1 /*
2 * Copyright (c) 2005, 2014, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "oops/klass.inline.hpp"
27 #include "oops/markOop.hpp"
28 #include "runtime/basicLock.hpp"
29 #include "runtime/biasedLocking.hpp"
30 #include "runtime/task.hpp"
31 #include "runtime/vframe.hpp"
32 #include "runtime/vmThread.hpp"
33 #include "runtime/vm_operations.hpp"
34 #if INCLUDE_JFR
35 #include "jfr/support/jfrThreadId.hpp"
36 #include "jfr/jfrEvents.hpp"
37 #endif
38
39 static bool _biased_locking_enabled = false;
40 BiasedLockingCounters BiasedLocking::_counters;
41
42 static GrowableArray<Handle>* _preserved_oop_stack = NULL;
43 static GrowableArray<markOop>* _preserved_mark_stack = NULL;
44
45 static void enable_biased_locking(Klass* k) {
46 k->set_prototype_header(markOopDesc::biased_locking_prototype());
47 }
48
49 class VM_EnableBiasedLocking: public VM_Operation {
50 private:
51 bool _is_cheap_allocated;
52 public:
53 VM_EnableBiasedLocking(bool is_cheap_allocated) { _is_cheap_allocated = is_cheap_allocated; }
54 VMOp_Type type() const { return VMOp_EnableBiasedLocking; }
55 Mode evaluation_mode() const { return _is_cheap_allocated ? _async_safepoint : _safepoint; }
56 bool is_cheap_allocated() const { return _is_cheap_allocated; }
57
58 void doit() {
59 // Iterate the system dictionary enabling biased locking for all
60 // currently loaded classes
61 SystemDictionary::classes_do(enable_biased_locking);
62 // Indicate that future instances should enable it as well
63 _biased_locking_enabled = true;
64
65 if (TraceBiasedLocking) {
66 tty->print_cr("Biased locking enabled");
67 }
68 }
69
70 bool allow_nested_vm_operations() const { return false; }
71 };
72
73
74 // One-shot PeriodicTask subclass for enabling biased locking
75 class EnableBiasedLockingTask : public PeriodicTask {
76 public:
77 EnableBiasedLockingTask(size_t interval_time) : PeriodicTask(interval_time) {}
78
79 virtual void task() {
80 // Use async VM operation to avoid blocking the Watcher thread.
81 // VM Thread will free C heap storage.
82 VM_EnableBiasedLocking *op = new VM_EnableBiasedLocking(true);
83 VMThread::execute(op);
84
85 // Reclaim our storage and disenroll ourself
86 delete this;
87 }
88 };
89
90
91 void BiasedLocking::init() {
92 // If biased locking is enabled, schedule a task to fire a few
93 // seconds into the run which turns on biased locking for all
94 // currently loaded classes as well as future ones. This is a
95 // workaround for startup time regressions due to a large number of
96 // safepoints being taken during VM startup for bias revocation.
97 // Ideally we would have a lower cost for individual bias revocation
98 // and not need a mechanism like this.
99 if (UseBiasedLocking) {
100 if (BiasedLockingStartupDelay > 0) {
101 EnableBiasedLockingTask* task = new EnableBiasedLockingTask(BiasedLockingStartupDelay);
102 task->enroll();
103 } else {
104 VM_EnableBiasedLocking op(false);
105 VMThread::execute(&op);
106 }
107 }
108 }
109
110
111 bool BiasedLocking::enabled() {
112 return _biased_locking_enabled;
113 }
114
115 // Returns MonitorInfos for all objects locked on this thread in youngest to oldest order
116 static GrowableArray<MonitorInfo*>* get_or_compute_monitor_info(JavaThread* thread) {
117 GrowableArray<MonitorInfo*>* info = thread->cached_monitor_info();
118 if (info != NULL) {
119 return info;
120 }
121
122 info = new GrowableArray<MonitorInfo*>();
123
124 // It's possible for the thread to not have any Java frames on it,
125 // i.e., if it's the main thread and it's already returned from main()
126 if (thread->has_last_Java_frame()) {
127 RegisterMap rm(thread);
128 for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {
129 GrowableArray<MonitorInfo*> *monitors = vf->monitors();
130 if (monitors != NULL) {
131 int len = monitors->length();
132 // Walk monitors youngest to oldest
133 for (int i = len - 1; i >= 0; i--) {
134 MonitorInfo* mon_info = monitors->at(i);
135 if (mon_info->eliminated()) continue;
136 oop owner = mon_info->owner();
137 if (owner != NULL) {
138 info->append(mon_info);
139 }
140 }
141 }
142 }
143 }
144
145 thread->set_cached_monitor_info(info);
146 return info;
147 }
148
149 // After the call, *biased_locker will be set to obj->mark()->biased_locker() if biased_locker != NULL,
150 // AND it is a living thread. Otherwise it will not be updated, (i.e. the caller is responsible for initialization).
