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

[Mika Penttilä]

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@xxxxxxx>
>
> +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

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