从wait的源码看撤销偏向锁的过程(revoke and rebias)

摘要：源码实现如下表示是否有异常如果是使用了偏向锁，要撤销偏向锁在中可以看到方法的实现。

wait源码实现如下

//TRAPS表示是否有异常
void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
  if (UseBiasedLocking) {
    //如果是使用了偏向锁，要撤销偏向锁
    BiasedLocking::revoke_and_rebias(obj, false, THREAD);
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
  }
 ...

在biasedLocking.cpp中可以看到方法的实现。整体结构划分如下

BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
//1：必须在安全点
assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
//2：读取对象头
markOop mark = obj->mark();
if (mark->is_biased_anonymously() && !attempt_rebias) {
//3:没有线程获取了偏向锁
} else if (mark->has_bias_pattern()) {
//4:已经偏向了
}
//5:没有执行偏向，通过启发式的方式决定到底是执行撤销还是执行rebias
HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
if (heuristics == HR_NOT_BIASED) {
//5.1:偏向状态改成了不需要偏向
} else if (heuristics == HR_SINGLE_REVOKE) {
//5.2:启发式决定执行单次的撤销
}
//6：等到虚拟机运行到safepoint,实际就是执行  VM_BulkRevokeBias 的doit的 bulk_revoke_or_rebias_at_safepoint方法
VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
                              (heuristics == HR_BULK_REBIAS),
                              attempt_rebias);
VMThread::execute(&bulk_revoke);
return bulk_revoke.status_code();
}

这里表示还没有被偏向，并且不是执行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;
//prootype本身构建的是 markOop( no_hash_in_place | no_lock_in_place );
markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
//执行CAS,如果当前对象的mark没有变更，就换成 unbiased_prototype
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) {
  //如果之前的和现在的一样，说明撤销成功，BIAS_REVOKED本身是一个枚举  
return BIAS_REVOKED;
}

//已经被线程偏向了，获取Klass对象，即类本身的头，obj则是它的实例 
Klass* k = Klass::cast(obj->klass());
 markOop prototype_header = k->prototype_header();
 if (!prototype_header->has_bias_pattern()) {
    //对象当前的偏向状态已经过期,并且是不可偏向的，直接设置成已经撤销偏向即可  
 // 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()) {
   //实例的epoch和类本身的epoch值不一样，说明它已经过期，也就是说这个对象当前处于未偏向但是可偏向的状态(rebiasable)   
// 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) {
    //执行rebias     wait希望直接撤销
        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) {
     //当前线程抢到了这个对象的偏向       
        return BIAS_REVOKED_AND_REBIASED;
     }
   } else {
     markOop biased_value       = mark;
     markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
    //CAS撤销偏向锁
     markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
     if (res_mark == biased_value) {
       //撤销了偏向       
    return BIAS_REVOKED;
     }
   }
 }

//启发式的方式决定要做那种操作static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {
  markOop mark = o->mark();
  if (!mark->has_bias_pattern()) {
    //不可偏向直接返回    return HR_NOT_BIASED;
  }
  //控制撤销的次数  

// 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->blueprint();
  jlong cur_time = os::javaTimeMillis();
  //获取上次执行bulk revication的时间  jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();
  //获取执行bulk revocation的次数  int revocation_count = k->biased_lock_revocation_count();
  //定义在globs.hpp,BiasedLockingBulkRebiasThreshold取值为20；BiasedLockingBulkRevokeThreshold取值为40,BiasedLockingDecayTime为25000毫秒  
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.   
 //在执行了一定时间之内，执行的撤销次数没有超过阈值，那么认为可以优先执行bulk rebias,因此将计数回归原始值    
    k->set_biased_lock_revocation_count(0);
    revocation_count = 0;
  }

  // Make revocation count saturate just beyond BiasedLockingBulkRevokeThreshold 
if (revocation_count <= BiasedLockingBulkRevokeThreshold) {
    //计算执行撤销的次数    

revocation_count = k->atomic_incr_biased_lock_revocation_count();
  }

  if (revocation_count == BiasedLockingBulkRevokeThreshold) {
    //达到执行bulk revoke的阈值，执行bulk revoke    return HR_BULK_REVOKE;
  }

  if (revocation_count == BiasedLockingBulkRebiasThreshold) {
    //达到 bulk rebias的阈值，执行bulk rebias   
return HR_BULK_REBIAS;
  }
  //默认执行单次的撤销  
return HR_SINGLE_REVOKE;
}

bulk revoke的关键在于它会遍历所有线程栈的每一帧

static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,
                                                                   bool bulk_rebias,
                                                                   bool attempt_rebias_of_object,
                                                                   JavaThread* requesting_thread) {
…
if (bulk_rebias) {
     ...
  // Now walk all threads" stacks and adjust epochs of any biased  
// and locked objects of this data type we encounter 
//遍历所有的线程
for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
  //遍历线程栈的每一帧，获取所有的监视器
    GrowableArray* 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");
        //更新所有栈中的有偏向锁的epoch        
        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);
}
…
if (attempt_rebias_of_object &&
    o->mark()->has_bias_pattern() &&
    klass->prototype_header()->has_bias_pattern()) {
 //bias_epoch本身则是表示获取了偏向锁
  markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),
                                         klass->prototype_header()->bias_epoch());
  o->set_mark(new_mark);
//执行rebiase
  status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;
 ...
}
}

revoke_bias的执行如下

static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread) {
markOop mark = obj->mark();
…
//偏向锁的头
markOop   biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);
//非偏向锁的头
markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);
…
//获取偏向的线程
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) {
  //没有线程获取，又需要执行rebias，改掉对象头即可
    obj->set_mark(unbiased_prototype);
  }
 ...
  //撤销完毕  
  return BiasedLocking::BIAS_REVOKED;
}
…
//线程活着
//遍历栈帧，获取所有这个线程的监视器,按照最年轻到最老的顺序 
GrowableArray* 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) {
   ...
     // Assume recursive case and fix up highest lock later    
 //当前栈帧存在了这个对象的锁     
    markOop mark = markOopDesc::encode((BasicLock*) NULL);
     highest_lock = mon_info->lock();
     //更新栈中的mark为NULL     
    highest_lock->set_displaced_header(mark);
   }
...
   }
 }
 if (highest_lock != NULL) {
   // Fix up highest lock to contain displaced header and point   
// object at it   
//将最久的那个lock更新为没有偏向  ,栈中  设置了锁记录 
highest_lock->set_displaced_header(unbiased_prototype);
   // Reset object header to point to displaced mark   
//将对象头指向栈中的位置，这样表示就没有偏向了
obj->set_mark(markOopDesc::encode(highest_lock));
   assert(!obj->mark()->has_bias_pattern(), "illegal mark state: stack lock used bias bit");
  ...
 } else {
  ...
   if (allow_rebias) {
     obj->set_mark(biased_prototype);
   } else {
     // Store the unlocked value into the object"s header.    
        obj->set_mark(unbiased_prototype);
   }
 }
//撤销完毕 return BiasedLocking::BIAS_REVOKED;
}