Re: [PATCH 8/8] dm-mpath: do not activate failed paths

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Feb 28 2014 at  4:52am -0500,
Hannes Reinecke <hare@suse.de> wrote:

> On 02/28/2014 10:37 AM, Junichi Nomura wrote:
> > On 02/28/14 18:32, Hannes Reinecke wrote:
> >> On 02/28/2014 09:49 AM, Junichi Nomura wrote:
> >>> On 02/27/14 16:30, Hannes Reinecke wrote:
> >>>> activate_path() is run without a lock, so the path might be
> >>>> set to failed before activate_path() had a chance to run.
> >>>>
> >>>> Signed-off-by: Hannes Reinecke <hare@suse.de>
> >>>> ---
> >>>>  drivers/md/dm-mpath.c | 7 +++++--
> >>>>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
> >>>> index 0a40fa9..22e7365 100644
> >>>> --- a/drivers/md/dm-mpath.c
> >>>> +++ b/drivers/md/dm-mpath.c
> >>>> @@ -1196,8 +1196,11 @@ static void activate_path(struct work_struct *work)
> >>>>  	struct pgpath *pgpath =
> >>>>  		container_of(work, struct pgpath, activate_path.work);
> >>>>  
> >>>> -	scsi_dh_activate(bdev_get_queue(pgpath->path.dev->bdev),
> >>>> -				pg_init_done, pgpath);
> >>>> +	if (pgpath->is_active)
> >>>> +		scsi_dh_activate(bdev_get_queue(pgpath->path.dev->bdev),
> >>>> +				 pg_init_done, pgpath);
> >>>> +	else
> >>>> +		pg_init_done(pgpath, SCSI_DH_DEV_OFFLINED);
> >>>
> >>> Hi Hannes,
> >>>
> >>> What problem are you going to fix with this?
> >>> It is still possible that the path is set to failed just after
> >>> "if (pgpath->is_active)" and before "scsi_dh_activate".
> >>>
> >> activate_path() is run from a workqueue, so there is a race window
> >> between the check ->is_active in __pg_init_all_paths() and the time
> >> the scsi_dh_activate() is executed. And as activate_path)() is run
> >> without any locks we don't have any synchronization with fail_path().
> >> So it's well possible that a path gets failed in between
> >> __pg_init_all_paths() and activate_paths().
> > 
> > What I pointed is that your patch makes the window very small
> > but doesn't close it.
> > If the race is a problem, this patch doesn't fix the problem.
> > 
> True. As said I just want to avoid unnecessary overhead by calling
> functions which are known to be failing.
> This patch actually makes more sense in combination with the 'accept
> failed paths' patch, where it's much more likely to trigger.

FYI, I still intend to review/take your "accept failed paths" patch.
Would be helpful if any rebase is needed for that patch that you do so
and re-post.

One thing I noticed is you're only converting MAJ:MIN paths to devices.
I think we should factor a helper out of DM core that does the full path
lookup check from dm_get_device() -- rather than you open coding an
older version of the MAJ:MIN device path processing.

But is there a reason for not using lookup_bdev()?  Because the device
is failed it cannot be found using lookup_bdev()?