From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759243AbYE0UZ2 (ORCPT ); Tue, 27 May 2008 16:25:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757951AbYE0UZU (ORCPT ); Tue, 27 May 2008 16:25:20 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:53075 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756379AbYE0UZU (ORCPT ); Tue, 27 May 2008 16:25:20 -0400 Date: Tue, 27 May 2008 17:24:46 -0300 From: Mauro Carvalho Chehab To: Arjan van de Ven Cc: Jonathan Corbet , Alan Cox , video4linux-list@redhat.com, linux-kernel@vger.kernel.org, Alan Cox Subject: Re: [PATCH] video4linux: Push down the BKL Message-ID: <20080527172446.6f856f21@gaivota> In-Reply-To: <20080527125041.0fc28fd4@infradead.org> 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> <20080527155942.7693c360@gaivota> <20080527125041.0fc28fd4@infradead.org> 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 12:50:41 -0700 Arjan van de Ven wrote: > On Tue, 27 May 2008 15:59:42 -0300 > Mauro Carvalho Chehab wrote: > > > 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. > > > \ > > please don't use rw/sems just because there MIGHT be parallel. > THey're more expensive than mutexes by quite a bit and you get a lot > less checking from lockdep. They make sense for very specific, very > read biased, contended cases. But please don't use them "just > because"... > Good point. The nature of get operations on V4L are not worthy enough to justify the loss of lockdep checking. Cheers, Mauro