I have some immutable data structures that I would like to manage using reference counts, sharing them across threads on an SMP system.

Here's what the release code looks like:

void avocado_release(struct avocado *p)
{
    if (atomic_dec(p->refcount) == 0) {
        free(p->pit);
        free(p->juicy_innards);
        free(p);
    }
}

Does atomic_dec need a memory barrier in it? If so, what kind of memory barrier?

Additional notes: The application must run on PowerPC and x86, so any processor-specific information is welcomed. I already know about the GCC atomic builtins. As for immutability, the refcount is the only field that changes over the duration of the object.