1 /*
   2  * Copyright (c) 2005, 2017, 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/threadSMR.hpp"
  36 #include "runtime/vframe.hpp"
  37 #include "runtime/vmThread.hpp"
  38 #include "runtime/vm_operations.hpp"
  39 
  40 static bool _biased_locking_enabled = false;
  41 BiasedLockingCounters BiasedLocking::_counters;
  42 
  43 static GrowableArray<Handle>*  _preserved_oop_stack  = NULL;
  44 static GrowableArray<markOop>* _preserved_mark_stack = NULL;
  45 
  46 static void enable_biased_locking(InstanceKlass* k) {
  47   k->set_prototype_header(markOopDesc::biased_locking_prototype());
  48 }
  49 
  50 class VM_EnableBiasedLocking: public VM_Operation {
  51  private:
  52   bool _is_cheap_allocated;
  53  public:
  54   VM_EnableBiasedLocking(bool is_cheap_allocated) { _is_cheap_allocated = is_cheap_allocated; }
  55   VMOp_Type type() const          { return VMOp_EnableBiasedLocking; }
  56   Mode evaluation_mode() const    { return _is_cheap_allocated ? _async_safepoint : _safepoint; }
  57   bool is_cheap_allocated() const { return _is_cheap_allocated; }
  58 
  59   void doit() {
  60     // Iterate the class loader data dictionaries enabling biased locking for all
  61     // currently loaded classes.
  62     ClassLoaderDataGraph::dictionary_classes_do(enable_biased_locking);
  63     // Indicate that future instances should enable it as well
  64     _biased_locking_enabled = true;
  65 
  66     log_info(biasedlocking)("Biased locking enabled");
  67   }
  68 
  69   bool allow_nested_vm_operations() const        { return false; }
  70 };
  71 
  72 
  73 // One-shot PeriodicTask subclass for enabling biased locking
  74 class EnableBiasedLockingTask : public PeriodicTask {
  75  public:
  76   EnableBiasedLockingTask(size_t interval_time) : PeriodicTask(interval_time) {}
  77 
  78   virtual void task() {
  79     // Use async VM operation to avoid blocking the Watcher thread.
  80     // VM Thread will free C heap storage.
  81     VM_EnableBiasedLocking *op = new VM_EnableBiasedLocking(true);
  82     VMThread::execute(op);
  83 
  84     // Reclaim our storage and disenroll ourself
  85     delete this;
  86   }
  87 };
  88 
  89 
  90 void BiasedLocking::init() {
  91   // If biased locking is enabled, schedule a task to fire a few
  92   // seconds into the run which turns on biased locking for all
  93   // currently loaded classes as well as future ones. This is a
  94   // workaround for startup time regressions due to a large number of
  95   // safepoints being taken during VM startup for bias revocation.
  96   // Ideally we would have a lower cost for individual bias revocation
  97   // and not need a mechanism like this.
  98   if (UseBiasedLocking) {
  99     if (BiasedLockingStartupDelay > 0) {
 100       EnableBiasedLockingTask* task = new EnableBiasedLockingTask(BiasedLockingStartupDelay);
 101       task->enroll();
 102     } else {
 103       VM_EnableBiasedLocking op(false);
 104       VMThread::execute(&op);
 105     }
 106   }
 107 }
 108 
 109 
 110 bool BiasedLocking::enabled() {
 111   return _biased_locking_enabled;
 112 }
 113 
 114 // Returns MonitorInfos for all objects locked on this thread in youngest to oldest order
 115 static GrowableArray<MonitorInfo*>* get_or_compute_monitor_info(JavaThread* thread) {
 116   GrowableArray<MonitorInfo*>* info = thread->cached_monitor_info();
 117   if (info != NULL) {
 118     return info;
 119   }
 120 
 121   info = new GrowableArray<MonitorInfo*>();
 122 
 123   // It's possible for the thread to not have any Java frames on it,
 124   // i.e., if it's the main thread and it's already returned from main()
 125   if (thread->has_last_Java_frame()) {
 126     RegisterMap rm(thread);
 127     for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {
 128       GrowableArray<MonitorInfo*> *monitors = vf->monitors();
 129       if (monitors != NULL) {
 130         int len = monitors->length();
 131         // Walk monitors youngest to oldest
 132         for (int i = len - 1; i >= 0; i--) {
 133           MonitorInfo* mon_info = monitors->at(i);
 134           if (mon_info->eliminated()) continue;
 135           oop owner = mon_info->owner();
 136           if (owner != NULL) {
 137             info->append(mon_info);
 138           }
 139         }
 140       }
 141     }
 142   }
 143 
 144   thread->set_cached_monitor_info(info);
 145   return info;
 146 }
 147 
 148 static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread) {
 149   markOop mark = obj->mark();
 150   if (!mark->has_bias_pattern()) {
 151     if (log_is_enabled(Info, biasedlocking)) {
