title: lightdm(lightdm-gtk-greeter) black screen when reboot/poweroff
body: <p>I have a extend monitor connected to my laptop.</p>
  
  <p>ArchLinux + systemd + lightdm + lightdm-gtk-greeter.</p>
  
  <p>When I execute reboot or poweroff command, Both of the 2 monitor will show an black screen(I can see an cursor on top-left corner of my laptop's screen).Nether reboot nor poweroff operation will be taken place.</p>
  
  <p>If I switch to an console tty with ctrl+alt+f1 hotkey, then execute reboot/poweroff command, then reboot/poweroff operation will be correctly processed.</p>
  
  <p>I'v found some error message in /var/log/lightdm/x-0.log :</p>
  
  <pre><code>[root@XSign hacksign]# cat /var/log/lightdm/x-0.log
  
  X.Org X Server 1.18.3
  Release Date: 2016-04-04
  X Protocol Version 11, Revision 0
  Build Operating System: Linux 4.2.5-1-ARCH x86_64 
  Current Operating System: Linux XSign 4.5.4-1-ARCH #1 SMP PREEMPT Wed May 11 22:21:28 CEST 2016 x86_64
  Kernel command line: initrd=\initramfs-linux.img root=/dev/sda3 rw
  Build Date: 05 April 2016  05:24:02PM
  
  Current version of pixman: 0.34.0
      Before reporting problems, check http://wiki.x.org
      to make sure that you have the latest version.
  Markers: (--) probed, (**) from config file, (==) default setting,
      (++) from command line, (!!) notice, (II) informational,
      (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
  (==) Log file: "/var/log/Xorg.0.log", Time: Mon May 30 13:08:38 2016
  (==) Using config directory: "/etc/X11/xorg.conf.d"
  (==) Using system config directory "/usr/share/X11/xorg.conf.d"
  xf86: remove device 0 /sys/devices
  </code></pre>
  
  <p>Any body knows how to solved this problem ?</p>