1 /*
   2  * Copyright (c) 2005, 2012, 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 
  35 static bool _biased_locking_enabled = false;
  36 BiasedLockingCounters BiasedLocking::_counters;
  37 
  38 static GrowableArray<Handle>*  _preserved_oop_stack  = NULL;
  39 static GrowableArray<markOop>* _preserved_mark_stack = NULL;
  40 
  41 static void enable_biased_locking(Klass* k) {
  42   k->set_prototype_header(markOopDesc::biased_locking_prototype());
  43 }
  44 
  45 class VM_EnableBiasedLocking: public VM_Operation {
  46  private:
  47   bool _is_cheap_allocated;
  48  public:
  49   VM_EnableBiasedLocking(bool is_cheap_allocated) { _is_cheap_allocated = is_cheap_allocated; }
  50   VMOp_Type type() const          { return VMOp_EnableBiasedLocking; }
  51   Mode evaluation_mode() const    { return _is_cheap_allocated ? _async_safepoint : _safepoint; }
  52   bool is_cheap_allocated() const { return _is_cheap_allocated; }
  53 
  54   void doit() {
  55     // Iterate the system dictionary enabling biased locking for all
  56     // currently loaded classes
  57     SystemDictionary::classes_do(enable_biased_locking);
  58     // Indicate that future instances should enable it as well
  59     _biased_locking_enabled = true;
  60 
  61     if (TraceBiasedLocking) {
  62       tty->print_cr("Biased locking enabled");
  63     }
  64   }
  65 
  66   bool allow_nested_vm_operations() const        { return false; }
  67 };
  68 
  69 
  70 // One-shot PeriodicTask subclass for enabling biased locking
  71 class EnableBiasedLockingTask : public PeriodicTask {
  72  public:
  73   EnableBiasedLockingTask(size_t interval_time) : PeriodicTask(interval_time) {}
  74 
  75   virtual void task() {
  76     // Use async VM operation to avoid blocking the Watcher thread.
  77     // VM Thread will free C heap storage.
  78     VM_EnableBiasedLocking *op = new VM_EnableBiasedLocking(true);
  79     VMThread::execute(op);
  80 
  81     // Reclaim our storage and disenroll ourself
  82     delete this;
  83   }
  84 };
  85 
  86 
  87 void BiasedLocking::init() {
  88   // If biased locking is enabled, schedule a task to fire a few
  89   // seconds into the run which turns on biased locking for all
  90   // currently loaded classes as well as future ones. This is a
  91   // workaround for startup time regressions due to a large number of
  92   // safepoints being taken during VM startup for bias revocation.
  93   // Ideally we would have a lower cost for individual bias revocation
  94   // and not need a mechanism like this.
  95   if (UseBiasedLocking) {
  96     if (BiasedLockingStartupDelay > 0) {
  97       EnableBiasedLockingTask* task = new EnableBiasedLockingTask(BiasedLockingStartupDelay);
  98       task->enroll();
  99     } else {
 100       VM_EnableBiasedLocking op(false);
 101       VMThread::execute(&op);
 102     }
 103   }
 104 }
 105 
 106 
 107 bool BiasedLocking::enabled() {
 108   return _biased_locking_enabled;
 109 }
 110 
 111 // Returns MonitorInfos for all objects locked on this thread in youngest to oldest order
 112 static GrowableArray<MonitorInfo*>* get_or_compute_monitor_info(JavaThread* thread) {
 113   GrowableArray<MonitorInfo*>* info = thread->cached_monitor_info();
 114   if (info != NULL) {
 115     return info;
 116   }
 117 
 118   info = new GrowableArray<MonitorInfo*>();
 119 
 120   // It's possible for the thread to not have any Java frames on it,
 121   // i.e., if it's the main thread and it's already returned from main()
