1 /*
   2  * Copyright (c) 2005, 2016, 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 "logging/log.hpp"
  27 #include "memory/resourceArea.hpp"
  28 #include "oops/klass.inline.hpp"
  29 #include "oops/markOop.hpp"
  30 #include "oops/oop.inline.hpp"
  31 #include "runtime/atomic.hpp"
  32 #include "runtime/basicLock.hpp"
  33 #include "runtime/biasedLocking.hpp"
  34 #include "runtime/task.hpp"
  35 #include "runtime/vframe.hpp"
  36 #include "runtime/vmThread.hpp"
  37 #include "runtime/vm_operations.hpp"
  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     log_info(biasedlocking)("Biased locking enabled");
  66   }
  67 
  68   bool allow_nested_vm_operations() const        { return false; }
  69 };
  70 
  71 
  72 // One-shot PeriodicTask subclass for enabling biased locking
  73 class EnableBiasedLockingTask : public PeriodicTask {
  74  public:
  75   EnableBiasedLockingTask(size_t interval_time) : PeriodicTask(interval_time) {}
  76 
  77   virtual void task() {
  78     // Use async VM operation to avoid blocking the Watcher thread.
  79     // VM Thread will free C heap storage.
  80     VM_EnableBiasedLocking *op = new VM_EnableBiasedLocking(true);
  81     VMThread::execute(op);
  82 
  83     // Reclaim our storage and disenroll ourself
  84     delete this;
  85   }
  86 };
  87 
  88 
  89 void BiasedLocking::init() {
  90   // If biased locking is enabled, schedule a task to fire a few
  91   // seconds into the run which turns on biased locking for all
  92   // currently loaded classes as well as future ones. This is a
  93   // workaround for startup time regressions due to a large number of
  94   // safepoints being taken during VM startup for bias revocation.
  95   // Ideally we would have a lower cost for individual bias revocation
  96   // and not need a mechanism like this.
  97   if (UseBiasedLocking) {
  98     if (BiasedLockingStartupDelay > 0) {
  99       EnableBiasedLockingTask* task = new EnableBiasedLockingTask(BiasedLockingStartupDelay);
 100       task->enroll();
 101     } else {
 102       VM_EnableBiasedLocking op(false);
 103       VMThread::execute(&op);
 104     }
 105   }
 106 }
 107 
 108 
 109 bool BiasedLocking::enabled() {
 110   return _biased_locking_enabled;
 111 }
 112 
 113 // Returns MonitorInfos for all objects locked on this thread in youngest to oldest order
 114 static GrowableArray<MonitorInfo*>* get_or_compute_monitor_info(JavaThread* thread) {
 115   GrowableArray<MonitorInfo*>* info = thread->cached_monitor_info();
 116   if (info != NULL) {
 117     return info;
 118   }
 119 
 120   info = new GrowableArray<MonitorInfo*>();
 121 
 122   // It's possible for the thread to not have any Java frames on it,
 123   // i.e., if it's the main thread and it's already returned from main()
 124   if (thread->has_last_Java_frame()) {
 125     RegisterMap rm(thread);
 126     for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {
 127       GrowableArray<MonitorInfo*> *monitors = vf->monitors();
 128       if (monitors != NULL) {
 129         int len = monitors->length();
 130         // Walk monitors youngest to oldest
 131         for (int i = len - 1; i >= 0; i--) {
 132           MonitorInfo* mon_info = monitors->at(i);
 133           if (mon_info->eliminated()) continue;
 134           oop owner = mon_info->owner();
 135           if (owner != NULL) {
 136             info->append(mon_info);
 137           }
 138         }
 139       }
 140     }
 141   }
 142 
 143   thread->set_cached_monitor_info(info);
 144   return info;
 145 }
 146 
 147 static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread) {
 148   markOop mark = obj->mark();
 149   if (!mark->has_bias_pattern()) {
 150     if (log_is_enabled(Info, biasedlocking)) {
 151       ResourceMark rm;
 152       log_info(biasedlocking)("  (Skipping revocation of object " INTPTR_FORMAT
 153                               ", mark " INTPTR_FORMAT ", type %s"
 154                               ", requesting thread " INTPTR_FORMAT
 155                               " because it's no longer biased)",
 156                               p2i((void *)obj), (intptr_t) mark,
