Those of you who are seeing similar problems; could you try to get a
core dump and a full stack trace?

Here's how I  managed to do it:

1. Prevent X from trapping SIGSEGV by putting the following in
/usr/share/X11/xorg.conf.d/00-no-trap-signals.conf:

Section "ServerFlags"
        Option "NoTrapSignals" "true"
EndSection

2. Tell X to dump core by adding the following to the [SeatDefaults]
section of /etc/lightdm.conf:

xserver-command=X -core

3. Make sure the X is allowed to dump core:
 - log out from your session
 - switch to another VT (ctrl-alt-f1)
 - sudo stop lightdm
 - ulimit -c unlimited
 - sudo lightdm

4. Check that the changes kicked in:
$ ps -fp $(pidof X)
UID        PID  PPID  C STIME TTY          TIME CMD
root     16043  6885  1 08:44 tty7     00:01:34 /usr/bin/X -core :7 -auth 
/var/run/lightdm/root/:7 -nolisten tcp vt7 -novtswitch
$ cat /proc/$(pidof X)/limits | grep core
Max core file size        unlimited            unlimited            bytes     
$ 

5. Switch back to VT7, log in and wait for X to crash again.

6. To get meaningful stacktraces, you'll want debug symbols installed
for X and friends (adjust for different video drivers):

sudo apt-get install libc6-dbg xserver-xorg-core-dbg 
xserver-xorg-video-intel-dbg
 
7. If/when X crashes, check for /etc/X11/core. If it exists, copy it to your 
home dir and run:

gdb /usr/bin/Xorg core

8. In gdb, say "bt full" to get a stacktrace.

-- 
You received this bug notification because you are a member of Ubuntu-X,
which is subscribed to xorg-server in Ubuntu.
https://bugs.launchpad.net/bugs/1026777

Title:
  Xorg segfault: kicks out to login screen

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1026777/+subscriptions

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