Memory ordering issues

Question

I'm experimenting with C++0x support and there is a problem, that I guess shouldn't be there. Either I don't understand the subject or gcc has a bug.

I have the following code, initially x and y are equal. Thread 1 always increments x first and then increments y. Both are atomic integer values, so there is no problem with the increment at all. Thread 2 is checking whether the x is less than y and displays an error message if so.

This code fails sometimes, but why? The issue here is probably memory reordering, but all atomic operations are sequentially consistent by default and I didn't explicitly relax of those any operations. I'm compiling this code on x86, which as far as I know shouldn't have any ordering issues. Can you please explain what the problem is?

#include <iostream>
#include <atomic>
#include <thread>

std::atomic_int x;
std::atomic_int y;

void f1()
{
    while (true)
    {
        ++x;
        ++y;
    }
}

void f2()
{
    while (true)
    {
        if (x < y)
        {
            std::cout << "error" << std::endl;
        }
    }
}

int main()
{
    x = 0;
    y = 0;

    std::thread t1(f1);
    std::thread t2(f2);

    t1.join();
    t2.join();
}

The result can be viewed here.

Answer 1

There is a problem with the comparison:

x < y

The order of evaluation of subexpressions (in this case, of x and y) is unspecified, so y may be evaluated before x or x may be evaluated before y.

If x is read first, you have a problem:

x = 0; y = 0;
t2 reads x (value = 0);
t1 increments x; x = 1;
t1 increments y; y = 1;
t2 reads y (value = 1);
t2 compares x < y as 0 < 1; test succeeds!

If you explicitly ensure that y is read first, you can avoid the problem:

int yval = y;
int xval = x;
if (xval < yval) { /* ... */ }

Answer 2

The problem could be in your test:

if (x < y)

the thread could evaluate x and not get around to evaluating y until much later.