We look to have had a hard recurrance of this issue on the system. After much messing about we appear to have gotten some real stack traces.  We see similar ones on a range of kernel versions in 3.13.0-* series:

NULL pointer dereference at 0000000000000010
[   15.316900] IP: [<ffffffff813700a1>] rb_next+0x1/0x50
[   15.316910] PGD 0
[   15.316913] Oops: 0000 [#1] SMP
[   15.316917] Modules linked in: btrfs xor raid6_pq libcrc32c hid_generic usbhid hid crct10dif_pclmul crc32_pclmul ixgbe igb aesni_intel aes_x86_64 lrw dca gf128mul glue_helper ablk_helper ptp cryptd ahci pps_core i2c_algo_bit libahci mdi
o megaraid_sas wmi
[   15.316940] CPU: 13 PID: 883 Comm: upstart-udev-br Not tainted 3.13.0-88-generic #135-Ubuntu
[   15.316942] Hardware name: Dell Inc. PowerEdge R630/0CNCJW, BIOS 1.0.4 08/28/2014
[   15.316943] task: ffff883fcd8d1800 ti: ffff883fcd870000 task.ti: ffff883fcd870000
[   15.316947] RIP: 0010:[<ffffffff813700a1>]  [<ffffffff813700a1>] rb_next+0x1/0x50
[   15.316948] RSP: 0018:ffff883fcd871870  EFLAGS: 00010046
[   15.316950] RAX: 0000000000000000 RBX: ffff883fd063f600 RCX: 0000000000000000
[   15.316951] RDX: ffff883ffecd3180 RSI: ffff883ffecd3228 RDI: 0000000000000010
[   15.316952] RBP: ffff883fcd8718b8 R08: 0000000000000000 R09: 0000000000000001
[   15.316953] R10: 0000000000000001 R11: 0000000000000304 R12: 0000000000000000
[   15.316954] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   15.316956] FS:  00007f63bc298740(0000) GS:ffff883ffecc0000(0000) knlGS:0000000000000000
[   15.316957] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   15.316958] CR2: 0000000000000010 CR3: 0000003fd0604000 CR4: 00000000001407e0
[   15.316959] Stack:
[   15.316965]  ffff883fcd8718b8 ffffffff810a2c72 0000000dcd8718a8 ffff883ffecd3180
[   15.316970]  ffff883fcd8d1c28 ffff883ffecd3180 000000000000000d 0000000000000007
[   15.316974]  0000000000000007 ffff883fcd871918 ffffffff8172d712 ffff883fcd8d1800
[   15.316975] Call Trace:
[   15.316986]  [<ffffffff810a2c72>] ? pick_next_task_fair+0x102/0x1b0
[   15.316994]  [<ffffffff8172d712>] __schedule+0x142/0x7f0
[   15.316997]  [<ffffffff8172dde9>] schedule+0x29/0x70
[   15.317001]  [<ffffffff8172d42d>] schedule_hrtimeout_range_clock+0x14d/0x170
[   15.317007]  [<ffffffff811d50df>] ? __pollwait+0x7f/0xf0
[   15.317014]  [<ffffffff8173190b>] ? _raw_spin_unlock_bh+0x1b/0x40
[   15.317019]  [<ffffffff81657cce>] ? netlink_poll+0x13e/0x1e0
[   15.317022]  [<ffffffff8172d463>] schedule_hrtimeout_range+0x13/0x20
[   15.317026]  [<ffffffff811d5199>] poll_schedule_timeout+0x49/0x70
[   15.317029]  [<ffffffff811d5b86>] do_select+0x5b6/0x780
[   15.317037]  [<ffffffff811a54ad>] ? kfree+0x11d/0x160
[   15.317041]  [<ffffffff811d5320>] ? poll_select_copy_remaining+0x130/0x130
[   15.317044]  [<ffffffff811d5320>] ? poll_select_copy_remaining+0x130/0x130
[   15.317047]  [<ffffffff811d5320>] ? poll_select_copy_remaining+0x130/0x130
[   15.317055]  [<ffffffff8115bac3>] ? __alloc_pages_nodemask+0x1a3/0xb90
[   15.317061]  [<ffffffff8161402a>] ? sock_recvmsg+0x9a/0xd0
[   15.317070]  [<ffffffff811524f3>] ? unlock_page+0x23/0x30
[   15.317078]  [<ffffffff811794b9>] ? __do_fault+0x429/0x530
[   15.317082]  [<ffffffff811d5f1c>] core_sys_select+0x1cc/0x2e0
[   15.317089]  [<ffffffff810ad5cd>] ? remove_wait_queue+0x4d/0x60
[   15.317096]  [<ffffffff8106b743>] ? do_wait+0x123/0x250
[   15.317100]  [<ffffffff811d60db>] SyS_select+0xab/0x100
[   15.317103]  [<ffffffff8106a410>] ? task_stopped_code+0x60/0x60
[   15.317109]  [<ffffffff8173a4dd>] system_call_fastpath+0x1a/0x1f
[   15.317130] Code: e5 48 85 c0 75 07 eb 19 66 90 48 89 d0 48 8b 50 10 48 85 d2 75 f4 48 8b 50 08 48 85 d2 75 eb 5d c3 31 c0 5d c3 0f 1f 44 00 00 55 <48> 8b 17 48 89 e5 48 39 d7 74 3b 48 8b 47 08 48 85 c0 75 0e eb
[   15.317133] RIP  [<ffffffff813700a1>] rb_next+0x1/0x50
[   15.317133]  RSP <ffff883fcd871870>
[   15.317134] CR2: 0000000000000010
[   15.317137] ---[ end trace 91af0079725c7159 ]---