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SubjectRe: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion
On 09/15, Al Viro wrote:
> On Thu, Sep 14, 2017 at 05:19:39PM -0700, Jaegeuk Kim wrote:
> > Instead, I put more traces in the reboot procedure, and got a clue to suspect
> > the below flow.
> >
> > delayed_fput() init
> > - umount
> > - mntput()
> > - mntput_no_expire() - mntput_no_expire()
> > - mnt_add_count(-1);
> > - mnt_get_count() return;
> > - return 0;
> > - mnt_add_count(-1);
> > - delayed_mntput_work
> > - device_shutdown
> > - ext4_put_super()
> > - EIO
> >
> > Does this make any sense?
>
> Which filesystem it is? With root I would've expected remount ro done
> by sys_umount(); with anything else... How has it managed to avoid
> -EBUSY? If it was umount -l (IOW, MNT_DETACH), I can see that happening,
> but... How would flushing prevent the scenario when the same opened
> file had remained open until after the umount(2) return?

It's ext4, and we use umount(0) and retry it several times if -EBUSY happens.
But, I don't see -EBUSY error in the log.

> In other words, where has that fput() come from and how had it managed
> to get past the umount(2)?

Huge number of fput() were called by system drivers when init kills all the
processes before umount(2). So, most of fput() were added in delayed_fput_list.
Then, it seems there is a race between delayed_fput() and umount(). Anyway,
even after umount returns zero, it seems ext4's superblock is still alive
and waiting for delayed_fput() which will finally call put_super.

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