #endif 315 return; 316 } 317 318 if (mark == (markOop) lock) { 319 // If the object is stack-locked by the current thread, try to 320 // swing the displaced header from the BasicLock back to the mark. 321 assert(dhw->is_neutral(), "invariant"); 322 if (object->cas_set_mark(dhw, mark) == mark) { 323 TEVENT(fast_exit: release stack-lock); 324 return; 325 } 326 } 327 328 // We have to take the slow-path of possible inflation and then exit. 329 ObjectSynchronizer::inflate(THREAD, 330 object, 331 inflate_cause_vm_internal)->exit(true, THREAD); 332 } 333 334 // ----------------------------------------------------------------------------- 335 // Interpreter/Compiler Slow Case 336 // This routine is used to handle interpreter/compiler slow case 337 // We don't need to use fast path here, because it must have been 338 // failed in the interpreter/compiler code. 339 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) { 340 markOop mark = obj->mark(); 341 assert(!mark->has_bias_pattern(), "should not see bias pattern here"); 342 343 if (mark->is_neutral()) { 344 // Anticipate successful CAS -- the ST of the displaced mark must 345 // be visible <= the ST performed by the CAS. 346 lock->set_displaced_header(mark); 347 if (mark == obj()->cas_set_mark((markOop) lock, mark)) { 348 TEVENT(slow_enter: release stacklock); 349 return; 350 } 351 // Fall through to inflate() ... 352 } else if (mark->has_locker() && 353 THREAD->is_lock_owned((address)mark->locker())) { 354 assert(lock != mark->locker(), "must not re-lock the same lock"); 355 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock"); 356 lock->set_displaced_header(NULL); 357 return; 358 } 359 360 // The object header will never be displaced to this lock, 361 // so it does not matter what the value is, except that it 362 // must be non-zero to avoid looking like a re-entrant lock, 363 // and must not look locked either. 364 lock->set_displaced_header(markOopDesc::unused_mark()); 365 ObjectSynchronizer::inflate(THREAD, 366 obj(), 367 inflate_cause_monitor_enter)->enter(THREAD); 368 } 369 370 // This routine is used to handle interpreter/compiler slow case 371 // We don't need to use fast path here, because it must have 372 // failed in the interpreter/compiler code. Simply use the heavy 373 // weight monitor should be ok, unless someone find otherwise. 374 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) { 375 fast_exit(object, lock, THREAD); 376 } 377 378 // ----------------------------------------------------------------------------- 379 // Class Loader support to workaround deadlocks on the class loader lock objects 380 // Also used by GC 381 // complete_exit()/reenter() are used to wait on a nested lock 382 // i.e. to give up an outer lock completely and then re-enter 383 // Used when holding nested locks - lock acquisition order: lock1 then lock2 384 // 1) complete_exit lock1 - saving recursion count 385 // 2) wait on lock2 386 // 3) when notified on lock2, unlock lock2 387 // 4) reenter lock1 with original recursion count 388 // 5) lock lock2 389 // NOTE: must use heavy weight monitor to handle complete_exit/reenter() 390 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) { 391 TEVENT(complete_exit); 392 if (UseBiasedLocking) { 393 BiasedLocking::revoke_and_rebias(obj, false, THREAD); 394 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 395 } 396 397 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, 398 obj(), 399 inflate_cause_vm_internal); 400 401 return monitor->complete_exit(THREAD); 402 } 403 404 // NOTE: must use heavy weight monitor to handle complete_exit/reenter() 405 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) { 406 TEVENT(reenter); 407 if (UseBiasedLocking) { 408 BiasedLocking::revoke_and_rebias(obj, false, THREAD); 409 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 410 } 411 412 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, 413 obj(), 414 inflate_cause_vm_internal); 415 416 monitor->reenter(recursion, THREAD); 417 } 418 // ----------------------------------------------------------------------------- 419 // JNI locks on java objects 420 // NOTE: must use heavy weight monitor to handle jni monitor enter 421 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { 422 // the current locking is from JNI instead of Java code 423 TEVENT(jni_enter); 424 if (UseBiasedLocking) { 425 BiasedLocking::revoke_and_rebias(obj, false, THREAD); 426 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 427 } 428 THREAD->set_current_pending_monitor_is_from_java(false); 429 ObjectSynchronizer::inflate(THREAD, obj(), inflate_cause_jni_enter)->enter(THREAD); 430 THREAD->set_current_pending_monitor_is_from_java(true); 431 } 432 433 // NOTE: must use heavy weight monitor to handle jni monitor exit 434 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) { 435 TEVENT(jni_exit); 436 if (UseBiasedLocking) { 437 Handle h_obj(THREAD, obj); 438 BiasedLocking::revoke_and_rebias(h_obj, false, THREAD); 439 obj = h_obj(); 440 } 441 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 442 443 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, 444 obj, 445 inflate_cause_jni_exit); 446 // If this thread has locked the object, exit the monitor. Note: can't use 447 // monitor->check(CHECK); must exit even if an exception is pending. 448 if (monitor->check(THREAD)) { 449 monitor->exit(true, THREAD); 450 } 451 } 452 453 // ----------------------------------------------------------------------------- 454 // Internal VM locks on java objects 455 // standard constructor, allows locking failures 456 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) { 457 _dolock = doLock; 458 _thread = thread; 459 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);) 460 _obj = obj; 461 462 if (_dolock) { 463 TEVENT(ObjectLocker); 464 465 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread); 466 } 467 } 468 469 ObjectLocker::~ObjectLocker() { 470 if (_dolock) { 471 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread); 472 } 473 } 474 475 476 // ----------------------------------------------------------------------------- 477 // Wait/Notify/NotifyAll 478 // NOTE: must use heavy weight monitor to handle wait() 479 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) { 480 if (UseBiasedLocking) { 481 BiasedLocking::revoke_and_rebias(obj, false, THREAD); 482 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 483 } 484 if (millis < 0) { 485 TEVENT(wait - throw IAX); 486 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative"); 487 } 488 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, 489 obj(), 490 inflate_cause_wait); 491 492 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis); 493 monitor->wait(millis, true, THREAD); 494 495 // This dummy call is in place to get around dtrace bug 6254741. Once 496 // that's fixed we can uncomment the following line, remove the call 497 // and change this function back into a "void" func. 498 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD); 499 return dtrace_waited_probe(monitor, obj, THREAD); 500 } 501 502 void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) { 503 if (UseBiasedLocking) { 504 BiasedLocking::revoke_and_rebias(obj, false, THREAD); 505 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 506 } 507 if (millis < 0) { 508 TEVENT(wait - throw IAX); 509 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative"); 510 } 511 ObjectSynchronizer::inflate(THREAD, 512 obj(), 513 inflate_cause_wait)->wait(millis, false, THREAD); 514 } 515 516 void ObjectSynchronizer::notify(Handle obj, TRAPS) { 517 if (UseBiasedLocking) { 518 BiasedLocking::revoke_and_rebias(obj, false, THREAD); 519 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 520 } 521 522 markOop mark = obj->mark(); 523 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { 524 return; 525 } 526 ObjectSynchronizer::inflate(THREAD, 527 obj(), 528 inflate_cause_notify)->notify(THREAD); 529 } 530 531 // NOTE: see comment of notify() 532 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) { 533 if (UseBiasedLocking) { 534 BiasedLocking::revoke_and_rebias(obj, false, THREAD); 535 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 536 } 537 538 markOop mark = obj->mark(); 539 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { 540 return; 541 } 542 ObjectSynchronizer::inflate(THREAD, 543 obj(), 544 inflate_cause_notify)->notifyAll(THREAD); 545 } 546 547 // ----------------------------------------------------------------------------- 548 // Hash Code handling 549 // 550 // Performance concern: 551 // OrderAccess::storestore() calls release() which at one time stored 0 552 // into the global volatile OrderAccess::dummy variable. This store was 553 // unnecessary for correctness. Many threads storing into a common location 554 // causes considerable cache migration or "sloshing" on large SMP systems. 