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 /*

  * Copyright (c) 2005, 2014, Oracle and/or its affiliates. All rights reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

  * or visit www.oracle.com if you need additional information or have any

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  */

 #include "precompiled.hpp"

 #include "oops/klass.inline.hpp"

 #include "oops/markOop.hpp"

 #include "runtime/basicLock.hpp"

 #include "runtime/biasedLocking.hpp"

 #include "runtime/task.hpp"

 #include "runtime/vframe.hpp"

 #include "runtime/vmThread.hpp"

 #include "runtime/vm_operations.hpp"

 #include "jfr/support/jfrThreadId.hpp"

 #include "jfr/jfrEvents.hpp"

 static bool _biased_locking_enabled = false;

 BiasedLockingCounters BiasedLocking::_counters;

 static GrowableArray<Handle>*  _preserved_oop_stack  = NULL;

 static GrowableArray<markOop>* _preserved_mark_stack = NULL;

 static void enable_biased_locking(Klass* k) {

   k->set_prototype_header(markOopDesc::biased_locking_prototype());

 }

 class VM_EnableBiasedLocking: public VM_Operation {

  private:

   bool _is_cheap_allocated;

  public:

   VM_EnableBiasedLocking(bool is_cheap_allocated) { _is_cheap_allocated = is_cheap_allocated; }

   VMOp_Type type() const          { return VMOp_EnableBiasedLocking; }

   Mode evaluation_mode() const    { return _is_cheap_allocated ? _async_safepoint : _safepoint; }

   bool is_cheap_allocated() const { return _is_cheap_allocated; }

   void doit() {

     // Iterate the system dictionary enabling biased locking for all

     // currently loaded classes

     SystemDictionary::classes_do(enable_biased_locking);

     // Indicate that future instances should enable it as well

     _biased_locking_enabled = true;

     if (TraceBiasedLocking) {

       tty->print_cr("Biased locking enabled");

     }

   }

   bool allow_nested_vm_operations() const        { return false; }

 };

 // One-shot PeriodicTask subclass for enabling biased locking

 class EnableBiasedLockingTask : public PeriodicTask {

  public:

   EnableBiasedLockingTask(size_t interval_time) : PeriodicTask(interval_time) {}

   virtual void task() {

     // Use async VM operation to avoid blocking the Watcher thread.

     // VM Thread will free C heap storage.

     VM_EnableBiasedLocking *op = new VM_EnableBiasedLocking(true);

     VMThread::execute(op);

     // Reclaim our storage and disenroll ourself

     delete this;

   }

 };

 void BiasedLocking::init() {

   // If biased locking is enabled, schedule a task to fire a few

   // seconds into the run which turns on biased locking for all

   // currently loaded classes as well as future ones. This is a

   // workaround for startup time regressions due to a large number of

   // safepoints being taken during VM startup for bias revocation.

   // Ideally we would have a lower cost for individual bias revocation

   // and not need a mechanism like this.

   if (UseBiasedLocking) {

     if (BiasedLockingStartupDelay > 0) {

       EnableBiasedLockingTask* task = new EnableBiasedLockingTask(BiasedLockingStartupDelay);

       task->enroll();

     } else {

       VM_EnableBiasedLocking op(false);

       VMThread::execute(&op);

     }

   }

 }

 bool BiasedLocking::enabled() {

   return _biased_locking_enabled;

 }

 // Returns MonitorInfos for all objects locked on this thread in youngest to oldest order

 static GrowableArray<MonitorInfo*>* get_or_compute_monitor_info(JavaThread* thread) {

   GrowableArray<MonitorInfo*>* info = thread->cached_monitor_info();

   if (info != NULL) {

     return info;

   }

   info = new GrowableArray<MonitorInfo*>();

   // It's possible for the thread to not have any Java frames on it,

   // i.e., if it's the main thread and it's already returned from main()

   if (thread->has_last_Java_frame()) {

     RegisterMap rm(thread);

     for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {

       GrowableArray<MonitorInfo*> *monitors = vf->monitors();

       if (monitors != NULL) {

         int len = monitors->length();

         // Walk monitors youngest to oldest

         for (int i = len - 1; i >= 0; i--) {

           MonitorInfo* mon_info = monitors->at(i);

           if (mon_info->eliminated()) continue;

           oop owner = mon_info->owner();

           if (owner != NULL) {

             info->append(mon_info);

