c# - ReaderWriterLockSlim LockRecursionPolicy.SupportsRecursion DeadLock

Question

我有一个数据库写入操作队列，由一个专用线程管理。我有很多线程可以随时从数据库中读取数据。

我正在使用 ReaderWriterLockSlim 进行读/写访问控制。

我的问题是——为什么不推荐 LockRecursionPolicy.SupportsRecursion？ MSDN 文档说:

The use of recursion is not recommended for new development, because it introduces unnecessary complications and makes your code more prone to deadlocks.

这里怎么会死锁呢？例如，如果我在 WriteReadLock 已经获取时尝试调用 EnterReadLock(并且我在 SupportsRecursion 策略下)我得到一个异常......

Answer 1

锁递归是指在同一个线程上多次获取同一个锁而不离开原来的锁。

这样做的主要问题是，首先要进入那种情况，您可能对由谁来处理必要的同步有严重的疑问 - 您的锁可能过于细化或过于全局。多线程很难，让它变得更难是彻头彻尾的傻瓜。

第二个大问题是锁与线程绑定(bind)。但是，如果您正在编写异步代码，您的代码可能会随意地在不同线程之间跳转，这可能意味着看起来要采用递归锁的代码实际上不是——外部锁结束由与内部锁不同的线程拥有，你将永远死锁，线程 A 等待线程 B 完成，而 B 正在等待 A 释放外部锁。

您提到即使启用了递归，ReaderWriterLockSlim 也会抛出大量递归异常。是的，这意味着使用递归锁比使用递归锁更安全一点。 ReaderWriterLock 或 Monitor。 MSDN 中清楚地概述了这些规则:

For a ReaderWriterLockSlim that allows recursion, the following can be said about the modes a thread can enter:

• A thread in read mode can enter read mode recursively, but cannot enter write mode or upgradeable mode. If it tries to do this, a LockRecursionException is thrown. Entering read mode and then entering write mode or upgradeable mode is a pattern with a strong probability of deadlocks, so it is not allowed. As discussed earlier, upgradeable mode is provided for cases where it is necessary to upgrade a lock.
• A thread in upgradeable mode can enter write mode and/or read mode, and can enter any of the three modes recursively. However, an attempt to enter write mode blocks if there are other threads in read mode.
• A thread in write mode can enter read mode and/or upgradeable mode, and can enter any of the three modes recursively.
• A thread that has not entered the lock can enter any mode. This attempt can block for the same reasons as an attempt to enter a non-recursive lock.

A thread can exit the modes it has entered in any order, as long as it exits each mode exactly as many times as it entered that mode. If a thread tries to exit a mode too many times, or to exit a mode it has not entered, a SynchronizationLockException is thrown.

他们尽最大努力彻底禁止几乎肯定会导致死锁的递归。然而，这并不意味着仍然没有被忽视的死锁(毕竟，您不需要递归来导致死锁——它只是给您很多难以找到的死锁机会)。更不用说在定期递归其锁的代码中很难做任何一致性保证——这可能意味着某些操作在从外部锁调用时是(半)原子的，但在直接调用它们时不再是原子的。

多线程已经够难了。不要仅仅因为你的对象设计被破坏就让它变得更难 :) Joe Albahari 的“C# 中的线程”是对多线程的一个很好的介绍(一般来说，特别是在 .NET 中)，可以在互联网上免费获得(谢谢，乔!)。 ReaderWriterLockSlim 特别在 http://www.albahari.com/threading/part4.aspx#_Reader_Writer_Locks 中处理