Is the spin lock implementation correct and optimal

cconcurrencymultithreadingmutual-exclusionspinlock

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.

Best Answer

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)
            ;
}
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