相互竞争的原子操作会导致彼此挨饿吗？

发布于 2024-08-20 12:08:45 字数 621 浏览 2 评论 0原文

想象一个有两个线程的程序。他们正在运行以下代码（CAS指的是比较和交换）：

// Visible to both threads
static int test;

// Run by thread A
void foo()
{
    // Check if value is 'test' and swap in 0xdeadbeef
    while(!CAS(&test, test, 0xdeadbeef)) {}
}

// Run by thread B
void bar()
{
    while(1) {
        // Perpetually atomically write rand() into the test variable
        atomic_write(&test, rand());
    }
}

是吗线程 B 是否有可能永久导致线程 A 的 CAS 失败，从而导致 0xdeadbeef 永远不会写入“测试”？或者自然调度抖动是否意味着在实践中这种情况永远不会发生？如果某些工作是在线程 A 的 while 循环内完成的怎么办？

原文

Imagine a program with two threads. They are running the following code (CAS refers to Compare and Swap):

// Visible to both threads
static int test;

// Run by thread A
void foo()
{
    // Check if value is 'test' and swap in 0xdeadbeef
    while(!CAS(&test, test, 0xdeadbeef)) {}
}

// Run by thread B
void bar()
{
    while(1) {
        // Perpetually atomically write rand() into the test variable
        atomic_write(&test, rand());
    }
}

Is it possible for thread B to perpetually cause thread A's CAS to fail such that 0xdeadbeef is never written to 'test'? Or does natural scheduling jitter mean that in practice this never happens? What if some work was done inside thread A's while loop?

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梦忆晨望 2024-08-27 12:08:45

从理论上讲，是的。如果你能设法让两个线程像这样同步运行，

    time     thread A     thread B
    ----     --------     --------
     ||       CAS
     ||                   atomic_write
     ||       CAS
     \/                   atomic_write

那么 CAS 永远不会返回 true。

实际上，当线程共享 CPU/核心时，这种情况永远不会发生，并且当线程在不同的 CPU 或核心上运行时，这种情况也不太可能发生。实际上，这种情况发生的可能性极小超过几个周期，而且在天文数字上也不太可能发生超过一个调度程序量程。

也就是说，如果这段代码

void foo()
{
    // Check if value is 'test' and swap in 0xdeadbeef
    while(!CAS(&test, test, 0xdeadbeef)) {}
}

执行了它看起来要执行的操作，即获取 test 的当前值，并将其与 test 进行比较以查看它是否已更改。在现实世界中，CAS 的迭代将由实际工作的代码分隔开。需要使用 volatile 关键字来确保编译器在调用 CAS 之前获取测试，而不是假设寄存器中可能仍然存在的副本仍然有效。

或者您要测试的值不是测试的当前值，而是某种最后已知值。

换句话说，此代码示例是对理论的测试，但在实践中您不会像这样使用 CAS，因此即使您可能会失败，它也不一定会告诉您在使用时它如何会失败现实世界的算法。

As a theoretical matter, yes. If you could manage somehow to get the two threads running in lockstep like this

    time     thread A     thread B
    ----     --------     --------
     ||       CAS
     ||                   atomic_write
     ||       CAS
     \/                   atomic_write

Then CAS would never return true.

In practice this will never happen when threads share a CPU/Core, and is unlikely to happen when threads are running on different CPUs or Cores. In practice it's incredibly unlikely to happen for more than few cycles, and astronomically unlikely to happen for more than a scheduler quantum.

And that's if this code

void foo()
{
    // Check if value is 'test' and swap in 0xdeadbeef
    while(!CAS(&test, test, 0xdeadbeef)) {}
}

does what it appears to do, which is to fetch the current value of test, and compare it to test to see if it has changed. In the real world iterations of CAS would be separated by code that did actual work. The volatile keyword would be needed to insure that the compiler fetched test before calling CAS rather than assuming that a copy it might still have in a register would still be valid.

Or the value that you would be testing against wouldn't be the current value of test, but rather some sort of last known value.

In other words, this code example is a test of the theory, but you wouldn't use CAS like this in practice, so even if you could get this to fail, it doesn't necessarily tell you how it could fail when used in real world algorithms.

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