           }

         }

       }

     }

   }

   thread->set_cached_monitor_info(info);

   return info;

 }

 // After the call, *biased_locker will be set to obj->mark()->biased_locker() if biased_locker != NULL,

 // AND it is a living thread. Otherwise it will not be updated, (i.e. the caller is responsible for initialization).

 static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread, JavaThread** biased_locker) {

   markOop mark = obj->mark();

   if (!mark->has_bias_pattern()) {

     if (TraceBiasedLocking) {

       ResourceMark rm;

       tty->print_cr("  (Skipping revocation of object of type %s because it's no longer biased)",

                     obj->klass()->external_name());

     }

     return BiasedLocking::NOT_BIASED;

   }

   uint age = mark->age();

   markOop   biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);

   markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);

   if (TraceBiasedLocking && (Verbose || !is_bulk)) {

     ResourceMark rm;

     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,

                   p2i((void *)obj), (intptr_t) mark, obj->klass()->external_name(), (intptr_t) obj->klass()->prototype_header(), (allow_rebias ? 1 : 0), (intptr_t) requesting_thread);

   }

   JavaThread* biased_thread = mark->biased_locker();

   if (biased_thread == NULL) {

     // Object is anonymously biased. We can get here if, for

     // example, we revoke the bias due to an identity hash code

     // being computed for an object.

     if (!allow_rebias) {

       obj->set_mark(unbiased_prototype);

     }

     if (TraceBiasedLocking && (Verbose || !is_bulk)) {

       tty->print_cr("  Revoked bias of anonymously-biased object");

     }

     return BiasedLocking::BIAS_REVOKED;

   }

   // Handle case where the thread toward which the object was biased has exited

   bool thread_is_alive = false;

   if (requesting_thread == biased_thread) {

     thread_is_alive = true;

   } else {

     for (JavaThread* cur_thread = Threads::first(); cur_thread != NULL; cur_thread = cur_thread->next()) {

       if (cur_thread == biased_thread) {

         thread_is_alive = true;

         break;

       }

     }

   }

   if (!thread_is_alive) {

     if (allow_rebias) {

       obj->set_mark(biased_prototype);

     } else {

       obj->set_mark(unbiased_prototype);

     }

     if (TraceBiasedLocking && (Verbose || !is_bulk)) {

       tty->print_cr("  Revoked bias of object biased toward dead thread");

     }

     return BiasedLocking::BIAS_REVOKED;

   }

   // Thread owning bias is alive.

   // Check to see whether it currently owns the lock and, if so,

   // write down the needed displaced headers to the thread's stack.

   // Otherwise, restore the object's header either to the unlocked

   // or unbiased state.

   GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(biased_thread);

   BasicLock* highest_lock = NULL;

   for (int i = 0; i < cached_monitor_info->length(); i++) {

     MonitorInfo* mon_info = cached_monitor_info->at(i);

     if (mon_info->owner() == obj) {

       if (TraceBiasedLocking && Verbose) {

         tty->print_cr("   mon_info->owner (" PTR_FORMAT ") == obj (" PTR_FORMAT ")",

                       p2i((void *) mon_info->owner()),

                       p2i((void *) obj));

       }

       // Assume recursive case and fix up highest lock later

       markOop mark = markOopDesc::encode((BasicLock*) NULL);

       highest_lock = mon_info->lock();

       highest_lock->set_displaced_header(mark);

     } else {

       if (TraceBiasedLocking && Verbose) {

         tty->print_cr("   mon_info->owner (" PTR_FORMAT ") != obj (" PTR_FORMAT ")",

                       p2i((void *) mon_info->owner()),

                       p2i((void *) obj));

       }

     }

   }

   if (highest_lock != NULL) {

     // Fix up highest lock to contain displaced header and point

     // object at it

     highest_lock->set_displaced_header(unbiased_prototype);

     // Reset object header to point to displaced mark.

