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列表上的原子操作

[英]Atomic operations over a list

原文 2013-02-28 19:53:34 7 1 c/ concurrency/ atomic/ test-and-set

Suppose I have a list, and I want to use a test_and_set operation a parameter of which is the calculation of some pointer address l->a.next->next . 假设我有一个列表，我想使用test_and_set操作，其参数是一些指针地址l->a.next->next 。 This, I think, will not be atomic, and the test_and_set will be useless. 我认为这不是原子的， test_and_set也没用。 Is there a way to calculate that pointer value atomically so that the TAS will work atomically? 有没有办法以原子方式计算指针值，以便TAS原子地工作？

1 个解决方案

You probably mean CAS (more useful). 你可能意味着CAS （更有用）。

In general: yes it is often used to implement transactional or wait-free datastructures, 通常：是的，它通常用于实现事务性或等待的数据结构，

Buf first things first: let's separate address-calculations from atomic operations on an address, you first get the specific address at which something should be swapped, the CAS does not care how you got there. 首先要做的事情是：让地址计算与地址上的原子操作分开，首先得到应该交换某些东西的具体地址，CAS不关心你是如何到达那里的。

Specifically you should first let each of your threads navigate the list until they find a place where they want to replace a next-pointer, then they can try-repeat this by using CAS. 具体来说，您应首先让每个线程导航列表，直到找到他们想要替换下一个指针的位置，然后他们可以尝试使用CAS重复此操作。 It depends on your scenario whether the thread has to re-walk the list for the retry. 这取决于您的场景线程是否必须重新遍历列表以进行重试。

The tricky part is actually at a different place in the code: where you free (or re-use) the list-nodes. 棘手的部分实际上是在代码中的不同位置：您释放（或重用）列表节点。 In general you have to assume that you cannot re-use or free node-chains that were disconnected from your list ever . 一般来说，你必须承担，你不能重复使用或从列表中断开以往任何时候都免费节点链。 In practice there are heursitics you can use but this depends on your use-case. 在实践中，你可以使用heursitics，但这取决于你的用例。