panic when disconnecting cdce device just before powering off PC

I'm running a fairly recent -current and got a panic when shutting
down.  I'd pressed the power button on my GTA02 and at about the same
time had pressed the power button on my PC too.  It seems the GTA02
CDCE device powered off first, just before FreeBSD was going to power
off:

Waiting (max 60 seconds) for system process 'vnlru' to stop...done
Waiting (max 60 seconds) for system process 'bufdaemon' to stop...done
Waiting (max 60 seconds) for system process 'syncer' to stop...
Syncing disks, vnodes remaining...1 1 1 0 0 0 done
All buffers synced.
Uptime: 10h12m10s
ugen3.2: <Linux 2.6.29-rc3 with s3c2410_udc> at usbus3 (disconnected)
cdce0: at uhub3, port 2, addr 2 (disconnected)


Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address  = 0x288
fault code             = supervisor read data, page not present
instruction pointer    = 0x20:0xffffffff80343d3e
stack pointer          = 0x28:0xfffffffe4001c8a0
frame pointer          = 0x28:0xfffffffe4001c8c0
code segment           = base 0x0, limit 0xfffff, type 0x1b
                       = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags       = interrupt enabled, resume, IOPL = 0
current process        = 1 (init)
[thread pid 1 tid 100002 ]
Stopped at     _mtx_lock_sleep+0x4e:   movl   0x288(%rcx),%esi
db> bt
Tracing pid 1 tid 100002 td 0xffffff00014d7a80
_mtx_lock_sleep() at _mtx_lock_sleep+0x4e
_sleep() at _sleep+0x232
usb2_proc_mwait() at usb2_proc_mwait+0x46
usb2_ether_ifshutdown() at usb2_ether_ifshutdown+0xaf
cdce_shutdown() at cdce_shutdown+0x11
bus_generic_shutdown() at bus_generic_shutdown+0x1a
bus_generic_shutdown() at bus_generic_shutdown+0x1a
bus_generic_shutdown() at bus_generic_shutdown+0x1a
bus_generic_shutdown() at bus_generic_shutdown+0x1a
bus_generic_shutdown() at bus_generic_shutdown+0x1a
bus_generic_shutdown() at bus_generic_shutdown+0x1a
acpi_shutdown() at acpi_shutdown+0x9
bus_generic_shutdown() at bus_generic_shutdown+0x1a
bus_generic_shutdown() at bus_generic_shutdown+0x1a
root_bus_module_handler() at root_bus_module_handler+0x11b
module_shutdown() at module_shutdown+0x84
boot() at boot+0x37b
reboot() at reboot+0x46
syscall() at syscall+0x264
Xfast_syscall() at Xfast_syscall+0xd0
--- syscall (55, FreeBSD ELF64, reboot), rip = 0x40897c, rsp =
0x7fffffffe738, rbp = 0x402420 ---

-- 
Bruce Cran