     // Must release storing the lock address for platforms without TSO

     // ordering (e.g. ppc).

     obj->release_set_mark(markOopDesc::encode(highest_lock));

     assert(!obj->mark()->has_bias_pattern(), "illegal mark state: stack lock used bias bit");

     if (TraceBiasedLocking && (Verbose || !is_bulk)) {

       tty->print_cr("  Revoked bias of currently-locked object");

     }

   } else {

     if (TraceBiasedLocking && (Verbose || !is_bulk)) {

       tty->print_cr("  Revoked bias of currently-unlocked object");

     }

     if (allow_rebias) {

       obj->set_mark(biased_prototype);

     } else {

       // Store the unlocked value into the object's header.

       obj->set_mark(unbiased_prototype);

     }

   }

   // If requested, return information on which thread held the bias

   if (biased_locker != NULL) {

     *biased_locker = biased_thread;

   }

   return BiasedLocking::BIAS_REVOKED;

 }

 enum HeuristicsResult {

   HR_NOT_BIASED    = 1,

   HR_SINGLE_REVOKE = 2,

   HR_BULK_REBIAS   = 3,

   HR_BULK_REVOKE   = 4

 };

 static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {

   markOop mark = o->mark();

   if (!mark->has_bias_pattern()) {

     return HR_NOT_BIASED;

   }

   // Heuristics to attempt to throttle the number of revocations.

   // Stages:

   // 1. Revoke the biases of all objects in the heap of this type,

   //    but allow rebiasing of those objects if unlocked.

   // 2. Revoke the biases of all objects in the heap of this type

   //    and don't allow rebiasing of these objects. Disable

   //    allocation of objects of that type with the bias bit set.

   Klass* k = o->klass();

   jlong cur_time = os::javaTimeMillis();

   jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();

   int revocation_count = k->biased_lock_revocation_count();

   if ((revocation_count >= BiasedLockingBulkRebiasThreshold) &&

       (revocation_count <  BiasedLockingBulkRevokeThreshold) &&

       (last_bulk_revocation_time != 0) &&

       (cur_time - last_bulk_revocation_time >= BiasedLockingDecayTime)) {

     // This is the first revocation we've seen in a while of an

     // object of this type since the last time we performed a bulk

     // rebiasing operation. The application is allocating objects in

     // bulk which are biased toward a thread and then handing them

     // off to another thread. We can cope with this allocation

     // pattern via the bulk rebiasing mechanism so we reset the

     // klass's revocation count rather than allow it to increase

     // monotonically. If we see the need to perform another bulk

     // rebias operation later, we will, and if subsequently we see

     // many more revocation operations in a short period of time we

     // will completely disable biasing for this type.

     k->set_biased_lock_revocation_count(0);

     revocation_count = 0;

   }

   // Make revocation count saturate just beyond BiasedLockingBulkRevokeThreshold

   if (revocation_count <= BiasedLockingBulkRevokeThreshold) {

     revocation_count = k->atomic_incr_biased_lock_revocation_count();

   }

   if (revocation_count == BiasedLockingBulkRevokeThreshold) {

     return HR_BULK_REVOKE;

   }

   if (revocation_count == BiasedLockingBulkRebiasThreshold) {

     return HR_BULK_REBIAS;

   }

   return HR_SINGLE_REVOKE;

 }

 static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,

                                                                    bool bulk_rebias,

                                                                    bool attempt_rebias_of_object,

                                                                    JavaThread* requesting_thread) {

   assert(SafepointSynchronize::is_at_safepoint(), "must be done at safepoint");

   if (TraceBiasedLocking) {

     tty->print_cr("* Beginning bulk revocation (kind == %s) because of object "

                   INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",

                   (bulk_rebias ? "rebias" : "revoke"),

                   p2i((void *) o), (intptr_t) o->mark(), o->klass()->external_name());

   }

   jlong cur_time = os::javaTimeMillis();

   o->klass()->set_last_biased_lock_bulk_revocation_time(cur_time);

   Klass* k_o = o->klass();

   Klass* klass = k_o;

   if (bulk_rebias) {

     // Use the epoch in the klass of the object to implicitly revoke

     // all biases of objects of this data type and force them to be

     // reacquired. However, we also need to walk the stacks of all

     // threads and update the headers of lightweight locked objects

     // with biases to have the current epoch.

