Dan, thanks for looking at the patch and for your question!
I tried to explain this a bit in comment #1, not too much detailed though:
"[...]closing session should be somehow added to gc [...] and then (the gc)
To include a session in gc, the function session_add_to_gc_queue() should
[...] (this) patch proactively puts a session in gc on session_remove_fifo().
This function is always called in the event of Release a session - when the ssh
ends, for example.
Being a bit more specific: when SSH ends, for example, a Release event is sent
through dbus and systemd-logind captures it, in the function
>From there, the function session_remove_fifo() is called. That point is our
to add the closing session on gc, our first addition there. But unfortunately
an unpredictable timing of cgroup become empty for that session, the gc might
to remove the session right in that moment. So, we need to re-add the closing
gc from...within the gc! In my experiments, 2 or 3 re-additions are enough to
get the session removed.
It wouldn't be necessary to do these 2 steps above IF the cgroup empty
was working fine. In that case, after the cgroup for the closing session become
a notification handler would be triggered in the systemd-logind and that
add the session to gc, and then gc would clean it up.
But, since in Trusty we have upstart as our init system (and systemd is highly
patched to co-exist in this fashion) and cgroup empty notifications were broken
in systemd (until a complete refactor of cgroup handling in systemd), our
approach here is the less expensive one.
In summary, the above 2 steps [adding the session to gc in
session_remove_fifo() and re-adding it from within gc] are the most
important parts of this patch - without them we leak entire sessions and
eventually the logind process gets OOM'ed.
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systemd-logind: memory leaks on session's connections (trusty-only)
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