 122   if (thread->has_last_Java_frame()) {
 123     RegisterMap rm(thread);
 124     for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {
 125       GrowableArray<MonitorInfo*> *monitors = vf->monitors();
 126       if (monitors != NULL) {
 127         int len = monitors->length();
 128         // Walk monitors youngest to oldest
 129         for (int i = len - 1; i >= 0; i--) {
 130           MonitorInfo* mon_info = monitors->at(i);
 131           if (mon_info->owner_is_scalar_replaced()) continue;
 132           oop owner = mon_info->owner();
 133           if (owner != NULL) {
 134             info->append(mon_info);
 135           }
 136         }
 137       }
 138     }
 139   }
 140 
 141   thread->set_cached_monitor_info(info);
 142   return info;
 143 }
 144 
 145 
 146 static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread) {
 147   markOop mark = obj->mark();
 148   if (!mark->has_bias_pattern()) {
 149     if (TraceBiasedLocking) {
 150       ResourceMark rm;
 151       tty->print_cr("  (Skipping revocation of object of type %s because it's no longer biased)",
 152                     obj->klass()->external_name());
 153     }
 154     return BiasedLocking::NOT_BIASED;
 155   }
 156 
 157   uint age = mark->age();
 158   markOop   biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);
 159   markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);
 160 
 161   if (TraceBiasedLocking && (Verbose || !is_bulk)) {
 162     ResourceMark rm;
 163     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,
 164                   (intptr_t) obj, (intptr_t) mark, obj->klass()->external_name(), (intptr_t) obj->klass()->prototype_header(), (allow_rebias ? 1 : 0), (intptr_t) requesting_thread);
 165   }
 166 
 167   JavaThread* biased_thread = mark->biased_locker();
 168   if (biased_thread == NULL) {
 169     // Object is anonymously biased. We can get here if, for
 170     // example, we revoke the bias due to an identity hash code
 171     // being computed for an object.
 172     if (!allow_rebias) {
 173       obj->set_mark(unbiased_prototype);
 174     }
 175     if (TraceBiasedLocking && (Verbose || !is_bulk)) {
 176       tty->print_cr("  Revoked bias of anonymously-biased object");
 177     }
 178     return BiasedLocking::BIAS_REVOKED;
 179   }
 180 
 181   // Handle case where the thread toward which the object was biased has exited
 182   bool thread_is_alive = false;
 183   if (requesting_thread == biased_thread) {
 184     thread_is_alive = true;
 185   } else {
 186     for (JavaThread* cur_thread = Threads::first(); cur_thread != NULL; cur_thread = cur_thread->next()) {
 187       if (cur_thread == biased_thread) {
 188         thread_is_alive = true;
 189         break;
 190       }
 191     }
 192   }
 193   if (!thread_is_alive) {
 194     if (allow_rebias) {
 195       obj->set_mark(biased_prototype);
 196     } else {
 197       obj->set_mark(unbiased_prototype);
 198     }
 199     if (TraceBiasedLocking && (Verbose || !is_bulk)) {
 200       tty->print_cr("  Revoked bias of object biased toward dead thread");
 201     }
 202     return BiasedLocking::BIAS_REVOKED;
 203   }
 204 
 205   // Thread owning bias is alive.
 206   // Check to see whether it currently owns the lock and, if so,
 207   // write down the needed displaced headers to the thread's stack.
 208   // Otherwise, restore the object's header either to the unlocked
 209   // or unbiased state.
