On Fri, Jun 15 2007, 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 of 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 <htejun@xxxxxxxxx> > Cc: Jenn Axboe <jens.axboe@xxxxxxxxxx> > Cc: David Greaves <david@xxxxxxxxxxxx> > --- > Okay, it took a lot of hours of debugging but boiled down to two liner > fix. I feel so empty. :-) RAID6 triggers this reliably because it > uses BIO_BARRIER heavily to update its superblock. The recent ATA > suspend/resume rewrite is hit by this because it uses SCSI internal > commands to spin down and up the drives for suspending and resuming. > > David, please test this. Jens, does it look okay? Yep looks good, except for the bad multi-line comment style, but that's minor stuff ;-) Acked-by: Jens Axboe <jens.axboe@xxxxxxxxxx> -- Jens Axboe _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
