synchronized源码分析

monitorenter

获取锁是通过monitorenter,我们来看下源码实现

IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
  if (PrintBiasedLockingStatistics) {
    Atomic::inc(BiasedLocking::slow_path_entry_count_addr());
  }
  Handle h_obj(thread, elem->obj());
  assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
         "must be NULL or an object");
  //是否使用偏向锁
  if (UseBiasedLocking) {
    // Retry fast entry if bias is revoked to avoid unnecessary inflation
    ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
  } else {
    //开始锁
    ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
  }
  assert(Universe::heap()->is_in_reserved_or_null(elem->obj()),
         "must be NULL or an object");
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
IRT_END

偏向锁

todo

void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
 if (UseBiasedLocking) {
    if (!SafepointSynchronize::is_at_safepoint()) {
      BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
      if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
        return;
      }
    } else {
      assert(!attempt_rebias, "can not rebias toward VM thread");
      BiasedLocking::revoke_at_safepoint(obj);
    }
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 }

 slow_enter (obj, lock, THREAD) ;
}

非偏向锁

void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
  markOop mark = obj->mark();
  assert(!mark->has_bias_pattern(), "should not see bias pattern here");

  //是否处于无锁状态
  if (mark->is_neutral()) {
    // Anticipate successful CAS -- the ST of the displaced mark must
    // be visible <= the ST performed by the CAS.
    //复制markword到栈
    lock->set_displaced_header(mark);
    //如果CAS更新指向栈中的指针成功则释放栈锁,并返回
    if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
      TEVENT (slow_enter: release stacklock) ;
      return ;
    }
    // Fall through to inflate() ...
  } else
  if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
    //如果处于有锁状态,就检查是不是当前线程持有锁,如果是当前线程持有的,就return,然后就能执行同步代码块中的代码了
    assert(lock != mark->locker(), "must not re-lock the same lock");
    assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
    lock->set_displaced_header(NULL);
    return;
  }

#if 0
  // The following optimization isn't particularly useful.
  if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
    lock->set_displaced_header (NULL) ;
    return ;
  }
#endif

  // The object header will never be displaced to this lock,
  // so it does not matter what the value is, except that it
  // must be non-zero to avoid looking like a re-entrant lock,
  // and must not look locked either.
  lock->set_displaced_header(markOopDesc::unused_mark());
  //锁膨胀
  ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
}

锁膨胀

ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
  // Inflate mutates the heap ...
  // Relaxing assertion for bug 6320749.
  assert (Universe::verify_in_progress() ||
          !SafepointSynchronize::is_at_safepoint(), "invariant") ;

    //自旋
  for (;;) {
      const markOop mark = object->mark() ;//获取对象的头部mark_word
      assert (!mark->has_bias_pattern(), "invariant") ;

      // CASE: inflated
      // 如果已经膨胀为重量级锁则直接返回
      if (mark->has_monitor()) {//当前对象已有monitor
          ObjectMonitor * inf = mark->monitor() ;
          assert (inf->header()->is_neutral(), "invariant");
          assert (inf->object() == object, "invariant") ;
          assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
          return inf ;
      }
      //如果当前锁正在膨胀,(轻量级锁膨胀为重量级锁),重新获取mark_word,进入下一次循环
      if (mark == markOopDesc::INFLATING()) {
         TEVENT (Inflate: spin while INFLATING) ;
         ReadStableMark(object) ;
         continue ;
      }

        //如果正在被轻量级锁标记
      if (mark->has_locker()) {
          ObjectMonitor * m = omAlloc (Self) ;//从当前线程中分配ObjectMonitor
          // Optimistically prepare the objectmonitor - anticipate successful CAS
          // We do this before the CAS in order to minimize the length of time
          // in which INFLATING appears in the mark.
          m->Recycle();//置回初始状态
          m->_Responsible  = NULL ;
          m->OwnerIsThread = 0 ;
          m->_recursions   = 0 ;
          m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;   // Consider: maintain by type/class

            //CAS更新状态为膨胀中,如果失败,进入下一次循环
          markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
          if (cmp != mark) {
             omRelease (Self, m, true) ;
             continue ;       // Interference -- just retry
          }

