bool atomic_compare_exchange( volatile A *obj, C* expected, C desired, memory_order succ, memory_order fail );
{
    fence_before(succ); // fence 1
    if(*obj == expected)
    {
        *obj = desired;
        fence_after(succ); // fence 2
        return true;
    }
    else
    {
        *expected = *obj; // strong 保证此语义一定发生；weak 依平台而定，某些平台
        fence_after(fail); // fence 3
        return false;
    }
}