 210   GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(biased_thread);
 211   BasicLock* highest_lock = NULL;
 212   for (int i = 0; i < cached_monitor_info->length(); i++) {
 213     MonitorInfo* mon_info = cached_monitor_info->at(i);
 214     if (mon_info->owner() == obj) {
 215       if (TraceBiasedLocking && Verbose) {
 216         tty->print_cr("   mon_info->owner (" PTR_FORMAT ") == obj (" PTR_FORMAT ")",
 217                       (intptr_t) mon_info->owner(),
 218                       (intptr_t) obj);
 219       }
 220       // Assume recursive case and fix up highest lock later
 221       markOop mark = markOopDesc::encode((BasicLock*) NULL);
 222       highest_lock = mon_info->lock();
 223       highest_lock->set_displaced_header(mark);
 224     } else {
 225       if (TraceBiasedLocking && Verbose) {
 226         tty->print_cr("   mon_info->owner (" PTR_FORMAT ") != obj (" PTR_FORMAT ")",
 227                       (intptr_t) mon_info->owner(),
 228                       (intptr_t) obj);
 229       }
 230     }
 231   }
 232   if (highest_lock != NULL) {
 233     // Fix up highest lock to contain displaced header and point
 234     // object at it
 235     highest_lock->set_displaced_header(unbiased_prototype);
 236     // Reset object header to point to displaced mark
 237     obj->set_mark(markOopDesc::encode(highest_lock));
 238     assert(!obj->mark()->has_bias_pattern(), "illegal mark state: stack lock used bias bit");
 239     if (TraceBiasedLocking && (Verbose || !is_bulk)) {
 240       tty->print_cr("  Revoked bias of currently-locked object");
 241     }
 242   } else {
 243     if (TraceBiasedLocking && (Verbose || !is_bulk)) {
 244       tty->print_cr("  Revoked bias of currently-unlocked object");
 245     }
 246     if (allow_rebias) {
 247       obj->set_mark(biased_prototype);
 248     } else {
 249       // Store the unlocked value into the object's header.
 250       obj->set_mark(unbiased_prototype);
 251     }
 252   }
 253 
 254   return BiasedLocking::BIAS_REVOKED;
 255 }
 256 
 257 
 258 enum HeuristicsResult {
 259   HR_NOT_BIASED    = 1,
 260   HR_SINGLE_REVOKE = 2,
 261   HR_BULK_REBIAS   = 3,
 262   HR_BULK_REVOKE   = 4
 263 };
 264 
 265 
 266 static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {
 267   markOop mark = o->mark();
 268   if (!mark->has_bias_pattern()) {
 269     return HR_NOT_BIASED;
 270   }
 271 
 272   // Heuristics to attempt to throttle the number of revocations.
 273   // Stages:
 274   // 1. Revoke the biases of all objects in the heap of this type,
 275   //    but allow rebiasing of those objects if unlocked.
 276   // 2. Revoke the biases of all objects in the heap of this type
 277   //    and don't allow rebiasing of these objects. Disable
 278   //    allocation of objects of that type with the bias bit set.
 279   Klass* k = o->klass();
 280   jlong cur_time = os::javaTimeMillis();
 281   jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();
 282   int revocation_count = k->biased_lock_revocation_count();
 283   if ((revocation_count >= BiasedLockingBulkRebiasThreshold) &&
 284       (revocation_count <  BiasedLockingBulkRevokeThreshold) &&
 285       (last_bulk_revocation_time != 0) &&
 286       (cur_time - last_bulk_revocation_time >= BiasedLockingDecayTime)) {
 287     // This is the first revocation we've seen in a while of an
 288     // object of this type since the last time we performed a bulk
 289     // rebiasing operation. The application is allocating objects in
 290     // bulk which are biased toward a thread and then handing them
 291     // off to another thread. We can cope with this allocation
 292     // pattern via the bulk rebiasing mechanism so we reset the
 293     // klass's revocation count rather than allow it to increase
 294     // monotonically. If we see the need to perform another bulk
 295     // rebias operation later, we will, and if subsequently we see
 296     // many more revocation operations in a short period of time we
 297     // will completely disable biasing for this type.