151 static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread, JavaThread** biased_locker) {
152 markOop mark = obj->mark();
153 if (!mark->has_bias_pattern()) {
154 if (TraceBiasedLocking) {
155 ResourceMark rm;
156 tty->print_cr(" (Skipping revocation of object of type %s because it's no longer biased)",
157 obj->klass()->external_name());
158 }
159 return BiasedLocking::NOT_BIASED;
160 }
161
162 uint age = mark->age();
163 markOop biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);
164 markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);
165
166 if (TraceBiasedLocking && (Verbose || !is_bulk)) {
167 ResourceMark rm;
168 tty->print_cr("Revoking bias of object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s , prototype header " INTPTR_FORMAT " , allow rebias %d , requesting thread " INTPTR_FORMAT,
169 p2i((void *)obj), (intptr_t) mark, obj->klass()->external_name(), (intptr_t) obj->klass()->prototype_header(), (allow_rebias ? 1 : 0), (intptr_t) requesting_thread);
170 }
171
172 JavaThread* biased_thread = mark->biased_locker();
173 if (biased_thread == NULL) {
174 // Object is anonymously biased. We can get here if, for
175 // example, we revoke the bias due to an identity hash code
176 // being computed for an object.
177 if (!allow_rebias) {
178 obj->set_mark(unbiased_prototype);
179 }
180 if (TraceBiasedLocking && (Verbose || !is_bulk)) {
181 tty->print_cr(" Revoked bias of anonymously-biased object");
182 }
183 return BiasedLocking::BIAS_REVOKED;
184 }
185
186 // Handle case where the thread toward which the object was biased has exited
187 bool thread_is_alive = false;
188 if (requesting_thread == biased_thread) {
189 thread_is_alive = true;
190 } else {
191 for (JavaThread* cur_thread = Threads::first(); cur_thread != NULL; cur_thread = cur_thread->next()) {
192 if (cur_thread == biased_thread) {
193 thread_is_alive = true;
194 break;
195 }
196 }
197 }
198 if (!thread_is_alive) {
199 if (allow_rebias) {
200 obj->set_mark(biased_prototype);
201 } else {
202 obj->set_mark(unbiased_prototype);
203 }
204 if (TraceBiasedLocking && (Verbose || !is_bulk)) {
205 tty->print_cr(" Revoked bias of object biased toward dead thread");
206 }
207 return BiasedLocking::BIAS_REVOKED;
208 }
209
210 // Thread owning bias is alive.
211 // Check to see whether it currently owns the lock and, if so,
212 // write down the needed displaced headers to the thread's stack.
213 // Otherwise, restore the object's header either to the unlocked
214 // or unbiased state.
