C# Monitor.Wait() 源码追踪 （转载）
source:
释放对象上的锁并阻止当前线程，直到它重新获取该锁。 如果已用指定的超时时间间隔，则线程进入就绪队列。 可以在等待之前退出同步上下文的同步域，随后重新获取该域。
[SecuritySafeCritical]
public static bool Wait(object obj, int millisecondsTimeout, bool exitContext)
{
if (obj == null)
{
throw new ArgumentNullException("obj");
}
return ObjWait(exitContext, millisecondsTimeout, obj);
}
[MethodImpl(MethodImplOptions.InternalCall), SecurityCritical]
private static extern bool ObjWait(bool exitContext, int millisecondsTimeout, object obj);
clr/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()
映射到ObjectNative的方法
clr/src/vm/comobject.cpp -- cpp存储实现
FCIMPL3(FC_BOOL_RET, ObjectNative::WaitTimeout, CLR_BOOL exitContext, INT32 Timeout, Object* pThisUNSAFE)
{
CONTRACTL
{
MODE_COOPERATIVE;
DISABLED(GC_TRIGGERS); // can't use this in an FCALL because we're in forbid gc mode until we setup a H_M_F.
SO_TOLERANT;
THROWS;
}
CONTRACTL_END;
BOOL retVal = FALSE;
OBJECTREF pThis = (OBJECTREF) pThisUNSAFE;
HELPER_METHOD_FRAME_BEGIN_RET_1(pThis);
//-[autocvtpro]-------------------------------------------------------
if (pThis == NULL)
COMPlusThrow(kNullReferenceException, L"NullReference_This");
if ((Timeout < 0) && (Timeout != INFINITE_TIMEOUT))
COMPlusThrowArgumentOutOfRange(L"millisecondsTimeout", L"ArgumentOutOfRange_NeedNonNegNum");
retVal = pThis->Wait(Timeout, exitContext);
//-[autocvtepi]-------------------------------------------------------
HELPER_METHOD_FRAME_END();
FC_RETURN_BOOL(retVal);
}
FCIMPLEND
现在我们看到函数体中最终调用的是pThis->Wait，pThis是个啥玩意呢，通过分析代码，发现它就是WaitTimeOut函数的最后一个参数Object* pThisUNSAFE的一个引用，原来是一个Object类型，那这里的Object和c#的object或者.Net的Object有啥关系，大胆猜想，这其实就是托管Object对应的native Object。而事实也应如此。
那麽废话不多说，我们要来看看此Object的Wait实现，依然避免不了搜索一番，首先我们在object.h中找到了Object类的定义，摘取其说明如下，也印证了刚才的猜想：
/*
* Object
*
* This is the underlying base on which objects are built. The MethodTable
* 这是构建对象的基础。的方法表
* 每个对象都要维护自己的方法表
*
* pointer and the sync block index live here. The sync block index is actually
* 指针和同步块索引在这里。同步块索引实际上是
* at a negative offset to the instance. See syncblk.h for details.
* *在实例的负偏移量处。详见syncbl .h。
*
*/
查看wait方法：
BOOL Wait(INT32 timeOut, BOOL exitContext)
{
WRAPPER_CONTRACT;
return GetHeader()->Wait(timeOut, exitContext);
}
哦，原来是先调用了GetHeader方法获取对象头，然后调用对象头的Wait方法，追下去，GetHeader方法的实现：
// Sync Block & Synchronization services
// 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));
}
看来要想往下追，还必须看对象头ObjHeader类的Wait方法实现：在syncblk.h中找到了其定义，在对应的cpp文件中找到了其相应的实现如下：
BOOL ObjHeader::Wait(INT32 timeOut, BOOL exitContext)
{
CONTRACTL
{
INSTANCE_CHECK;
THROWS;
GC_TRIGGERS;
MODE_ANY;
INJECT_FAULT(COMPlusThrowOM(););
}
CONTRACTL_END;
// The following code may cause GC, so we must fetch the sync block from
// the object now in case it moves.
SyncBlock *pSB = GetBaseObject()->GetSyncBlock();
// GetSyncBlock throws on failure
_ASSERTE(pSB != NULL);
// make sure we own the crst
if (!pSB->DoesCurrentThreadOwnMonitor())
COMPlusThrow(kSynchronizationLockException);
#ifdef _DEBUG
Thread *pThread = GetThread();
DWORD curLockCount = pThread->m_dwLockCount;
#endif
BOOL result = pSB->Wait(timeOut,exitContext);
_ASSERTE (curLockCount == pThread->m_dwLockCount);
return result;
}
看到了嘛！！！！该Wait实现最重要的两行代码终于浮现出来了，它们就是加横线的两行。
第一行 SyncBlock *pSB = GetBaseObject()->GetSyncBlock(); 用来获取对象的索引块；
第二行 BOOL result = pSB->Wait(timeOut,exitContext); 嗯，越来越接近真相，原来又调用了索引块对象的Wait方法。
那继续吧，看看SyncBlock 类型的Wait方法实现，依旧在syncblk.cpp中，如下：
// We maintain two queues for SyncBlock::Wait.
