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>

8 files changed, 28 insertions, 27 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 0eb79e35671f..ca4cdef74a1b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2015,7 +2015,7 @@ static struct netdev_queue *dev_pick_tx(struct net_device *dev,
                         if (dev->real_num_tx_queues > 1)
                                 queue_index = skb_tx_hash(dev, skb);
 
-                        if (sk && sk->sk_dst_cache)
+                        if (sk && rcu_dereference_check(sk->sk_dst_cache, 1))
                                 sk_tx_queue_set(sk, queue_index);
                 }
         }
diff --git a/net/core/sock.c b/net/core/sock.c
index c5812bbc2cc9..7effa1e689df 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -364,11 +364,11 @@ EXPORT_SYMBOL(sk_reset_txq);
 
 struct dst_entry *__sk_dst_check(struct sock *sk, u32 cookie)
 {
-        struct dst_entry *dst = sk->sk_dst_cache;
+        struct dst_entry *dst = __sk_dst_get(sk);
 
         if (dst && dst->obsolete && dst->ops->check(dst, cookie) == NULL) {
                 sk_tx_queue_clear(sk);
-                sk->sk_dst_cache = NULL;
+                rcu_assign_pointer(sk->sk_dst_cache, NULL);
                 dst_release(dst);
                 return NULL;
         }
@@ -1157,7 +1157,7 @@ struct sock *sk_clone(const struct sock *sk, const gfp_t priority)
                 skb_queue_head_init(&newsk->sk_async_wait_queue);
 #endif
 
-                rwlock_init(&newsk->sk_dst_lock);
+                spin_lock_init(&newsk->sk_dst_lock);
                 rwlock_init(&newsk->sk_callback_lock);
                 lockdep_set_class_and_name(&newsk->sk_callback_lock,
                                 af_callback_keys + newsk->sk_family,
@@ -1898,7 +1898,7 @@ void sock_init_data(struct socket *sock, struct sock *sk)
         } else
                 sk->sk_sleep    =       NULL;
 
-        rwlock_init(&sk->sk_dst_lock);
+        spin_lock_init(&sk->sk_dst_lock);
         rwlock_init(&sk->sk_callback_lock);
         lockdep_set_class_and_name(&sk->sk_callback_lock,
                         af_callback_keys + sk->sk_family,
diff --git a/net/dccp/timer.c b/net/dccp/timer.c
index bbfeb5eae46a..1a9aa05d4dc4 100644
--- a/net/dccp/timer.c
+++ b/net/dccp/timer.c
@@ -38,7 +38,7 @@ static int dccp_write_timeout(struct sock *sk)
 
         if (sk->sk_state == DCCP_REQUESTING || sk->sk_state == DCCP_PARTOPEN) {
                 if (icsk->icsk_retransmits != 0)
-                        dst_negative_advice(&sk->sk_dst_cache, sk);
+                        dst_negative_advice(sk);
                 retry_until = icsk->icsk_syn_retries ?
                             : sysctl_dccp_request_retries;
         } else {
@@ -63,7 +63,7 @@ static int dccp_write_timeout(struct sock *sk)
                            Golden words :-).
                    */
 
-                        dst_negative_advice(&sk->sk_dst_cache, sk);
+                        dst_negative_advice(sk);
                 }
 
                 retry_until = sysctl_dccp_retries2;
diff --git a/net/decnet/af_decnet.c b/net/decnet/af_decnet.c
index 2b494fac9468..55e3b6b0061a 100644
--- a/net/decnet/af_decnet.c
+++ b/net/decnet/af_decnet.c
@@ -446,7 +446,7 @@ static void dn_destruct(struct sock *sk)
         skb_queue_purge(&scp->other_xmit_queue);
         skb_queue_purge(&scp->other_receive_queue);
 
-        dst_release(xchg(&sk->sk_dst_cache, NULL));
+        dst_release(rcu_dereference_check(sk->sk_dst_cache, 1));
 }
 
 static int dn_memory_pressure;
@@ -1105,7 +1105,7 @@ static int dn_accept(struct socket *sock, struct socket *newsock, int flags)
         release_sock(sk);
 
         dst = skb_dst(skb);
-        dst_release(xchg(&newsk->sk_dst_cache, dst));
+        sk_dst_set(newsk, dst);
         skb_dst_set(skb, NULL);
 