215 GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(biased_thread);
216 BasicLock* highest_lock = NULL;
217 for (int i = 0; i < cached_monitor_info->length(); i++) {
218 MonitorInfo* mon_info = cached_monitor_info->at(i);
219 if (mon_info->owner() == obj) {
220 if (TraceBiasedLocking && Verbose) {
221 tty->print_cr(" mon_info->owner (" PTR_FORMAT ") == obj (" PTR_FORMAT ")",
222 p2i((void *) mon_info->owner()),
223 p2i((void *) obj));
224 }
225 // Assume recursive case and fix up highest lock later
226 markOop mark = markOopDesc::encode((BasicLock*) NULL);
227 highest_lock = mon_info->lock();
228 highest_lock->set_displaced_header(mark);
229 } else {
230 if (TraceBiasedLocking && Verbose) {
231 tty->print_cr(" mon_info->owner (" PTR_FORMAT ") != obj (" PTR_FORMAT ")",
232 p2i((void *) mon_info->owner()),
233 p2i((void *) obj));
234 }
235 }
236 }
237 if (highest_lock != NULL) {
238 // Fix up highest lock to contain displaced header and point
239 // object at it
240 highest_lock->set_displaced_header(unbiased_prototype);
241 // Reset object header to point to displaced mark.
242 // Must release storing the lock address for platforms without TSO
243 // ordering (e.g. ppc).
244 obj->release_set_mark(markOopDesc::encode(highest_lock));
245 assert(!obj->mark()->has_bias_pattern(), "illegal mark state: stack lock used bias bit");
246 if (TraceBiasedLocking && (Verbose || !is_bulk)) {
247 tty->print_cr(" Revoked bias of currently-locked object");
248 }
249 } else {
250 if (TraceBiasedLocking && (Verbose || !is_bulk)) {
251 tty->print_cr(" Revoked bias of currently-unlocked object");
252 }
253 if (allow_rebias) {
254 obj->set_mark(biased_prototype);
255 } else {
256 // Store the unlocked value into the object's header.
257 obj->set_mark(unbiased_prototype);
258 }
259 }
260
261 // If requested, return information on which thread held the bias
262 if (biased_locker != NULL) {
263 *biased_locker = biased_thread;
264 }
265
266 return BiasedLocking::BIAS_REVOKED;
267 }
268
269
270 enum HeuristicsResult {
271 HR_NOT_BIASED = 1,
272 HR_SINGLE_REVOKE = 2,
273 HR_BULK_REBIAS = 3,
274 HR_BULK_REVOKE = 4
275 };
276
277
278 static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {
279 markOop mark = o->mark();
280 if (!mark->has_bias_pattern()) {
281 return HR_NOT_BIASED;
282 }
283
284 // Heuristics to attempt to throttle the number of revocations.
285 // Stages:
286 // 1. Revoke the biases of all objects in the heap of this type,
287 // but allow rebiasing of those objects if unlocked.
288 // 2. Revoke the biases of all objects in the heap of this type
289 // and don't allow rebiasing of these objects. Disable
290 // allocation of objects of that type with the bias bit set.
291 Klass* k = o->klass();
292 jlong cur_time = os::javaTimeMillis();
293 jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();
294 int revocation_count = k->biased_lock_revocation_count();
295 if ((revocation_count >= BiasedLockingBulkRebiasThreshold) &&
296 (revocation_count < BiasedLockingBulkRevokeThreshold) &&
297 (last_bulk_revocation_time != 0) &&
298 (cur_time - last_bulk_revocation_time >= BiasedLockingDecayTime)) {
299 // This is the first revocation we've seen in a while of an
300 // object of this type since the last time we performed a bulk
301 // rebiasing operation. The application is allocating objects in
302 // bulk which are biased toward a thread and then handing them
303 // off to another thread. We can cope with this allocation
304 // pattern via the bulk rebiasing mechanism so we reset the
305 // klass's revocation count rather than allow it to increase
306 // monotonically. If we see the need to perform another bulk
307 // rebias operation later, we will, and if subsequently we see
308 // many more revocation operations in a short period of time we
309 // will completely disable biasing for this type.
