[513][root.p4: ROUTETABLES]$ # kgdb kernel.debug /var/crash/vmcore.2
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Unread portion of the kernel message buffer:


Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address   = 0x0
fault code              = supervisor read, page not present
instruction pointer     = 0x20:0xc0746b9d
stack pointer           = 0x28:0xd85acbdc
frame pointer           = 0x28:0xd85acbf4
code segment            = base rx0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 15 (usbus4)
trap number             = 12
panic: page fault
cpuid = 0
KDB: stack backtrace:
#0 0xc069421a at kdb_backtrace+0x43
#1 0xc0663652 at panic+0x114
#2 0xc08fbcb4 at trap_fatal+0x320
#3 0xc08fbd49 at trap_pfault+0x89
#4 0xc08fca67 at trap+0x437
#5 0xc08e6e7c at calltrap+0x6
#6 0xc072b61a at ieee80211_process_callback+0x46
#7 0xc0574743 at urtw_bulk_tx_callback+0x96
#8 0xc056f8ab at usbd_callback_wrapper+0x70c
#9 0xc056bda4 at usb_command_wrapper+0xc5
#10 0xc056e7ce at usb_callback_proc+0x100
#11 0xc0568c8e at usb_process+0xf5
#12 0xc06375db at fork_exit+0x91
#13 0xc08e6ef4 at fork_trampoline+0x8
Uptime: 3h44m34s
Physical memory: 1006 MB
Dumping 209 MB: 194 178 162 146 130 114 98 82 66 50 34 18 2

Reading symbols from /boot/kernel/geom_mirror.ko...Reading symbols from /boot/kernel/geom_mirror.ko.symbols...done.
done.
Loaded symbols for /boot/kernel/geom_mirror.ko
#0  doadump (textdump=1) at pcpu.h:244
244             __asm("movl %%fs:0,%0" : "=r" (td));
(kgdb) list *0xc0746b9d
0xc0746b9d is in ieee80211_tx_mgt_cb (/usr/src/sys/net80211/ieee80211_output.c:2505).
2500    }
2501
2502    static void
2503    ieee80211_tx_mgt_cb(struct ieee80211_node *ni, void *arg, int status)
2504    {
2505            struct ieee80211vap *vap = ni->ni_vap;
2506            enum ieee80211_state ostate = (enum ieee80211_state) arg;
2507
2508            /*
2509             * Frame transmit completed; arrange timer callback.  If
(kgdb) backtrace
#0  doadump (textdump=1) at pcpu.h:244
#1  0xc06633fe in kern_reboot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:442
#2  0xc066368f in panic (fmt=Variable "fmt" is not available.
) at /usr/src/sys/kern/kern_shutdown.c:607
#3  0xc08fbcb4 in trap_fatal (frame=0xd85acb9c, eva=0) at /usr/src/sys/i386/i386/trap.c:975
#4  0xc08fbd49 in trap_pfault (frame=0xd85acb9c, usermode=0, eva=0) at /usr/src/sys/i386/i386/trap.c:839
#5  0xc08fca67 in trap (frame=0xd85acb9c) at /usr/src/sys/i386/i386/trap.c:558
#6  0xc08e6e7c in calltrap () at /usr/src/sys/i386/i386/exception.s:168
#7  0xc0746b9d in ieee80211_tx_mgt_cb (ni=0x0, arg=0x2, status=0) at /usr/src/sys/net80211/ieee80211_output.c:2504
#8  0xc072b61a in ieee80211_process_callback (ni=0x0, m=0xc818f100, status=0) at /usr/src/sys/net80211/ieee80211_freebsd.c:478
#9  0xc0574743 in urtw_bulk_tx_callback (xfer=0xc3cc9168, error=USB_ERR_NORMAL_COMPLETION) at /usr/src/sys/dev/usb/wlan/if_urtw.c:4176
#10 0xc056f8ab in usbd_callback_wrapper (pq=0xc3cc9030) at /usr/src/sys/dev/usb/usb_transfer.c:2231
#11 0xc056bda4 in usb_command_wrapper (pq=0xc3cc9030, xfer=0x0) at /usr/src/sys/dev/usb/usb_transfer.c:2860
#12 0xc056e7ce in usb_callback_proc (_pm=0xc3cc9044) at /usr/src/sys/dev/usb/usb_transfer.c:2096
#13 0xc0568c8e in usb_process (arg=0xc3a96ccc) at /usr/src/sys/dev/usb/usb_process.c:170
#14 0xc06375db in fork_exit (callout=0xc0568b99 <usb_process>, arg=0xc3a96ccc, frame=0xd85acd28) at /usr/src/sys/kern/kern_fork.c:995
#15 0xc08e6ef4 in fork_trampoline () at /usr/src/sys/i386/i386/exception.s:275
(kgdb)

How-To-Repeat: Not sure, seems to happen randomly.  I did notice that that it happened about 15 min after the wireless link went down.  The adapter is an Alfa AWUS036H.

urtw0: <vendor 0x0bda product 0x8187, class 0/0, rev 2.00/1.00, addr 2> on usbus4
urtw0: unknown RTL8187L type: 0x8000000


Feb 16 05:51:17 p4 kernel: wlan0: link state changed to DOWN
Feb 16 06:06:36 p4 syslogd: kernel boot file is /boot/kernel/kernel
Feb 16 06:06:37 p4 kernel: Copyright (c) 1992-2012 The FreeBSD Project.
Feb 16 06:06:37 p4 kernel: Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994