// 1. Inside SyncBlock we queue all threads that are waiting on the SyncBlock.
// When we pulse, we pick the thread from this queue using FIFO.
// 2. We queue all SyncBlocks that a thread is waiting for in Thread::m_WaitEventLink.
// When we pulse a thread, we find the event from this queue to set, and we also
// or in a 1 bit in the syncblock value saved in the queue, so that we can return
// immediately from SyncBlock::Wait if the syncblock has been pulsed.
BOOL SyncBlock::Wait(INT32 timeOut, BOOL exitContext)
{
CONTRACTL
{
INSTANCE_CHECK;
THROWS;
GC_TRIGGERS;
MODE_ANY;
INJECT_FAULT(COMPlusThrowOM());
}
CONTRACTL_END;
Thread *pCurThread = GetThread();
BOOL isTimedOut = FALSE;
BOOL isEnqueued = FALSE;
WaitEventLink waitEventLink;
WaitEventLink *pWaitEventLink;
// As soon as we flip the switch, we are in a race with the GC, which could clean
// up the SyncBlock underneath us -- unless we report the object.
_ASSERTE(pCurThread->PreemptiveGCDisabled());
// Does this thread already wait for this SyncBlock?
WaitEventLink *walk = pCurThread->WaitEventLinkForSyncBlock(this); ✨🤔😀😎✨
if (walk->m_Next) {
if (walk->m_Next->m_WaitSB == this) {
// Wait on the same lock again.
walk->m_Next->m_RefCount ++;
pWaitEventLink = walk->m_Next;
}
else if ((SyncBlock*)(((DWORD_PTR)walk->m_Next->m_WaitSB) & ~1)== this) {
// This thread has been pulsed. No need to wait.
return TRUE;
}
}
else {
// First time this thread is going to wait for this SyncBlock. ✨🤔😀😎✨
CLREvent* hEvent;
if (pCurThread->m_WaitEventLink.m_Next == NULL) {
hEvent = &(pCurThread->m_EventWait); ✨🤔😀😎✨
}
else {
hEvent = GetEventFromEventStore(); ✨🤔😀😎✨
}
waitEventLink.m_WaitSB = this;
waitEventLink.m_EventWait = hEvent;
waitEventLink.m_Thread = pCurThread;
waitEventLink.m_Next = NULL;
waitEventLink.m_LinkSB.m_pNext = NULL;
waitEventLink.m_RefCount = 1;
pWaitEventLink = &waitEventLink;
walk->m_Next = pWaitEventLink;
// Before we enqueue it (and, thus, before it can be dequeued), reset the event
// that will awaken us.
hEvent->Reset();
// This thread is now waiting on this sync block
ThreadQueue::EnqueueThread(pWaitEventLink, this);✨🤔😀😎✨
isEnqueued = TRUE;
}
_ASSERTE ((SyncBlock*)((DWORD_PTR)walk->m_Next->m_WaitSB & ~1)== this);
PendingSync syncState(walk);
OBJECTREF obj = m_Monitor.GetOwningObject();
m_Monitor.IncrementTransientPrecious();
GCPROTECT_BEGIN(obj);
{
GCX_PREEMP();
// remember how many times we synchronized
syncState.m_EnterCount = LeaveMonitorCompletely();
_ASSERTE(syncState.m_EnterCount > 0);
Context* targetContext = pCurThread->GetContext();
_ASSERTE(targetContext);
Context* defaultContext = pCurThread->GetDomain()->GetDefaultContext();
_ASSERTE(defaultContext);
if (exitContext &&
targetContext != defaultContext)
{
Context::MonitorWaitArgs waitArgs = {timeOut, &syncState, &isTimedOut};
Context::CallBackInfo callBackInfo = {Context::MonitorWait_callback, (void*) &waitArgs};
Context::RequestCallBack(CURRENT_APPDOMAIN_ID, defaultContext, &callBackInfo);
}
else
{
isTimedOut = pCurThread->Block(timeOut, &syncState); ✨🤔😀😎✨
}
}
GCPROTECT_END();
m_Monitor.DecrementTransientPrecious();
return !isTimedOut;
}
拜托，当你看到函数又臭又长的时候..尤其时还不熟悉的时候，一定要看函数的描述，该函数开头之前的函数说明解释了两件事情：
1.在SyncBlock 内部维护了一个等待所有这个SyncBlock 的线程队列，当调用pulse的时候(如Monitor.Pulse)会从该队列取出下一个线程，方式是先进先出。
2.使用另外一个队列维护所有有线程正在waiting的SyncBlock ,队列类型为WaitEventLink(也即是Thread::m_WaitEventLink的类型），一旦有pulse调用，会从该队列取出一个Event并set.
