panic: blockable sleep lock (sleep mutex) ehci0

[Peter Jeremy]

My work desktop has suddenly suddenly started reporting the above
panic.  I'm running 8.2-stable/amd64  from end of February.  It
was "hung" when I came in this morning and "hung" again soon after
(I was in X on both occasions and so couldn't investigate further
but now suspect it was in DDB).  I've since kept in in VTY mode
and it's paniced twice more as above.  In both cases, the
relevant part of the backtrace is:

#10 0xffffffff8034d8bb in witness_checkorder (lock=0xffffff800030af50, 
    flags=9, file=0xffffffff805df848 "/usr/src/sys/kern/kern_mutex.c", 
    line=147, interlock=Variable "interlock" is not available.
) at /usr/src/sys/kern/subr_witness.c:1064
#11 0xffffffff80301622 in _mtx_lock_flags (m=0xffffff800030af50, opts=0, 
    file=0xffffffff805df848 "/usr/src/sys/kern/kern_mutex.c", line=147)
    at /usr/src/sys/kern/kern_mutex.c:200
#12 0xffffffff802d3e2c in _cv_wait (cvp=Variable "cvp" is not available.
)
    at /usr/src/sys/kern/kern_condvar.c:147
#13 0xffffffff802a34e2 in usb_process (arg=Variable "arg" is not available.
)
    at /usr/src/sys/dev/usb/usb_process.c:183
#14 0xffffffff802e9d02 in fork_exit (
    callout=0xffffffff802a3380 <usb_process>, arg=0xffffff800030adb0, 
    frame=0xffffff8000123c40) at /usr/src/sys/kern/kern_fork.c:859
#15 0xffffffff804f979e in fork_trampoline ()
    at /usr/src/sys/amd64/amd64/exception.S:565

The latest time, the system was idle other than a ZFS scrub - I was
working on another box.  There were no USB2 peripherals attached
(the keyboard and mouse are USB1.1)

Any suggestions on how to fix this?

Relevant bits of dmesg:
uhci0: <Intel 82801G (ICH7) USB controller USB-A> port 0xff80-0xff9f irq 21 at device 29.0 on pci0
uhci0: [ITHREAD]
uhci0: LegSup = 0x3000
usbus0: <Intel 82801G (ICH7) USB controller USB-A> on uhci0
uhci1: <Intel 82801G (ICH7) USB controller USB-B> port 0xff60-0xff7f irq 22 at device 29.1 on pci0
uhci1: [ITHREAD]
usbus1: <Intel 82801G (ICH7) USB controller USB-B> on uhci1
uhci2: <Intel 82801G (ICH7) USB controller USB-C> port 0xff40-0xff5f irq 18 at device 29.2 on pci0
uhci2: [ITHREAD]
usbus2: <Intel 82801G (ICH7) USB controller USB-C> on uhci2
uhci3: <Intel 82801G (ICH7) USB controller USB-D> port 0xff20-0xff3f irq 23 at device 29.3 on pci0
uhci3: [ITHREAD]
usbus3: <Intel 82801G (ICH7) USB controller USB-D> on uhci3
ehci0: <Intel 82801GB/R (ICH7) USB 2.0 controller> mem 0xffa80800-0xffa80bff irq 21 at device 29.7 on pci0
ehci0: [ITHREAD]
usbus4: EHCI version 1.0
usbus4: <Intel 82801GB/R (ICH7) USB 2.0 controller> on ehci0
...
usbus0: 12Mbps Full Speed USB v1.0
usbus1: 12Mbps Full Speed USB v1.0
usbus2: 12Mbps Full Speed USB v1.0
usbus3: 12Mbps Full Speed USB v1.0
usbus4: 480Mbps High Speed USB v2.0
...
ugen0.1: <Intel> at usbus0
uhub0: <Intel UHCI root HUB, class 9/0, rev 1.00/1.00, addr 1> on usbus0
ugen1.1: <Intel> at usbus1
uhub1: <Intel UHCI root HUB, class 9/0, rev 1.00/1.00, addr 1> on usbus1
ugen2.1: <Intel> at usbus2
uhub2: <Intel UHCI root HUB, class 9/0, rev 1.00/1.00, addr 1> on usbus2
ugen3.1: <Intel> at usbus3
uhub3: <Intel UHCI root HUB, class 9/0, rev 1.00/1.00, addr 1> on usbus3
ugen4.1: <Intel> at usbus4
uhub4: <Intel EHCI root HUB, class 9/0, rev 2.00/1.00, addr 1> on usbus4
...
Root mount waiting for: usbus4 usbus3 usbus2 usbus1 usbus0
uhub0: 2 ports with 2 removable, self powered
uhub1: 2 ports with 2 removable, self powered
uhub2: 2 ports with 2 removable, self powered
uhub3: 2 ports with 2 removable, self powered
Root mount waiting for: usbus4
Root mount waiting for: usbus4
Root mount waiting for: usbus4
uhub4: 8 ports with 8 removable, self powered
Root mount waiting for: usbus4
ugen4.2: <NEC Corporation> at usbus4
uhub5: <NEC Corporation USB2.0 Hub Controller, class 9/0, rev 2.00/1.00, addr 2> on usbus4
ugen0.2: <Dell> at usbus0
uhub6: <Dell USB Keyboard Hub> on usbus0
Root mount waiting for: usbus4
uhub5: 4 ports with 4 removable, self powered
Trying to mount root from zfs:zroot
uhub6: 3 ports with 2 removable, bus powered
ugen0.3: <Dell> at usbus0
ukbd0: <Dell USB Keyboard> on usbus0
kbd1 at ukbd0
ugen0.4: <vendor 0x413c> at usbus0
ums0: <vendor 0x413c product 0x3010, class 0/0, rev 2.00/2.30, addr 4> on usbus0
ums0: 3 buttons and [XYZ] coordinates ID=0

-- 
Peter Jeremy
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