Comment 2 for bug 1485996

Same thing happens here, running 15.04 64bit, Kernel 3.19.0-26-generic on a ATI (AMD Radeon HD 7800 Series) card. 3.19.0-25-generic runs just fine. The screen stays black, when SSHing into the machine and reading syslog, it repeatedly posts these lines:

Aug 18 18:31:52 paul-desktop gdm[1240]: GLib: Source ID 61 was not found when attempting to remove it
Aug 18 18:31:52 paul-desktop gdm[1240]: Child process 3888 was already dead.
Aug 18 18:31:52 paul-desktop gdm[1240]: Unable to kill session worker process
Aug 18 18:31:52 paul-desktop org.a11y.Bus[3898]: g_dbus_connection_real_closed: Remote peer vanished with error: Underlying GIOStream returned 0 bytes on an async read (g-io-error-quark, 0). Exiting.
Aug 18 18:31:52 paul-desktop gdm[1240]: Child process 3888 was already dead.
Aug 18 18:31:52 paul-desktop gdm[1240]: Unable to kill session worker process
Aug 18 18:31:52 paul-desktop org.a11y.atspi.Registry[3904]: g_dbus_connection_real_closed: Remote peer vanished with error: Underlying GIOStream returned 0 bytes on an async read (g-io-error-quark, 0). Exiting.
Aug 18 18:31:52 paul-desktop systemd[1]: Stopping User Manager for UID 116...
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopped target Default.
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopping Default.
Aug 18 18:31:52 paul-desktop systemd[3891]: Reached target Shutdown.
Aug 18 18:31:52 paul-desktop systemd[3891]: Starting Shutdown.
Aug 18 18:31:52 paul-desktop systemd[3891]: Starting Exit the Session...
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopped target Basic System.
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopping Basic System.
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopped target Paths.
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopping Paths.
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopped target Timers.
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopping Timers.
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopped target Sockets.
Aug 18 18:31:52 paul-desktop systemd[3891]: Stopping Sockets.
Aug 18 18:31:52 paul-desktop systemd[3891]: Received SIGRTMIN+24 from PID 3917 (kill).
Aug 18 18:31:52 paul-desktop systemd[1]: Stopped User Manager for UID 116.
Aug 18 18:31:52 paul-desktop systemd[1]: Removed slice user-116.slice.
Aug 18 18:31:52 paul-desktop systemd[1]: Stopping user-116.slice.
Aug 18 18:31:52 paul-desktop systemd[1]: Created slice user-1000.slice.
Aug 18 18:31:52 paul-desktop systemd[1]: Starting user-1000.slice.
Aug 18 18:31:52 paul-desktop systemd[1]: Starting User Manager for UID 1000...
Aug 18 18:31:52 paul-desktop systemd[1]: Started Session 1 of user paul.
Aug 18 18:31:52 paul-desktop systemd[1]: Starting Session 1 of user paul.
Aug 18 18:31:52 paul-desktop systemd[3923]: Reached target Paths.
Aug 18 18:31:52 paul-desktop systemd[3923]: Starting Paths.
Aug 18 18:31:52 paul-desktop systemd[3923]: Reached target Timers.
Aug 18 18:31:52 paul-desktop systemd[3923]: Starting Timers.
Aug 18 18:31:52 paul-desktop systemd[3923]: Reached target Sockets.
Aug 18 18:31:52 paul-desktop systemd[3923]: Starting Sockets.
Aug 18 18:31:52 paul-desktop systemd[3923]: Reached target Basic System.
Aug 18 18:31:52 paul-desktop systemd[3923]: Starting Basic System.
Aug 18 18:31:52 paul-desktop systemd[3923]: Reached target Default.
Aug 18 18:31:52 paul-desktop systemd[3923]: Startup finished in 138ms.
Aug 18 18:31:52 paul-desktop systemd[3923]: Starting Default.
Aug 18 18:31:52 paul-desktop systemd[1]: Started User Manager for UID 1000.
Aug 18 18:31:53 paul-desktop gdm[1240]: Failed to give slave programs access to the display. Trying to proceed.
