Unable to Login as root authentication failure error in Linux

If you get Unable to Login as root authentication failure error in Linux

Modify two files 

1. vi /etc/pam.d/gdm-password 
in this file put comment on line "auth        required      pam_succeed_if.so user != root quiet" like below

auth     [success=done ignore=ignore default=bad] pam_selinux_permit.so
auth        substack      password-auth
auth        required      pam_succeed_if.so user != root quiet
auth        optional      pam_gnome_keyring.so
.....
.....
To

auth     [success=done ignore=ignore default=bad] pam_selinux_permit.so
auth        substack      password-auth
#auth        required      pam_succeed_if.so user != root quiet
auth        optional      pam_gnome_keyring.so
....
...

2. vi /etc/pam.d/gdm-fingerprint
in this file put comment on line "auth        required      pam_succeed_if.so user != root quiet" like below

auth        substack      fingerprint-auth
auth        required      pam_succeed_if.so user != root quiet

account     required      pam_nologin.so
account     include       fingerprint-auth

password    include       fingerprint-auth
....
....
To

auth        substack      fingerprint-auth
#auth        required      pam_succeed_if.so user != root quiet

account     required      pam_nologin.so
account     include       fingerprint-auth

password    include       fingerprint-auth
....
....

3. Logout and Re-Login as root