大于比较和交换 [英] Greater-than compare-and-swap

问题描述

正如标题所暗示的，我正在寻找一个比较交换方案，但具有大于比较的结果:

As the title suggests, I'm looking for a compare-and-swap implementation, but with greater-than comparison:

if(newValue > oldValue) {
    oldValue = newValue;
}

其中oldValue是某些全局共享状态，而newValue是每个线程专用的，而无需执行以下操作:

where oldValue is some global shared state and newValue is private to each thread, without doing this:

synchronized(locker) {
    if(newValue > oldValue) {
        oldValue = newValue;
    }       
}

因为我想要一个非阻塞的解决方案.通过研究其他非阻塞操作的源代码，我想到了这一点(假设值是整数):

because I want a non-blocking solution. From studying source codes of other non-blocking operations, I've come up with this (assuming the values are integers):

AtomicInteger oldValue; // shared global variable

...

public boolean GreaterThanCAS(int newValue) {

    while(true) {
        int local = oldValue;
        if(local == oldValue) {
            if(newValue > local) {
                 if(oldValue.compareAndSet(local, newValue) {
                     return true;  // swap successful
                 } // else keep looping
            } else {
                 return false; // swap failed
            }
        } // else keep looping
    }
}

当// else keep looping发生时，这意味着同时有另一个线程更改了oldValue，因此我需要循环并重试.

when // else keep looping happens, it means that another thread has changed the oldValue in the meantime and so I need to loop and try again.

此实现是否正确(线程安全)?

Is this implementation correct (thread-safe)?

推荐答案

我发现您的实现没有问题，假设没有线程会降低AtomicInteger 的值.如果是这样，您的代码就可以接受竞争条件.

I see no problems with your implementation, provided that no thread ever decreases the value of the AtomicInteger. If they do, your code is open to race conditions.

请注意，代码可以简化如下:

Note that the code can be simplified as follows:

public boolean GreaterThanCAS(int newValue) {
    while(true) {
        int local = oldValue.get();
        if(newValue <= local) {
             return false; // swap failed
        }
        if(oldValue.compareAndSet(local, newValue)) {
             return true;  // swap successful
        }
        // keep trying
    }
}

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