I have a fresh install of 11.04 64bit, with 2 administrator accounts and 4 normal accounts. The 4 normal accounts (the kids' accounts) don't have passwords, they can login simply by clicking on their names.
When any of the users -- either admin or normal -- tries to switch to another account by clicking in the top-right corner of the screen and selecting another user, the screen goes black and the entire system locks up. Even CTRL+ALT+F1 through F7 does nothing. This is reproducible 100% of the time on this system.
I can ssh into the box when the console locks up, and by running top, I see that Xorg is consuming about 100% of the CPU. Looking at the output of "ps axfu" in bash while the system is in this "locked up" state, here is the lightdm and X process tree:
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1153 0.0 0.1 183508 4292 ? Ssl Dec26 0:00 lightdm
root 2187 0.4 4.6 265976 164168 tty7 Ss+ 00:43 0:21 \_ /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
stephane 2612 0.0 0.3 266400 10736 ? Ssl 01:52 0:00 \_ /usr/bin/gnome-session --session=ubuntu
stephane 2650 0.0 0.0 12264 276 ? Ss 01:52 0:00 | \_ /usr/bin/ssh-agent /usr/bin/dbus-launch --exit-with-session /usr/bin/gnome-session --session=ubuntu
stephane 2703 0.8 3.0 562068 106548 ? Sl 01:52 0:08 | \_ compiz
stephane 2801 0.0 0.0 4264 584 ? Ss 01:52 0:00 | | \_ /bin/sh -c /usr/bin/compiz-decorator
stephane 2802 0.0 0.3 265744 13772 ? Sl 01:52 0:00 | | \_ /usr/bin/unity-window-decorator
...cut...
root 3024 80.6 0.3 107928 13088 tty8 Rs+ 01:53 12:34 \_ /usr/bin/X :1 -auth /var/run/lightdm/root/:1 -nolisten tcp vt8 -novtswitch
That last process, pid #3024 in this case, is what has the CPU pegged.
In case it matters (I suspect it might) here is what I think may be the relevant information for my video card, taken from /var/log/Xorg.0.log:
[ 3392.653] (II) Loading /usr/lib/x86_64-linux-gnu/xorg/extra-modules/extra-modules.dpkg-tmp/modules/extensions/libglx.so
[ 3392.653] (II) Module glx: vendor="FireGL - AMD Technologies Inc."
[ 3392.653] compiled for 6.9.0, module version = 1.0.0
...
[ 3392.655] (II) LoadModule: "fglrx"
[ 3392.655] (II) Loading /usr/lib/x86_64-linux-gnu/xorg/extra-modules/extra-modules.dpkg-tmp/modules/drivers/fglrx_drv.so
[ 3392.672] (II) Module fglrx: vendor="FireGL - ATI Technologies Inc."
[ 3392.672] compiled for 1.4.99.906, module version = 8.88.7
[ 3392.672] Module class: X.Org Video Driver
...
[ 3392.759] (==) fglrx(0): ATI 2D Acceleration Architecture enabled
[ 3392.759] (--) fglrx(0): Chipset: "AMD Radeon HD 6410D" (Chipset = 0x9644)
Lastly: I did see this posting: Change user on 11.10 hangs system
...but I checked, and the libpam-smbpass package isn't installed on this system.
