I use a desktop environment MATE. When I try to shut down the computer or log off the screen freezes for 30 seconds, then turns black. The computer does not respond to keyboard and mouse, I can turn it off only with the aid of keys Alt + SysRq + O.
After installing the kernel http://kernel.ubuntu.com/~kernel-ppa/mainline/v3.17.1-utopic/ problem disappeared. If you are using the latest kernel from the repository or http://kernel.ubuntu.com/~kernel-ppa/mainline/v3.16.7-ckt2-utopic/ problem exists.
ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: lightdm 1.12.2-0ubuntu1
Uname: Linux 3.17.1-031701-generic x86_64
ApportVersion: 2.14.7-0ubuntu8
Architecture: amd64
Date: Sat Dec 6 05:57:38 2014
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)
modified.conffile..etc.init.lightdm.conf: [modified]
mtime.conffile..etc.init.lightdm.conf: 2014-11-12T21:32:04.127872
[Expired for lightdm (Ubuntu) because there has been no activity for 60 days.]