Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name

[Linus Torvalds]

On Wed, Aug 23, 2017 at 5:24 PM, Stefan Lippers-Hollmann <s.l-h@xxxxxx> wrote:
>
> This patch[1] as part of 4.13-rc6 (up to, at least,
> v4.13-rc6-45-g6470812e2226) introduces a regression for me when using
> pbuilder 0.228.7[2] (a helper to build Debian packages in a chroot and
> to create and update its chroots) when trying to umount /dev/ptmx (inside
> the chroot) on Debian/ unstable (full log and pbuilder configuration
> file[3] attached).
>
> [...]
> Setting up build-essential (12.3) ...
> Processing triggers for libc-bin (2.24-15) ...
> I: unmounting dev/ptmx filesystem
> W: Could not unmount dev/ptmx: umount: /var/cache/pbuilder/build/1340/dev/ptmx: target is busy

Yes, that patch definitely keeps a reference to the pts filesystem
around while a pty is open.

We always used to do that, but we did it differently - we would keep
the 's_active' count elevated so that the superblock never went away,
even after it was unmounted.

Now it does an actual mntget(), and that makes umount _notice_ that
the filesystem is still busy.

How annoying.

Because in a very real sehse the filesystem really is busy, but we
used to hide it (perhaps on purpose - it's possible that people hit
this problem before).

Let me try to think about alteratives. Clearly this is a regression
and I need to fix it, I just need to figure out _how_.

                   Linus