     // If the prototype header doesn't have the bias pattern, don't

     // try to update the epoch -- assume another VM operation came in

     // and reset the header to the unbiased state, which will

     // implicitly cause all existing biases to be revoked

     if (klass->prototype_header()->has_bias_pattern()) {

       int prev_epoch = klass->prototype_header()->bias_epoch();

       klass->set_prototype_header(klass->prototype_header()->incr_bias_epoch());

       int cur_epoch = klass->prototype_header()->bias_epoch();

       // Now walk all threads' stacks and adjust epochs of any biased

       // and locked objects of this data type we encounter

       for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {

         GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);

         for (int i = 0; i < cached_monitor_info->length(); i++) {

           MonitorInfo* mon_info = cached_monitor_info->at(i);

           oop owner = mon_info->owner();

           markOop mark = owner->mark();

           if ((owner->klass() == k_o) && mark->has_bias_pattern()) {

             // We might have encountered this object already in the case of recursive locking

             assert(mark->bias_epoch() == prev_epoch || mark->bias_epoch() == cur_epoch, "error in bias epoch adjustment");

             owner->set_mark(mark->set_bias_epoch(cur_epoch));

           }

         }

       }

     }

     // At this point we're done. All we have to do is potentially

     // adjust the header of the given object to revoke its bias.

     revoke_bias(o, attempt_rebias_of_object && klass->prototype_header()->has_bias_pattern(), true, requesting_thread, NULL);

   } else {

     if (TraceBiasedLocking) {

       ResourceMark rm;

       tty->print_cr("* Disabling biased locking for type %s", klass->external_name());

     }

     // Disable biased locking for this data type. Not only will this

     // cause future instances to not be biased, but existing biased

     // instances will notice that this implicitly caused their biases

     // to be revoked.

     klass->set_prototype_header(markOopDesc::prototype());

     // Now walk all threads' stacks and forcibly revoke the biases of

     // any locked and biased objects of this data type we encounter.

     for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {

       GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);

       for (int i = 0; i < cached_monitor_info->length(); i++) {

         MonitorInfo* mon_info = cached_monitor_info->at(i);

         oop owner = mon_info->owner();

         markOop mark = owner->mark();

         if ((owner->klass() == k_o) && mark->has_bias_pattern()) {

           revoke_bias(owner, false, true, requesting_thread, NULL);

         }

       }

     }

     // Must force the bias of the passed object to be forcibly revoked

     // as well to ensure guarantees to callers

     revoke_bias(o, false, true, requesting_thread, NULL);

   }

   if (TraceBiasedLocking) {

     tty->print_cr("* Ending bulk revocation");

   }

   BiasedLocking::Condition status_code = BiasedLocking::BIAS_REVOKED;

   if (attempt_rebias_of_object &&

       o->mark()->has_bias_pattern() &&

       klass->prototype_header()->has_bias_pattern()) {

     markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),

                                            klass->prototype_header()->bias_epoch());

     o->set_mark(new_mark);

     status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;

     if (TraceBiasedLocking) {

       tty->print_cr("  Rebiased object toward thread " INTPTR_FORMAT, (intptr_t) requesting_thread);

     }

   }

   assert(!o->mark()->has_bias_pattern() ||

          (attempt_rebias_of_object && (o->mark()->biased_locker() == requesting_thread)),

          "bug in bulk bias revocation");

   return status_code;

 }

 static void clean_up_cached_monitor_info() {

   // Walk the thread list clearing out the cached monitors

   for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {

     thr->set_cached_monitor_info(NULL);

   }

 }

 class VM_RevokeBias : public VM_Operation {

 protected:

   Handle* _obj;

   GrowableArray<Handle>* _objs;

   JavaThread* _requesting_thread;

   BiasedLocking::Condition _status_code;

   traceid _biased_locker_id;

 public:

   VM_RevokeBias(Handle* obj, JavaThread* requesting_thread)

     : _obj(obj)

     , _objs(NULL)

     , _requesting_thread(requesting_thread)

     , _status_code(BiasedLocking::NOT_BIASED)

     , _biased_locker_id(0) {}

   VM_RevokeBias(GrowableArray<Handle>* objs, JavaThread* requesting_thread)

     : _obj(NULL)

     , _objs(objs)

     , _requesting_thread(requesting_thread)

     , _status_code(BiasedLocking::NOT_BIASED)

     , _biased_locker_id(0) {}

   virtual VMOp_Type type() const { return VMOp_RevokeBias; }

   virtual bool doit_prologue() {

     // Verify that there is actual work to do since the callers just

     // give us locked object(s). If we don't find any biased objects

     // there is nothing to do and we avoid a safepoint.