 152       ResourceMark rm;
 153       log_info(biasedlocking)("  (Skipping revocation of object " INTPTR_FORMAT
 154                               ", mark " INTPTR_FORMAT ", type %s"
 155                               ", requesting thread " INTPTR_FORMAT
 156                               " because it's no longer biased)",
 157                               p2i((void *)obj), (intptr_t) mark,
 158                               obj->klass()->external_name(),
 159                               (intptr_t) requesting_thread);
 160     }
 161     return BiasedLocking::NOT_BIASED;
 162   }
 163 
 164   uint age = mark->age();
 165   markOop   biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);
 166   markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);
 167 
 168   // Log at "info" level if not bulk, else "trace" level
 169   if (!is_bulk) {
 170     ResourceMark rm;
 171     log_info(biasedlocking)("Revoking bias of object " INTPTR_FORMAT ", mark "
 172                             INTPTR_FORMAT ", type %s, prototype header " INTPTR_FORMAT
 173                             ", allow rebias %d, requesting thread " INTPTR_FORMAT,
 174                             p2i((void *)obj),
 175                             (intptr_t) mark,
 176                             obj->klass()->external_name(),
 177                             (intptr_t) obj->klass()->prototype_header(),
 178                             (allow_rebias ? 1 : 0),
 179                             (intptr_t) requesting_thread);
 180   } else {
 181     ResourceMark rm;
 182     log_trace(biasedlocking)("Revoking bias of object " INTPTR_FORMAT " , mark "
 183                              INTPTR_FORMAT " , type %s , prototype header " INTPTR_FORMAT
 184                              " , allow rebias %d , requesting thread " INTPTR_FORMAT,
 185                              p2i((void *)obj),
 186                              (intptr_t) mark,
 187                              obj->klass()->external_name(),
 188                              (intptr_t) obj->klass()->prototype_header(),
 189                              (allow_rebias ? 1 : 0),
 190                              (intptr_t) requesting_thread);
 191   }
 192 
 193   JavaThread* biased_thread = mark->biased_locker();
 194   if (biased_thread == NULL) {
 195     // Object is anonymously biased. We can get here if, for
 196     // example, we revoke the bias due to an identity hash code
 197     // being computed for an object.
 198     if (!allow_rebias) {
 199       obj->set_mark(unbiased_prototype);
 200     }
 201     // Log at "info" level if not bulk, else "trace" level
 202     if (!is_bulk) {
 203       log_info(biasedlocking)("  Revoked bias of anonymously-biased object");
 204     } else {
 205       log_trace(biasedlocking)("  Revoked bias of anonymously-biased object");
 206     }
 207     return BiasedLocking::BIAS_REVOKED;
 208   }
 209 
 210   // Handle case where the thread toward which the object was biased has exited
 211   bool thread_is_alive = false;
 212   if (requesting_thread == biased_thread) {
 213     thread_is_alive = true;
 214   } else {
 215     ThreadsListHandle tlh;
 216     JavaThreadIterator jti(tlh.list());
 217     for (JavaThread* cur_thread = jti.first(); cur_thread != NULL; cur_thread = jti.next()) {
 218       if (cur_thread == biased_thread) {
 219         thread_is_alive = true;
 220         break;
 221       }
 222     }
 223   }
 224   if (!thread_is_alive) {
 225     if (allow_rebias) {
 226       obj->set_mark(biased_prototype);
 227     } else {
 228       obj->set_mark(unbiased_prototype);
 229     }
 230     // Log at "info" level if not bulk, else "trace" level
 231     if (!is_bulk) {
 232       log_info(biasedlocking)("  Revoked bias of object biased toward dead thread ("
 233                               PTR_FORMAT ")", p2i(biased_thread));
 234     } else {
 235       log_trace(biasedlocking)("  Revoked bias of object biased toward dead thread ("
 236                                PTR_FORMAT ")", p2i(biased_thread));
 237     }
 238     return BiasedLocking::BIAS_REVOKED;
 239   }
 240 
 241   // Log at "info" level if not bulk, else "trace" level
 242   if (!is_bulk) {
 243     log_info(biasedlocking)("  Revoked bias of object biased toward live thread ("
 244                             PTR_FORMAT ")", p2i(biased_thread));
 245   } else {
 246     log_trace(biasedlocking)("  Revoked bias of object biased toward live thread ("
 247                                PTR_FORMAT ")", p2i(biased_thread));
 248   }
 249 
 250   // Thread owning bias is alive.
