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Date: Sat, 18 Aug 2007 10:17:38 -0700
From: Alfred Perlstein <alfred@freebsd.org>
To: Pawel Jakub Dawidek <pjd@FreeBSD.org>
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Cc: freebsd-arch@FreeBSD.org
Subject: Re: Lockless uidinfo.
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* Pawel Jakub Dawidek <pjd@FreeBSD.org> [070818 09:14] wrote:
> On Sat, Aug 18, 2007 at 08:50:41AM -0700, Alfred Perlstein wrote:
> > * Pawel Jakub Dawidek <pjd@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?

I don't know, that's why I was asking you. :)

-- 
- Alfred Perlstein