可以在没有硬件支持的情况下以软件实施“测试和设置"吗? [英] Can Test and Set be implemented in software without hardware support?

问题描述

这是用软件编写的测试和设置:

Here's the Test and Set written in software:

boolean TestAndSet(boolean *target) {
    boolean rv = *target;
    *target = TRUE;
    return rv;
}

和

do {
    while(TestAndSetLock(&lock))
        ; // do nothing
        // critical section
    lock = FALSE;
        // remainder section
} while(TRUE);

我们可以在不支持硬件级别测试设置的CPU中使用该机制吗?如果是这样，如何确保原子性?

Can we use the mechanism in CPUs that do not support test-and-set at the hardware level? If so, how is atomicity assured?

推荐答案

在没有硬件支持的情况下，测试和设置无法在软件中实现.

Test and Set cannot be implemented in software without hardware support the way you proposed.

这是因为上面的TestAndSet被认为是用于确保相互排斥的机制，但它不是原子的.但是，原子性只是互斥的另一个名称:TestAndSet的内部变量必须通过确保两个进程不能同时执行其代码来加以保护.

It is because TestAndSet above is supposed to be a mechanism to be used to ensure mutual exclusion, but it is not atomic. Atomicity, however, is just another name for mutual exclusion: the internal variables of TestAndSet must be protected by ensuring that two processes cannot execute its code concurrently.

因此，您以这种方式定义了一种确保相互排斥的方法，它本身需要某种机制来确保相互排斥.这个技巧可以玩很多次，但是如果没有某种硬件支持，就不会有任何实质性进展.

So this way you defined a method for ensuring mutual exclusion which itself requires some mechanism to ensure mutual exclusion. This trick can be played a number of times, but there will not be any real progress without some sort of hardware support.

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