pasting one trace here for the record

Jan 24 19:59:19 localhost kernel: [ 1028.120275] ------------[ cut here ]------------
Jan 24 19:59:19 localhost kernel: [ 1028.120848] kernel BUG at /build/buildd/linux-3.2.0/fs/buffer.c:2917!
Jan 24 19:59:19 localhost kernel: [ 1028.121602] invalid opcode: 0000 [#1] SMP 
Jan 24 19:59:19 localhost kernel: [ 1028.122098] CPU 1 
Jan 24 19:59:19 localhost kernel: [ 1028.122314] Modules linked in: xt_mac xt_physdev xt_conntrack ipt_REDIRECT xfs rmd160 crypto_null xfrm_user ah6 ah4 esp6 esp4 xfrm4_mode_beet xfrm4_tunnel tunnel4 xfrm4_mode_tunnel xfrm4_mode_transport xfrm6_mode_transport xfrm6_mode_ro xfrm6_mode_beet xfrm6_mode_tunnel ipcomp ipcomp6 xfrm_ipcomp xfrm6_tunnel tunnel6 af_key camellia lzo cast6 cast5 deflate zlib_deflate cts ctr gcm ccm serpent blowfish_generic blowfish_x86_64 blowfish_common twofish_generic twofish_x86_64_3way twofish_x86_64 twofish_common xcbc sha512_generic des_generic cryptd aes_x86_64 kvm ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat xt_CHECKSUM iptable_mangle bridge stp openvswitch(O) iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi dm_multipath nbd ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_filter ip6_tables ipt_REJECT xt_tcpudp nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack iptable_filter ip_tables x_tables psmouse serio_raw virtio_balloon acpiphp floppy
Jan 24 19:59:19 localhost kernel: [ 1028.130686] 
Jan 24 19:59:19 localhost kernel: [ 1028.130686] Pid: 12224, comm: dnsmasq Tainted: G           O 3.2.0-58-virtual #88-Ubuntu Bochs Bochs
Jan 24 19:59:19 localhost kernel: [ 1028.130686] RIP: 0010:[<ffffffff811a76d3>]  [<ffffffff811a76d3>] submit_bh+0x113/0x120
Jan 24 19:59:19 localhost kernel: [ 1028.130686] RSP: 0000:ffff88015b2df738  EFLAGS: 00010246
Jan 24 19:59:19 localhost kernel: [ 1028.130686] RAX: 0000000000000005 RBX: ffff880072d71f08 RCX: 00000000000b2a1e
Jan 24 19:59:19 localhost kernel: [ 1028.130686] RDX: 000000000000008d RSI: ffff880072d71f08 RDI: 0000000000000211
Jan 24 19:59:19 localhost kernel: [ 1028.130686] RBP: ffff88015b2df758 R08: ffffffff811190c7 R09: 0000000180150015
Jan 24 19:59:19 localhost kernel: [ 1028.130686] R10: ffff8801ec485180 R11: 0000000001000000 R12: 0000000000000211
Jan 24 19:59:19 localhost kernel: [ 1028.130686] R13: ffff880004a8c824 R14: 0000000000000001 R15: ffff880072d71f08
Jan 24 19:59:19 localhost kernel: [ 1028.130686] FS:  00007fd15c3aa700(0000) GS:ffff88021fc80000(0000) knlGS:0000000000000000
Jan 24 19:59:19 localhost kernel: [ 1028.130686] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
Jan 24 19:59:19 localhost kernel: [ 1028.130686] CR2: 00000000042c5560 CR3: 000000020f9b6000 CR4: 00000000000006e0
Jan 24 19:59:19 localhost kernel: [ 1028.130686] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Jan 24 19:59:19 localhost kernel: [ 1028.130686] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Jan 24 19:59:19 localhost kernel: [ 1028.130686] Process dnsmasq (pid: 12224, threadinfo ffff88015b2de000, task ffff8801a7178000)
Jan 24 19:59:19 localhost kernel: [ 1028.130686] Stack:
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  ffff8801ec485180 ffff880072d71f08 0000000000000211 ffff880004a8c824
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  ffff88015b2df788 ffffffff811a9442 ffff88015b2df7b8 ffff8801ec485180
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  ffff880004a8c800 ffff8800c3726c00 ffff88015b2df798 ffffffff811a94d3
Jan 24 19:59:19 localhost kernel: [ 1028.130686] Call Trace:
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff811a9442>] __sync_dirty_buffer+0x52/0xd0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff811a94d3>] sync_dirty_buffer+0x13/0x20
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8125c578>] journal_update_superblock+0x118/0x220
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff811ad575>] ? bio_fs_destructor+0x15/0x20
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff811ac6eb>] ? bio_put+0x2b/0x30