现在再来看函数代码部分，重点看横线的代码行：
WaitEventLink *walk = pCurThread->WaitEventLinkForSyncBlock(this);
先检查当前线程是否已经在等待对象的同步索引块，本示例中显然是第一次，然后通过
hEvent = &(pCurThread->m_EventWait);或者
hEvent = GetEventFromEventStore();获取一个等待事件对象
之后会走 ThreadQueue::EnqueueThread(pWaitEventLink, this);
从而把当前线程加入到等待队列，这时候我的脑海中又想起来MSDN上对Monitor.Wait的描述：
当线程调用 Wait 时，它释放对象的锁并进入对象的等待队列。 对象的就绪队列中的下一个线程（如果有）获取锁并拥有对对象的独占使用。
这下大概能对上号了吧。
在函数最后，还是调用了isTimedOut = pCurThread->Block(timeOut, &syncState);以实现实现当前线程的等待（或曰阻塞）。
所以依旧要看看这个Block方法的实现：
// Called out of SyncBlock::Wait() to block this thread until the Notify occurs.
BOOL Thread::Block(INT32 timeOut, PendingSync *syncState)
{
WRAPPER_CONTRACT;
_ASSERTE(this == GetThread());
// Before calling Block, the SyncBlock queued us onto it's list of waiting threads.
// However, before calling Block the SyncBlock temporarily left the synchronized
// region. This allowed threads to enter the region and call Notify, in which
// case we may have been signalled before we entered the Wait. So we aren't in the
// m_WaitSB list any longer. Not a problem: the following Wait will return
// immediately. But it means we cannot enforce the following assertion:
// _ASSERTE(m_WaitSB != NULL);
return (Wait(syncState->m_WaitEventLink->m_Next->m_EventWait, timeOut, syncState) != WAIT_OBJECT_0);
}
Block又调用了Thread的Wait方法：
// Return whether or not a timeout occured. TRUE=>we waited successfully
DWORD Thread::Wait(CLREvent *pEvent, INT32 timeOut, PendingSync *syncInfo)
{
WRAPPER_CONTRACT;
DWORD dwResult;
DWORD dwTimeOut32;
_ASSERTE(timeOut >= 0 || timeOut == INFINITE_TIMEOUT);
dwTimeOut32 = (timeOut == INFINITE_TIMEOUT
? INFINITE
: (DWORD) timeOut);
dwResult = pEvent->Wait(dwTimeOut32, TRUE /*alertable*/, syncInfo);✨🤔😀😎✨
// Either we succeeded in the wait, or we timed out
_ASSERTE((dwResult == WAIT_OBJECT_0) ||
(dwResult == WAIT_TIMEOUT));
return dwResult;
}
Wait又调用了pEvent的Wait方法，注意这里的pEvent是CLREvent类型，而该参数的值则是之前在SyncBlock::Wait获取的等待事件对象。这里我们可以大胆猜测CLREvent对应的其实是一个内核事件对象。
CLREvent的Wait实现如下，有点长，看关键的横线代码行：
DWORD CLREvent::Wait(DWORD dwMilliseconds, BOOL alertable, PendingSync *syncState)
{
WRAPPER_CONTRACT;
return WaitEx(dwMilliseconds, alertable?WaitMode_Alertable:WaitMode_None,syncState);
}
紧接着WaitEx的实现如下：
DWORD CLREvent::WaitEx(DWORD dwMilliseconds, WaitMode mode, PendingSync *syncState)
{
BOOL alertable = (mode & WaitMode_Alertable)!=0;
CONTRACTL
{
if (alertable)
{
THROWS; // Thread::DoAppropriateWait can throw
}
else
{
NOTHROW;
}
if (GetThread())
{
if (alertable)
GC_TRIGGERS;
else
GC_NOTRIGGER;
}
else
{
DISABLED(GC_TRIGGERS);
}
SO_TOLERANT;
PRECONDITION(m_handle != INVALID_HANDLE_VALUE); // Handle has to be valid
}
CONTRACTL_END;
_ASSERTE(Thread::AllowCallout());
Thread *pThread = GetThread();
#ifdef _DEBUG
// If a CLREvent is OS event only, we can not wait for the event on a managed thread
if (IsOSEvent())
_ASSERTE (!pThread);
#endif
_ASSERTE (pThread || !g_fEEStarted || dbgOnly_IsSpecialEEThread());
if (IsOSEvent() || !CLRSyncHosted()) {
if (pThread && alertable) {
DWORD dwRet = WAIT_FAILED;
BEGIN_SO_INTOLERANT_CODE_NOTHROW (pThread, return WAIT_FAILED;);
dwRet = pThread->DoAppropriateWait(1, &m_handle, FALSE, dwMilliseconds, ✨🤔😀😎✨
mode, ✨🤔😀😎✨
syncState); ✨🤔😀😎✨
END_SO_INTOLERANT_CODE;
return dwRet;
}
else {
_ASSERTE (syncState == NULL);
return CLREventWaitHelper(m_handle,dwMilliseconds,alertable);
}
}
else {
if (pThread && alertable) {
DWORD dwRet = WAIT_FAILED;
BEGIN_SO_INTOLERANT_CODE_NOTHROW (pThread, return WAIT_FAILED;);
dwRet = pThread->DoAppropriateWait(IsAutoEvent()?HostAutoEventWait:HostManualEventWait, ✨🤔😀😎✨
m_handle,dwMilliseconds, ✨🤔😀😎✨
mode, ✨🤔😀😎✨
syncState); ✨🤔😀😎✨
END_SO_INTOLERANT_CODE;
return dwRet;
}
else {
_ASSERTE (syncState == NULL);
DWORD option = 0;