 157                               obj->klass()->external_name(),
 158                               (intptr_t) requesting_thread);
 159     }
 160     return BiasedLocking::NOT_BIASED;
 161   }
 162 
 163   uint age = mark->age();
 164   markOop   biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);
 165   markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);
 166 
 167   // Log at "info" level if not bulk, else "trace" level
 168   if (!is_bulk) {
 169     ResourceMark rm;
 170     log_info(biasedlocking)("Revoking bias of object " INTPTR_FORMAT ", mark "
 171                             INTPTR_FORMAT ", type %s, prototype header " INTPTR_FORMAT
 172                             ", allow rebias %d, requesting thread " INTPTR_FORMAT,
 173                             p2i((void *)obj),
 174                             (intptr_t) mark,
 175                             obj->klass()->external_name(),
 176                             (intptr_t) obj->klass()->prototype_header(),
 177                             (allow_rebias ? 1 : 0),
 178                             (intptr_t) requesting_thread);
 179   } else {
 180     ResourceMark rm;
 181     log_trace(biasedlocking)("Revoking bias of object " INTPTR_FORMAT " , mark "
 182                              INTPTR_FORMAT " , type %s , prototype header " INTPTR_FORMAT
 183                              " , allow rebias %d , requesting thread " INTPTR_FORMAT,
 184                              p2i((void *)obj),
 185                              (intptr_t) mark,
 186                              obj->klass()->external_name(),
 187                              (intptr_t) obj->klass()->prototype_header(),
 188                              (allow_rebias ? 1 : 0),
 189                              (intptr_t) requesting_thread);
 190   }
 191 
 192   JavaThread* biased_thread = mark->biased_locker();
 193   if (biased_thread == NULL) {
 194     // Object is anonymously biased. We can get here if, for
 195     // example, we revoke the bias due to an identity hash code
 196     // being computed for an object.
 197     if (!allow_rebias) {
 198       obj->set_mark(unbiased_prototype);
 199     }
 200     // Log at "info" level if not bulk, else "trace" level
 201     if (!is_bulk) {
 202       log_info(biasedlocking)("  Revoked bias of anonymously-biased object");
 203     } else {
 204       log_trace(biasedlocking)("  Revoked bias of anonymously-biased object");
 205     }
 206     return BiasedLocking::BIAS_REVOKED;
 207   }
 208 
 209   // Handle case where the thread toward which the object was biased has exited
 210   bool thread_is_alive = false;
 211   if (requesting_thread == biased_thread) {
 212     thread_is_alive = true;
 213   } else {
 214     for (JavaThread* cur_thread = Threads::first(); cur_thread != NULL; cur_thread = cur_thread->next()) {
 215       if (cur_thread == biased_thread) {
 216         thread_is_alive = true;
 217         break;
 218       }
 219     }
 220   }
 221   if (!thread_is_alive) {
 222     if (allow_rebias) {
 223       obj->set_mark(biased_prototype);
 224     } else {
 225       obj->set_mark(unbiased_prototype);
 226     }
 227     // Log at "info" level if not bulk, else "trace" level
 228     if (!is_bulk) {
 229       log_info(biasedlocking)("  Revoked bias of object biased toward dead thread ("
 230                               PTR_FORMAT ")", p2i(biased_thread));
 231     } else {
 232       log_trace(biasedlocking)("  Revoked bias of object biased toward dead thread ("
 233                                PTR_FORMAT ")", p2i(biased_thread));
 234     }
 235     return BiasedLocking::BIAS_REVOKED;
 236   }
 237 
 238   // Log at "info" level if not bulk, else "trace" level
 239   if (!is_bulk) {
 240     log_info(biasedlocking)("  Revoked bias of object biased toward live thread ("
 241                             PTR_FORMAT ")", p2i(biased_thread));
 242   } else {
 243     log_trace(biasedlocking)("  Revoked bias of object biased toward live thread ("
 244                                PTR_FORMAT ")", p2i(biased_thread));
 245   }
 246 
 247   // Thread owning bias is alive.