 298     k->set_biased_lock_revocation_count(0);
 299     revocation_count = 0;
 300   }
 301 
 302   // Make revocation count saturate just beyond BiasedLockingBulkRevokeThreshold
 303   if (revocation_count <= BiasedLockingBulkRevokeThreshold) {
 304     revocation_count = k->atomic_incr_biased_lock_revocation_count();
 305   }
 306 
 307   if (revocation_count == BiasedLockingBulkRevokeThreshold) {
 308     return HR_BULK_REVOKE;
 309   }
 310 
 311   if (revocation_count == BiasedLockingBulkRebiasThreshold) {
 312     return HR_BULK_REBIAS;
 313   }
 314 
 315   return HR_SINGLE_REVOKE;
 316 }
 317 
 318 
 319 static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,
 320                                                                    bool bulk_rebias,
 321                                                                    bool attempt_rebias_of_object,
 322                                                                    JavaThread* requesting_thread) {
 323   assert(SafepointSynchronize::is_at_safepoint(), "must be done at safepoint");
 324 
 325   if (TraceBiasedLocking) {
 326     tty->print_cr("* Beginning bulk revocation (kind == %s) because of object "
 327                   INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
 328                   (bulk_rebias ? "rebias" : "revoke"),
 329                   (intptr_t) o, (intptr_t) o->mark(), o->klass()->external_name());
 330   }
 331 
 332   jlong cur_time = os::javaTimeMillis();
 333   o->klass()->set_last_biased_lock_bulk_revocation_time(cur_time);
 334 
 335 
 336   Klass* k_o = o->klass();
 337   Klass* klass = k_o;
 338 
 339   if (bulk_rebias) {
 340     // Use the epoch in the klass of the object to implicitly revoke
 341     // all biases of objects of this data type and force them to be
 342     // reacquired. However, we also need to walk the stacks of all
 343     // threads and update the headers of lightweight locked objects
 344     // with biases to have the current epoch.
 345 
 346     // If the prototype header doesn't have the bias pattern, don't
 347     // try to update the epoch -- assume another VM operation came in
 348     // and reset the header to the unbiased state, which will
 349     // implicitly cause all existing biases to be revoked
 350     if (klass->prototype_header()->has_bias_pattern()) {
 351       int prev_epoch = klass->prototype_header()->bias_epoch();
 352       klass->set_prototype_header(klass->prototype_header()->incr_bias_epoch());
 353       int cur_epoch = klass->prototype_header()->bias_epoch();
 354 
 355       // Now walk all threads' stacks and adjust epochs of any biased
 356       // and locked objects of this data type we encounter
 357       for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
 358         GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
 359         for (int i = 0; i < cached_monitor_info->length(); i++) {
 360           MonitorInfo* mon_info = cached_monitor_info->at(i);
 361           oop owner = mon_info->owner();
 362           markOop mark = owner->mark();
 363           if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
 364             // We might have encountered this object already in the case of recursive locking
 365             assert(mark->bias_epoch() == prev_epoch || mark->bias_epoch() == cur_epoch, "error in bias epoch adjustment");
 366             owner->set_mark(mark->set_bias_epoch(cur_epoch));
 367           }
 368         }
 369       }
 370     }
 371 
 372     // At this point we're done. All we have to do is potentially
 373     // adjust the header of the given object to revoke its bias.
 374     revoke_bias(o, attempt_rebias_of_object && klass->prototype_header()->has_bias_pattern(), true, requesting_thread);
 375   } else {
 376     if (TraceBiasedLocking) {
 377       ResourceMark rm;
 378       tty->print_cr("* Disabling biased locking for type %s", klass->external_name());
 379     }
 380 
 381     // Disable biased locking for this data type. Not only will this
 382     // cause future instances to not be biased, but existing biased
 383     // instances will notice that this implicitly caused their biases
 384     // to be revoked.
 385     klass->set_prototype_header(markOopDesc::prototype());
 386 
 387     // Now walk all threads' stacks and forcibly revoke the biases of
 388     // any locked and biased objects of this data type we encounter.
