Andrew Morton a écrit :
On Wed, 13 Dec 2006 16:12:46 -0800
Greg KH <[email protected]> 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.
I agree this 'optimization' is not "good" (I was the guy who suggested it
http://lkml.org/lkml/2006/1/30/4 )
After Eric Biederman message (http://lkml.org/lkml/2006/1/30/292) I remember
adding some stat counters and telling Greg to not put the patch in because
kref_put() was mostly called with refcount=1. But the patch did its way. I
*did* ask Greg to revert it, but cannot find this mail archived somewhere...
But I believe Venkatesh problem comes from its release() function : It is
supposed to free the object.
If not, it should properly setup it so that further uses are OK.
ie doing in release(kref)
atomic_set(&kref->count, 0);
Eric
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