310 k->set_biased_lock_revocation_count(0);
311 revocation_count = 0;
312 }
313
314 // Make revocation count saturate just beyond BiasedLockingBulkRevokeThreshold
315 if (revocation_count <= BiasedLockingBulkRevokeThreshold) {
316 revocation_count = k->atomic_incr_biased_lock_revocation_count();
317 }
318
319 if (revocation_count == BiasedLockingBulkRevokeThreshold) {
320 return HR_BULK_REVOKE;
321 }
322
323 if (revocation_count == BiasedLockingBulkRebiasThreshold) {
324 return HR_BULK_REBIAS;
325 }
326
327 return HR_SINGLE_REVOKE;
328 }
329
330
331 static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,
332 bool bulk_rebias,
333 bool attempt_rebias_of_object,
334 JavaThread* requesting_thread) {
335 assert(SafepointSynchronize::is_at_safepoint(), "must be done at safepoint");
336
337 if (TraceBiasedLocking) {
338 tty->print_cr("* Beginning bulk revocation (kind == %s) because of object "
339 INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
340 (bulk_rebias ? "rebias" : "revoke"),
341 p2i((void *) o), (intptr_t) o->mark(), o->klass()->external_name());
342 }
343
344 jlong cur_time = os::javaTimeMillis();
345 o->klass()->set_last_biased_lock_bulk_revocation_time(cur_time);
346
347
348 Klass* k_o = o->klass();
349 Klass* klass = k_o;
350
351 if (bulk_rebias) {
352 // Use the epoch in the klass of the object to implicitly revoke
353 // all biases of objects of this data type and force them to be
354 // reacquired. However, we also need to walk the stacks of all
355 // threads and update the headers of lightweight locked objects
356 // with biases to have the current epoch.
357
358 // If the prototype header doesn't have the bias pattern, don't
359 // try to update the epoch -- assume another VM operation came in
360 // and reset the header to the unbiased state, which will
361 // implicitly cause all existing biases to be revoked
362 if (klass->prototype_header()->has_bias_pattern()) {
363 int prev_epoch = klass->prototype_header()->bias_epoch();
364 klass->set_prototype_header(klass->prototype_header()->incr_bias_epoch());
365 int cur_epoch = klass->prototype_header()->bias_epoch();
366
367 // Now walk all threads' stacks and adjust epochs of any biased
368 // and locked objects of this data type we encounter
369 for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
370 GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
371 for (int i = 0; i < cached_monitor_info->length(); i++) {
372 MonitorInfo* mon_info = cached_monitor_info->at(i);
373 oop owner = mon_info->owner();
374 markOop mark = owner->mark();
375 if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
376 // We might have encountered this object already in the case of recursive locking
377 assert(mark->bias_epoch() == prev_epoch || mark->bias_epoch() == cur_epoch, "error in bias epoch adjustment");
378 owner->set_mark(mark->set_bias_epoch(cur_epoch));
379 }
380 }
381 }
382 }
383
384 // At this point we're done. All we have to do is potentially
385 // adjust the header of the given object to revoke its bias.
386 revoke_bias(o, attempt_rebias_of_object && klass->prototype_header()->has_bias_pattern(), true, requesting_thread, NULL);
387 } else {
388 if (TraceBiasedLocking) {
389 ResourceMark rm;
390 tty->print_cr("* Disabling biased locking for type %s", klass->external_name());
391 }
392
393 // Disable biased locking for this data type. Not only will this
394 // cause future instances to not be biased, but existing biased
395 // instances will notice that this implicitly caused their biases
396 // to be revoked.
397 klass->set_prototype_header(markOopDesc::prototype());
398
399 // Now walk all threads' stacks and forcibly revoke the biases of
400 // any locked and biased objects of this data type we encounter.
