[dm-devel] 4.1-rc2 dm-multipath-mq kernel warning

[Mike Snitzer]

On Wed, May 06 2015 at  3:45am -0400,
Bart Van Assche <bart.vanassche at sandisk.com> wrote:

> On 05/06/15 04:23, Mike Snitzer wrote:
> > On Tue, May 05 2015 at 10:04am -0400,
> > Bart Van Assche <bart.vanassche at sandisk.com> wrote:
> >> While retesting my SRP initiator patches on top of kernel v4.1-rc2
> >> with DM_MQ_DEFAULT=y I ran into the kernel warning below. Does this
> >> mean that I'm missing any device mapper related patches ? This
> >> warning was reported shortly after scsi_remove_host() had been
> >> invoked.
> > 
> > I put the warning in place because, to me, if it triggers it speaks to
> > unsafe teardown occuring (request is still completing but the queue it
> > was issued from no longer exists).
> > 
> > Like I said before I'm open to removing the WARN_ON_ONCE() if this
> > scenario is perfectly valid.  But I just haven't had time to revisit
> > what appears to be a potentially serious problem with the underlying
> > paths' teardown vs upper level mpath IO.
> > 
> > I'll try to revisit this week.  But I welcome input from others too.
> > 
> > (Just thinking about it further now, it could be that the way the clone
> > request is allocated in the case of blk-mq DM is as part of the original
> > request's pdu... meaning there isn't a proper get_request() call against
> > the underlying queue.. so the expected refcounting likely isn't
> > happening.  And given the request won't be free'd from that underlying
> > request_queue there really isn't a need to artificially link these
> > cloned requests with the underlying request_queue... so I'm now leaning
> > toward just removing the WARN_ON_ONCE.. but I'll look closer tomorrow)
> 
> Hello Mike,
> 
> With CONFIG_SCSI_MQ_DEFAULT=y and CONFIG_DM_MQ_DEFAULT=n I just ran into
> the bug report below. I will continue my v4.1-rc2 tests with SCSI_MQ=n.

What were you doing when this happened?  Quite a strange place to get a
NULL pointer (it should be noted that for 4.2 hch's patch does away with
cloning the request's bios).  Is there an easy reproducer (unlikely
considering I've tested CONFIG_SCSI_MQ_DEFAULT=y and
CONFIG_DM_MQ_DEFAULT=n a fair amount).

BTW, my "Just thinking about it further now" above was relative to
CONFIG_DM_MQ_DEFAULT=y and CONFIG_SCSI_MQ_DEFAULT=n.

Mike