555 // As such, I avoided using OrderAccess::storestore(). In some cases 556 // OrderAccess::fence() -- which incurs local latency on the executing 557 // processor -- is a better choice as it scales on SMP systems. 558 // 559 // See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for 560 // a discussion of coherency costs. Note that all our current reference 561 // platforms provide strong ST-ST order, so the issue is moot on IA32, 562 // x64, and SPARC. 563 // 564 // As a general policy we use "volatile" to control compiler-based reordering 565 // and explicit fences (barriers) to control for architectural reordering 566 // performed by the CPU(s) or platform. 567 568 struct SharedGlobals { 569 char _pad_prefix[DEFAULT_CACHE_LINE_SIZE]; 570 // These are highly shared mostly-read variables. 571 // To avoid false-sharing they need to be the sole occupants of a cache line. 572 volatile int stwRandom; 573 volatile int stwCycle; 574 DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2); 575 // Hot RW variable -- Sequester to avoid false-sharing 576 volatile int hcSequence; 577 DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int)); 578 }; 579 580 static SharedGlobals GVars; 581 static int MonitorScavengeThreshold = 1000000; 582 static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending 583 584 static markOop ReadStableMark(oop obj) { 585 markOop mark = obj->mark(); 586 if (!mark->is_being_inflated()) { 587 return mark; // normal fast-path return 588 } 589 590 int its = 0; 591 for (;;) { 592 markOop mark = obj->mark(); 593 if (!mark->is_being_inflated()) { 594 return mark; // normal fast-path return 595 } 596 597 // The object is being inflated by some other thread. 598 // The caller of ReadStableMark() must wait for inflation to complete. 599 // Avoid live-lock 600 // TODO: consider calling SafepointSynchronize::do_call_back() while 601 // spinning to see if there's a safepoint pending. If so, immediately 602 // yielding or blocking would be appropriate. Avoid spinning while 603 // there is a safepoint pending. 604 // TODO: add inflation contention performance counters. 605 // TODO: restrict the aggregate number of spinners. 606 607 ++its; 608 if (its > 10000 || !os::is_MP()) { 609 if (its & 1) { 610 os::naked_yield(); 611 TEVENT(Inflate: INFLATING - yield); 612 } else { 613 // Note that the following code attenuates the livelock problem but is not 614 // a complete remedy. A more complete solution would require that the inflating 615 // thread hold the associated inflation lock. The following code simply restricts 616 // the number of spinners to at most one. We'll have N-2 threads blocked 617 // on the inflationlock, 1 thread holding the inflation lock and using 618 // a yield/park strategy, and 1 thread in the midst of inflation. 619 // A more refined approach would be to change the encoding of INFLATING 620 // to allow encapsulation of a native thread pointer. Threads waiting for 621 // inflation to complete would use CAS to push themselves onto a singly linked 622 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag 623 // and calling park(). When inflation was complete the thread that accomplished inflation 624 // would detach the list and set the markword to inflated with a single CAS and 625 // then for each thread on the list, set the flag and unpark() the thread. 626 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease 627 // wakes at most one thread whereas we need to wake the entire list. 628 int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1); 629 int YieldThenBlock = 0; 630 assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant"); 631 assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant"); 632 Thread::muxAcquire(gInflationLocks + ix, "gInflationLock"); 633 while (obj->mark() == markOopDesc::INFLATING()) { 634 // Beware: NakedYield() is advisory and has almost no effect on some platforms 635 // so we periodically call Self->_ParkEvent->park(1). 636 // We use a mixed spin/yield/block mechanism. 637 if ((YieldThenBlock++) >= 16) { 638 Thread::current()->_ParkEvent->park(1); 639 } else { 640 os::naked_yield(); 641 } 642 } 643 Thread::muxRelease(gInflationLocks + ix); 644 TEVENT(Inflate: INFLATING - yield/park); 645 } 646 } else { 647 SpinPause(); // SMP-polite spinning 648 } 649 } 650 } 651 652 // hashCode() generation : 653 // 654 // Possibilities: 655 // * MD5Digest of {obj,stwRandom} 656 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function. 657 // * A DES- or AES-style SBox[] mechanism 658 // * One of the Phi-based schemes, such as: 659 // 2654435761 = 2^32 * Phi (golden ratio) 660 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ; 661 // * A variation of Marsaglia's shift-xor RNG scheme. 662 // * (obj ^ stwRandom) is appealing, but can result 663 // in undesirable regularity in the hashCode values of adjacent objects 664 // (objects allocated back-to-back, in particular). This could potentially 665 // result in hashtable collisions and reduced hashtable efficiency. 666 // There are simple ways to "diffuse" the middle address bits over the 667 // generated hashCode values: 668 669 static inline intptr_t get_next_hash(Thread * Self, oop obj) { 670 intptr_t value = 0; 671 if (hashCode == 0) { 672 // This form uses global Park-Miller RNG. 673 // On MP system we'll have lots of RW access to a global, so the 674 // mechanism induces lots of coherency traffic. 675 value = os::random(); 676 } else if (hashCode == 1) { 677 // This variation has the property of being stable (idempotent) 678 // between STW operations. This can be useful in some of the 1-0 679 // synchronization schemes. 680 intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3; 681 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom; 682 } else if (hashCode == 2) { 683 value = 1; // for sensitivity testing 684 } else if (hashCode == 3) { 685 value = ++GVars.hcSequence; 686 } else if (hashCode == 4) { 687 value = cast_from_oop<intptr_t>(obj); 688 } else { 689 // Marsaglia's xor-shift scheme with thread-specific state 690 // This is probably the best overall implementation -- we'll 691 // likely make this the default in future releases. 692 unsigned t = Self->_hashStateX; 693 t ^= (t << 11); 694 Self->_hashStateX = Self->_hashStateY; 695 Self->_hashStateY = Self->_hashStateZ; 696 Self->_hashStateZ = Self->_hashStateW; 697 unsigned v = Self->_hashStateW; 698 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)); 699 Self->_hashStateW = v; 700 value = v; 701 } 702 703 value &= markOopDesc::hash_mask; 704 if (value == 0) value = 0xBAD; 705 assert(value != markOopDesc::no_hash, "invariant"); 706 TEVENT(hashCode: GENERATE); 707 return value; 708 } 709 710 intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) { 711 if (UseBiasedLocking) { 712 // NOTE: many places throughout the JVM do not expect a safepoint 713 // to be taken here, in particular most operations on perm gen 714 // objects. However, we only ever bias Java instances and all of 715 // the call sites of identity_hash that might revoke biases have 716 // been checked to make sure they can handle a safepoint. The 717 // added check of the bias pattern is to avoid useless calls to 718 // thread-local storage. 