401 for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
402 GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
403 for (int i = 0; i < cached_monitor_info->length(); i++) {
404 MonitorInfo* mon_info = cached_monitor_info->at(i);
405 oop owner = mon_info->owner();
406 markOop mark = owner->mark();
407 if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
408 revoke_bias(owner, false, true, requesting_thread, NULL);
409 }
410 }
411 }
412
413 // Must force the bias of the passed object to be forcibly revoked
414 // as well to ensure guarantees to callers
415 revoke_bias(o, false, true, requesting_thread, NULL);
416 }
417
418 if (TraceBiasedLocking) {
419 tty->print_cr("* Ending bulk revocation");
420 }
421
422 BiasedLocking::Condition status_code = BiasedLocking::BIAS_REVOKED;
423
424 if (attempt_rebias_of_object &&
425 o->mark()->has_bias_pattern() &&
426 klass->prototype_header()->has_bias_pattern()) {
427 markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),
428 klass->prototype_header()->bias_epoch());
429 o->set_mark(new_mark);
430 status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;
431 if (TraceBiasedLocking) {
432 tty->print_cr(" Rebiased object toward thread " INTPTR_FORMAT, (intptr_t) requesting_thread);
433 }
434 }
435
436 assert(!o->mark()->has_bias_pattern() ||
437 (attempt_rebias_of_object && (o->mark()->biased_locker() == requesting_thread)),
438 "bug in bulk bias revocation");
439
440 return status_code;
441 }
442
443
444 static void clean_up_cached_monitor_info() {
445 // Walk the thread list clearing out the cached monitors
446 for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
447 thr->set_cached_monitor_info(NULL);
448 }
449 }
450
451
452 class VM_RevokeBias : public VM_Operation {
453 protected:
454 Handle* _obj;
455 GrowableArray<Handle>* _objs;
456 JavaThread* _requesting_thread;
457 BiasedLocking::Condition _status_code;
458 JFR_ONLY(traceid _biased_locker_id;)
459
460 public:
461 VM_RevokeBias(Handle* obj, JavaThread* requesting_thread)
462 : _obj(obj)
463 , _objs(NULL)
464 , _requesting_thread(requesting_thread)
465 , _status_code(BiasedLocking::NOT_BIASED)
466 #if INCLUDE_JFR
467 , _biased_locker_id(0)
468 #endif
469 {}
470
471 VM_RevokeBias(GrowableArray<Handle>* objs, JavaThread* requesting_thread)
472 : _obj(NULL)
473 , _objs(objs)
474 , _requesting_thread(requesting_thread)
475 , _status_code(BiasedLocking::NOT_BIASED)
476 #if INCLUDE_JFR
477 , _biased_locker_id(0)
478 #endif
479 {}
480
481 virtual VMOp_Type type() const { return VMOp_RevokeBias; }
482
483 virtual bool doit_prologue() {
484 // Verify that there is actual work to do since the callers just
485 // give us locked object(s). If we don't find any biased objects
486 // there is nothing to do and we avoid a safepoint.
487 if (_obj != NULL) {
488 markOop mark = (*_obj)()->mark();
489 if (mark->has_bias_pattern()) {
490 return true;
491 }
492 } else {
493 for ( int i = 0 ; i < _objs->length(); i++ ) {
494 markOop mark = (_objs->at(i))()->mark();
495 if (mark->has_bias_pattern()) {
496 return true;
497 }
498 }
499 }
500 return false;
501 }
502
503 virtual void doit() {
504 if (_obj != NULL) {
505 if (TraceBiasedLocking) {
506 tty->print_cr("Revoking bias with potentially per-thread safepoint:");
507 }
508 JavaThread* biased_locker = NULL;
509 _status_code = revoke_bias((*_obj)(), false, false, _requesting_thread, &biased_locker);
510 #if INCLUDE_JFR
511 if (biased_locker != NULL) {
512 _biased_locker_id = JFR_THREAD_ID(biased_locker);
513 }
514 #endif
515 clean_up_cached_monitor_info();
516 return;
517 } else {
518 if (TraceBiasedLocking) {
519 tty->print_cr("Revoking bias with global safepoint:");
520 }
521 BiasedLocking::revoke_at_safepoint(_objs);
522 }
523 }
524
525 BiasedLocking::Condition status_code() const {
526 return _status_code;
527 }
528
529 #if INCLUDE_JFR
530 traceid biased_locker() const {
531 return _biased_locker_id;
532 }
533 #endif
534 };
535
536
537 class VM_BulkRevokeBias : public VM_RevokeBias {
538 private:
539 bool _bulk_rebias;
540 bool _attempt_rebias_of_object;
541
542 public:
543 VM_BulkRevokeBias(Handle* obj, JavaThread* requesting_thread,
544 bool bulk_rebias,
545 bool attempt_rebias_of_object)
546 : VM_RevokeBias(obj, requesting_thread)
547 , _bulk_rebias(bulk_rebias)
548 , _attempt_rebias_of_object(attempt_rebias_of_object) {}
549
550 virtual VMOp_Type type() const { return VMOp_BulkRevokeBias; }
551 virtual bool doit_prologue() { return true; }
552
553 virtual void doit() {
554 _status_code = bulk_revoke_or_rebias_at_safepoint((*_obj)(), _bulk_rebias, _attempt_rebias_of_object, _requesting_thread);
555 clean_up_cached_monitor_info();
556 }
557 };
558
559
560 BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
561 assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
562
563 // We can revoke the biases of anonymously-biased objects
564 // efficiently enough that we should not cause these revocations to
565 // update the heuristics because doing so may cause unwanted bulk
566 // revocations (which are expensive) to occur.
