Comment 21 for bug 250696

I think the problem still exists with Xsession when use gdm to login, at least in 12.04.5 LTS:
(Yeah I know the release is not officially supported any more, just in case anybody is interested to trace back)
(For the moment I leave the comment here, before somebody is sure this is related to some other issue)
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Part of the command ps axjf (The full result of this command is in the attachment):
 1543 2227 1539 1539 ? -1 Sl 0 0:00 \_ /usr/lib/gdm/gdm-session-worker
 2227 2502 2502 2502 ? -1 Ssl 1001 0:00 \_ gnome-session --session=ubuntu
 2502 2547 2502 2502 ? -1 Z 1001 0:00 \_ [Xsession] <defunct>

The zombie Xsession process, listed with the ps -elf command:
$ ps -elf | grep defunct
1 Z theloginuser 2547 2502 0 80 0 - 0 exit 06:06 ? 00:00:00 [Xsession] <defunct>

The gnome-session parent process of the zombie, listed with the ps -elf command:
4 S theloginuser 2502 2227 0 80 0 - 98064 poll_s 06:06 ? 00:00:00 gnome-session --session=ubuntu
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Version infomation:

$ apt-cache policy gnome-session
gnome-session:
  Installed: 3.2.1-0ubuntu8
  Candidate: 3.2.1-0ubuntu8
  Version table:
 *** 3.2.1-0ubuntu8 0
        500 http://mirror.aarnet.edu.au/pub/ubuntu/archive/ precise/main amd64 Packages
        100 /var/lib/dpkg/status

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 12.04.5 LTS
Release: 12.04
Codename: precise
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There are reports on the Internet that this phenomenon may also occur in some old version of Fedora.