 389     for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
 390       GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
 391       for (int i = 0; i < cached_monitor_info->length(); i++) {
 392         MonitorInfo* mon_info = cached_monitor_info->at(i);
 393         oop owner = mon_info->owner();
 394         markOop mark = owner->mark();
 395         if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
 396           revoke_bias(owner, false, true, requesting_thread);
 397         }
 398       }
 399     }
 400 
 401     // Must force the bias of the passed object to be forcibly revoked
 402     // as well to ensure guarantees to callers
 403     revoke_bias(o, false, true, requesting_thread);
 404   }
 405 
 406   if (TraceBiasedLocking) {
 407     tty->print_cr("* Ending bulk revocation");
 408   }
 409 
 410   BiasedLocking::Condition status_code = BiasedLocking::BIAS_REVOKED;
 411 
 412   if (attempt_rebias_of_object &&
 413       o->mark()->has_bias_pattern() &&
 414       klass->prototype_header()->has_bias_pattern()) {
 415     markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),
 416                                            klass->prototype_header()->bias_epoch());
 417     o->set_mark(new_mark);
 418     status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;
 419     if (TraceBiasedLocking) {
 420       tty->print_cr("  Rebiased object toward thread " INTPTR_FORMAT, (intptr_t) requesting_thread);
 421     }
 422   }
 423 
 424   assert(!o->mark()->has_bias_pattern() ||
 425          (attempt_rebias_of_object && (o->mark()->biased_locker() == requesting_thread)),
 426          "bug in bulk bias revocation");
 427 
 428   return status_code;
 429 }
 430 
 431 
 432 static void clean_up_cached_monitor_info() {
 433   // Walk the thread list clearing out the cached monitors
 434   for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
 435     thr->set_cached_monitor_info(NULL);
 436   }
 437 }
 438 
 439 
 440 class VM_RevokeBias : public VM_Operation {
 441 protected:
 442   Handle* _obj;
 443   GrowableArray<Handle>* _objs;
 444   JavaThread* _requesting_thread;
 445   BiasedLocking::Condition _status_code;
 446 
 447 public:
 448   VM_RevokeBias(Handle* obj, JavaThread* requesting_thread)
 449     : _obj(obj)
 450     , _objs(NULL)
 451     , _requesting_thread(requesting_thread)
 452     , _status_code(BiasedLocking::NOT_BIASED) {}
 453 
 454   VM_RevokeBias(GrowableArray<Handle>* objs, JavaThread* requesting_thread)
 455     : _obj(NULL)
 456     , _objs(objs)
 457     , _requesting_thread(requesting_thread)
 458     , _status_code(BiasedLocking::NOT_BIASED) {}
 459 
 460   virtual VMOp_Type type() const { return VMOp_RevokeBias; }
 461 
 462   virtual bool doit_prologue() {
 463     // Verify that there is actual work to do since the callers just
 464     // give us locked object(s). If we don't find any biased objects
 465     // there is nothing to do and we avoid a safepoint.
 466     if (_obj != NULL) {
 467       markOop mark = (*_obj)()->mark();
 468       if (mark->has_bias_pattern()) {
 469         return true;
 470       }
 471     } else {
 472       for ( int i = 0 ; i < _objs->length(); i++ ) {
 473         markOop mark = (_objs->at(i))()->mark();
 474         if (mark->has_bias_pattern()) {
 475           return true;
 476         }
 477       }
 478     }
 479     return false;
 480   }
 481 
 482   virtual void doit() {
 483     if (_obj != NULL) {
 484       if (TraceBiasedLocking) {
 485         tty->print_cr("Revoking bias with potentially per-thread safepoint:");
 486       }
 487       _status_code = revoke_bias((*_obj)(), false, false, _requesting_thread);
 488       clean_up_cached_monitor_info();
 489       return;
 490     } else {
 491       if (TraceBiasedLocking) {
 492         tty->print_cr("Revoking bias with global safepoint:");
 493       }
 494       BiasedLocking::revoke_at_safepoint(_objs);
 495     }
 496   }
 497 
 498   BiasedLocking::Condition status_code() const {
 499     return _status_code;
 500   }
 501 };
 502 
 503 
 504 class VM_BulkRevokeBias : public VM_RevokeBias {
 505 private:
 506   bool _bulk_rebias;
 507   bool _attempt_rebias_of_object;
 508 
 509 public:
 510   VM_BulkRevokeBias(Handle* obj, JavaThread* requesting_thread,
 511                     bool bulk_rebias,
 512                     bool attempt_rebias_of_object)
 513     : VM_RevokeBias(obj, requesting_thread)
 514     , _bulk_rebias(bulk_rebias)
 515     , _attempt_rebias_of_object(attempt_rebias_of_object) {}
 516 
 517   virtual VMOp_Type type() const { return VMOp_BulkRevokeBias; }
 518   virtual bool doit_prologue()   { return true; }
 519 
 520   virtual void doit() {
 521     _status_code = bulk_revoke_or_rebias_at_safepoint((*_obj)(), _bulk_rebias, _attempt_rebias_of_object, _requesting_thread);
 522     clean_up_cached_monitor_info();
 523   }
 524 };
 525 
 526 
 527 BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
 528   assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
 529 
 530   // We can revoke the biases of anonymously-biased objects
 531   // efficiently enough that we should not cause these revocations to
 532   // update the heuristics because doing so may cause unwanted bulk
 533   // revocations (which are expensive) to occur.
