Mounting all filesystems...
Reader / writer lock error: rw_vector_exit: assertion failed: 
RW_COUNT(rw) != 0
lock address : 0xffffa00024733240
current cpu  :                  0
current lwp  : 0xffffa000248f9bc0
owner/count  : 000000000000000000 flags    : 000000000000000000
panic: lock error
fatal breakpoint trap in supervisor mode
trap type 1 code 0 rip ffffffff80129aad cs e030 rflags 246 cr2  
7f7ffd96abc0 cpl
0 rsp ffffa0002491d690
Stopped in pid 159.1 (mount_ffs) at     netbsd:breakpoint+0x5:  leave
breakpoint() at netbsd:breakpoint+0x5
panic() at netbsd:panic+0x242
lockdebug_abort() at netbsd:lockdebug_abort+0x42
rw_vector_exit() at netbsd:rw_vector_exit+0xa6
vlockmgr() at netbsd:vlockmgr+0xd8
VOP_UNLOCK() at netbsd:VOP_UNLOCK+0x28
spec_open() at netbsd:spec_open+0x33e
VOP_OPEN() at netbsd:VOP_OPEN+0x29
ffs_mount() at netbsd:ffs_mount+0x1c7
do_sys_mount() at netbsd:do_sys_mount+0x62d
sys___mount50() at netbsd:sys___mount50+0x33
syscall() at netbsd:syscall+0xb4
ds          0x6fc7
es          0xe02b
fs          0xd658
gs          0x246
rdi         0
rsi         0xd
rbp         0xffffa0002491d690
rbx         0xffffa0002491d6a0
rdx         0
rcx         0
rax         0x1
r8          0xffffffff80588940  cpu_info_primary
r9          0x1
r10         0xffffa0002491d5b0
r11         0xffffffff803abf80  xenconscn_putc
r12         0x104
r13         0xffffffff80406fc7  copyright+0xe907
r14         0xffffffff80590540  rwlock_lockops
r15         0xffffa000246f9608
rip         0xffffffff80129aad  breakpoint+0x5
cs          0xe030
rflags      0x246
rsp         0xffffa0002491d690
ss          0xe02b
netbsd:breakpoint+0x5:  leave
db>
db> ps/l
PID         LID S     FLAGS       STRUCT LWP *               NAME WAIT
>159       >   1 7         4   ffffa000248f9bc0          mount_ffs
158           1 3        84   ffffa000245dc7c0              mount wait
157           1 3        84   ffffa000232397c0                 sh wait
153           1 3        84   ffffa000245dc000      rpc.yppasswdd select
143           1 3        84   ffffa000245dc3e0             ypbind select
144           1 3        84   ffffa000245dcba0             ypserv select
144           1 3        84   ffffa000245dcba0             ypserv select
108           1 3        84   ffffa00023239ba0            syslogd kqueue
2             1 3        84   ffffa000232393e0                 sh wait
1             1 3        84   ffffa0002323a400               init wait
0            29 3       204   ffffa0002323b040            physiod physiod
             28 3       204   ffffa00023239000        vmem_rehash 
vmem_rehash
             27 3       204   ffffa0002323abc0           aiodoned 
aiodoned
             26 3       204   ffffa0002323a7e0            ioflush syncer
             25 3       204   ffffa0002323a020           pgdaemon 
pgdaemon
             24 3       204   ffffa00023231040          cryptoret 
crypto_wait
             23 2       204   ffffa0002323b800             xenbus
             22 3       204   ffffa0002323bbe0           xenwatch evtsq
             12 3       204   ffffa00023231420           pmfevent 
pmfevent
             11 3       204   ffffa00023231800           nfssilly 
nfssilly
             10 3       204   ffffa00023231be0            cachegc cachegc
              9 3       204   ffffa0002322e020              vrele vrele
              8 3       204   ffffa0002322e400          modunload 
modunload
              7 3       204   ffffa0002322e7e0            xcall/0 xcall
              6 1       204   ffffa0002322ebc0          softser/0
              5 1       204   ffffa0002322c000          softclk/0
              4 1       204   ffffa0002322c3e0          softbio/0
              3 1       204   ffffa0002322c7c0          softnet/0
              2 1       205   ffffa0002322cba0             idle/0
              1 3       204   ffffffff80590000            swapper 
schedule
db>
I got the same panic last night when I'd left the image/vnd attached 
and then started the domu, so I detached it, started ... and that's 
what happened. Just in case the vnd was still attached on the auto 
reboot, I'll fsck manually and try starting it again.