 248   // Check to see whether it currently owns the lock and, if so,
 249   // write down the needed displaced headers to the thread's stack.
 250   // Otherwise, restore the object's header either to the unlocked
 251   // or unbiased state.
 252   GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(biased_thread);
 253   BasicLock* highest_lock = NULL;
 254   for (int i = 0; i < cached_monitor_info->length(); i++) {
 255     MonitorInfo* mon_info = cached_monitor_info->at(i);
 256     if (mon_info->owner() == obj) {
 257       log_trace(biasedlocking)("   mon_info->owner (" PTR_FORMAT ") == obj (" PTR_FORMAT ")",
 258                                p2i((void *) mon_info->owner()),
 259                                p2i((void *) obj));
 260       // Assume recursive case and fix up highest lock later
 261       markOop mark = markOopDesc::encode((BasicLock*) NULL);
 262       highest_lock = mon_info->lock();
 263       highest_lock->set_displaced_header(mark);
 264     } else {
 265       log_trace(biasedlocking)("   mon_info->owner (" PTR_FORMAT ") != obj (" PTR_FORMAT ")",
 266                                p2i((void *) mon_info->owner()),
 267                                p2i((void *) obj));
 268     }
 269   }
 270   if (highest_lock != NULL) {
 271     // Fix up highest lock to contain displaced header and point
 272     // object at it
 273     highest_lock->set_displaced_header(unbiased_prototype);
 274     // Reset object header to point to displaced mark.
 275     // Must release storing the lock address for platforms without TSO
 276     // ordering (e.g. ppc).
 277     obj->release_set_mark(markOopDesc::encode(highest_lock));
 278     assert(!obj->mark()->has_bias_pattern(), "illegal mark state: stack lock used bias bit");
 279     // Log at "info" level if not bulk, else "trace" level
 280     if (!is_bulk) {
 281       log_info(biasedlocking)("  Revoked bias of currently-locked object");
 282     } else {
 283       log_trace(biasedlocking)("  Revoked bias of currently-locked object");
 284     }
 285   } else {
 286     // Log at "info" level if not bulk, else "trace" level
 287     if (!is_bulk) {
 288       log_info(biasedlocking)("  Revoked bias of currently-unlocked object");
 289     } else {
 290       log_trace(biasedlocking)("  Revoked bias of currently-unlocked object");
 291     }
 292     if (allow_rebias) {
 293       obj->set_mark(biased_prototype);
 294     } else {
 295       // Store the unlocked value into the object's header.
 296       obj->set_mark(unbiased_prototype);
 297     }
 298   }
 299 
 300   return BiasedLocking::BIAS_REVOKED;
 301 }
 302 
 303 
 304 enum HeuristicsResult {
 305   HR_NOT_BIASED    = 1,
 306   HR_SINGLE_REVOKE = 2,
 307   HR_BULK_REBIAS   = 3,
 308   HR_BULK_REVOKE   = 4
 309 };
 310 
 311 
 312 static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {
 313   markOop mark = o->mark();
 314   if (!mark->has_bias_pattern()) {
 315     return HR_NOT_BIASED;
 316   }
 317 
 318   // Heuristics to attempt to throttle the number of revocations.
 319   // Stages:
 320   // 1. Revoke the biases of all objects in the heap of this type,
 321   //    but allow rebiasing of those objects if unlocked.
 322   // 2. Revoke the biases of all objects in the heap of this type
 323   //    and don't allow rebiasing of these objects. Disable
 324   //    allocation of objects of that type with the bias bit set.
 325   Klass* k = o->klass();
 326   jlong cur_time = os::javaTimeMillis();
 327   jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();
 328   int revocation_count = k->biased_lock_revocation_count();
 329   if ((revocation_count >= BiasedLockingBulkRebiasThreshold) &&
 330       (revocation_count <  BiasedLockingBulkRevokeThreshold) &&
 331       (last_bulk_revocation_time != 0) &&
 332       (cur_time - last_bulk_revocation_time >= BiasedLockingDecayTime)) {
 333     // This is the first revocation we've seen in a while of an
 334     // object of this type since the last time we performed a bulk
 335     // rebiasing operation. The application is allocating objects in
 336     // bulk which are biased toward a thread and then handing them
 337     // off to another thread. We can cope with this allocation
 338     // pattern via the bulk rebiasing mechanism so we reset the
 339     // klass's revocation count rather than allow it to increase
 340     // monotonically. If we see the need to perform another bulk
 341     // rebias operation later, we will, and if subsequently we see
 342     // many more revocation operations in a short period of time we
 343     // will completely disable biasing for this type.
