kern/48213: ehci assertion failure during boot

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         48213
>Category:       kern
>Synopsis:       ehci assertion failure during boot
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Sep 15 10:50:00 +0000 2013
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date 2013.09.12.07.26.13
>Organization:
>Environment:
System: NetBSD guido.araneus.fi
Architecture: x86_64
Machine: amd64
>Description:

While doing reboots in a loop trying to reproduce a different bug, my
x86_64 machine paniced.  The following console output is manually
transcribed, so there may be typos:

  uhidev0: GASIA PS2toUSB Adapter, rev 1.10/2.80, addr 3, iclass 3/1
  ukbd0 at uhidev0: 8 modifier keys, 6 key codes

  This port is broken, it does not call cnpollc() before calling cngetc().
  This should be fixed, but it will work anyway (for now).
  panic: kernel diagnostic assertion "mutex_owned(&sc->sx_lock)" failed: file 
"/tmp/bracket/build/2013.09.12.07.26.13-amd64/sys/dev/usb/ehci.c", line 3302
  fatal breakpoint trap in supervisor mode
  trap type 1 code 0 rip ffffffff8026a46d cs 8 rflags 246 cr2 0 ilevel 8 rsp 
ffffffff810e3970
  curlwp 0xffffffff80ec7460 pid 0.1 lowest kstack 0xffffffff810dc280
  Stopped in pid 0,1 (system) at  netbsd:breakpoint+0x5:   leave
  db{0}> ehci_done: ex=0xfffffe8419c6b2e0 is done!

I am unable to obtain a stack trace, because when I try to type a ddb
command on the USB keyboard, each keypress only results in printing
another copy of the last line,

  ehci_done: ex=0xfffffe8419c6b2e0 is done!

This is an Intel DH67CLB3 motherboard with BIOS version 
BLH6710H.86A.0131.2011.0926.1945.
The same machine is also afflicted by PRs 38970, 46596, 46696, and 47153.

>How-To-Repeat:

Not sure since it has only happened once so far, but presumably
doing reboots in a loop with a similar motherboard and a USB
keyboard/mouse will eventually hit it.

>Fix: