> On Fri, Nov 24, 2017 at 2:36 AM, Reshetova, Elena
> <elena.reshetova@intel.com> wrote:
> >> On Fri, Oct 20, 2017 at 10:37:38AM +0300, Elena Reshetova wrote:
> >> >     } else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
> >> >             r = upgrade_mode(dd, mode, t->md);
> >> >             if (r)
> >> >                     return r;
> >> > +           refcount_inc(&dd->count);
> >> >     }
> >>
> >> Missing here:
> >>
> >>         else
> >>               refcount_inc(&dd->count);
> >>
> >> ?
> >
> > Oh, yes, thanks for catching this! I think this got unnoticed so far and patch was
> merged, so I am going to send a followup patch now.
> 
> I pushed this fix and will send to Linus next week:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-
> dm.git/commit/?h=dm-4.15&id=d908af82d06cc420f9581c97c6db941cb87e4434


I guess you mean this commit:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=c2318d07ead871f058dda62e942ed7b6b1c1cfcf

Unfortunately it is not correct:

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 88130b5..f6d32ee 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -451,15 +451,15 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
 			return r;
 		}
 
-		refcount_set(&dd->count, 1);
+		refcount_set(&dd->count, 0);
 		list_add(&dd->list, &t->devices);
 
 	} else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
 		r = upgrade_mode(dd, mode, t->md);
 		if (r)
 			return r;
-		refcount_inc(&dd->count);
 	}
+	refcount_inc(&dd->count);
 
Problem will be here if you hit this refcount_inc() after the refcount_set(&dd->count, 0) earlier. 
refcount_inc() does not increment on zero value *ever* for security reasons and instead people
should initialize refcounters to 1 always and do increments from there if needed. 
This was the reason for the initial change I did, my mistake was just to forget to increment it also
in case condition (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) fails. 

I have issues with my intel smpt server for sending patches (I will get it fixed tomorrow from internal network),
so I am attaching the patch I did end of last week to this thread instead (or alternatively can properly send it tomorrow after fix). 
Sorry for the delay!

Best Regards,
Elena.