I've got a cdrom as /cdrom on  a CATS board with 1.3F UVM kernel. This was
then mounted over NFS (read-write) onto a RiscPC running 1.3F. I
accidentally tried to write to something in /cdrom and the CATS machine
bombed out with:

panic: locking against myself

typing trace at the db> prompt showed that the panic was in lockmgr()
being called from the cd9660 functions. While I accept full responsibility
for the stupidity of not exporting /cdrom read-only, the panic was not
very helpful.

Stephen