if (alertable) {
option |= WAIT_ALERTABLE;
}
if (IsAutoEvent()) {
return HostAutoEventWait((IHostAutoEvent*)m_handle,dwMilliseconds, option);
}
else {
return HostManualEventWait((IHostManualEvent*)m_handle,dwMilliseconds, option);
}
}
}
}
这里又调用了Thread的DoAppropriateWait；
DoAppropriateWait的实现如下：
DWORD Thread::DoAppropriateWait(int countHandles, HANDLE *handles, BOOL waitAll,
DWORD millis, WaitMode mode, PendingSync *syncState)
{
STATIC_CONTRACT_THROWS;
STATIC_CONTRACT_GC_TRIGGERS;
INDEBUG(BOOL alertable = (mode & WaitMode_Alertable) != 0;);
_ASSERTE(alertable || syncState == 0);
DWORD dwRet = (DWORD) -1;
EE_TRY_FOR_FINALLY {
dwRet =DoAppropriateWaitWorker(countHandles, handles, waitAll, millis, mode); ✨🤔😀😎✨
}
EE_FINALLY {
if (syncState) {
if (!GOT_EXCEPTION() &&
dwRet >= WAIT_OBJECT_0 && dwRet < (DWORD)(WAIT_OBJECT_0 + countHandles)) {
// This thread has been removed from syncblk waiting list by the signalling thread
syncState->Restore(FALSE);
}
else
syncState->Restore(TRUE);
}
_ASSERTE (dwRet != WAIT_IO_COMPLETION);
}
EE_END_FINALLY;
return(dwRet);
}
then，DoAppropriateWaitWorker的实现如下，有点长，只看最关键那一句：
DWORD Thread::DoAppropriateWaitWorker(int countHandles, HANDLE *handles, BOOL waitAll,
DWORD millis, WaitMode mode)
{
CONTRACTL {
THROWS;
GC_TRIGGERS;
}
CONTRACTL_END;
DWORD ret = 0;
BOOL alertable = (mode & WaitMode_Alertable)!= 0;
BOOL ignoreSyncCtx = (mode & WaitMode_IgnoreSyncCtx)!= 0;
// Unless the ignoreSyncCtx flag is set, first check to see if there is a synchronization
// context on the current thread and if there is, dispatch to it to do the wait.
// If the wait is non alertable we cannot forward the call to the sync context
// since fundamental parts of the system (such as the GC) rely on non alertable
// waits not running any managed code. Also if we are past the point in shutdown were we
// are allowed to run managed code then we can't forward the call to the sync context.
if (!ignoreSyncCtx && alertable && CanRunManagedCode(FALSE))
{
GCX_COOP();
BOOL fSyncCtxPresent = FALSE;
OBJECTREF SyncCtxObj = NULL;
GCPROTECT_BEGIN(SyncCtxObj)
{
GetSynchronizationContext(&SyncCtxObj);
if (SyncCtxObj != NULL)
{
SYNCHRONIZATIONCONTEXTREF syncRef = (SYNCHRONIZATIONCONTEXTREF)SyncCtxObj;
if (syncRef->IsWaitNotificationRequired())
{
fSyncCtxPresent = TRUE;
ret = DoSyncContextWait(&SyncCtxObj, countHandles, handles, waitAll, millis);
}
}
}
GCPROTECT_END();
if (fSyncCtxPresent)
return ret;
}
GCX_PREEMP();
if(alertable)
{
DoAppropriateWaitWorkerAlertableHelper(mode);
}
LeaveRuntimeHolder holder((size_t)WaitForMultipleObjectsEx);
StateHolder<MarkOSAlertableWait,UnMarkOSAlertableWait> OSAlertableWait(alertable);
ThreadStateHolder tsh(alertable, TS_Interruptible | TS_Interrupted);
ULONGLONG dwStart = 0, dwEnd;
retry:
if (millis != INFINITE)
{
dwStart = CLRGetTickCount64();
}
ret = DoAppropriateAptStateWait(countHandles, handles, waitAll, millis, mode);✨🤔😀😎✨
if (ret == WAIT_IO_COMPLETION)
{
_ASSERTE (alertable);
if (m_State & TS_Interrupted)
{
HandleThreadInterrupt(mode & WaitMode_ADUnload);
}
// We could be woken by some spurious APC or an EE APC queued to
// interrupt us. In the latter case the TS_Interrupted bit will be set
// in the thread state bits. Otherwise we just go back to sleep again.
if (millis != INFINITE)
{
dwEnd = CLRGetTickCount64();
if (dwEnd >= dwStart + millis)
{
ret = WAIT_TIMEOUT;
goto WaitCompleted;
}
else
{
millis -= (DWORD)(dwEnd - dwStart);
}
}
goto retry;
}
_ASSERTE((ret >= WAIT_OBJECT_0 && ret < (WAIT_OBJECT_0 + (DWORD)countHandles)) ||
(ret >= WAIT_ABANDONED && ret < (WAIT_ABANDONED + (DWORD)countHandles)) ||
(ret == WAIT_TIMEOUT) || (ret == WAIT_FAILED));
// countHandles is used as an unsigned -- it should never be negative.
