信号量P和V操作是原子吗？ [英] Are Semaphore P and V operations atomic?

问题描述

能够对信号量保证原子执行的P（）和V（）操作吗？一个信号量阻止两个进程进入P（）？

Are the P() and V() operations that can be performed on a semaphore guarantee atomic? Can a semaphore prevent two processes getting into the P()?

推荐答案

假设我们有一个二进制信号量，值1，并且两个进程同时尝试执行P on。只有其中一个操作能够在下一个V操作之前完成;

Suppose we have a binary semaphore, s, which has the value 1, and two processes simultaneously attempt to execute P on s. Only one of these operations will be able to complete before the next V operation on s; the other process attempting to perform a P operation is suspended.

取自我的大学注意事项：

Taken from my university notes:

我们可以认为如果P和V控制
访问资源：

We can think if P and V as controlling access to a resource:

当进程要使用
资源，它执行P操作：
如果成功，它减少
的可用资源量，并且
进程继续;如果所有
资源当前正在使用，则
进程必须等待。

When a process wants to use the resource, it performs a P operation: if this succeeds, it decrements the amount of resource available and the process continues; if all the resource is currently in use, the process has to wait.

当进程完成
资源时，它执行V操作：
如果有进程在
资源上等待，其中一个被唤醒;

如果没有等待进程，
信号量增加
，表明现在有更多的
资源空闲。注意，V的
定义没有指定如果多个
进程在同一个
信号量上暂停，那么哪个
进程被唤醒。

When a process is finished with the resource, it performs a V operation: if there were processes waiting on the resource, one of these is woken up;
if there were no waiting processes, the semaphore is incremented indicating that there is now more of the resource free. Note that the definition of V doesn’t specify which process is woken up if more than one process has been suspended on the same semaphore.

Semaphores可以解决互斥和条件同步问题。因此，您的问题的答案是：是的。

Semaphores can solve both mutual exclusion and condition synchronization problems. So the answer to both your questions is: yes.

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