panic: mtx_lock() of spin mutex in ip_output.c

I have a Dell 4150 laptop using dhcp and ntpd on the
xl0 interface.  I did "ifconfig xl0 down" and received
the following panic (hand transcribed :-( ).

panic: mtx_lock() of spin mutex (null) _at_ /usr/src/sys/netinet/ip_output.c:266
Stack backtrace:
backtrace()
panic()
panic: process 414(ntpd):2 Giant but isn't blocked on a mutex

In ddb> I get,

Debugger()
panic()
propagate_priority()
_mtx_lock_sleep()
_mtx_lock_flags()
softclock()
ithread_loop()
fork_exit()
fork_trampoline()

I have a crash dump and kernel.debug if further info is need

-- 
Steve
http://troutmask.apl.washington.edu/~kargl/