_ASSERTE(countHandles >= 0);
if (ret == WAIT_FAILED)
{
DWORD errorCode = ::GetLastError();
if (errorCode == ERROR_INVALID_PARAMETER)
{
if (CheckForDuplicateHandles(countHandles, handles))
COMPlusThrow(kDuplicateWaitObjectException);
else
COMPlusThrowHR(HRESULT_FROM_WIN32(errorCode));
}
else if (errorCode == ERROR_ACCESS_DENIED)
{
// A Win32 ACL could prevent us from waiting on the handle.
COMPlusThrow(kUnauthorizedAccessException);
}
_ASSERTE(errorCode == ERROR_INVALID_HANDLE);
if (countHandles == 1)
ret = WAIT_OBJECT_0;
else if (waitAll)
{
// Probe all handles with a timeout of zero. When we find one that's
// invalid, move it out of the list and retry the wait.
#ifdef _DEBUG
BOOL fFoundInvalid = FALSE;
#endif
for (int i = 0; i < countHandles; i++)
{
// WaitForSingleObject won't pump memssage; we already probe enough space
// before calling this function and we don't want to fail here, so we don't
// do a transition to tolerant code here
DWORD subRet = WaitForSingleObject (handles[i], 0);
if (subRet != WAIT_FAILED)
continue;
_ASSERTE(::GetLastError() == ERROR_INVALID_HANDLE);
if ((countHandles - i - 1) > 0)
memmove(&handles[i], &handles[i+1], (countHandles - i - 1) * sizeof(HANDLE));
countHandles--;
#ifdef _DEBUG
fFoundInvalid = TRUE;
#endif
break;
}
_ASSERTE(fFoundInvalid);
// Compute the new timeout value by assume that the timeout
// is not large enough for more than one wrap
dwEnd = CLRGetTickCount64();
if (millis != INFINITE)
{
if (dwEnd >= dwStart + millis)
{
ret = WAIT_TIMEOUT;
goto WaitCompleted;
}
else
{
millis -= (DWORD)(dwEnd - dwStart);
}
}
goto retry;
}
else
{
// Probe all handles with a timeout as zero, succeed with the first
// handle that doesn't timeout.
ret = WAIT_OBJECT_0;
int i;
for (i = 0; i < countHandles; i++)
{
TryAgain:
// WaitForSingleObject won't pump memssage; we already probe enough space
// before calling this function and we don't want to fail here, so we don't
// do a transition to tolerant code here
DWORD subRet = WaitForSingleObject (handles[i], 0);
if ((subRet == WAIT_OBJECT_0) || (subRet == WAIT_FAILED))
break;
if (subRet == WAIT_ABANDONED)
{
ret = (ret - WAIT_OBJECT_0) + WAIT_ABANDONED;
break;
}
// If we get alerted it just masks the real state of the current
// handle, so retry the wait.
if (subRet == WAIT_IO_COMPLETION)
goto TryAgain;
_ASSERTE(subRet == WAIT_TIMEOUT);
ret++;
}
_ASSERTE(i != countHandles);
}
}
WaitCompleted:
_ASSERTE((ret != WAIT_TIMEOUT) || (millis != INFINITE));
return ret;
}
then， 还要看 DoAppropriateAptStateWait(countHandles, handles, waitAll, millis, mode)的实现：
DWORD Thread::DoAppropriateAptStateWait(int numWaiters, HANDLE* pHandles, BOOL bWaitAll,
DWORD timeout, WaitMode mode)
{
CONTRACTL {
THROWS;
GC_TRIGGERS;
}
CONTRACTL_END;
BOOL alertable = (mode&WaitMode_Alertable)!=0;
return WaitForMultipleObjectsEx_SO_TOLERANT(numWaiters, pHandles,bWaitAll, timeout,alertable);
}
then，再看WaitForMultipleObjectsEx_SO_TOLERANT的实现：
DWORD WaitForMultipleObjectsEx_SO_TOLERANT (DWORD nCount, HANDLE *lpHandles, BOOL bWaitAll,DWORD dwMilliseconds, BOOL bAlertable)
{
DWORD dwRet = WAIT_FAILED;
DWORD lastError = 0;
BEGIN_SO_TOLERANT_CODE (GetThread ());
dwRet = ::WaitForMultipleObjectsEx (nCount, lpHandles, bWaitAll, dwMilliseconds, bAlertable);
lastError = ::GetLastError();
END_SO_TOLERANT_CODE;
// END_SO_TOLERANT_CODE overwrites lasterror. Let's reset it.
