On Tue 30-07-19 12:16:46, Jan Kara wrote:
> On Tue 30-07-19 10:36:59, John Lenton wrote:
> > On Tue, 30 Jul 2019 at 10:29, Jan Kara <jack@suse.cz> wrote:
> > >
> > > Thanks for the notice and the references. What's your version of
> > > util-linux? What your test script does is indeed racy. You have there:
> > >
> > > echo Running:
> > > for i in {a..z}{a..z}; do
> > >     mount $i.squash /mnt/$i &
> > > done
> > >
> > > So all mount(8) commands will run in parallel and race to setup loop
> > > devices with LOOP_SET_FD and mount them. However util-linux (at least in
> > > the current version) seems to handle EBUSY from LOOP_SET_FD just fine and
> > > retries with the new loop device. So at this point I don't see why the patch
> > > makes difference... I guess I'll need to reproduce and see what's going on
> > > in detail.
> > 
> > We've observed this in arch with util-linux 2.34, and ubuntu 19.10
> > (eoan ermine) with util-linux 2.33.
> > 
> > just to be clear, the initial reports didn't involve a zany loop of
> > mounts, but were triggered by effectively the same thing as systemd
> > booted a system with a lot of snaps. The reroducer tries to makes
> > things simpler to reproduce :-). FWIW,  systemd versions were 244 and
> > 242 for those systems, respectively.
> 
> Thanks for info! So I think I see what's going on. The two mounts race
> like:
> 
> MOUNT1					MOUNT2
> num = ioctl(LOOP_CTL_GET_FREE)
> 					num = ioctl(LOOP_CTL_GET_FREE)
> ioctl("/dev/loop$num", LOOP_SET_FD, ..)
>  - returns OK
> 					ioctl("/dev/loop$num", LOOP_SET_FD, ..)
> 					  - acquires exclusine loop$num
> 					    reference
> mount("/dev/loop$num", ...)
>  - sees exclusive reference from MOUNT2 and fails
> 					  - sees loop device is already
> 					    bound and fails
> 
> It is a bug in the scheme I've chosen that racing LOOP_SET_FD can block
> perfectly valid mount. I'll think how to fix this...

So how about attached patch? It fixes the regression for me.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR