Question

我正在尝试实现一个使用简单缓存来保存从内部服务检索的数据的类。我正在使用ManualResetEvent来阻止多个线程，这些线程可能会尝试同时刷新缓存的数据，第一个线程成功通过调用Set（）然后调用Reset（）来检索其他数据后再继续检索数据。在测试时我注意到有时所有线程都被释放，有时一个或多个线程没有被释放，几乎就像我在所有线程被释放之前调用Reset一样。有人可以解释我做错了吗？

我已经包含了以下代码的缩减版本。

    private bool _updating;
    private const int WaitTimeout = 20000;
    private DateTime _lastRefresh;
    private object _cacheData;
    private readonly ManualResetEvent _signaller = new ManualResetEvent(false);

private void RefreshCachedData()
        {
            Console.WriteLine("ThreadId {0}: Refreshing Cache", Thread.CurrentThread.ManagedThreadId);
        if (_updating)
        {
            Console.WriteLine("ThreadId {0}: Cache Refresh in progress, waiting on signal.", Thread.CurrentThread.ManagedThreadId);

            // another thread is currently updating the cache so wait for a signal to continue
            if (!_signaller.WaitOne(WaitTimeout))
                Console.WriteLine("ThreadId {0}: Thread timed out ({1}s) waiting for a signal that the cache had been refreshed",
                    Thread.CurrentThread.ManagedThreadId,WaitTimeout);

            Console.WriteLine("ThreadId {0}: Signal recieved to use refreshed cache.", Thread.CurrentThread.ManagedThreadId);
        }
        else
        {
            try
            {
                _updating = true;

                var result = _requestHandler.GetNewData();

                if (!result.Success)
                {
                        Console.WriteLine("Failed to retrieve new data.");
                }
                else
                {
                    // switch the cache with the new data
                    _cacheData = result.Data;

                    Console.WriteLine(
                        "ThreadId {0}: Cache refresh success.",
                        Thread.CurrentThread.ManagedThreadId);
                    Thread.Sleep(8000);
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error occured: {0}", ex);
            }
            finally
            {
                // Set the refresh date time regardless of whether we succeded or not
                _lastRefresh = DateTime.Now;
                _updating = false;

                // signal any other threads to to carry on and use the refreshed cache
                Console.WriteLine("ThreadId {0}: Signalling other threads that cache is refreshed.", Thread.CurrentThread.ManagedThreadId);
                _signaller.Set();
                _signaller.Reset();
            }
        }
    }

Answer 1

看起来你的线程在重置之前没有从ResetEvent释放。

您可以通过创建事件打开并让第一个线程输入您的方法重置它来解决问题。

或者，您可以通过执行以下操作来避免出现ManualResetEvent行为的变幻莫测：

private object _latch = new object();
private bool _updating;

private void UpdateIfRequired() 
{
    lock (_latch) 
    {
        if (_updating) 
        {
            //wait here and short circuit out when the work is done
            while (_updating)
                Monitor.Wait(_latch);

            return;
        }

        _updating = true;
    }

    //do lots of expensive work here

    lock (_latch)
    {
        _updating = false;
        Monitor.PulseAll(_latch); //let the other threads go
    }
}

请查看此页面，了解有关其工作原理http://www.albahari.com/threading/part4.aspx#_Signaling_with_Wait_and_Pulse

的详细说明

ManualResetEvent问题不能始终如一地释放所有等待的线程

1 个答案: