c - Is my spin lock implementation correct and optimal?

Question

I'm using a spin lock to protect a very small critical section. Contention happens very rarely so a spin lock is more appropriate than a regular mutex.

My current code is as follows, and assumes x86 and GCC:

volatile int exclusion = 0;

void lock() {
    while (__sync_lock_test_and_set(&exclusion, 1)) {
        // Do nothing. This GCC builtin instruction
        // ensures memory barrier.
    }
}

void unlock() {
    __sync_synchronize(); // Memory barrier.
    exclusion = 0;
}

So I'm wondering:

• Is this code correct? Does it correctly ensure mutual exclusion?
• Does it work on all x86 operating systems?
• Does it work on x86_64 too? On all operating systems?
• Is it optimal?
  • I've seen spin lock implementations using compare-and-swap but I'm not sure which is better.
  • According to the GCC atomic builtins documentation (http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html) there's also __sync_lock_release. I'm not an expert on memory barriers so I'm not sure whether it's okay for me to use this instead of __sync_synchronize.
  • I'm optimizing for the case in which there's no contention.

I do not care at all about contention. There may be 1, maybe 2 other threads trying to lock the spin lock once every few days.

Answer 1

Looks fine to me. Btw, here is the textbook implementation that is more efficient even in the contended case.

void lock(volatile int *exclusion)
{
    while (__sync_lock_test_and_set(exclusion, 1))
        while (*exclusion)
            ;
}