567 markOop mark = obj->mark();
568 if (mark->is_biased_anonymously() && !attempt_rebias) {
569 // We are probably trying to revoke the bias of this object due to
570 // an identity hash code computation. Try to revoke the bias
571 // without a safepoint. This is possible if we can successfully
572 // compare-and-exchange an unbiased header into the mark word of
573 // the object, meaning that no other thread has raced to acquire
574 // the bias of the object.
575 markOop biased_value = mark;
576 markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
577 markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
578 if (res_mark == biased_value) {
579 return BIAS_REVOKED;
580 }
581 } else if (mark->has_bias_pattern()) {
582 Klass* k = obj->klass();
583 markOop prototype_header = k->prototype_header();
584 if (!prototype_header->has_bias_pattern()) {
585 // This object has a stale bias from before the bulk revocation
586 // for this data type occurred. It's pointless to update the
587 // heuristics at this point so simply update the header with a
588 // CAS. If we fail this race, the object's bias has been revoked
589 // by another thread so we simply return and let the caller deal
590 // with it.
591 markOop biased_value = mark;
592 markOop res_mark = (markOop) Atomic::cmpxchg_ptr(prototype_header, obj->mark_addr(), mark);
593 assert(!(*(obj->mark_addr()))->has_bias_pattern(), "even if we raced, should still be revoked");
594 return BIAS_REVOKED;
595 } else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
596 // The epoch of this biasing has expired indicating that the
597 // object is effectively unbiased. Depending on whether we need
598 // to rebias or revoke the bias of this object we can do it
599 // efficiently enough with a CAS that we shouldn't update the
600 // heuristics. This is normally done in the assembly code but we
601 // can reach this point due to various points in the runtime
602 // needing to revoke biases.
603 if (attempt_rebias) {
604 assert(THREAD->is_Java_thread(), "");
605 markOop biased_value = mark;
606 markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());
607 markOop res_mark = (markOop) Atomic::cmpxchg_ptr(rebiased_prototype, obj->mark_addr(), mark);
608 if (res_mark == biased_value) {
609 return BIAS_REVOKED_AND_REBIASED;
610 }
611 } else {
612 markOop biased_value = mark;
613 markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
614 markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
615 if (res_mark == biased_value) {
616 return BIAS_REVOKED;
617 }
618 }
619 }
620 }
621
622 HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
623 if (heuristics == HR_NOT_BIASED) {
624 return NOT_BIASED;
625 } else if (heuristics == HR_SINGLE_REVOKE) {
626 Klass *k = obj->klass();
627 markOop prototype_header = k->prototype_header();
628 if (mark->biased_locker() == THREAD &&
629 prototype_header->bias_epoch() == mark->bias_epoch()) {
630 // A thread is trying to revoke the bias of an object biased
631 // toward it, again likely due to an identity hash code
632 // computation. We can again avoid a safepoint in this case
633 // since we are only going to walk our own stack. There are no
634 // races with revocations occurring in other threads because we
635 // reach no safepoints in the revocation path.
636 // Also check the epoch because even if threads match, another thread
637 // can come in with a CAS to steal the bias of an object that has a
638 // stale epoch.