719 if (obj->mark()->has_bias_pattern()) { 720 // Handle for oop obj in case of STW safepoint 721 Handle hobj(Self, obj); 722 // Relaxing assertion for bug 6320749. 723 assert(Universe::verify_in_progress() || 724 !SafepointSynchronize::is_at_safepoint(), 725 "biases should not be seen by VM thread here"); 726 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current()); 727 obj = hobj(); 728 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 729 } 730 } 731 732 // hashCode() is a heap mutator ... 733 // Relaxing assertion for bug 6320749. 734 assert(Universe::verify_in_progress() || DumpSharedSpaces || 735 !SafepointSynchronize::is_at_safepoint(), "invariant"); 736 assert(Universe::verify_in_progress() || DumpSharedSpaces || 737 Self->is_Java_thread() , "invariant"); 738 assert(Universe::verify_in_progress() || DumpSharedSpaces || 739 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant"); 740 741 ObjectMonitor* monitor = NULL; 742 markOop temp, test; 743 intptr_t hash; 744 markOop mark = ReadStableMark(obj); 745 746 // object should remain ineligible for biased locking 747 assert(!mark->has_bias_pattern(), "invariant"); 748 749 if (mark->is_neutral()) { 750 hash = mark->hash(); // this is a normal header 751 if (hash) { // if it has hash, just return it 752 return hash; 753 } 754 hash = get_next_hash(Self, obj); // allocate a new hash code 755 temp = mark->copy_set_hash(hash); // merge the hash code into header 756 // use (machine word version) atomic operation to install the hash 757 test = obj->cas_set_mark(temp, mark); 758 if (test == mark) { 759 return hash; 760 } 761 // If atomic operation failed, we must inflate the header 762 // into heavy weight monitor. We could add more code here 763 // for fast path, but it does not worth the complexity. 764 } else if (mark->has_monitor()) { 765 monitor = mark->monitor(); 766 temp = monitor->header(); 767 assert(temp->is_neutral(), "invariant"); 768 hash = temp->hash(); 769 if (hash) { 770 return hash; 771 } 772 // Skip to the following code to reduce code size 773 } else if (Self->is_lock_owned((address)mark->locker())) { 774 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned 775 assert(temp->is_neutral(), "invariant"); 776 hash = temp->hash(); // by current thread, check if the displaced 777 if (hash) { // header contains hash code 778 return hash; 779 } 780 // WARNING: 781 // The displaced header is strictly immutable. 782 // It can NOT be changed in ANY cases. So we have 783 // to inflate the header into heavyweight monitor 784 // even the current thread owns the lock. The reason 785 // is the BasicLock (stack slot) will be asynchronously 786 // read by other threads during the inflate() function. 787 // Any change to stack may not propagate to other threads 788 // correctly. 789 } 790 791 // Inflate the monitor to set hash code 792 monitor = ObjectSynchronizer::inflate(Self, obj, inflate_cause_hash_code); 793 // Load displaced header and check it has hash code 794 mark = monitor->header(); 795 assert(mark->is_neutral(), "invariant"); 796 hash = mark->hash(); 797 if (hash == 0) { 798 hash = get_next_hash(Self, obj); 799 temp = mark->copy_set_hash(hash); // merge hash code into header 800 assert(temp->is_neutral(), "invariant"); 801 test = Atomic::cmpxchg(temp, monitor->header_addr(), mark); 802 if (test != mark) { 803 // The only update to the header in the monitor (outside GC) 804 // is install the hash code. If someone add new usage of 805 // displaced header, please update this code 806 hash = test->hash(); 807 assert(test->is_neutral(), "invariant"); 808 assert(hash != 0, "Trivial unexpected object/monitor header usage."); 809 } 810 } 811 // We finally get the hash 812 return hash; 813 } 814 815 // Deprecated -- use FastHashCode() instead. 816 817 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) { 818 return FastHashCode(Thread::current(), obj()); 819 } 820 821 822 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread, 823 Handle h_obj) { 824 if (UseBiasedLocking) { 825 BiasedLocking::revoke_and_rebias(h_obj, false, thread); 826 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 827 } 828 829 assert(thread == JavaThread::current(), "Can only be called on current thread"); 830 oop obj = h_obj(); 831 832 markOop mark = ReadStableMark(obj); 833 834 // Uncontended case, header points to stack 835 if (mark->has_locker()) { 836 return thread->is_lock_owned((address)mark->locker()); 837 } 838 // Contended case, header points to ObjectMonitor (tagged pointer) 839 if (mark->has_monitor()) { 840 ObjectMonitor* monitor = mark->monitor(); 841 return monitor->is_entered(thread) != 0; 842 } 843 // Unlocked case, header in place 844 assert(mark->is_neutral(), "sanity check"); 845 return false; 846 } 847 848 // Be aware of this method could revoke bias of the lock object. 849 // This method queries the ownership of the lock handle specified by 'h_obj'. 850 // If the current thread owns the lock, it returns owner_self. If no 851 // thread owns the lock, it returns owner_none. Otherwise, it will return 852 // owner_other. 853 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership 854 (JavaThread *self, Handle h_obj) { 855 // The caller must beware this method can revoke bias, and 856 // revocation can result in a safepoint. 857 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 858 assert(self->thread_state() != _thread_blocked, "invariant"); 859 860 // Possible mark states: neutral, biased, stack-locked, inflated 861 862 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) { 863 // CASE: biased 864 BiasedLocking::revoke_and_rebias(h_obj, false, self); 865 assert(!h_obj->mark()->has_bias_pattern(), 866 "biases should be revoked by now"); 867 } 868 869 assert(self == JavaThread::current(), "Can only be called on current thread"); 870 oop obj = h_obj(); 871 markOop mark = ReadStableMark(obj); 872 873 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack. 874 if (mark->has_locker()) { 875 return self->is_lock_owned((address)mark->locker()) ? 876 owner_self : owner_other; 877 } 878 879 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor. 880 // The Object:ObjectMonitor relationship is stable as long as we're 881 // not at a safepoint. 882 if (mark->has_monitor()) { 883 void * owner = mark->monitor()->_owner; 884 if (owner == NULL) return owner_none; 885 return (owner == self || 886 self->is_lock_owned((address)owner)) ? owner_self : owner_other; 887 } 888 889 // CASE: neutral 890 assert(mark->is_neutral(), "sanity check"); 891 return owner_none; // it's unlocked 892 } 893 894 // FIXME: jvmti should call this 895 JavaThread* ObjectSynchronizer::get_lock_owner(ThreadsList * t_list, Handle h_obj) { 896 if (UseBiasedLocking) { 897 if (SafepointSynchronize::is_at_safepoint()) { 898 BiasedLocking::revoke_at_safepoint(h_obj); 899 } else { 900 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current()); 901 } 902 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now"); 903 } 904 905 oop obj = h_obj(); 906 address owner = NULL; 907 908 markOop mark = ReadStableMark(obj); 909 910 // Uncontended case, header points to stack 911 if (mark->has_locker()) { 912 owner = (address) mark->locker(); 913 } 914 915 // Contended case, header points to ObjectMonitor (tagged pointer) 916 if (mark->has_monitor()) { 917 ObjectMonitor* monitor = mark->monitor(); 918 assert(monitor != NULL, "monitor should be non-null"); 919 owner = (address) monitor->owner(); 920 } 921 922 if (owner != NULL) { 923 // owning_thread_from_monitor_owner() may also return NULL here 924 return Threads::owning_thread_from_monitor_owner(t_list, owner); 925 } 926 927 // Unlocked case, header in place 928 // Cannot have assertion since this object may have been 929 // locked by another thread when reaching here. 930 // assert(mark->is_neutral(), "sanity check"); 931 932 return NULL; 933 } 934 935 // Visitors ... 