Re: [PATCH 20/32] vfs: Make close() unmount the attached mount if so flagged [ver #8]

[Al Viro]

On Fri, May 25, 2018 at 01:07:34AM +0100, David Howells wrote:
> +     if (unlikely(file->f_mode & FMODE_NEED_UNMOUNT))
> +             __detach_mounts(dentry);
> +

This is completely wrong.  First of all, you want to dissolve the mount tree
on file->f_path.mount, not every tree rooted at dentry equal to 
file->f_path.dentry.
This is easily done - it would be a simple call of drop_collected_mounts(mnt)
if not for one detail.  You want it to happen only if the sucker isn't attached
anywhere by that point.  IOW,
        namespace_lock();
        lock_mount_hash();
        if (!real_mount(mnt)->mnt_ns)
                umount_tree(real_mount(mnt), UMOUNT_SYNC);
        unlock_mount_hash();
        namespace_unlock();
and that's it.  You don't need that magical mystery turd in move_mount() later
in the series and all the infrastructure you grow for it.

FWIW, I would've suggested this
 void drop_collected_mounts(struct vfsmount *mnt)
 {
        namespace_lock();
        lock_mount_hash();
+       if (!real_mount(mnt)->mnt_ns)
+               umount_tree(real_mount(mnt), UMOUNT_SYNC);
-       umount_tree(real_mount(mnt), UMOUNT_SYNC);
        unlock_mount_hash();
        namespace_unlock();
 }

and in __fput()
        if (unlikely(file->f_mode & FMODE_NEED_UNMOUNT))
                drop_collected_mounts(mnt);

All there is to it, AFAICS...