I have the same issue here. It only happens when using the NVidia display driver.

I get this in Kern.log when it happens:
Jun  7 00:17:48 laptop kernel: [  480.797985] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jun  7 00:17:48 laptop kernel: [  480.799932] fsnotify_mark   D ffff88022eed4440     0    80      2 0x00000000
Jun  7 00:17:48 laptop kernel: [  480.799940]  ffff88021ee49ce0 0000000000000002 ffff88021edf5fc0 ffff88021ee49fd8
Jun  7 00:17:48 laptop kernel: [  480.799952]  0000000000014440 0000000000014440 ffff88021edf5fc0 ffff88021ee49e10
Jun  7 00:17:48 laptop kernel: [  480.799964]  ffff88021ee49e18 7fffffffffffffff ffff88021edf5fc0 0000000000000000
Jun  7 00:17:48 laptop kernel: [  480.799970] Call Trace:
Jun  7 00:17:48 laptop kernel: [  480.799983]  [<ffffffff8171e639>] schedule+0x29/0x70
Jun  7 00:17:48 laptop kernel: [  480.799988]  [<ffffffff8171d889>] schedule_timeout+0x239/0x2d0
Jun  7 00:17:48 laptop kernel: [  480.799996]  [<ffffffff81075e8b>] ? mod_timer+0x13b/0x210
Jun  7 00:17:48 laptop kernel: [  480.800001]  [<ffffffff81075f78>] ? add_timer+0x18/0x20
Jun  7 00:17:48 laptop kernel: [  480.800007]  [<ffffffff8171f156>] wait_for_completion+0xa6/0x160
Jun  7 00:17:48 laptop kernel: [  480.800014]  [<ffffffff8109a7a0>] ? wake_up_state+0x20/0x20
Jun  7 00:17:48 laptop kernel: [  480.800022]  [<ffffffff810c67de>] __synchronize_srcu+0xfe/0x180
Jun  7 00:17:48 laptop kernel: [  480.800028]  [<ffffffff810c62f0>] ? call_srcu+0x70/0x70
Jun  7 00:17:48 laptop kernel: [  480.800052]  [<ffffffff810c687d>] synchronize_srcu+0x1d/0x20
Jun  7 00:17:48 laptop kernel: [  480.800067]  [<ffffffff811ff7f7>] fsnotify_mark_destroy+0x87/0x150
Jun  7 00:17:48 laptop kernel: [  480.800075]  [<ffffffff810aaea0>] ? prepare_to_wait_event+0x100/0x100
Jun  7 00:17:48 laptop kernel: [  480.800081]  [<ffffffff811ff770>] ? fsnotify_put_mark+0x40/0x40
Jun  7 00:17:48 laptop kernel: [  480.800087]  [<ffffffff8108b322>] kthread+0xd2/0xf0
Jun  7 00:17:48 laptop kernel: [  480.800100]  [<ffffffff8108b250>] ? kthread_create_on_node+0x1d0/0x1d0
Jun  7 00:17:48 laptop kernel: [  480.800107]  [<ffffffff8172ab3c>] ret_from_fork+0x7c/0xb0
Jun  7 00:17:48 laptop kernel: [  480.800117]  [<ffffffff8108b250>] ? kthread_create_on_node+0x1d0/0x1d0
Jun  7 00:17:48 laptop kernel: [  480.800172] INFO: task kio_trash:3826 blocked for more than 120 seconds.
