memcg: killed threads should not invoke memcg OOM killer

If a memory cgroup contains a single process with many threads
(including different process group sharing the mm) then it is possible
to trigger a race when the oom killer complains that there are no oom
elible tasks and complain into the log which is both annoying and
confusing because there is no actual problem.  The race looks as
follows:

P1				oom_reaper		P2
try_charge						try_charge
  mem_cgroup_out_of_memory
    mutex_lock(oom_lock)
      out_of_memory
        oom_kill_process(P1,P2)
         wake_oom_reaper
    mutex_unlock(oom_lock)
    				oom_reap_task
							  mutex_lock(oom_lock)
							    select_bad_process # no victim

The problem is more visible with many threads.

Fix this by checking for fatal_signal_pending from
mem_cgroup_out_of_memory when the oom_lock is already held.

The oom bypass is safe because we do the same early in the try_charge
path already.  The situation migh have changed in the mean time.  It
should be safe to check for fatal_signal_pending and tsk_is_oom_victim
but for a better code readability abstract the current charge bypass
condition into should_force_charge and reuse it from that path.  "

Link: http://lkml.kernel.org/r/01370f70-e1f6-ebe4-b95e-0df21a0bc15e@i-love.sakura.ne.jp


Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>