     if (_obj != NULL) {

       markOop mark = (*_obj)()->mark();

       if (mark->has_bias_pattern()) {

         return true;

       }

     } else {

       for ( int i = 0 ; i < _objs->length(); i++ ) {

         markOop mark = (_objs->at(i))()->mark();

         if (mark->has_bias_pattern()) {

           return true;

         }

       }

     }

     return false;

   }

   virtual void doit() {

     if (_obj != NULL) {

       if (TraceBiasedLocking) {

         tty->print_cr("Revoking bias with potentially per-thread safepoint:");

       }

       JavaThread* biased_locker = NULL;

       _status_code = revoke_bias((*_obj)(), false, false, _requesting_thread, &biased_locker);

       if (biased_locker != NULL) {

         _biased_locker_id = JFR_THREAD_ID(biased_locker);

       }

       clean_up_cached_monitor_info();

       return;

     } else {

       if (TraceBiasedLocking) {

         tty->print_cr("Revoking bias with global safepoint:");

       }

       BiasedLocking::revoke_at_safepoint(_objs);

     }

   }

   BiasedLocking::Condition status_code() const {

     return _status_code;

   }

   traceid biased_locker() const {

     return _biased_locker_id;

   }

 };

 class VM_BulkRevokeBias : public VM_RevokeBias {

 private:

   bool _bulk_rebias;

   bool _attempt_rebias_of_object;

 public:

   VM_BulkRevokeBias(Handle* obj, JavaThread* requesting_thread,

                     bool bulk_rebias,

                     bool attempt_rebias_of_object)

     : VM_RevokeBias(obj, requesting_thread)

     , _bulk_rebias(bulk_rebias)

     , _attempt_rebias_of_object(attempt_rebias_of_object) {}

   virtual VMOp_Type type() const { return VMOp_BulkRevokeBias; }

   virtual bool doit_prologue()   { return true; }

   virtual void doit() {

     _status_code = bulk_revoke_or_rebias_at_safepoint((*_obj)(), _bulk_rebias, _attempt_rebias_of_object, _requesting_thread);

     clean_up_cached_monitor_info();

   }

 };

 BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {

   assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");

   // We can revoke the biases of anonymously-biased objects

   // efficiently enough that we should not cause these revocations to

   // update the heuristics because doing so may cause unwanted bulk

   // revocations (which are expensive) to occur.

   markOop mark = obj->mark();

   if (mark->is_biased_anonymously() && !attempt_rebias) {

     // We are probably trying to revoke the bias of this object due to

     // an identity hash code computation. Try to revoke the bias

     // without a safepoint. This is possible if we can successfully

     // compare-and-exchange an unbiased header into the mark word of

     // the object, meaning that no other thread has raced to acquire

     // the bias of the object.

     markOop biased_value       = mark;

     markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());

     markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);

     if (res_mark == biased_value) {

       return BIAS_REVOKED;

     }

   } else if (mark->has_bias_pattern()) {

     Klass* k = obj->klass();

     markOop prototype_header = k->prototype_header();

     if (!prototype_header->has_bias_pattern()) {

       // This object has a stale bias from before the bulk revocation

       // for this data type occurred. It's pointless to update the

       // heuristics at this point so simply update the header with a

       // CAS. If we fail this race, the object's bias has been revoked

       // by another thread so we simply return and let the caller deal

       // with it.

       markOop biased_value       = mark;

       markOop res_mark = (markOop) Atomic::cmpxchg_ptr(prototype_header, obj->mark_addr(), mark);

       assert(!(*(obj->mark_addr()))->has_bias_pattern(), "even if we raced, should still be revoked");

       return BIAS_REVOKED;

     } else if (prototype_header->bias_epoch() != mark->bias_epoch()) {

       // The epoch of this biasing has expired indicating that the

       // object is effectively unbiased. Depending on whether we need

       // to rebias or revoke the bias of this object we can do it

       // efficiently enough with a CAS that we shouldn't update the

       // heuristics. This is normally done in the assembly code but we

       // can reach this point due to various points in the runtime

       // needing to revoke biases.

       if (attempt_rebias) {

         assert(THREAD->is_Java_thread(), "");

         markOop biased_value       = mark;

         markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());

         markOop res_mark = (markOop) Atomic::cmpxchg_ptr(rebiased_prototype, obj->mark_addr(), mark);

         if (res_mark == biased_value) {

           return BIAS_REVOKED_AND_REBIASED;