         DN_SK(newsk)->state        = DN_CR;
@@ -1956,7 +1956,7 @@ static int dn_sendmsg(struct kiocb *iocb, struct socket *sock,
         }
 
         if ((flags & MSG_TRYHARD) && sk->sk_dst_cache)
-                dst_negative_advice(&sk->sk_dst_cache, sk);
+                dst_negative_advice(sk);
 
         mss = scp->segsize_rem;
         fctype = scp->services_rem & NSP_FC_MASK;
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index a0beb32beaa3..193dcd6ed64f 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -154,7 +154,7 @@ void inet_sock_destruct(struct sock *sk)
         WARN_ON(sk->sk_forward_alloc);
 
         kfree(inet->opt);
-        dst_release(sk->sk_dst_cache);
+        dst_release(rcu_dereference_check(sk->sk_dst_cache, 1));
         sk_refcnt_debug_dec(sk);
 }
 EXPORT_SYMBOL(inet_sock_destruct);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 4000b10610b7..ae3ec15fb630 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3710,7 +3710,7 @@ static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag)
         }
 
         if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
-                dst_confirm(sk->sk_dst_cache);
+                dst_confirm(__sk_dst_get(sk));
 
         return 1;
 
@@ -5833,7 +5833,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                         if (tp->snd_una == tp->write_seq) {
                                 tcp_set_state(sk, TCP_FIN_WAIT2);
                                 sk->sk_shutdown |= SEND_SHUTDOWN;
-                                dst_confirm(sk->sk_dst_cache);
+                                dst_confirm(__sk_dst_get(sk));
 
                                 if (!sock_flag(sk, SOCK_DEAD))
                                         /* Wake up lingering close() */
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 8a0ab2977f1f..c732be00606b 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -172,14 +172,14 @@ static int tcp_write_timeout(struct sock *sk)
 
         if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
                 if (icsk->icsk_retransmits)
-                        dst_negative_advice(&sk->sk_dst_cache, sk);
+                        dst_negative_advice(sk);
                 retry_until = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries;
         } else {
                 if (retransmits_timed_out(sk, sysctl_tcp_retries1)) {
                         /* Black hole detection */
                         tcp_mtu_probing(icsk, sk);
 
-                        dst_negative_advice(&sk->sk_dst_cache, sk);
+                        dst_negative_advice(sk);
                 }
 
                 retry_until = sysctl_tcp_retries2;
diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 33f60fca7aa7..1160400e9dbd 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -114,9 +114,9 @@ struct ipv6_txoptions *ipv6_update_options(struct sock *sk,
                 }
                 opt = xchg(&inet6_sk(sk)->opt, opt);
         } else {
-                write_lock(&sk->sk_dst_lock);
+                spin_lock(&sk->sk_dst_lock);
                 opt = xchg(&inet6_sk(sk)->opt, opt);
-                write_unlock(&sk->sk_dst_lock);
+                spin_unlock(&sk->sk_dst_lock);
         }
         sk_dst_reset(sk);
 
@@ -971,14 +971,13 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
         case IPV6_MTU:
         {
                 struct dst_entry *dst;
+
                 val = 0;
-                lock_sock(sk);
-                dst = sk_dst_get(sk);
-                if (dst) {
+                rcu_read_lock();
+                dst = __sk_dst_get(sk);
+                if (dst)
                         val = dst_mtu(dst);
-                        dst_release(dst);
-                }
-                release_sock(sk);
+                rcu_read_unlock();
                 if (!val)
                         return -ENOTCONN;
                 break;
@@ -1066,12 +1065,14 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
                 else
                         val = np->mcast_hops;
 
-                dst = sk_dst_get(sk);
-                if (dst) {
-                        if (val < 0)
+                if (val < 0) {
+                        rcu_read_lock();
+                        dst = __sk_dst_get(sk);
+                        if (dst)
                                 val = ip6_dst_hoplimit(dst);
-                        dst_release(dst);
+                        rcu_read_unlock();
                 }
+
                 if (val < 0)
                         val = sock_net(sk)->ipv6.devconf_all->hop_limit;
                 break;