 251   // Check to see whether it currently owns the lock and, if so,
 252   // write down the needed displaced headers to the thread's stack.
 253   // Otherwise, restore the object's header either to the unlocked
 254   // or unbiased state.
 255   GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(biased_thread);
 256   BasicLock* highest_lock = NULL;
 257   for (int i = 0; i < cached_monitor_info->length(); i++) {
 258     MonitorInfo* mon_info = cached_monitor_info->at(i);
 259     if (mon_info->owner() == obj) {
 260       log_trace(biasedlocking)("   mon_info->owner (" PTR_FORMAT ") == obj (" PTR_FORMAT ")",
 261                                p2i((void *) mon_info->owner()),
 262                                p2i((void *) obj));
 263       // Assume recursive case and fix up highest lock later
 264       markOop mark = markOopDesc::encode((BasicLock*) NULL);
 265       highest_lock = mon_info->lock();
 266       highest_lock->set_displaced_header(mark);
 267     } else {
 268       log_trace(biasedlocking)("   mon_info->owner (" PTR_FORMAT ") != obj (" PTR_FORMAT ")",
 269                                p2i((void *) mon_info->owner()),
 270                                p2i((void *) obj));
 271     }
 272   }
 273   if (highest_lock != NULL) {
 274     // Fix up highest lock to contain displaced header and point
 275     // object at it
 276     highest_lock->set_displaced_header(unbiased_prototype);
 277     // Reset object header to point to displaced mark.
 278     // Must release storing the lock address for platforms without TSO
 279     // ordering (e.g. ppc).
 280     obj->release_set_mark(markOopDesc::encode(highest_lock));
 281     assert(!obj->mark()->has_bias_pattern(), "illegal mark state: stack lock used bias bit");
 282     // Log at "info" level if not bulk, else "trace" level
 283     if (!is_bulk) {
 284       log_info(biasedlocking)("  Revoked bias of currently-locked object");
 285     } else {
 286       log_trace(biasedlocking)("  Revoked bias of currently-locked object");
 287     }
 288   } else {
 289     // Log at "info" level if not bulk, else "trace" level
 290     if (!is_bulk) {
 291       log_info(biasedlocking)("  Revoked bias of currently-unlocked object");
 292     } else {
 293       log_trace(biasedlocking)("  Revoked bias of currently-unlocked object");
 294     }
 295     if (allow_rebias) {
 296       obj->set_mark(biased_prototype);
 297     } else {
 298       // Store the unlocked value into the object's header.
 299       obj->set_mark(unbiased_prototype);
 300     }
 301   }
 302 
 303   return BiasedLocking::BIAS_REVOKED;
 304 }
 305 
 306 
 307 enum HeuristicsResult {
 308   HR_NOT_BIASED    = 1,
 309   HR_SINGLE_REVOKE = 2,
 310   HR_BULK_REBIAS   = 3,
 311   HR_BULK_REVOKE   = 4
 312 };
 313 
 314 
 315 static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {
 316   markOop mark = o->mark();
 317   if (!mark->has_bias_pattern()) {
 318     return HR_NOT_BIASED;
 319   }
 320 
 321   // Heuristics to attempt to throttle the number of revocations.
 322   // Stages:
 323   // 1. Revoke the biases of all objects in the heap of this type,
 324   //    but allow rebiasing of those objects if unlocked.
 325   // 2. Revoke the biases of all objects in the heap of this type
 326   //    and don't allow rebiasing of these objects. Disable
 327   //    allocation of objects of that type with the bias bit set.