 344     k->set_biased_lock_revocation_count(0);
 345     revocation_count = 0;
 346   }
 347 
 348   // Make revocation count saturate just beyond BiasedLockingBulkRevokeThreshold
 349   if (revocation_count <= BiasedLockingBulkRevokeThreshold) {
 350     revocation_count = k->atomic_incr_biased_lock_revocation_count();
 351   }
 352 
 353   if (revocation_count == BiasedLockingBulkRevokeThreshold) {
 354     return HR_BULK_REVOKE;
 355   }
 356 
 357   if (revocation_count == BiasedLockingBulkRebiasThreshold) {
 358     return HR_BULK_REBIAS;
 359   }
 360 
 361   return HR_SINGLE_REVOKE;
 362 }
 363 
 364 
 365 static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,
 366                                                                    bool bulk_rebias,
 367                                                                    bool attempt_rebias_of_object,
 368                                                                    JavaThread* requesting_thread) {
 369   assert(SafepointSynchronize::is_at_safepoint(), "must be done at safepoint");
 370 
 371   log_info(biasedlocking)("* Beginning bulk revocation (kind == %s) because of object "
 372                           INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
 373                           (bulk_rebias ? "rebias" : "revoke"),
 374                           p2i((void *) o),
 375                           (intptr_t) o->mark(),
 376                           o->klass()->external_name());
 377 
 378   jlong cur_time = os::javaTimeMillis();
 379   o->klass()->set_last_biased_lock_bulk_revocation_time(cur_time);
 380 
 381 
 382   Klass* k_o = o->klass();
 383   Klass* klass = k_o;
 384 
 385   if (bulk_rebias) {
 386     // Use the epoch in the klass of the object to implicitly revoke
 387     // all biases of objects of this data type and force them to be
 388     // reacquired. However, we also need to walk the stacks of all
 389     // threads and update the headers of lightweight locked objects
 390     // with biases to have the current epoch.
 391 
 392     // If the prototype header doesn't have the bias pattern, don't
 393     // try to update the epoch -- assume another VM operation came in
 394     // and reset the header to the unbiased state, which will
 395     // implicitly cause all existing biases to be revoked
 396     if (klass->prototype_header()->has_bias_pattern()) {
 397       int prev_epoch = klass->prototype_header()->bias_epoch();
 398       klass->set_prototype_header(klass->prototype_header()->incr_bias_epoch());
 399       int cur_epoch = klass->prototype_header()->bias_epoch();
 400 
 401       // Now walk all threads' stacks and adjust epochs of any biased
 402       // and locked objects of this data type we encounter
 403       for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
 404         GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
 405         for (int i = 0; i < cached_monitor_info->length(); i++) {
 406           MonitorInfo* mon_info = cached_monitor_info->at(i);
 407           oop owner = mon_info->owner();
 408           markOop mark = owner->mark();
 409           if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
 410             // We might have encountered this object already in the case of recursive locking
 411             assert(mark->bias_epoch() == prev_epoch || mark->bias_epoch() == cur_epoch, "error in bias epoch adjustment");
 412             owner->set_mark(mark->set_bias_epoch(cur_epoch));
 413           }
 414         }
 415       }
 416     }
 417 
 418     // At this point we're done. All we have to do is potentially
 419     // adjust the header of the given object to revoke its bias.
 420     revoke_bias(o, attempt_rebias_of_object && klass->prototype_header()->has_bias_pattern(), true, requesting_thread);
 421   } else {
 422     if (log_is_enabled(Info, biasedlocking)) {
 423       ResourceMark rm;
 424       log_info(biasedlocking)("* Disabling biased locking for type %s", klass->external_name());
 425     }
 426 
 427     // Disable biased locking for this data type. Not only will this
 428     // cause future instances to not be biased, but existing biased
 429     // instances will notice that this implicitly caused their biases
 430     // to be revoked.
