ReaderWriterLock.UpgradeToWriterLock在超时后不会抛出异常

在编写单元测试时，我偶然发现了ReaderWriterLock的一个非常奇怪的问题。我尝试测试UpgradeToWriterLock方法，超时选项设置为50毫秒。

在主线程上，我取reader锁，然后启动许多任务。在任务中，我也采用reader锁，然后尝试使用超时升级到writer。当主线程持有读锁时，这应该在每个线程上都失败。由于超时为50毫秒，任务应该抛出超时异常并完成。如果我开始10个以上的任务，它们就不会。它们会卡在UpgradeToWriterLock上。

有人能解释一下吗?完整的源代码如下。

    [TestMethod]
    public void UpgradeLockFailTest()
    {
        // strangely when more than 10 threads then it gets stuck on UpgradeToWriterLock regardless of the timeout
        const int THREADS_COUNT = 20;
        // 50 milliseconds
        const int TIMEOUT = 50;
        // create the main reader writer lock
        ReaderWriterLock rwl = new ReaderWriterLock();
        // acquire the reader lock on the main thread
        rwl.AcquireReaderLock(TIMEOUT);
        // create and start all the tasks
        Task[] tasks = new Task[THREADS_COUNT];
        for (int i = 0; i < THREADS_COUNT; i++)
        {
            tasks[i] = Task.Factory.StartNew(() =>
            {
                try
                {
                    // acquire the reader lock on the worker thread
                    rwl.AcquireReaderLock(TIMEOUT);
                    // acquire the writer lock on the worker thread 
                    rwl.UpgradeToWriterLock(TIMEOUT); // <-- GETS STUCK HERE AND DOESN'T RESPECT TIMEOUT
                }
                finally
                {
                    rwl.ReleaseLock();
                }
            });
        }
        // should be enough for all the tasks to be created
        Thread.Sleep(2000);
        try
        {
            // wait for all tasks
            Task.WaitAll(tasks); // <-- GETS STUCK HERE BECAUSE THE TASKS ARE STUCK ON UpgradeToWriterLock
        }
        catch (AggregateException ae)
        {
            Assert.AreEqual(THREADS_COUNT, ae.InnerExceptions.Count);
        }
        // release all the locks on the main thread
        rwl.ReleaseLock();
    }

有趣的是，如果我在等待任务之前释放主线程读取器锁，一切都像预期的那样工作。抛出正确数量的超时异常。