[PATCH 4/8] mm,oom: Close oom_has_pending_mm race.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Previous patch ignored a situation where oom_has_pending_mm() returns
false due to all threads which mm->oom_mm.victim belongs to have reached
TASK_DEAD state, for there might be other thread groups sharing that mm.

This patch handles such situation by always updating mm->oom_mm.victim.
By applying this patch, the comm/pid pair printed at oom_kill_process()
and oom_reap_task() might differ. But that will not be a critical
problem.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 mm/oom_kill.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 07e8c1a..0b78133 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -688,6 +688,7 @@ subsys_initcall(oom_init)
 void mark_oom_victim(struct task_struct *tsk)
 {
 	struct mm_struct *mm = tsk->mm;
+	struct task_struct *old_tsk;
 
 	WARN_ON(oom_killer_disabled);
 	/* OOM killer might race with memcg OOM */
@@ -705,15 +706,26 @@ void mark_oom_victim(struct task_struct *tsk)
 	/*
 	 * Since mark_oom_victim() is called from multiple threads,
 	 * connect this mm to oom_mm_list only if not yet connected.
+	 *
+	 * But task_in_oom_domain(mm->oom_mm.victim, memcg, nodemask) in
+	 * oom_has_pending_mm() might return false after all threads in one
+	 * thread group (which mm->oom_mm.victim belongs to) reached TASK_DEAD
+	 * state. In that case, the same mm will be selected by another thread
+	 * group (which mm->oom_mm.victim does not belongs to). Therefore,
+	 * we need to replace the old task with the new task (at least when
+	 * task_in_oom_domain() returned false).
 	 */
-	if (!mm->oom_mm.victim) {
+	get_task_struct(tsk);
+	spin_lock(&oom_mm_lock);
+	old_tsk = mm->oom_mm.victim;
+	mm->oom_mm.victim = tsk;
+	if (!old_tsk) {
 		atomic_inc(&mm->mm_count);
-		get_task_struct(tsk);
-		mm->oom_mm.victim = tsk;
-		spin_lock(&oom_mm_lock);
 		list_add_tail(&mm->oom_mm.list, &oom_mm_list);
-		spin_unlock(&oom_mm_lock);
 	}
+	spin_unlock(&oom_mm_lock);
+	if (old_tsk)
+		put_task_struct(old_tsk);
 }
 
 /**
-- 
1.8.3.1

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