 431     klass->set_prototype_header(markOopDesc::prototype());
 432 
 433     // Now walk all threads' stacks and forcibly revoke the biases of
 434     // any locked and biased objects of this data type we encounter.
 435     for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
 436       GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
 437       for (int i = 0; i < cached_monitor_info->length(); i++) {
 438         MonitorInfo* mon_info = cached_monitor_info->at(i);
 439         oop owner = mon_info->owner();
 440         markOop mark = owner->mark();
 441         if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
 442           revoke_bias(owner, false, true, requesting_thread);
 443         }
 444       }
 445     }
 446 
 447     // Must force the bias of the passed object to be forcibly revoked
 448     // as well to ensure guarantees to callers
 449     revoke_bias(o, false, true, requesting_thread);
 450   }
 451 
 452   log_info(biasedlocking)("* Ending bulk revocation");
 453 
 454   BiasedLocking::Condition status_code = BiasedLocking::BIAS_REVOKED;
 455 
 456   if (attempt_rebias_of_object &&
 457       o->mark()->has_bias_pattern() &&
 458       klass->prototype_header()->has_bias_pattern()) {
 459     markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),
 460                                            klass->prototype_header()->bias_epoch());
 461     o->set_mark(new_mark);
 462     status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;
 463     log_info(biasedlocking)("  Rebiased object toward thread " INTPTR_FORMAT, (intptr_t) requesting_thread);
 464   }
 465 
 466   assert(!o->mark()->has_bias_pattern() ||
 467          (attempt_rebias_of_object && (o->mark()->biased_locker() == requesting_thread)),
 468          "bug in bulk bias revocation");
 469 
 470   return status_code;
 471 }
 472 
 473 
 474 static void clean_up_cached_monitor_info() {
 475   // Walk the thread list clearing out the cached monitors
 476   for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
 477     thr->set_cached_monitor_info(NULL);
 478   }
 479 }
 480 
 481 
 482 class VM_RevokeBias : public VM_Operation {
 483 protected:
 484   Handle* _obj;
 485   GrowableArray<Handle>* _objs;
 486   JavaThread* _requesting_thread;
 487   BiasedLocking::Condition _status_code;
 488 
 489 public:
 490   VM_RevokeBias(Handle* obj, JavaThread* requesting_thread)
 491     : _obj(obj)
 492     , _objs(NULL)
 493     , _requesting_thread(requesting_thread)
 494     , _status_code(BiasedLocking::NOT_BIASED) {}
 495 
 496   VM_RevokeBias(GrowableArray<Handle>* objs, JavaThread* requesting_thread)
 497     : _obj(NULL)
 498     , _objs(objs)
 499     , _requesting_thread(requesting_thread)
 500     , _status_code(BiasedLocking::NOT_BIASED) {}
 501 
 502   virtual VMOp_Type type() const { return VMOp_RevokeBias; }
 503 
 504   virtual bool doit_prologue() {
 505     // Verify that there is actual work to do since the callers just
 506     // give us locked object(s). If we don't find any biased objects
 507     // there is nothing to do and we avoid a safepoint.
