Re: [PATCH 13/28] VFS: Introduce soft reference counts

["Mika Penttilä" <mika.penttila@kolumbus.fi>]

Mike Waychison wrote:

>This patch introduces the concept of a 'soft' reference count for a vfsmount.
>This type of reference count allows for references to be held on mountpoints
>that do not affect their busy states for userland unmounting.  Some might
>argue that this is wrong because 'when I unmount a filesystem, I want the
>resources associated with it to go away too', but this way of thinking was
>deprecated with the addition of namespaces and --bind back in the 2.4 series.
>
>A future addition may see a callback mechanism so that in kernel users can
>use a given mountpoint and have it deregistered some way (quota and
>accounting come to mind).
>
>These soft reference counts are used by a later patch that adds an interface
>for holding and manipulating mountpoints using filedescriptors.
>
>Signed-off-by: Mike Waychison <michael.waychison@sun.com>
> 
>+static inline struct vfsmount *mntsoftget(struct vfsmount *mnt)
>+{
>+	if (mnt) {
>+		read_lock(&vfsmountref_lock);
>+		atomic_inc(&mnt->mnt_softcount);
>+		mntgroupget(mnt);
>+		read_unlock(&vfsmountref_lock);
>+	}
>+	return mnt;
>+}
>+
>+static inline void mntsoftput(struct vfsmount *mnt)
>+{
>+	struct vfsmount *cleanup;
>+	might_sleep();
>+	if (mnt) {
>+		if (atomic_dec_and_test(&mnt->mnt_count))
>+			__mntput(mnt);
>+		read_lock(&vfsmountref_lock);
>+		cleanup = mntgroupput(mnt);
>+		atomic_dec(&mnt->mnt_softcount);
>+		read_unlock(&vfsmountref_lock);
>+		if (cleanup)
>+			__mntgroupput(cleanup);
>+	}
>+}
>+
> extern void free_vfsmnt(struct vfsmount *mnt);
>  
>
What is this against? What are mntgroupput and mntgroupget? Why does 
soft put decrement mnt_count which isn't increment by soft get? How do 
soft references allow userland umount? I don't see soft references used 
anywhere...

--Mika