Re: [PATCH] loop: Don't change loop device under exclusive opener

From: Jan Kara <jack@suse.cz>
To: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>, John Lenton <john.lenton@canonical.com>,
	Kai-Heng Feng <kaihengfeng@me.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	linux-block@vger.kernel.org,
	jean-baptiste.lallement@canonical.com
Subject: Re: [PATCH] loop: Don't change loop device under exclusive opener
Date: Wed, 31 Jul 2019 10:56:31 +0200	[thread overview]
Message-ID: <20190731085631.GA15806@quack2.suse.cz> (raw)
In-Reply-To: <009a7a06-67c5-6b3c-5aa3-7d67ca35fc3a@kernel.dk>

On Tue 30-07-19 13:17:28, Jens Axboe wrote:
> On 7/30/19 7:36 AM, Jan Kara 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@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.
> 
> Jan, I've applied this patch - and also marked it for stable, so it'll
> end up in 5.2-stable. Thanks.

Thanks Jens!

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