 328   Klass* k = o->klass();
 329   jlong cur_time = os::javaTimeMillis();
 330   jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();
 331   int revocation_count = k->biased_lock_revocation_count();
 332   if ((revocation_count >= BiasedLockingBulkRebiasThreshold) &&
 333       (revocation_count <  BiasedLockingBulkRevokeThreshold) &&
 334       (last_bulk_revocation_time != 0) &&
 335       (cur_time - last_bulk_revocation_time >= BiasedLockingDecayTime)) {
 336     // This is the first revocation we've seen in a while of an
 337     // object of this type since the last time we performed a bulk
 338     // rebiasing operation. The application is allocating objects in
 339     // bulk which are biased toward a thread and then handing them
 340     // off to another thread. We can cope with this allocation
 341     // pattern via the bulk rebiasing mechanism so we reset the
 342     // klass's revocation count rather than allow it to increase
 343     // monotonically. If we see the need to perform another bulk
 344     // rebias operation later, we will, and if subsequently we see
 345     // many more revocation operations in a short period of time we
 346     // will completely disable biasing for this type.
 347     k->set_biased_lock_revocation_count(0);
 348     revocation_count = 0;
 349   }
 350 
 351   // Make revocation count saturate just beyond BiasedLockingBulkRevokeThreshold
 352   if (revocation_count <= BiasedLockingBulkRevokeThreshold) {
 353     revocation_count = k->atomic_incr_biased_lock_revocation_count();
 354   }
 355 
 356   if (revocation_count == BiasedLockingBulkRevokeThreshold) {
 357     return HR_BULK_REVOKE;
 358   }
 359 
 360   if (revocation_count == BiasedLockingBulkRebiasThreshold) {
 361     return HR_BULK_REBIAS;
 362   }
 363 
 364   return HR_SINGLE_REVOKE;
 365 }
 366 
 367 
 368 static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,
 369                                                                    bool bulk_rebias,
 370                                                                    bool attempt_rebias_of_object,
 371                                                                    JavaThread* requesting_thread) {
 372   assert(SafepointSynchronize::is_at_safepoint(), "must be done at safepoint");
 373 
 374   log_info(biasedlocking)("* Beginning bulk revocation (kind == %s) because of object "
 375                           INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
 376                           (bulk_rebias ? "rebias" : "revoke"),
 377                           p2i((void *) o),
 378                           (intptr_t) o->mark(),
 379                           o->klass()->external_name());
 380 
 381   jlong cur_time = os::javaTimeMillis();
 382   o->klass()->set_last_biased_lock_bulk_revocation_time(cur_time);
 383 
 384 
 385   Klass* k_o = o->klass();
 386   Klass* klass = k_o;
 387 
 388   {
 389     ThreadsListHandle tlh;
 390     JavaThreadIterator jti(tlh.list());
 391 
 392     if (bulk_rebias) {
 393       // Use the epoch in the klass of the object to implicitly revoke
 394       // all biases of objects of this data type and force them to be
 395       // reacquired. However, we also need to walk the stacks of all
 396       // threads and update the headers of lightweight locked objects
 397       // with biases to have the current epoch.
 398 
 399       // If the prototype header doesn't have the bias pattern, don't
 400       // try to update the epoch -- assume another VM operation came in
 401       // and reset the header to the unbiased state, which will
 402       // implicitly cause all existing biases to be revoked
 403       if (klass->prototype_header()->has_bias_pattern()) {
 404         int prev_epoch = klass->prototype_header()->bias_epoch();
 405         klass->set_prototype_header(klass->prototype_header()->incr_bias_epoch());
 406         int cur_epoch = klass->prototype_header()->bias_epoch();
 407 
 408         // Now walk all threads' stacks and adjust epochs of any biased
 409         // and locked objects of this data type we encounter
 410         for (JavaThread* thr = jti.first(); thr != NULL; thr = jti.next()) {
 411           GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
 412           for (int i = 0; i < cached_monitor_info->length(); i++) {
 413             MonitorInfo* mon_info = cached_monitor_info->at(i);
 414             oop owner = mon_info->owner();
 415             markOop mark = owner->mark();
 416             if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
 417               // We might have encountered this object already in the case of recursive locking
 418               assert(mark->bias_epoch() == prev_epoch || mark->bias_epoch() == cur_epoch, "error in bias epoch adjustment");
 419               owner->set_mark(mark->set_bias_epoch(cur_epoch));
 420             }
 421           }
 422         }
 423       }
 424 
 425       // At this point we're done. All we have to do is potentially
 426       // adjust the header of the given object to revoke its bias.