639 ResourceMark rm;
640 if (TraceBiasedLocking) {
641 tty->print_cr("Revoking bias by walking my own stack:");
642 }
643 #if INCLUDE_JFR
644 EventBiasedLockSelfRevocation event;
645 #endif
646 BiasedLocking::Condition cond = revoke_bias(obj(), false, false, (JavaThread*) THREAD, NULL);
647 ((JavaThread*) THREAD)->set_cached_monitor_info(NULL);
648 assert(cond == BIAS_REVOKED, "why not?");
649 #if INCLUDE_JFR
650 if (event.should_commit()) {
651 event.set_lockClass(k);
652 event.commit();
653 }
654 #endif
655 return cond;
656 } else {
657 #if INCLUDE_JFR
658 EventBiasedLockRevocation event;
659 #endif
660 VM_RevokeBias revoke(&obj, (JavaThread*) THREAD);
661 VMThread::execute(&revoke);
662 #if INCLUDE_JFR
663 if (event.should_commit() && (revoke.status_code() != NOT_BIASED)) {
664 event.set_lockClass(k);
665 // Subtract 1 to match the id of events committed inside the safepoint
666 event.set_safepointId(SafepointSynchronize::safepoint_counter() - 1);
667 event.set_previousOwner(revoke.biased_locker());
668 event.commit();
669 }
670 #endif
671 return revoke.status_code();
672 }
673 }
674
675 assert((heuristics == HR_BULK_REVOKE) ||
676 (heuristics == HR_BULK_REBIAS), "?");
677 #if INCLUDE_JFR
678 EventBiasedLockClassRevocation event;
679 #endif
680 VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
681 (heuristics == HR_BULK_REBIAS),
682 attempt_rebias);
683 VMThread::execute(&bulk_revoke);
684 #if INCLUDE_JFR
685 if (event.should_commit()) {
686 event.set_revokedClass(obj->klass());
687 event.set_disableBiasing((heuristics != HR_BULK_REBIAS));
688 // Subtract 1 to match the id of events committed inside the safepoint
689 event.set_safepointId(SafepointSynchronize::safepoint_counter() - 1);
690 event.commit();
691 }
692 #endif
693 return bulk_revoke.status_code();
694 }
695
696
697 void BiasedLocking::revoke(GrowableArray<Handle>* objs) {
698 assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
699 if (objs->length() == 0) {
700 return;
701 }
702 VM_RevokeBias revoke(objs, JavaThread::current());
703 VMThread::execute(&revoke);
704 }
705
706
707 void BiasedLocking::revoke_at_safepoint(Handle h_obj) {
708 assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
709 oop obj = h_obj();
710 HeuristicsResult heuristics = update_heuristics(obj, false);
711 if (heuristics == HR_SINGLE_REVOKE) {
712 revoke_bias(obj, false, false, NULL, NULL);
713 } else if ((heuristics == HR_BULK_REBIAS) ||
714 (heuristics == HR_BULK_REVOKE)) {
715 bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
716 }
717 clean_up_cached_monitor_info();
718 }
719
720
721 void BiasedLocking::revoke_at_safepoint(GrowableArray<Handle>* objs) {
722 assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
723 int len = objs->length();
724 for (int i = 0; i < len; i++) {
725 oop obj = (objs->at(i))();
726 HeuristicsResult heuristics = update_heuristics(obj, false);
727 if (heuristics == HR_SINGLE_REVOKE) {
728 revoke_bias(obj, false, false, NULL, NULL);
729 } else if ((heuristics == HR_BULK_REBIAS) ||
730 (heuristics == HR_BULK_REVOKE)) {
731 bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
732 }
733 }
734 clean_up_cached_monitor_info();
735 }
736
737
738 void BiasedLocking::preserve_marks() {
739 if (!UseBiasedLocking)
740 return;
741
742 assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
743
744 assert(_preserved_oop_stack == NULL, "double initialization");
745 assert(_preserved_mark_stack == NULL, "double initialization");
746
747 // In order to reduce the number of mark words preserved during GC
748 // due to the presence of biased locking, we reinitialize most mark
749 // words to the class's prototype during GC -- even those which have
750 // a currently valid bias owner. One important situation where we
751 // must not clobber a bias is when a biased object is currently
752 // locked. To handle this case we iterate over the currently-locked
753 // monitors in a prepass and, if they are biased, preserve their
754 // mark words here. This should be a relatively small set of objects
755 // especially compared to the number of objects in the heap.
