From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S265288AbUGGSmT (ORCPT ); Wed, 7 Jul 2004 14:42:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S265292AbUGGSmT (ORCPT ); Wed, 7 Jul 2004 14:42:19 -0400 Received: from mail-relay-1.tiscali.it ([212.123.84.91]:48297 "EHLO mail-relay-1.tiscali.it") by vger.kernel.org with ESMTP id S265288AbUGGSmP (ORCPT ); Wed, 7 Jul 2004 14:42:15 -0400 Date: Wed, 7 Jul 2004 20:42:02 +0200 From: Andrea Arcangeli To: Andrew Morton Cc: marcelo.tosatti@cyclades.com, mason@suse.com, linux-kernel@vger.kernel.org Subject: Re: Unnecessary barrier in sync_page()? Message-ID: <20040707184202.GN28479@dualathlon.random> References: <20040707175724.GB3106@logos.cnet> <20040707182025.GJ28479@dualathlon.random> <20040707112953.0157383e.akpm@osdl.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20040707112953.0157383e.akpm@osdl.org> X-GPG-Key: 1024D/68B9CB43 13D9 8355 295F 4823 7C49 C012 DFA1 686E 68B9 CB43 X-PGP-Key: 1024R/CB4660B9 CC A0 71 81 F4 A0 63 AC C0 4B 81 1D 8C 15 C8 E5 User-Agent: Mutt/1.5.6i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 07, 2004 at 11:29:53AM -0700, Andrew Morton wrote: > Andrea Arcangeli wrote: > > > > however the smp_mb() isn't needed in sync_page, simply because it's > > perfectly ok if we start running sync_page before reading pagelocked. > > All we care about is to run sync_page _before_ io_schedule() and that we > > read PageLocked _after_ prepare_to_wait_exclusive. > > > > So the locking in between PageLocked and sync_page is _absolutely_ > > worthless and the smp_mb() can go away. > > IIRC, Chris added that barrier (and several similar) for the reads in > page_mapping(). how does it help to know the page is not locked before executing page_mapped?