 427       revoke_bias(o, attempt_rebias_of_object && klass->prototype_header()->has_bias_pattern(), true, requesting_thread);
 428     } else {
 429       if (log_is_enabled(Info, biasedlocking)) {
 430         ResourceMark rm;
 431         log_info(biasedlocking)("* Disabling biased locking for type %s", klass->external_name());
 432       }
 433 
 434       // Disable biased locking for this data type. Not only will this
 435       // cause future instances to not be biased, but existing biased
 436       // instances will notice that this implicitly caused their biases
 437       // to be revoked.
 438       klass->set_prototype_header(markOopDesc::prototype());
 439 
 440       // Now walk all threads' stacks and forcibly revoke the biases of
 441       // any locked and biased objects of this data type we encounter.
 442       for (JavaThread* thr = jti.first(); thr != NULL; thr = jti.next()) {
 443         GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
 444         for (int i = 0; i < cached_monitor_info->length(); i++) {
 445           MonitorInfo* mon_info = cached_monitor_info->at(i);
 446           oop owner = mon_info->owner();
 447           markOop mark = owner->mark();
 448           if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
 449             revoke_bias(owner, false, true, requesting_thread);
 450           }
 451         }
 452       }
 453 
 454       // Must force the bias of the passed object to be forcibly revoked
 455       // as well to ensure guarantees to callers
 456       revoke_bias(o, false, true, requesting_thread);
 457     }
 458   } // ThreadsListHandle is destroyed here.
 459 
 460   log_info(biasedlocking)("* Ending bulk revocation");
 461 
 462   BiasedLocking::Condition status_code = BiasedLocking::BIAS_REVOKED;
 463 
 464   if (attempt_rebias_of_object &&
 465       o->mark()->has_bias_pattern() &&
 466       klass->prototype_header()->has_bias_pattern()) {
 467     markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),
 468                                            klass->prototype_header()->bias_epoch());
 469     o->set_mark(new_mark);
 470     status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;
 471     log_info(biasedlocking)("  Rebiased object toward thread " INTPTR_FORMAT, (intptr_t) requesting_thread);
 472   }
 473 
 474   assert(!o->mark()->has_bias_pattern() ||
 475          (attempt_rebias_of_object && (o->mark()->biased_locker() == requesting_thread)),
 476          "bug in bulk bias revocation");
 477 
 478   return status_code;
 479 }
 480 
 481 
 482 static void clean_up_cached_monitor_info() {
 483   // Walk the thread list clearing out the cached monitors
 484   ThreadsListHandle tlh;
 485   JavaThreadIterator jti(tlh.list());
 486   for (JavaThread* thr = jti.first(); thr != NULL; thr = jti.next()) {
 487     thr->set_cached_monitor_info(NULL);
 488   }
 489 }
 490 
 491 
 492 class VM_RevokeBias : public VM_Operation {
 493 protected:
 494   Handle* _obj;
 495   GrowableArray<Handle>* _objs;
 496   JavaThread* _requesting_thread;
 497   BiasedLocking::Condition _status_code;
 498 
 499 public:
 500   VM_RevokeBias(Handle* obj, JavaThread* requesting_thread)
 501     : _obj(obj)
 502     , _objs(NULL)
 503     , _requesting_thread(requesting_thread)
 504     , _status_code(BiasedLocking::NOT_BIASED) {}
 505 
 506   VM_RevokeBias(GrowableArray<Handle>* objs, JavaThread* requesting_thread)
 507     : _obj(NULL)
 508     , _objs(objs)
 509     , _requesting_thread(requesting_thread)
 510     , _status_code(BiasedLocking::NOT_BIASED) {}
 511 
 512   virtual VMOp_Type type() const { return VMOp_RevokeBias; }
 513 
 514   virtual bool doit_prologue() {
 515     // Verify that there is actual work to do since the callers just
 516     // give us locked object(s). If we don't find any biased objects
 517     // there is nothing to do and we avoid a safepoint.