 508     if (_obj != NULL) {
 509       markOop mark = (*_obj)()->mark();
 510       if (mark->has_bias_pattern()) {
 511         return true;
 512       }
 513     } else {
 514       for ( int i = 0 ; i < _objs->length(); i++ ) {
 515         markOop mark = (_objs->at(i))()->mark();
 516         if (mark->has_bias_pattern()) {
 517           return true;
 518         }
 519       }
 520     }
 521     return false;
 522   }
 523 
 524   virtual void doit() {
 525     if (_obj != NULL) {
 526       log_info(biasedlocking)("Revoking bias with potentially per-thread safepoint:");
 527       _status_code = revoke_bias((*_obj)(), false, false, _requesting_thread);
 528       clean_up_cached_monitor_info();
 529       return;
 530     } else {
 531       log_info(biasedlocking)("Revoking bias with global safepoint:");
 532       BiasedLocking::revoke_at_safepoint(_objs);
 533     }
 534   }
 535 
 536   BiasedLocking::Condition status_code() const {
 537     return _status_code;
 538   }
 539 };
 540 
 541 
 542 class VM_BulkRevokeBias : public VM_RevokeBias {
 543 private:
 544   bool _bulk_rebias;
 545   bool _attempt_rebias_of_object;
 546 
 547 public:
 548   VM_BulkRevokeBias(Handle* obj, JavaThread* requesting_thread,
 549                     bool bulk_rebias,
 550                     bool attempt_rebias_of_object)
 551     : VM_RevokeBias(obj, requesting_thread)
 552     , _bulk_rebias(bulk_rebias)
 553     , _attempt_rebias_of_object(attempt_rebias_of_object) {}
 554 
 555   virtual VMOp_Type type() const { return VMOp_BulkRevokeBias; }
 556   virtual bool doit_prologue()   { return true; }
 557 
 558   virtual void doit() {
 559     _status_code = bulk_revoke_or_rebias_at_safepoint((*_obj)(), _bulk_rebias, _attempt_rebias_of_object, _requesting_thread);
 560     clean_up_cached_monitor_info();
 561   }
 562 };
 563 
 564 
 565 BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
 566   assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
 567 
 568   // We can revoke the biases of anonymously-biased objects
 569   // efficiently enough that we should not cause these revocations to
 570   // update the heuristics because doing so may cause unwanted bulk
 571   // revocations (which are expensive) to occur.
 572   markOop mark = obj->mark();
 573   if (mark->is_biased_anonymously() && !attempt_rebias) {
 574     // We are probably trying to revoke the bias of this object due to
 575     // an identity hash code computation. Try to revoke the bias
 576     // without a safepoint. This is possible if we can successfully
 577     // compare-and-exchange an unbiased header into the mark word of
 578     // the object, meaning that no other thread has raced to acquire
 579     // the bias of the object.
 580     markOop biased_value       = mark;
 581     markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
 582     markOop res_mark = obj->cas_set_mark(unbiased_prototype, mark);
 583     if (res_mark == biased_value) {
 584       return BIAS_REVOKED;
 585     }
 586   } else if (mark->has_bias_pattern()) {
 587     Klass* k = obj->klass();
 588     markOop prototype_header = k->prototype_header();
 589     if (!prototype_header->has_bias_pattern()) {
 590       // This object has a stale bias from before the bulk revocation
 591       // for this data type occurred. It's pointless to update the
 592       // heuristics at this point so simply update the header with a
 593       // CAS. If we fail this race, the object's bias has been revoked
 594       // by another thread so we simply return and let the caller deal
 595       // with it.
 596       markOop biased_value       = mark;
 597       markOop res_mark = obj->cas_set_mark(prototype_header, mark);
 598       assert(!(*(obj->mark_addr()))->has_bias_pattern(), "even if we raced, should still be revoked");
 599       return BIAS_REVOKED;
 600     } else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
 601       // The epoch of this biasing has expired indicating that the
 602       // object is effectively unbiased. Depending on whether we need
 603       // to rebias or revoke the bias of this object we can do it
 604       // efficiently enough with a CAS that we shouldn't update the
 605       // heuristics. This is normally done in the assembly code but we
 606       // can reach this point due to various points in the runtime
 607       // needing to revoke biases.
 608       if (attempt_rebias) {
 609         assert(THREAD->is_Java_thread(), "");
 610         markOop biased_value       = mark;
 611         markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());
 612         markOop res_mark = obj->cas_set_mark(rebiased_prototype, mark);
 613         if (res_mark == biased_value) {
 614           return BIAS_REVOKED_AND_REBIASED;
 615         }
 616       } else {
 617         markOop biased_value       = mark;
 618         markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
 619         markOop res_mark = obj->cas_set_mark(unbiased_prototype, mark);
 620         if (res_mark == biased_value) {
 621           return BIAS_REVOKED;
 622         }
 623       }
 624     }
 625   }
 626 
 627   HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
 628   if (heuristics == HR_NOT_BIASED) {
 629     return NOT_BIASED;
 630   } else if (heuristics == HR_SINGLE_REVOKE) {
 631     Klass *k = obj->klass();
 632     markOop prototype_header = k->prototype_header();
 633     if (mark->biased_locker() == THREAD &&
 634         prototype_header->bias_epoch() == mark->bias_epoch()) {
 635       // A thread is trying to revoke the bias of an object biased
 636       // toward it, again likely due to an identity hash code
 637       // computation. We can again avoid a safepoint in this case
 638       // since we are only going to walk our own stack. There are no
 639       // races with revocations occurring in other threads because we
 640       // reach no safepoints in the revocation path.
