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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 1ae08b8..90b2a0b 100644 (file)
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -140,7 +140,8 @@ static inline bool in_vfork(struct task_struct *tsk)
         * 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;