读写器锁

Question

我正在通过这里提到的多个读取器/写入器锁的实现 MultiplereadersWriterLock

这里 EnterReader 的代码是

void EnterReader(void)
    {
        EnterCriticalSection(&m_csWrite);
        EnterCriticalSection(&m_csReaderCount);
        if (++m_cReaders == 1)
            ResetEvent(m_hevReadersCleared);
        LeaveCriticalSection(&m_csReaderCount);
        LeaveCriticalSection(&m_csWrite);
    }

根据我对这个锁的理解，我们正在解决我们可以通过多个线程读取共享资源的问题。所以这意味着多个线程可以调用 EnterReader 函数并且不会阻塞它们可以继续读取。

例子

Thread T1 calls EnterReader()
It acquires m_csWrite
It acquires m_csReaderCount
Interrupted by CPU and thread T2 starts

    Thread T2 calls EnterReader()
    Since m_csWrite already acquired by T1 so how thread T2 can perform read. 
T2 cannot proceed further since m_csWrite is already acquired that means T2 reader thread is blocked and waiting for T1 to get finish.

我很困惑这个实现如何解决多个读者访问共享资源的问题。

Answer 1

EnterReader 只是快速锁定以增加读者的数量。请注意，它会在返回之前释放锁。阅读时，m_csWrite 不会像在 EnterWriter 和 LeaveWriter 之间那样在 EnterReader 和 LeaveReader 调用之间保持锁定状态。

使用示例：

lock->EnterReader();
/*Do the reading*/ //No locks here. m_csReaderCount is blocking writes.
lock->LeaveReader();