 641       // Also check the epoch because even if threads match, another thread
 642       // can come in with a CAS to steal the bias of an object that has a
 643       // stale epoch.
 644       ResourceMark rm;
 645       log_info(biasedlocking)("Revoking bias by walking my own stack:");
 646       BiasedLocking::Condition cond = revoke_bias(obj(), false, false, (JavaThread*) THREAD);
 647       ((JavaThread*) THREAD)->set_cached_monitor_info(NULL);
 648       assert(cond == BIAS_REVOKED, "why not?");
 649       return cond;
 650     } else {
 651       VM_RevokeBias revoke(&obj, (JavaThread*) THREAD);
 652       VMThread::execute(&revoke);
 653       return revoke.status_code();
 654     }
 655   }
 656 
 657   assert((heuristics == HR_BULK_REVOKE) ||
 658          (heuristics == HR_BULK_REBIAS), "?");
 659   VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
 660                                 (heuristics == HR_BULK_REBIAS),
 661                                 attempt_rebias);
 662   VMThread::execute(&bulk_revoke);
 663   return bulk_revoke.status_code();
 664 }
 665 
 666 
 667 void BiasedLocking::revoke(GrowableArray<Handle>* objs) {
 668   assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
 669   if (objs->length() == 0) {
 670     return;
 671   }
 672   VM_RevokeBias revoke(objs, JavaThread::current());
 673   VMThread::execute(&revoke);
 674 }
 675 
 676 
 677 void BiasedLocking::revoke_at_safepoint(Handle h_obj) {
 678   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 679   oop obj = h_obj();
 680   HeuristicsResult heuristics = update_heuristics(obj, false);
 681   if (heuristics == HR_SINGLE_REVOKE) {
 682     revoke_bias(obj, false, false, NULL);
 683   } else if ((heuristics == HR_BULK_REBIAS) ||
 684              (heuristics == HR_BULK_REVOKE)) {
 685     bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
 686   }
 687   clean_up_cached_monitor_info();
 688 }
 689 
 690 
 691 void BiasedLocking::revoke_at_safepoint(GrowableArray<Handle>* objs) {
 692   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 693   int len = objs->length();
 694   for (int i = 0; i < len; i++) {
 695     oop obj = (objs->at(i))();
 696     HeuristicsResult heuristics = update_heuristics(obj, false);
 697     if (heuristics == HR_SINGLE_REVOKE) {
 698       revoke_bias(obj, false, false, NULL);
 699     } else if ((heuristics == HR_BULK_REBIAS) ||
 700                (heuristics == HR_BULK_REVOKE)) {
 701       bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
 702     }
 703   }
 704   clean_up_cached_monitor_info();
 705 }
 706 
 707 
 708 void BiasedLocking::preserve_marks() {
 709   if (!UseBiasedLocking)
 710     return;
 711 
 712   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 713 
 714   assert(_preserved_oop_stack  == NULL, "double initialization");
 715   assert(_preserved_mark_stack == NULL, "double initialization");
 716 
 717   // In order to reduce the number of mark words preserved during GC
 718   // due to the presence of biased locking, we reinitialize most mark
 719   // words to the class's prototype during GC -- even those which have
 720   // a currently valid bias owner. One important situation where we
 721   // must not clobber a bias is when a biased object is currently
 722   // locked. To handle this case we iterate over the currently-locked
 723   // monitors in a prepass and, if they are biased, preserve their
 724   // mark words here. This should be a relatively small set of objects
 725   // especially compared to the number of objects in the heap.
