This problem has caused more serious damage recently.  When nbd dies and
printk()s like mad, the serial console is not fast enough to display it.

The kernel keeps allocating buffer space for serial output, which we see
as 13G kmalloc-256 or kmalloc-512 kernel threads.

Eventually the OOMkiller tries to free up space, but it can only kill
userspace programs so ultimately the system dies altogether.

This is more dire than mere CPU load or lockup warning messages.

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  Soft lockup with "block nbdX: Attempted send on closed socket" spam

Status in linux package in Ubuntu:
  In Progress

Bug description:
  Some of our nova compute hosts regularly freeze, sometimes for a few
  hours, with kern.log getting spammed with:

  block nbdX: Attempted send on closed socket

  and a few "CPU soft lockup" messages (see attached log). This clears
  up when the queue gets cleared, eg :

  block nbdX: queue cleared

  trusty hosts with kernel version 3.19.0-30-generic.
   total 0
   crw-rw---- 1 root audio 116,  1 Nov 24 12:23 seq
   crw-rw---- 1 root audio 116, 33 Nov 24 12:23 timer
  AplayDevices: Error: [Errno 2] No such file or directory
  ApportVersion: 2.14.1-0ubuntu3.19
  Architecture: amd64
  ArecordDevices: Error: [Errno 2] No such file or directory
  AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', 
'/dev/snd/timer'] failed with exit code 1:
  DistroRelease: Ubuntu 14.04
  IwConfig: Error: [Errno 2] No such file or directory
  MachineType: HP ProLiant DL385 G7
  Package: linux (not installed)
   PATH=(custom, no user)
  ProcFB: 0 radeondrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.19.0-36-generic 
root=UUID=13289ac9-8dc9-4feb-b6bd-ca7db66b21d6 ro console=tty0 
console=ttyS1,38400 nosplash crashkernel=384M-:512M nox2apic intremap=off
  ProcVersionSignature: Ubuntu 
3.19.0-36.41~14.04.1hf00090138v20151122b1-generic 3.19.8-ckt9
   linux-restricted-modules-3.19.0-36-generic N/A
   linux-backports-modules-3.19.0-36-generic  N/A
   linux-firmware                             1.127.18
  RfKill: Error: [Errno 2] No such file or directory
  Tags:  trusty uec-images
  Uname: Linux 3.19.0-36-generic x86_64
  UpgradeStatus: No upgrade log present (probably fresh install)
  _MarkForUpload: True 02/02/2014
  dmi.bios.vendor: HP
  dmi.bios.version: A18
  dmi.chassis.type: 23
  dmi.chassis.vendor: HP
dmi:bvnHP:bvrA18:bd02/02/2014:svnHP:pnProLiantDL385G7:pvr:cvnHP:ct23:cvr: ProLiant DL385 G7
  dmi.sys.vendor: HP

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