Tetsuo has reported the following potential oom_killer_disable vs.
oom_reaper race:

 (1) freeze_processes() starts freezing user space threads.
 (2) Somebody (maybe a kenrel thread) calls out_of_memory().
 (3) The OOM killer calls mark_oom_victim() on a user space thread
     P1 which is already in __refrigerator().
 (4) oom_killer_disable() sets oom_killer_disabled = true.
 (5) P1 leaves __refrigerator() and enters do_exit().
 (6) The OOM reaper calls exit_oom_victim(P1) before P1 can call
     exit_oom_victim(P1).
 (7) oom_killer_disable() returns while P1 not yet finished
 (8) P1 perform IO/interfere with the freezer.

This situation is unfortunate.  We cannot move oom_killer_disable after
all the freezable kernel threads are frozen because the oom victim might
depend on some of those kthreads to make a forward progress to exit so
we could deadlock.  It is also far from trivial to teach the oom_reaper
to not call exit_oom_victim() because then we would lose a guarantee of
the OOM killer and oom_killer_disable forward progress because
exit_mm->mmput might block and never call exit_oom_victim.

It seems the easiest way forward is to workaround this race by calling
try_to_freeze_tasks again after oom_killer_disable.  This will make sure
that all the tasks are frozen or it bails out.

Fixes: 449d777d7ad6 ("mm, oom_reaper: clear TIF_MEMDIE for all tasks queued for oom_reaper")
Link: http://lkml.kernel.org/r/1466597634-16199-1-git-send-email-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>