data rac

最新推荐文章于 2023-11-25 23:13:58 发布

翻译最新推荐文章于 2023-11-25 23:13:58 发布 · 572 阅读

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本文探讨了多线程编程中的数据竞争问题，即多个线程同时访问并尝试修改共享数据时可能出现的不确定行为。文章通过检查-然后行动的具体例子说明了问题，并提出了使用锁来防止此类问题的方法。

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最近在看effective STL，其中看到一个重要的概念就是在多线程编程中常常出现的data racing 的问题。也就是当两个不同的线程试图同时去修改他们共同享有的数据的时候，会发生不可预知的结果。例如当一个线程在check 和act 之间的这个时间内，可能其他的线程会去修改带这个数据。所以另一个线程操作的结果同期预期的效果是不同的。为了避免data race的发生，我们在一个线程访问数据的时候，对数据加上一个lock，以阻止其他的线程去修改这个数据。只有该线程的这个修改操作完成之后，才释放掉这个lock。 stackoverflow的解释如下：

287down voteaccepted

A race condition occurs when two or more threads can access shared data and they try to change it at the same time. Because the thread scheduling algorithm can swap between threads at any time, you don't know the order in which the threads will attempt to access the shared data. Therefore, the result of the change in data is dependent on the thread scheduling algorithm, i.e. both threads are "racing" to access/change the data.

Problems often occur when one thread does a "check-then-act" (e.g. "check" if the value is X, then "act" to do something that depends on the value being X) and another thread does something to the value in between the "check" and the "act". E.g:

if (x == 5) // The "Check"
{
   y = x * 2; // The "Act"

   // If another thread changed x in between "if (x == 5)" and "y = x * 2" above,
   // y will not be equal to 10.
}

The point being, y could be 10, or it could be anything, depending on whether another thread changed x in between the check and act. You have no real way of knowing.

In order to prevent race conditions from occurring, you would typically put a lock around the shared data to ensure only one thread can access the data at a time. This would mean something like this:

// Obtain lock for x
if (x == 5)
{
   y = x * 2; // Now, nothing can change x until the lock is released. 
              // Therefore y = 10
}
// release lock for x

287down voteaccepted

if (x == 5) // The "Check"
{
   y = x * 2; // The "Act"

   // If another thread changed x in between "if (x == 5)" and "y = x * 2" above,
   // y will not be equal to 10.
}

The point being, y could be 10, or it could be anything, depending on whether another thread changed x in between the check and act. You have no real way of knowing.

In order to prevent race conditions from occurring, you would typically put a lock around the shared data to ensure only one thread can access the data at a time. This would mean something like this:

// Obtain lock for x
if (x == 5)
{
   y = x * 2; // Now, nothing can change x until the lock is released. 
              // Therefore y = 10
}
// release lock for x