 518     if (_obj != NULL) {
 519       markOop mark = (*_obj)()->mark();
 520       if (mark->has_bias_pattern()) {
 521         return true;
 522       }
 523     } else {
 524       for ( int i = 0 ; i < _objs->length(); i++ ) {
 525         markOop mark = (_objs->at(i))()->mark();
 526         if (mark->has_bias_pattern()) {
 527           return true;
 528         }
 529       }
 530     }
 531     return false;
 532   }
 533 
 534   virtual void doit() {
 535     if (_obj != NULL) {
 536       log_info(biasedlocking)("Revoking bias with potentially per-thread safepoint:");
 537       _status_code = revoke_bias((*_obj)(), false, false, _requesting_thread);
 538       clean_up_cached_monitor_info();
 539       return;
 540     } else {
 541       log_info(biasedlocking)("Revoking bias with global safepoint:");
 542       BiasedLocking::revoke_at_safepoint(_objs);
 543     }
 544   }
 545 
 546   BiasedLocking::Condition status_code() const {
 547     return _status_code;
 548   }
 549 };
 550 
 551 
 552 class VM_BulkRevokeBias : public VM_RevokeBias {
 553 private:
 554   bool _bulk_rebias;
 555   bool _attempt_rebias_of_object;
 556 
 557 public:
 558   VM_BulkRevokeBias(Handle* obj, JavaThread* requesting_thread,
 559                     bool bulk_rebias,
 560                     bool attempt_rebias_of_object)
 561     : VM_RevokeBias(obj, requesting_thread)
 562     , _bulk_rebias(bulk_rebias)
 563     , _attempt_rebias_of_object(attempt_rebias_of_object) {}
 564 
 565   virtual VMOp_Type type() const { return VMOp_BulkRevokeBias; }
 566   virtual bool doit_prologue()   { return true; }
 567 
 568   virtual void doit() {
 569     _status_code = bulk_revoke_or_rebias_at_safepoint((*_obj)(), _bulk_rebias, _attempt_rebias_of_object, _requesting_thread);
 570     clean_up_cached_monitor_info();
 571   }
 572 };
 573 
 574 
 575 BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
 576   assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
 577 
 578   // We can revoke the biases of anonymously-biased objects
 579   // efficiently enough that we should not cause these revocations to
 580   // update the heuristics because doing so may cause unwanted bulk
 581   // revocations (which are expensive) to occur.
 582   markOop mark = obj->mark();
 583   if (mark->is_biased_anonymously() && !attempt_rebias) {
 584     // We are probably trying to revoke the bias of this object due to
 585     // an identity hash code computation. Try to revoke the bias
 586     // without a safepoint. This is possible if we can successfully
 587     // compare-and-exchange an unbiased header into the mark word of
 588     // the object, meaning that no other thread has raced to acquire
 589     // the bias of the object.
 590     markOop biased_value       = mark;
 591     markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
 592     markOop res_mark = obj->cas_set_mark(unbiased_prototype, mark);
 593     if (res_mark == biased_value) {
 594       return BIAS_REVOKED;
 595     }
 596   } else if (mark->has_bias_pattern()) {
 597     Klass* k = obj->klass();
 598     markOop prototype_header = k->prototype_header();
 599     if (!prototype_header->has_bias_pattern()) {
 600       // This object has a stale bias from before the bulk revocation
 601       // for this data type occurred. It's pointless to update the
 602       // heuristics at this point so simply update the header with a
 603       // CAS. If we fail this race, the object's bias has been revoked
 604       // by another thread so we simply return and let the caller deal
 605       // with it.
 606       markOop biased_value       = mark;
 607       markOop res_mark = obj->cas_set_mark(prototype_header, mark);
 608       assert(!(*(obj->mark_addr()))->has_bias_pattern(), "even if we raced, should still be revoked");
 609       return BIAS_REVOKED;
 610     } else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
 611       // The epoch of this biasing has expired indicating that the
 612       // object is effectively unbiased. Depending on whether we need
 613       // to rebias or revoke the bias of this object we can do it
 614       // efficiently enough with a CAS that we shouldn't update the
 615       // heuristics. This is normally done in the assembly code but we
 616       // can reach this point due to various points in the runtime
 617       // needing to revoke biases.
