>  panic: kernel diagnostic assertion "p->p_nrlwps == 0" failed: file "/usr/src/sys/kern/kern_exit.c", line 781

as p_nrlwps currently has no locking afaik, the panic is not surprising. :)

proc.h claims it's protected by p_lock, but i think
sched_lock is more straightforward lock to use.

YAMAMOTO Takashi