panic: sleeping thread & bufwrite: buffer is not busy???

i386 soekris 5501
current as of Dec 11 00:27 gmt

Unread portion of the kernel message buffer:
Sleeping thread (tid 100054, pid 646) owns a non-sleepable lock
panic: sleeping thread
panic: bufwrite: buffer is not busy???
Uptime: 2m1s
Physical memory: 503 MB

#0  doadump () at pcpu.h:246
#1  0xc0571f33 in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:420
#2  0xc05720f7 in panic (fmt=Variable "fmt" is not available.)
     at /usr/src/sys/kern/kern_shutdown.c:576
#3  0xc06fc75c in ffs_bufwrite (bp=0xc2937e20)
     at /usr/src/sys/ufs/ffs/ffs_vfsops.c:1786
#4  0xc05c7250 in vfs_bio_awrite (bp=0xc2937e20) at buf.h:385
#5  0xc05cfee0 in vop_stdfsync (ap=0xc2a89b34)
     at /usr/src/sys/kern/vfs_default.c:479
#6  0xc0520ad3 in devfs_fsync (ap=0xc2a89b34)
     at /usr/src/sys/fs/devfs/devfs_vnops.c:485
#7  0xc074e50e in VOP_FSYNC_APV (vop=0xc07cc800, a=0xc2a89b34)
     at vnode_if.c:1007
#8  0xc06fd0a2 in ffs_sync (mp=0xc2e33c80, waitfor=2, td=0xc2c28480)
     at vnode_if.h:529
#9  0xc05e0803 in sync (td=0xc2c28480, uap=0x0)
     at /usr/src/sys/kern/vfs_syscalls.c:149
#10 0xc0571ad2 in boot (howto=256) at /usr/src/sys/kern/kern_shutdown.c:312
#11 0xc05720f7 in panic (fmt=Variable "fmt" is not available.)
     at /usr/src/sys/kern/kern_shutdown.c:576
#12 0xc059ec81 in propagate_priority (td=0xc2e696c0)
     at /usr/src/sys/kern/subr_turnstile.c:222
#13 0xc059f551 in turnstile_wait (ts=0xc2c0ee10, owner=0xc2e696c0, 
queue=Variable "queue" is not available.)
     at /usr/src/sys/kern/subr_turnstile.c:738
#14 0xc0570c73 in _rw_wlock_hard (rw=0xc2cdcd80, tid=3267527808, 
file=0x0, line=0)
     at /usr/src/sys/kern/kern_rwlock.c:705
#15 0xc06bdcb6 in in6_mtutimo (rock=0xc2cdcd00)
     at /usr/src/sys/netinet6/in6_rmx.c:437
#16 0xc0580f77 in softclock (arg=0xc07ffbe0)
     at /usr/src/sys/kern/kern_timeout.c:398
#17 0xc055790a in intr_event_execute_handlers (p=0xc2c257ec, ie=0xc2c22400)
     at /usr/src/sys/kern/kern_intr.c:1134
#18 0xc0558933 in ithread_loop (arg=0xc2c07ba0)
     at /usr/src/sys/kern/kern_intr.c:1147
#19 0xc0555cb6 in fork_exit (callout=0xc05588d0 <ithread_loop>,
     arg=0xc2c07ba0, frame=0xc2a89d38) at /usr/src/sys/kern/kern_fork.c:821
#20 0xc0730a50 in fork_trampoline () at 
/usr/src/sys/i386/i386/exception.s:270