Aug 18 18:31:53 paul-desktop systemd[1]: Created slice user-116.slice.
Aug 18 18:31:53 paul-desktop systemd[1]: Starting user-116.slice.
Aug 18 18:31:53 paul-desktop systemd[1]: Starting User Manager for UID 116...
Aug 18 18:31:53 paul-desktop systemd[1]: Started Session c33 of user gdm.
Aug 18 18:31:53 paul-desktop systemd[1]: Starting Session c33 of user gdm.
Aug 18 18:31:53 paul-desktop systemd[3964]: Reached target Paths.
Aug 18 18:31:53 paul-desktop systemd[3964]: Starting Paths.
Aug 18 18:31:53 paul-desktop systemd[3964]: Reached target Sockets.
Aug 18 18:31:53 paul-desktop systemd[3964]: Starting Sockets.
Aug 18 18:31:53 paul-desktop systemd[3964]: Reached target Timers.
Aug 18 18:31:53 paul-desktop systemd[3964]: Starting Timers.
Aug 18 18:31:53 paul-desktop systemd[3964]: Reached target Basic System.
Aug 18 18:31:53 paul-desktop systemd[3964]: Starting Basic System.
Aug 18 18:31:53 paul-desktop systemd[3964]: Reached target Default.
Aug 18 18:31:53 paul-desktop systemd[3964]: Startup finished in 10ms.
Aug 18 18:31:53 paul-desktop systemd[3964]: Starting Default.
Aug 18 18:31:53 paul-desktop systemd[1]: Started User Manager for UID 116.
Aug 18 18:31:53 paul-desktop org.a11y.Bus[3971]: Activating service name='org.a11y.atspi.Registry'
Aug 18 18:31:53 paul-desktop org.a11y.Bus[3971]: Successfully activated service 'org.a11y.atspi.Registry'
Aug 18 18:31:53 paul-desktop org.a11y.atspi.Registry[3977]: SpiRegistry daemon is running with well-known name - org.a11y.atspi.Registry
Aug 18 18:31:53 paul-desktop gnome-session[3967]: X Error of failed request: BadRequest (invalid request code or no such operation)
Aug 18 18:31:53 paul-desktop gnome-session[3967]: Major opcode of failed request: 156 (ATIFGLEXTENSION)
Aug 18 18:31:53 paul-desktop gnome-session[3967]: Minor opcode of failed request: 66 ()
Aug 18 18:31:53 paul-desktop gnome-session[3967]: Serial number of failed request: 15
Aug 18 18:31:53 paul-desktop gnome-session[3967]: Current serial number in output stream: 15
Aug 18 18:31:53 paul-desktop gnome-session[3967]: gnome-session-check-accelerated: Helper exited with code 256
Aug 18 18:31:58 paul-desktop gnome-session[3967]: X Error of failed request: BadRequest (invalid request code or no such operation)
Aug 18 18:31:58 paul-desktop gnome-session[3967]: Major opcode of failed request: 156 (ATIFGLEXTENSION)
Aug 18 18:31:58 paul-desktop gnome-session[3967]: Minor opcode of failed request: 66 ()
Aug 18 18:31:58 paul-desktop gnome-session[3967]: Serial number of failed request: 15
Aug 18 18:31:58 paul-desktop gnome-session[3967]: Current serial number in output stream: 15
Aug 18 18:31:58 paul-desktop gnome-session[3967]: gnome-session-check-accelerated: Helper exited with code 256
Aug 18 18:31:58 paul-desktop gnome-session[3967]: gnome-session[3967]: WARNING: software acceleration check failed: Child process exited with code 1
Aug 18 18:31:58 paul-desktop gnome-session[3967]: gnome-session[3967]: CRITICAL: We failed, but the fail whale is dead. Sorry....
Aug 18 18:31:58 paul-desktop gnome-session[3967]: WARNING: software acceleration check failed: Child process exited with code 1
Aug 18 18:31:58 paul-desktop gnome-session[3967]: CRITICAL: We failed, but the fail whale is dead. Sorry....