netdev - [PATCH net-next 3/6] net: avoid potential false sharing in neighbor related code

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Date:   Tue,  5 Nov 2019 14:11:51 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next 3/6] net: avoid potential false sharing in neighbor
 related code

There are common instances of the following construct :

	if (n->confirmed != now)
		n->confirmed = now;

A C compiler could legally remove the conditional.

Use READ_ONCE()/WRITE_ONCE() to avoid this problem.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/net/arp.h   |  4 ++--
 include/net/ndisc.h |  8 ++++----
 include/net/sock.h  | 12 ++++++------
 3 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/include/net/arp.h b/include/net/arp.h
index c8f580a0e6b1f5c0853cda1605336fa8eb90917c..4950191f6b2bf424519c7ecf3483336739fea143 100644
--- a/include/net/arp.h
+++ b/include/net/arp.h
@@ -57,8 +57,8 @@ static inline void __ipv4_confirm_neigh(struct net_device *dev, u32 key)
 		unsigned long now = jiffies;
 
 		/* avoid dirtying neighbour */
-		if (n->confirmed != now)
-			n->confirmed = now;
+		if (READ_ONCE(n->confirmed) != now)
+			WRITE_ONCE(n->confirmed, now);
 	}
 	rcu_read_unlock_bh();
 }
diff --git a/include/net/ndisc.h b/include/net/ndisc.h
index b2f715ca056726d75033083d1bccee693cea9672..b5ebeb3b0de0e9de3fa495148ab19830d9767e89 100644
--- a/include/net/ndisc.h
+++ b/include/net/ndisc.h
@@ -414,8 +414,8 @@ static inline void __ipv6_confirm_neigh(struct net_device *dev,
 		unsigned long now = jiffies;
 
 		/* avoid dirtying neighbour */
-		if (n->confirmed != now)
-			n->confirmed = now;
+		if (READ_ONCE(n->confirmed) != now)
+			WRITE_ONCE(n->confirmed, now);
 	}
 	rcu_read_unlock_bh();
 }
@@ -431,8 +431,8 @@ static inline void __ipv6_confirm_neigh_stub(struct net_device *dev,
 		unsigned long now = jiffies;
 
 		/* avoid dirtying neighbour */
-		if (n->confirmed != now)
-			n->confirmed = now;
+		if (READ_ONCE(n->confirmed) != now)
+			WRITE_ONCE(n->confirmed, now);
 	}
 	rcu_read_unlock_bh();
 }
diff --git a/include/net/sock.h b/include/net/sock.h
index ac6042d0af328a7f6e18f8882d2ee0679a0e59c5..f2f853439b6576925e39f6db010964762e39ccf2 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1939,8 +1939,8 @@ struct dst_entry *sk_dst_check(struct sock *sk, u32 cookie);
 
 static inline void sk_dst_confirm(struct sock *sk)
 {
-	if (!sk->sk_dst_pending_confirm)
-		sk->sk_dst_pending_confirm = 1;
+	if (!READ_ONCE(sk->sk_dst_pending_confirm))
+		WRITE_ONCE(sk->sk_dst_pending_confirm, 1);
 }
 
 static inline void sock_confirm_neigh(struct sk_buff *skb, struct neighbour *n)
@@ -1950,10 +1950,10 @@ static inline void sock_confirm_neigh(struct sk_buff *skb, struct neighbour *n)
 		unsigned long now = jiffies;
 
 		/* avoid dirtying neighbour */
-		if (n->confirmed != now)
-			n->confirmed = now;
-		if (sk && sk->sk_dst_pending_confirm)
-			sk->sk_dst_pending_confirm = 0;
+		if (READ_ONCE(n->confirmed) != now)
+			WRITE_ONCE(n->confirmed, now);
+		if (sk && READ_ONCE(sk->sk_dst_pending_confirm))
+			WRITE_ONCE(sk->sk_dst_pending_confirm, 0);
 	}
 }
 
-- 
2.24.0.rc1.363.gb1bccd3e3d-goog