[dm-devel] v3.15 dm-mpath regression: cable pull test causes I/O hang

[Mike Snitzer]

On Fri, Jun 27 2014 at  9:33am -0400,
Mike Snitzer <snitzer at redhat.com> wrote:

> On Fri, Jun 27 2014 at  9:02am -0400,
> Bart Van Assche <bvanassche at acm.org> wrote:
> 
> > Hello,
> > 
> > While running a cable pull simulation test with dm_multipath on top of
> > the SRP initiator driver I noticed that after a few iterations I/O locks
> > up instead of dm_multipath processing the path failure properly (see also
> > below for a call trace). At least kernel versions 3.15 and 3.16-rc2 are
> > vulnerable. This issue does not occur with kernel 3.14. I have tried to
> > bisect this but gave up when I noticed that I/O locked up completely with
> > a kernel built from git commit ID e809917735ebf1b9a56c24e877ce0d320baee2ec
> > (dm mpath: push back requests instead of queueing). But with the bisect I
> > have been able to narrow down this issue to one of the patches in "Merge
> > tag 'dm-3.15-changes' of git://git.kernel.org/pub/scm/linux/kernel/git/
> > device-mapper/linux-dm". Does anyone have a suggestion how to analyze this
> > further or how to fix this ?

I still don't have a _known_ fix for your issue but I reviewed commit
e809917735ebf1b9a56c24e877ce0d320baee2ec closer and identified what
looks to be a regression in logic for multipath_busy, it now calls
!pg_ready() instead of directly checking pg_init_in_progress.  I think
this is needed (Hannes, what do you think?):

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index 3f6fd9d..561ead6 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -373,7 +373,7 @@ static int __must_push_back(struct multipath *m)
 		 dm_noflush_suspending(m->ti)));
 }
 
-#define pg_ready(m) (!(m)->queue_io && !(m)->pg_init_required)
+#define pg_ready(m) (!(m)->queue_io && !(m)->pg_init_required && !(m)->pg_init_in_progress)
 
 /*
  * Map cloned requests