         }

       } else {

         markOop biased_value       = mark;

         markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());

         markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);

         if (res_mark == biased_value) {

           return BIAS_REVOKED;

         }

       }

     }

   }

   HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);

   if (heuristics == HR_NOT_BIASED) {

     return NOT_BIASED;

   } else if (heuristics == HR_SINGLE_REVOKE) {

     Klass *k = obj->klass();

     markOop prototype_header = k->prototype_header();

     if (mark->biased_locker() == THREAD &&

         prototype_header->bias_epoch() == mark->bias_epoch()) {

       // A thread is trying to revoke the bias of an object biased

       // toward it, again likely due to an identity hash code

       // computation. We can again avoid a safepoint in this case

       // since we are only going to walk our own stack. There are no

       // races with revocations occurring in other threads because we

       // reach no safepoints in the revocation path.

       // Also check the epoch because even if threads match, another thread

       // can come in with a CAS to steal the bias of an object that has a

       // stale epoch.

       ResourceMark rm;

       if (TraceBiasedLocking) {

         tty->print_cr("Revoking bias by walking my own stack:");

       }

       EventBiasedLockSelfRevocation event;

       BiasedLocking::Condition cond = revoke_bias(obj(), false, false, (JavaThread*) THREAD, NULL);

       ((JavaThread*) THREAD)->set_cached_monitor_info(NULL);

       assert(cond == BIAS_REVOKED, "why not?");

       if (event.should_commit()) {

         event.set_lockClass(k);

         event.commit();

       }

       return cond;

     } else {

       EventBiasedLockRevocation event;

       VM_RevokeBias revoke(&obj, (JavaThread*) THREAD);

       VMThread::execute(&revoke);

       if (event.should_commit() && (revoke.status_code() != NOT_BIASED)) {

         event.set_lockClass(k);

         // Subtract 1 to match the id of events committed inside the safepoint

         event.set_safepointId(SafepointSynchronize::safepoint_counter() - 1);

         event.set_previousOwner(revoke.biased_locker());

         event.commit();

       }

       return revoke.status_code();

     }

   }

   assert((heuristics == HR_BULK_REVOKE) ||

          (heuristics == HR_BULK_REBIAS), "?");

   EventBiasedLockClassRevocation event;

   VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,

                                 (heuristics == HR_BULK_REBIAS),

                                 attempt_rebias);

   VMThread::execute(&bulk_revoke);

   if (event.should_commit()) {

     event.set_revokedClass(obj->klass());

     event.set_disableBiasing((heuristics != HR_BULK_REBIAS));

     // Subtract 1 to match the id of events committed inside the safepoint

     event.set_safepointId(SafepointSynchronize::safepoint_counter() - 1);

     event.commit();

   }

   return bulk_revoke.status_code();

 }

 void BiasedLocking::revoke(GrowableArray<Handle>* objs) {

   assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");

   if (objs->length() == 0) {

     return;

   }

   VM_RevokeBias revoke(objs, JavaThread::current());

   VMThread::execute(&revoke);

 }

 void BiasedLocking::revoke_at_safepoint(Handle h_obj) {

   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");

   oop obj = h_obj();

   HeuristicsResult heuristics = update_heuristics(obj, false);

   if (heuristics == HR_SINGLE_REVOKE) {

     revoke_bias(obj, false, false, NULL, NULL);

   } else if ((heuristics == HR_BULK_REBIAS) ||

              (heuristics == HR_BULK_REVOKE)) {

     bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);

   }

   clean_up_cached_monitor_info();

 }

 void BiasedLocking::revoke_at_safepoint(GrowableArray<Handle>* objs) {

   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");

   int len = objs->length();

   for (int i = 0; i < len; i++) {

     oop obj = (objs->at(i))();

     HeuristicsResult heuristics = update_heuristics(obj, false);

     if (heuristics == HR_SINGLE_REVOKE) {

       revoke_bias(obj, false, false, NULL, NULL);

     } else if ((heuristics == HR_BULK_REBIAS) ||

                (heuristics == HR_BULK_REVOKE)) {

       bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);

     }

   }

   clean_up_cached_monitor_info();

 }

 void BiasedLocking::preserve_marks() {

   if (!UseBiasedLocking)

     return;

   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");

   assert(_preserved_oop_stack  == NULL, "double initialization");

   assert(_preserved_mark_stack == NULL, "double initialization");

   // In order to reduce the number of mark words preserved during GC

   // due to the presence of biased locking, we reinitialize most mark

   // words to the class's prototype during GC -- even those which have

   // a currently valid bias owner. One important situation where we

   // must not clobber a bias is when a biased object is currently

   // locked. To handle this case we iterate over the currently-locked

   // monitors in a prepass and, if they are biased, preserve their

   // mark words here. This should be a relatively small set of objects

   // especially compared to the number of objects in the heap.