::SetLastError(lastError);
return dwRet;
}
到这里，万水千山，我们终于搞清楚Monitor.Wait的大概实现原理（事实上我们只捋了一遍本文示例中Monitor.Enter的调用stack），内部最终还是调用了WaitForMultipleObjectsEx，不过要注意CLREvent::WaitEx的实现有好几个分支，根据情况的不同，最后调的并不一定是WaitForMultipleObjectsEx，也有可能是CLREventWaitHelper->WaitForSingleObjectEx等等。
转载
再来加深一下印象，每一个Object实例都维护一个SyncBlock并通过这个玩意来进行线程的同步,所以Monitor.Wait最终走到这个BOOL SyncBlock::Wait(INT32 timeOut, BOOL exitContext)并不足奇。在SyncBlock内部我们维护了一个所有正在等待此同步索引块的线程的队列，那具体是通过什麽来控制的呢，通过阅读SyncBlock::Wait源码，我们知道SyncBlock内部的这个维护链表就是SLink m_Link;
// We thread two different lists through this link. When the SyncBlock is
// active, we create a list of waiting threads here. When the SyncBlock is
// released (we recycle them), the SyncBlockCache maintains a free list of
// SyncBlocks here.
//
// We can't afford to use an SList<> here because we only want to burn
// space for the minimum, which is the pointer within an SLink.
SLink m_Link;
在SyncBlock::Wait中通过调用ThreadQueue::EnqueueThread把当前线程的WaitEventLink加入到SyncBlock的m_Link之中：
// Enqueue is the slow one. We have to find the end of the Q since we don't
// want to burn storage for this in the SyncBlock.
/* static */
inline void ThreadQueue::EnqueueThread(WaitEventLink *pWaitEventLink, SyncBlock *psb)
{
LEAF_CONTRACT;
COUNTER_ONLY(GetPrivatePerfCounters().m_LocksAndThreads.cQueueLength++);
_ASSERTE (pWaitEventLink->m_LinkSB.m_pNext == NULL);
SyncBlockCache::LockHolder lh(SyncBlockCache::GetSyncBlockCache());
SLink *pPrior = &psb->m_Link;
while (pPrior->m_pNext)
{
// We shouldn't already be in the waiting list!
_ASSERTE(pPrior->m_pNext != &pWaitEventLink->m_LinkSB);
pPrior = pPrior->m_pNext;
}
pPrior->m_pNext = &pWaitEventLink->m_LinkSB;
}
通过分析Thread的结构，我们知道Thread的两个私有字段：
// For Object::Wait, Notify and NotifyAll, we use an Event inside the
// thread and we queue the threads onto the SyncBlock of the object they
// are waiting for.
CLREvent m_EventWait;
WaitEventLink m_WaitEventLink;
WaitEventLink是一个struct用来管理线程等待的事件，而CLREvent m_EventWait显然就是当前用来阻塞线程或者线程用来等待的事件对象:
// Used inside Thread class to chain all events that a thread is waiting for by Object::Wait
struct WaitEventLink {
SyncBlock *m_WaitSB;
CLREvent *m_EventWait;
Thread *m_Thread; // Owner of this WaitEventLink.
WaitEventLink *m_Next; // Chain to the next waited SyncBlock.
SLink m_LinkSB; // Chain to the next thread waiting on the same SyncBlock.
DWORD m_RefCount; // How many times Object::Wait is called on the same SyncBlock.
};
再返回到BOOL SyncBlock::Wait(INT32 timeOut, BOOL exitContext)
我们看到刚开始就需要检查是否已经有线程在等待本SyncBlock，方法就是：
// Does this thread already wait for this SyncBlock?
WaitEventLink *walk = pCurThread->WaitEventLinkForSyncBlock(this);
若果已经有了，引用数加1：
// Wait on the same lock again.
walk->m_Next->m_RefCount ++;
如没有，则属于第一次，需要先创建一个事件对象CLREvent，创建过程：
// First time this thread is going to wait for this SyncBlock.
CLREvent* hEvent;
if (pCurThread->m_WaitEventLink.m_Next == NULL) {
hEvent = &(pCurThread->m_EventWait);
}
else {
hEvent = GetEventFromEventStore();
}
而这个事件对最后真正用来WaitForMultipleObjects的那个句柄至关重要。为什麽这麽说，我们继续看SyncBlock::Wait最后调用了pCurThread->Block(timeOut, &syncState);
// Called out of SyncBlock::Wait() to block this thread until the Notify occurs.
BOOL Thread::Block(INT32 timeOut, PendingSync *syncState)
{
WRAPPER_CONTRACT;
_ASSERTE(this == GetThread());
// Before calling Block, the SyncBlock queued us onto it's list of waiting threads.
