Lockless uidinfo.

[Pawel Jakub Dawidek]

On Sat, Aug 18, 2007 at 08:50:41AM -0700, Alfred Perlstein wrote:
> * Pawel Jakub Dawidek <pjd at FreeBSD.org> [070818 07:59] wrote:
> > Yes, to lookup uidinfo you need to hold uihashtbl_mtx mutex, so once you
> > hold it and ui_ref is 0, noone will be able to reference it, because it
> > has to wait to look it up.
> 
> And the field doesn't need to be volatile to prevent cached/opportunitic
> reads?

The only chance of something like this will be the scenario below:

thread1 (uifind)		thread2 (uifree)
----------------		----------------
				refcount_release(&uip->ui_ref))
				/* ui_ref == 0 */
mtx_lock(&uihashtbl_mtx);
refcount_acquire(&uip->ui_ref);
/* ui_ref == 1 */
mtx_unlock(&uihashtbl_mtx);
				mtx_lock(&uihashtbl_mtx);
				if (uip->ui_ref > 0) {
					mtx_unlock(&uihashtbl_mtx);
					return;
				}

Now, you suggest that ui_ref in 'if (uip->ui_ref > 0)' may still have
cached 0? I don't think it is possible, first refcount_acquire() uses
read memory bariers (but we may still need ui_ref to volatile for this
to make any difference) and second, think of ui_ref as a field protected
by uihashtbl_mtx mutex in this very case.

Is my thinking correct?

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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