Re: [PATCH] block: Check that queue is alive inblk_insert_cloned_request()

[James Bottomley]

On Tue, 2011-07-12 at 14:02 -0400, Vivek Goyal wrote:
> 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().

I don't quite follow.  blk_cleanup_queue() doesn't free anything (well,
except the elevator).  Final put will free the queue structure which
contains the lock, but if it's really a final put, you have no other
possible references, so no-one is using the lock ... well, assuming
there isn't a programming error, of course ...

> 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.

As I said: cleanup doesn't free the structure containing the lock,
release does, so that piece wouldn't be altered by putting
blk_cleanup_queue() elsewhere.

James


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