ReactOS分析CriticalSection

首先要区分的是，之前提过的critical region和critical section的区别，前者是主要是通过IRQL阻止线程被打断，而后者则是通过包装内核对象来防止线程被打断。

为了减少不必要的代码量，直接跳过一些函数的封装。下面是critical section的初始化实现。

NTSTATUS
NTAPI
RtlInitializeCriticalSectionAndSpinCount(PRTL_CRITICAL_SECTION CriticalSection,ULONG SpinCount)
{
    PRTL_CRITICAL_SECTION_DEBUG CritcalSectionDebugData;
    CriticalSection->LockCount = -1;
    CriticalSection->RecursionCount = 0;
    CriticalSection->OwningThread = 0;
    CriticalSection->SpinCount = (NtCurrentPeb()->NumberOfProcessors > 1) ? SpinCount : 0;
    CriticalSection->LockSemaphore = 0;
    CritcalSectionDebugData = RtlpAllocateDebugInfo();
    if (!CritcalSectionDebugData) {
        return STATUS_NO_MEMORY;
    }
    CritcalSectionDebugData->Type = RTL_CRITSECT_TYPE;
    CritcalSectionDebugData->ContentionCount = 0;
    CritcalSectionDebugData->EntryCount = 0;
    CritcalSectionDebugData->CriticalSection = CriticalSection;
    CriticalSection->DebugInfo = CritcalSectionDebugData;

    if ((CriticalSection != &RtlCriticalSectionLock) && (RtlpCritSectInitialized)) {
        RtlEnterCriticalSection(&RtlCriticalSectionLock);
        InsertTailList(&RtlCriticalSectionList,&CritcalSectionDebugData->ProcessLocksList);
        RtlLeaveCriticalSection(&RtlCriticalSectionLock);

    } else {
        InsertTailList(&RtlCriticalSectionList,&CritcalSectionDebugData->ProcessLocksList);
    }

    return STATUS_SUCCESS;
}

在初始化调试数据之后，就将critical secction插入到队列的尾部。这里之所以需要有一个判断，是因为RtlCriticalSectionLock是系统提供的专用于保护访问互斥；而这个数据同样也是经过这个函数初始化，所以需要有一个分支过程。（个人觉得这里在定义的时候直接进行初始化，然后定义一个特殊的函数进行初始化，毕竟分支影响处理器的流水线，个人理解，还望高手指点迷津）

NTSTATUS
NTAPI
RtlEnterCriticalSection(PRTL_CRITICAL_SECTION CriticalSection)
{
    HANDLE Thread = (HANDLE)NtCurrentTeb()->ClientId.UniqueThread;
    if (InterlockedIncrement(&CriticalSection->LockCount) != 0) {
        if (Thread == CriticalSection->OwningThread) {
            CriticalSection->RecursionCount++;
            return STATUS_SUCCESS;
        }
        RtlpWaitForCriticalSection(CriticalSection);
    }
    CriticalSection->OwningThread = Thread;
    CriticalSection->RecursionCount = 1;
    return STATUS_SUCCESS;
}

NTSTATUS
NTAPI
RtlpWaitForCriticalSection(PRTL_CRITICAL_SECTION CriticalSection)
{
    NTSTATUS Status;
    EXCEPTION_RECORD ExceptionRecord;
    BOOLEAN LastChance = FALSE;
    LARGE_INTEGER Timeout;
    Timeout.QuadPart = 150000L * (ULONGLONG)10000;
    Timeout.QuadPart = -Timeout.QuadPart;
    if (!CriticalSection->LockSemaphore) {
        RtlpCreateCriticalSectionSem(CriticalSection);
    }
    if (CriticalSection->DebugInfo)
        CriticalSection->DebugInfo->EntryCount++;
    for (;;) {
        if (CriticalSection->DebugInfo)
            CriticalSection->DebugInfo->ContentionCount++;
        Status = NtWaitForSingleObject(CriticalSection->LockSemaphore,FALSE,&Timeout);
        if (Status == STATUS_TIMEOUT) {
            LastChance = TRUE;
        } else {
            return STATUS_SUCCESS;
        }
    }
}

<pre code_snippet_id="461785" snippet_file_name="blog_20140901_5_3253223" name="code" class="cpp">NTSTATUS
NTAPI
RtlLeaveCriticalSection(PRTL_CRITICAL_SECTION CriticalSection)
{
    if (--CriticalSection->RecursionCount) {
        InterlockedDecrement(&CriticalSection->LockCount);
    } else {
        CriticalSection->OwningThread = 0;
        if (-1 != InterlockedDecrement(&CriticalSection->LockCount)) {
            RtlpUnWaitCriticalSection(CriticalSection);
        }
    }
    return STATUS_SUCCESS;
}

退出critical section段的操作不需要加锁，因为可以执行这一语句的肯定是进入到critical section的线程。首先会递减递归访问计数，然后原子递减当前的LockCount（这里主要是对应于前面的原子操作自加）。如果RecursionCount自减之后等于0，则清除当前的线程句柄，然后递减LockCount。在这里有一种可能是Critical Section的线程句柄等于0。在前面进入到等待的时候，如果某个线程在OwingThread等于0的时候，请求进入到critical section，由于LockCount大于0，所以判断是否等于当前线程，肯定线程句柄是不等于0的，所以进入等待状态。唤醒等待的线程很简单，设置事件就可以了。

完整的源代码网址：http://doxygen.reactos.org/d0/d06/critical_8c_source.html

（编辑：李大同）

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