936 937 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) { 938 PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList); 939 while (block != NULL) { 940 assert(block->object() == CHAINMARKER, "must be a block header"); 941 for (int i = _BLOCKSIZE - 1; i > 0; i--) { 942 ObjectMonitor* mid = (ObjectMonitor *)(block + i); 943 oop object = (oop)mid->object(); 944 if (object != NULL) { 945 closure->do_monitor(mid); 946 } 947 } 948 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext; 949 } 950 } 951 952 // Get the next block in the block list. 953 static inline PaddedEnd<ObjectMonitor>* next(PaddedEnd<ObjectMonitor>* block) { 954 assert(block->object() == CHAINMARKER, "must be a block header"); 955 block = (PaddedEnd<ObjectMonitor>*) block->FreeNext; 956 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header"); 957 return block; 958 } 959 960 static bool monitors_used_above_threshold() { 961 if (gMonitorPopulation == 0) { 962 return false; 963 } 964 int monitors_used = gMonitorPopulation - gMonitorFreeCount; 965 int monitor_usage = (monitors_used * 100LL) / gMonitorPopulation; 966 return monitor_usage > MonitorUsedDeflationThreshold; 967 } 968 969 bool ObjectSynchronizer::is_cleanup_needed() { 970 if (MonitorUsedDeflationThreshold > 0) { 971 return monitors_used_above_threshold(); 972 } 973 return false; 974 } 975 976 void ObjectSynchronizer::oops_do(OopClosure* f) { 977 if (MonitorInUseLists) { 978 // When using thread local monitor lists, we only scan the 979 // global used list here (for moribund threads), and 980 // the thread-local monitors in Thread::oops_do(). 981 global_used_oops_do(f); 982 } else { 983 global_oops_do(f); 984 } 985 } 986 987 void ObjectSynchronizer::global_oops_do(OopClosure* f) { 988 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); 989 PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList); 990 for (; block != NULL; block = next(block)) { 991 assert(block->object() == CHAINMARKER, "must be a block header"); 992 for (int i = 1; i < _BLOCKSIZE; i++) { 993 ObjectMonitor* mid = (ObjectMonitor *)&block[i]; 994 if (mid->object() != NULL) { 995 f->do_oop((oop*)mid->object_addr()); 996 } 997 } 998 } 999 } 1000 1001 void ObjectSynchronizer::global_used_oops_do(OopClosure* f) { 1002 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); 1003 list_oops_do(gOmInUseList, f); 1004 } 1005 1006 void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) { 1007 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); 1008 list_oops_do(thread->omInUseList, f); 1009 } 1010 1011 void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) { 1012 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); 1013 ObjectMonitor* mid; 1014 for (mid = list; mid != NULL; mid = mid->FreeNext) { 1015 if (mid->object() != NULL) { 1016 f->do_oop((oop*)mid->object_addr()); 1017 } 1018 } 1019 } 1020 1021 1022 // ----------------------------------------------------------------------------- 1023 // ObjectMonitor Lifecycle 1024 // ----------------------- 1025 // Inflation unlinks monitors from the global gFreeList and 1026 // associates them with objects. Deflation -- which occurs at 1027 // STW-time -- disassociates idle monitors from objects. Such 1028 // scavenged monitors are returned to the gFreeList. 1029 // 1030 // The global list is protected by gListLock. All the critical sections 1031 // are short and operate in constant-time. 1032 // 1033 // ObjectMonitors reside in type-stable memory (TSM) and are immortal. 1034 // 1035 // Lifecycle: 1036 // -- unassigned and on the global free list 1037 // -- unassigned and on a thread's private omFreeList 1038 // -- assigned to an object. The object is inflated and the mark refers 1039 // to the objectmonitor. 1040 1041 1042 // Constraining monitor pool growth via MonitorBound ... 1043 // 1044 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the 1045 // the rate of scavenging is driven primarily by GC. As such, we can find 1046 // an inordinate number of monitors in circulation. 1047 // To avoid that scenario we can artificially induce a STW safepoint 1048 // if the pool appears to be growing past some reasonable bound. 1049 // Generally we favor time in space-time tradeoffs, but as there's no 1050 // natural back-pressure on the # of extant monitors we need to impose some 1051 // type of limit. Beware that if MonitorBound is set to too low a value 1052 // we could just loop. In addition, if MonitorBound is set to a low value 1053 // we'll incur more safepoints, which are harmful to performance. 1054 // See also: GuaranteedSafepointInterval 1055 // 1056 // The current implementation uses asynchronous VM operations. 1057 1058 static void InduceScavenge(Thread * Self, const char * Whence) { 1059 // Induce STW safepoint to trim monitors 1060 // Ultimately, this results in a call to deflate_idle_monitors() in the near future. 1061 // More precisely, trigger an asynchronous STW safepoint as the number 1062 // of active monitors passes the specified threshold. 1063 // TODO: assert thread state is reasonable 1064 1065 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) { 1066 if (ObjectMonitor::Knob_Verbose) { 1067 tty->print_cr("INFO: Monitor scavenge - Induced STW @%s (%d)", 1068 Whence, ForceMonitorScavenge) ; 1069 tty->flush(); 1070 } 1071 // Induce a 'null' safepoint to scavenge monitors 1072 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted 1073 // to the VMthread and have a lifespan longer than that of this activation record. 1074 // The VMThread will delete the op when completed. 1075 VMThread::execute(new VM_ScavengeMonitors()); 1076 1077 if (ObjectMonitor::Knob_Verbose) { 1078 tty->print_cr("INFO: Monitor scavenge - STW posted @%s (%d)", 1079 Whence, ForceMonitorScavenge) ; 1080 tty->flush(); 1081 } 1082 } 1083 } 1084 1085 void ObjectSynchronizer::verifyInUse(Thread *Self) { 1086 ObjectMonitor* mid; 1087 int in_use_tally = 0; 1088 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) { 1089 in_use_tally++; 1090 } 1091 assert(in_use_tally == Self->omInUseCount, "in-use count off"); 1092 1093 int free_tally = 0; 1094 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) { 1095 free_tally++; 1096 } 1097 assert(free_tally == Self->omFreeCount, "free count off"); 1098 } 1099 1100 ObjectMonitor* ObjectSynchronizer::omAlloc(Thread * Self) { 1101 // A large MAXPRIVATE value reduces both list lock contention 1102 // and list coherency traffic, but also tends to increase the 1103 // number of objectMonitors in circulation as well as the STW 1104 // scavenge costs. As usual, we lean toward time in space-time 1105 // tradeoffs. 1106 const int MAXPRIVATE = 1024; 1107 for (;;) { 1108 ObjectMonitor * m; 1109 1110 // 1: try to allocate from the thread's local omFreeList. 1111 // Threads will attempt to allocate first from their local list, then 1112 // from the global list, and only after those attempts fail will the thread 1113 // attempt to instantiate new monitors. Thread-local free lists take 1114 // heat off the gListLock and improve allocation latency, as well as reducing 1115 // coherency traffic on the shared global list. 1116 m = Self->omFreeList; 1117 if (m != NULL) { 1118 Self->omFreeList = m->FreeNext; 1119 Self->omFreeCount--; 1120 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene 1121 guarantee(m->object() == NULL, "invariant"); 1122 if (MonitorInUseLists) { 1123 m->FreeNext = Self->omInUseList; 1124 Self->omInUseList = m; 1125 Self->omInUseCount++; 1126 if (ObjectMonitor::Knob_VerifyInUse) { 1127 verifyInUse(Self); 1128 } 1129 } else { 1130 m->FreeNext = NULL; 1131 } 1132 return m; 1133 } 1134 1135 // 2: try to allocate from the global gFreeList 1136 // CONSIDER: use muxTry() instead of muxAcquire(). 1137 // If the muxTry() fails then drop immediately into case 3. 1138 // If we're using thread-local free lists then try 1139 // to reprovision the caller's free list. 1140 if (gFreeList != NULL) { 1141 // Reprovision the thread's omFreeList. 1142 // Use bulk transfers to reduce the allocation rate and heat 1143 // on various locks. 