/var/log/kern.log (with -63 kernel):
[   21.617846] Disabling lock debugging due to kernel taint
[   21.624593] nvidia: module verification failed: signature and/or  required key missing - tainting kernel
[   21.727721] input: HDA NVidia HDMI/DP,pcm=9 as /devices/pci0000:00/0000:00:05.0/0000:08:00.1/sound/card0/input10
[   21.727943] input: HDA NVidia HDMI/DP,pcm=8 as /devices/pci0000:00/0000:00:05.0/0000:08:00.1/sound/card0/input9
[   21.728198] input: HDA NVidia HDMI/DP,pcm=7 as /devices/pci0000:00/0000:00:05.0/0000:08:00.1/sound/card0/input8
[   21.728454] input: HDA NVidia HDMI/DP,pcm=3 as /devices/pci0000:00/0000:00:05.0/0000:08:00.1/sound/card0/input7
[   21.729674] hda_intel: Disabling MSI
[   21.729683] hda-intel 0000:03:00.1: Handle VGA-switcheroo audio client
[   21.729715] hda-intel 0000:03:00.1: Disabling 64bit DMA
[   21.729755] vgaarb: device changed decodes: PCI:0000:08:00.0,olddecodes=io+mem,decodes=none:owns=none
[   21.733177] hda-intel 0000:03:00.1: Enable delay in RIRB handling
[   21.733664] BUG: unable to handle kernel NULL pointer dereference at           (null)
[   21.733695] IP: [<ffffffff8172a54b>] __down_common+0x4c/0x144
[   21.733719] PGD 81d8a2067 PUD 81d86f067 PMD 0 
[   21.733739] Oops: 0002 [#1] SMP 
[   21.733753] Modules linked in: nvidia(POX+) snd_hda_codec_hdmi snd_hda_intel(+) snd_hda_codec snd_hwdep snd_pcm snd_page_alloc snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer gpio_ich parport_pc snd ppdev dm_multipath intel_powerclamp soundcore w83795 i2c_i801 ioatdma scsi_dh coretemp i7core_edac kvm_intel joydev lp serio_raw kvm shpchp lpc_ich mac_hid edac_core parport hid_generic radeon ttm drm_kms_helper igb drm dca psmouse usbhid ptp hid pps_core pata_acpi pata_jmicron i2c_algo_bit floppy
[   21.733997] CPU: 0 PID: 862 Comm: nvidia-persiste Tainted: P           OX 3.13.0-63-generic #103-Ubuntu
[   21.734038] Hardware name: Supermicro X8DTN/X8DTN, BIOS 080016  10/06/2010
[   21.734064] task: ffff88081d82b000 ti: ffff88083747e000 task.ti: ffff88083747e000
[   21.734102] RIP: 0010:[<ffffffff8172a54b>]  [<ffffffff8172a54b>] __down_common+0x4c/0x144
[   21.734145] RSP: 0018:ffff88083747fb48  EFLAGS: 00010096
[   21.734168] RAX: 0000000000000000 RBX: ffffffffa0ce0970 RCX: 0000000000000000
[   21.734194] RDX: ffffffffa0ce0978 RSI: ffff88083747fb50 RDI: ffffffffa0ce0970
[   21.734220] RBP: ffff88083747fb98 R08: 0000000000000296 R09: ffffffffa0a460bb
[   21.734246] R10: 0000000000000008 R11: 00000000000000ff R12: 7fffffffffffffff
[   21.734272] R13: ffff88081d82b000 R14: 0000000000000002 R15: 0000000000000000
[   21.734298] FS:  00007f0bcff56740(0000) GS:ffff88056ae00000(0000) knlGS:0000000000000000
[   21.734337] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   21.734360] CR2: 0000000000000000 CR3: 0000000831555000 CR4: 00000000000007f0
[   21.734386] Stack:
[   21.734404]  ffff88083747fb68 ffffffffa0ce0978 0000000000000000 ffff88055dad7700
[   21.734453]  0000000000000000 ffffffffa0ce0970 ffff88055ff00000 0000000000000003
[   21.734503]  ffff8805600aa7b8 0000000000000002 ffff88083747fba8 ffffffff8172a660
[   21.734552] Call Trace:
[   21.734572]  [<ffffffff8172a660>] __down+0x1d/0x1f
[   21.734597]  [<ffffffff810b0f71>] down+0x41/0x50
[   21.734664]  [<ffffffffa0a463b7>] nvidia_open+0x387/0x8b0 [nvidia]
[   21.734728]  [<ffffffffa0a537a9>] nvidia_frontend_open+0x49/0xa0 [nvidia]
[   21.734756]  [<ffffffff811c2dbf>] chrdev_open+0x9f/0x1d0
