nullfs on 5.1

[Florian Weimer]

I'm trying to use nullfs on FreeBSD 5.1, and run into deadlocks in quite
simple situations (chroot to tree that contains an alias mount, read
access to the mounted tree -- no recursive/loop mounts).  Processes get
stuck and hang forever in D state (holding a UFS log according to ps).

Is nullfs essentially unsuported, or is it just broken?  What's the
status of unionfs (the manpage is rather discouraging).