> > > What "tricks" could merge locks A and B into B?
> > 
> > something like this:
> > 	ffs_lock(...)
> > 	{
> > 
> > 		genfs_lock(vp, ...);
> > 		if (vp is snapshot) {
> > 			lockmgr(snaplk, ...);
> > 		}
> > 	}
> 
> That 1) forces all callers to pay a (slight) penalty for snapshot support 
> being present, even if they never use it,

and transferlockers() forces all lockmgr users to pay the penalty.

> and 2) it destroys our ability 
> to use v_vnlock. v_vnlock makes life much nicer for layered file systems, 
> as it lets us readily access one lock for a stack of vnodes.

i think that it's better to remove v_vnlock because:
- it's only useful in the limited cases.
- it involves complexity in layered filesystems
  as they should deal with two cases. (exported and not exported)
- it violates VOP abstraction.
  sharing a lock can be achieved using locking VOPs.

YAMAMOTO Takashi