[   21.734781]  [<ffffffff811bb8f3>] do_dentry_open+0x233/0x2e0
[   21.734805]  [<ffffffff811c2d20>] ? cdev_put+0x30/0x30
[   21.734828]  [<ffffffff811bbc29>] vfs_open+0x49/0x50
[   21.734853]  [<ffffffff811ccfd4>] do_last+0x564/0x1240
[   21.734877]  [<ffffffff811caa91>] ? link_path_walk+0x71/0x880
[   21.734902]  [<ffffffff8131618b>] ? apparmor_file_alloc_security+0x5b/0x180
[   21.734929]  [<ffffffff811cdd6b>] path_openat+0xbb/0x650
[   21.734954]  [<ffffffff810c949d>] ? call_rcu_sched+0x1d/0x20
[   21.734978]  [<ffffffff811cf16a>] do_filp_open+0x3a/0x90
[   21.735003]  [<ffffffff811dbfe7>] ? __alloc_fd+0xa7/0x130
[   21.735028]  [<ffffffff811bd749>] do_sys_open+0x129/0x280
[   21.735052]  [<ffffffff811bd8be>] SyS_open+0x1e/0x20
[   21.735076]  [<ffffffff8173489d>] system_call_fastpath+0x1a/0x1f
[   21.735100] Code: 54 49 89 d4 48 8d 57 08 53 48 89 fb 48 83 e4 f0 48 83 ec 28 48 8b 47 10 48 8d 74 24 08 48 89 54 24 08 48 89 44 24 10 48 89 77 10 <48> 89 30 4c 89 f0 4c 89 6c 24 18 83 e0 01 c6 44 24 20 00 48 89 
[   21.735352] RIP  [<ffffffff8172a54b>] __down_common+0x4c/0x144
[   21.735379]  RSP <ffff88083747fb48>
[   21.735399] CR2: 0000000000000000
[   21.735420] ---[ end trace 3e512bcf3204bb86 ]---
[   21.812982] EXT4-fs (sda2): re-mounted. Opts: errors=remount-ro
[   22.215651] input: HDA NVidia HDMI/DP,pcm=9 as /devices/pci0000:00/0000:00:1c.0/0000:03:00.1/sound/card1/input14
[   22.215928] input: HDA NVidia HDMI/DP,pcm=8 as /devices/pci0000:00/0000:00:1c.0/0000:03:00.1/sound/card1/input13
[   22.216175] input: HDA NVidia HDMI/DP,pcm=7 as /devices/pci0000:00/0000:00:1c.0/0000:03:00.1/sound/card1/input12
[   22.216435] input: HDA NVidia HDMI/DP,pcm=3 as /devices/pci0000:00/0000:00:1c.0/0000:03:00.1/sound/card1/input11
[   22.217657] nvidia 0000:03:00.0: enabling device (0000 -> 0003)
[   22.217755] vgaarb: device changed decodes: PCI:0000:03:00.0,olddecodes=io+mem,decodes=none:owns=none
[   22.314154] Bluetooth: Core ver 2.17
[   22.314191] NET: Registered protocol family 31
[   22.314213] Bluetooth: HCI device and connection manager initialized
[   22.314245] Bluetooth: HCI socket layer initialized
[   22.314268] Bluetooth: L2CAP socket layer initialized
[   22.314293] Bluetooth: SCO socket layer initialized
[   22.381787] Bluetooth: BNEP (Ethernet Emulation) ver 1.3
[   22.381812] Bluetooth: BNEP filters: protocol multicast
[   22.381837] Bluetooth: BNEP socket layer initialized
[   22.393654] Bluetooth: RFCOMM TTY layer initialized
[   22.393691] Bluetooth: RFCOMM socket layer initialized
[   22.393721] Bluetooth: RFCOMM ver 1.11
[   23.179742] igb: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX/TX
[   23.180011] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[   23.686385] init: failsafe main process (942) killed by TERM signal
[   24.703154] init: gdm main process (1368) killed by TERM signal
[   25.850750] Ebtables v2.0 registered
[   25.922333] ip_tables: (C) 2000-2006 Netfilter Core Team
[   25.957520] ip6_tables: (C) 2000-2006 Netfilter Core Team
[   27.794812] Bridge firewalling registered
[   27.962400] nf_conntrack version 0.5.0 (16384 buckets, 65536 max)
[   28.005490] IPv6: ADDRCONF(NETDEV_UP): virbr0: link is not ready
[  157.723254] cgroup: systemd-logind (1015) created nested cgroup for controller "memory" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.
[  157.723258] cgroup: "memory" requires setting use_hierarchy to 1 on the root.