On Wed 01-06-16 15:53:13, Andrew Morton wrote:

On Sat, 28 May 2016 17:16:05 +0900 Tetsuo Handa [off-list ref] wrote:

quoted

Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task")
reduced frequency of needlessly selecting next OOM victim, but was
calling mmput_async() when atomic_inc_not_zero() failed.

Changelog fail.

quoted

--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c

@@ -478,6 +478,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	mm = p->mm;
 	if (!atomic_inc_not_zero(&mm->mm_users)) {
 		task_unlock(p);
+		mm = NULL;
 		goto unlock_oom;
 	}

This looks like a pretty fatal bug.  I assume the result of hitting
that race will be a kernel crash, yes?

Yes it is a nasty bug. It was (re)introduced by the final touch to the
goto paths. And yes it can cause a crash.

Is it even possible to hit that race? 

It is, we can have a concurrent mmput followed by mmdrop.

find_lock_task_mm() takes some
care to prevent a NULL ->mm.  But I guess a concurrent mmput() doesn't
require task_lock().  Kinda makes me wonder what's the point in even
having find_lock_task_mm() if its guarantee on ->mm is useless...

find_lock_task_mm makes sure that the mm stays non-NULL while we hold
the lock. We have to do all the necessary pinning while holding it.
atomic_inc_not_zero will guarantee we are not racing with the finall
mmput.

Does that make more sense now?
-- 
Michal Hocko
SUSE Labs

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