synchronize偏向锁底层实现原理

Posted 2022-03-08 JavaEdge.

1 偏向锁的意义

无多线程竞争时，减少不必要的轻量级锁执行路径。大多数情况下，锁不仅不存在多线程竞争，而且总是由同一条线程去多次获得锁，为了让线程获得锁的性能代价更低而引入了偏向锁。

偏向锁主要用来优化同一线程多次申请同一个锁的竞争，即当对象被当做同步锁并有一个线程抢到了锁时，则在Mark Word设置该线程的线程ID、是否偏向锁设置1、锁标志位设置01等信息，此时的Mark Word 存储的就是偏向锁状态信息。

在：

• 创建一个线程并在线程中执行循环监听的场景下

• 或单线程操作一个线程安全集合时

同一线程每次都需获取和释放锁，每次操作都会发生用户态与内核态的切换。

获取偏向锁的场景：

在自己的线程栈生成一条Lock Record，然后Object Reference指向对象头，此时Lock Record与对象头就建立了联系：

① : 先判断Mard Word的Thread ID是否有值

• 没有，则表示当前资源没有被其他线程占用，把当前线程ID等信息记录到Mark Word（这需CAS，可能多条线程修改Mark Word，需要保证原子性）
• 有，则表示当前资源被线程占用，需要判断该线程是不是自己
  • 该线程ID是自己的，则表示可重入，直接获取(此时在自己的线程栈中继续生成一条新的Lock Record)
  • 该线程ID不是自己的，说明出现其他线程竞争，当前持有偏向锁的线程就需要撤销了，即当其他线程尝试获取偏向锁才释放锁

轻量级锁的获取及释放依赖多次的CAS操作，而偏向锁只依赖一次CAS置换ThreadID。

一旦出现多个线程竞争时必须撤销偏向锁，所以：

撤销偏向锁消耗的性能必须 ＜ 之前节省下来的CAS原子操作的性能消耗

不然得不偿失!

JDK6默认开启偏向锁，可通过-XX:-UseBiasedLocking禁用偏向锁。

2 偏向锁的获取

偏向锁的入口，synchronizer.cpp 文件的

ObjectSynchronizer::fast_enter

由BiasedLocking::revoke_and_rebias实现

2.1 markOop mark = obj->mark()

获取对象的markOop数据mark，即对象头的Mark Word

2.2 判断mark是否为可偏向状态

mark的偏向锁的锁标志位为 01

2.3 判断mark中JavaThread的状态

• 若指向当前线程，则执行同步代码块
• 若为空，则走4
• 若指向其它线程，则走5

2.4 执行CAS原子指令

设置mark中JavaThread为当前线程ID。

若CAS成功，则执行同步代码块，否则走5。

2.5 执行CAS失败

说明当前存在多个线程竞争锁，当达到全局安全点（safepoint），获得偏向锁的线程就会被挂起，撤销偏向锁，并升级为轻量级锁。

升级完成后被阻塞在安全点的线程继续执行同步代码块。

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.
  // step1
  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();

3 偏向锁的撤销

只有当其它线程尝试竞争偏向锁时，持有偏向锁的线程才会释放锁。

偏向锁的撤销由BiasedLocking::revoke_at_safepoint实现：

void BiasedLocking::revoke_at_safepoint(Handle h_obj) 
  assert(SafepointSynchronize::is_at_safepoint(), "must only be called 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();

1. 偏向锁的撤销动作必须等待全局安全点（safepoint，GC时会让所有线程阻塞的停顿点）
2. 暂停拥有偏向锁的线程，判断锁对象是否处于被锁定状态
3. 撤销偏向锁，恢复到无锁（标志位 01）或轻量级锁（标志位 00）状态

偏向锁在Java 1.6后默认启用，但在应用程序启动几s后才激活，可关闭延迟：

-XX:BiasedLockingStartupDelay=0

若确定应用程序中所有锁通常情况下处于竞争状态，可关闭偏向锁：

XX:-UseBiasedLocking=false（默认打开）

偏向锁的释放

遍历线程栈的所有Lock Record,把ObjectReference切断,即ObjectReference = null.

把ObjectReference置null，但锁对象的对象头的Mark Word还是没改变，依然偏向之前的线程，那还是没释放锁的嘛，的确是，线程退出临界区时候，并没有释放偏向锁，这么做是为 : 当再次需要获取锁时，只需要简单判断是否是重入，即可快速获取锁，而不用每次都CAS，这也是偏向锁在只有一个线程访问锁的情景下高效的核心。

总结

• 当出现锁资源访问的时候，都会在当前线程栈生成一条Lock Record，并且ObjectReference将指向锁对象的对象头 的Mark Word，该设置可能出现多线程，需CAS操作
• 多线程情况下竞争同一个锁资源，偏向锁的撤销会影响效率
• 偏向锁的重入计数依靠线程栈里Lock Record个数
• 偏向锁撤销失败，最终会升级为轻量级锁
• 偏向锁退出时并没有修改Mark Word，也就是没有释放锁
• 偏向锁相对轻量级锁来说，当同一线程去再次获取锁的时候,不用进行CAS操作,提高了性能.(轻量级锁在同一线程情况下每次去获取锁,在无锁的状态下,每次都要进行一次CAS操作)
• 偏向锁只有遇到其他线程尝试竞争偏向锁时,持有偏向锁的线程才会释放锁,线程不会主动去释放偏向锁
• 偏向锁的撤销是很复杂，成为理解代码的障碍，也阻碍了对同步系统重构，而且现如今基本都是多核系统，偏向锁的劣势越来越明显，所以在Java 15废弃了偏向锁