Ubuntu 12.04: cannot login after enter correct password, the system returns back you login screen

After I upgraded my ubuntu from 11.10 to 12.04 for few days, I cannot login even I entered the correct password. It always returned back to login screen.

After spent 30 minutes research on google, and found what is the possible issue and the solution which works for me.

Issue: lightdm might be the problem

Solution: change to use gdm

1. Go to recovery mode.
2. enable network.
3. re mount / by the using this command: #mount -o rw,remount /
4. remove the file in your home directory: rm /home/userName/.Xauthority*
5. install gdm: apt-get install gdm.
6. change to use gdm as the default login manager: dpkg-reconfigure gdm
7. reboot

This solution works for me and I can login use my account. But the problem backs when I change back to use lightdm as default. So I have to go with gdm from now on to login successfully.