C# Monitor.Enter() 源码追踪 - HelloWorld开发者社区

start

[MethodImpl(MethodImplOptions.InternalCall), SecuritySafeCritical, __DynamicallyInvokable]
public static extern void Enter(object obj);

src/vm/ecall.cpp

FCFuncStart(gMonitorFuncs)
    FCFuncElement("Enter", JIT_MonEnter)
    FCFuncElement("Exit", JIT_MonExit)
    FCFuncElement("TryEnterTimeout", JIT_MonTryEnter)
    FCFuncElement("ObjWait", ObjectNative::WaitTimeout)
    FCFuncElement("ObjPulse", ObjectNative::Pulse)
    FCFuncElement("ObjPulseAll", ObjectNative::PulseAll)
    FCFuncElement("ReliableEnter", JIT_MonReliableEnter)
FCFuncEnd()

next -> JIT_MonEnter

clr/src/vm/jithelpers.cpp

HCIMPL_MONHELPER (JIT_MonEnterWorker_Portable, Object* obj)
{
    .......
}

源码追踪

1.GetHeader()方法获取对象头ObjHeader，在ObjHeader里有对EnterObjMonitor()方法的定义：

clr/src/vm/object.cpp

// 获取请求头
// 访问对象上负偏移量的ObjHeader(因为高速缓存线路)
// Access the ObjHeader which is at a negative offset on the object (because of
// cache lines)
ObjHeader   *GetHeader()
{
    LEAF_CONTRACT;
    return PTR_ObjHeader(PTR_HOST_TO_TADDR(this) - sizeof(ObjHeader));
}

next->查看ObjHeader的EnterObjMonitor的定义

clr/src/vm/syncblk.cpp

void ObjHeader::EnterObjMonitor()
{
    WRAPPER_CONTRACT;
    GetSyncBlock()->EnterMonitor();
}

又调用了GetSyncBlock的EnterMonitor 追下去。

clr/src/vm/syncblk.h

void EnterMonitor()
{
    WRAPPER_CONTRACT;
    m_Monitor.Enter();
}

调用了m_Monitor的Enter

查看m_Monitor的定义：

protected:
AwareLock  m_Monitor;                    // the actual monitor

继续查看AwareLock的Enter方法定义，感觉已经越来越近了😢

void AwareLock::Enter()
{
    CONTRACTL
    {
        INSTANCE_CHECK;
        THROWS;
        GC_TRIGGERS;
        MODE_ANY;
        INJECT_FAULT(COMPlusThrowOM(););
    }
    CONTRACTL_END;

    Thread  *pCurThread = GetThread();

    for (;;) 
    {
        // Read existing lock state.
        volatile LONG state = m_MonitorHeld;

        if (state == 0) 
        {
            // Common case: lock not held, no waiters. Attempt to acquire lock by
            //常见情况:锁没锁，没有服务员。试图获得锁定
            // switching lock bit.
            //开关锁。
            if (FastInterlockCompareExchange((LONG*)&m_MonitorHeld, 1, 0) == 0)//cas 修改值。
            {
                break;
            }
        } 
        else 
        {
            // It's possible to get here with waiters but no lock held, but in this
            // case a signal is about to be fired which will wake up a waiter. So
            // for fairness sake we should wait too.
            // Check first for recursive lock attempts on the same thread.
            if (m_HoldingThread == pCurThread)//如果为当前线程
            {    
                goto Recursion;
            }

            // Attempt to increment this count of waiters then goto contention
            // handling code.
            if (FastInterlockCompareExchange((LONG*)&m_MonitorHeld, (state + 2), state) == state)
            {
                goto MustWait;
            }
        }

    }

    // We get here if we successfully acquired the mutex.
    m_HoldingThread = pCurThread;
    m_Recursion = 1;
    pCurThread->IncLockCount();

#if defined(_DEBUG) && defined(TRACK_SYNC)
    {
        // The best place to grab this is from the ECall frame
        Frame   *pFrame = pCurThread->GetFrame();
        int      caller = (pFrame && pFrame != FRAME_TOP
                            ? (int) pFrame->GetReturnAddress()
                            : -1);
        pCurThread->m_pTrackSync->EnterSync(caller, this);
    }
#endif

    return;

MustWait:
    // Didn't manage to get the mutex, must wait. //继续等待
    EnterEpilog(pCurThread);
    return;

Recursion:
    // Got the mutex via recursive locking on the same thread.
    _ASSERTE(m_Recursion >= 1);
    m_Recursion++;//递归次数加1
#if defined(_DEBUG) && defined(TRACK_SYNC)
    // The best place to grab this is from the ECall frame
    Frame   *pFrame = pCurThread->GetFrame();
    int      caller = (pFrame && pFrame != FRAME_TOP ? (int) pFrame->GetReturnAddress() : -1);
    pCurThread->m_pTrackSync->EnterSync(caller, this);
#endif
}

从上面的代码我们可以看到，先使用GetThread()获取当前的线程，然后取出m_MonitorHeld字段，如果现在没有线程进入临界区，则设置该字段的状态，然后将m_HoldingThread设置为当前线程，从这一点上来这与Win32的过程应该是一样的。

如果从m_MonitorHeld字段看，有线程已经进入临界区则分两种情况：第一，是否已进入的线程如当前线程是同一个线程，如果是，则把m_Recursion递加，如果不是，则通过EnterEpilog( pCurThread)方法，当前线程进入线程等待队列。

通过上面的文字描述和代码的跟踪，在我们的大脑中应该有这样一张图了：

C# Monitor.Enter() 源码追踪