Re: INFO: task hung in __get_super

[Jan Kara]

On Wed 15-05-19 20:32:27, Tetsuo Handa wrote:
> On 2019/05/15 19:21, Jan Kara wrote:
> > The question is how to fix this problem. The simplest fix I can see is that
> > we'd just refuse to do LOOP_SET_FD if someone has the block device
> > exclusively open as there are high chances such user will be unpleasantly
> > surprised by the device changing under him. OTOH this has some potential
> > for userspace visible regressions. But I guess it's worth a try. Something
> > like attached patch?
> 
> (1) If I understand correctly, FMODE_EXCL is set at blkdev_open() only if
> O_EXCL is specified.

Yes.

> How can we detect if O_EXCL was not used, for the reproducer (
> https://syzkaller.appspot.com/text?tag=ReproC&x=135385a8a00000 ) is not
> using O_EXCL ?

mount_bdev() is using O_EXCL and that's what matters.

> (2) There seems to be no serialization. What guarantees that mount_bdev()
>     does not start due to preempted after the check added by this patch?

That's a good question. lo_ctl_mutex actually synchronizes most of this
(taken in both loop_set_fd() and lo_open()) but you're right that there's
still a small race window where loop_set_fd() need not see bdev->bd_holders
elevated while blkdev_get() will succeed. So I need to think a bit more
about proper synchronization of this.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR