From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030401AbWGNLXL (ORCPT ); Fri, 14 Jul 2006 07:23:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932408AbWGNLXL (ORCPT ); Fri, 14 Jul 2006 07:23:11 -0400 Received: from pne-smtpout2-sn2.hy.skanova.net ([81.228.8.164]:20118 "EHLO pne-smtpout2-sn2.hy.skanova.net") by vger.kernel.org with ESMTP id S932402AbWGNLXK (ORCPT ); Fri, 14 Jul 2006 07:23:10 -0400 To: Arjan van de Ven Cc: Laurent Riffard , Kernel development list , axboe@suse.de Subject: Re: 2.6.17-rc6-mm1/pktcdvd - BUG: possible circular locking References: <448875D1.5080905@free.fr> <448D84C0.1070400@linux.intel.com> <1151000451.3120.56.camel@laptopd505.fenrus.org> From: Peter Osterlund Date: 14 Jul 2006 13:22:57 +0200 In-Reply-To: <1151000451.3120.56.camel@laptopd505.fenrus.org> Message-ID: User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.4 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Arjan van de Ven writes: > On Thu, 2006-06-22 at 16:50 +0200, Peter Osterlund wrote: > > Arjan van de Ven writes: > > > > > Laurent Riffard wrote: > > > > Hello, > > > > This BUG happened while pktcdvd service was starting. Basically, the > > > > 2 following commands were issued: > > > > - modprobe ptkcdvd > > > > - pktsetup dvd /dev/dvd > > > > > > This appears to be a real bug: > > > > > > A normal pkt dvd block dev open takes the > > > bdev_mutex in the regular block device open path, which takes > > > ctl_mutex in the pkt_open function which gets called then from > > > the block layer. > > > > > > HOWEVER the IOCTL path does it the other way around: > > > > > > mutex_lock(&ctl_mutex); > > > ret = pkt_setup_dev(&ctrl_cmd); > > > mutex_unlock(&ctl_mutex); > > > > > > where pkt_setup_dev in term calls pkt_new_dev which > > > calls blkdev_get(), which takes the bdev_mutex. > > > > > > Looks very much like a AB-BA deadlock to me... > > > > I don't understand how this could deadlock. If the device is already > > setup, pkt_new_dev() returns before calling blkdev_get(). If the > > device is not already setup, the block device doesn't exist yet so > > there can not be another caller in the pkt_open() path. > > and what locking prevents this? And via multiple opens? You are right that my reasoning was incorrect. If someone is doing "pktsetup ; pktsetup -d" quickly in a loop while someone else is trying to open the device, one thread could be at the start of pkt_open() at the same time as another thread is in pkt_new_dev(). However, I added a 5s delay in pkt_open() to enlarge the race window. I still couldn't make the driver lock up though. The explanation is that pkt_new_dev() calls blkdev_get() with the CD device (eg /dev/hdc) as bdev parameter, while do_open() locks the bd_mutex for the pktcdvd device (eg /dev/pktcdvd/0). Do you still think this could deadlock? If not, how should the code be annotated to make this warning go away? -- Peter Osterlund - petero2@telia.com http://web.telia.com/~u89404340