使用CAS（比较和交换）时，如何确保旧值实际上是旧的？

Question

请考虑以下事项：

    int grab_next_target(int* target) { 
            do {    
                    /* Intention: store current value of *target into old, so as  
                       to ensure that old never changes */ 
                    int old = *target; 
                    /* get new value based on old -- note that old is assumed not to change here */ 
                    int new; 
                    if (1 == old) { /* imagine that old is 1 so new is now 20 */ 
                            new = 20; 
                    } else if (2 == old) { 
                            new = 300; 
                    } else if (3 == old) { 
                            new = -20; 
                    } else if (4 == old) { 
                            new = 400; 
                    } 
                    /* but the compiler has optimized 
                       old to just read from *target, so *target could be 
                       changed by another thread to be 4.  The CAS will succeed 
                       and now target will hold the wrong value (it will hold 20, instead of 400)  */ 
            } while (!AO_compare_and_swap(target, old, new)); 
    } 

我需要一种方法来将* target读入局部变量，并确保局部变量不会被优化而只是* target。是不稳定的答案？

Answer 1

是的，那（正是那个）就是volatile所做的。

int grab_next_target(volatile int *target) {
    ...
    int old = *target; // Guaranteed to access "target" exactly once
    ...
}