>nullfs & umapfs would then just point the struct lock * in the upper node
>to the lock in the lower node. I think for unionfs, the union lock should
>point to the upper vndoe if present, and the lower vnode otherwise.
>
>Thoughts?

Sounds very elegant. but weren't stackable filesystems also meant to
support putting the acutal filesystem code into userspace (or via
userspace)? Like the Solaris inkernel support for automount points?

In such cases we really need to go the full RPC route like we do now
(or rather, like nullfs doesn't), *and* enforce the VOP_LOOKUP- style
calls acquire locks in the same order as VOP_LOCK.

Do we have  any such cases now? In future?