commit 25c7a6d1f90e208ec27ca854b1381ed39842ec57
author Eric Dumazet <edumazet@google.com> Tue Nov 05 14:11:51 2019 -0800
committer David S. Miller <davem@davemloft.net> Wed Nov 06 16:14:48 2019 -0800
tree a645c406ef309589a1c6f851ca1ba761575a00b4
parent 3828a93f5cfdf5d8a4ff9dead741e9a2871ff57b

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@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/arp.h

diff --git a/include/net/arp.h b/include/net/arp.h
index c8f580a..4950191 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();
 }

include/net/ndisc.h

diff --git a/include/net/ndisc.h b/include/net/ndisc.h
index b2f715c..b5ebeb3 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();
 }

include/net/sock.h

diff --git a/include/net/sock.h b/include/net/sock.h
index ac6042d..f2f8534 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);
 	}
 }