Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT_DETACH umount

[Alexander Larsson <alexl@redhat.com>]

On Mon, Jul 1, 2024 at 7:50 AM Christian Brauner <brauner@kernel.org> wrote:
>
> > I always thought the rcu delay was to ensure concurrent path walks "see" the
> >
> > umount not to ensure correct operation of the following mntput()(s).
> >
> >
> > Isn't the sequence of operations roughly, resolve path, lock, deatch,
> > release
> >
> > lock, rcu wait, mntput() subordinate mounts, put path.
>
> The crucial bit is really that synchronize_rcu_expedited() ensures that
> the final mntput() won't happen until path walk leaves RCU mode.
>
> This allows caller's like legitimize_mnt() which are called with only
> the RCU read-lock during lazy path walk to simple check for
> MNT_SYNC_UMOUNT and see that the mnt is about to be killed. If they see
> that this mount is MNT_SYNC_UMOUNT then they know that the mount won't
> be freed until an RCU grace period is up and so they know that they can
> simply put the reference count they took _without having to actually
> call mntput()_.
>
> Because if they did have to call mntput() they might end up shutting the
> filesystem down instead of umount() and that will cause said EBUSY
> errors I mentioned in my earlier mails.

But such behaviour could be kept even without an expedited RCU sync.
Such as in my alternative patch for this:
https://www.spinics.net/lists/linux-fsdevel/msg270117.html

I.e. we would still guarantee the final mput is called, but not block
the return of the unmount call.


-- 
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 Alexander Larsson                                Red Hat, Inc
       alexl@redhat.com         alexander.larsson@gmail.com