What do you do if a kernel dumped core but you did not expect
it, and it is therefore not compiled using config -g?
Not everything is lost here. Do not panic!
Of course, you still need to enable crash dumps. See above on the options you have to specify in order to do this.
Go to your kernel compile directory, and edit the line
containing COPTFLAGS?=-O. Add the -g option
there (but do not change anything on the level of
optimization). If you do already know roughly the probable
location of the failing piece of code (e.g., the pcvt
driver in the example above), remove all the object files for
this code. Rebuild the kernel. Due to the time stamp change on
the Makefile, there will be some other object files rebuild,
for example trap.o. With a bit of luck, the added
-g option will not change anything for the generated
code, so you will finally get a new kernel with similar code to
the faulting one but some debugging symbols. You should at
least verify the old and new sizes with the size(1) command. If
there is a mismatch, you probably need to give up here.
Go and examine the dump as described above. The debugging
symbols might be incomplete for some places, as can be seen in
the stack trace in the example above where some functions are
displayed without line numbers and argument lists. If you need
more debugging symbols, remove the appropriate object files and
repeat the kgdb session until you know enough.
All this is not guaranteed to work, but it will do it fine in most cases.