Re: kref refcnt and false positives

[Andrew Morton <akpm@osdl.org>]

On Wed, 13 Dec 2006 16:12:46 -0800
Greg KH <gregkh@suse.de> wrote:

> > Original comment seemed to indicate that this conditional thing was
> > performance related. Is it really? If not, we should consider the below patch.
> 
> Yes, it's a performance gain and I don't see how this patch would change
> the above warning.

I suspect it's a false optimisation.

int kref_put(struct kref *kref, void (*release)(struct kref *kref))
{
	WARN_ON(release == NULL);
	WARN_ON(release == (void (*)(struct kref *))kfree);

	/*
	 * if current count is one, we are the last user and can release object
	 * right now, avoiding an atomic operation on 'refcount'
	 */
 	if ((atomic_read(&kref->refcount) == 1) ||
	    (atomic_dec_and_test(&kref->refcount))) {
		release(kref);
		return 1;
	}
	return 0;
}

The only time we avoid the atomic_dec_and_test() is when the object is
about to be freed.  ie: once in its entire lifetime.  And freeing the
object is part of an expensive (and rare) operation anyway.

otoh, we've gone and added a test-n-branch to the common case: those cases
where the object will not be freed.