I am having the same problem. My dmesg does not show the trap reported by the OP. But the symptoms are exactly the same. I have subscribed to this bug report. I am using systemd and xfce. Here are some files whose content may be relevant:
/usr/share/lightdm/lightdm.conf.d/01_debian.conf: ----- # Debian specific defaults # # - use lightdm-greeter session greeter, points to the etc-alternatives managed # greeter # - hide users list by default, we don't want to expose them # - use Debian specific session wrapper, to gain support for # /etc/X11/Xsession.d scripts [Seat:*] greeter-session=lightdm-greeter greeter-hide-users=true session-wrapper=/etc/X11/Xsession ----- /etc/alternatives/lightdm-greeter (a symbolic link to /usr/share/xgreeters/lightdm-gtk-greeter.desktop): ----- [Desktop Entry] Name=LightDM GTK+ Greeter Comment=This runs the GTK+ greeter, it should only be run from LightDM Exec=lightdm-gtk-greeter Type=Application X-Ubuntu-Gettext-Domain=lightdm ----- Note that I am running pure Debian, not Ubuntu. I wonder if X-Ubuntu-Gettext-Domain should be in the above file? /etc/lightdm/users.conf: ----- # # User accounts configuration # # NOTE: If you have AccountsService installed on your system, then LightDM will # use this instead and these settings will be ignored # # minimum-uid = Minimum UID required to be shown in greeter # hidden-users = Users that are not shown to the user # hidden-shells = Shells that indicate a user cannot login # [UserList] minimum-uid=500 hidden-users=nobody nobody4 noaccess hidden-shells=/bin/false /usr/sbin/nologin ----- Note that the lightdm user on my system has a uid of 113. /var/log/lightdm/lightdm.log: ----- [+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log [+0.00s] DEBUG: Starting Light Display Manager 1.18.2, UID=0 PID=3285 [+0.00s] DEBUG: Loading configuration dirs from /usr/share/lightdm/lightdm.conf.d [+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/01_debian.conf [+0.00s] DEBUG: Loading configuration dirs from /usr/local/share/lightdm/lightdm.conf.d [+0.00s] DEBUG: Loading configuration dirs from /etc/xdg/lightdm/lightdm.conf.d [+0.00s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf [+0.00s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager [+0.00s] DEBUG: Registered seat module xlocal [+0.00s] DEBUG: Registered seat module xremote [+0.00s] DEBUG: Registered seat module unity [+0.02s] DEBUG: Monitoring logind for seats [+0.02s] DEBUG: New seat added from logind: seat0 [+0.02s] DEBUG: Seat seat0: Loading properties from config section Seat:* [+0.02s] DEBUG: Seat seat0: Starting [+0.02s] DEBUG: Seat seat0: Creating greeter session [+0.02s] DEBUG: Seat seat0: Creating display server of type x [+0.02s] DEBUG: Could not run plymouth --ping: Failed to execute child process "plymouth" (No such file or directory) [+0.02s] DEBUG: Using VT 7 [+0.02s] DEBUG: Seat seat0: Starting local X display on VT 7 [+0.02s] DEBUG: DisplayServer x-0: Logging to /var/log/lightdm/x-0.log [+0.02s] DEBUG: DisplayServer x-0: Writing X server authority to /var/run/lightdm/root/:0 [+0.02s] DEBUG: DisplayServer x-0: Launching X Server [+0.02s] DEBUG: Launching process 3292: /usr/bin/X :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch [+0.02s] DEBUG: DisplayServer x-0: Waiting for ready signal from X server :0 [+0.02s] DEBUG: Acquired bus name org.freedesktop.DisplayManager [+0.02s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0 [+0.04s] DEBUG: Loading users from org.freedesktop.Accounts [+0.04s] DEBUG: User /org/freedesktop/Accounts/User1001 added [+0.04s] DEBUG: User /org/freedesktop/Accounts/User1000 added [+0.10s] DEBUG: Seat seat0 changes active session to [+0.10s] CRITICAL: session_get_login1_session_id: assertion 'session != NULL' failed [+0.64s] DEBUG: Got signal 10 from process 3292 [+0.64s] DEBUG: DisplayServer x-0: Got signal from X server :0 [+0.64s] DEBUG: DisplayServer x-0: Connecting to XServer :0 [+0.64s] DEBUG: Seat seat0: Display server ready, starting session authentication [+0.65s] DEBUG: Session pid=3311: Started with service 'lightdm-greeter', username 'lightdm' [+0.68s] DEBUG: Session pid=3311: Authentication complete with return value 0: Success [+0.68s] DEBUG: Seat seat0: Session authenticated, running command [+0.68s] DEBUG: Session pid=3311: Running command /usr/sbin/lightdm-gtk-greeter [+0.68s] DEBUG: Creating shared data directory /var/lib/lightdm/data/lightdm [+0.68s] DEBUG: Session pid=3311: Logging to /var/log/lightdm/seat0-greeter.log [+0.82s] DEBUG: Activating VT 7 [+0.82s] DEBUG: Activating login1 session c2 [+1.34s] DEBUG: Greeter connected version=1.18.2 resettable=false [+2.58s] DEBUG: Greeter start authentication [+2.58s] DEBUG: Session pid=3336: Started with service 'lightdm', username '(null)' [+2.60s] DEBUG: Session pid=3336: Got 1 message(s) from PAM [+2.60s] DEBUG: Prompt greeter with 1 message(s) [+11.76s] DEBUG: Seat seat0 changes active session to 4 [+21.93s] DEBUG: Got signal 15 from process 1 [+21.93s] DEBUG: Caught Terminated signal, shutting down [+21.93s] DEBUG: Stopping display manager [+21.93s] DEBUG: Seat seat0: Stopping [+21.93s] DEBUG: Seat seat0: Stopping display server [+21.93s] DEBUG: Sending signal 15 to process 3292 [+21.93s] DEBUG: Seat seat0: Stopping session [+21.93s] DEBUG: Session pid=3311: Sending SIGTERM [+21.93s] DEBUG: Seat seat0: Stopping session [+21.93s] DEBUG: Session pid=3336: Sending SIGTERM [+21.94s] DEBUG: Session pid=3336: Terminated with signal 15 [+21.94s] DEBUG: Session: Failed during authentication [+21.94s] DEBUG: Seat seat0: Session stopped [+21.96s] DEBUG: Process 3292 exited with return value 0 [+21.96s] DEBUG: DisplayServer x-0: X server stopped [+21.96s] DEBUG: Releasing VT 7 [+21.96s] DEBUG: DisplayServer x-0: Removing X server authority /var/run/lightdm/root/:0 [+21.96s] DEBUG: Seat seat0: Display server stopped [+22.01s] DEBUG: Greeter closed communication channel [+22.01s] DEBUG: Session pid=3311: Exited with return value 0 [+22.01s] DEBUG: Seat seat0: Session stopped [+22.01s] DEBUG: Seat seat0: Stopped [+22.01s] DEBUG: Display manager stopped [+22.01s] DEBUG: Stopping daemon [+22.03s] DEBUG: Exiting with return value 0 ----- /var/log/lightdm/seat0-greeter.log: ----- ** Message: Starting lightdm-gtk-greeter 2.0.1 (Jun 21 2015, 09:50:04) ** Message: [Configuration] Reading file: /usr/share/lightdm/lightdm-gtk-greeter.conf.d/01_debian.conf ** Message: [Configuration] Reading file: /etc/lightdm/lightdm-gtk-greeter.conf ** (lightdm-gtk-greeter:3317): WARNING **: [PIDs] Failed to execute command: upstart ** (process:3322): WARNING **: Failed to register client: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files (lightdm-gtk-greeter:3317): GdkPixbuf-CRITICAL **: gdk_pixbuf_new: assertion 'width > 0' failed (lightdm-gtk-greeter:3317): GdkPixbuf-CRITICAL **: gdk_pixbuf_composite: assertion 'GDK_IS_PIXBUF (dest)' failed (lightdm-gtk-greeter:3317): GLib-GObject-CRITICAL **: g_object_ref: assertion 'G_IS_OBJECT (object)' failed ** (lightdm-gtk-greeter:3317): WARNING **: [Background] Failed to read wallpaper: /usr/share/images/desktop-base/login-background.svg (lightdm-gtk-greeter:3317): GLib-GObject-CRITICAL **: g_object_unref: assertion 'G_IS_OBJECT (object)' failed (lightdm-gtk-greeter:3317): GdkPixbuf-CRITICAL **: gdk_pixbuf_new: assertion 'width > 0' failed (lightdm-gtk-greeter:3317): GdkPixbuf-CRITICAL **: gdk_pixbuf_composite: assertion 'GDK_IS_PIXBUF (dest)' failed (lightdm-gtk-greeter:3317): GLib-GObject-CRITICAL **: g_object_ref: assertion 'G_IS_OBJECT (object)' failed ** (lightdm-gtk-greeter:3317): WARNING **: [Background] Failed to read wallpaper: /usr/share/images/desktop-base/login-background.svg (lightdm-gtk-greeter:3317): GLib-GObject-CRITICAL **: g_object_unref: assertion 'G_IS_OBJECT (object)' failed (lightdm-gtk-greeter:3317): GdkPixbuf-CRITICAL **: gdk_pixbuf_new: assertion 'width > 0' failed (lightdm-gtk-greeter:3317): GdkPixbuf-CRITICAL **: gdk_pixbuf_composite: assertion 'GDK_IS_PIXBUF (dest)' failed (lightdm-gtk-greeter:3317): GLib-GObject-CRITICAL **: g_object_ref: assertion 'G_IS_OBJECT (object)' failed ** (lightdm-gtk-greeter:3317): WARNING **: [Background] Failed to read wallpaper: /usr/share/images/desktop-base/login-background.svg (lightdm-gtk-greeter:3317): GLib-GObject-CRITICAL **: g_object_unref: assertion 'G_IS_OBJECT (object)' failed (lightdm-gtk-greeter:3317): Gtk-WARNING **: Drawing a gadget with negative dimensions. Did you forget to allocate a size? (node menubar owner GreeterMenuBar) (lightdm-gtk-greeter:3317): Gtk-WARNING **: Drawing a gadget with negative dimensions. Did you forget to allocate a size? (node menubar owner GreeterMenuBar) (lightdm-gtk-greeter:3317): Gdk-WARNING **: lightdm-gtk-greeter: Fatal IO error 11 (Resource temporarily unavailable) on X server :0. ----- /var/log/lightdm/x-0-greeter.log: ----- ** Message: Starting lightdm-gtk-greeter 2.0.1 (Jun 21 2015, 09:50:04) ** Message: [Configuration] Reading file: /usr/share/lightdm/lightdm-gtk-greeter.conf.d/01_debian.conf ** Message: [Configuration] Reading file: /etc/lightdm/lightdm-gtk-greeter.conf ** (lightdm-gtk-greeter:18795): WARNING **: [PIDs] Failed to execute command: upstart ** (process:18800): WARNING **: Failed to register client: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files (lightdm-gtk-greeter:18795): Gtk-WARNING **: Drawing a gadget with negative dimensions. Did you forget to allocate a size? (node menubar owner GreeterMenuBar) (lightdm-gtk-greeter:18795): Gdk-WARNING **: lightdm-gtk-greeter: Fatal IO error 11 (Resource temporarily unavailable) on X server :0. ----- x-0.log shows no errors. Is there any other file you wish to see? -- .''`. Stephen Powell <zlinux...@fastmail.com> : :' : `. `'` `-
