From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758477AbYE0TAT (ORCPT ); Tue, 27 May 2008 15:00:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756440AbYE0TAF (ORCPT ); Tue, 27 May 2008 15:00:05 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:45202 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756707AbYE0TAE (ORCPT ); Tue, 27 May 2008 15:00:04 -0400 Date: Tue, 27 May 2008 15:59:42 -0300 From: Mauro Carvalho Chehab To: Jonathan Corbet Cc: Alan Cox , video4linux-list@redhat.com, linux-kernel@vger.kernel.org, Alan Cox Subject: Re: [PATCH] video4linux: Push down the BKL Message-ID: <20080527155942.7693c360@gaivota> In-Reply-To: <20080527103755.1fd67ec1@bike.lwn.net> References: <20080522223700.2f103a14@core> <20080526135951.7989516d@gaivota> <20080526202317.GA12793@devserv.devel.redhat.com> <20080526181027.1ff9c758@gaivota> <20080526220154.GA15487@devserv.devel.redhat.com> <20080527101039.1c0a3804@gaivota> <20080527094144.1189826a@bike.lwn.net> <20080527133100.6a9302fb@gaivota> <20080527103755.1fd67ec1@bike.lwn.net> X-Mailer: Claws Mail 3.4.0 (GTK+ 2.12.9; x86_64-mandriva-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 27 May 2008 10:37:55 -0600 Jonathan Corbet wrote: > On Tue, 27 May 2008 13:31:00 -0300 > Mauro Carvalho Chehab wrote: > > > Since the other methods don't explicitly call BKL (and, AFAIK, kernel > > open handler don't call it neither), if a program 1 is opening a > > device and initializing some data, and a program 2 starts doing > > ioctl, interrupting program 1 execution in the middle of a data > > initialization procedure, you may have a race condition, since some > > devices initialize some device global data during open [1]. > > In fact, 2.6.26 and prior kernels *do* acquire the BKL on open (for > char devices) - that's the behavior that the bkl-removal tree is there > to do away with. So, for example, I've pushed that acquisition down > into video_open() instead. > > So, for now, open() is serialized against ioctl() in video drivers. As > soon as you take the BKL out of ioctl(), though, that won't happen, > unless the mutex you use is also acquired in the open path. Ok. A few drivers seem to be almost read to work without BKL. For example, em28xx has already a lock at the operations that change values at "dev" struct, including open() method. However, since the lock is not called at get operations, it needs to be fixed. I would also change it from mutex to a read/write semaphore, since two (or more) get operations can safely happen in parallel. Cheers, Mauro