   _preserved_mark_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<markOop>(10, true);

   _preserved_oop_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<Handle>(10, true);

   ResourceMark rm;

   Thread* cur = Thread::current();

   for (JavaThread* thread = Threads::first(); thread != NULL; thread = thread->next()) {

     if (thread->has_last_Java_frame()) {

       RegisterMap rm(thread);

       for (javaVFrame* vf = thread->last_java_vframe(&rm); vf != NULL; vf = vf->java_sender()) {

         GrowableArray<MonitorInfo*> *monitors = vf->monitors();

         if (monitors != NULL) {

           int len = monitors->length();

           // Walk monitors youngest to oldest

           for (int i = len - 1; i >= 0; i--) {

             MonitorInfo* mon_info = monitors->at(i);

             if (mon_info->owner_is_scalar_replaced()) continue;

             oop owner = mon_info->owner();

             if (owner != NULL) {

               markOop mark = owner->mark();

               if (mark->has_bias_pattern()) {

                 _preserved_oop_stack->push(Handle(cur, owner));

                 _preserved_mark_stack->push(mark);

               }

             }

           }

         }

       }

     }

   }

 }

 void BiasedLocking::restore_marks() {

   if (!UseBiasedLocking)

     return;

   assert(_preserved_oop_stack  != NULL, "double free");

   assert(_preserved_mark_stack != NULL, "double free");

   int len = _preserved_oop_stack->length();

   for (int i = 0; i < len; i++) {

     Handle owner = _preserved_oop_stack->at(i);

     markOop mark = _preserved_mark_stack->at(i);

     owner->set_mark(mark);

   }

   delete _preserved_oop_stack;

   _preserved_oop_stack = NULL;

   delete _preserved_mark_stack;

   _preserved_mark_stack = NULL;

 }

 int* BiasedLocking::total_entry_count_addr()                   { return _counters.total_entry_count_addr(); }

 int* BiasedLocking::biased_lock_entry_count_addr()             { return _counters.biased_lock_entry_count_addr(); }

 int* BiasedLocking::anonymously_biased_lock_entry_count_addr() { return _counters.anonymously_biased_lock_entry_count_addr(); }

 int* BiasedLocking::rebiased_lock_entry_count_addr()           { return _counters.rebiased_lock_entry_count_addr(); }

 int* BiasedLocking::revoked_lock_entry_count_addr()            { return _counters.revoked_lock_entry_count_addr(); }

 int* BiasedLocking::fast_path_entry_count_addr()               { return _counters.fast_path_entry_count_addr(); }

 int* BiasedLocking::slow_path_entry_count_addr()               { return _counters.slow_path_entry_count_addr(); }

 // BiasedLockingCounters

 int BiasedLockingCounters::slow_path_entry_count() {

   if (_slow_path_entry_count != 0) {

     return _slow_path_entry_count;

   }

   int sum = _biased_lock_entry_count   + _anonymously_biased_lock_entry_count +

             _rebiased_lock_entry_count + _revoked_lock_entry_count +

             _fast_path_entry_count;

   return _total_entry_count - sum;

 }

 void BiasedLockingCounters::print_on(outputStream* st) {

   tty->print_cr("# total entries: %d", _total_entry_count);

   tty->print_cr("# biased lock entries: %d", _biased_lock_entry_count);

   tty->print_cr("# anonymously biased lock entries: %d", _anonymously_biased_lock_entry_count);

   tty->print_cr("# rebiased lock entries: %d", _rebiased_lock_entry_count);

   tty->print_cr("# revoked lock entries: %d", _revoked_lock_entry_count);

   tty->print_cr("# fast path lock entries: %d", _fast_path_entry_count);

   tty->print_cr("# slow path lock entries: %d", slow_path_entry_count());

 }