> playing more with unionfs on my ss20 I get the above panic shutting
> down the system (shutdown -r now) w/o umounting my union mounted
> /usr/src. 

Let me guess: you ran `shutdown -r now ' while your current directory
was inside the union?  This one's been biting me for a while..

It's trivially easy to reproduce, at least on my test system (i386, so
it's pretty clear it's an MI problem):

     mount -t union -o rw,-r /usr /mnt
     cd /mnt/bin
     /sbin/reboot

If cwd isn't in the union, it doesn't happen, so a workaround is to
always "cd /" before the reboot.

I'm not yet sure why it's happening; it looks like a bug in how forced
unmounts (done at shutdown time) work in unionfs; it's probably a
reference counting glitch, because it does this before the system even
tries to unmount either of the filesystems involved in the union.

I'm going to poke at this with kgdb when I get a chance..

					- Bill