From owner-freebsd-arch@FreeBSD.ORG Sat Aug 18 17:28:26 2007 Return-Path: Delivered-To: freebsd-arch@FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 7A61F16A420; Sat, 18 Aug 2007 17:28:26 +0000 (UTC) (envelope-from pjd@garage.freebsd.pl) Received: from mail.garage.freebsd.pl (arm132.internetdsl.tpnet.pl [83.17.198.132]) by mx1.freebsd.org (Postfix) with ESMTP id D1F2E13C442; Sat, 18 Aug 2007 17:28:25 +0000 (UTC) (envelope-from pjd@garage.freebsd.pl) Received: by mail.garage.freebsd.pl (Postfix, from userid 65534) id 9131D487F0; Sat, 18 Aug 2007 19:28:24 +0200 (CEST) Received: from localhost (154.81.datacomsa.pl [195.34.81.154]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by mail.garage.freebsd.pl (Postfix) with ESMTP id 26E0D4569A; Sat, 18 Aug 2007 19:28:20 +0200 (CEST) Date: Sat, 18 Aug 2007 19:27:19 +0200 From: Pawel Jakub Dawidek To: Alfred Perlstein Message-ID: <20070818172719.GF6498@garage.freebsd.pl> References: <20070818120056.GA6498@garage.freebsd.pl> <20070818142337.GW90381@elvis.mu.org> <20070818150028.GD6498@garage.freebsd.pl> <20070818155041.GY90381@elvis.mu.org> <20070818161449.GE6498@garage.freebsd.pl> <20070818171738.GB90381@elvis.mu.org> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="eDB11BtaWSyaBkpc" Content-Disposition: inline In-Reply-To: <20070818171738.GB90381@elvis.mu.org> User-Agent: Mutt/1.4.2.3i X-PGP-Key-URL: http://people.freebsd.org/~pjd/pjd.asc X-OS: FreeBSD 7.0-CURRENT i386 X-Spam-Checker-Version: SpamAssassin 3.0.4 (2005-06-05) on mail.garage.freebsd.pl X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=BAYES_00 autolearn=ham version=3.0.4 Cc: freebsd-arch@FreeBSD.org Subject: Re: Lockless uidinfo. X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 18 Aug 2007 17:28:26 -0000 --eDB11BtaWSyaBkpc Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sat, Aug 18, 2007 at 10:17:38AM -0700, Alfred Perlstein wrote: > * Pawel Jakub Dawidek [070818 09:14] wrote: > > On Sat, Aug 18, 2007 at 08:50:41AM -0700, Alfred Perlstein wrote: > > > * Pawel Jakub Dawidek [070818 07:59] wrote: > > > > Yes, to lookup uidinfo you need to hold uihashtbl_mtx mutex, so onc= e you > > > > hold it and ui_ref is 0, noone will be able to reference it, becaus= e it > > > > has to wait to look it up. > > >=20 > > > And the field doesn't need to be volatile to prevent cached/opportuni= tic > > > reads? > >=20 > > The only chance of something like this will be the scenario below: > >=20 > > 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; > > } > >=20 > > 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. > >=20 > > Is my thinking correct? >=20 > I don't know, that's why I was asking you. :) In previous version of the patch I had atomic_load() in there, but after some thinking I decided it's not needed and I change it. Now, that you asked about it I was afraid that maybe my thinking isn't correct. Anyway, it'll be good if someone could confirm it's ok. --=20 Pawel Jakub Dawidek http://www.wheel.pl pjd@FreeBSD.org http://www.FreeBSD.org FreeBSD committer Am I Evil? Yes, I Am! --eDB11BtaWSyaBkpc Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.4 (FreeBSD) iD8DBQFGxyv3ForvXbEpPzQRAsJXAJ0e00hB+95tdLvWgtkiorckarjC0gCfVsDf pWL2e7NV3LNx7NhWl9B6D2s= =4DHl -----END PGP SIGNATURE----- --eDB11BtaWSyaBkpc--