1144 Thread::muxAcquire(&gListLock, "omAlloc"); 1145 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) { 1146 gMonitorFreeCount--; 1147 ObjectMonitor * take = gFreeList; 1148 gFreeList = take->FreeNext; 1149 guarantee(take->object() == NULL, "invariant"); 1150 guarantee(!take->is_busy(), "invariant"); 1151 take->Recycle(); 1152 omRelease(Self, take, false); 1153 } 1154 Thread::muxRelease(&gListLock); 1155 Self->omFreeProvision += 1 + (Self->omFreeProvision/2); 1156 if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE; 1157 TEVENT(omFirst - reprovision); 1158 1159 const int mx = MonitorBound; 1160 if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) { 1161 // We can't safely induce a STW safepoint from omAlloc() as our thread 1162 // state may not be appropriate for such activities and callers may hold 1163 // naked oops, so instead we defer the action. 1164 InduceScavenge(Self, "omAlloc"); 1165 } 1166 continue; 1167 } 1168 1169 // 3: allocate a block of new ObjectMonitors 1170 // Both the local and global free lists are empty -- resort to malloc(). 1171 // In the current implementation objectMonitors are TSM - immortal. 1172 // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want 1173 // each ObjectMonitor to start at the beginning of a cache line, 1174 // so we use align_up(). 1175 // A better solution would be to use C++ placement-new. 1176 // BEWARE: As it stands currently, we don't run the ctors! 1177 assert(_BLOCKSIZE > 1, "invariant"); 1178 size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE; 1179 PaddedEnd<ObjectMonitor> * temp; 1180 size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1); 1181 void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size, 1182 mtInternal); 1183 temp = (PaddedEnd<ObjectMonitor> *) 1184 align_up(real_malloc_addr, DEFAULT_CACHE_LINE_SIZE); 1185 1186 // NOTE: (almost) no way to recover if allocation failed. 1187 // We might be able to induce a STW safepoint and scavenge enough 1188 // objectMonitors to permit progress. 1189 if (temp == NULL) { 1190 vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR, 1191 "Allocate ObjectMonitors"); 1192 } 1193 (void)memset((void *) temp, 0, neededsize); 1194 1195 // Format the block. 1196 // initialize the linked list, each monitor points to its next 1197 // forming the single linked free list, the very first monitor 1198 // will points to next block, which forms the block list. 1199 // The trick of using the 1st element in the block as gBlockList 1200 // linkage should be reconsidered. A better implementation would 1201 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; } 1202 1203 for (int i = 1; i < _BLOCKSIZE; i++) { 1204 temp[i].FreeNext = (ObjectMonitor *)&temp[i+1]; 1205 } 1206 1207 // terminate the last monitor as the end of list 1208 temp[_BLOCKSIZE - 1].FreeNext = NULL; 1209 1210 // Element [0] is reserved for global list linkage 1211 temp[0].set_object(CHAINMARKER); 1212 1213 // Consider carving out this thread's current request from the 1214 // block in hand. This avoids some lock traffic and redundant 1215 // list activity. 1216 1217 // Acquire the gListLock to manipulate gBlockList and gFreeList. 1218 // An Oyama-Taura-Yonezawa scheme might be more efficient. 1219 Thread::muxAcquire(&gListLock, "omAlloc [2]"); 1220 gMonitorPopulation += _BLOCKSIZE-1; 1221 gMonitorFreeCount += _BLOCKSIZE-1; 1222 1223 // Add the new block to the list of extant blocks (gBlockList). 1224 // The very first objectMonitor in a block is reserved and dedicated. 1225 // It serves as blocklist "next" linkage. 1226 temp[0].FreeNext = gBlockList; 1227 // There are lock-free uses of gBlockList so make sure that 1228 // the previous stores happen before we update gBlockList. 1229 OrderAccess::release_store(&gBlockList, temp); 1230 1231 // Add the new string of objectMonitors to the global free list 1232 temp[_BLOCKSIZE - 1].FreeNext = gFreeList; 1233 gFreeList = temp + 1; 1234 Thread::muxRelease(&gListLock); 1235 TEVENT(Allocate block of monitors); 1236 } 1237 } 1238 1239 // Place "m" on the caller's private per-thread omFreeList. 1240 // In practice there's no need to clamp or limit the number of 1241 // monitors on a thread's omFreeList as the only time we'll call 1242 // omRelease is to return a monitor to the free list after a CAS 1243 // attempt failed. This doesn't allow unbounded #s of monitors to 1244 // accumulate on a thread's free list. 1245 // 1246 // Key constraint: all ObjectMonitors on a thread's free list and the global 1247 // free list must have their object field set to null. This prevents the 1248 // scavenger -- deflate_idle_monitors -- from reclaiming them. 1249 1250 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m, 1251 bool fromPerThreadAlloc) { 1252 guarantee(m->object() == NULL, "invariant"); 1253 guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor"); 1254 // Remove from omInUseList 1255 if (MonitorInUseLists && fromPerThreadAlloc) { 1256 ObjectMonitor* cur_mid_in_use = NULL; 1257 bool extracted = false; 1258 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) { 1259 if (m == mid) { 1260 // extract from per-thread in-use list 1261 if (mid == Self->omInUseList) { 1262 Self->omInUseList = mid->FreeNext; 1263 } else if (cur_mid_in_use != NULL) { 1264 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list 1265 } 1266 extracted = true; 1267 Self->omInUseCount--; 1268 if (ObjectMonitor::Knob_VerifyInUse) { 1269 verifyInUse(Self); 1270 } 1271 break; 1272 } 1273 } 1274 assert(extracted, "Should have extracted from in-use list"); 1275 } 1276 1277 // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new 1278 m->FreeNext = Self->omFreeList; 1279 Self->omFreeList = m; 1280 Self->omFreeCount++; 1281 } 1282 1283 // Return the monitors of a moribund thread's local free list to 1284 // the global free list. Typically a thread calls omFlush() when 1285 // it's dying. We could also consider having the VM thread steal 1286 // monitors from threads that have not run java code over a few 1287 // consecutive STW safepoints. Relatedly, we might decay 1288 // omFreeProvision at STW safepoints. 1289 // 1290 // Also return the monitors of a moribund thread's omInUseList to 1291 // a global gOmInUseList under the global list lock so these 1292 // will continue to be scanned. 1293 // 1294 // We currently call omFlush() from Threads::remove() _before the thread 1295 // has been excised from the thread list and is no longer a mutator. 1296 // This means that omFlush() can not run concurrently with a safepoint and 1297 // interleave with the scavenge operator. In particular, this ensures that 1298 // the thread's monitors are scanned by a GC safepoint, either via 1299 // Thread::oops_do() (if safepoint happens before omFlush()) or via 1300 // ObjectSynchronizer::oops_do() (if it happens after omFlush() and the thread's 1301 // monitors have been transferred to the global in-use list). 1302 1303 void ObjectSynchronizer::omFlush(Thread * Self) { 1304 ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL 1305 Self->omFreeList = NULL; 1306 ObjectMonitor * tail = NULL; 1307 int tally = 0; 1308 if (list != NULL) { 1309 ObjectMonitor * s; 1310 // The thread is going away, the per-thread free monitors 1311 // are freed via set_owner(NULL) 1312 // Link them to tail, which will be linked into the global free list 1313 // gFreeList below, under the gListLock 1314 for (s = list; s != NULL; s = s->FreeNext) { 1315 tally++; 1316 tail = s; 1317 guarantee(s->object() == NULL, "invariant"); 1318 guarantee(!s->is_busy(), "invariant"); 1319 s->set_owner(NULL); // redundant but good hygiene 1320 TEVENT(omFlush - Move one); 1321 } 1322 guarantee(tail != NULL && list != NULL, "invariant"); 1323 } 1324 1325 ObjectMonitor * inUseList = Self->omInUseList; 1326 ObjectMonitor * inUseTail = NULL; 1327 int inUseTally = 0; 1328 if (inUseList != NULL) { 1329 Self->omInUseList = NULL; 1330 ObjectMonitor *cur_om; 1331 // The thread is going away, however the omInUseList inflated 1332 // monitors may still be in-use by other threads. 