Jun  7 00:17:48 laptop kernel: [  480.801999]       Tainted: PF          O 3.13.0-29-generic #53-Ubuntu
Jun  7 00:17:48 laptop kernel: [  480.803729] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jun  7 00:17:48 laptop kernel: [  480.805470] kio_trash       D ffff88022efd4440     0  3826   3569 0x00000002
Jun  7 00:17:48 laptop kernel: [  480.805475]  ffff880222fb3c80 0000000000000002 ffff88021e2947d0 ffff880222fb3fd8
Jun  7 00:17:48 laptop kernel: [  480.805481]  0000000000014440 0000000000014440 ffff88021e2947d0 ffff880222fb3db0
Jun  7 00:17:48 laptop kernel: [  480.805486]  ffff880222fb3db8 7fffffffffffffff ffff88021e2947d0 ffff8802239cc160
Jun  7 00:17:48 laptop kernel: [  480.805491] Call Trace:
Jun  7 00:17:48 laptop kernel: [  480.805497]  [<ffffffff8171e639>] schedule+0x29/0x70
Jun  7 00:17:48 laptop kernel: [  480.805502]  [<ffffffff8171d889>] schedule_timeout+0x239/0x2d0
Jun  7 00:17:48 laptop kernel: [  480.805508]  [<ffffffff810aaa44>] ? __wake_up+0x44/0x50
Jun  7 00:17:48 laptop kernel: [  480.805515]  [<ffffffff81201741>] ? inotify_remove_from_idr+0xa1/0x1f0
Jun  7 00:17:48 laptop kernel: [  480.805521]  [<ffffffff8171f156>] wait_for_completion+0xa6/0x160
Jun  7 00:17:48 laptop kernel: [  480.805527]  [<ffffffff8109a7a0>] ? wake_up_state+0x20/0x20
Jun  7 00:17:48 laptop kernel: [  480.805533]  [<ffffffff810c67de>] __synchronize_srcu+0xfe/0x180
Jun  7 00:17:48 laptop kernel: [  480.805538]  [<ffffffff810c62f0>] ? call_srcu+0x70/0x70
Jun  7 00:17:48 laptop kernel: [  480.805544]  [<ffffffff810c687d>] synchronize_srcu+0x1d/0x20
Jun  7 00:17:48 laptop kernel: [  480.805549]  [<ffffffff811fefae>] fsnotify_destroy_group+0x1e/0x40
Jun  7 00:17:48 laptop kernel: [  480.805555]  [<ffffffff81201032>] inotify_release+0x22/0x50
Jun  7 00:17:48 laptop kernel: [  480.805562]  [<ffffffff811be384>] __fput+0xe4/0x260
Jun  7 00:17:48 laptop kernel: [  480.805567]  [<ffffffff811be54e>] ____fput+0xe/0x10
Jun  7 00:17:48 laptop kernel: [  480.805575]  [<ffffffff810880c4>] task_work_run+0xc4/0xe0
Jun  7 00:17:48 laptop kernel: [  480.805580]  [<ffffffff81069c08>] do_exit+0x2b8/0xa50
Jun  7 00:17:48 laptop kernel: [  480.805586]  [<ffffffff8109a790>] ? wake_up_state+0x10/0x20
Jun  7 00:17:48 laptop kernel: [  480.805591]  [<ffffffff81077b4e>] ? signal_wake_up_state+0x1e/0x30
Jun  7 00:17:48 laptop kernel: [  480.805597]  [<ffffffff81078eb2>] ? zap_other_threads+0x82/0xa0
Jun  7 00:17:48 laptop kernel: [  480.805601]  [<ffffffff8106a41f>] do_group_exit+0x3f/0xa0
Jun  7 00:17:48 laptop kernel: [  480.805606]  [<ffffffff8106a494>] SyS_exit_group+0x14/0x20
Jun  7 00:17:48 laptop kernel: [  480.805612]  [<ffffffff8172adff>] tracesys+0xe1/0xe6
Jun  7 00:17:48 laptop kernel: [  480.805618] INFO: task kio_trash:4195 blocked for more than 120 seconds.
Jun  7 00:17:48 laptop kernel: [  480.807360]       Tainted: PF          O 3.13.0-29-generic #53-Ubuntu
Jun  7 00:17:48 laptop kernel: [  480.809084] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jun  7 00:17:48 laptop kernel: [  480.810826] kio_trash       D ffff88022ee14440     0  4195   3569 0x00000002
Jun  7 00:17:48 laptop kernel: [  480.810831]  ffff8801f879dc80 0000000000000002 ffff880223358000 ffff8801f879dfd8
Jun  7 00:17:48 laptop kernel: [  480.810836]  0000000000014440 0000000000014440 ffff880223358000 ffff8801f879ddb0
Jun  7 00:17:48 laptop kernel: [  480.810841]  ffff8801f879ddb8 7fffffffffffffff ffff880223358000 ffff8802239cc160
Jun  7 00:17:48 laptop kernel: [  480.810846] Call Trace: