Mercurial > hg > truffle
annotate src/share/vm/runtime/objectMonitor.cpp @ 7212:291ffc492eb6
Merge with http://hg.openjdk.java.net/hsx/hsx25/hotspot/
| author | Doug Simon <doug.simon@oracle.com> |
|---|---|
| date | Fri, 14 Dec 2012 14:35:13 +0100 |
| parents | f34d701e952e |
| children | 22ba8c8ce6a6 |
| rev | line source |
|---|---|
| 1878 | 1 /* |
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2 * Copyright (c) 1998, 2012, Oracle and/or its affiliates. All rights reserved. |
| 1878 | 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 | |
| 1972 | 25 #include "precompiled.hpp" |
| 26 #include "classfile/vmSymbols.hpp" | |
| 27 #include "memory/resourceArea.hpp" | |
| 28 #include "oops/markOop.hpp" | |
| 29 #include "oops/oop.inline.hpp" | |
| 30 #include "runtime/handles.inline.hpp" | |
| 31 #include "runtime/interfaceSupport.hpp" | |
| 32 #include "runtime/mutexLocker.hpp" | |
| 33 #include "runtime/objectMonitor.hpp" | |
| 34 #include "runtime/objectMonitor.inline.hpp" | |
| 35 #include "runtime/osThread.hpp" | |
| 36 #include "runtime/stubRoutines.hpp" | |
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37 #include "runtime/thread.inline.hpp" |
| 1972 | 38 #include "services/threadService.hpp" |
| 39 #include "utilities/dtrace.hpp" | |
| 40 #include "utilities/preserveException.hpp" | |
| 41 #ifdef TARGET_OS_FAMILY_linux | |
| 42 # include "os_linux.inline.hpp" | |
| 43 #endif | |
| 44 #ifdef TARGET_OS_FAMILY_solaris | |
| 45 # include "os_solaris.inline.hpp" | |
| 46 #endif | |
| 47 #ifdef TARGET_OS_FAMILY_windows | |
| 48 # include "os_windows.inline.hpp" | |
| 49 #endif | |
| 3960 | 50 #ifdef TARGET_OS_FAMILY_bsd |
| 51 # include "os_bsd.inline.hpp" | |
| 52 #endif | |
| 1878 | 53 |
| 54 #if defined(__GNUC__) && !defined(IA64) | |
| 55 // Need to inhibit inlining for older versions of GCC to avoid build-time failures | |
| 56 #define ATTR __attribute__((noinline)) | |
| 57 #else | |
| 58 #define ATTR | |
| 59 #endif | |
| 60 | |
| 61 | |
| 62 #ifdef DTRACE_ENABLED | |
| 63 | |
| 64 // Only bother with this argument setup if dtrace is available | |
| 65 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. | |
| 66 | |
| 4006 | 67 |
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68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ |
| 4006 | 69 char* bytes = NULL; \ |
| 70 int len = 0; \ | |
| 71 jlong jtid = SharedRuntime::get_java_tid(thread); \ | |
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72 Symbol* klassname = ((oop)obj)->klass()->name(); \ |
| 4006 | 73 if (klassname != NULL) { \ |
| 74 bytes = (char*)klassname->bytes(); \ | |
| 75 len = klassname->utf8_length(); \ | |
| 76 } | |
| 77 | |
| 78 #ifndef USDT2 | |
| 79 | |
| 1878 | 80 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify, |
| 81 jlong, uintptr_t, char*, int); | |
| 82 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll, | |
| 83 jlong, uintptr_t, char*, int); | |
| 84 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter, | |
| 85 jlong, uintptr_t, char*, int); | |
| 86 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered, | |
| 87 jlong, uintptr_t, char*, int); | |
| 88 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit, | |
| 89 jlong, uintptr_t, char*, int); | |
| 90 | |
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91 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ |
| 1878 | 92 { \ |
| 93 if (DTraceMonitorProbes) { \ | |
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94 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ |
| 1878 | 95 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \ |
| 96 (monitor), bytes, len, (millis)); \ | |
| 97 } \ | |
| 98 } | |
| 99 | |
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100 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ |
| 1878 | 101 { \ |
| 102 if (DTraceMonitorProbes) { \ | |
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103 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ |
| 1878 | 104 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \ |
| 105 (uintptr_t)(monitor), bytes, len); \ | |
| 106 } \ | |
| 107 } | |
| 108 | |
| 4006 | 109 #else /* USDT2 */ |
| 110 | |
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111 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ |
| 4006 | 112 { \ |
| 113 if (DTraceMonitorProbes) { \ | |
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114 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ |
| 4006 | 115 HOTSPOT_MONITOR_WAIT(jtid, \ |
| 116 (monitor), bytes, len, (millis)); \ | |
| 117 } \ | |
| 118 } | |
| 119 | |
| 120 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER | |
| 121 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED | |
| 122 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT | |
| 123 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY | |
| 124 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL | |
| 125 | |
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126 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ |
| 4006 | 127 { \ |
| 128 if (DTraceMonitorProbes) { \ | |
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129 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ |
| 4006 | 130 HOTSPOT_MONITOR_##probe(jtid, \ |
| 131 (uintptr_t)(monitor), bytes, len); \ | |
| 132 } \ | |
| 133 } | |
| 134 | |
| 135 #endif /* USDT2 */ | |
| 1878 | 136 #else // ndef DTRACE_ENABLED |
| 137 | |
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138 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} |
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139 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} |
| 1878 | 140 |
| 141 #endif // ndef DTRACE_ENABLED | |
| 142 | |
| 143 // Tunables ... | |
| 144 // The knob* variables are effectively final. Once set they should | |
| 145 // never be modified hence. Consider using __read_mostly with GCC. | |
| 146 | |
| 147 int ObjectMonitor::Knob_Verbose = 0 ; | |
| 148 int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool - | |
| 149 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins | |
| 150 static int Knob_HandOff = 0 ; | |
| 151 static int Knob_ReportSettings = 0 ; | |
| 152 | |
| 153 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin | |
| 154 static int Knob_SpinBackOff = 0 ; // spin-loop backoff | |
| 155 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS | |
| 156 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change | |
| 157 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field | |
| 158 static int Knob_SpinEarly = 1 ; | |
| 159 static int Knob_SuccEnabled = 1 ; // futile wake throttling | |
| 160 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one | |
| 161 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs | |
| 162 static int Knob_Bonus = 100 ; // spin success bonus | |
| 163 static int Knob_BonusB = 100 ; // spin success bonus | |
| 164 static int Knob_Penalty = 200 ; // spin failure penalty | |
| 165 static int Knob_Poverty = 1000 ; | |
| 166 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park() | |
| 167 static int Knob_FixedSpin = 0 ; | |
| 168 static int Knob_OState = 3 ; // Spinner checks thread state of _owner | |
| 169 static int Knob_UsePause = 1 ; | |
| 170 static int Knob_ExitPolicy = 0 ; | |
| 171 static int Knob_PreSpin = 10 ; // 20-100 likely better | |
| 172 static int Knob_ResetEvent = 0 ; | |
| 173 static int BackOffMask = 0 ; | |
| 174 | |
| 175 static int Knob_FastHSSEC = 0 ; | |
| 176 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee | |
| 177 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline | |
| 178 static volatile int InitDone = 0 ; | |
| 179 | |
| 180 #define TrySpin TrySpin_VaryDuration | |
| 181 | |
| 182 // ----------------------------------------------------------------------------- | |
| 183 // Theory of operations -- Monitors lists, thread residency, etc: | |
| 184 // | |
| 185 // * A thread acquires ownership of a monitor by successfully | |
| 186 // CAS()ing the _owner field from null to non-null. | |
| 187 // | |
| 188 // * Invariant: A thread appears on at most one monitor list -- | |
| 189 // cxq, EntryList or WaitSet -- at any one time. | |
| 190 // | |
| 191 // * Contending threads "push" themselves onto the cxq with CAS | |
| 192 // and then spin/park. | |
| 193 // | |
| 194 // * After a contending thread eventually acquires the lock it must | |
| 195 // dequeue itself from either the EntryList or the cxq. | |
| 196 // | |
| 197 // * The exiting thread identifies and unparks an "heir presumptive" | |
| 198 // tentative successor thread on the EntryList. Critically, the | |
| 199 // exiting thread doesn't unlink the successor thread from the EntryList. | |
| 200 // After having been unparked, the wakee will recontend for ownership of | |
| 201 // the monitor. The successor (wakee) will either acquire the lock or | |
| 202 // re-park itself. | |
| 203 // | |
| 204 // Succession is provided for by a policy of competitive handoff. | |
| 205 // The exiting thread does _not_ grant or pass ownership to the | |
| 206 // successor thread. (This is also referred to as "handoff" succession"). | |
| 207 // Instead the exiting thread releases ownership and possibly wakes | |
| 208 // a successor, so the successor can (re)compete for ownership of the lock. | |
| 209 // If the EntryList is empty but the cxq is populated the exiting | |
| 210 // thread will drain the cxq into the EntryList. It does so by | |
| 211 // by detaching the cxq (installing null with CAS) and folding | |
| 212 // the threads from the cxq into the EntryList. The EntryList is | |
| 213 // doubly linked, while the cxq is singly linked because of the | |
| 214 // CAS-based "push" used to enqueue recently arrived threads (RATs). | |
| 215 // | |
| 216 // * Concurrency invariants: | |
| 217 // | |
| 218 // -- only the monitor owner may access or mutate the EntryList. | |
| 219 // The mutex property of the monitor itself protects the EntryList | |
| 220 // from concurrent interference. | |
| 221 // -- Only the monitor owner may detach the cxq. | |
| 222 // | |
| 223 // * The monitor entry list operations avoid locks, but strictly speaking | |
| 224 // they're not lock-free. Enter is lock-free, exit is not. | |
| 225 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html | |
| 226 // | |
| 227 // * The cxq can have multiple concurrent "pushers" but only one concurrent | |
| 228 // detaching thread. This mechanism is immune from the ABA corruption. | |
| 229 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. | |
| 230 // | |
| 231 // * Taken together, the cxq and the EntryList constitute or form a | |
| 232 // single logical queue of threads stalled trying to acquire the lock. | |
| 233 // We use two distinct lists to improve the odds of a constant-time | |
| 234 // dequeue operation after acquisition (in the ::enter() epilog) and | |
| 235 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). | |
| 236 // A key desideratum is to minimize queue & monitor metadata manipulation | |
| 237 // that occurs while holding the monitor lock -- that is, we want to | |
| 238 // minimize monitor lock holds times. Note that even a small amount of | |
| 239 // fixed spinning will greatly reduce the # of enqueue-dequeue operations | |
| 240 // on EntryList|cxq. That is, spinning relieves contention on the "inner" | |
| 241 // locks and monitor metadata. | |
| 242 // | |
| 243 // Cxq points to the the set of Recently Arrived Threads attempting entry. | |
| 244 // Because we push threads onto _cxq with CAS, the RATs must take the form of | |
| 245 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when | |
| 246 // the unlocking thread notices that EntryList is null but _cxq is != null. | |
| 247 // | |
| 248 // The EntryList is ordered by the prevailing queue discipline and | |
| 249 // can be organized in any convenient fashion, such as a doubly-linked list or | |
| 250 // a circular doubly-linked list. Critically, we want insert and delete operations | |
| 251 // to operate in constant-time. If we need a priority queue then something akin | |
| 252 // to Solaris' sleepq would work nicely. Viz., | |
| 253 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. | |
| 254 // Queue discipline is enforced at ::exit() time, when the unlocking thread | |
| 255 // drains the cxq into the EntryList, and orders or reorders the threads on the | |
| 256 // EntryList accordingly. | |
| 257 // | |
| 258 // Barring "lock barging", this mechanism provides fair cyclic ordering, | |
| 259 // somewhat similar to an elevator-scan. | |
| 260 // | |
| 261 // * The monitor synchronization subsystem avoids the use of native | |
| 262 // synchronization primitives except for the narrow platform-specific | |
| 263 // park-unpark abstraction. See the comments in os_solaris.cpp regarding | |
| 264 // the semantics of park-unpark. Put another way, this monitor implementation | |
| 265 // depends only on atomic operations and park-unpark. The monitor subsystem | |
| 266 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the | |
| 267 // underlying OS manages the READY<->RUN transitions. | |
| 268 // | |
| 269 // * Waiting threads reside on the WaitSet list -- wait() puts | |
| 270 // the caller onto the WaitSet. | |
| 271 // | |
| 272 // * notify() or notifyAll() simply transfers threads from the WaitSet to | |
| 273 // either the EntryList or cxq. Subsequent exit() operations will | |
| 274 // unpark the notifyee. Unparking a notifee in notify() is inefficient - | |
| 275 // it's likely the notifyee would simply impale itself on the lock held | |
| 276 // by the notifier. | |
| 277 // | |
| 278 // * An interesting alternative is to encode cxq as (List,LockByte) where | |
| 279 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary | |
| 280 // variable, like _recursions, in the scheme. The threads or Events that form | |
| 281 // the list would have to be aligned in 256-byte addresses. A thread would | |
| 282 // try to acquire the lock or enqueue itself with CAS, but exiting threads | |
| 283 // could use a 1-0 protocol and simply STB to set the LockByte to 0. | |
| 284 // Note that is is *not* word-tearing, but it does presume that full-word | |
| 285 // CAS operations are coherent with intermix with STB operations. That's true | |
| 286 // on most common processors. | |
| 287 // | |
| 288 // * See also http://blogs.sun.com/dave | |
| 289 | |
| 290 | |
| 291 // ----------------------------------------------------------------------------- | |
| 292 // Enter support | |
| 293 | |
| 294 bool ObjectMonitor::try_enter(Thread* THREAD) { | |
| 295 if (THREAD != _owner) { | |
| 296 if (THREAD->is_lock_owned ((address)_owner)) { | |
| 297 assert(_recursions == 0, "internal state error"); | |
| 298 _owner = THREAD ; | |
| 299 _recursions = 1 ; | |
| 300 OwnerIsThread = 1 ; | |
| 301 return true; | |
| 302 } | |
| 303 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { | |
| 304 return false; | |
| 305 } | |
| 306 return true; | |
| 307 } else { | |
| 308 _recursions++; | |
| 309 return true; | |
| 310 } | |
| 311 } | |
| 312 | |
| 313 void ATTR ObjectMonitor::enter(TRAPS) { | |
| 314 // The following code is ordered to check the most common cases first | |
| 315 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. | |
| 316 Thread * const Self = THREAD ; | |
| 317 void * cur ; | |
| 318 | |
| 319 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; | |
| 320 if (cur == NULL) { | |
| 321 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0. | |
| 322 assert (_recursions == 0 , "invariant") ; | |
| 323 assert (_owner == Self, "invariant") ; | |
| 324 // CONSIDER: set or assert OwnerIsThread == 1 | |
| 325 return ; | |
| 326 } | |
| 327 | |
| 328 if (cur == Self) { | |
| 329 // TODO-FIXME: check for integer overflow! BUGID 6557169. | |
| 330 _recursions ++ ; | |
| 331 return ; | |
| 332 } | |
| 333 | |
| 334 if (Self->is_lock_owned ((address)cur)) { | |
| 335 assert (_recursions == 0, "internal state error"); | |
| 336 _recursions = 1 ; | |
| 337 // Commute owner from a thread-specific on-stack BasicLockObject address to | |
| 338 // a full-fledged "Thread *". | |
| 339 _owner = Self ; | |
| 340 OwnerIsThread = 1 ; | |
| 341 return ; | |
| 342 } | |
| 343 | |
| 344 // We've encountered genuine contention. | |
| 345 assert (Self->_Stalled == 0, "invariant") ; | |
| 346 Self->_Stalled = intptr_t(this) ; | |
| 347 | |
| 348 // Try one round of spinning *before* enqueueing Self | |
| 349 // and before going through the awkward and expensive state | |
| 350 // transitions. The following spin is strictly optional ... | |
| 351 // Note that if we acquire the monitor from an initial spin | |
| 352 // we forgo posting JVMTI events and firing DTRACE probes. | |
| 353 if (Knob_SpinEarly && TrySpin (Self) > 0) { | |
| 354 assert (_owner == Self , "invariant") ; | |
| 355 assert (_recursions == 0 , "invariant") ; | |
| 356 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; | |
| 357 Self->_Stalled = 0 ; | |
| 358 return ; | |
| 359 } | |
| 360 | |
| 361 assert (_owner != Self , "invariant") ; | |
| 362 assert (_succ != Self , "invariant") ; | |
| 363 assert (Self->is_Java_thread() , "invariant") ; | |
| 364 JavaThread * jt = (JavaThread *) Self ; | |
| 365 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ; | |
| 366 assert (jt->thread_state() != _thread_blocked , "invariant") ; | |
| 367 assert (this->object() != NULL , "invariant") ; | |
| 368 assert (_count >= 0, "invariant") ; | |
| 369 | |
| 370 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy(). | |
| 371 // Ensure the object-monitor relationship remains stable while there's contention. | |
| 372 Atomic::inc_ptr(&_count); | |
| 373 | |
| 374 { // Change java thread status to indicate blocked on monitor enter. | |
| 375 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); | |
| 376 | |
| 377 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); | |
| 378 if (JvmtiExport::should_post_monitor_contended_enter()) { | |
| 379 JvmtiExport::post_monitor_contended_enter(jt, this); | |
| 380 } | |
| 381 | |
| 382 OSThreadContendState osts(Self->osthread()); | |
| 383 ThreadBlockInVM tbivm(jt); | |
| 384 | |
| 385 Self->set_current_pending_monitor(this); | |
| 386 | |
| 387 // TODO-FIXME: change the following for(;;) loop to straight-line code. | |
| 388 for (;;) { | |
| 389 jt->set_suspend_equivalent(); | |
| 390 // cleared by handle_special_suspend_equivalent_condition() | |
| 391 // or java_suspend_self() | |
| 392 | |
| 393 EnterI (THREAD) ; | |
| 394 | |
| 395 if (!ExitSuspendEquivalent(jt)) break ; | |
| 396 | |
| 397 // | |
| 398 // We have acquired the contended monitor, but while we were | |
| 399 // waiting another thread suspended us. We don't want to enter | |
| 400 // the monitor while suspended because that would surprise the | |
| 401 // thread that suspended us. | |
| 402 // | |
| 403 _recursions = 0 ; | |
| 404 _succ = NULL ; | |
| 405 exit (Self) ; | |
| 406 | |
| 407 jt->java_suspend_self(); | |
| 408 } | |
| 409 Self->set_current_pending_monitor(NULL); | |
| 410 } | |
| 411 | |
| 412 Atomic::dec_ptr(&_count); | |
| 413 assert (_count >= 0, "invariant") ; | |
| 414 Self->_Stalled = 0 ; | |
| 415 | |
| 416 // Must either set _recursions = 0 or ASSERT _recursions == 0. | |
| 417 assert (_recursions == 0 , "invariant") ; | |
| 418 assert (_owner == Self , "invariant") ; | |
| 419 assert (_succ != Self , "invariant") ; | |
| 420 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; | |
| 421 | |
| 422 // The thread -- now the owner -- is back in vm mode. | |
| 423 // Report the glorious news via TI,DTrace and jvmstat. | |
| 424 // The probe effect is non-trivial. All the reportage occurs | |
| 425 // while we hold the monitor, increasing the length of the critical | |
| 426 // section. Amdahl's parallel speedup law comes vividly into play. | |
| 427 // | |
| 428 // Another option might be to aggregate the events (thread local or | |
| 429 // per-monitor aggregation) and defer reporting until a more opportune | |
| 430 // time -- such as next time some thread encounters contention but has | |
| 431 // yet to acquire the lock. While spinning that thread could | |
| 432 // spinning we could increment JVMStat counters, etc. | |
| 433 | |
| 434 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); | |
| 435 if (JvmtiExport::should_post_monitor_contended_entered()) { | |
| 436 JvmtiExport::post_monitor_contended_entered(jt, this); | |
| 437 } | |
| 438 if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) { | |
| 439 ObjectMonitor::_sync_ContendedLockAttempts->inc() ; | |
| 440 } | |
| 441 } | |
| 442 | |
| 443 | |
| 444 // Caveat: TryLock() is not necessarily serializing if it returns failure. | |
| 445 // Callers must compensate as needed. | |
| 446 | |
| 447 int ObjectMonitor::TryLock (Thread * Self) { | |
| 448 for (;;) { | |
| 449 void * own = _owner ; | |
| 450 if (own != NULL) return 0 ; | |
| 451 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) { | |
| 452 // Either guarantee _recursions == 0 or set _recursions = 0. | |
| 453 assert (_recursions == 0, "invariant") ; | |
| 454 assert (_owner == Self, "invariant") ; | |
| 455 // CONSIDER: set or assert that OwnerIsThread == 1 | |
| 456 return 1 ; | |
| 457 } | |
| 458 // The lock had been free momentarily, but we lost the race to the lock. | |
| 459 // Interference -- the CAS failed. | |
| 460 // We can either return -1 or retry. | |
| 461 // Retry doesn't make as much sense because the lock was just acquired. | |
| 462 if (true) return -1 ; | |
| 463 } | |
| 464 } | |
| 465 | |
| 466 void ATTR ObjectMonitor::EnterI (TRAPS) { | |
| 467 Thread * Self = THREAD ; | |
| 468 assert (Self->is_Java_thread(), "invariant") ; | |
| 469 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ; | |
| 470 | |
| 471 // Try the lock - TATAS | |
| 472 if (TryLock (Self) > 0) { | |
| 473 assert (_succ != Self , "invariant") ; | |
| 474 assert (_owner == Self , "invariant") ; | |
| 475 assert (_Responsible != Self , "invariant") ; | |
| 476 return ; | |
| 477 } | |
| 478 | |
| 479 DeferredInitialize () ; | |
| 480 | |
| 481 // We try one round of spinning *before* enqueueing Self. | |
| 482 // | |
| 483 // If the _owner is ready but OFFPROC we could use a YieldTo() | |
| 484 // operation to donate the remainder of this thread's quantum | |
| 485 // to the owner. This has subtle but beneficial affinity | |
| 486 // effects. | |
| 487 | |
| 488 if (TrySpin (Self) > 0) { | |
| 489 assert (_owner == Self , "invariant") ; | |
| 490 assert (_succ != Self , "invariant") ; | |
| 491 assert (_Responsible != Self , "invariant") ; | |
| 492 return ; | |
| 493 } | |
| 494 | |
| 495 // The Spin failed -- Enqueue and park the thread ... | |
| 496 assert (_succ != Self , "invariant") ; | |
| 497 assert (_owner != Self , "invariant") ; | |
| 498 assert (_Responsible != Self , "invariant") ; | |
| 499 | |
| 500 // Enqueue "Self" on ObjectMonitor's _cxq. | |
| 501 // | |
| 502 // Node acts as a proxy for Self. | |
| 503 // As an aside, if were to ever rewrite the synchronization code mostly | |
| 504 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class | |
| 505 // Java objects. This would avoid awkward lifecycle and liveness issues, | |
| 506 // as well as eliminate a subset of ABA issues. | |
| 507 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. | |
| 508 // | |
| 509 | |
| 510 ObjectWaiter node(Self) ; | |
| 511 Self->_ParkEvent->reset() ; | |
| 512 node._prev = (ObjectWaiter *) 0xBAD ; | |
| 513 node.TState = ObjectWaiter::TS_CXQ ; | |
| 514 | |
| 515 // Push "Self" onto the front of the _cxq. | |
| 516 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. | |
| 517 // Note that spinning tends to reduce the rate at which threads | |
| 518 // enqueue and dequeue on EntryList|cxq. | |
| 519 ObjectWaiter * nxt ; | |
| 520 for (;;) { | |
| 521 node._next = nxt = _cxq ; | |
| 522 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ; | |
| 523 | |
| 524 // Interference - the CAS failed because _cxq changed. Just retry. | |
| 525 // As an optional optimization we retry the lock. | |
| 526 if (TryLock (Self) > 0) { | |
| 527 assert (_succ != Self , "invariant") ; | |
| 528 assert (_owner == Self , "invariant") ; | |
| 529 assert (_Responsible != Self , "invariant") ; | |
| 530 return ; | |
| 531 } | |
| 532 } | |
| 533 | |
| 534 // Check for cxq|EntryList edge transition to non-null. This indicates | |
| 535 // the onset of contention. While contention persists exiting threads | |
| 536 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit | |
| 537 // operations revert to the faster 1-0 mode. This enter operation may interleave | |
| 538 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we | |
| 539 // arrange for one of the contending thread to use a timed park() operations | |
| 540 // to detect and recover from the race. (Stranding is form of progress failure | |
| 541 // where the monitor is unlocked but all the contending threads remain parked). | |
| 542 // That is, at least one of the contended threads will periodically poll _owner. | |
| 543 // One of the contending threads will become the designated "Responsible" thread. | |
| 544 // The Responsible thread uses a timed park instead of a normal indefinite park | |
| 545 // operation -- it periodically wakes and checks for and recovers from potential | |
| 546 // strandings admitted by 1-0 exit operations. We need at most one Responsible | |
| 547 // thread per-monitor at any given moment. Only threads on cxq|EntryList may | |
| 548 // be responsible for a monitor. | |
| 549 // | |
| 550 // Currently, one of the contended threads takes on the added role of "Responsible". | |
| 551 // A viable alternative would be to use a dedicated "stranding checker" thread | |
| 552 // that periodically iterated over all the threads (or active monitors) and unparked | |
| 553 // successors where there was risk of stranding. This would help eliminate the | |
| 554 // timer scalability issues we see on some platforms as we'd only have one thread | |
| 555 // -- the checker -- parked on a timer. | |
| 556 | |
| 557 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) { | |
| 558 // Try to assume the role of responsible thread for the monitor. | |
| 559 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } | |
| 560 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; | |
| 561 } | |
| 562 | |
| 563 // The lock have been released while this thread was occupied queueing | |
| 564 // itself onto _cxq. To close the race and avoid "stranding" and | |
| 565 // progress-liveness failure we must resample-retry _owner before parking. | |
| 566 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. | |
| 567 // In this case the ST-MEMBAR is accomplished with CAS(). | |
| 568 // | |
| 569 // TODO: Defer all thread state transitions until park-time. | |
| 570 // Since state transitions are heavy and inefficient we'd like | |
| 571 // to defer the state transitions until absolutely necessary, | |
| 572 // and in doing so avoid some transitions ... | |
| 573 | |
| 574 TEVENT (Inflated enter - Contention) ; | |
| 575 int nWakeups = 0 ; | |
| 576 int RecheckInterval = 1 ; | |
| 577 | |
| 578 for (;;) { | |
| 579 | |
| 580 if (TryLock (Self) > 0) break ; | |
| 581 assert (_owner != Self, "invariant") ; | |
| 582 | |
| 583 if ((SyncFlags & 2) && _Responsible == NULL) { | |
| 584 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; | |
| 585 } | |
| 586 | |
| 587 // park self | |
| 588 if (_Responsible == Self || (SyncFlags & 1)) { | |
| 589 TEVENT (Inflated enter - park TIMED) ; | |
| 590 Self->_ParkEvent->park ((jlong) RecheckInterval) ; | |
| 591 // Increase the RecheckInterval, but clamp the value. | |
| 592 RecheckInterval *= 8 ; | |
| 593 if (RecheckInterval > 1000) RecheckInterval = 1000 ; | |
| 594 } else { | |
| 595 TEVENT (Inflated enter - park UNTIMED) ; | |
| 596 Self->_ParkEvent->park() ; | |
| 597 } | |
| 598 | |
| 599 if (TryLock(Self) > 0) break ; | |
| 600 | |
| 601 // The lock is still contested. | |
