[patch 18/29] include/linux/sched/mm.h: use rcu_dereference in in_vfork()

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Subject: include/linux/sched/mm.h: use rcu_dereference in in_vfork()

Fix a sparse warning by using rcu_dereference().  Technically this is a
bug and a sufficiently aggressive compiler could reload the `real_parent'
pointer outside the protection of the rcu lock (and access freed memory),
but I think it's pretty unlikely to happen.

Link: https://lkml.kernel.org/r/20210221194207.1351703-1-willy@infradead.org
Fixes: b18dc5f291c0 ("mm, oom: skip vforked tasks from being selected")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/sched/mm.h |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/include/linux/sched/mm.h~mm-use-rcu_dereference-in-in_vfork
+++ a/include/linux/sched/mm.h

@@ -140,7 +140,8 @@ static inline bool in_vfork(struct task_
 	 * another oom-unkillable task does this it should blame itself.
 	 */
 	rcu_read_lock();
-	ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm;
+	ret = tsk->vfork_done &&
+			rcu_dereference(tsk->real_parent)->mm == tsk->mm;
 	rcu_read_unlock();
 
 	return ret;

_