From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Garzik Subject: Re: [PATCH] block: always requeue !fs requests at the front Date: Fri, 15 Jun 2007 11:08:47 -0400 Message-ID: <4672AB7F.7030702@garzik.org> References: <200706020122.49989.rjw@sisk.pl> <46706968.7000703@dgreaves.com> <200706140115.58733.rjw@sisk.pl> <46714ECF.8080203@gmail.com> <46715A66.8030806@suse.de> <20070615094246.GN29122@htj.dyndns.org> <20070615110544.GR6149@kernel.dk> <20070615111728.GO29122@htj.dyndns.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20070615111728.GO29122@htj.dyndns.org> Sender: linux-kernel-owner@vger.kernel.org To: Tejun Heo Cc: Jens Axboe , David Greaves , "Rafael J. Wysocki" , Linus Torvalds , David Chinner , xfs@oss.sgi.com, "'linux-kernel@vger.kernel.org'" , linux-pm , Neil Brown List-Id: linux-pm@vger.kernel.org Tejun Heo wrote: > SCSI marks internal commands with REQ_PREEMPT and push it at the front > of the request queue using blk_execute_rq(). When entering suspended > or frozen state, SCSI devices are quiesced using > scsi_device_quiesce(). In quiesced state, only REQ_PREEMPT requests > are processed. This is how SCSI blocks other requests out while > suspending and resuming. As all internal commands are pushed at the > front of the queue, this usually works. > > Unfortunately, this interacts badly with ordered requeueing. To > preserve request order on requeueing (due to busy device, active EH or > other failures), requests are sorted according to ordered sequence on > requeue if IO barrier is in progress. > > The following sequence deadlocks. > > 1. IO barrier sequence issues. > > 2. Suspend requested. Queue is quiesced with part or all of IO > barrier sequence at the front. > > 3. During suspending or resuming, SCSI issues internal command which > gets deferred and requeued for some reason. As the command is > issued after the IO barrier in #1, ordered requeueing code puts the > request after IO barrier sequence. > > 4. The device is ready to process requests again but still is in > quiesced state and the first request of the queue isn't > REQ_PREEMPT, so command processing is deadlocked - > suspending/resuming waits for the issued request to complete while > the request can't be processed till device is put back into > running state by resuming. > > This can be fixed by always putting !fs requests at the front when > requeueing. > > The following thread reports this deadlock. > > http://thread.gmane.org/gmane.linux.kernel/537473 > > Signed-off-by: Tejun Heo > Cc: Jenn Axboe > Cc: David Greaves Acked-by: Jeff Garzik Thanks Tejun, you kick ass as usual. Jeff