| 602 // Keep a tally of the # of futile wakeups. | |
| 603 // Note that the counter is not protected by a lock or updated by atomics. | |
| 604 // That is by design - we trade "lossy" counters which are exposed to | |
| 605 // races during updates for a lower probe effect. | |
| 606 TEVENT (Inflated enter - Futile wakeup) ; | |
| 607 if (ObjectMonitor::_sync_FutileWakeups != NULL) { | |
| 608 ObjectMonitor::_sync_FutileWakeups->inc() ; | |
| 609 } | |
| 610 ++ nWakeups ; | |
| 611 | |
| 612 // Assuming this is not a spurious wakeup we'll normally find _succ == Self. | |
| 613 // We can defer clearing _succ until after the spin completes | |
| 614 // TrySpin() must tolerate being called with _succ == Self. | |
| 615 // Try yet another round of adaptive spinning. | |
| 616 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ; | |
| 617 | |
| 618 // We can find that we were unpark()ed and redesignated _succ while | |
| 619 // we were spinning. That's harmless. If we iterate and call park(), | |
| 620 // park() will consume the event and return immediately and we'll | |
| 621 // just spin again. This pattern can repeat, leaving _succ to simply | |
| 622 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks(). | |
| 623 // Alternately, we can sample fired() here, and if set, forgo spinning | |
| 624 // in the next iteration. | |
| 625 | |
| 626 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) { | |
| 627 Self->_ParkEvent->reset() ; | |
| 628 OrderAccess::fence() ; | |
| 629 } | |
| 630 if (_succ == Self) _succ = NULL ; | |
| 631 | |
| 632 // Invariant: after clearing _succ a thread *must* retry _owner before parking. | |
| 633 OrderAccess::fence() ; | |
| 634 } | |
| 635 | |
| 636 // Egress : | |
| 637 // Self has acquired the lock -- Unlink Self from the cxq or EntryList. | |
| 638 // Normally we'll find Self on the EntryList . | |
| 639 // From the perspective of the lock owner (this thread), the | |
| 640 // EntryList is stable and cxq is prepend-only. | |
| 641 // The head of cxq is volatile but the interior is stable. | |
| 642 // In addition, Self.TState is stable. | |
| 643 | |
| 644 assert (_owner == Self , "invariant") ; | |
| 645 assert (object() != NULL , "invariant") ; | |
| 646 // I'd like to write: | |
| 647 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; | |
| 648 // but as we're at a safepoint that's not safe. | |
| 649 | |
| 650 UnlinkAfterAcquire (Self, &node) ; | |
| 651 if (_succ == Self) _succ = NULL ; | |
| 652 | |
| 653 assert (_succ != Self, "invariant") ; | |
| 654 if (_Responsible == Self) { | |
| 655 _Responsible = NULL ; | |
| 656 // Dekker pivot-point. | |
| 657 // Consider OrderAccess::storeload() here | |
| 658 | |
| 659 // We may leave threads on cxq|EntryList without a designated | |
| 660 // "Responsible" thread. This is benign. When this thread subsequently | |
| 661 // exits the monitor it can "see" such preexisting "old" threads -- | |
| 662 // threads that arrived on the cxq|EntryList before the fence, above -- | |
| 663 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads | |
| 664 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible | |
| 665 // non-null and elect a new "Responsible" timer thread. | |
| 666 // | |
| 667 // This thread executes: | |
| 668 // ST Responsible=null; MEMBAR (in enter epilog - here) | |
| 669 // LD cxq|EntryList (in subsequent exit) | |
| 670 // | |
| 671 // Entering threads in the slow/contended path execute: | |
| 672 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) | |
| 673 // The (ST cxq; MEMBAR) is accomplished with CAS(). | |
| 674 // | |
| 675 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent | |
| 676 // exit operation from floating above the ST Responsible=null. | |
| 677 // | |
| 678 // In *practice* however, EnterI() is always followed by some atomic | |
| 679 // operation such as the decrement of _count in ::enter(). Those atomics | |
| 680 // obviate the need for the explicit MEMBAR, above. | |
| 681 } | |
| 682 | |
| 683 // We've acquired ownership with CAS(). | |
| 684 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. | |
| 685 // But since the CAS() this thread may have also stored into _succ, | |
| 686 // EntryList, cxq or Responsible. These meta-data updates must be | |
| 687 // visible __before this thread subsequently drops the lock. | |
| 688 // Consider what could occur if we didn't enforce this constraint -- | |
| 689 // STs to monitor meta-data and user-data could reorder with (become | |
| 690 // visible after) the ST in exit that drops ownership of the lock. | |
| 691 // Some other thread could then acquire the lock, but observe inconsistent | |
| 692 // or old monitor meta-data and heap data. That violates the JMM. | |
| 693 // To that end, the 1-0 exit() operation must have at least STST|LDST | |
| 694 // "release" barrier semantics. Specifically, there must be at least a | |
| 695 // STST|LDST barrier in exit() before the ST of null into _owner that drops | |
| 696 // the lock. The barrier ensures that changes to monitor meta-data and data | |
| 697 // protected by the lock will be visible before we release the lock, and | |
| 698 // therefore before some other thread (CPU) has a chance to acquire the lock. | |
| 699 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. | |
| 700 // | |
| 701 // Critically, any prior STs to _succ or EntryList must be visible before | |
| 702 // the ST of null into _owner in the *subsequent* (following) corresponding | |
| 703 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily | |
| 704 // execute a serializing instruction. | |
| 705 | |
| 706 if (SyncFlags & 8) { | |
| 707 OrderAccess::fence() ; | |
| 708 } | |
| 709 return ; | |
| 710 } | |
| 711 | |
| 712 // ReenterI() is a specialized inline form of the latter half of the | |
| 713 // contended slow-path from EnterI(). We use ReenterI() only for | |
| 714 // monitor reentry in wait(). | |
| 715 // | |
| 716 // In the future we should reconcile EnterI() and ReenterI(), adding | |
| 717 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the | |
| 718 // loop accordingly. | |
| 719 | |
| 720 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) { | |
| 721 assert (Self != NULL , "invariant") ; | |
| 722 assert (SelfNode != NULL , "invariant") ; | |
| 723 assert (SelfNode->_thread == Self , "invariant") ; | |
| 724 assert (_waiters > 0 , "invariant") ; | |
| 725 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ; | |
| 726 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ; | |
| 727 JavaThread * jt = (JavaThread *) Self ; | |
| 728 | |
| 729 int nWakeups = 0 ; | |
| 730 for (;;) { | |
| 731 ObjectWaiter::TStates v = SelfNode->TState ; | |
| 732 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; | |
| 733 assert (_owner != Self, "invariant") ; | |
| 734 | |
| 735 if (TryLock (Self) > 0) break ; | |
| 736 if (TrySpin (Self) > 0) break ; | |
| 737 | |
| 738 TEVENT (Wait Reentry - parking) ; | |
| 739 | |
| 740 // State transition wrappers around park() ... | |
| 741 // ReenterI() wisely defers state transitions until | |
| 742 // it's clear we must park the thread. | |
| 743 { | |
| 744 OSThreadContendState osts(Self->osthread()); | |
| 745 ThreadBlockInVM tbivm(jt); | |
| 746 | |
| 747 // cleared by handle_special_suspend_equivalent_condition() | |
| 748 // or java_suspend_self() | |
| 749 jt->set_suspend_equivalent(); | |
| 750 if (SyncFlags & 1) { | |
| 751 Self->_ParkEvent->park ((jlong)1000) ; | |
| 752 } else { | |
| 753 Self->_ParkEvent->park () ; | |
| 754 } | |
| 755 | |
| 756 // were we externally suspended while we were waiting? | |
| 757 for (;;) { | |
| 758 if (!ExitSuspendEquivalent (jt)) break ; | |
| 759 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } | |
| 760 jt->java_suspend_self(); | |
| 761 jt->set_suspend_equivalent(); | |
| 762 } | |
| 763 } | |
| 764 | |
| 765 // Try again, but just so we distinguish between futile wakeups and | |
| 766 // successful wakeups. The following test isn't algorithmically | |
| 767 // necessary, but it helps us maintain sensible statistics. | |
| 768 if (TryLock(Self) > 0) break ; | |
| 769 | |
| 770 // The lock is still contested. | |
| 771 // Keep a tally of the # of futile wakeups. | |
| 772 // Note that the counter is not protected by a lock or updated by atomics. | |
| 773 // That is by design - we trade "lossy" counters which are exposed to | |
| 774 // races during updates for a lower probe effect. | |
| 775 TEVENT (Wait Reentry - futile wakeup) ; | |
| 776 ++ nWakeups ; | |
| 777 | |
| 778 // Assuming this is not a spurious wakeup we'll normally | |
| 779 // find that _succ == Self. | |
| 780 if (_succ == Self) _succ = NULL ; | |
| 781 | |
| 782 // Invariant: after clearing _succ a contending thread | |
| 783 // *must* retry _owner before parking. | |
| 784 OrderAccess::fence() ; | |
| 785 | |
| 786 if (ObjectMonitor::_sync_FutileWakeups != NULL) { | |
| 787 ObjectMonitor::_sync_FutileWakeups->inc() ; | |
| 788 } | |
| 789 } | |
| 790 | |
| 791 // Self has acquired the lock -- Unlink Self from the cxq or EntryList . | |
| 792 // Normally we'll find Self on the EntryList. | |
| 793 // Unlinking from the EntryList is constant-time and atomic-free. | |
| 794 // From the perspective of the lock owner (this thread), the | |
| 795 // EntryList is stable and cxq is prepend-only. | |
| 796 // The head of cxq is volatile but the interior is stable. | |
| 797 // In addition, Self.TState is stable. | |
| 798 | |
| 799 assert (_owner == Self, "invariant") ; | |
| 800 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; | |
| 801 UnlinkAfterAcquire (Self, SelfNode) ; | |
| 802 if (_succ == Self) _succ = NULL ; | |
| 803 assert (_succ != Self, "invariant") ; | |
| 804 SelfNode->TState = ObjectWaiter::TS_RUN ; | |
| 805 OrderAccess::fence() ; // see comments at the end of EnterI() | |
| 806 } | |
| 807 | |
| 808 // after the thread acquires the lock in ::enter(). Equally, we could defer | |
| 809 // unlinking the thread until ::exit()-time. | |
| 810 | |
| 811 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode) | |
| 812 { | |
| 813 assert (_owner == Self, "invariant") ; | |
| 814 assert (SelfNode->_thread == Self, "invariant") ; | |
| 815 | |
| 816 if (SelfNode->TState == ObjectWaiter::TS_ENTER) { | |
| 817 // Normal case: remove Self from the DLL EntryList . | |
| 818 // This is a constant-time operation. | |
| 819 ObjectWaiter * nxt = SelfNode->_next ; | |
| 820 ObjectWaiter * prv = SelfNode->_prev ; | |
| 821 if (nxt != NULL) nxt->_prev = prv ; | |
| 822 if (prv != NULL) prv->_next = nxt ; | |
| 823 if (SelfNode == _EntryList ) _EntryList = nxt ; | |
| 824 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ; | |
| 825 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ; | |
| 826 TEVENT (Unlink from EntryList) ; | |
| 827 } else { | |
| 828 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ; | |
| 829 // Inopportune interleaving -- Self is still on the cxq. | |
| 830 // This usually means the enqueue of self raced an exiting thread. | |
| 831 // Normally we'll find Self near the front of the cxq, so | |
| 832 // dequeueing is typically fast. If needbe we can accelerate | |
| 833 // this with some MCS/CHL-like bidirectional list hints and advisory | |
| 834 // back-links so dequeueing from the interior will normally operate | |
| 835 // in constant-time. | |
| 836 // Dequeue Self from either the head (with CAS) or from the interior | |
| 837 // with a linear-time scan and normal non-atomic memory operations. | |
| 838 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList | |
| 839 // and then unlink Self from EntryList. We have to drain eventually, | |
| 840 // so it might as well be now. | |
| 841 | |
| 842 ObjectWaiter * v = _cxq ; | |
| 843 assert (v != NULL, "invariant") ; | |
| 844 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) { | |
| 845 // The CAS above can fail from interference IFF a "RAT" arrived. | |
| 846 // In that case Self must be in the interior and can no longer be | |
| 847 // at the head of cxq. | |
| 848 if (v == SelfNode) { | |
| 849 assert (_cxq != v, "invariant") ; | |
| 850 v = _cxq ; // CAS above failed - start scan at head of list | |
| 851 } | |
| 852 ObjectWaiter * p ; | |
| 853 ObjectWaiter * q = NULL ; | |
| 854 for (p = v ; p != NULL && p != SelfNode; p = p->_next) { | |
| 855 q = p ; | |
| 856 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ; | |
| 857 } | |
| 858 assert (v != SelfNode, "invariant") ; | |
| 859 assert (p == SelfNode, "Node not found on cxq") ; | |
| 860 assert (p != _cxq, "invariant") ; | |
| 861 assert (q != NULL, "invariant") ; | |
| 862 assert (q->_next == p, "invariant") ; | |
| 863 q->_next = p->_next ; | |
| 864 } | |
| 865 TEVENT (Unlink from cxq) ; | |
| 866 } | |
| 867 | |
| 868 // Diagnostic hygiene ... | |
| 869 SelfNode->_prev = (ObjectWaiter *) 0xBAD ; | |
| 870 SelfNode->_next = (ObjectWaiter *) 0xBAD ; | |
| 871 SelfNode->TState = ObjectWaiter::TS_RUN ; | |
| 872 } | |
| 873 | |
| 874 // ----------------------------------------------------------------------------- | |
| 875 // Exit support | |
| 876 // | |
| 877 // exit() | |
| 878 // ~~~~~~ | |
| 879 // Note that the collector can't reclaim the objectMonitor or deflate | |
| 880 // the object out from underneath the thread calling ::exit() as the | |
| 881 // thread calling ::exit() never transitions to a stable state. | |
| 882 // This inhibits GC, which in turn inhibits asynchronous (and | |
| 883 // inopportune) reclamation of "this". | |
| 884 // | |
| 885 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; | |
| 886 // There's one exception to the claim above, however. EnterI() can call | |
| 887 // exit() to drop a lock if the acquirer has been externally suspended. | |
| 888 // In that case exit() is called with _thread_state as _thread_blocked, | |
| 889 // but the monitor's _count field is > 0, which inhibits reclamation. | |
| 890 // | |
| 891 // 1-0 exit | |
| 892 // ~~~~~~~~ | |
| 893 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of | |
