Strange hang on 4.99.49

[Paul Goyette <paul%whooppee.com@localhost>]

I'm running a kernel + userland from sources as of just a few hours ago.

I started up a build.sh with -j 4 and all of a sudden, after an hour or 
more of running, the system just went completely idle even though the 
build is nowhere near complete.

Looking around, I found three cc1 processes (only 3, not 4).  One of 
them was in a 'vnode' state while the other two are in 'vnlock'.  Both 
my /usr/src and /usr/obj are null-mounts

        /dev/wd0a on / type ffs (NFS exported, local)
        /dev/wd0e on /var type ffs (local)
        /dev/wd0g on /home type ffs (NFS exported, local)
> > >         /dev/wd0h on /build type ffs (soft dependencies, NFS exported, local)
        /dev/wd1a on /amanda type ffs (local)
> > >         /build/src on /usr/src type null (local)
> > >         /build/obj on /usr/obj type null (local)
        /build/xsrc on /usr/xsrc type null (local)
        /build/pkgsrc on /usr/pkgsrc type null (local)
        kernfs on /kern type kernfs (local)
        ptyfs on /dev/pts type ptyfs (local)
        tmpfs on /tmp type tmpfs (local)

Is there some sort of locking race condition that I've managed to hit? 
If so, is there a way to avoid it?

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