如何在writer尝试输入写锁定时避免阻塞ReaderWriterLockSlim读取器

Question

我正在ReaderWriterLockSlim用来保护一些操作.我想赞成读者而不是作家,这样当读者长时间持有锁并且作家试图获得写锁时,未来的读者也不会被作者的尝试所阻挡(如果是这样的话会发生这种情况.作家被封锁了lock.EnterWriteLock()).

为此,我虽然编写器可以TryEnterWriteLock在循环中使用短暂的超时,以便后续的读者仍然能够获得读锁定而编写者不能.然而,令我惊讶的是,我发现一个不成功的呼叫TryEnterWriteLock改变了锁的状态,无论如何阻止了未来的读者.概念证明代码:

System.Threading.ReaderWriterLockSlim myLock = new System.Threading.ReaderWriterLockSlim(System.Threading.LockRecursionPolicy.NoRecursion);

System.Threading.Thread t1 = new System.Threading.Thread(() =>
{
    Console.WriteLine("T1:{0}: entering read lock...", DateTime.Now);
    myLock.EnterReadLock();
    Console.WriteLine("T1:{0}: ...entered read lock.", DateTime.Now);

    System.Threading.Thread.Sleep(10000);
});

System.Threading.Thread t2 = new System.Threading.Thread(() =>
{
    System.Threading.Thread.Sleep(1000);

    while (true)
    {
        Console.WriteLine("T2:{0}: try-entering write lock...", DateTime.Now);
        bool result = myLock.TryEnterWriteLock(TimeSpan.FromMilliseconds(1500));
        Console.WriteLine("T2:{0}: ...try-entered write lock, result={1}.", DateTime.Now, result);

        if (result)
        {
            // Got it!
            break;
        }

        System.Threading.Thread.Yield();
    }

    System.Threading.Thread.Sleep(9000);
});

System.Threading.Thread t3 = new System.Threading.Thread(() =>
{
    System.Threading.Thread.Sleep(2000);

    Console.WriteLine("T3:{0}: entering read lock...", DateTime.Now);
    myLock.EnterReadLock();
    Console.WriteLine("T3:{0}: ...entered read lock!!!!!!!!!!!!!!!!!!!", DateTime.Now);

    System.Threading.Thread.Sleep(8000);
});

此代码的输出是:

T1:18-09-2015 16:29:49: entering read lock...
T1:18-09-2015 16:29:49: ...entered read lock.
T2:18-09-2015 16:29:50: try-entering write lock...
T3:18-09-2015 16:29:51: entering read lock...
T2:18-09-2015 16:29:51: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:51: try-entering write lock...
T2:18-09-2015 16:29:53: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:53: try-entering write lock...
T2:18-09-2015 16:29:54: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:54: try-entering write lock...
T2:18-09-2015 16:29:56: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:56: try-entering write lock...
T2:18-09-2015 16:29:57: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:57: try-entering write lock...
T2:18-09-2015 16:29:59: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:59: try-entering write lock...

正如您所看到的,即使线程2("Writer")没有获得写入锁定而且它不在EnterWriteLock调用中,线程3 也会被阻止.我可以看到类似的行为ReaderWriterLock.

我做错了吗？如果没有,当作家排队时,我有什么选择支持读者？

Answer 1

我可以\xe2\x80\x99t 提供帮助，但我相信这是一个 .NET Framework 错误（更新：我已报告该错误）。以下简单的程序（上述程序的简化版本）说明了这一点：

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var myLock = new ReaderWriterLockSlim(LockRecursionPolicy.NoRecursion);\n\nvar t1 = new Thread(() =>\n{\n    Console.WriteLine("T1:{0}: entering read lock...", DateTime.Now);\n    myLock.EnterReadLock();\n    Console.WriteLine("T1:{0}: ...entered read lock.", DateTime.Now);\n\n    Thread.Sleep(50000);\n\n    Console.WriteLine("T1:{0}: exiting", DateTime.Now);\n    myLock.ExitReadLock();\n});\n\nvar t2 = new Thread(() =>\n{\n    Thread.Sleep(1000);\n\n    Console.WriteLine("T2:{0}: try-entering write lock...", DateTime.Now);\n    bool result = myLock.TryEnterWriteLock(3000);\n    Console.WriteLine("T2:{0}: ...try-entered write lock, result={1}.", DateTime.Now, result);\n\n    Thread.Sleep(50000);\n\n    if (result)\n    {\n        myLock.ExitWriteLock();\n    }\n    Console.WriteLine("T2:{0}: exiting", DateTime.Now);\n});\n\nvar t3 = new Thread(() =>\n{\n    Thread.Sleep(2000);\n\n    Console.WriteLine("T3:{0}: entering read lock...", DateTime.Now);\n    myLock.EnterReadLock();\n    Console.WriteLine("T3:{0}: ...entered read lock!!!!!!!!!!!!!!!!!!!", DateTime.Now);\n\n    Thread.Sleep(50000);\n\n    myLock.ExitReadLock();\n    Console.WriteLine("T3:{0}: exiting", DateTime.Now);\n});\n\nt1.Start();\nt2.Start();\nt3.Start();\n\nt1.Join();\nt2.Join();\nt3.Join();\n

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以下内容以简单的顺序发生，没有锁车队，没有竞赛，没有循环或任何东西。

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1. T1获取读锁。
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2. T2尝试获取写锁并阻塞，等待超时（如T1持有锁）。
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3. T3尝试获取读锁并阻塞（因为T2被阻塞等待写锁，并且根据文档，这意味着所有其他读取器都被阻塞直到超时）。
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4. T2\xe2\x80\x99s 超时到期。根据文档，T3现在应该唤醒并获取读锁。然而，这种情况不会发生，并且T3会被永远阻塞（或者，在本示例中，在T1离开读锁之前的 50 秒内）。
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AFAICT，\xe2\x80\x99sExitMyLock中的应该是.ReaderWriterLockSlimWaitOnEventExitAndWakeUpAppropriateWaiters

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