1333 // Link them to inUseTail, which will be linked into the global in-use list 1334 // gOmInUseList below, under the gListLock 1335 for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) { 1336 inUseTail = cur_om; 1337 inUseTally++; 1338 } 1339 assert(Self->omInUseCount == inUseTally, "in-use count off"); 1340 Self->omInUseCount = 0; 1341 guarantee(inUseTail != NULL && inUseList != NULL, "invariant"); 1342 } 1343 1344 Thread::muxAcquire(&gListLock, "omFlush"); 1345 if (tail != NULL) { 1346 tail->FreeNext = gFreeList; 1347 gFreeList = list; 1348 gMonitorFreeCount += tally; 1349 assert(Self->omFreeCount == tally, "free-count off"); 1350 Self->omFreeCount = 0; 1351 } 1352 1353 if (inUseTail != NULL) { 1354 inUseTail->FreeNext = gOmInUseList; 1355 gOmInUseList = inUseList; 1356 gOmInUseCount += inUseTally; 1357 } 1358 1359 Thread::muxRelease(&gListLock); 1360 TEVENT(omFlush); 1361 } 1362 1363 static void post_monitor_inflate_event(EventJavaMonitorInflate* event, 1364 const oop obj, 1365 ObjectSynchronizer::InflateCause cause) { 1366 assert(event != NULL, "invariant"); 1367 assert(event->should_commit(), "invariant"); 1368 event->set_monitorClass(obj->klass()); 1369 event->set_address((uintptr_t)(void*)obj); 1370 event->set_cause((u1)cause); 1371 event->commit(); 1372 } 1373 1374 // Fast path code shared by multiple functions 1375 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) { 1376 markOop mark = obj->mark(); 1377 if (mark->has_monitor()) { 1378 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid"); 1379 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header"); 1380 return mark->monitor(); 1381 } 1382 return ObjectSynchronizer::inflate(Thread::current(), 1383 obj, 1384 inflate_cause_vm_internal); 1385 } 1386 1387 ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self, 1388 oop object, 1389 const InflateCause cause) { 1390 1391 // Inflate mutates the heap ... 1392 // Relaxing assertion for bug 6320749. 1393 assert(Universe::verify_in_progress() || 1394 !SafepointSynchronize::is_at_safepoint(), "invariant"); 1395 1396 EventJavaMonitorInflate event; 1397 1398 for (;;) { 1399 const markOop mark = object->mark(); 1400 assert(!mark->has_bias_pattern(), "invariant"); 1401 1402 // The mark can be in one of the following states: 1403 // * Inflated - just return 1404 // * Stack-locked - coerce it to inflated 1405 // * INFLATING - busy wait for conversion to complete 1406 // * Neutral - aggressively inflate the object. 1407 // * BIASED - Illegal. We should never see this 1408 1409 // CASE: inflated 1410 if (mark->has_monitor()) { 1411 ObjectMonitor * inf = mark->monitor(); 1412 assert(inf->header()->is_neutral(), "invariant"); 1413 assert(oopDesc::equals((oop) inf->object(), object), "invariant"); 1414 assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid"); 1415 return inf; 1416 } 1417 1418 // CASE: inflation in progress - inflating over a stack-lock. 1419 // Some other thread is converting from stack-locked to inflated. 1420 // Only that thread can complete inflation -- other threads must wait. 1421 // The INFLATING value is transient. 1422 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish. 1423 // We could always eliminate polling by parking the thread on some auxiliary list. 1424 if (mark == markOopDesc::INFLATING()) { 1425 TEVENT(Inflate: spin while INFLATING); 1426 ReadStableMark(object); 1427 continue; 1428 } 1429 1430 // CASE: stack-locked 1431 // Could be stack-locked either by this thread or by some other thread. 1432 // 1433 // Note that we allocate the objectmonitor speculatively, _before_ attempting 1434 // to install INFLATING into the mark word. We originally installed INFLATING, 1435 // allocated the objectmonitor, and then finally STed the address of the 1436 // objectmonitor into the mark. This was correct, but artificially lengthened 1437 // the interval in which INFLATED appeared in the mark, thus increasing 1438 // the odds of inflation contention. 1439 // 1440 // We now use per-thread private objectmonitor free lists. 1441 // These list are reprovisioned from the global free list outside the 1442 // critical INFLATING...ST interval. A thread can transfer 1443 // multiple objectmonitors en-mass from the global free list to its local free list. 1444 // This reduces coherency traffic and lock contention on the global free list. 1445 // Using such local free lists, it doesn't matter if the omAlloc() call appears 1446 // before or after the CAS(INFLATING) operation. 1447 // See the comments in omAlloc(). 1448 1449 if (mark->has_locker()) { 1450 ObjectMonitor * m = omAlloc(Self); 1451 // Optimistically prepare the objectmonitor - anticipate successful CAS 1452 // We do this before the CAS in order to minimize the length of time 1453 // in which INFLATING appears in the mark. 1454 m->Recycle(); 1455 m->_Responsible = NULL; 1456 m->_recursions = 0; 1457 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class 1458 1459 markOop cmp = object->cas_set_mark(markOopDesc::INFLATING(), mark); 1460 if (cmp != mark) { 1461 omRelease(Self, m, true); 1462 continue; // Interference -- just retry 1463 } 1464 1465 // We've successfully installed INFLATING (0) into the mark-word. 1466 // This is the only case where 0 will appear in a mark-word. 1467 // Only the singular thread that successfully swings the mark-word 1468 // to 0 can perform (or more precisely, complete) inflation. 1469 // 1470 // Why do we CAS a 0 into the mark-word instead of just CASing the 1471 // mark-word from the stack-locked value directly to the new inflated state? 1472 // Consider what happens when a thread unlocks a stack-locked object. 1473 // It attempts to use CAS to swing the displaced header value from the 1474 // on-stack basiclock back into the object header. Recall also that the 1475 // header value (hashcode, etc) can reside in (a) the object header, or 1476 // (b) a displaced header associated with the stack-lock, or (c) a displaced 1477 // header in an objectMonitor. The inflate() routine must copy the header 1478 // value from the basiclock on the owner's stack to the objectMonitor, all 1479 // the while preserving the hashCode stability invariants. If the owner 1480 // decides to release the lock while the value is 0, the unlock will fail 1481 // and control will eventually pass from slow_exit() to inflate. The owner 1482 // will then spin, waiting for the 0 value to disappear. Put another way, 1483 // the 0 causes the owner to stall if the owner happens to try to 1484 // drop the lock (restoring the header from the basiclock to the object) 1485 // while inflation is in-progress. This protocol avoids races that might 1486 // would otherwise permit hashCode values to change or "flicker" for an object. 1487 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable. 1488 // 0 serves as a "BUSY" inflate-in-progress indicator. 1489 1490 1491 // fetch the displaced mark from the owner's stack. 1492 // The owner can't die or unwind past the lock while our INFLATING 1493 // object is in the mark. Furthermore the owner can't complete 1494 // an unlock on the object, either. 1495 markOop dmw = mark->displaced_mark_helper(); 1496 assert(dmw->is_neutral(), "invariant"); 1497 1498 // Setup monitor fields to proper values -- prepare the monitor 1499 m->set_header(dmw); 1500 1501 // Optimization: if the mark->locker stack address is associated 1502 // with this thread we could simply set m->_owner = Self. 1503 // Note that a thread can inflate an object 1504 // that it has stack-locked -- as might happen in wait() -- directly 1505 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom. 1506 m->set_owner(mark->locker()); 1507 m->set_object(object); 1508 // TODO-FIXME: assert BasicLock->dhw != 0. 1509 1510 // Must preserve store ordering. The monitor state must 1511 // be stable at the time of publishing the monitor address. 1512 guarantee(object->mark() == markOopDesc::INFLATING(), "invariant"); 1513 object->release_set_mark(markOopDesc::encode(m)); 1514 1515 // Hopefully the performance counters are allocated on distinct cache lines 1516 // to avoid false sharing on MP systems ... 