Re: [PATCH] cgroup: Don't expose dead tasks in cgroup

[Tejun Heo <tj@kernel.org>]

So, I think we can fix this in the iterator without moving the unlink.
css_task_iter_advance() already skips dying leaders w/ no live threads
but only on the dying_tasks list, which gets populated too late. We can
extend it to catch PF_EXITING tasks on the regular tasks list too:

  if ((task->flags & PF_EXITING) &&
      !atomic_read(&task->signal->live))
          goto repeat;

PF_EXITING is set in exit_signals() which is before exit_notify(), so
by the time the parent wakes up, the flag is already set. The
signal->live check keeps zombie leaders with live threads visible. I
can't see anything that would break by this being checked earlier than
cgroup_task_exit() - everything between PF_EXITING and
cgroup_task_exit() is just teardown (mm, files, etc.) and it should
close the race window.

Haven't tested this yet. What do you think?

Thanks.

--
tejun