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff812f4e9d>] ? blkdev_issue_flush+0xbd/0xe0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81258463>] cleanup_journal_tail.part.6+0x113/0x210
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8125857d>] cleanup_journal_tail+0x1d/0x30
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81258d41>] log_do_checkpoint+0x21/0x1f0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8105053d>] ? ttwu_do_wakeup+0x3d/0x120
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8103dd05>] ? pvclock_clocksource_read+0x55/0xf0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81055db3>] ? update_curr+0x143/0x1f0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8103dc39>] ? default_spin_lock_flags+0x9/0x10
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81661680>] ? kprobe_flush_task+0xc0/0x150
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8105588f>] ? finish_task_switch+0xcf/0xf0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff816588cc>] ? __schedule+0x3cc/0x6f0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8103dc39>] ? default_spin_lock_flags+0x9/0x10
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8103dc39>] ? default_spin_lock_flags+0x9/0x10
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8108a6e7>] ? finish_wait+0x67/0x80
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8165ab3e>] ? _raw_spin_lock+0xe/0x20
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8125ab8a>] ? journal_kill_thread+0xfa/0x110
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8108a5f0>] ? add_wait_queue+0x60/0x60
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8125cc22>] journal_destroy+0x52/0x130
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81207557>] ext3_put_super+0x47/0x2a0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff811795f2>] generic_shutdown_super+0x62/0xe0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff811796a0>] kill_block_super+0x30/0x80
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81179d4c>] deactivate_locked_super+0x3c/0xa0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8117a5ce>] deactivate_super+0x4e/0x70
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81196abd>] mntput_no_expire+0x9d/0xf0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81196b2f>] mntput+0x1f/0x30
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8119760a>] release_mounts+0x7a/0x90
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81197866>] put_mnt_ns+0x66/0x80
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8108f3bf>] free_nsproxy+0x1f/0x90
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8108f5d8>] switch_task_namespaces+0x38/0x40
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8108f5f0>] exit_task_namespaces+0x10/0x20
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8106ad4f>] exit_notify+0x1f/0x110
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8106b5d2>] do_exit+0x1f2/0x450
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81079afa>] ? __dequeue_signal+0x6a/0xb0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8106b9d4>] do_group_exit+0x44/0xa0
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff8107c67c>] get_signal_to_deliver+0x21c/0x420
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81014825>] do_signal+0x45/0x130
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81014ad5>] do_notify_resume+0x65/0x80
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  [<ffffffff81663450>] int_signal+0x12/0x17
Jan 24 19:59:19 localhost kernel: [ 1028.130686] Code: 8d 14 00 49 8b 5d 18 4c 89 ef e8 09 50 00 00 48 83 c4 08 c1 e3 18 c1 fb 1f 83 e3 a1 89 d8 5b 41 5c 41 5d 5d c3 0f 0b 0f 0b 0f 0b <0f> 0b 0f 0b 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 53 48 83 ec 
Jan 24 19:59:19 localhost kernel: [ 1028.130686] RIP  [<ffffffff811a76d3>] submit_bh+0x113/0x120
Jan 24 19:59:19 localhost kernel: [ 1028.130686]  RSP <ffff88015b2df738>
Jan 24 19:59:19 localhost kernel: [ 1028.182334] ---[ end trace dcd1b28a51f9e8d7 ]---