Re: Layered fs, vnode locking and v_vnlock removal

[David Holland <dholland-tech%netbsd.org@localhost>]

On Sun, May 23, 2010 at 04:11:25PM +0200, Juergen Hannken-Illjes wrote:
 > With our current vnode lock implementation VOP_LOCK() and VOP_UNLOCK()
 > are not symmetric.  A vnode may be locked from one file system and
 > unlocked from another one.  Is there any reason left to have layered
 > file systems share the vnode lock with lower file systems via v_vnlock?
 > 
 > The attached patch will remove v_vnlock and change layered file systems
 > to always pass the locking VOP's down to the leaf file system.

I'm not sure. This is fine for things like umapfs and nullfs where
there's a 1-1 mapping between upper and lower vnodes; but what if
there isn't? Consider for example a versioning FS layer that stores
versions as diffs, or a layer that turns /var/mail into maildirs. It's
not immediately clear how either of these ought to work, so I'm
concerned that making the infrastructure less general will lead to
problems.

In the long run I intend to make all the vnode ops symmetric with
respect to locking, which should make a lot of this less toxic, but at
the rate I've been able to work on this stuff we won't be there
anytime soon.

-- 
David A. Holland
dholland%netbsd.org@localhost