panic with clangbsd kernel in kern/vfs_bio.c

Hi,

on my Acer 7738G laptop running FreeBSD 9.0-amd64 r209980 (with latest
clangbsd kernel),
I encountered this panic (recovered from /var/log/messages), while
doing some moderately
light load (portmaster, openoffice, firefox, thunderbird in an xfce4 session):

Jul 21 22:29:47 acer kernel: panic: buf 0xffffff80526d00c0 already
counted as free
Jul 21 22:29:47 acer kernel: cpuid = 1
Jul 21 22:29:47 acer kernel: KDB: enter: panic
Jul 21 22:29:47 acer kernel:
Jul 21 22:29:47 acer kernel: 0xffffff0005093b40: tag devfs, type VCHR
Jul 21 22:29:47 acer kernel: usecount 1, writecount 0, refcount 588
mountedhere 0xffffff0004c84200
Jul 21 22:29:47 acer kernel: flags ()
Jul 21 22:29:47 acer kernel: v_object 0xffffff00050adbd0 ref 0 pages 12590
Jul 21 22:29:47 acer kernel: lock type devfs: EXCL by thread
0xffffff0004c59880 (pid 17)
Jul 21 22:29:47 acer kernel: dev ad4s1f

I have the following modules loaded:
acer % kldstat
Id Refs Address            Size     Name
 1   32 0xffffffff80100000 f925f0   kernel (GENERIC)
 2    1 0xffffffff81093000 1bd28    if_iwn.ko
 3    1 0xffffffff810af000 296f8    snd_hda.ko
 4    2 0xffffffff810d9000 85fe8    sound.ko
 5    1 0xffffffff8115f000 570f8    iwn5000fw.ko
 6    1 0xffffffff811b7000 dc29d0   nvidia.ko
(ports/x11/nvidia-driver, version 256.35 with patches from current_at_)
 7    3 0xffffffff81f7a000 423c8    linux.ko
 8    1 0xffffffff81fbd000 d38      biosfont.ko (ports/sysutils/biosfont)
 9    1 0xffffffff82012000 3a73     linprocfs.ko

acer % grep -nr "already counted as free" ~/freebsd/clangbsd/sys/* |
grep -v \.svn
kern/vfs_bio.c:401:	    ("buf %p already counted as free", bp));
acer % ident kern/vfs_bio.c
kern/vfs_bio.c:
     $FreeBSD: projects/clangbsd/sys/kern/vfs_bio.c 209170 2010-06-14
18:45:33Z rdivacky $
acer % ident /sys/kern/vfs_bio.c
/sys/kern/vfs_bio.c:
     $FreeBSD: head/sys/kern/vfs_bio.c 209902 2010-07-11 20:11:44Z alc $

So it might be fixed already.

Let me know if you need more information. Unfortunately I didn't get a
core dump,
although dumpdev="AUTO" in /etc/rc.conf.  The laptop rebooted by itself.

Regards,
Rene