 618       if (attempt_rebias) {
 619         assert(THREAD->is_Java_thread(), "");
 620         markOop biased_value       = mark;
 621         markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());
 622         markOop res_mark = obj->cas_set_mark(rebiased_prototype, mark);
 623         if (res_mark == biased_value) {
 624           return BIAS_REVOKED_AND_REBIASED;
 625         }
 626       } else {
 627         markOop biased_value       = mark;
 628         markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
 629         markOop res_mark = obj->cas_set_mark(unbiased_prototype, mark);
 630         if (res_mark == biased_value) {
 631           return BIAS_REVOKED;
 632         }
 633       }
 634     }
 635   }
 636 
 637   HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
 638   if (heuristics == HR_NOT_BIASED) {
 639     return NOT_BIASED;
 640   } else if (heuristics == HR_SINGLE_REVOKE) {
 641     Klass *k = obj->klass();
 642     markOop prototype_header = k->prototype_header();
 643     if (mark->biased_locker() == THREAD &&
 644         prototype_header->bias_epoch() == mark->bias_epoch()) {
 645       // A thread is trying to revoke the bias of an object biased
 646       // toward it, again likely due to an identity hash code
 647       // computation. We can again avoid a safepoint in this case
 648       // since we are only going to walk our own stack. There are no
 649       // races with revocations occurring in other threads because we
 650       // reach no safepoints in the revocation path.
 651       // Also check the epoch because even if threads match, another thread
 652       // can come in with a CAS to steal the bias of an object that has a
 653       // stale epoch.
 654       ResourceMark rm;
 655       log_info(biasedlocking)("Revoking bias by walking my own stack:");
 656       BiasedLocking::Condition cond = revoke_bias(obj(), false, false, (JavaThread*) THREAD);
 657       ((JavaThread*) THREAD)->set_cached_monitor_info(NULL);
 658       assert(cond == BIAS_REVOKED, "why not?");
 659       return cond;
 660     } else {
 661       VM_RevokeBias revoke(&obj, (JavaThread*) THREAD);
 662       VMThread::execute(&revoke);
 663       return revoke.status_code();
 664     }
 665   }
 666 
 667   assert((heuristics == HR_BULK_REVOKE) ||
 668          (heuristics == HR_BULK_REBIAS), "?");
 669   VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
 670                                 (heuristics == HR_BULK_REBIAS),
 671                                 attempt_rebias);
 672   VMThread::execute(&bulk_revoke);
 673   return bulk_revoke.status_code();
 674 }
 675 
 676 
 677 void BiasedLocking::revoke(GrowableArray<Handle>* objs) {
 678   assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
 679   if (objs->length() == 0) {
 680     return;
 681   }
 682   VM_RevokeBias revoke(objs, JavaThread::current());
 683   VMThread::execute(&revoke);
 684 }
 685 
 686 
 687 void BiasedLocking::revoke_at_safepoint(Handle h_obj) {
 688   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 689   oop obj = h_obj();
 690   HeuristicsResult heuristics = update_heuristics(obj, false);
 691   if (heuristics == HR_SINGLE_REVOKE) {
 692     revoke_bias(obj, false, false, NULL);
 693   } else if ((heuristics == HR_BULK_REBIAS) ||
 694              (heuristics == HR_BULK_REVOKE)) {
 695     bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
 696   }
 697   clean_up_cached_monitor_info();
 698 }
 699 
 700 
 701 void BiasedLocking::revoke_at_safepoint(GrowableArray<Handle>* objs) {
 702   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 703   int len = objs->length();
 704   for (int i = 0; i < len; i++) {
 705     oop obj = (objs->at(i))();
 706     HeuristicsResult heuristics = update_heuristics(obj, false);
 707     if (heuristics == HR_SINGLE_REVOKE) {
 708       revoke_bias(obj, false, false, NULL);
 709     } else if ((heuristics == HR_BULK_REBIAS) ||
 710                (heuristics == HR_BULK_REVOKE)) {
 711       bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
 712     }
 713   }
 714   clean_up_cached_monitor_info();
 715 }
 716 
 717 
 718 void BiasedLocking::preserve_marks() {
 719   if (!UseBiasedLocking)
 720     return;
 721 
 722   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
 723 
 724   assert(_preserved_oop_stack  == NULL, "double initialization");
 725   assert(_preserved_mark_stack == NULL, "double initialization");
 726 
 727   // In order to reduce the number of mark words preserved during GC
 728   // due to the presence of biased locking, we reinitialize most mark
 729   // words to the class's prototype during GC -- even those which have
 730   // a currently valid bias owner. One important situation where we
 731   // must not clobber a bias is when a biased object is currently
 732   // locked. To handle this case we iterate over the currently-locked
 733   // monitors in a prepass and, if they are biased, preserve their
 734   // mark words here. This should be a relatively small set of objects
 735   // especially compared to the number of objects in the heap.
