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Subject: [PATCH 2/4] exit: Use rcu instead of get_task_struct() in
 mm_update_next_owner()
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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        ktkhai@virtuozzo.com
Date:   Thu, 26 Apr 2018 14:00:49 +0300
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Since release_task() puts final task_struct::usage counter
at least one rcu grace period after removing from task list:

	__exit_signal()
	  __unhash_process()
	call_rcu(&p->rcu, delayed_put_task_struct)

rcu_read_lock() guarantees nobody release task_struct memory.
So, it's possible to use this primitive instead of get_task_struct().

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 kernel/exit.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 9fb7b699bdeb..5c42a9b9c1d7 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -464,7 +464,7 @@ void mm_update_next_owner(struct mm_struct *mm)
 	return;
 
 assign_new_owner:
-	get_task_struct(c);
+	rcu_read_lock();
 	read_unlock(&tasklist_lock);
 	BUG_ON(c == p);
 
@@ -475,12 +475,12 @@ void mm_update_next_owner(struct mm_struct *mm)
 	task_lock(c);
 	if (c->mm != mm) {
 		task_unlock(c);
-		put_task_struct(c);
+		rcu_read_unlock();
 		goto retry;
 	}
 	mm->owner = c;
 	task_unlock(c);
-	put_task_struct(c);
+	rcu_read_unlock();
 }
 #endif /* CONFIG_MEMCG */