On Tue 30-07-19 10:36:59, John Lenton wrote:
> On Tue, 30 Jul 2019 at 10:29, Jan Kara <jack@xxxxxxx> 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...
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