1517 OM_PERFDATA_OP(Inflations, inc()); 1518 TEVENT(Inflate: overwrite stacklock); 1519 if (log_is_enabled(Debug, monitorinflation)) { 1520 if (object->is_instance()) { 1521 ResourceMark rm; 1522 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", 1523 p2i(object), p2i(object->mark()), 1524 object->klass()->external_name()); 1525 } 1526 } 1527 if (event.should_commit()) { 1528 post_monitor_inflate_event(&event, object, cause); 1529 } 1530 return m; 1531 } 1532 1533 // CASE: neutral 1534 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner. 1535 // If we know we're inflating for entry it's better to inflate by swinging a 1536 // pre-locked objectMonitor pointer into the object header. A successful 1537 // CAS inflates the object *and* confers ownership to the inflating thread. 1538 // In the current implementation we use a 2-step mechanism where we CAS() 1539 // to inflate and then CAS() again to try to swing _owner from NULL to Self. 1540 // An inflateTry() method that we could call from fast_enter() and slow_enter() 1541 // would be useful. 1542 1543 assert(mark->is_neutral(), "invariant"); 1544 ObjectMonitor * m = omAlloc(Self); 1545 // prepare m for installation - set monitor to initial state 1546 m->Recycle(); 1547 m->set_header(mark); 1548 m->set_owner(NULL); 1549 m->set_object(object); 1550 m->_recursions = 0; 1551 m->_Responsible = NULL; 1552 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class 1553 1554 if (object->cas_set_mark(markOopDesc::encode(m), mark) != mark) { 1555 m->set_object(NULL); 1556 m->set_owner(NULL); 1557 m->Recycle(); 1558 omRelease(Self, m, true); 1559 m = NULL; 1560 continue; 1561 // interference - the markword changed - just retry. 1562 // The state-transitions are one-way, so there's no chance of 1563 // live-lock -- "Inflated" is an absorbing state. 1564 } 1565 1566 // Hopefully the performance counters are allocated on distinct 1567 // cache lines to avoid false sharing on MP systems ... 1568 OM_PERFDATA_OP(Inflations, inc()); 1569 TEVENT(Inflate: overwrite neutral); 1570 if (log_is_enabled(Debug, monitorinflation)) { 1571 if (object->is_instance()) { 1572 ResourceMark rm; 1573 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", 1574 p2i(object), p2i(object->mark()), 1575 object->klass()->external_name()); 1576 } 1577 } 1578 if (event.should_commit()) { 1579 post_monitor_inflate_event(&event, object, cause); 1580 } 1581 return m; 1582 } 1583 } 1584 1585 1586 // Deflate_idle_monitors() is called at all safepoints, immediately 1587 // after all mutators are stopped, but before any objects have moved. 1588 // It traverses the list of known monitors, deflating where possible. 1589 // The scavenged monitor are returned to the monitor free list. 1590 // 1591 // Beware that we scavenge at *every* stop-the-world point. 1592 // Having a large number of monitors in-circulation negatively 1593 // impacts the performance of some applications (e.g., PointBase). 1594 // Broadly, we want to minimize the # of monitors in circulation. 1595 // 1596 // We have added a flag, MonitorInUseLists, which creates a list 1597 // of active monitors for each thread. deflate_idle_monitors() 1598 // only scans the per-thread in-use lists. omAlloc() puts all 1599 // assigned monitors on the per-thread list. deflate_idle_monitors() 1600 // returns the non-busy monitors to the global free list. 1601 // When a thread dies, omFlush() adds the list of active monitors for 1602 // that thread to a global gOmInUseList acquiring the 1603 // global list lock. deflate_idle_monitors() acquires the global 1604 // list lock to scan for non-busy monitors to the global free list. 1605 // An alternative could have used a single global in-use list. The 1606 // downside would have been the additional cost of acquiring the global list lock 1607 // for every omAlloc(). 1608 // 1609 // Perversely, the heap size -- and thus the STW safepoint rate -- 1610 // typically drives the scavenge rate. Large heaps can mean infrequent GC, 1611 // which in turn can mean large(r) numbers of objectmonitors in circulation. 1612 // This is an unfortunate aspect of this design. 1613 1614 enum ManifestConstants { 1615 ClearResponsibleAtSTW = 0 1616 }; 1617 1618 // Deflate a single monitor if not in-use 1619 // Return true if deflated, false if in-use 1620 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj, 1621 ObjectMonitor** freeHeadp, 1622 ObjectMonitor** freeTailp) { 1623 bool deflated; 1624 // Normal case ... The monitor is associated with obj. 1625 guarantee(obj->mark() == markOopDesc::encode(mid), "invariant"); 1626 guarantee(mid == obj->mark()->monitor(), "invariant"); 1627 guarantee(mid->header()->is_neutral(), "invariant"); 1628 1629 if (mid->is_busy()) { 1630 if (ClearResponsibleAtSTW) mid->_Responsible = NULL; 1631 deflated = false; 1632 } else { 1633 // Deflate the monitor if it is no longer being used 1634 // It's idle - scavenge and return to the global free list 1635 // plain old deflation ... 1636 TEVENT(deflate_idle_monitors - scavenge1); 1637 if (log_is_enabled(Debug, monitorinflation)) { 1638 if (obj->is_instance()) { 1639 ResourceMark rm; 1640 log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , " 1641 "mark " INTPTR_FORMAT " , type %s", 1642 p2i(obj), p2i(obj->mark()), 1643 obj->klass()->external_name()); 1644 } 1645 } 1646 1647 // Restore the header back to obj 1648 obj->release_set_mark(mid->header()); 1649 mid->clear(); 1650 1651 assert(mid->object() == NULL, "invariant"); 1652 1653 // Move the object to the working free list defined by freeHeadp, freeTailp 1654 if (*freeHeadp == NULL) *freeHeadp = mid; 1655 if (*freeTailp != NULL) { 1656 ObjectMonitor * prevtail = *freeTailp; 1657 assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); 1658 prevtail->FreeNext = mid; 1659 } 1660 *freeTailp = mid; 1661 deflated = true; 1662 } 1663 return deflated; 1664 } 1665 1666 // Walk a given monitor list, and deflate idle monitors 1667 // The given list could be a per-thread list or a global list 1668 // Caller acquires gListLock. 1669 // 1670 // In the case of parallel processing of thread local monitor lists, 1671 // work is done by Threads::parallel_threads_do() which ensures that 1672 // each Java thread is processed by exactly one worker thread, and 1673 // thus avoid conflicts that would arise when worker threads would 1674 // process the same monitor lists concurrently. 1675 // 1676 // See also ParallelSPCleanupTask and 1677 // SafepointSynchronize::do_cleanup_tasks() in safepoint.cpp and 1678 // Threads::parallel_java_threads_do() in thread.cpp. 1679 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp, 1680 ObjectMonitor** freeHeadp, 1681 ObjectMonitor** freeTailp) { 1682 ObjectMonitor* mid; 1683 ObjectMonitor* next; 1684 ObjectMonitor* cur_mid_in_use = NULL; 1685 int deflated_count = 0; 1686 1687 for (mid = *listHeadp; mid != NULL;) { 1688 oop obj = (oop) mid->object(); 1689 if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) { 1690 // if deflate_monitor succeeded, 1691 // extract from per-thread in-use list 1692 if (mid == *listHeadp) { 1693 *listHeadp = mid->FreeNext; 1694 } else if (cur_mid_in_use != NULL) { 1695 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list 1696 } 1697 next = mid->FreeNext; 1698 mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list 1699 mid = next; 1700 deflated_count++; 1701 } else { 1702 cur_mid_in_use = mid; 1703 mid = mid->FreeNext; 1704 } 1705 } 1706 return deflated_count; 1707 } 1708 1709 void ObjectSynchronizer::prepare_deflate_idle_monitors(DeflateMonitorCounters* counters) { 1710 counters->nInuse = 0; // currently associated with objects 1711 counters->nInCirculation = 0; // extant 1712 counters->nScavenged = 0; // reclaimed 1713 } 1714 1715 void ObjectSynchronizer::deflate_idle_monitors(DeflateMonitorCounters* counters) { 1716 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); 1717 bool deflated = false; 1718 1719 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors 1720 ObjectMonitor * freeTailp = NULL; 1721 1722 TEVENT(deflate_idle_monitors); 1723 // Prevent omFlush from changing mids in Thread dtor's during deflation 1724 // And in case the vm thread is acquiring a lock during a safepoint 1725 // See e.g. 6320749 1726 Thread::muxAcquire(&gListLock, "scavenge - return"); 1727 1728 if (MonitorInUseLists) { 1729 // Note: the thread-local monitors lists get deflated in 1730 // a separate pass. See deflate_thread_local_monitors(). 