C# Monitor.Enter() 源码追踪

start

1[MethodImpl(MethodImplOptions.InternalCall), SecuritySafeCritical, __DynamicallyInvokable]
2public static extern void Enter(object obj);

src/vm/ecall.cpp

1FCFuncStart(gMonitorFuncs)
2    FCFuncElement("Enter", JIT_MonEnter)
3    FCFuncElement("Exit", JIT_MonExit)
4    FCFuncElement("TryEnterTimeout", JIT_MonTryEnter)
5    FCFuncElement("ObjWait", ObjectNative::WaitTimeout)
6    FCFuncElement("ObjPulse", ObjectNative::Pulse)
7    FCFuncElement("ObjPulseAll", ObjectNative::PulseAll)
8    FCFuncElement("ReliableEnter", JIT_MonReliableEnter)
9FCFuncEnd()

next -> JIT_MonEnter

clr/src/vm/jithelpers.cpp

1HCIMPL_MONHELPER (JIT_MonEnterWorker_Portable, Object* obj)
2{
3	.......
4}

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

1// 获取请求头
2// 访问对象上负偏移量的ObjHeader(因为高速缓存线路)
3// Access the ObjHeader which is at a negative offset on the object (because of
4// cache lines)
5ObjHeader   *GetHeader()
6{
7    LEAF_CONTRACT;
8    return PTR_ObjHeader(PTR_HOST_TO_TADDR(this) - sizeof(ObjHeader));
9}

next->查看ObjHeader的EnterObjMonitor的定义

clr/src/vm/syncblk.cpp

1void ObjHeader::EnterObjMonitor()
2{
3    WRAPPER_CONTRACT;
4    GetSyncBlock()->EnterMonitor();
5}

又调用了GetSyncBlock的EnterMonitor 追下去。

clr/src/vm/syncblk.h

1void EnterMonitor()
2{
3    WRAPPER_CONTRACT;
4    m_Monitor.Enter();
5}

调用了m_Monitor的Enter

查看m_Monitor的定义：

1protected:
2AwareLock  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😢

1void AwareLock::Enter()
2{
3    CONTRACTL
4    {
5        INSTANCE_CHECK;
6        THROWS;
7        GC_TRIGGERS;
8        MODE_ANY;
9        INJECT_FAULT(COMPlusThrowOM(););
10    }
11    CONTRACTL_END;
12
13    Thread  *pCurThread = GetThread();
14
15    for (;;) 
16    {
17        // Read existing lock state.
18        volatile LONG state = m_MonitorHeld;
19
20        if (state == 0) 
21        {
22            // Common case: lock not held, no waiters. Attempt to acquire lock by
23			//常见情况:锁没锁，没有服务员。试图获得锁定
24            // switching lock bit.
25			//开关锁。
26            if (FastInterlockCompareExchange((LONG*)&m_MonitorHeld, 1, 0) == 0)//cas 修改值。
27            {
28                break;
29            }
30        } 
31        else 
32        {
33            // It's possible to get here with waiters but no lock held, but in this
34            // case a signal is about to be fired which will wake up a waiter. So
35            // for fairness sake we should wait too.
36            // Check first for recursive lock attempts on the same thread.
37            if (m_HoldingThread == pCurThread)//如果为当前线程
38            {    
39                goto Recursion;
40            }
41
42            // Attempt to increment this count of waiters then goto contention
43            // handling code.
44            if (FastInterlockCompareExchange((LONG*)&m_MonitorHeld, (state + 2), state) == state)
45            {
46                goto MustWait;
47            }
48        }
49
50    }
51
52    // We get here if we successfully acquired the mutex.
53    m_HoldingThread = pCurThread;
54    m_Recursion = 1;
55    pCurThread->IncLockCount();
56
57#if defined(_DEBUG) && defined(TRACK_SYNC)
58    {
59        // The best place to grab this is from the ECall frame
60        Frame   *pFrame = pCurThread->GetFrame();
61        int      caller = (pFrame && pFrame != FRAME_TOP
62                            ? (int) pFrame->GetReturnAddress()
63                            : -1);
64        pCurThread->m_pTrackSync->EnterSync(caller, this);
65    }
66#endif
67
68    return;
69
70MustWait:
71    // Didn't manage to get the mutex, must wait. //继续等待
72    EnterEpilog(pCurThread);
73    return;
74
75Recursion:
76    // Got the mutex via recursive locking on the same thread.
77    _ASSERTE(m_Recursion >= 1);
78    m_Recursion++;//递归次数加1
79#if defined(_DEBUG) && defined(TRACK_SYNC)
80    // The best place to grab this is from the ECall frame
81    Frame   *pFrame = pCurThread->GetFrame();
82    int      caller = (pFrame && pFrame != FRAME_TOP ? (int) pFrame->GetReturnAddress() : -1);
83    pCurThread->m_pTrackSync->EnterSync(caller, this);
84#endif
85}

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：

confirm

clr/src/vm/syncblk.h

ObjHeader has an index to a SyncBlock. This index is 0 for the bulk of all

ObjHeader有一个指向同步块的索引。大多数情况下，这个指数是0

C# Monitor.Enter() 源码追踪

源码追踪

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