net: sk_dst_cache RCUification

With latest CONFIG_PROVE_RCU stuff, I felt more comfortable to make this
work.

sk->sk_dst_cache is currently protected by a rwlock (sk_dst_lock)

This rwlock is readlocked for a very small amount of time, and dst
entries are already freed after RCU grace period. This calls for RCU
again :)

This patch converts sk_dst_lock to a spinlock, and use RCU for readers.

__sk_dst_get() is supposed to be called with rcu_read_lock() or if
socket locked by user, so use appropriate rcu_dereference_check()
condition (rcu_read_lock_held() || sock_owned_by_user(sk))

This patch avoids two atomic ops per tx packet on UDP connected sockets,
for example, and permits sk_dst_lock to be much less dirtied.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 32 insertions, 34 deletions

diff --git a/include/net/dst.h b/include/net/dst.h
index ce078cda..aac5a5f 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -225,21 +225,6 @@ static inline void dst_confirm(struct dst_entry *dst)
 		neigh_confirm(dst->neighbour);
 }
 
-static inline void dst_negative_advice(struct dst_entry **dst_p,
-				       struct sock *sk)
-{
-	struct dst_entry * dst = *dst_p;
-	if (dst && dst->ops->negative_advice) {
-		*dst_p = dst->ops->negative_advice(dst);
-
-		if (dst != *dst_p) {
-			extern void sk_reset_txq(struct sock *sk);
-
-			sk_reset_txq(sk);
-		}
-	}
-}
-
 static inline void dst_link_failure(struct sk_buff *skb)
 {
 	struct dst_entry *dst = skb_dst(skb);
diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 68f67836..278312c 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -152,9 +152,9 @@ static inline void __ip6_dst_store(struct sock *sk, struct dst_entry *dst,
 static inline void ip6_dst_store(struct sock *sk, struct dst_entry *dst,
 				 struct in6_addr *daddr, struct in6_addr *saddr)
 {
-	write_lock(&sk->sk_dst_lock);
+	spin_lock(&sk->sk_dst_lock);
 	__ip6_dst_store(sk, dst, daddr, saddr);
-	write_unlock(&sk->sk_dst_lock);
+	spin_unlock(&sk->sk_dst_lock);
 }
 
 static inline int ipv6_unicast_destination(struct sk_buff *skb)
diff --git a/include/net/sock.h b/include/net/sock.h
index b4603cd..56df440 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -262,7 +262,7 @@ struct sock {
 #ifdef CONFIG_XFRM
 	struct xfrm_policy	*sk_policy[2];
 #endif
-	rwlock_t		sk_dst_lock;
+	spinlock_t		sk_dst_lock;
 	atomic_t		sk_rmem_alloc;
 	atomic_t		sk_wmem_alloc;
 	atomic_t		sk_omem_alloc;
@@ -1192,7 +1192,8 @@ extern unsigned long sock_i_ino(struct sock *sk);
 static inline struct dst_entry *
 __sk_dst_get(struct sock *sk)
 {
-	return sk->sk_dst_cache;
+	return rcu_dereference_check(sk->sk_dst_cache, rcu_read_lock_held() ||
+						       sock_owned_by_user(sk));
 }
 
 static inline struct dst_entry *
@@ -1200,50 +1201,62 @@ sk_dst_get(struct sock *sk)
 {
 	struct dst_entry *dst;
 
-	read_lock(&sk->sk_dst_lock);
-	dst = sk->sk_dst_cache;
+	rcu_read_lock();
+	dst = rcu_dereference(sk->sk_dst_cache);
 	if (dst)
 		dst_hold(dst);
-	read_unlock(&sk->sk_dst_lock);
+	rcu_read_unlock();
 	return dst;
 }
 
+extern void sk_reset_txq(struct sock *sk);
+
+static inline void dst_negative_advice(struct sock *sk)
+{
+	struct dst_entry *ndst, *dst = __sk_dst_get(sk);
+
+	if (dst && dst->ops->negative_advice) {
+		ndst = dst->ops->negative_advice(dst);
+
+		if (ndst != dst) {
+			rcu_assign_pointer(sk->sk_dst_cache, ndst);
+			sk_reset_txq(sk);
+		}
+	}
+}
+
 static inline void
 __sk_dst_set(struct sock *sk, struct dst_entry *dst)
 {
 	struct dst_entry *old_dst;
 
 	sk_tx_queue_clear(sk);
-	old_dst = sk->sk_dst_cache;
-	sk->sk_dst_cache = dst;
+	old_dst = rcu_dereference_check(sk->sk_dst_cache,
+					lockdep_is_held(&sk->sk_dst_lock));
+	rcu_assign_pointer(sk->sk_dst_cache, dst);
 	dst_release(old_dst);
 }
 
 static inline void
 sk_dst_set(struct sock *sk, struct dst_entry *dst)
 {
-	write_lock(&sk->sk_dst_lock);
+	spin_lock(&sk->sk_dst_lock);
 	__sk_dst_set(sk, dst);
-	write_unlock(&sk->sk_dst_lock);
+	spin_unlock(&sk->sk_dst_lock);
 }
 
 static inline void
 __sk_dst_reset(struct sock *sk)
 {
-	struct dst_entry *old_dst;
-
-	sk_tx_queue_clear(sk);
-	old_dst = sk->sk_dst_cache;
-	sk->sk_dst_cache = NULL;
-	dst_release(old_dst);
+	__sk_dst_set(sk, NULL);
 }
 
 static inline void
 sk_dst_reset(struct sock *sk)
 {
-	write_lock(&sk->sk_dst_lock);
+	spin_lock(&sk->sk_dst_lock);
 	__sk_dst_reset(sk);
-	write_unlock(&sk->sk_dst_lock);
+	spin_unlock(&sk->sk_dst_lock);
 }
 
 extern struct dst_entry *__sk_dst_check(struct sock *sk, u32 cookie);