 726   _preserved_mark_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<markOop>(10, true);
 727   _preserved_oop_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<Handle>(10, true);
 728 
 729   ResourceMark rm;
 730   Thread* cur = Thread::current();
 731   for (JavaThread* thread = Threads::first(); thread != NULL; thread = thread->next()) {
 732     if (thread->has_last_Java_frame()) {
 733       RegisterMap rm(thread);
 734       for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {
 735         GrowableArray<MonitorInfo*> *monitors = vf->monitors();
 736         if (monitors != NULL) {
 737           int len = monitors->length();
 738           // Walk monitors youngest to oldest
 739           for (int i = len - 1; i >= 0; i--) {
 740             MonitorInfo* mon_info = monitors->at(i);
 741             if (mon_info->owner_is_scalar_replaced()) continue;
 742             oop owner = mon_info->owner();
 743             if (owner != NULL) {
 744               markOop mark = owner->mark();
 745               if (mark->has_bias_pattern()) {
 746                 _preserved_oop_stack->push(Handle(cur, owner));
 747                 _preserved_mark_stack->push(mark);
 748               }
 749             }
 750           }
 751         }
 752       }
 753     }
 754   }
 755 }
 756 
 757 
 758 void BiasedLocking::restore_marks() {
 759   if (!UseBiasedLocking)
 760     return;
 761 
 762   assert(_preserved_oop_stack  != NULL, "double free");
 763   assert(_preserved_mark_stack != NULL, "double free");
 764 
 765   int len = _preserved_oop_stack->length();
 766   for (int i = 0; i < len; i++) {
 767     Handle owner = _preserved_oop_stack->at(i);
 768     markOop mark = _preserved_mark_stack->at(i);
 769     owner->set_mark(mark);
 770   }
 771 
 772   delete _preserved_oop_stack;
 773   _preserved_oop_stack = NULL;
 774   delete _preserved_mark_stack;
 775   _preserved_mark_stack = NULL;
 776 }
 777 
 778 
 779 int* BiasedLocking::total_entry_count_addr()                   { return _counters.total_entry_count_addr(); }
 780 int* BiasedLocking::biased_lock_entry_count_addr()             { return _counters.biased_lock_entry_count_addr(); }
 781 int* BiasedLocking::anonymously_biased_lock_entry_count_addr() { return _counters.anonymously_biased_lock_entry_count_addr(); }
 782 int* BiasedLocking::rebiased_lock_entry_count_addr()           { return _counters.rebiased_lock_entry_count_addr(); }
 783 int* BiasedLocking::revoked_lock_entry_count_addr()            { return _counters.revoked_lock_entry_count_addr(); }
 784 int* BiasedLocking::fast_path_entry_count_addr()               { return _counters.fast_path_entry_count_addr(); }
 785 int* BiasedLocking::slow_path_entry_count_addr()               { return _counters.slow_path_entry_count_addr(); }
 786 
 787 
 788 // BiasedLockingCounters
 789 
 790 int BiasedLockingCounters::slow_path_entry_count() {
 791   if (_slow_path_entry_count != 0) {
 792     return _slow_path_entry_count;
 793   }
 794   int sum = _biased_lock_entry_count   + _anonymously_biased_lock_entry_count +
 795             _rebiased_lock_entry_count + _revoked_lock_entry_count +
 796             _fast_path_entry_count;
 797 
 798   return _total_entry_count - sum;
 799 }
 800 
 801 void BiasedLockingCounters::print_on(outputStream* st) {
 802   tty->print_cr("# total entries: %d", _total_entry_count);
 803   tty->print_cr("# biased lock entries: %d", _biased_lock_entry_count);
 804   tty->print_cr("# anonymously biased lock entries: %d", _anonymously_biased_lock_entry_count);
 805   tty->print_cr("# rebiased lock entries: %d", _rebiased_lock_entry_count);
 806   tty->print_cr("# revoked lock entries: %d", _revoked_lock_entry_count);
 807   tty->print_cr("# fast path lock entries: %d", _fast_path_entry_count);
 808   tty->print_cr("# slow path lock entries: %d", slow_path_entry_count());
 809 }