at 21:36, Jan Kara <jack@xxxxxxx> wrote:
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@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...
So how about attached patch? It fixes the regression for me.
Tested-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
<0001-loop-Fix-mount-2-failure-due-to-race-with-LOOP_SET_F.patch>
