> I finally got to try out the kernel I built Tuesday afternoon, and
> it has some significant problems compared to the one I have from
> the 10th.
> 
> In particular, the Jan10 kernel would report that it could not ping
> a cpu no more often than every couple of minutes, and sometimes
> not for a few hours.  The Jan13 kernel, on the other hand, reported
> about a hundred such logs between each line of the regular boot
> messages, finally ending in a panic about "simple_lock: locking
> against myself", before allowing a login.

You have LOCKDEBUG on, which has a problem of its own (well, on sparc at least)
If you want it, apply this patch:

Index: kern_lock.c
===================================================================
RCS file: /cvsroot/src/sys/kern/kern_lock.c,v
retrieving revision 1.67
diff -r1.67 kern_lock.c
1052a1053
>       splx(s);
1053a1055
>       s = spllock();