1731 1732 // For moribund threads, scan gOmInUseList 1733 if (gOmInUseList) { 1734 counters->nInCirculation += gOmInUseCount; 1735 int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp); 1736 gOmInUseCount -= deflated_count; 1737 counters->nScavenged += deflated_count; 1738 counters->nInuse += gOmInUseCount; 1739 } 1740 1741 } else { 1742 PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList); 1743 for (; block != NULL; block = next(block)) { 1744 // Iterate over all extant monitors - Scavenge all idle monitors. 1745 assert(block->object() == CHAINMARKER, "must be a block header"); 1746 counters->nInCirculation += _BLOCKSIZE; 1747 for (int i = 1; i < _BLOCKSIZE; i++) { 1748 ObjectMonitor* mid = (ObjectMonitor*)&block[i]; 1749 oop obj = (oop)mid->object(); 1750 1751 if (obj == NULL) { 1752 // The monitor is not associated with an object. 1753 // The monitor should either be a thread-specific private 1754 // free list or the global free list. 1755 // obj == NULL IMPLIES mid->is_busy() == 0 1756 guarantee(!mid->is_busy(), "invariant"); 1757 continue; 1758 } 1759 deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp); 1760 1761 if (deflated) { 1762 mid->FreeNext = NULL; 1763 counters->nScavenged++; 1764 } else { 1765 counters->nInuse++; 1766 } 1767 } 1768 } 1769 } 1770 1771 // Move the scavenged monitors back to the global free list. 1772 if (freeHeadp != NULL) { 1773 guarantee(freeTailp != NULL && counters->nScavenged > 0, "invariant"); 1774 assert(freeTailp->FreeNext == NULL, "invariant"); 1775 // constant-time list splice - prepend scavenged segment to gFreeList 1776 freeTailp->FreeNext = gFreeList; 1777 gFreeList = freeHeadp; 1778 } 1779 Thread::muxRelease(&gListLock); 1780 1781 } 1782 1783 void ObjectSynchronizer::finish_deflate_idle_monitors(DeflateMonitorCounters* counters) { 1784 gMonitorFreeCount += counters->nScavenged; 1785 1786 // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree. 1787 1788 if (ObjectMonitor::Knob_Verbose) { 1789 tty->print_cr("INFO: Deflate: InCirc=%d InUse=%d Scavenged=%d " 1790 "ForceMonitorScavenge=%d : pop=%d free=%d", 1791 counters->nInCirculation, counters->nInuse, counters->nScavenged, ForceMonitorScavenge, 1792 gMonitorPopulation, gMonitorFreeCount); 1793 tty->flush(); 1794 } 1795 1796 ForceMonitorScavenge = 0; // Reset 1797 1798 OM_PERFDATA_OP(Deflations, inc(counters->nScavenged)); 1799 OM_PERFDATA_OP(MonExtant, set_value(counters->nInCirculation)); 1800 1801 // TODO: Add objectMonitor leak detection. 1802 // Audit/inventory the objectMonitors -- make sure they're all accounted for. 1803 GVars.stwRandom = os::random(); 1804 GVars.stwCycle++; 1805 } 1806 1807 void ObjectSynchronizer::deflate_thread_local_monitors(Thread* thread, DeflateMonitorCounters* counters) { 1808 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); 1809 if (!MonitorInUseLists) return; 1810 1811 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors 1812 ObjectMonitor * freeTailp = NULL; 1813 1814 int deflated_count = deflate_monitor_list(thread->omInUseList_addr(), &freeHeadp, &freeTailp); 1815 1816 Thread::muxAcquire(&gListLock, "scavenge - return"); 1817 1818 // Adjust counters 1819 counters->nInCirculation += thread->omInUseCount; 1820 thread->omInUseCount -= deflated_count; 1821 if (ObjectMonitor::Knob_VerifyInUse) { 1822 verifyInUse(thread); 1823 } 1824 counters->nScavenged += deflated_count; 1825 counters->nInuse += thread->omInUseCount; 1826 1827 // Move the scavenged monitors back to the global free list. 1828 if (freeHeadp != NULL) { 1829 guarantee(freeTailp != NULL && deflated_count > 0, "invariant"); 1830 assert(freeTailp->FreeNext == NULL, "invariant"); 1831 1832 // constant-time list splice - prepend scavenged segment to gFreeList 1833 freeTailp->FreeNext = gFreeList; 1834 gFreeList = freeHeadp; 1835 } 1836 Thread::muxRelease(&gListLock); 1837 } 1838 1839 // Monitor cleanup on JavaThread::exit 1840 1841 // Iterate through monitor cache and attempt to release thread's monitors 1842 // Gives up on a particular monitor if an exception occurs, but continues 1843 // the overall iteration, swallowing the exception. 1844 class ReleaseJavaMonitorsClosure: public MonitorClosure { 1845 private: 1846 TRAPS; 1847 1848 public: 1849 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {} 1850 void do_monitor(ObjectMonitor* mid) { 1851 if (mid->owner() == THREAD) { 1852 if (ObjectMonitor::Knob_VerifyMatch != 0) { 1853 ResourceMark rm; 1854 Handle obj(THREAD, (oop) mid->object()); 1855 tty->print("INFO: unexpected locked object:"); 1856 javaVFrame::print_locked_object_class_name(tty, obj, "locked"); 1857 fatal("exiting JavaThread=" INTPTR_FORMAT 1858 " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT, 1859 p2i(THREAD), p2i(mid)); 1860 } 1861 (void)mid->complete_exit(CHECK); 1862 } 1863 } 1864 }; 1865 1866 // Release all inflated monitors owned by THREAD. Lightweight monitors are 1867 // ignored. This is meant to be called during JNI thread detach which assumes 1868 // all remaining monitors are heavyweight. All exceptions are swallowed. 1869 // Scanning the extant monitor list can be time consuming. 1870 // A simple optimization is to add a per-thread flag that indicates a thread 1871 // called jni_monitorenter() during its lifetime. 1872 // 1873 // Instead of No_Savepoint_Verifier it might be cheaper to 1874 // use an idiom of the form: 1875 // auto int tmp = SafepointSynchronize::_safepoint_counter ; 1876 // <code that must not run at safepoint> 1877 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ; 1878 // Since the tests are extremely cheap we could leave them enabled 1879 // for normal product builds. 1880 1881 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) { 1882 assert(THREAD == JavaThread::current(), "must be current Java thread"); 1883 NoSafepointVerifier nsv; 1884 ReleaseJavaMonitorsClosure rjmc(THREAD); 1885 Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread"); 1886 ObjectSynchronizer::monitors_iterate(&rjmc); 1887 Thread::muxRelease(&gListLock); 1888 THREAD->clear_pending_exception(); 1889 } 1890 1891 const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) { 1892 switch (cause) { 1893 case inflate_cause_vm_internal: return "VM Internal"; 1894 case inflate_cause_monitor_enter: return "Monitor Enter"; 1895 case inflate_cause_wait: return "Monitor Wait"; 1896 case inflate_cause_notify: return "Monitor Notify"; 1897 case inflate_cause_hash_code: return "Monitor Hash Code"; 1898 case inflate_cause_jni_enter: return "JNI Monitor Enter"; 1899 case inflate_cause_jni_exit: return "JNI Monitor Exit"; 1900 default: 1901 ShouldNotReachHere(); 1902 } 1903 return "Unknown"; 1904 } 1905 1906 //------------------------------------------------------------------------------ 1907 // Debugging code 1908 1909 u_char* ObjectSynchronizer::get_gvars_addr() { 1910 return (u_char*)&GVars; 1911 } 1912 1913 u_char* ObjectSynchronizer::get_gvars_hcSequence_addr() { 1914 return (u_char*)&GVars.hcSequence; 1915 } 1916 1917 size_t ObjectSynchronizer::get_gvars_size() { 1918 return sizeof(SharedGlobals); 1919 } 1920 1921 u_char* ObjectSynchronizer::get_gvars_stwRandom_addr() { 1922 return (u_char*)&GVars.stwRandom; 1923 } 1924 1925 #ifndef PRODUCT 1926 1927 // Check if monitor belongs to the monitor cache 1928 // The list is grow-only so it's *relatively* safe to traverse 1929 // the list of extant blocks without taking a lock. 1930 1931 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) { 1932 PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList); 1933 while (block != NULL) { 1934 assert(block->object() == CHAINMARKER, "must be a block header"); 1935 if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) { 1936 address mon = (address)monitor; 1937 address blk = (address)block; 1938 size_t diff = mon - blk; 1939 assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "must be aligned"); 1940 return 1; 1941 } 1942 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext; 1943 } 1944 return 0; 1945 } 1946 1947 #endif