| 894 // the fast-path operators have been optimized so the common ::exit() | |
| 895 // operation is 1-0. See i486.ad fast_unlock(), for instance. | |
| 896 // The code emitted by fast_unlock() elides the usual MEMBAR. This | |
| 897 // greatly improves latency -- MEMBAR and CAS having considerable local | |
| 898 // latency on modern processors -- but at the cost of "stranding". Absent the | |
| 899 // MEMBAR, a thread in fast_unlock() can race a thread in the slow | |
| 900 // ::enter() path, resulting in the entering thread being stranding | |
| 901 // and a progress-liveness failure. Stranding is extremely rare. | |
| 902 // We use timers (timed park operations) & periodic polling to detect | |
| 903 // and recover from stranding. Potentially stranded threads periodically | |
| 904 // wake up and poll the lock. See the usage of the _Responsible variable. | |
| 905 // | |
| 906 // The CAS() in enter provides for safety and exclusion, while the CAS or | |
| 907 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking | |
| 908 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding. | |
| 909 // We detect and recover from stranding with timers. | |
| 910 // | |
| 911 // If a thread transiently strands it'll park until (a) another | |
| 912 // thread acquires the lock and then drops the lock, at which time the | |
| 913 // exiting thread will notice and unpark the stranded thread, or, (b) | |
| 914 // the timer expires. If the lock is high traffic then the stranding latency | |
| 915 // will be low due to (a). If the lock is low traffic then the odds of | |
| 916 // stranding are lower, although the worst-case stranding latency | |
| 917 // is longer. Critically, we don't want to put excessive load in the | |
| 918 // platform's timer subsystem. We want to minimize both the timer injection | |
| 919 // rate (timers created/sec) as well as the number of timers active at | |
| 920 // any one time. (more precisely, we want to minimize timer-seconds, which is | |
| 921 // the integral of the # of active timers at any instant over time). | |
| 922 // Both impinge on OS scalability. Given that, at most one thread parked on | |
| 923 // a monitor will use a timer. | |
| 924 | |
| 925 void ATTR ObjectMonitor::exit(TRAPS) { | |
| 926 Thread * Self = THREAD ; | |
| 927 if (THREAD != _owner) { | |
| 928 if (THREAD->is_lock_owned((address) _owner)) { | |
| 929 // Transmute _owner from a BasicLock pointer to a Thread address. | |
| 930 // We don't need to hold _mutex for this transition. | |
| 931 // Non-null to Non-null is safe as long as all readers can | |
| 932 // tolerate either flavor. | |
| 933 assert (_recursions == 0, "invariant") ; | |
| 934 _owner = THREAD ; | |
| 935 _recursions = 0 ; | |
| 936 OwnerIsThread = 1 ; | |
| 937 } else { | |
| 938 // NOTE: we need to handle unbalanced monitor enter/exit | |
| 939 // in native code by throwing an exception. | |
| 940 // TODO: Throw an IllegalMonitorStateException ? | |
| 941 TEVENT (Exit - Throw IMSX) ; | |
| 942 assert(false, "Non-balanced monitor enter/exit!"); | |
| 943 if (false) { | |
| 944 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); | |
| 945 } | |
| 946 return; | |
| 947 } | |
| 948 } | |
| 949 | |
| 950 if (_recursions != 0) { | |
| 951 _recursions--; // this is simple recursive enter | |
| 952 TEVENT (Inflated exit - recursive) ; | |
| 953 return ; | |
| 954 } | |
| 955 | |
| 956 // Invariant: after setting Responsible=null an thread must execute | |
| 957 // a MEMBAR or other serializing instruction before fetching EntryList|cxq. | |
| 958 if ((SyncFlags & 4) == 0) { | |
| 959 _Responsible = NULL ; | |
| 960 } | |
| 961 | |
| 962 for (;;) { | |
| 963 assert (THREAD == _owner, "invariant") ; | |
| 964 | |
| 965 | |
| 966 if (Knob_ExitPolicy == 0) { | |
| 967 // release semantics: prior loads and stores from within the critical section | |
| 968 // must not float (reorder) past the following store that drops the lock. | |
| 969 // On SPARC that requires MEMBAR #loadstore|#storestore. | |
| 970 // But of course in TSO #loadstore|#storestore is not required. | |
| 971 // I'd like to write one of the following: | |
| 972 // A. OrderAccess::release() ; _owner = NULL | |
| 973 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL; | |
| 974 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both | |
| 975 // store into a _dummy variable. That store is not needed, but can result | |
| 976 // in massive wasteful coherency traffic on classic SMP systems. | |
| 977 // Instead, I use release_store(), which is implemented as just a simple | |
| 978 // ST on x64, x86 and SPARC. | |
| 979 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock | |
| 980 OrderAccess::storeload() ; // See if we need to wake a successor | |
| 981 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { | |
| 982 TEVENT (Inflated exit - simple egress) ; | |
| 983 return ; | |
| 984 } | |
| 985 TEVENT (Inflated exit - complex egress) ; | |
| 986 | |
| 987 // Normally the exiting thread is responsible for ensuring succession, | |
| 988 // but if other successors are ready or other entering threads are spinning | |
| 989 // then this thread can simply store NULL into _owner and exit without | |
| 990 // waking a successor. The existence of spinners or ready successors | |
| 991 // guarantees proper succession (liveness). Responsibility passes to the | |
| 992 // ready or running successors. The exiting thread delegates the duty. | |
| 993 // More precisely, if a successor already exists this thread is absolved | |
| 994 // of the responsibility of waking (unparking) one. | |
| 995 // | |
| 996 // The _succ variable is critical to reducing futile wakeup frequency. | |
| 997 // _succ identifies the "heir presumptive" thread that has been made | |
| 998 // ready (unparked) but that has not yet run. We need only one such | |
| 999 // successor thread to guarantee progress. | |
| 1000 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf | |
| 1001 // section 3.3 "Futile Wakeup Throttling" for details. | |
| 1002 // | |
| 1003 // Note that spinners in Enter() also set _succ non-null. | |
| 1004 // In the current implementation spinners opportunistically set | |
| 1005 // _succ so that exiting threads might avoid waking a successor. | |
| 1006 // Another less appealing alternative would be for the exiting thread | |
| 1007 // to drop the lock and then spin briefly to see if a spinner managed | |
| 1008 // to acquire the lock. If so, the exiting thread could exit | |
| 1009 // immediately without waking a successor, otherwise the exiting | |
| 1010 // thread would need to dequeue and wake a successor. | |
| 1011 // (Note that we'd need to make the post-drop spin short, but no | |
| 1012 // shorter than the worst-case round-trip cache-line migration time. | |
| 1013 // The dropped lock needs to become visible to the spinner, and then | |
| 1014 // the acquisition of the lock by the spinner must become visible to | |
| 1015 // the exiting thread). | |
| 1016 // | |
| 1017 | |
| 1018 // It appears that an heir-presumptive (successor) must be made ready. | |
| 1019 // Only the current lock owner can manipulate the EntryList or | |
| 1020 // drain _cxq, so we need to reacquire the lock. If we fail | |
| 1021 // to reacquire the lock the responsibility for ensuring succession | |
| 1022 // falls to the new owner. | |
| 1023 // | |
| 1024 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { | |
| 1025 return ; | |
| 1026 } | |
| 1027 TEVENT (Exit - Reacquired) ; | |
| 1028 } else { | |
| 1029 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { | |
| 1030 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock | |
| 1031 OrderAccess::storeload() ; | |
| 1032 // Ratify the previously observed values. | |
| 1033 if (_cxq == NULL || _succ != NULL) { | |
| 1034 TEVENT (Inflated exit - simple egress) ; | |
| 1035 return ; | |
| 1036 } | |
| 1037 | |
| 1038 // inopportune interleaving -- the exiting thread (this thread) | |
| 1039 // in the fast-exit path raced an entering thread in the slow-enter | |
| 1040 // path. | |
| 1041 // We have two choices: | |
| 1042 // A. Try to reacquire the lock. | |
| 1043 // If the CAS() fails return immediately, otherwise | |
| 1044 // we either restart/rerun the exit operation, or simply | |
| 1045 // fall-through into the code below which wakes a successor. | |
| 1046 // B. If the elements forming the EntryList|cxq are TSM | |
| 1047 // we could simply unpark() the lead thread and return | |
| 1048 // without having set _succ. | |
| 1049 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { | |
| 1050 TEVENT (Inflated exit - reacquired succeeded) ; | |
| 1051 return ; | |
| 1052 } | |
| 1053 TEVENT (Inflated exit - reacquired failed) ; | |
| 1054 } else { | |
| 1055 TEVENT (Inflated exit - complex egress) ; | |
| 1056 } | |
| 1057 } | |
| 1058 | |
| 1059 guarantee (_owner == THREAD, "invariant") ; | |
| 1060 | |
| 1061 ObjectWaiter * w = NULL ; | |
| 1062 int QMode = Knob_QMode ; | |
| 1063 | |
| 1064 if (QMode == 2 && _cxq != NULL) { | |
| 1065 // QMode == 2 : cxq has precedence over EntryList. | |
| 1066 // Try to directly wake a successor from the cxq. | |
| 1067 // If successful, the successor will need to unlink itself from cxq. | |
| 1068 w = _cxq ; | |
| 1069 assert (w != NULL, "invariant") ; | |
| 1070 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ; | |
| 1071 ExitEpilog (Self, w) ; | |
| 1072 return ; | |
| 1073 } | |
| 1074 | |
| 1075 if (QMode == 3 && _cxq != NULL) { | |
| 1076 // Aggressively drain cxq into EntryList at the first opportunity. | |
| 1077 // This policy ensure that recently-run threads live at the head of EntryList. | |
| 1078 // Drain _cxq into EntryList - bulk transfer. | |
| 1079 // First, detach _cxq. | |
| 1080 // The following loop is tantamount to: w = swap (&cxq, NULL) | |
| 1081 w = _cxq ; | |
| 1082 for (;;) { | |
| 1083 assert (w != NULL, "Invariant") ; | |
| 1084 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; | |
| 1085 if (u == w) break ; | |
| 1086 w = u ; | |
| 1087 } | |
| 1088 assert (w != NULL , "invariant") ; | |
| 1089 | |
| 1090 ObjectWaiter * q = NULL ; | |
| 1091 ObjectWaiter * p ; | |
| 1092 for (p = w ; p != NULL ; p = p->_next) { | |
| 1093 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; | |
| 1094 p->TState = ObjectWaiter::TS_ENTER ; | |
| 1095 p->_prev = q ; | |
| 1096 q = p ; | |
| 1097 } | |
| 1098 | |
| 1099 // Append the RATs to the EntryList | |
| 1100 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time. | |
| 1101 ObjectWaiter * Tail ; | |
| 1102 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ; | |
| 1103 if (Tail == NULL) { | |
| 1104 _EntryList = w ; | |
| 1105 } else { | |
| 1106 Tail->_next = w ; | |
| 1107 w->_prev = Tail ; | |
| 1108 } | |
| 1109 | |
| 1110 // Fall thru into code that tries to wake a successor from EntryList | |
| 1111 } | |
| 1112 | |
| 1113 if (QMode == 4 && _cxq != NULL) { | |
| 1114 // Aggressively drain cxq into EntryList at the first opportunity. | |
| 1115 // This policy ensure that recently-run threads live at the head of EntryList. | |
| 1116 | |
| 1117 // Drain _cxq into EntryList - bulk transfer. | |
| 1118 // First, detach _cxq. | |
| 1119 // The following loop is tantamount to: w = swap (&cxq, NULL) | |
| 1120 w = _cxq ; | |
| 1121 for (;;) { | |
| 1122 assert (w != NULL, "Invariant") ; | |
| 1123 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; | |
| 1124 if (u == w) break ; | |
| 1125 w = u ; | |
| 1126 } | |
| 1127 assert (w != NULL , "invariant") ; | |
| 1128 | |
| 1129 ObjectWaiter * q = NULL ; | |
| 1130 ObjectWaiter * p ; | |
| 1131 for (p = w ; p != NULL ; p = p->_next) { | |
| 1132 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; | |
| 1133 p->TState = ObjectWaiter::TS_ENTER ; | |
| 1134 p->_prev = q ; | |
| 1135 q = p ; | |
| 1136 } | |
| 1137 | |
| 1138 // Prepend the RATs to the EntryList | |
| 1139 if (_EntryList != NULL) { | |
| 1140 q->_next = _EntryList ; | |
| 1141 _EntryList->_prev = q ; | |
| 1142 } | |
| 1143 _EntryList = w ; | |
| 1144 | |
| 1145 // Fall thru into code that tries to wake a successor from EntryList | |
| 1146 } | |
| 1147 | |
| 1148 w = _EntryList ; | |
| 1149 if (w != NULL) { | |
| 1150 // I'd like to write: guarantee (w->_thread != Self). | |
| 1151 // But in practice an exiting thread may find itself on the EntryList. | |
| 1152 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and | |
| 1153 // then calls exit(). Exit release the lock by setting O._owner to NULL. | |
| 1154 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The | |
| 1155 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then | |
| 1156 // release the lock "O". T2 resumes immediately after the ST of null into | |
| 1157 // _owner, above. T2 notices that the EntryList is populated, so it | |
| 1158 // reacquires the lock and then finds itself on the EntryList. | |
| 1159 // Given all that, we have to tolerate the circumstance where "w" is | |
| 1160 // associated with Self. | |
| 1161 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; | |
| 1162 ExitEpilog (Self, w) ; | |
| 1163 return ; | |
| 1164 } | |
| 1165 | |
| 1166 // If we find that both _cxq and EntryList are null then just | |
| 1167 // re-run the exit protocol from the top. | |
| 1168 w = _cxq ; | |
| 1169 if (w == NULL) continue ; | |
| 1170 | |
| 1171 // Drain _cxq into EntryList - bulk transfer. | |
| 1172 // First, detach _cxq. | |
| 1173 // The following loop is tantamount to: w = swap (&cxq, NULL) | |
| 1174 for (;;) { | |
| 1175 assert (w != NULL, "Invariant") ; | |
| 1176 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; | |
| 1177 if (u == w) break ; | |
| 1178 w = u ; | |
| 1179 } | |
| 1180 TEVENT (Inflated exit - drain cxq into EntryList) ; | |
| 1181 | |
| 1182 assert (w != NULL , "invariant") ; | |
| 1183 assert (_EntryList == NULL , "invariant") ; | |
| 1184 | |
| 1185 // Convert the LIFO SLL anchored by _cxq into a DLL. | |
| 1186 // The list reorganization step operates in O(LENGTH(w)) time. | |
