From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932112AbZKRReD (ORCPT ); Wed, 18 Nov 2009 12:34:03 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758022AbZKRReD (ORCPT ); Wed, 18 Nov 2009 12:34:03 -0500 Received: from moutng.kundenserver.de ([212.227.17.10]:61811 "EHLO moutng.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758020AbZKRReB (ORCPT ); Wed, 18 Nov 2009 12:34:01 -0500 From: Arnd Bergmann To: Jamie Lokier Subject: Re: [PATCH 1/2] BKL: Remove BKL from default_llseek() Date: Wed, 18 Nov 2009 18:33:37 +0100 User-Agent: KMail/1.12.2 (Linux/2.6.31-14-generic; KDE/4.3.2; x86_64; ; ) Cc: Alan Cox , Jan Blunck , linux-fsdevel@vger.kernel.org, "Linux-Kernel Mailinglist" , Andrew Morton , jkacur@redhat.com, Thomas Gleixner , Christoph Hellwig , =?iso-8859-1?q?Fr=E9d=E9ric_Weisbecker?= , Alexander Viro References: <1258560457-15129-1-git-send-email-jblunck@suse.de> <20091118171524.4d2f8cec@lxorguk.ukuu.org.uk> <20091118172730.GD28723@shareable.org> In-Reply-To: <20091118172730.GD28723@shareable.org> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <200911181833.37531.arnd@arndb.de> X-Provags-ID: V01U2FsdGVkX1+WGSD/naf6Ptlnvd1fQ3LA2vn+205MnkbaDl9 NPtORZb8PyRIU3rydxkG7cGuRSzPNVI4Hx51qlsy3StFtS0RSG cfxS0As5DGEYFSLj5TndA== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wednesday 18 November 2009, Jamie Lokier wrote: > Alan Cox wrote: > > > Using the BKL in llseek() does not protect the inode's i_size from > > > modification since the i_size is protected by a seqlock nowadays. Since > > > default_llseek() is already using the i_size_read() wrapper it is not the > > > BKL which is serializing the access here. > > > The access to file->f_pos is not protected by the BKL either since its > > > access in vfs_write()/vfs_read() is not protected by any lock. If the BKL > > > is not protecting anything here it can clearly get removed. > > > > No. Your logic is flawed > > > > The BKL is protected something here - it protects the change of offset > > with respect to other BKL users within drivers. The question is what if > > anything in any other driver code depends upon the BKL and uses it to > > protect f_pos. Probably very little if anything but a grep for f_pos > > through the drivers might not be a bad idea before assuming this. Very > > few touch f_pos except in their own llseek method. > > Of course, drivers shouldn't be using f_pos outside their llseek > method, as they should all behave the same with pread/pwrite as with > llseek+read/write. > > Is that mistaken? There are drivers touching f_pos in ioctl() methods, which is vaguely reasonable. There are also driver touching it in their read()/write() methods, which has no effect whatsoever. I started grepping through the kernel trying to find any instances of the first case that uses the BKL, but I only found three instances of the second case and got heavily demotivated by that. Arnd <><