On Tue, Jul 12, 2011 at 12:41:30PM -0500, James Bottomley wrote: > On Tue, 2011-07-12 at 13:06 -0400, Vivek Goyal wrote: > > On Mon, Jul 11, 2011 at 06:40:11PM -0400, Mike Snitzer wrote: > > > [cc'ing dm-devel, vivek and tejun] > > > > > > On Fri, Jul 8, 2011 at 7:04 PM, Roland Dreier <roland@xxxxxxxxxx> wrote: > > > > From: Roland Dreier <roland@xxxxxxxxxxxxxxx> > > > > > > > > This fixes crashes such as the below that I see when the storage > > > > underlying a dm-multipath device is hot-removed. The problem is that > > > > dm requeues a request to a device whose block queue has already been > > > > cleaned up, and blk_insert_cloned_request() doesn't check if the queue > > > > is alive, but rather goes ahead and tries to queue the request. This > > > > ends up dereferencing the elevator that was already freed in > > > > blk_cleanup_queue(). > > > > > > Your patch looks fine to me: > > > Acked-by: Mike Snitzer <snitzer@xxxxxxxxxx> > > > > > > And I looked at various code paths to arrive at the references DM takes. > > > > > > A reference is taken on the underlying devices' block_device via > > > drivers/md/dm-table.c:open_dev() with blkdev_get_by_dev(). open_dev() > > > also does bd_link_disk_holder(), resulting in the mpath device > > > becoming a holder of the underlying devices. e.g.: > > > /sys/block/sda/holders/dm-4 > > > > > > But at no point does DM-mpath get a reference to the underlying > > > devices' request_queue that gets assigned to clone->q (in > > > drivers/md/dm-mpath.c:map_io). > > > > > > Seems we should, though AFAIK it won't help with the issue you've > > > pointed out (because the hotplugged device's driver already called > > > blk_cleanup_queue and nuked the elevator). > > > > [Thinking loud] > > > > Could it be a driver specific issue that it cleaned up the request > > queue too early? > > One could glibly answer yes to this. However, the fact is that it's > currently SCSI which manages the queue, so SCSI cleans it up. Now, the > only real thing dm is interested in is the queue itself, hence the need > to take a reference to the queue. However, queue references don't pin > SCSI devices, so you can hold a queue reference all you like and SCSI > will still clean up the queue. > > I think a better question is what should cleaning up the queue do? SCSI > uses it to indicate that we're no longer processing requests, which > happens when the device goes into a DEL state, but queue cleanup tears > down the elevators and really makes the request queue non functional. > In this case, holding a reference isn't particularly helpful. > > I'm starting to wonder if there's actually any value to > blk_cleanup_queue() and whether its functionality wouldn't be better > assumed by the queue release function on last put. I think one problem point is q->queue_lock. If driver drops its reference on queue and cleans up its data structures, then it will free up memory associated with q->queue_lock too. (If driver provided its own queue lock). In that case anything which is dependent on queue lock, needs to be freed up on blk_cleanup_queue(). If we can make sure that request queue reference will keep the spin lock alive, then i guess all cleanup part might be able to go in release queue function. Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
