[PATCH] oom, suspend: fix oom_reaper vs. oom_killer_disable race

[Michal Hocko]

From: Michal Hocko <mhocko@xxxxxxxx>

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")
Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---

Hi,
this is a temporal fix that is easy enough for the current 4.7 cycle.
Even though I suspect PM suspend race with the OOM killer is super
unlikely it is better to not risk it considering the patch is quite
trivial. I plan to come up with a proper solution later on after some
other OOM killer/reaper changes settle down.  It is really desirable
that oom_killer_disable() is a full "barrier" rather than working around
potential issues.

So if this looks like a proper fix to you Rafael I would route this via Andrew.
What do you think?

 kernel/power/process.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/power/process.c b/kernel/power/process.c
index df058bed53ce..0c2ee9761d57 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -146,6 +146,18 @@ int freeze_processes(void)
 	if (!error && !oom_killer_disable())
 		error = -EBUSY;
 
+	/*
+	 * There is a hard to fix race between oom_reaper kernel thread
+	 * and oom_killer_disable. oom_reaper calls exit_oom_victim
+	 * before the victim reaches exit_mm so try to freeze all the tasks
+	 * again and catch such a left over task.
+	 */
+	if (!error) {
+		pr_info("Double checking all user space processes after OOM killer disable... ");
+		error = try_to_freeze_tasks(true);
+		pr_cont("\n");
+	}
+
 	if (error)
 		thaw_processes();
 	return error;
-- 
2.8.1