From owner-freebsd-arch@FreeBSD.ORG Sat Aug 18 16:15:54 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 918A316A41B; Sat, 18 Aug 2007 16:15:54 +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 1E13713C48D; Sat, 18 Aug 2007 16:15:53 +0000 (UTC) (envelope-from pjd@garage.freebsd.pl) Received: by mail.garage.freebsd.pl (Postfix, from userid 65534) id 3941945CD9; Sat, 18 Aug 2007 18:15:51 +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 C36AE45685; Sat, 18 Aug 2007 18:15:45 +0200 (CEST) Date: Sat, 18 Aug 2007 18:14:49 +0200 From: Pawel Jakub Dawidek To: Alfred Perlstein Message-ID: <20070818161449.GE6498@garage.freebsd.pl> References: <20070818120056.GA6498@garage.freebsd.pl> <20070818142337.GW90381@elvis.mu.org> <20070818150028.GD6498@garage.freebsd.pl> <20070818155041.GY90381@elvis.mu.org> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="brEuL7wsLY8+TuWz" Content-Disposition: inline In-Reply-To: <20070818155041.GY90381@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 16:15:54 -0000 --brEuL7wsLY8+TuWz Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 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! --brEuL7wsLY8+TuWz Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.4 (FreeBSD) iD8DBQFGxxr5ForvXbEpPzQRAlMmAKDKLAdY8FdL3zxvKydBiyBkSYglGACfR6Bz VWMnsUbbb/z6hvcfToxJWic= =z16K -----END PGP SIGNATURE----- --brEuL7wsLY8+TuWz--