From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752633AbZBCXUE (ORCPT ); Tue, 3 Feb 2009 18:20:04 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754541AbZBCXTg (ORCPT ); Tue, 3 Feb 2009 18:19:36 -0500 Received: from vena.lwn.net ([206.168.112.25]:58184 "EHLO vena.lwn.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752952AbZBCXTe (ORCPT ); Tue, 3 Feb 2009 18:19:34 -0500 Date: Tue, 3 Feb 2009 16:19:31 -0700 From: Jonathan Corbet To: Andrew Morton 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: <20090203161931.4054a25e@bike.lwn.net> In-Reply-To: <20090203145346.8df40277.akpm@linux-foundation.org> 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> Organization: LWN.net X-Mailer: Claws Mail 3.7.0 (GTK+ 2.15.2; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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? > 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? jon