ReaderWriterLockSlim.EnterUpgradeableReadLock()总是死锁吗？

Question

我对ReaderWriterLockSlim非常熟悉，但最近我尝试在一个类中实现EnterUpgradeableReadLock() .当我意识到当两个或更多线程运行代码时，这几乎肯定是一个有保证的死锁：

Thread A --> enter upgradeable read lock
Thread B --> enter upgradeable read lock
Thread A --> tries to enter write lock, blocks for B to leave read
Thread B --> tries to enter write lock, blocks for A to leave read
Thread A --> waiting for B to exit read lock
Thread B --> waiting for A to exit read lock

我在这里错过了什么？

编辑

添加了我的场景的代码示例。Run()方法将由2个或多个线程并发调用。

public class Deadlocker
{
    private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);

    public void Run()
    {
        _lock.EnterUpgradeableReadLock();
        try
        {
            _lock.EnterWriteLock();
            try
            {
                // Do something
            }
            finally
            {
                _lock.ExitWriteLock();
            }
        }
        finally
        {
            _lock.ExitUpgradeableReadLock();
        }
    }
}

Answer 1

我建议避免使用EnterUpgradeableReadLock()。只需使用EnterWriteLock()即可。我知道这似乎是低效的，可升级的读锁几乎和写锁一样糟糕。

• http://ayende.com/blog/4349/using-readerwriterlockslims-enterupgradeablereadlock
• http://joeduffyblog.com/2007/02/07/introducing-the-new-readerwriterlockslim-in-orcas
• http://ayende.com/blog/4349/using-readerwriterlockslims-enterupgradeablereadlock
• Is ReaderWriterLockSlim.EnterUpgradeableReadLock() essentially the same as Monitor.Enter()?

Answer 2

您的示例中有一个错误

private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);

它应该是

private static readonly ReaderWriterLockSlim _lock = new  ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);

现在，在您的代码中，每当一个类被实例化时，它都在创建ReaderWriterLockSlim的新实例，它无法锁定任何东西，因为每个线程都有它自己的实例。使其静态将强制所有线程使用一个实例，该实例将按其应有的方式工作。