| 1187 // It's critical that this step operate quickly as | |
| 1188 // "Self" still holds the outer-lock, restricting parallelism | |
| 1189 // and effectively lengthening the critical section. | |
| 1190 // Invariant: s chases t chases u. | |
| 1191 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so | |
| 1192 // we have faster access to the tail. | |
| 1193 | |
| 1194 if (QMode == 1) { | |
| 1195 // QMode == 1 : drain cxq to EntryList, reversing order | |
| 1196 // We also reverse the order of the list. | |
| 1197 ObjectWaiter * s = NULL ; | |
| 1198 ObjectWaiter * t = w ; | |
| 1199 ObjectWaiter * u = NULL ; | |
| 1200 while (t != NULL) { | |
| 1201 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ; | |
| 1202 t->TState = ObjectWaiter::TS_ENTER ; | |
| 1203 u = t->_next ; | |
| 1204 t->_prev = u ; | |
| 1205 t->_next = s ; | |
| 1206 s = t; | |
| 1207 t = u ; | |
| 1208 } | |
| 1209 _EntryList = s ; | |
| 1210 assert (s != NULL, "invariant") ; | |
| 1211 } else { | |
| 1212 // QMode == 0 or QMode == 2 | |
| 1213 _EntryList = w ; | |
| 1214 ObjectWaiter * q = NULL ; | |
| 1215 ObjectWaiter * p ; | |
| 1216 for (p = w ; p != NULL ; p = p->_next) { | |
| 1217 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; | |
| 1218 p->TState = ObjectWaiter::TS_ENTER ; | |
| 1219 p->_prev = q ; | |
| 1220 q = p ; | |
| 1221 } | |
| 1222 } | |
| 1223 | |
| 1224 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL | |
| 1225 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). | |
| 1226 | |
| 1227 // See if we can abdicate to a spinner instead of waking a thread. | |
| 1228 // A primary goal of the implementation is to reduce the | |
| 1229 // context-switch rate. | |
| 1230 if (_succ != NULL) continue; | |
| 1231 | |
| 1232 w = _EntryList ; | |
| 1233 if (w != NULL) { | |
| 1234 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; | |
| 1235 ExitEpilog (Self, w) ; | |
| 1236 return ; | |
| 1237 } | |
| 1238 } | |
| 1239 } | |
| 1240 | |
| 1241 // ExitSuspendEquivalent: | |
| 1242 // A faster alternate to handle_special_suspend_equivalent_condition() | |
| 1243 // | |
| 1244 // handle_special_suspend_equivalent_condition() unconditionally | |
| 1245 // acquires the SR_lock. On some platforms uncontended MutexLocker() | |
| 1246 // operations have high latency. Note that in ::enter() we call HSSEC | |
| 1247 // while holding the monitor, so we effectively lengthen the critical sections. | |
| 1248 // | |
| 1249 // There are a number of possible solutions: | |
| 1250 // | |
| 1251 // A. To ameliorate the problem we might also defer state transitions | |
| 1252 // to as late as possible -- just prior to parking. | |
| 1253 // Given that, we'd call HSSEC after having returned from park(), | |
| 1254 // but before attempting to acquire the monitor. This is only a | |
| 1255 // partial solution. It avoids calling HSSEC while holding the | |
| 1256 // monitor (good), but it still increases successor reacquisition latency -- | |
| 1257 // the interval between unparking a successor and the time the successor | |
| 1258 // resumes and retries the lock. See ReenterI(), which defers state transitions. | |
| 1259 // If we use this technique we can also avoid EnterI()-exit() loop | |
| 1260 // in ::enter() where we iteratively drop the lock and then attempt | |
| 1261 // to reacquire it after suspending. | |
| 1262 // | |
| 1263 // B. In the future we might fold all the suspend bits into a | |
| 1264 // composite per-thread suspend flag and then update it with CAS(). | |
| 1265 // Alternately, a Dekker-like mechanism with multiple variables | |
| 1266 // would suffice: | |
| 1267 // ST Self->_suspend_equivalent = false | |
| 1268 // MEMBAR | |
| 1269 // LD Self_>_suspend_flags | |
| 1270 // | |
| 1271 | |
| 1272 | |
| 1273 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) { | |
| 1274 int Mode = Knob_FastHSSEC ; | |
| 1275 if (Mode && !jSelf->is_external_suspend()) { | |
| 1276 assert (jSelf->is_suspend_equivalent(), "invariant") ; | |
| 1277 jSelf->clear_suspend_equivalent() ; | |
| 1278 if (2 == Mode) OrderAccess::storeload() ; | |
| 1279 if (!jSelf->is_external_suspend()) return false ; | |
| 1280 // We raced a suspension -- fall thru into the slow path | |
| 1281 TEVENT (ExitSuspendEquivalent - raced) ; | |
| 1282 jSelf->set_suspend_equivalent() ; | |
| 1283 } | |
| 1284 return jSelf->handle_special_suspend_equivalent_condition() ; | |
| 1285 } | |
| 1286 | |
| 1287 | |
| 1288 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) { | |
| 1289 assert (_owner == Self, "invariant") ; | |
| 1290 | |
| 1291 // Exit protocol: | |
| 1292 // 1. ST _succ = wakee | |
| 1293 // 2. membar #loadstore|#storestore; | |
| 1294 // 2. ST _owner = NULL | |
| 1295 // 3. unpark(wakee) | |
| 1296 | |
| 1297 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ; | |
| 1298 ParkEvent * Trigger = Wakee->_event ; | |
| 1299 | |
| 1300 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. | |
| 1301 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be | |
| 1302 // out-of-scope (non-extant). | |
| 1303 Wakee = NULL ; | |
| 1304 | |
| 1305 // Drop the lock | |
| 1306 OrderAccess::release_store_ptr (&_owner, NULL) ; | |
| 1307 OrderAccess::fence() ; // ST _owner vs LD in unpark() | |
| 1308 | |
| 1309 if (SafepointSynchronize::do_call_back()) { | |
| 1310 TEVENT (unpark before SAFEPOINT) ; | |
| 1311 } | |
| 1312 | |
| 1313 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); | |
| 1314 Trigger->unpark() ; | |
| 1315 | |
| 1316 // Maintain stats and report events to JVMTI | |
| 1317 if (ObjectMonitor::_sync_Parks != NULL) { | |
| 1318 ObjectMonitor::_sync_Parks->inc() ; | |
| 1319 } | |
| 1320 } | |
| 1321 | |
| 1322 | |
| 1323 // ----------------------------------------------------------------------------- | |
| 1324 // Class Loader deadlock handling. | |
| 1325 // | |
| 1326 // complete_exit exits a lock returning recursion count | |
| 1327 // complete_exit/reenter operate as a wait without waiting | |
| 1328 // complete_exit requires an inflated monitor | |
| 1329 // The _owner field is not always the Thread addr even with an | |
| 1330 // inflated monitor, e.g. the monitor can be inflated by a non-owning | |
| 1331 // thread due to contention. | |
| 1332 intptr_t ObjectMonitor::complete_exit(TRAPS) { | |
| 1333 Thread * const Self = THREAD; | |
| 1334 assert(Self->is_Java_thread(), "Must be Java thread!"); | |
| 1335 JavaThread *jt = (JavaThread *)THREAD; | |
| 1336 | |
| 1337 DeferredInitialize(); | |
| 1338 | |
| 1339 if (THREAD != _owner) { | |
| 1340 if (THREAD->is_lock_owned ((address)_owner)) { | |
| 1341 assert(_recursions == 0, "internal state error"); | |
| 1342 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ | |
| 1343 _recursions = 0 ; | |
| 1344 OwnerIsThread = 1 ; | |
| 1345 } | |
| 1346 } | |
| 1347 | |
| 1348 guarantee(Self == _owner, "complete_exit not owner"); | |
| 1349 intptr_t save = _recursions; // record the old recursion count | |
| 1350 _recursions = 0; // set the recursion level to be 0 | |
| 1351 exit (Self) ; // exit the monitor | |
| 1352 guarantee (_owner != Self, "invariant"); | |
| 1353 return save; | |
| 1354 } | |
| 1355 | |
| 1356 // reenter() enters a lock and sets recursion count | |
| 1357 // complete_exit/reenter operate as a wait without waiting | |
| 1358 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) { | |
| 1359 Thread * const Self = THREAD; | |
| 1360 assert(Self->is_Java_thread(), "Must be Java thread!"); | |
| 1361 JavaThread *jt = (JavaThread *)THREAD; | |
| 1362 | |
| 1363 guarantee(_owner != Self, "reenter already owner"); | |
| 1364 enter (THREAD); // enter the monitor | |
| 1365 guarantee (_recursions == 0, "reenter recursion"); | |
| 1366 _recursions = recursions; | |
| 1367 return; | |
| 1368 } | |
| 1369 | |
| 1370 | |
| 1371 // ----------------------------------------------------------------------------- | |
| 1372 // A macro is used below because there may already be a pending | |
| 1373 // exception which should not abort the execution of the routines | |
| 1374 // which use this (which is why we don't put this into check_slow and | |
| 1375 // call it with a CHECK argument). | |
| 1376 | |
| 1377 #define CHECK_OWNER() \ | |
| 1378 do { \ | |
| 1379 if (THREAD != _owner) { \ | |
| 1380 if (THREAD->is_lock_owned((address) _owner)) { \ | |
| 1381 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \ | |
| 1382 _recursions = 0; \ | |
| 1383 OwnerIsThread = 1 ; \ | |
| 1384 } else { \ | |
| 1385 TEVENT (Throw IMSX) ; \ | |
| 1386 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \ | |
| 1387 } \ | |
| 1388 } \ | |
| 1389 } while (false) | |
| 1390 | |
| 1391 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception. | |
| 1392 // TODO-FIXME: remove check_slow() -- it's likely dead. | |
| 1393 | |
| 1394 void ObjectMonitor::check_slow(TRAPS) { | |
| 1395 TEVENT (check_slow - throw IMSX) ; | |
| 1396 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner"); | |
| 1397 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner"); | |
| 1398 } | |
| 1399 | |
| 1400 static int Adjust (volatile int * adr, int dx) { | |
| 1401 int v ; | |
| 1402 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ; | |
| 1403 return v ; | |
| 1404 } | |
| 1405 // ----------------------------------------------------------------------------- | |
| 1406 // Wait/Notify/NotifyAll | |
| 1407 // | |
| 1408 // Note: a subset of changes to ObjectMonitor::wait() | |
| 1409 // will need to be replicated in complete_exit above | |
| 1410 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { | |
| 1411 Thread * const Self = THREAD ; | |
| 1412 assert(Self->is_Java_thread(), "Must be Java thread!"); | |
| 1413 JavaThread *jt = (JavaThread *)THREAD; | |
| 1414 | |
| 1415 DeferredInitialize () ; | |
| 1416 | |
| 1417 // Throw IMSX or IEX. | |
| 1418 CHECK_OWNER(); | |
| 1419 | |
| 1420 // check for a pending interrupt | |
| 1421 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { | |
| 1422 // post monitor waited event. Note that this is past-tense, we are done waiting. | |
| 1423 if (JvmtiExport::should_post_monitor_waited()) { | |
| 1424 // Note: 'false' parameter is passed here because the | |
| 1425 // wait was not timed out due to thread interrupt. | |
| 1426 JvmtiExport::post_monitor_waited(jt, this, false); | |
| 1427 } | |
| 1428 TEVENT (Wait - Throw IEX) ; | |
| 1429 THROW(vmSymbols::java_lang_InterruptedException()); | |
| 1430 return ; | |
| 1431 } | |
| 1432 TEVENT (Wait) ; | |
| 1433 | |
| 1434 assert (Self->_Stalled == 0, "invariant") ; | |
| 1435 Self->_Stalled = intptr_t(this) ; | |
| 1436 jt->set_current_waiting_monitor(this); | |
| 1437 | |
| 1438 // create a node to be put into the queue | |
| 1439 // Critically, after we reset() the event but prior to park(), we must check | |
| 1440 // for a pending interrupt. | |
| 1441 ObjectWaiter node(Self); | |
| 1442 node.TState = ObjectWaiter::TS_WAIT ; | |
| 1443 Self->_ParkEvent->reset() ; | |
| 1444 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag | |
| 1445 | |
| 1446 // Enter the waiting queue, which is a circular doubly linked list in this case | |
| 1447 // but it could be a priority queue or any data structure. | |
| 1448 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only | |
| 1449 // by the the owner of the monitor *except* in the case where park() | |
| 1450 // returns because of a timeout of interrupt. Contention is exceptionally rare | |
| 1451 // so we use a simple spin-lock instead of a heavier-weight blocking lock. | |
| 1452 | |
| 1453 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ; | |
| 1454 AddWaiter (&node) ; | |
| 1455 Thread::SpinRelease (&_WaitSetLock) ; | |
| 1456 | |
| 1457 if ((SyncFlags & 4) == 0) { | |
| 1458 _Responsible = NULL ; | |
| 1459 } | |
| 1460 intptr_t save = _recursions; // record the old recursion count | |
| 1461 _waiters++; // increment the number of waiters | |
| 1462 _recursions = 0; // set the recursion level to be 1 | |
| 1463 exit (Self) ; // exit the monitor | |
| 1464 guarantee (_owner != Self, "invariant") ; | |
| 1465 | |
| 1466 // As soon as the ObjectMonitor's ownership is dropped in the exit() | |
| 1467 // call above, another thread can enter() the ObjectMonitor, do the | |
| 1468 // notify(), and exit() the ObjectMonitor. If the other thread's | |
| 1469 // exit() call chooses this thread as the successor and the unpark() | |
| 1470 // call happens to occur while this thread is posting a | |
| 1471 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event | |
| 1472 // handler using RawMonitors and consuming the unpark(). | |
| 1473 // | |
| 1474 // To avoid the problem, we re-post the event. This does no harm | |
| 1475 // even if the original unpark() was not consumed because we are the | |
| 1476 // chosen successor for this monitor. | |
| 1477 if (node._notified != 0 && _succ == Self) { | |
| 1478 node._event->unpark(); | |
| 1479 } | |
| 1480 | |
| 1481 // The thread is on the WaitSet list - now park() it. | |
| 1482 // On MP systems it's conceivable that a brief spin before we park | |
| 1483 // could be profitable. | |
| 1484 // | |
| 1485 // TODO-FIXME: change the following logic to a loop of the form | |
| 1486 // while (!timeout && !interrupted && _notified == 0) park() | |
| 1487 | |
| 1488 int ret = OS_OK ; | |
| 1489 int WasNotified = 0 ; | |
| 1490 { // State transition wrappers | |
| 1491 OSThread* osthread = Self->osthread(); | |
| 1492 OSThreadWaitState osts(osthread, true); | |
| 1493 { | |
| 1494 ThreadBlockInVM tbivm(jt); | |
| 1495 // Thread is in thread_blocked state and oop access is unsafe. | |
| 1496 jt->set_suspend_equivalent(); | |
| 1497 | |
| 1498 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) { | |
| 1499 // Intentionally empty | |
| 1500 } else | |
| 1501 if (node._notified == 0) { | |
| 1502 if (millis <= 0) { | |
| 1503 Self->_ParkEvent->park () ; | |
| 1504 } else { | |
| 1505 ret = Self->_ParkEvent->park (millis) ; | |
| 1506 } | |
| 1507 } | |
| 1508 | |
| 1509 // were we externally suspended while we were waiting? | |
