lightdm - Ubuntu gets stuck in a login loop

my ubuntu stuck in login loop when trying enter desktop. when login, screen gets black , after login screen comes back.

i've read problem might caused error depending on graphics, here's graphics card: ati radeon 7670m

did end here after running sudo startx? nevertheless:

press ctrl+alt+f3 , login shell.

now run ls -la. if see line

-rw-------  1 root root   53 nov 29 10:19 .xauthority

then need chown username:username .xauthority , try logging in (you may need same for .iceauthority).

else, ls -ld /tmp. check first 10 letters in left: should read so: drwxrwxrwt.

drwxrwxrwt 15 root root 4096 nov 30 04:17 /tmp

else, need sudo chmod a+wt /tmp , check again.

if not both, i'd recommend either

now press alt+-> until reach login screen again, , restart.

Primitatvve