Sorry for the delay. I've just installed the apparent latest kernel,
which is v4.9-rc1. So far, I haven't been able to reproduce the problem
with it, unlike previous kernels 4.8-rc2 and 4.4.0-{31,34,43}. Looks
like it might be fixed!

** Changed in: linux (Ubuntu)
       Status: Incomplete => Confirmed

** Tags added: kernel-fixed-upstream

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  Display freezes on 2nd+subsequent logins

Status in linux package in Ubuntu:

Bug description:
  First boot after a fresh install of 16.04 from the 16.04.1 amd64
  desktop ISO, with 'download updates' enabled. This is alongside an
  existing 14.04 installation, which continues to work flawlessly. In
  particular, this issue cannot be reproduced there. (I've installed the
  -lts-xenial kernel package there, so it's version, but NOT
  the xorg ones.)

  The first login on the new system worked fine. After simply logging
  out and in again, the display freezes just after drawing my user's
  desktop. (This first time, everything was drawn, but on subsequent
  attempts the background was black and the top Unity panel was empty.)
  The mouse cursor is movable, but no button or key presses are

  Eventually, I press Ctrl+Alt+F1. The mouse cursor disappears
  immediately; the display stays frozen and otherwise unchanged. After
  another two minutes or so of waiting, I am finally dumped to the
  virtual tty, which works fine. At this point a couple of kernel stack
  traces appear in dmesg (see below), with timestamps corresponding to
  "just now," i.e. when the vty switch finally occurred.

  Switching back to the X vty via Alt+F7 brings me back to the still-
  frozen display, at which point all I can do is Ctrl+Alt+F1 and wait ~2
  minutes again. Stopping, then starting the lightdm service brings me
  back to a working login screen, but logging in just freezes right back
  again. Finally, rebooting the system allows me to login to a working
  desktop exactly once -- logging out and in again reproduces this
  problem reliably.

  The full dmesg is attached, but the short version is that stack traces
  appear in pairs, summarized by the following lines on the first

  [  242.116429] WARNING: CPU: 2 PID: 2539 at 
drm_wait_one_vblank+0x1b5/0x1c0 [drm]()
  [  242.116498] CPU: 2 PID: 2539 Comm: Xorg Not tainted 4.4.0-31-generic 

  and by the following, slightly different ones, on the second and

  [  380.232443] WARNING: CPU: 1 PID: 4173 at 
intel_crtc_wait_for_pending_flips+0x1e2/0x240 [i915]()
  [  380.232563] CPU: 1 PID: 4173 Comm: Xorg Tainted: G        W       
4.4.0-31-generic #50-Ubuntu

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: linux-image-4.4.0-31-generic 4.4.0-31.50 [modified: 
  ProcVersionSignature: Ubuntu 4.4.0-31.50-generic 4.4.13
  Uname: Linux 4.4.0-31-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.1
  Architecture: amd64
   /dev/snd/controlC0:  who        3445 F.... pulseaudio
  CurrentDesktop: Unity
  Date: Fri Aug 12 19:51:28 2016
  HibernationDevice: RESUME=UUID=2065ee3e-d689-4432-a805-4a7822b06d0e
  InstallationDate: Installed on 2016-08-13 (0 days ago)
  InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 
  MachineType: LENOVO 4236AR5
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-31-generic 
root=UUID=093a7652-dfb2-4f9e-9ceb-23761b18c8ef ro quiet splash
   linux-restricted-modules-4.4.0-31-generic N/A
   linux-backports-modules-4.4.0-31-generic  N/A
   linux-firmware                            1.157.2
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install) 10/31/2011
  dmi.bios.vendor: LENOVO
  dmi.bios.version: 83ET66WW (1.36 )
  dmi.board.asset.tag: Not Available 4236AR5
  dmi.board.vendor: LENOVO
  dmi.board.version: Not Available
  dmi.chassis.asset.tag: No Asset Information
  dmi.chassis.type: 10
  dmi.chassis.vendor: LENOVO
  dmi.chassis.version: Not Available
dmi:bvnLENOVO:bvr83ET66WW(1.36):bd10/31/2011:svnLENOVO:pn4236AR5:pvrThinkPadT420:rvnLENOVO:rn4236AR5:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable: 4236AR5
  dmi.product.version: ThinkPad T420
  dmi.sys.vendor: LENOVO

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