| 1510 if (ExitSuspendEquivalent (jt)) { | |
| 1511 // TODO-FIXME: add -- if succ == Self then succ = null. | |
| 1512 jt->java_suspend_self(); | |
| 1513 } | |
| 1514 | |
| 1515 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm | |
| 1516 | |
| 1517 | |
| 1518 // Node may be on the WaitSet, the EntryList (or cxq), or in transition | |
| 1519 // from the WaitSet to the EntryList. | |
| 1520 // See if we need to remove Node from the WaitSet. | |
| 1521 // We use double-checked locking to avoid grabbing _WaitSetLock | |
| 1522 // if the thread is not on the wait queue. | |
| 1523 // | |
| 1524 // Note that we don't need a fence before the fetch of TState. | |
| 1525 // In the worst case we'll fetch a old-stale value of TS_WAIT previously | |
| 1526 // written by the is thread. (perhaps the fetch might even be satisfied | |
| 1527 // by a look-aside into the processor's own store buffer, although given | |
| 1528 // the length of the code path between the prior ST and this load that's | |
| 1529 // highly unlikely). If the following LD fetches a stale TS_WAIT value | |
| 1530 // then we'll acquire the lock and then re-fetch a fresh TState value. | |
| 1531 // That is, we fail toward safety. | |
| 1532 | |
| 1533 if (node.TState == ObjectWaiter::TS_WAIT) { | |
| 1534 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ; | |
| 1535 if (node.TState == ObjectWaiter::TS_WAIT) { | |
| 1536 DequeueSpecificWaiter (&node) ; // unlink from WaitSet | |
| 1537 assert(node._notified == 0, "invariant"); | |
| 1538 node.TState = ObjectWaiter::TS_RUN ; | |
| 1539 } | |
| 1540 Thread::SpinRelease (&_WaitSetLock) ; | |
| 1541 } | |
| 1542 | |
| 1543 // The thread is now either on off-list (TS_RUN), | |
| 1544 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). | |
| 1545 // The Node's TState variable is stable from the perspective of this thread. | |
| 1546 // No other threads will asynchronously modify TState. | |
| 1547 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ; | |
| 1548 OrderAccess::loadload() ; | |
| 1549 if (_succ == Self) _succ = NULL ; | |
| 1550 WasNotified = node._notified ; | |
| 1551 | |
| 1552 // Reentry phase -- reacquire the monitor. | |
| 1553 // re-enter contended monitor after object.wait(). | |
| 1554 // retain OBJECT_WAIT state until re-enter successfully completes | |
| 1555 // Thread state is thread_in_vm and oop access is again safe, | |
| 1556 // although the raw address of the object may have changed. | |
| 1557 // (Don't cache naked oops over safepoints, of course). | |
| 1558 | |
| 1559 // post monitor waited event. Note that this is past-tense, we are done waiting. | |
| 1560 if (JvmtiExport::should_post_monitor_waited()) { | |
| 1561 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); | |
| 1562 } | |
| 1563 OrderAccess::fence() ; | |
| 1564 | |
| 1565 assert (Self->_Stalled != 0, "invariant") ; | |
| 1566 Self->_Stalled = 0 ; | |
| 1567 | |
| 1568 assert (_owner != Self, "invariant") ; | |
| 1569 ObjectWaiter::TStates v = node.TState ; | |
| 1570 if (v == ObjectWaiter::TS_RUN) { | |
| 1571 enter (Self) ; | |
| 1572 } else { | |
| 1573 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; | |
| 1574 ReenterI (Self, &node) ; | |
| 1575 node.wait_reenter_end(this); | |
| 1576 } | |
| 1577 | |
| 1578 // Self has reacquired the lock. | |
| 1579 // Lifecycle - the node representing Self must not appear on any queues. | |
| 1580 // Node is about to go out-of-scope, but even if it were immortal we wouldn't | |
| 1581 // want residual elements associated with this thread left on any lists. | |
| 1582 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ; | |
| 1583 assert (_owner == Self, "invariant") ; | |
| 1584 assert (_succ != Self , "invariant") ; | |
| 1585 } // OSThreadWaitState() | |
| 1586 | |
| 1587 jt->set_current_waiting_monitor(NULL); | |
| 1588 | |
| 1589 guarantee (_recursions == 0, "invariant") ; | |
| 1590 _recursions = save; // restore the old recursion count | |
| 1591 _waiters--; // decrement the number of waiters | |
| 1592 | |
| 1593 // Verify a few postconditions | |
| 1594 assert (_owner == Self , "invariant") ; | |
| 1595 assert (_succ != Self , "invariant") ; | |
| 1596 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; | |
| 1597 | |
| 1598 if (SyncFlags & 32) { | |
| 1599 OrderAccess::fence() ; | |
| 1600 } | |
| 1601 | |
| 1602 // check if the notification happened | |
| 1603 if (!WasNotified) { | |
| 1604 // no, it could be timeout or Thread.interrupt() or both | |
| 1605 // check for interrupt event, otherwise it is timeout | |
| 1606 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { | |
| 1607 TEVENT (Wait - throw IEX from epilog) ; | |
| 1608 THROW(vmSymbols::java_lang_InterruptedException()); | |
| 1609 } | |
| 1610 } | |
| 1611 | |
| 1612 // NOTE: Spurious wake up will be consider as timeout. | |
| 1613 // Monitor notify has precedence over thread interrupt. | |
| 1614 } | |
| 1615 | |
| 1616 | |
| 1617 // Consider: | |
| 1618 // If the lock is cool (cxq == null && succ == null) and we're on an MP system | |
| 1619 // then instead of transferring a thread from the WaitSet to the EntryList | |
| 1620 // we might just dequeue a thread from the WaitSet and directly unpark() it. | |
| 1621 | |
| 1622 void ObjectMonitor::notify(TRAPS) { | |
| 1623 CHECK_OWNER(); | |
| 1624 if (_WaitSet == NULL) { | |
| 1625 TEVENT (Empty-Notify) ; | |
| 1626 return ; | |
| 1627 } | |
| 1628 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); | |
| 1629 | |
| 1630 int Policy = Knob_MoveNotifyee ; | |
| 1631 | |
| 1632 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ; | |
| 1633 ObjectWaiter * iterator = DequeueWaiter() ; | |
| 1634 if (iterator != NULL) { | |
| 1635 TEVENT (Notify1 - Transfer) ; | |
| 1636 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; | |
| 1637 guarantee (iterator->_notified == 0, "invariant") ; | |
| 1638 if (Policy != 4) { | |
| 1639 iterator->TState = ObjectWaiter::TS_ENTER ; | |
| 1640 } | |
| 1641 iterator->_notified = 1 ; | |
| 1642 | |
| 1643 ObjectWaiter * List = _EntryList ; | |
| 1644 if (List != NULL) { | |
| 1645 assert (List->_prev == NULL, "invariant") ; | |
| 1646 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; | |
| 1647 assert (List != iterator, "invariant") ; | |
| 1648 } | |
| 1649 | |
| 1650 if (Policy == 0) { // prepend to EntryList | |
| 1651 if (List == NULL) { | |
| 1652 iterator->_next = iterator->_prev = NULL ; | |
| 1653 _EntryList = iterator ; | |
| 1654 } else { | |
| 1655 List->_prev = iterator ; | |
| 1656 iterator->_next = List ; | |
| 1657 iterator->_prev = NULL ; | |
| 1658 _EntryList = iterator ; | |
| 1659 } | |
| 1660 } else | |
| 1661 if (Policy == 1) { // append to EntryList | |
| 1662 if (List == NULL) { | |
| 1663 iterator->_next = iterator->_prev = NULL ; | |
| 1664 _EntryList = iterator ; | |
| 1665 } else { | |
| 1666 // CONSIDER: finding the tail currently requires a linear-time walk of | |
| 1667 // the EntryList. We can make tail access constant-time by converting to | |
| 1668 // a CDLL instead of using our current DLL. | |
| 1669 ObjectWaiter * Tail ; | |
| 1670 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; | |
| 1671 assert (Tail != NULL && Tail->_next == NULL, "invariant") ; | |
| 1672 Tail->_next = iterator ; | |
| 1673 iterator->_prev = Tail ; | |
| 1674 iterator->_next = NULL ; | |
| 1675 } | |
| 1676 } else | |
| 1677 if (Policy == 2) { // prepend to cxq | |
| 1678 // prepend to cxq | |
| 1679 if (List == NULL) { | |
| 1680 iterator->_next = iterator->_prev = NULL ; | |
| 1681 _EntryList = iterator ; | |
| 1682 } else { | |
| 1683 iterator->TState = ObjectWaiter::TS_CXQ ; | |
| 1684 for (;;) { | |
| 1685 ObjectWaiter * Front = _cxq ; | |
| 1686 iterator->_next = Front ; | |
| 1687 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { | |
| 1688 break ; | |
| 1689 } | |
| 1690 } | |
| 1691 } | |
| 1692 } else | |
| 1693 if (Policy == 3) { // append to cxq | |
| 1694 iterator->TState = ObjectWaiter::TS_CXQ ; | |
| 1695 for (;;) { | |
| 1696 ObjectWaiter * Tail ; | |
| 1697 Tail = _cxq ; | |
| 1698 if (Tail == NULL) { | |
| 1699 iterator->_next = NULL ; | |
| 1700 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { | |
| 1701 break ; | |
| 1702 } | |
| 1703 } else { | |
| 1704 while (Tail->_next != NULL) Tail = Tail->_next ; | |
| 1705 Tail->_next = iterator ; | |
| 1706 iterator->_prev = Tail ; | |
| 1707 iterator->_next = NULL ; | |
| 1708 break ; | |
| 1709 } | |
| 1710 } | |
| 1711 } else { | |
| 1712 ParkEvent * ev = iterator->_event ; | |
| 1713 iterator->TState = ObjectWaiter::TS_RUN ; | |
| 1714 OrderAccess::fence() ; | |
| 1715 ev->unpark() ; | |
| 1716 } | |
| 1717 | |
| 1718 if (Policy < 4) { | |
| 1719 iterator->wait_reenter_begin(this); | |
| 1720 } | |
| 1721 | |
| 1722 // _WaitSetLock protects the wait queue, not the EntryList. We could | |
| 1723 // move the add-to-EntryList operation, above, outside the critical section | |
| 1724 // protected by _WaitSetLock. In practice that's not useful. With the | |
| 1725 // exception of wait() timeouts and interrupts the monitor owner | |
| 1726 // is the only thread that grabs _WaitSetLock. There's almost no contention | |
| 1727 // on _WaitSetLock so it's not profitable to reduce the length of the | |
| 1728 // critical section. | |
| 1729 } | |
| 1730 | |
| 1731 Thread::SpinRelease (&_WaitSetLock) ; | |
| 1732 | |
| 1733 if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) { | |
| 1734 ObjectMonitor::_sync_Notifications->inc() ; | |
| 1735 } | |
| 1736 } | |
| 1737 | |
| 1738 | |
| 1739 void ObjectMonitor::notifyAll(TRAPS) { | |
| 1740 CHECK_OWNER(); | |
| 1741 ObjectWaiter* iterator; | |
| 1742 if (_WaitSet == NULL) { | |
| 1743 TEVENT (Empty-NotifyAll) ; | |
| 1744 return ; | |
| 1745 } | |
| 1746 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); | |
| 1747 | |
| 1748 int Policy = Knob_MoveNotifyee ; | |
| 1749 int Tally = 0 ; | |
| 1750 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ; | |
| 1751 | |
| 1752 for (;;) { | |
| 1753 iterator = DequeueWaiter () ; | |
| 1754 if (iterator == NULL) break ; | |
| 1755 TEVENT (NotifyAll - Transfer1) ; | |
| 1756 ++Tally ; | |
| 1757 | |
| 1758 // Disposition - what might we do with iterator ? | |
| 1759 // a. add it directly to the EntryList - either tail or head. | |
| 1760 // b. push it onto the front of the _cxq. | |
| 1761 // For now we use (a). | |
| 1762 | |
| 1763 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; | |
| 1764 guarantee (iterator->_notified == 0, "invariant") ; | |
| 1765 iterator->_notified = 1 ; | |
| 1766 if (Policy != 4) { | |
| 1767 iterator->TState = ObjectWaiter::TS_ENTER ; | |
| 1768 } | |
| 1769 | |
| 1770 ObjectWaiter * List = _EntryList ; | |
| 1771 if (List != NULL) { | |
| 1772 assert (List->_prev == NULL, "invariant") ; | |
| 1773 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; | |
| 1774 assert (List != iterator, "invariant") ; | |
| 1775 } | |
| 1776 | |
| 1777 if (Policy == 0) { // prepend to EntryList | |
| 1778 if (List == NULL) { | |
| 1779 iterator->_next = iterator->_prev = NULL ; | |
| 1780 _EntryList = iterator ; | |
| 1781 } else { | |
| 1782 List->_prev = iterator ; | |
| 1783 iterator->_next = List ; | |
| 1784 iterator->_prev = NULL ; | |
| 1785 _EntryList = iterator ; | |
| 1786 } | |
| 1787 } else | |
| 1788 if (Policy == 1) { // append to EntryList | |
| 1789 if (List == NULL) { | |
| 1790 iterator->_next = iterator->_prev = NULL ; | |
| 1791 _EntryList = iterator ; | |
| 1792 } else { | |
| 1793 // CONSIDER: finding the tail currently requires a linear-time walk of | |
| 1794 // the EntryList. We can make tail access constant-time by converting to | |
| 1795 // a CDLL instead of using our current DLL. | |
| 1796 ObjectWaiter * Tail ; | |
| 1797 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; | |
| 1798 assert (Tail != NULL && Tail->_next == NULL, "invariant") ; | |
| 1799 Tail->_next = iterator ; | |
| 1800 iterator->_prev = Tail ; | |
| 1801 iterator->_next = NULL ; | |
| 1802 } | |
| 1803 } else | |
| 1804 if (Policy == 2) { // prepend to cxq | |
| 1805 // prepend to cxq | |
| 1806 iterator->TState = ObjectWaiter::TS_CXQ ; | |
| 1807 for (;;) { | |
| 1808 ObjectWaiter * Front = _cxq ; | |
| 1809 iterator->_next = Front ; | |
| 1810 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { | |
| 1811 break ; | |
| 1812 } | |
| 1813 } | |
| 1814 } else | |
| 1815 if (Policy == 3) { // append to cxq | |
| 1816 iterator->TState = ObjectWaiter::TS_CXQ ; | |
| 1817 for (;;) { | |
| 1818 ObjectWaiter * Tail ; | |
| 1819 Tail = _cxq ; | |
| 1820 if (Tail == NULL) { | |
| 1821 iterator->_next = NULL ; | |
| 1822 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { | |
| 1823 break ; | |
| 1824 } | |
| 1825 } else { | |
| 1826 while (Tail->_next != NULL) Tail = Tail->_next ; | |
| 1827 Tail->_next = iterator ; | |
| 1828 iterator->_prev = Tail ; | |
| 1829 iterator->_next = NULL ; | |
| 1830 break ; | |
| 1831 } | |
| 1832 } | |
| 1833 } else { | |
| 1834 ParkEvent * ev = iterator->_event ; | |
| 1835 iterator->TState = ObjectWaiter::TS_RUN ; | |
| 1836 OrderAccess::fence() ; | |
| 1837 ev->unpark() ; | |
| 1838 } | |
| 1839 | |
| 1840 if (Policy < 4) { | |
| 1841 iterator->wait_reenter_begin(this); | |
| 1842 } | |
| 1843 | |
| 1844 // _WaitSetLock protects the wait queue, not the EntryList. We could | |
| 1845 // move the add-to-EntryList operation, above, outside the critical section | |
| 1846 // protected by _WaitSetLock. In practice that's not useful. With the | |
| 1847 // exception of wait() timeouts and interrupts the monitor owner | |
| 1848 // is the only thread that grabs _WaitSetLock. There's almost no contention | |
| 1849 // on _WaitSetLock so it's not profitable to reduce the length of the | |
| 1850 // critical section. | |
| 1851 } | |
| 1852 | |
| 1853 Thread::SpinRelease (&_WaitSetLock) ; | |
| 1854 | |
| 1855 if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) { | |
| 1856 ObjectMonitor::_sync_Notifications->inc(Tally) ; | |
