multithreading - 升级锁如何帮助避免读写器锁死锁

Question

我知道升级锁试图解决经典的死锁示例，其中两个并发线程同时保持读取锁并尝试获取写锁将导致死锁。

Thread A: S lock (acquired)

Thread B: S lock (acquired)

Thread A: X lock (waiting for B to release S lock)

Thread B: X lock (waiting for A to release S lock) DEADLOCKED

为什么同样的问题不在于升级锁？ This文档说，只有一个线程可以被授予升级锁，而其他线程仍保持S锁时，可以这样做。有人可以解释为什么运行此模式的两个并发线程不会陷入死锁吗？

  lock.AcquireSharedLock()      // Thread A and B both acquired this S lock

  lock.EnterUpgradeableReadLock();  // Only one (e.g. A) acquired U lock and other (e.g. B) is blocked here.

  if(somecondition)
  {
    lock.AcquireExclusiveLock();   // A tries to acquire X lock. But wouldn't it get blocked since B is already holding onto S lock? B is already blocked, so this has to be a deadlock
  }

Answer 1

有趣的是，我刚刚针对SQL Server和UPDATE语句回答了相同的问题:How do UPDATE locks prevent a common form of deadlock?

答案的关键方面是，可升级的事务永远不会采用S锁。他们立即U锁。因此，S锁和X锁之间永远不会发生冲突。

在您的示例中，

lock.AcquireSharedLock()

不应该存在。您可以使用S锁或U锁。不是按顺序的，而是它们中的任何一个。