xenstore panic

Fatal trap 9: general protection fault while in kernel mode
cpuid = 0; apic id = 00
instruction pointer     = 0x20:0xffffffff8084b4f0
stack pointer           = 0x28:0xfffffe011b2d49f0
frame pointer           = 0x28:0xfffffe011b2d4a60
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         = 0 (thread taskq)
[ thread pid 0 tid 100028 ]
Stopped at      xs_unregister_watch+0x90:       movq    (%rcx),%rcx
db> lock order reversal: (Giant after non-sleepable)
 1st 0xffffffff816759d0 watches (watches) @ /usr/src/sys/dev/xen/xenstore/xenstore.c:1669
 2nd 0xffffffff81894860 Giant (Giant) @ /usr/src/sys/dev/usb/usb_transfer.c:2265
stack backtrace:
#0 0xffffffff809d4490 at witness_checkorder+0xf70
#1 0xffffffff809d4391 at witness_checkorder+0xe71
#2 0xffffffff80963244 at __mtx_lock_flags+0xa4
#3 0xffffffff807eeb3c at usbd_transfer_setup+0x11ec
#4 0xffffffff807f101b at usbd_transfer_poll+0x22b
#5 0xffffffff807f21e7 at usb_make_str_desc+0xdf7
#6 0xffffffff807f198e at usb_make_str_desc+0x59e
#7 0xffffffff805ccf95 at genkbd_keyaction+0x1755
#8 0xffffffff808144ac at vtbuf_scroll_mode+0x7ac
#9 0xffffffff80929a32 at cngetc+0x62
#10 0xffffffff803672fc at db_readline+0x9c
#11 0xffffffff80367a25 at db_read_line+0x15
#12 0xffffffff80365b4a at db_command_loop+0x5a
#13 0xffffffff803685eb at db_fetch_ksymtab+0x28b
#14 0xffffffff809b76a3 at kdb_trap+0x193
#15 0xffffffff80d9e961 at trap+0xb61
#16 0xffffffff80d9e63a at trap+0x83a
#17 0xffffffff80d7ea07 at calltrap+0x8
bt
Tracing pid 0 tid 100028 td 0xfffff80003ed69a0
xs_unregister_watch() at xs_unregister_watch+0x90/frame 0xfffffe011b2d4a60
xenbusb_free_child_ivars() at xenbusb_free_child_ivars+0x18/frame 0xfffffe011b2d4a80
xenbusb_probe_children() at xenbusb_probe_children+0x18a/frame 0xfffffe011b2d4b00
xenbusb_probe_children_cb() at xenbusb_probe_children_cb+0x2b/frame 0xfffffe011b2d4b20
taskqueue_run_locked() at taskqueue_run_locked+0xf0/frame 0xfffffe011b2d4b80
taskqueue_thread_loop() at taskqueue_thread_loop+0x88/frame 0xfffffe011b2d4bb0
fork_exit() at fork_exit+0x84/frame 0xfffffe011b2d4bf0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe011b2d4bf0