释放互斥锁

Question

我有一个web应用程序，它需要利用应用程序缓存来存储数据(由于在逐个请求的基础上获取数据的高开销)。请参阅https://stackoverflow.com/a/16961962/236860上的前一篇文章

这种方法似乎工作得很好，但我在网站的错误中看到了以下偶尔的错误：

System.ApplicationException: Object synchronization method was called from an
unsynchronized block of code.
at System.Threading.Mutex.ReleaseMutex()
at InboxInsight.Web.Web_Controls.Twitter.TwitterFeed.GetTwitterData(HttpContext context)
at InboxInsight.Web.Web_Controls.Twitter.TwitterFeed.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

作为参考，下面是代码块：

public string GetData(HttpContext context)
{
    var cache      = context.Cache;
    Mutex mutex    = null;
    string data    = (string)cache[CacheKey];

    // Start check to see if available on cache
    if (data == null)
    {
       try
       {
           // Lock base on resource key
           // (note that not all chars are valid for name)
           mutex = new Mutex(true, CacheKey);

           // Wait until it is safe to enter (someone else might already be
           // doing this), but also add 30 seconds max.
           mutex.WaitOne(30000);

           // Now let's see if some one else has added it...
           data = (string)cache[CacheKey];

           // They did, so send it...
           if (data != null)
           {
              return data;
           }


           // Still not there, so now is the time to look for it!
           data = GetSlowFeed(context);

           cache.Remove(CacheKey);
           cache.Add(CacheKey, data, null, GetExpiryDate(),
              TimeSpan.Zero, CacheItemPriority.Normal, null);
       }
       finally
       {
           // Release the Mutex.
           if (mutex != null)
           {
              mutex.ReleaseMutex();
           }
       }
    }

    return data;
}

从我的研究来看，它表明这个问题是由一个进程线程试图释放它没有创建的Mutex引起的，但我不明白这是怎么发生的。

有没有人能建议我如何重新组织代码以避免这个问题？

Answer 1

您没有处理mutex.WaitOne返回false的情况，即超时。如果WaitOne返回false，那么您不拥有互斥锁，因此不需要释放它。

bool iOwnTheMutex;

try {
    // set up mutex here...
    iOwnTheMutex = mutex.WaitOne(2000);
    if (iOwnTheMutex) {
       // do what you need to do
    }
}
finally {
    if (mutex != null && iOwnTheMutex) {
       mutex.ReleaseMutex();
    }
}