| 1857 } | |
| 1858 } | |
| 1859 | |
| 1860 // ----------------------------------------------------------------------------- | |
| 1861 // Adaptive Spinning Support | |
| 1862 // | |
| 1863 // Adaptive spin-then-block - rational spinning | |
| 1864 // | |
| 1865 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS | |
| 1866 // algorithm. On high order SMP systems it would be better to start with | |
| 1867 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, | |
| 1868 // a contending thread could enqueue itself on the cxq and then spin locally | |
| 1869 // on a thread-specific variable such as its ParkEvent._Event flag. | |
| 1870 // That's left as an exercise for the reader. Note that global spinning is | |
| 1871 // not problematic on Niagara, as the L2$ serves the interconnect and has both | |
| 1872 // low latency and massive bandwidth. | |
| 1873 // | |
| 1874 // Broadly, we can fix the spin frequency -- that is, the % of contended lock | |
| 1875 // acquisition attempts where we opt to spin -- at 100% and vary the spin count | |
| 1876 // (duration) or we can fix the count at approximately the duration of | |
| 1877 // a context switch and vary the frequency. Of course we could also | |
| 1878 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. | |
| 1879 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html. | |
| 1880 // | |
| 1881 // This implementation varies the duration "D", where D varies with | |
| 1882 // the success rate of recent spin attempts. (D is capped at approximately | |
| 1883 // length of a round-trip context switch). The success rate for recent | |
| 1884 // spin attempts is a good predictor of the success rate of future spin | |
| 1885 // attempts. The mechanism adapts automatically to varying critical | |
| 1886 // section length (lock modality), system load and degree of parallelism. | |
| 1887 // D is maintained per-monitor in _SpinDuration and is initialized | |
| 1888 // optimistically. Spin frequency is fixed at 100%. | |
| 1889 // | |
| 1890 // Note that _SpinDuration is volatile, but we update it without locks | |
| 1891 // or atomics. The code is designed so that _SpinDuration stays within | |
| 1892 // a reasonable range even in the presence of races. The arithmetic | |
| 1893 // operations on _SpinDuration are closed over the domain of legal values, | |
| 1894 // so at worst a race will install and older but still legal value. | |
| 1895 // At the very worst this introduces some apparent non-determinism. | |
| 1896 // We might spin when we shouldn't or vice-versa, but since the spin | |
| 1897 // count are relatively short, even in the worst case, the effect is harmless. | |
| 1898 // | |
| 1899 // Care must be taken that a low "D" value does not become an | |
| 1900 // an absorbing state. Transient spinning failures -- when spinning | |
| 1901 // is overall profitable -- should not cause the system to converge | |
| 1902 // on low "D" values. We want spinning to be stable and predictable | |
| 1903 // and fairly responsive to change and at the same time we don't want | |
| 1904 // it to oscillate, become metastable, be "too" non-deterministic, | |
| 1905 // or converge on or enter undesirable stable absorbing states. | |
| 1906 // | |
| 1907 // We implement a feedback-based control system -- using past behavior | |
| 1908 // to predict future behavior. We face two issues: (a) if the | |
| 1909 // input signal is random then the spin predictor won't provide optimal | |
| 1910 // results, and (b) if the signal frequency is too high then the control | |
| 1911 // system, which has some natural response lag, will "chase" the signal. | |
| 1912 // (b) can arise from multimodal lock hold times. Transient preemption | |
| 1913 // can also result in apparent bimodal lock hold times. | |
| 1914 // Although sub-optimal, neither condition is particularly harmful, as | |
| 1915 // in the worst-case we'll spin when we shouldn't or vice-versa. | |
| 1916 // The maximum spin duration is rather short so the failure modes aren't bad. | |
| 1917 // To be conservative, I've tuned the gain in system to bias toward | |
| 1918 // _not spinning. Relatedly, the system can sometimes enter a mode where it | |
| 1919 // "rings" or oscillates between spinning and not spinning. This happens | |
| 1920 // when spinning is just on the cusp of profitability, however, so the | |
| 1921 // situation is not dire. The state is benign -- there's no need to add | |
| 1922 // hysteresis control to damp the transition rate between spinning and | |
| 1923 // not spinning. | |
| 1924 // | |
| 1925 | |
| 1926 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ; | |
| 1927 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ; | |
| 1928 | |
| 1929 // Spinning: Fixed frequency (100%), vary duration | |
| 1930 | |
| 1931 | |
| 1932 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) { | |
| 1933 | |
| 1934 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. | |
| 1935 int ctr = Knob_FixedSpin ; | |
| 1936 if (ctr != 0) { | |
| 1937 while (--ctr >= 0) { | |
| 1938 if (TryLock (Self) > 0) return 1 ; | |
| 1939 SpinPause () ; | |
| 1940 } | |
| 1941 return 0 ; | |
| 1942 } | |
| 1943 | |
| 1944 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) { | |
| 1945 if (TryLock(Self) > 0) { | |
| 1946 // Increase _SpinDuration ... | |
| 1947 // Note that we don't clamp SpinDuration precisely at SpinLimit. | |
| 1948 // Raising _SpurDuration to the poverty line is key. | |
| 1949 int x = _SpinDuration ; | |
| 1950 if (x < Knob_SpinLimit) { | |
| 1951 if (x < Knob_Poverty) x = Knob_Poverty ; | |
| 1952 _SpinDuration = x + Knob_BonusB ; | |
| 1953 } | |
| 1954 return 1 ; | |
| 1955 } | |
| 1956 SpinPause () ; | |
| 1957 } | |
| 1958 | |
| 1959 // Admission control - verify preconditions for spinning | |
| 1960 // | |
| 1961 // We always spin a little bit, just to prevent _SpinDuration == 0 from | |
| 1962 // becoming an absorbing state. Put another way, we spin briefly to | |
| 1963 // sample, just in case the system load, parallelism, contention, or lock | |
| 1964 // modality changed. | |
| 1965 // | |
| 1966 // Consider the following alternative: | |
| 1967 // Periodically set _SpinDuration = _SpinLimit and try a long/full | |
| 1968 // spin attempt. "Periodically" might mean after a tally of | |
| 1969 // the # of failed spin attempts (or iterations) reaches some threshold. | |
| 1970 // This takes us into the realm of 1-out-of-N spinning, where we | |
| 1971 // hold the duration constant but vary the frequency. | |
| 1972 | |
| 1973 ctr = _SpinDuration ; | |
| 1974 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ; | |
| 1975 if (ctr <= 0) return 0 ; | |
| 1976 | |
| 1977 if (Knob_SuccRestrict && _succ != NULL) return 0 ; | |
| 1978 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) { | |
| 1979 TEVENT (Spin abort - notrunnable [TOP]); | |
| 1980 return 0 ; | |
| 1981 } | |
| 1982 | |
| 1983 int MaxSpin = Knob_MaxSpinners ; | |
| 1984 if (MaxSpin >= 0) { | |
| 1985 if (_Spinner > MaxSpin) { | |
| 1986 TEVENT (Spin abort -- too many spinners) ; | |
| 1987 return 0 ; | |
| 1988 } | |
| 1989 // Slighty racy, but benign ... | |
| 1990 Adjust (&_Spinner, 1) ; | |
| 1991 } | |
| 1992 | |
| 1993 // We're good to spin ... spin ingress. | |
| 1994 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades | |
| 1995 // when preparing to LD...CAS _owner, etc and the CAS is likely | |
| 1996 // to succeed. | |
| 1997 int hits = 0 ; | |
| 1998 int msk = 0 ; | |
| 1999 int caspty = Knob_CASPenalty ; | |
| 2000 int oxpty = Knob_OXPenalty ; | |
| 2001 int sss = Knob_SpinSetSucc ; | |
| 2002 if (sss && _succ == NULL ) _succ = Self ; | |
| 2003 Thread * prv = NULL ; | |
| 2004 | |
| 2005 // There are three ways to exit the following loop: | |
| 2006 // 1. A successful spin where this thread has acquired the lock. | |
| 2007 // 2. Spin failure with prejudice | |
| 2008 // 3. Spin failure without prejudice | |
| 2009 | |
| 2010 while (--ctr >= 0) { | |
| 2011 | |
| 2012 // Periodic polling -- Check for pending GC | |
| 2013 // Threads may spin while they're unsafe. | |
| 2014 // We don't want spinning threads to delay the JVM from reaching | |
| 2015 // a stop-the-world safepoint or to steal cycles from GC. | |
| 2016 // If we detect a pending safepoint we abort in order that | |
| 2017 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) | |
| 2018 // this thread, if safe, doesn't steal cycles from GC. | |
| 2019 // This is in keeping with the "no loitering in runtime" rule. | |
| 2020 // We periodically check to see if there's a safepoint pending. | |
| 2021 if ((ctr & 0xFF) == 0) { | |
| 2022 if (SafepointSynchronize::do_call_back()) { | |
| 2023 TEVENT (Spin: safepoint) ; | |
| 2024 goto Abort ; // abrupt spin egress | |
| 2025 } | |
| 2026 if (Knob_UsePause & 1) SpinPause () ; | |
| 2027 | |
| 2028 int (*scb)(intptr_t,int) = SpinCallbackFunction ; | |
| 2029 if (hits > 50 && scb != NULL) { | |
| 2030 int abend = (*scb)(SpinCallbackArgument, 0) ; | |
| 2031 } | |
| 2032 } | |
| 2033 | |
| 2034 if (Knob_UsePause & 2) SpinPause() ; | |
| 2035 | |
| 2036 // Exponential back-off ... Stay off the bus to reduce coherency traffic. | |
| 2037 // This is useful on classic SMP systems, but is of less utility on | |
| 2038 // N1-style CMT platforms. | |
| 2039 // | |
| 2040 // Trade-off: lock acquisition latency vs coherency bandwidth. | |
| 2041 // Lock hold times are typically short. A histogram | |
| 2042 // of successful spin attempts shows that we usually acquire | |
| 2043 // the lock early in the spin. That suggests we want to | |
| 2044 // sample _owner frequently in the early phase of the spin, | |
| 2045 // but then back-off and sample less frequently as the spin | |
| 2046 // progresses. The back-off makes a good citizen on SMP big | |
| 2047 // SMP systems. Oversampling _owner can consume excessive | |
| 2048 // coherency bandwidth. Relatedly, if we _oversample _owner we | |
| 2049 // can inadvertently interfere with the the ST m->owner=null. | |
| 2050 // executed by the lock owner. | |
| 2051 if (ctr & msk) continue ; | |
| 2052 ++hits ; | |
| 2053 if ((hits & 0xF) == 0) { | |
| 2054 // The 0xF, above, corresponds to the exponent. | |
| 2055 // Consider: (msk+1)|msk | |
| 2056 msk = ((msk << 2)|3) & BackOffMask ; | |
| 2057 } | |
| 2058 | |
| 2059 // Probe _owner with TATAS | |
| 2060 // If this thread observes the monitor transition or flicker | |
| 2061 // from locked to unlocked to locked, then the odds that this | |
| 2062 // thread will acquire the lock in this spin attempt go down | |
| 2063 // considerably. The same argument applies if the CAS fails | |
| 2064 // or if we observe _owner change from one non-null value to | |
| 2065 // another non-null value. In such cases we might abort | |
| 2066 // the spin without prejudice or apply a "penalty" to the | |
| 2067 // spin count-down variable "ctr", reducing it by 100, say. | |
| 2068 | |
| 2069 Thread * ox = (Thread *) _owner ; | |
| 2070 if (ox == NULL) { | |
| 2071 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; | |
| 2072 if (ox == NULL) { | |
| 2073 // The CAS succeeded -- this thread acquired ownership | |
| 2074 // Take care of some bookkeeping to exit spin state. | |
| 2075 if (sss && _succ == Self) { | |
| 2076 _succ = NULL ; | |
| 2077 } | |
| 2078 if (MaxSpin > 0) Adjust (&_Spinner, -1) ; | |
| 2079 | |
| 2080 // Increase _SpinDuration : | |
| 2081 // The spin was successful (profitable) so we tend toward | |
| 2082 // longer spin attempts in the future. | |
| 2083 // CONSIDER: factor "ctr" into the _SpinDuration adjustment. | |
| 2084 // If we acquired the lock early in the spin cycle it | |
| 2085 // makes sense to increase _SpinDuration proportionally. | |
| 2086 // Note that we don't clamp SpinDuration precisely at SpinLimit. | |
| 2087 int x = _SpinDuration ; | |
| 2088 if (x < Knob_SpinLimit) { | |
| 2089 if (x < Knob_Poverty) x = Knob_Poverty ; | |
| 2090 _SpinDuration = x + Knob_Bonus ; | |
| 2091 } | |
| 2092 return 1 ; | |
| 2093 } | |
| 2094 | |
| 2095 // The CAS failed ... we can take any of the following actions: | |
| 2096 // * penalize: ctr -= Knob_CASPenalty | |
| 2097 // * exit spin with prejudice -- goto Abort; | |
| 2098 // * exit spin without prejudice. | |
| 2099 // * Since CAS is high-latency, retry again immediately. | |
| 2100 prv = ox ; | |
| 2101 TEVENT (Spin: cas failed) ; | |
| 2102 if (caspty == -2) break ; | |
| 2103 if (caspty == -1) goto Abort ; | |
| 2104 ctr -= caspty ; | |
| 2105 continue ; | |
| 2106 } | |
| 2107 | |
| 2108 // Did lock ownership change hands ? | |
| 2109 if (ox != prv && prv != NULL ) { | |
| 2110 TEVENT (spin: Owner changed) | |
| 2111 if (oxpty == -2) break ; | |
| 2112 if (oxpty == -1) goto Abort ; | |
| 2113 ctr -= oxpty ; | |
| 2114 } | |
| 2115 prv = ox ; | |
| 2116 | |
| 2117 // Abort the spin if the owner is not executing. | |
| 2118 // The owner must be executing in order to drop the lock. | |
| 2119 // Spinning while the owner is OFFPROC is idiocy. | |
| 2120 // Consider: ctr -= RunnablePenalty ; | |
| 2121 if (Knob_OState && NotRunnable (Self, ox)) { | |
| 2122 TEVENT (Spin abort - notrunnable); | |
| 2123 goto Abort ; | |
| 2124 } | |
| 2125 if (sss && _succ == NULL ) _succ = Self ; | |
| 2126 } | |
| 2127 | |
| 2128 // Spin failed with prejudice -- reduce _SpinDuration. | |
| 2129 // TODO: Use an AIMD-like policy to adjust _SpinDuration. | |
| 2130 // AIMD is globally stable. | |
| 2131 TEVENT (Spin failure) ; | |
| 2132 { | |
| 2133 int x = _SpinDuration ; | |
| 2134 if (x > 0) { | |
| 2135 // Consider an AIMD scheme like: x -= (x >> 3) + 100 | |
| 2136 // This is globally sample and tends to damp the response. | |
| 2137 x -= Knob_Penalty ; | |
| 2138 if (x < 0) x = 0 ; | |
| 2139 _SpinDuration = x ; | |
| 2140 } | |
| 2141 } | |
| 2142 | |
| 2143 Abort: | |
| 2144 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ; | |
| 2145 if (sss && _succ == Self) { | |
| 2146 _succ = NULL ; | |
| 2147 // Invariant: after setting succ=null a contending thread | |
