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Date:   Thu, 31 May 2018 20:19:55 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     David Howells <dhowells@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-afs@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 20/32] vfs: Make close() unmount the attached mount if so
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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...