[54400.606291] BUG: unable to handle kernel NULL pointer dereference at 0000000000000088
[54400.606695] IP: [<ffffffff81011719>] xen_spin_lock+0x29/0x60
[54400.608063] PGD 5529067 PUD 546f067 PMD 0
[54400.608063] Oops: 0002 [#1] SMP
[54400.608063] CPU 0
[54400.608063] Modules linked in: ip6_tunnel tunnel6 xenfs lp parport xen_blkfront xen_netfront xenbus_probe_frontend
[54400.608063]
[54400.608063] Pid: 5674, comm: update-apt-xapi Not tainted 3.2.0-030200rc2-generic #201111151435
[54400.608063] RIP: e030:[<ffffffff81011719>]  [<ffffffff81011719>] xen_spin_lock+0x29/0x60
[54400.608063] RSP: e02b:ffff880000b95958  EFLAGS: 00010282
[54400.608063] RAX: 0000000000000400 RBX: 0000000000000088 RCX: 0000000000000000
[54400.608063] RDX: 0000000000000001 RSI: ffff88000262c000 RDI: 0000000000000088
[54400.608063] RBP: ffff880000b95978 R08: 0000000000000000 R09: ffff880000e54a28
[54400.608063] R10: ffff880005123958 R11: 0000000000000000 R12: 0000000000000001
[54400.608063] R13: 0000000000000400 R14: 0000000000000fff R15: ffff880006bc91e0
[54400.608063] FS:  0000000000000000(0000) GS:ffff880007cd6000(0063) knlGS:0000000000000000
[54400.608063] CS:  e033 DS: 002b ES: 002b CR0: 000000008005003b
[54400.608063] CR2: 0000000000000088 CR3: 0000000004644000 CR4: 0000000000002660
[54400.608063] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[54400.608063] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[54400.608063] Process update-apt-xapi (pid: 5674, threadinfo ffff880000b94000, task ffff880005dd0000)
[54400.608063] Stack:
[54400.608063]  ffffffff81c9ad08 ffffea0000098b00 0000000000000000 ffff880000e54a28
[54400.608063]  ffff880000b95988 ffffffff8160e7fe ffff880000b959b8 ffffffff811a884b
[54400.608063]  0000000000001213 0000000000001000 00000000000011ca 000000000000000c
[54400.608063] Call Trace:
[54400.608063]  [<ffffffff8160e7fe>] _raw_spin_lock+0xe/0x20
[54400.608063]  [<ffffffff811a884b>] create_empty_buffers+0x4b/0xd0
[54400.608063]  [<ffffffff8122400e>] ext4_discard_partial_page_buffers_no_lock+0xbe/0x480
[54400.608063]  [<ffffffff81134de8>] ? bdi_wakeup_thread_delayed+0x38/0x40
[54400.608063]  [<ffffffff8119f282>] ? __mark_inode_dirty+0x1b2/0x240
[54400.608063]  [<ffffffff811a9eab>] ? generic_write_end+0x6b/0xa0
[54400.608063]  [<ffffffff812263ed>] ext4_da_write_end+0x10d/0x330
[54400.608063]  [<ffffffff81114d14>] ? iov_iter_copy_from_user_atomic+0x84/0x160
[54400.608063]  [<ffffffff811150a2>] generic_perform_write+0x122/0x1d0
[54400.608063]  [<ffffffff811151b4>] generic_file_buffered_write+0x64/0xa0
[54400.608063]  [<ffffffff81117ea8>] __generic_file_aio_write+0x238/0x470
[54400.608063]  [<ffffffff811729f3>] ? swap_cgroup_record+0x93/0xb0
[54400.608063]  [<ffffffff81150125>] ? swap_entry_free+0x115/0x150
[54400.608063]  [<ffffffff81118156>] generic_file_aio_write+0x76/0xf0
[54400.608063]  [<ffffffff8121d519>] ext4_file_write+0x69/0x280
[54400.608063]  [<ffffffff8100995d>] ? xen_force_evtchn_callback+0xd/0x10
[54400.608063]  [<ffffffff8100a162>] ? check_events+0x12/0x20
[54400.608063]  [<ffffffff81176e02>] do_sync_write+0xe2/0x120
[54400.608063]  [<ffffffff8100a14f>] ? xen_restore_fl_direct_reloc+0x4/0x4
[54400.608063]  [<ffffffff812e4108>] ? apparmor_file_permission+0x18/0x20
[54400.608063]  [<ffffffff812aae83>] ? security_file_permission+0x23/0x90
[54400.608063]  [<ffffffff81177398>] vfs_write+0xc8/0x190
[54400.608063]  [<ffffffff811774f2>] sys_pwrite64+0x92/0xa0
[54400.608063]  [<ffffffff81047dbc>] sys32_pwrite+0x1c/0x20
[54400.608063]  [<ffffffff816190b0>] sysenter_dispatch+0x7/0x2e
[54400.608063] Code: 00 00 55 48 89 e5 48 83 ec 20 48 89 5d e8 4c 89 65 f0 48 89 fb 4c 89 6d f8 41 bc 01 00 00 00 41 bd 00 04 00 00 44 89 e8 44 89 e2
[54400.608063]  13 84 d2 74 0b f3 90 80 3b 00 74 f3 ff c8 75 f5 84 d2 75 0e
[54400.608063] RIP  [<ffffffff81011719>] xen_spin_lock+0x29/0x60
[54400.608063]  RSP <ffff880000b95958>
[54400.608063] CR2: 0000000000000088
[54400.608063] ---[ end trace 04488bc483621861 ]---