 534   markOop mark = obj->mark();
 535   if (mark->is_biased_anonymously() && !attempt_rebias) {
 536     // We are probably trying to revoke the bias of this object due to
 537     // an identity hash code computation. Try to revoke the bias
 538     // without a safepoint. This is possible if we can successfully
 539     // compare-and-exchange an unbiased header into the mark word of
 540     // the object, meaning that no other thread has raced to acquire
 541     // the bias of the object.
 542     markOop biased_value       = mark;
 543     markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
 544     markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
 545     if (res_mark == biased_value) {
 546       return BIAS_REVOKED;
 547     }
 548   } else if (mark->has_bias_pattern()) {
 549     Klass* k = obj->klass();
 550     markOop prototype_header = k->prototype_header();
 551     if (!prototype_header->has_bias_pattern()) {
 552       // This object has a stale bias from before the bulk revocation
 553       // for this data type occurred. It's pointless to update the
 554       // heuristics at this point so simply update the header with a
 555       // CAS. If we fail this race, the object's bias has been revoked
 556       // by another thread so we simply return and let the caller deal
 557       // with it.
 558       markOop biased_value       = mark;
 559       markOop res_mark = (markOop) Atomic::cmpxchg_ptr(prototype_header, obj->mark_addr(), mark);
 560       assert(!(*(obj->mark_addr()))->has_bias_pattern(), "even if we raced, should still be revoked");
 561       return BIAS_REVOKED;
 562     } else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
 563       // The epoch of this biasing has expired indicating that the
 564       // object is effectively unbiased. Depending on whether we need
 565       // to rebias or revoke the bias of this object we can do it
 566       // efficiently enough with a CAS that we shouldn't update the
 567       // heuristics. This is normally done in the assembly code but we
 568       // can reach this point due to various points in the runtime
 569       // needing to revoke biases.
 570       if (attempt_rebias) {
 571         assert(THREAD->is_Java_thread(), "");
 572         markOop biased_value       = mark;
 573         markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());
 574         markOop res_mark = (markOop) Atomic::cmpxchg_ptr(rebiased_prototype, obj->mark_addr(), mark);
 575         if (res_mark == biased_value) {
 576           return BIAS_REVOKED_AND_REBIASED;
 577         }
 578       } else {
 579         markOop biased_value       = mark;
 580         markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
 581         markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
 582         if (res_mark == biased_value) {
 583           return BIAS_REVOKED;
 584         }
 585       }
 586     }
 587   }
 588 
 589   HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
 590   if (heuristics == HR_NOT_BIASED) {
 591     return NOT_BIASED;
 592   } else if (heuristics == HR_SINGLE_REVOKE) {
 593     Klass *k = obj->klass();
 594     markOop prototype_header = k->prototype_header();
 595     if (mark->biased_locker() == THREAD &&
 596         prototype_header->bias_epoch() == mark->bias_epoch()) {
 597       // A thread is trying to revoke the bias of an object biased
 598       // toward it, again likely due to an identity hash code
 599       // computation. We can again avoid a safepoint in this case
 600       // since we are only going to walk our own stack. There are no
 601       // races with revocations occurring in other threads because we
 602       // reach no safepoints in the revocation path.
