From kern.log:

Nov 15 00:00:15 lister kernel: [576836.444343] EXT4-fs (sdc1): warning: checktime reached, running e2fsck is recommended
Nov 15 00:00:15 lister kernel: [576836.448622] EXT4-fs (sdc1): mounted filesystem with ordered data mode. Opts: (null)
Nov 15 00:24:38 lister kernel: [578296.072009] BUG: soft lockup - CPU#0 stuck for 23s! [umount:30662]
Nov 15 00:24:38 lister kernel: [578296.072015] Modules linked in: btusb vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) isofs nls_iso8859_1 nls_cp437 vfat fat nls_utf8 udf crc_itu_t       pci_stub bnep rfcomm bluetooth parport_pc ppdev nfsd nfs lockd binfmt_misc fscache auth_rpcgss nfs_acl sunrpc vesafb snd_hda_codec_hdmi snd_hda_codec_realtek snd_usb_audio snd_usbmidi_lib   snd_seq_midi psmouse snd_rawmidi snd_hda_intel snd_hda_codec snd_hwdep snd_seq_midi_event snd_pcm snd_seq lp ses uvcvideo videodev edac_core snd_timer pl2303 sp5100_tco enclosure            v4l2_compat_ioctl32 hid_logitech_dj snd_seq_device usbserial snd soundcore i2c_piix4 edac_mce_amd asus_atk0110 joydev fglrx(P) mac_hid snd_page_alloc k10temp wmi serio_raw parport           usb_storage usbhid hid pata_atiixp sky2 [last unloaded: vboxdrv]
Nov 15 00:24:38 lister kernel: [578296.072099] CPU 0
Nov 15 00:24:38 lister kernel: [578296.072101] Modules linked in: btusb vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) isofs nls_iso8859_1 nls_cp437 vfat fat nls_utf8 udf crc_itu_t       pci_stub bnep rfcomm bluetooth parport_pc ppdev nfsd nfs lockd binfmt_misc fscache auth_rpcgss nfs_acl sunrpc vesafb snd_hda_codec_hdmi snd_hda_codec_realtek snd_usb_audio snd_usbmidi_lib   snd_seq_midi psmouse snd_rawmidi snd_hda_intel snd_hda_codec snd_hwdep snd_seq_midi_event snd_pcm snd_seq lp ses uvcvideo videodev edac_core snd_timer pl2303 sp5100_tco enclosure            v4l2_compat_ioctl32 hid_logitech_dj snd_seq_device usbserial snd soundcore i2c_piix4 edac_mce_amd asus_atk0110 joydev fglrx(P) mac_hid snd_page_alloc k10temp wmi serio_raw parport           usb_storage usbhid hid pata_atiixp sky2 [last unloaded: vboxdrv]
Nov 15 00:24:38 lister kernel: [578296.072166]
Nov 15 00:24:38 lister kernel: [578296.072171] Pid: 30662, comm: umount Tainted: P           O 3.2.0-32-generic #51-Ubuntu System manufacturer System Product Name/M3A78-T
Nov 15 00:24:38 lister kernel: [578296.072180] RIP: 0010:[<ffffffff8103dc4d>]  [<ffffffff8103dc4d>] __ticket_spin_lock+0xd/0x30
Nov 15 00:24:38 lister kernel: [578296.072193] RSP: 0018:ffff88017c747e28  EFLAGS: 00000282
Nov 15 00:24:38 lister kernel: [578296.072197] RAX: 00000000b9d0b9d0 RBX: ffff880100000000 RCX: 0000000000000002
Nov 15 00:24:38 lister kernel: [578296.072201] RDX: 000000000000b9cf RSI: ffff88002f227830 RDI: ffff88002f226de0
Nov 15 00:24:38 lister kernel: [578296.072204] RBP: ffff88017c747e28 R08: 0000000000000000 R09: 0000000000000000
Nov 15 00:24:38 lister kernel: [578296.072208] R10: 0000000000001000 R11: 0000000000000000 R12: ffff88017c747d90
Nov 15 00:24:38 lister kernel: [578296.072212] R13: ffff88017c747d80 R14: ffff88017c747d80 R15: 0000000091827364
Nov 15 00:24:38 lister kernel: [578296.072216] FS:  00007fdd7889e800(0000) GS:ffff88022fc00000(0000) knlGS:00000000f75786c0
Nov 15 00:24:38 lister kernel: [578296.072220] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Nov 15 00:24:38 lister kernel: [578296.072224] CR2: 00007fdd77ecc100 CR3: 0000000128aed000 CR4:Nov 15 07:20:22 lister kernel: imklog 5.8.6, log source = /proc/kmsg started.