linux-kernel - [PATCH rcu 6/6] rcu: Use WRITE_ONCE() for assignments to ->next for rculist

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Message-Id: <20230717180317.1097590-6-paulmck@kernel.org>
Date:   Mon, 17 Jul 2023 11:03:17 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        rostedt@...dmis.org, Alan Huang <mmpgouride@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 6/6] rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls

From: Alan Huang <mmpgouride@...il.com>

When the objects managed by rculist_nulls are allocated with
SLAB_TYPESAFE_BY_RCU, old readers may still hold references to an object
even though it is just now being added, which means the modification of
->next is visible to readers.  This patch therefore uses WRITE_ONCE()
for assignments to ->next.

Signed-off-by: Alan Huang <mmpgouride@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 include/linux/rculist_nulls.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index ba4c00dd8005..89186c499dd4 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -101,7 +101,7 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
 {
 	struct hlist_nulls_node *first = h->first;
 
-	n->next = first;
+	WRITE_ONCE(n->next, first);
 	WRITE_ONCE(n->pprev, &h->first);
 	rcu_assign_pointer(hlist_nulls_first_rcu(h), n);
 	if (!is_a_nulls(first))
@@ -137,7 +137,7 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
 		last = i;
 
 	if (last) {
-		n->next = last->next;
+		WRITE_ONCE(n->next, last->next);
 		n->pprev = &last->next;
 		rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
 	} else {
-- 
2.40.1