From: Kai-Heng Feng <kai.heng.feng@canonical.com>
To: Jan Kara <jack@suse.cz>
Cc: John Lenton <john.lenton@canonical.com>,
Jens Axboe <axboe@kernel.dk>,
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 01:59:20 +0800 [thread overview]
Message-ID: <A5D409B1-A8D2-4C06-ACA3-775594759FDE@canonical.com> (raw)
In-Reply-To: <20190730133607.GD28829@quack2.suse.cz>
at 21:36, Jan Kara <jack@suse.cz> 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.
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
>
> Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR
> <0001-loop-Fix-mount-2-failure-due-to-race-with-LOOP_SET_F.patch>
next prev parent reply other threads:[~2019-07-30 17:59 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-16 14:01 [PATCH] loop: Don't change loop device under exclusive opener Jan Kara
2019-05-16 20:44 ` Jens Axboe
2019-05-27 12:29 ` Jan Kara
2019-05-27 13:34 ` Jens Axboe
2019-07-18 8:15 ` Kai-Heng Feng
2019-07-30 9:29 ` Jan Kara
2019-07-30 9:36 ` John Lenton
2019-07-30 10:16 ` Jan Kara
2019-07-30 13:36 ` Jan Kara
2019-07-30 17:59 ` Kai-Heng Feng [this message]
2019-07-30 19:17 ` Jens Axboe
2019-07-30 21:11 ` John Lenton
2019-07-31 8:56 ` Jan Kara
2019-08-05 16:41 ` Bart Van Assche
2019-08-05 21:01 ` Tetsuo Handa
2019-08-07 9:45 ` Jan Kara
2019-08-07 18:45 ` Bart Van Assche
2019-08-08 13:37 ` Jens Axboe
2019-07-30 10:16 ` Tetsuo Handa
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