 603       // Also check the epoch because even if threads match, another thread
 604       // can come in with a CAS to steal the bias of an object that has a
 605       // stale epoch.
 606       ResourceMark rm;
 607       if (TraceBiasedLocking) {
 608         tty->print_cr("Revoking bias by walking my own stack:");
 609       }
 610       BiasedLocking::Condition cond = revoke_bias(obj(), false, false, (JavaThread*) THREAD);
 611       ((JavaThread*) THREAD)->set_cached_monitor_info(NULL);
 612       assert(cond == BIAS_REVOKED, "why not?");
 613       return cond;
 614     } else {
 615       VM_RevokeBias revoke(&obj, (JavaThread*) THREAD);
 616       VMThread::execute(&revoke);
 617       return revoke.status_code();
 618     }
 619   }
 620 
 621   assert((heuristics == HR_BULK_REVOKE) ||
 622          (heuristics == HR_BULK_REBIAS), "?");
 623   VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
 624                                 (heuristics == HR_BULK_REBIAS),
 625                                 attempt_rebias);
 626   VMThread::execute(&bulk_revoke);
 627   return bulk_revoke.status_code();
 628 }
 629 
 630 
 631 void BiasedLocking::revoke(GrowableArray<Handle>* objs) {
 632   assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
 633   if (objs->length() == 0) {
 634     return;
 635   }
 636   VM_RevokeBias revoke(objs, JavaThread::current());
 637   VMThread::execute(&revoke);
 638 }
 639 
 640 
 641 void BiasedLocking::revoke_at_safepoint(Handle h_obj) {
 642   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 643   oop obj = h_obj();
 644   HeuristicsResult heuristics = update_heuristics(obj, false);
 645   if (heuristics == HR_SINGLE_REVOKE) {
 646     revoke_bias(obj, false, false, NULL);
 647   } else if ((heuristics == HR_BULK_REBIAS) ||
 648              (heuristics == HR_BULK_REVOKE)) {
 649     bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
 650   }
 651   clean_up_cached_monitor_info();
 652 }
 653 
 654 
 655 void BiasedLocking::revoke_at_safepoint(GrowableArray<Handle>* objs) {
 656   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 657   int len = objs->length();
 658   for (int i = 0; i < len; i++) {
 659     oop obj = (objs->at(i))();
 660     HeuristicsResult heuristics = update_heuristics(obj, false);
 661     if (heuristics == HR_SINGLE_REVOKE) {
 662       revoke_bias(obj, false, false, NULL);
 663     } else if ((heuristics == HR_BULK_REBIAS) ||
 664                (heuristics == HR_BULK_REVOKE)) {
 665       bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
 666     }
 667   }
 668   clean_up_cached_monitor_info();
 669 }
 670 
 671 
 672 void BiasedLocking::preserve_marks() {
 673   if (!UseBiasedLocking)
 674     return;
 675 
 676   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 677 
 678   assert(_preserved_oop_stack  == NULL, "double initialization");
 679   assert(_preserved_mark_stack == NULL, "double initialization");
 680 
 681   // In order to reduce the number of mark words preserved during GC
 682   // due to the presence of biased locking, we reinitialize most mark
 683   // words to the class's prototype during GC -- even those which have
 684   // a currently valid bias owner. One important situation where we
 685   // must not clobber a bias is when a biased object is currently
 686   // locked. To handle this case we iterate over the currently-locked
 687   // monitors in a prepass and, if they are biased, preserve their
 688   // mark words here. This should be a relatively small set of objects
 689   // especially compared to the number of objects in the heap.
