Bug#803013: systemd should not destroy application created cgroups

[paul.szabo at sydney.edu.au]

Dear Michael,

> I'm not able to reproduce the issue either.
> ... this looks like something specific to your system configuration, a
> default jessie system doesn't seem to be affected.

I found what causes reproducibility: editing the file
  /etc/pam.d/common-session
and removing or commenting out the optional(?!!) line
  session       optional        pam_systemd.so
makes the issue reproducible. - Curiously, removing that line also
causes GDM3 to become unusable (and I was thinking it was just too buggy
to be used... which it is, really).

I wonder how that line came to be missed on my machines: I upgraded from
wheezy (which was upgraded from previous releases).

So, to reproduce the issue, on a "plain jessie" machine (e.g. freshly
installed from debian-8.2.0-amd64-netinst.iso):
 - Edit the file
     /etc/pam.d/common-session
   and remove or comment out the line
     session optional pam_systemd.so
   then reboot.
 - Log in to the machine; probably not via GDM3 as that might not work
   at all; not via getty as then the issue will not show(?!!); but log
   in via XDM, or via telnetd or sshd. (Wonder if sshd login with keys
   would bypass PAM and then not need the editing above.)
 - Become root (log in as such, or use su).
 - As root, do commands:
     # Set things up
     mkdir /sys/fs/cgroup/cpu/mytest
     echo $$ > /sys/fs/cgroup/cpu/mytest/tasks
     # Check it is there
     grep . /sys/fs/cgroup/cpu/mytest/tasks
     # Do the systemd thing
     systemctl daemon-reload
     systemctl start anacron
     # See it gone
     grep . /sys/fs/cgroup/cpu/mytest/tasks

Cheers, Paul

Paul Szabo   psz at maths.usyd.edu.au   http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics   University of Sydney    Australia