Public bug reported:

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 4.4.0.34.24, 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 recognized.

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
occurrence:

[  242.116429] WARNING: CPU: 2 PID: 2539 at 
/build/linux-dcxD3m/linux-4.4.0/drivers/gpu/drm/drm_irq.c:1326 
drm_wait_one_vblank+0x1b5/0x1c0 [drm]()
[  242.116498] CPU: 2 PID: 2539 Comm: Xorg Not tainted 4.4.0-31-generic 
#50-Ubuntu

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

[  380.232443] WARNING: CPU: 1 PID: 4173 at 
/build/linux-dcxD3m/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:3963 
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: 
boot/vmlinuz-4.4.0-31-generic]
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
AudioDevicesInUse:
 USER        PID ACCESS COMMAND
 /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 (20160719)
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
RelatedPackageVersions:
 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)
dmi.bios.date: 10/31/2011
dmi.bios.vendor: LENOVO
dmi.bios.version: 83ET66WW (1.36 )
dmi.board.asset.tag: Not Available
dmi.board.name: 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.modalias: 
dmi:bvnLENOVO:bvr83ET66WW(1.36):bd10/31/2011:svnLENOVO:pn4236AR5:pvrThinkPadT420:rvnLENOVO:rn4236AR5:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
dmi.product.name: 4236AR5
dmi.product.version: ThinkPad T420
dmi.sys.vendor: LENOVO

** Affects: linux (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug xenial

** Attachment added: "dmesg.log"
   https://bugs.launchpad.net/bugs/1612895/+attachment/4720010/+files/dmesg.log

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https://bugs.launchpad.net/bugs/1612895

Title:
  Display freezes on 2nd+subsequent logins

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