          //已经成功更新了markword状态为膨胀中,它是锁状态更新为0的唯一途径,只有成功更新状态的单线程可以进行锁膨胀。

          //获取栈中的markword
          markOop dmw = mark->displaced_mark_helper() ;
          assert (dmw->is_neutral(), "invariant") ;

          // Setup monitor fields to proper values -- prepare the monitor
          //将监视器字段设置为适当的值
          m->set_header(dmw) ;
          //设置拥有锁的线程
          m->set_owner(mark->locker());
          //设置监视器的对象
          m->set_object(object);
          guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
          object->release_set_mark(markOopDesc::encode(m));//将monitor地址set到对象头部
          if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
          TEVENT(Inflate: overwrite stacklock) ;
          if (TraceMonitorInflation) {
            if (object->is_instance()) {
              ResourceMark rm;
              tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
                (void *) object, (intptr_t) object->mark(),
                object->klass()->external_name());
            }
          }
          return m ;
      }

    // 无锁场景
      assert (mark->is_neutral(), "invariant");
      //分配一个有效对象监视器
      ObjectMonitor * m = omAlloc (Self) ;
      // prepare m for installation - set monitor to initial state
      //重置监视器
      m->Recycle();
      m->set_header(mark);
      m->set_owner(NULL);
      m->set_object(object);
      m->OwnerIsThread = 1 ;
      m->_recursions   = 0 ;
      m->_Responsible  = NULL ;
      m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;       // consider: keep metastats by type/class

      if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
          m->set_object (NULL) ;
          m->set_owner  (NULL) ;
          m->OwnerIsThread = 0 ;
          m->Recycle() ;
          omRelease (Self, m, true) ;
          m = NULL ;
          continue ;
      }

      // Hopefully the performance counters are allocated on distinct
      // cache lines to avoid false sharing on MP systems ...
      if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
      TEVENT(Inflate: overwrite neutral) ;
      if (TraceMonitorInflation) {
        if (object->is_instance()) {
          ResourceMark rm;
          tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
            (void *) object, (intptr_t) object->mark(),
            object->klass()->external_name());
        }
      }
      return m ;
  }
}

简单来说就是在不断重试中获取ObjectMonitor对象,如果已经被锁了则返回,否则如果正在膨胀则继续重试,否则尝试CAS为膨胀中状态,如果成功则继续设置owner等,最后返回。

enter方法

我们看到slow_enter获取ObjectMonitor后调用enter方法,接下来看一下enter方法。

void ATTR ObjectMonitor::enter(TRAPS) {
  // The following code is ordered to check the most common cases first
  // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
  Thread * const Self = THREAD ;
  void * cur ;

    //CAS将owner设置为自己
  cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
  if (cur == NULL) {
     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
     assert (_recursions == 0   , "invariant") ;
     assert (_owner      == Self, "invariant") ;
     // CONSIDER: set or assert OwnerIsThread == 1
     return ;
  }

    //设置owner线程为自己成功了,接下来是重入锁的逻辑
  if (cur == Self) {
     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
     _recursions ++ ;
     return ;
  }
    //第一次设置_owner成功
  if (Self->is_lock_owned ((address)cur)) {
    assert (_recursions == 0, "internal state error");
    _recursions = 1 ;
    // Commute owner from a thread-specific on-stack BasicLockObject address to
    // a full-fledged "Thread *".
    _owner = Self ;
    OwnerIsThread = 1 ;
    return ;
  }

  // We've encountered genuine contention.
  assert (Self->_Stalled == 0, "invariant") ;
  Self->_Stalled = intptr_t(this) ;

  // Try one round of spinning *before* enqueueing Self
  // and before going through the awkward and expensive state
  // transitions.  The following spin is strictly optional ...
  // Note that if we acquire the monitor from an initial spin
  // we forgo posting JVMTI events and firing DTRACE probes.
  if (Knob_SpinEarly && TrySpin (Self) > 0) {
     assert (_owner == Self      , "invariant") ;
     assert (_recursions == 0    , "invariant") ;
     assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
     Self->_Stalled = 0 ;
     return ;
  }

  assert (_owner != Self          , "invariant") ;
  assert (_succ  != Self          , "invariant") ;
  assert (Self->is_Java_thread()  , "invariant") ;
  JavaThread * jt = (JavaThread *) Self ;
  assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
  assert (jt->thread_state() != _thread_blocked   , "invariant") ;
  assert (this->object() != NULL  , "invariant") ;
  assert (_count >= 0, "invariant") ;

  // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
  // Ensure the object-monitor relationship remains stable while there's contention.
  Atomic::inc_ptr(&_count);

  EventJavaMonitorEnter event;

  { // Change java thread status to indicate blocked on monitor enter.
    JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);

    DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
    if (JvmtiExport::should_post_monitor_contended_enter()) {
      JvmtiExport::post_monitor_contended_enter(jt, this);
    }

    OSThreadContendState osts(Self->osthread());
    ThreadBlockInVM tbivm(jt);

    Self->set_current_pending_monitor(this);

    // TODO-FIXME: change the following for(;;) loop to straight-line code.
    for (;;) {
      jt->set_suspend_equivalent();
      // cleared by handle_special_suspend_equivalent_condition()
      // or java_suspend_self()

//竞争锁,如果失败则把本thread包装为node放入到_cxq链表中
      EnterI (THREAD) ;

      if (!ExitSuspendEquivalent(jt)) break ;

      //
      // We have acquired the contended monitor, but while we were
      // waiting another thread suspended us. We don't want to enter
      // the monitor while suspended because that would surprise the
      // thread that suspended us.
      //
          _recursions = 0 ;
      _succ = NULL ;
      exit (false, Self) ;

      jt->java_suspend_self();
    }
    Self->set_current_pending_monitor(NULL);
  }

  Atomic::dec_ptr(&_count);
  assert (_count >= 0, "invariant") ;
  Self->_Stalled = 0 ;

  // Must either set _recursions = 0 or ASSERT _recursions == 0.
  assert (_recursions == 0     , "invariant") ;
  assert (_owner == Self       , "invariant") ;
  assert (_succ  != Self       , "invariant") ;
  assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;

  // The thread -- now the owner -- is back in vm mode.
  // Report the glorious news via TI,DTrace and jvmstat.
  // The probe effect is non-trivial.  All the reportage occurs
  // while we hold the monitor, increasing the length of the critical
  // section.  Amdahl's parallel speedup law comes vividly into play.
  //
  // Another option might be to aggregate the events (thread local or
  // per-monitor aggregation) and defer reporting until a more opportune
  // time -- such as next time some thread encounters contention but has
  // yet to acquire the lock.  While spinning that thread could
  // spinning we could increment JVMStat counters, etc.

  DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
  if (JvmtiExport::should_post_monitor_contended_entered()) {
    JvmtiExport::post_monitor_contended_entered(jt, this);
  }

  if (event.should_commit()) {
    event.set_klass(((oop)this->object())->klass());
    event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
    event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
    event.commit();
  }

  if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
     ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
  }
}
//竞争锁,如果失败则把本thread包装为node放入到_cxq链表中
void ATTR ObjectMonitor::EnterI (TRAPS) {
    Thread * Self = THREAD ;
    assert (Self->is_Java_thread(), "invariant") ;
    assert (((JavaThread *) Self)->thread_state() == _thread_blocked   , "invariant") ;

    // Try the lock - TATAS
    if (TryLock (Self) > 0) {
        assert (_succ != Self              , "invariant") ;
        assert (_owner == Self             , "invariant") ;
        assert (_Responsible != Self       , "invariant") ;
        return ;
    }

    DeferredInitialize () ;

    if (TrySpin (Self) > 0) {
        assert (_owner == Self        , "invariant") ;
        assert (_succ != Self         , "invariant") ;
        assert (_Responsible != Self  , "invariant") ;
        return ;
    }

    // The Spin failed -- Enqueue and park the thread ...
    assert (_succ  != Self            , "invariant") ;
    assert (_owner != Self            , "invariant") ;
    assert (_Responsible != Self      , "invariant") ;