756 _preserved_mark_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<markOop>(10, true);
757 _preserved_oop_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<Handle>(10, true);
758
759 ResourceMark rm;
760 Thread* cur = Thread::current();
761 for (JavaThread* thread = Threads::first(); thread != NULL; thread = thread->next()) {
762 if (thread->has_last_Java_frame()) {
763 RegisterMap rm(thread);
764 for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {
765 GrowableArray<MonitorInfo*> *monitors = vf->monitors();
766 if (monitors != NULL) {
767 int len = monitors->length();
768 // Walk monitors youngest to oldest
769 for (int i = len - 1; i >= 0; i--) {
770 MonitorInfo* mon_info = monitors->at(i);
771 if (mon_info->owner_is_scalar_replaced()) continue;
772 oop owner = mon_info->owner();
773 if (owner != NULL) {
774 markOop mark = owner->mark();
775 if (mark->has_bias_pattern()) {
776 _preserved_oop_stack->push(Handle(cur, owner));
777 _preserved_mark_stack->push(mark);
778 }
779 }
780 }
781 }
782 }
783 }
784 }
785 }
786
787
788 void BiasedLocking::restore_marks() {
789 if (!UseBiasedLocking)
790 return;
791
792 assert(_preserved_oop_stack != NULL, "double free");
793 assert(_preserved_mark_stack != NULL, "double free");
794
795 int len = _preserved_oop_stack->length();
796 for (int i = 0; i < len; i++) {
797 Handle owner = _preserved_oop_stack->at(i);
798 markOop mark = _preserved_mark_stack->at(i);
799 owner->set_mark(mark);
800 }
801
802 delete _preserved_oop_stack;
803 _preserved_oop_stack = NULL;
804 delete _preserved_mark_stack;
805 _preserved_mark_stack = NULL;
806 }
807
808
809 int* BiasedLocking::total_entry_count_addr() { return _counters.total_entry_count_addr(); }
810 int* BiasedLocking::biased_lock_entry_count_addr() { return _counters.biased_lock_entry_count_addr(); }
811 int* BiasedLocking::anonymously_biased_lock_entry_count_addr() { return _counters.anonymously_biased_lock_entry_count_addr(); }
812 int* BiasedLocking::rebiased_lock_entry_count_addr() { return _counters.rebiased_lock_entry_count_addr(); }
813 int* BiasedLocking::revoked_lock_entry_count_addr() { return _counters.revoked_lock_entry_count_addr(); }
814 int* BiasedLocking::fast_path_entry_count_addr() { return _counters.fast_path_entry_count_addr(); }
815 int* BiasedLocking::slow_path_entry_count_addr() { return _counters.slow_path_entry_count_addr(); }
816
817
818 // BiasedLockingCounters
819
820 int BiasedLockingCounters::slow_path_entry_count() {
821 if (_slow_path_entry_count != 0) {
822 return _slow_path_entry_count;
823 }
824 int sum = _biased_lock_entry_count + _anonymously_biased_lock_entry_count +
825 _rebiased_lock_entry_count + _revoked_lock_entry_count +
826 _fast_path_entry_count;
827
828 return _total_entry_count - sum;
829 }
830
831 void BiasedLockingCounters::print_on(outputStream* st) {
832 tty->print_cr("# total entries: %d", _total_entry_count);
833 tty->print_cr("# biased lock entries: %d", _biased_lock_entry_count);
834 tty->print_cr("# anonymously biased lock entries: %d", _anonymously_biased_lock_entry_count);
835 tty->print_cr("# rebiased lock entries: %d", _rebiased_lock_entry_count);
836 tty->print_cr("# revoked lock entries: %d", _revoked_lock_entry_count);
837 tty->print_cr("# fast path lock entries: %d", _fast_path_entry_count);
838 tty->print_cr("# slow path lock entries: %d", slow_path_entry_count());
839 }