// However, before calling Block the SyncBlock temporarily left the synchronized
// region. This allowed threads to enter the region and call Notify, in which
// case we may have been signalled before we entered the Wait. So we aren't in the
// m_WaitSB list any longer. Not a problem: the following Wait will return
// immediately. But it means we cannot enforce the following assertion:
// _ASSERTE(m_WaitSB != NULL);
return (Wait(syncState->m_WaitEventLink->m_Next->m_EventWait, timeOut, syncState) != WAIT_OBJECT_0);
}
这时候又紧接着调用了Wait(syncState->m_WaitEventLink->m_Next->m_EventWait, timeOut, syncState)，第一个参数明显就是刚才的CLREvent,
// Return whether or not a timeout occured. TRUE=>we waited successfully
DWORD Thread::Wait(CLREvent *pEvent, INT32 timeOut, PendingSync *syncInfo)
{
WRAPPER_CONTRACT;
DWORD dwResult;
DWORD dwTimeOut32;
_ASSERTE(timeOut >= 0 || timeOut == INFINITE_TIMEOUT);
dwTimeOut32 = (timeOut == INFINITE_TIMEOUT
? INFINITE
: (DWORD) timeOut);
dwResult = pEvent->Wait(dwTimeOut32, TRUE /*alertable*/, syncInfo);
// Either we succeeded in the wait, or we timed out
_ASSERTE((dwResult == WAIT_OBJECT_0) ||
(dwResult == WAIT_TIMEOUT));
return dwResult;
}
而最后真正的Wait还是发生在CLREvent内部，看看它的Wait：
DWORD CLREvent::Wait(DWORD dwMilliseconds, BOOL alertable, PendingSync *syncState)
{
WRAPPER_CONTRACT;
return WaitEx(dwMilliseconds, alertable?WaitMode_Alertable:WaitMode_None,syncState);
}
再往下看就和之前的重复了，但是这里我们要着重的地方是CLREvent的私有字段
HANDLE m_handle;
其实你会发现这才是最后调用WaitForMupltipleObjectEx函数需要的那个句柄对象，而它就封装在CLREvent之中，这里的Handle就代表一个内核事件对象，
那麽那麽！这里的WaitForMupltipleObjectEx在什麽情况下返回呢？对的，需要事件对象的Set之后才能返回，ok，现在再让我们回忆一下Monitor.Wait在什麽
时候返回，没错，就是需要在其它的线程中调用Monitor.Pulse之后才能返回，这个Pulse名字起得很形象。由此，我们自然能推断出Pulse最后其实只不过是Event.Set,现在让我们看看Pulse：
void SyncBlock::Pulse()
{
CONTRACTL
{
INSTANCE_CHECK;
NOTHROW;
GC_NOTRIGGER;
MODE_ANY;
}
CONTRACTL_END;
WaitEventLink *pWaitEventLink;
if ((pWaitEventLink = ThreadQueue::DequeueThread(this)) != NULL)
pWaitEventLink->m_EventWait->Set();
}
看到这段代码，我们再对照Monitor.Pulse的描述：从队列中取到排在最前面的线程，这里其实等价于取到那个线程的Event事件对象并Set之，由此一来，正在WaitForMupltipeObjects这个事件的线程将获得释放，对于有多个线程等待同一个Event的情况，究竟是哪个线程会被释放，还应该取决于线程的优先级等属性，但是anyway，这样的调度过程已经交给操作系统定夺了。
同理PulseAll：
void SyncBlock::PulseAll()
{
CONTRACTL
{
INSTANCE_CHECK;
NOTHROW;
GC_NOTRIGGER;
MODE_ANY;
}
CONTRACTL_END;
WaitEventLink *pWaitEventLink;
while ((pWaitEventLink = ThreadQueue::DequeueThread(this)) != NULL)
pWaitEventLink->m_EventWait->Set();
}
转载
现在我们再回到最初的示例上来，ThreadProc1和ThreadProc2之间通过lock关键字进行同步，加在在这两个线程上的lock就好比两扇大门，而这两扇门同时只允许打开一扇。我们先在第一个线程中打开了第一扇门，那第二个线程就要在第二扇门外徘徊。而要打开第二扇门就应该等待第一扇门的Monitor.Exit，Exit的调用就好比是关上当前的门，通知另外的门可以打开了。
但是现在似乎出了点”意外“。
但是现在第一扇门打开之后，突然蹦出个Monitor.Wait,这玩意是个人物，它除了让第一个线程处于阻塞状态，还通知第二扇门可以打开了。这也就是说：并不需要等到第一扇门调用Monitor.Exit,第二扇门就可以打开了。
这一切究竟是怎麽发生的？带着种种疑惑，我们慢慢来拨开云雾见青天。
还需要从BOOL SyncBlock::Wait(INT32 timeOut, BOOL exitContext)开头，
该函数在真正的Block当前线程也即是调用isTimedOut = pCurThread->Block(timeOut, &syncState)之前，有一行代码值得研究一番：
syncState.m_EnterCount = LeaveMonitorCompletely();
单看这行代码所调用的函数名称，直译成：彻底离开Monitor，听起来和Monitor.Exit有点异曲同工之妙。
再来看看其实现：
LONG LeaveMonitorCompletely()
{
WRAPPER_CONTRACT;
return m_Monitor.LeaveCompletely();
}
嗯，又调用了
m_Monitor.LeaveCompletely();
这个m_Monitor在SyncBlock类中的定义：
protected:
AwareLock m_Monitor; // the actual monitor
注释说这是实际的Monitor，所以我们应该能猜出这就是Monitor.Enter/Exit所涉及的类（事实上也是如此，因为我很快看到了Monitor.Enter对应的实现就是AwareLock.Enter），是一个AwareLock 的变量。
Ok，我们再来看AwareLock 的LeaveCompletely实现：
LONG AwareLock::LeaveCompletely()
{
WRAPPER_CONTRACT;
LONG count = 0;
while (Leave()) {
count++;
}
