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

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2019/07/30 18:29, Jan Kara wrote:
>> This patch introduced a regression [1].
>> A reproducer can be found at [2].
>>
>> [1] https://bugs.launchpad.net/bugs/1836914
>> [2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1836914/comments/4
> 
> 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.

Firstly, why not to check the return value of blkdev_get() ?
EBUSY is not the only error code blkdev_get() might return.

 	/*
 	 * If we don't hold exclusive handle for the device, upgrade to it
 	 * here to avoid changing device under exclusive owner.
 	 */
 	if (!(mode & FMODE_EXCL)) {
 		bdgrab(bdev);
 		error = blkdev_get(bdev, mode | FMODE_EXCL, loop_set_fd);
-		if (error)
+		if (error) {
+			printk("loop_set_fd: %d\n", error);
 			goto out_putf;
+		}
 	}

And try finding which line is returning an error
(like https://marc.info/?l=linux-xfs&m=156437221703110 does).