具有非原子变量的C ++ 11原子存储顺序 [英] C++11 Atomic memory order with non-atomic variables

问题描述

我不确定c ++ 11中原子变量的内存排序保证如何影响对其他内存的操作.

I am unsure about how the memory ordering guarantees of atomic variables in c++11 affect operations to other memory.

假设我有一个线程定期调用write函数以更新值，而另一个线程调用read以获取当前值.

Let's say I have one thread which periodically calls the write function to update a value, and another thread which calls read to get the current value. Is it guaranteed that the effects of d = value; will not be seen before effects of a = version;, and will be seen before the effects of b = version;?

atomic<int> a {0};
atomic<int> b {0};
double d;

void write(int version, double value) {
    a = version;
    d = value;
    b = version;
}

double read() {
    int x,y;
    double ret;
    do {
        x = b;
        ret = d;
        y = a;
    } while (x != y);
    return ret;
}

推荐答案

是否保证d = value;的效果不会在a = version;的效果之前看到，并且会在b = version;的效果之前看到?

Is it guaranteed that the effects of d = value; will not be seen before effects of a = version;, and will be seen before the effects of b = version;?

是的.这是因为在读取或写入atomic<>变量时暗含了顺序一致性障碍.

Yes, it is. This is because sequensial consistency barrier is implied when read or write atomic<> variable.

在修改值之前和之后，不必将version标记存储到两个原子变量中，而是可以在修改之前和之后将单个原子变量递增:

Instead of storing version tag into two atomic variables before value's modification and after it, you can increment single atomic variable before and after modification:

atomic<int> a = {0};
double d;

void write(double value)
{
     a = a + 1; // 'a' become odd
     d = value; //or other modification of protected value(s)
     a = a + 1; // 'a' become even, but not equal to the one before modification
}

double read(void)
{
     int x;
     double ret;
     do
     {
         x = a;
         ret = value; // or other action with protected value(s)
     } while((x & 2) || (x != a));
     return ret;
}

在Linux内核中称为 seqlock : http://en .wikipedia.org/wiki/Seqlock

This is known as seqlock in the Linux kernel: http://en.wikipedia.org/wiki/Seqlock

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