这个锁+ ManualResetEvent使用线程安全吗? [英] Is this lock + ManualResetEvent usage thread safe?

问题描述

这是基于

This is a separate question based off of this question. To recap, say I have two functions that manipulate a count, and an OnTimer function that fires at a regular interval. My desire is that if/when OverwriteCount is called, IncrementCount can't be executed until the timer function executes.

建议的解决方案是:

private int _myCount = 0;
private readonly object _sync = new object();
private ManualResetEventSlim mre = new ManualResetEventSlim(initialState: true);

void IncrementCount()
{
    mre.Wait(); // all threads wait until the event is signaled

    lock (_sync)
    {
        _myCount++;
    }
}

void OverwriteCount(int newValue)
{
    lock (_sync)
    {
        mre.Reset(); // unsignal the event, blocking threads
        _myCount = newValue;
    }
}

void OnTimer()
{
    lock (_sync)
    {
        Console.WriteLine(_myCount);
        mre.Set(); // signal the event
    }
}

ManualResetEventSlim尝试确保一旦OverwriteCount()解除事件的信号，对_myCount的任何修改都必须等到OnTimer()执行之后.

The ManualResetEventSlim tries to ensure that once OverwriteCount() unsignals the event, any modifications to _myCount must wait until OnTimer() executes.

问题:

1. 说线程A输入IncrementCount()并传递事件的wait()-已经指示了ManualResetEvent的初始状态.
然后，
2. 线程B启动并执行所有OverwriteCount().
3. 然后，线程A通过获取锁并递增_myCount来继续.

这违反了我的目标，因为_myCount会在OnTimer运行之前在调用OverwriteCount()之后更改.

This violates my goal as _myCount would change after a call to OverwriteCount(), prior to OnTimer running.

被拒绝的选择:我可以将mre.Wait()移入lock(_sync)内，但这会带来死锁风险.如果线程A调用IncrementCount()并在等待时阻塞，则其他线程都无法获取该锁来释放它.

Rejected Alternative: I could move mre.Wait() within lock(_sync) but that poses a deadlock risk. If thread A calls IncrementCount() and blocks on the wait, no other threads can acquire the lock to release it.

问题:我是否需要其他同步原语才能实现目标?或者，我是否对线程安全性有疑问?

Question: Do I need a different synchronization primitive to achieve my goal? Alternatively, am I wrong about the thread safety concern?

推荐答案

我认为您只需使用标准的Monitor和附加标志即可实现您的目标.

I think you can achieve your goal with just the standard Monitor and an additional flag.

private readonly object _sync = new object();
private int _myCount = 0;
private bool _canIncrement = true;

void IncrementCount()
{
    lock (_sync)
    {
        // If the flag indicates we can't increment, unlock _sync and wait for a pulse.
        // Use a loop here to ensure that if Wait() returns following the PulseAll() below
        // (after re-acquiring the lock on _sync), but a call to OverwriteCount managed to
        // occur in-between, that we wait again.
        while (!_canIncrement)
        {
            Monitor.Wait(_sync);
        }

        _myCount++;
    }
}

void OverwriteCount(int newValue)
{
    lock (_sync)
    {
        _canIncrement = false;
        _myCount = newValue;
    }
}

void OnTimer()
{
    lock (_sync)
    {
        Console.WriteLine(_myCount);
        _canIncrement = true;
        // Ready any threads waiting on _sync in IncrementCount() above
        Monitor.PulseAll(_sync);
    }
}

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