From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754581AbZBCXad (ORCPT ); Tue, 3 Feb 2009 18:30:33 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751122AbZBCXaM (ORCPT ); Tue, 3 Feb 2009 18:30:12 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:60783 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753905AbZBCXaK (ORCPT ); Tue, 3 Feb 2009 18:30:10 -0500 Date: Tue, 3 Feb 2009 15:29:08 -0800 From: Andrew Morton To: Jonathan Corbet Cc: mpm@selenic.com, dada1@cosmosbay.com, linux-kernel@vger.kernel.org, andi@firstfloor.org, oleg@redhat.com, viro@ZenIV.linux.org.uk, davidel@xmailserver.org, davem@davemloft.net, hch@lst.de, linux-api@vger.kernel.org, alan@lxorguk.ukuu.org.uk Subject: Re: [PATCH 2/4] Convert epoll to a bitlock Message-Id: <20090203152908.355699e0.akpm@linux-foundation.org> In-Reply-To: <20090203161931.4054a25e@bike.lwn.net> References: <1233598811-6871-1-git-send-email-corbet@lwn.net> <1233598811-6871-3-git-send-email-corbet@lwn.net> <20090203133942.2ecec281.akpm@linux-foundation.org> <4988BD4E.8080206@cosmosbay.com> <20090203140543.6e915f97.akpm@linux-foundation.org> <1233699722.3243.127.camel@calx> <20090203153740.363d0a04@bike.lwn.net> <20090203145346.8df40277.akpm@linux-foundation.org> <20090203161931.4054a25e@bike.lwn.net> X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 3 Feb 2009 16:19:31 -0700 Jonathan Corbet wrote: > On Tue, 3 Feb 2009 14:53:46 -0800 > Andrew Morton wrote: > > > Well. We _could_ whack part of this nut with my usual hammer: protect > > f_flags with file->f_dentry->d_inode->i_lock. IIRC there was some > > objection to that - performance? > > Andi has objected to the addition of locks, but i_lock is maybe > sufficiently dispersed to pass muster there. Hope so. I'd wrap it in a lock_file_flags(file*) thing so we can change it later on (add a lock to struct file, take a global, lock, etc). > I had an instinctive > reaction to using a lock which is three pointers away, but I can get > over that. I'll admit a bit of ignorance, though: if a given struct > file exists, do we know for sure that file->f_dentry->d_inode exists? It should. A NULL ->d_inode especially signifies a negative dentry. > > One problem here seems to be that we're trying to change multiple > > things at the same time. We can blame the BKL for that. > > > > Can we break the problem into manageable chunks? Your patchset did > > that, I guess. What were those chunks again? ;) > > I'm not really sure how to break it down any further. If we take the > i_lock approach, the chunks would be something like: > > 1) Use i_lock to protect accesses to f_flags. This would enable some > BKL usage to be removed, but would not fix fasync. > > 2) Move responsibility for the FASYNC bit into ->fasync(), with > fasync_helper() doing it in almost all situations. The remaining > BKL usage would then go away. > > 3) The same optional fasync() return values cleanup. > > Make sense? yup. If the ->i_lock think is no good then we can trivially switch over to a global lock. Heck, we could even go back to lock_kernel() ;)