Date: Sat, 18 Aug 2007 18:14:49 +0200 From: Pawel Jakub Dawidek <pjd@FreeBSD.org> To: Alfred Perlstein <alfred@freebsd.org> Cc: freebsd-arch@FreeBSD.org Subject: Re: Lockless uidinfo. Message-ID: <20070818161449.GE6498@garage.freebsd.pl> In-Reply-To: <20070818155041.GY90381@elvis.mu.org> References: <20070818120056.GA6498@garage.freebsd.pl> <20070818142337.GW90381@elvis.mu.org> <20070818150028.GD6498@garage.freebsd.pl> <20070818155041.GY90381@elvis.mu.org>
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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.
>=20
> 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 =3D=3D 0 */
mtx_lock(&uihashtbl_mtx);
refcount_acquire(&uip->ui_ref);
/* ui_ref =3D=3D 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?
--=20
Pawel Jakub Dawidek http://www.wheel.pl
pjd@FreeBSD.org http://www.FreeBSD.org
FreeBSD committer Am I Evil? Yes, I Am!
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