 736   _preserved_mark_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<markOop>(10, true);
 737   _preserved_oop_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<Handle>(10, true);
 738 
 739   ResourceMark rm;
 740   Thread* cur = Thread::current();
 741   ThreadsListHandle tlh;
 742   JavaThreadIterator jti(tlh.list());
 743   for (JavaThread* thread = jti.first(); thread != NULL; thread = jti.next()) {
 744     if (thread->has_last_Java_frame()) {
 745       RegisterMap rm(thread);
 746       for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {
 747         GrowableArray<MonitorInfo*> *monitors = vf->monitors();
 748         if (monitors != NULL) {
 749           int len = monitors->length();
 750           // Walk monitors youngest to oldest
 751           for (int i = len - 1; i >= 0; i--) {
 752             MonitorInfo* mon_info = monitors->at(i);
 753             if (mon_info->owner_is_scalar_replaced()) continue;
 754             oop owner = mon_info->owner();
 755             if (owner != NULL) {
 756               markOop mark = owner->mark();
 757               if (mark->has_bias_pattern()) {
 758                 _preserved_oop_stack->push(Handle(cur, owner));
 759                 _preserved_mark_stack->push(mark);
 760               }
 761             }
 762           }
 763         }
 764       }
 765     }
 766   }
 767 }
 768 
 769 
 770 void BiasedLocking::restore_marks() {
 771   if (!UseBiasedLocking)
 772     return;
 773 
 774   assert(_preserved_oop_stack  != NULL, "double free");
 775   assert(_preserved_mark_stack != NULL, "double free");
 776 
 777   int len = _preserved_oop_stack->length();
 778   for (int i = 0; i < len; i++) {
 779     Handle owner = _preserved_oop_stack->at(i);
 780     markOop mark = _preserved_mark_stack->at(i);
 781     owner->set_mark(mark);
 782   }
 783 
 784   delete _preserved_oop_stack;
 785   _preserved_oop_stack = NULL;
 786   delete _preserved_mark_stack;
 787   _preserved_mark_stack = NULL;
 788 }
 789 
 790 
 791 int* BiasedLocking::total_entry_count_addr()                   { return _counters.total_entry_count_addr(); }
 792 int* BiasedLocking::biased_lock_entry_count_addr()             { return _counters.biased_lock_entry_count_addr(); }
 793 int* BiasedLocking::anonymously_biased_lock_entry_count_addr() { return _counters.anonymously_biased_lock_entry_count_addr(); }
 794 int* BiasedLocking::rebiased_lock_entry_count_addr()           { return _counters.rebiased_lock_entry_count_addr(); }
 795 int* BiasedLocking::revoked_lock_entry_count_addr()            { return _counters.revoked_lock_entry_count_addr(); }
 796 int* BiasedLocking::fast_path_entry_count_addr()               { return _counters.fast_path_entry_count_addr(); }
 797 int* BiasedLocking::slow_path_entry_count_addr()               { return _counters.slow_path_entry_count_addr(); }
 798 
 799 
 800 // BiasedLockingCounters
 801 
 802 int BiasedLockingCounters::slow_path_entry_count() {
 803   if (_slow_path_entry_count != 0) {
 804     return _slow_path_entry_count;
 805   }
 806   int sum = _biased_lock_entry_count   + _anonymously_biased_lock_entry_count +
 807             _rebiased_lock_entry_count + _revoked_lock_entry_count +
 808             _fast_path_entry_count;
 809 
 810   return _total_entry_count - sum;
 811 }
 812 
 813 void BiasedLockingCounters::print_on(outputStream* st) {
 814   tty->print_cr("# total entries: %d", _total_entry_count);
 815   tty->print_cr("# biased lock entries: %d", _biased_lock_entry_count);
 816   tty->print_cr("# anonymously biased lock entries: %d", _anonymously_biased_lock_entry_count);
 817   tty->print_cr("# rebiased lock entries: %d", _rebiased_lock_entry_count);
 818   tty->print_cr("# revoked lock entries: %d", _revoked_lock_entry_count);
 819   tty->print_cr("# fast path lock entries: %d", _fast_path_entry_count);
 820   tty->print_cr("# slow path lock entries: %d", slow_path_entry_count());
 821 }