There's kdebug which is very convenient if you want to change
variables in the running kernel. The kernel must not even be compiled
in debug mode!
However, it's not a real debugger. For this, you need 2 computers,
connected by a null modem cable, and a patch named gdb-stub. Look
at the online version of the Kernel Hacker's Guide
http://www.redhat.com:8080/HyperNews/get/khg.html
and look for gdb-stub
Hans-Joachim
-- Uncle Ed's Rule of Thumb: Never use your thumb for a rule. You'll either hit it with a hammer or get a splinter in it.