    ObjectWaiter node(Self) ;
    Self->_ParkEvent->reset() ;
    node._prev   = (ObjectWaiter *) 0xBAD ;
    node.TState  = ObjectWaiter::TS_CXQ ;

    // Push "Self" onto the front of the _cxq.
    // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
    // Note that spinning tends to reduce the rate at which threads
    // enqueue and dequeue on EntryList|cxq.
    //用CAS将本node放入到_cxq链表中
    ObjectWaiter * nxt ;
    for (;;) {
        node._next = nxt = _cxq ;
        if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;

        // Interference - the CAS failed because _cxq changed.  Just retry.
        // As an optional optimization we retry the lock.
        if (TryLock (Self) > 0) {
            assert (_succ != Self         , "invariant") ;
            assert (_owner == Self        , "invariant") ;
            assert (_Responsible != Self  , "invariant") ;
            return ;
        }
    }

    if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
        // Try to assume the role of responsible thread for the monitor.
        // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
        Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
    }


    TEVENT (Inflated enter - Contention) ;
    int nWakeups = 0 ;
    int RecheckInterval = 1 ;

    //首先竞争一次锁,如果失败了则调用Self->_ParkEvent->park方法使线程挂起
    for (;;) {

        if (TryLock (Self) > 0) break ;
        assert (_owner != Self, "invariant") ;

        if ((SyncFlags & 2) && _Responsible == NULL) {
           Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
        }

        // park self
        if (_Responsible == Self || (SyncFlags & 1)) {
            TEVENT (Inflated enter - park TIMED) ;
            Self->_ParkEvent->park ((jlong) RecheckInterval) ;
            // Increase the RecheckInterval, but clamp the value.
            RecheckInterval *= 8 ;
            if (RecheckInterval > 1000) RecheckInterval = 1000 ;
        } else {
            TEVENT (Inflated enter - park UNTIMED) ;
            Self->_ParkEvent->park() ;
        }

        if (TryLock(Self) > 0) break ;

        TEVENT (Inflated enter - Futile wakeup) ;
        if (ObjectMonitor::_sync_FutileWakeups != NULL) {
           ObjectMonitor::_sync_FutileWakeups->inc() ;
        }
        ++ nWakeups ;

        if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;


        if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
           Self->_ParkEvent->reset() ;
           OrderAccess::fence() ;
        }
        if (_succ == Self) _succ = NULL ;

        // Invariant: after clearing _succ a thread *must* retry _owner before parking.
        OrderAccess::fence() ;
    }

    assert (_owner == Self      , "invariant") ;
    assert (object() != NULL    , "invariant") ;
    // I'd like to write:
    //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
    // but as we're at a safepoint that's not safe.

    UnlinkAfterAcquire (Self, &node) ;
    if (_succ == Self) _succ = NULL ;

    assert (_succ != Self, "invariant") ;
    if (_Responsible == Self) {
        _Responsible = NULL ;
        OrderAccess::fence(); // Dekker pivot-point
    }

    if (SyncFlags & 8) {
       OrderAccess::fence() ;
    }
    return ;
}

将ObjectMoniter的owner设置为自己,然后

  1. 如果是重入锁,则++重入次数返回
  2. 如果无锁,则设置相关的变量后返回
  3. 无线循环中竞争锁,如果失败则把本thread包装为node放入到_cxq链表中
  4. 开始无限循环,竞争锁成功就退出循环,竞争失败线程挂起,等待被唤醒后继续竞争

refer:https://mp.weixin.qq.com/s?__biz=MzI2NzY4MjM1OQ==&mid=2247485074&idx=1&sn=6a8c0b3cfcc34196f4b2576452700e7c&chksm=eafa51cadd8dd8dc59b6b2599940dde9c27bea9c44b785703200e5de2badae9918caea491bb8&scene=21

https://blog.csdn.net/boling_cavalry/article/details/77793224
https://blog.csdn.net/boling_cavalry/article/details/77897108
https://blog.csdn.net/boling_cavalry/article/details/77995069

https://blog.csdn.net/chengzhang1989/article/details/78703818

https://sq.163yun.com/blog/article/188805878260191232

comments powered by Disqus