 690   _preserved_mark_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<markOop>(10, true);
 691   _preserved_oop_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<Handle>(10, true);
 692 
 693   ResourceMark rm;
 694   Thread* cur = Thread::current();
 695   for (JavaThread* thread = Threads::first(); thread != NULL; thread = thread->next()) {
 696     if (thread->has_last_Java_frame()) {
 697       RegisterMap rm(thread);
 698       for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {
 699         GrowableArray<MonitorInfo*> *monitors = vf->monitors();
 700         if (monitors != NULL) {
 701           int len = monitors->length();
 702           // Walk monitors youngest to oldest
 703           for (int i = len - 1; i >= 0; i--) {
 704             MonitorInfo* mon_info = monitors->at(i);
 705             if (mon_info->owner_is_scalar_replaced()) continue;
 706             oop owner = mon_info->owner();
 707             if (owner != NULL) {
 708               markOop mark = owner->mark();
 709               if (mark->has_bias_pattern()) {
 710                 _preserved_oop_stack->push(Handle(cur, owner));
 711                 _preserved_mark_stack->push(mark);
 712               }
 713             }
 714           }
 715         }
 716       }
 717     }
 718   }
 719 }
 720 
 721 
 722 void BiasedLocking::restore_marks() {
 723   if (!UseBiasedLocking)
 724     return;
 725 
 726   assert(_preserved_oop_stack  != NULL, "double free");
 727   assert(_preserved_mark_stack != NULL, "double free");
 728 
 729   int len = _preserved_oop_stack->length();
 730   for (int i = 0; i < len; i++) {
 731     Handle owner = _preserved_oop_stack->at(i);
 732     markOop mark = _preserved_mark_stack->at(i);
 733     owner->set_mark(mark);
 734   }
 735 
 736   delete _preserved_oop_stack;
 737   _preserved_oop_stack = NULL;
 738   delete _preserved_mark_stack;
 739   _preserved_mark_stack = NULL;
 740 }
 741 
 742 
 743 int* BiasedLocking::total_entry_count_addr()                   { return _counters.total_entry_count_addr(); }
 744 int* BiasedLocking::biased_lock_entry_count_addr()             { return _counters.biased_lock_entry_count_addr(); }
 745 int* BiasedLocking::anonymously_biased_lock_entry_count_addr() { return _counters.anonymously_biased_lock_entry_count_addr(); }
 746 int* BiasedLocking::rebiased_lock_entry_count_addr()           { return _counters.rebiased_lock_entry_count_addr(); }
 747 int* BiasedLocking::revoked_lock_entry_count_addr()            { return _counters.revoked_lock_entry_count_addr(); }
 748 int* BiasedLocking::fast_path_entry_count_addr()               { return _counters.fast_path_entry_count_addr(); }
 749 int* BiasedLocking::slow_path_entry_count_addr()               { return _counters.slow_path_entry_count_addr(); }
 750 
 751 
 752 // BiasedLockingCounters
 753 
 754 int BiasedLockingCounters::slow_path_entry_count() {
 755   if (_slow_path_entry_count != 0) {
 756     return _slow_path_entry_count;
 757   }
 758   int sum = _biased_lock_entry_count   + _anonymously_biased_lock_entry_count +
 759             _rebiased_lock_entry_count + _revoked_lock_entry_count +
 760             _fast_path_entry_count;
 761 
 762   return _total_entry_count - sum;
 763 }
 764 
 765 void BiasedLockingCounters::print_on(outputStream* st) {
 766   tty->print_cr("# total entries: %d", _total_entry_count);
 767   tty->print_cr("# biased lock entries: %d", _biased_lock_entry_count);
 768   tty->print_cr("# anonymously biased lock entries: %d", _anonymously_biased_lock_entry_count);
 769   tty->print_cr("# rebiased lock entries: %d", _rebiased_lock_entry_count);
 770   tty->print_cr("# revoked lock entries: %d", _revoked_lock_entry_count);
 771   tty->print_cr("# fast path lock entries: %d", _fast_path_entry_count);
 772   tty->print_cr("# slow path lock entries: %d", slow_path_entry_count());
 773 }