_ASSERTE(count > 0); // otherwise we were never in the lock
return count;
}
再看Leave：
BOOL AwareLock::Leave()
{
CONTRACTL
{
INSTANCE_CHECK;
NOTHROW;
GC_NOTRIGGER;
MODE_ANY;
}
CONTRACTL_END;
Thread* pThread = GetThread();
AwareLock::LeaveHelperAction action = LeaveHelper(pThread);
switch(action)
{
case AwareLock::LeaveHelperAction_None:
// We are done
return TRUE;
case AwareLock::LeaveHelperAction_Signal:
// Signal the event
Signal();
return TRUE;
default:
// Must be an error otherwise
_ASSERTE(action == AwareLock::LeaveHelperAction_Error);
return FALSE;
}
}
由此可以看出所谓彻底离开不过就是遍历+Signal();那麽这个Signal函数究竟做了啥，看名字和注释知其一二：Signal the event
void Signal()
{
WRAPPER_CONTRACT;
// CLREvent::SetMonitorEvent works even if the event has not been intialized yet
m_SemEvent.SetMonitorEvent();
}
现在问题又来了，m_SemEvent是啥？首先，定义：
CLREvent m_SemEvent;
是个CLREvent，然后看看其初始化，是在void AwareLock::AllocLockSemEvent()中：
m_SemEvent.CreateMonitorEvent((SIZE_T)this);
啊哈，只看名字就知道这一个Monitor专用的Event，那麽AllocLockSemEvent又被谁调用呢，是BOOL AwareLock::EnterEpilog(Thread* pCurThread, INT32 timeOut)，而EnterEpilog又为AwareLock::Enter所调用，事实上当EnterEpilog就是第二扇门的徘回函数。我们来看看怎麽徘徊的：
for (;;)
{
// We might be interrupted during the wait (Thread.Interrupt), so we need an
// exception handler round the call.
EE_TRY_FOR_FINALLY
{
// Measure the time we wait so that, in the case where we wake up
// and fail to acquire the mutex, we can adjust remaining timeout
// accordingly.
start = CLRGetTickCount64();
ret = m_SemEvent.Wait(timeOut, TRUE);
_ASSERTE((ret == WAIT_OBJECT_0) || (ret == WAIT_TIMEOUT));
if (timeOut != (INT32) INFINITE)
{
end = CLRGetTickCount64();
if (end == start)
{
duration = 1;
}
else
{
duration = end - start;
}
duration = min(duration, (DWORD)timeOut);
timeOut -= (INT32)duration;
}
}
要注意关键行
ret = m_SemEvent.Wait(timeOut, TRUE); 下文还会讲到。这明显是在等待事件对象的信号有状态。
再来看看SetMonitorEvent的实现:
void CLREvent::SetMonitorEvent()
{
CONTRACTL
{
NOTHROW;
GC_NOTRIGGER;
}
CONTRACTL_END;
// SetMonitorEvent is robust against initialization races. It is possible to
// call CLREvent::SetMonitorEvent on event that has not been initialialized yet by CreateMonitorEvent.
// CreateMonitorEvent will signal the event once it is created if it happens.
for (;;)
{
LONG oldFlags = m_dwFlags;
if (oldFlags & CLREVENT_FLAGS_MONITOREVENT_ALLOCATED)
{
// Event has been allocated already. Use the regular codepath.
Set();
break;
}
LONG newFlags = oldFlags | CLREVENT_FLAGS_MONITOREVENT_SIGNALLED;
if (FastInterlockCompareExchange((LONG*)&m_dwFlags, newFlags, oldFlags) != oldFlags)
{
// We lost the race
continue;
}
break;
}
}
又调用了Set函数：
BOOL CLREvent::Set()
{
CONTRACTL
{
NOTHROW;
GC_NOTRIGGER;
PRECONDITION((m_handle != INVALID_HANDLE_VALUE));
}
CONTRACTL_END;
_ASSERTE(Thread::AllowCallout());
if (IsOSEvent() || !CLRSyncHosted()) {
return UnsafeSetEvent(m_handle);
}
else {
if (IsAutoEvent()) {
HRESULT hr;
BEGIN_SO_TOLERANT_CODE_CALLING_HOST(GetThread());
hr = ((IHostAutoEvent*)m_handle)->Set();
END_SO_TOLERANT_CODE_CALLING_HOST;
return hr == S_OK;
}
else {
HRESULT hr;
BEGIN_SO_TOLERANT_CODE_CALLING_HOST(GetThread());
hr = ((IHostManualEvent*)m_handle)->Set();
END_SO_TOLERANT_CODE_CALLING_HOST;
return hr == S_OK;
}
}
}
在Set函数中我们看到最终是对m_handle的Set。从而使得事件状态被置成有信号状态，也即释放了所有的lock而使得它们重新处于被调度状态。
现在再回过头来看看AwareLock::EnterEpilog的逻辑，已经知道是通过ret = m_SemEvent.Wait(timeOut, TRUE)等待事件对象的信号状态，而我麽也已经知道在调用Monitor.Wait之后会调用事件对象的Set函数从而使得等待的线程得到锁。那麽为了加深印象，我还想通过Windbg走走。
LINK
https://github.com/SSCLI/sscli20_20060311
https://www.cnblogs.com/dancewithautomation/archive/2012/03/25/2416260.html