| 2148 // must recheck-retry _owner before parking. This usually happens | |
| 2149 // in the normal usage of TrySpin(), but it's safest | |
| 2150 // to make TrySpin() as foolproof as possible. | |
| 2151 OrderAccess::fence() ; | |
| 2152 if (TryLock(Self) > 0) return 1 ; | |
| 2153 } | |
| 2154 return 0 ; | |
| 2155 } | |
| 2156 | |
| 2157 // NotRunnable() -- informed spinning | |
| 2158 // | |
| 2159 // Don't bother spinning if the owner is not eligible to drop the lock. | |
| 2160 // Peek at the owner's schedctl.sc_state and Thread._thread_values and | |
| 2161 // spin only if the owner thread is _thread_in_Java or _thread_in_vm. | |
| 2162 // The thread must be runnable in order to drop the lock in timely fashion. | |
| 2163 // If the _owner is not runnable then spinning will not likely be | |
| 2164 // successful (profitable). | |
| 2165 // | |
| 2166 // Beware -- the thread referenced by _owner could have died | |
| 2167 // so a simply fetch from _owner->_thread_state might trap. | |
| 2168 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. | |
| 2169 // Because of the lifecycle issues the schedctl and _thread_state values | |
| 2170 // observed by NotRunnable() might be garbage. NotRunnable must | |
| 2171 // tolerate this and consider the observed _thread_state value | |
| 2172 // as advisory. | |
| 2173 // | |
| 2174 // Beware too, that _owner is sometimes a BasicLock address and sometimes | |
| 2175 // a thread pointer. We differentiate the two cases with OwnerIsThread. | |
| 2176 // Alternately, we might tag the type (thread pointer vs basiclock pointer) | |
| 2177 // with the LSB of _owner. Another option would be to probablistically probe | |
| 2178 // the putative _owner->TypeTag value. | |
| 2179 // | |
| 2180 // Checking _thread_state isn't perfect. Even if the thread is | |
| 2181 // in_java it might be blocked on a page-fault or have been preempted | |
| 2182 // and sitting on a ready/dispatch queue. _thread state in conjunction | |
| 2183 // with schedctl.sc_state gives us a good picture of what the | |
| 2184 // thread is doing, however. | |
| 2185 // | |
| 2186 // TODO: check schedctl.sc_state. | |
| 2187 // We'll need to use SafeFetch32() to read from the schedctl block. | |
| 2188 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/ | |
| 2189 // | |
| 2190 // The return value from NotRunnable() is *advisory* -- the | |
| 2191 // result is based on sampling and is not necessarily coherent. | |
| 2192 // The caller must tolerate false-negative and false-positive errors. | |
| 2193 // Spinning, in general, is probabilistic anyway. | |
| 2194 | |
| 2195 | |
| 2196 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) { | |
| 2197 // Check either OwnerIsThread or ox->TypeTag == 2BAD. | |
| 2198 if (!OwnerIsThread) return 0 ; | |
| 2199 | |
| 2200 if (ox == NULL) return 0 ; | |
| 2201 | |
| 2202 // Avoid transitive spinning ... | |
| 2203 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. | |
| 2204 // Immediately after T1 acquires L it's possible that T2, also | |
| 2205 // spinning on L, will see L.Owner=T1 and T1._Stalled=L. | |
| 2206 // This occurs transiently after T1 acquired L but before | |
| 2207 // T1 managed to clear T1.Stalled. T2 does not need to abort | |
| 2208 // its spin in this circumstance. | |
| 2209 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ; | |
| 2210 | |
| 2211 if (BlockedOn == 1) return 1 ; | |
| 2212 if (BlockedOn != 0) { | |
| 2213 return BlockedOn != intptr_t(this) && _owner == ox ; | |
| 2214 } | |
| 2215 | |
| 2216 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ; | |
| 2217 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ; | |
| 2218 // consider also: jst != _thread_in_Java -- but that's overspecific. | |
| 2219 return jst == _thread_blocked || jst == _thread_in_native ; | |
| 2220 } | |
| 2221 | |
| 2222 | |
| 2223 // ----------------------------------------------------------------------------- | |
| 2224 // WaitSet management ... | |
| 2225 | |
| 2226 ObjectWaiter::ObjectWaiter(Thread* thread) { | |
| 2227 _next = NULL; | |
| 2228 _prev = NULL; | |
| 2229 _notified = 0; | |
| 2230 TState = TS_RUN ; | |
| 2231 _thread = thread; | |
| 2232 _event = thread->_ParkEvent ; | |
| 2233 _active = false; | |
| 2234 assert (_event != NULL, "invariant") ; | |
| 2235 } | |
| 2236 | |
| 2237 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) { | |
| 2238 JavaThread *jt = (JavaThread *)this->_thread; | |
| 2239 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); | |
| 2240 } | |
| 2241 | |
| 2242 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) { | |
| 2243 JavaThread *jt = (JavaThread *)this->_thread; | |
| 2244 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); | |
| 2245 } | |
| 2246 | |
| 2247 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { | |
| 2248 assert(node != NULL, "should not dequeue NULL node"); | |
| 2249 assert(node->_prev == NULL, "node already in list"); | |
| 2250 assert(node->_next == NULL, "node already in list"); | |
| 2251 // put node at end of queue (circular doubly linked list) | |
| 2252 if (_WaitSet == NULL) { | |
| 2253 _WaitSet = node; | |
| 2254 node->_prev = node; | |
| 2255 node->_next = node; | |
| 2256 } else { | |
| 2257 ObjectWaiter* head = _WaitSet ; | |
| 2258 ObjectWaiter* tail = head->_prev; | |
| 2259 assert(tail->_next == head, "invariant check"); | |
| 2260 tail->_next = node; | |
| 2261 head->_prev = node; | |
| 2262 node->_next = head; | |
| 2263 node->_prev = tail; | |
| 2264 } | |
| 2265 } | |
| 2266 | |
| 2267 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { | |
| 2268 // dequeue the very first waiter | |
| 2269 ObjectWaiter* waiter = _WaitSet; | |
| 2270 if (waiter) { | |
| 2271 DequeueSpecificWaiter(waiter); | |
| 2272 } | |
| 2273 return waiter; | |
| 2274 } | |
| 2275 | |
| 2276 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { | |
| 2277 assert(node != NULL, "should not dequeue NULL node"); | |
| 2278 assert(node->_prev != NULL, "node already removed from list"); | |
| 2279 assert(node->_next != NULL, "node already removed from list"); | |
| 2280 // when the waiter has woken up because of interrupt, | |
| 2281 // timeout or other spurious wake-up, dequeue the | |
| 2282 // waiter from waiting list | |
| 2283 ObjectWaiter* next = node->_next; | |
| 2284 if (next == node) { | |
| 2285 assert(node->_prev == node, "invariant check"); | |
| 2286 _WaitSet = NULL; | |
| 2287 } else { | |
| 2288 ObjectWaiter* prev = node->_prev; | |
| 2289 assert(prev->_next == node, "invariant check"); | |
| 2290 assert(next->_prev == node, "invariant check"); | |
| 2291 next->_prev = prev; | |
| 2292 prev->_next = next; | |
| 2293 if (_WaitSet == node) { | |
| 2294 _WaitSet = next; | |
| 2295 } | |
| 2296 } | |
| 2297 node->_next = NULL; | |
| 2298 node->_prev = NULL; | |
| 2299 } | |
| 2300 | |
| 2301 // ----------------------------------------------------------------------------- | |
| 2302 // PerfData support | |
| 2303 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ; | |
| 2304 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ; | |
| 2305 PerfCounter * ObjectMonitor::_sync_Parks = NULL ; | |
| 2306 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ; | |
| 2307 PerfCounter * ObjectMonitor::_sync_Notifications = NULL ; | |
| 2308 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ; | |
| 2309 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ; | |
| 2310 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ; | |
| 2311 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ; | |
| 2312 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ; | |
| 2313 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ; | |
| 2314 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ; | |
| 2315 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ; | |
| 2316 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ; | |
| 2317 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ; | |
| 2318 PerfCounter * ObjectMonitor::_sync_Inflations = NULL ; | |
| 2319 PerfCounter * ObjectMonitor::_sync_Deflations = NULL ; | |
| 2320 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ; | |
| 2321 | |
| 2322 // One-shot global initialization for the sync subsystem. | |
| 2323 // We could also defer initialization and initialize on-demand | |
| 2324 // the first time we call inflate(). Initialization would | |
| 2325 // be protected - like so many things - by the MonitorCache_lock. | |
| 2326 | |
| 2327 void ObjectMonitor::Initialize () { | |
| 2328 static int InitializationCompleted = 0 ; | |
| 2329 assert (InitializationCompleted == 0, "invariant") ; | |
| 2330 InitializationCompleted = 1 ; | |
| 2331 if (UsePerfData) { | |
| 2332 EXCEPTION_MARK ; | |
| 2333 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); } | |
| 2334 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); } | |
| 2335 NEWPERFCOUNTER(_sync_Inflations) ; | |
| 2336 NEWPERFCOUNTER(_sync_Deflations) ; | |
| 2337 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ; | |
| 2338 NEWPERFCOUNTER(_sync_FutileWakeups) ; | |
| 2339 NEWPERFCOUNTER(_sync_Parks) ; | |
| 2340 NEWPERFCOUNTER(_sync_EmptyNotifications) ; | |
| 2341 NEWPERFCOUNTER(_sync_Notifications) ; | |
| 2342 NEWPERFCOUNTER(_sync_SlowEnter) ; | |
| 2343 NEWPERFCOUNTER(_sync_SlowExit) ; | |
| 2344 NEWPERFCOUNTER(_sync_SlowNotify) ; | |
| 2345 NEWPERFCOUNTER(_sync_SlowNotifyAll) ; | |
| 2346 NEWPERFCOUNTER(_sync_FailedSpins) ; | |
| 2347 NEWPERFCOUNTER(_sync_SuccessfulSpins) ; | |
| 2348 NEWPERFCOUNTER(_sync_PrivateA) ; | |
| 2349 NEWPERFCOUNTER(_sync_PrivateB) ; | |
| 2350 NEWPERFCOUNTER(_sync_MonInCirculation) ; | |
| 2351 NEWPERFCOUNTER(_sync_MonScavenged) ; | |
| 2352 NEWPERFVARIABLE(_sync_MonExtant) ; | |
| 2353 #undef NEWPERFCOUNTER | |
| 2354 } | |
| 2355 } | |
| 2356 | |
| 2357 | |
| 2358 // Compile-time asserts | |
| 2359 // When possible, it's better to catch errors deterministically at | |
| 2360 // compile-time than at runtime. The down-side to using compile-time | |
| 2361 // asserts is that error message -- often something about negative array | |
| 2362 // indices -- is opaque. | |
| 2363 | |
| 2364 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); } | |
| 2365 | |
| 2366 void ObjectMonitor::ctAsserts() { | |
| 2367 CTASSERT(offset_of (ObjectMonitor, _header) == 0); | |
| 2368 } | |
| 2369 | |
| 2370 | |
| 2371 static char * kvGet (char * kvList, const char * Key) { | |
| 2372 if (kvList == NULL) return NULL ; | |
| 2373 size_t n = strlen (Key) ; | |
| 2374 char * Search ; | |
| 2375 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) { | |
| 2376 if (strncmp (Search, Key, n) == 0) { | |
| 2377 if (Search[n] == '=') return Search + n + 1 ; | |
| 2378 if (Search[n] == 0) return (char *) "1" ; | |
| 2379 } | |
| 2380 } | |
| 2381 return NULL ; | |
| 2382 } | |
| 2383 | |
| 2384 static int kvGetInt (char * kvList, const char * Key, int Default) { | |
| 2385 char * v = kvGet (kvList, Key) ; | |
| 2386 int rslt = v ? ::strtol (v, NULL, 0) : Default ; | |
| 2387 if (Knob_ReportSettings && v != NULL) { | |
| 2388 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ; | |
| 2389 ::fflush (stdout) ; | |
| 2390 } | |
| 2391 return rslt ; | |
| 2392 } | |
| 2393 | |
| 2394 void ObjectMonitor::DeferredInitialize () { | |
| 2395 if (InitDone > 0) return ; | |
| 2396 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) { | |
| 2397 while (InitDone != 1) ; | |
| 2398 return ; | |
| 2399 } | |
| 2400 | |
| 2401 // One-shot global initialization ... | |
| 2402 // The initialization is idempotent, so we don't need locks. | |
| 2403 // In the future consider doing this via os::init_2(). | |
| 2404 // SyncKnobs consist of <Key>=<Value> pairs in the style | |
| 2405 // of environment variables. Start by converting ':' to NUL. | |
| 2406 | |
| 2407 if (SyncKnobs == NULL) SyncKnobs = "" ; | |
| 2408 | |
| 2409 size_t sz = strlen (SyncKnobs) ; | |
| 2410 char * knobs = (char *) malloc (sz + 2) ; | |
| 2411 if (knobs == NULL) { | |
| 2412 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ; | |
| 2413 guarantee (0, "invariant") ; | |
| 2414 } | |
| 2415 strcpy (knobs, SyncKnobs) ; | |
| 2416 knobs[sz+1] = 0 ; | |
| 2417 for (char * p = knobs ; *p ; p++) { | |
| 2418 if (*p == ':') *p = 0 ; | |
| 2419 } | |
| 2420 | |
| 2421 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); } | |
| 2422 SETKNOB(ReportSettings) ; | |
| 2423 SETKNOB(Verbose) ; | |
| 2424 SETKNOB(FixedSpin) ; | |
| 2425 SETKNOB(SpinLimit) ; | |
| 2426 SETKNOB(SpinBase) ; | |
| 2427 SETKNOB(SpinBackOff); | |
| 2428 SETKNOB(CASPenalty) ; | |
| 2429 SETKNOB(OXPenalty) ; | |
| 2430 SETKNOB(LogSpins) ; | |
| 2431 SETKNOB(SpinSetSucc) ; | |
| 2432 SETKNOB(SuccEnabled) ; | |
| 2433 SETKNOB(SuccRestrict) ; | |
| 2434 SETKNOB(Penalty) ; | |
| 2435 SETKNOB(Bonus) ; | |
| 2436 SETKNOB(BonusB) ; | |
| 2437 SETKNOB(Poverty) ; | |
| 2438 SETKNOB(SpinAfterFutile) ; | |
| 2439 SETKNOB(UsePause) ; | |
| 2440 SETKNOB(SpinEarly) ; | |
| 2441 SETKNOB(OState) ; | |
| 2442 SETKNOB(MaxSpinners) ; | |
| 2443 SETKNOB(PreSpin) ; | |
| 2444 SETKNOB(ExitPolicy) ; | |
| 2445 SETKNOB(QMode); | |
| 2446 SETKNOB(ResetEvent) ; | |
| 2447 SETKNOB(MoveNotifyee) ; | |
| 2448 SETKNOB(FastHSSEC) ; | |
| 2449 #undef SETKNOB | |
| 2450 | |
| 2451 if (os::is_MP()) { | |
| 2452 BackOffMask = (1 << Knob_SpinBackOff) - 1 ; | |
| 2453 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ; | |
| 2454 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1) | |
| 2455 } else { | |
| 2456 Knob_SpinLimit = 0 ; | |
| 2457 Knob_SpinBase = 0 ; | |
| 2458 Knob_PreSpin = 0 ; | |
| 2459 Knob_FixedSpin = -1 ; | |
| 2460 } | |
| 2461 | |
| 2462 if (Knob_LogSpins == 0) { | |
| 2463 ObjectMonitor::_sync_FailedSpins = NULL ; | |
| 2464 } | |
| 2465 | |
| 2466 free (knobs) ; | |
| 2467 OrderAccess::fence() ; | |
| 2468 InitDone = 1 ; | |
| 2469 } | |
| 2470 | |
| 2471 #ifndef PRODUCT | |
| 2472 void ObjectMonitor::verify() { | |
| 2473 } | |
| 2474 | |
| 2475 void ObjectMonitor::print() { | |
| 2476 } | |
| 2477 #endif |