tcp: annotate tp->snd_nxt lockless reads

There are few places where we fetch tp->snd_nxt while
this field can change from IRQ or other cpu.

We need to add READ_ONCE() annotations, and also make
sure write sides use corresponding WRITE_ONCE() to avoid
store-tearing.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1917,7 +1917,8 @@ static inline u32 tcp_notsent_lowat(const struct tcp_sock *tp)
 static inline bool tcp_stream_memory_free(const struct sock *sk, int wake)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
-	u32 notsent_bytes = READ_ONCE(tp->write_seq) - tp->snd_nxt;
+	u32 notsent_bytes = READ_ONCE(tp->write_seq) -
+			    READ_ONCE(tp->snd_nxt);
 
 	return (notsent_bytes << wake) < tcp_notsent_lowat(tp);
 }

net/ipv4/tcp.c

@@ -625,7 +625,8 @@ int tcp_ioctl(struct sock *sk, int cmd, unsigned long arg)
 		if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))
 			answ = 0;
 		else
-			answ = READ_ONCE(tp->write_seq) - tp->snd_nxt;
+			answ = READ_ONCE(tp->write_seq) -
+			       READ_ONCE(tp->snd_nxt);
 		break;
 	default:
 		return -ENOIOCTLCMD;

net/ipv4/tcp_minisocks.c

@@ -482,8 +482,10 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
 	WRITE_ONCE(newtp->rcv_nxt, seq);
 	newtp->segs_in = 1;
 
-	newtp->snd_sml = newtp->snd_una =
-	newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
+	seq = treq->snt_isn + 1;
+	newtp->snd_sml = newtp->snd_una = seq;
+	WRITE_ONCE(newtp->snd_nxt, seq);
+	newtp->snd_up = seq;
 
 	INIT_LIST_HEAD(&newtp->tsq_node);
 	INIT_LIST_HEAD(&newtp->tsorted_sent_queue);

net/ipv4/tcp_output.c

@@ -67,7 +67,7 @@ static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
 	struct tcp_sock *tp = tcp_sk(sk);
 	unsigned int prior_packets = tp->packets_out;
 
-	tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
+	WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(skb)->end_seq);
 
 	__skb_unlink(skb, &sk->sk_write_queue);
 	tcp_rbtree_insert(&sk->tcp_rtx_queue, skb);
@@ -3142,7 +3142,7 @@ void tcp_send_fin(struct sock *sk)
 			 * if FIN had been sent. This is because retransmit path
 			 * does not change tp->snd_nxt.
 			 */
-			tp->snd_nxt++;
+			WRITE_ONCE(tp->snd_nxt, tp->snd_nxt + 1);
 			return;
 		}
 	} else {
@@ -3426,7 +3426,7 @@ static void tcp_connect_init(struct sock *sk)
 	tp->snd_una = tp->write_seq;
 	tp->snd_sml = tp->write_seq;
 	tp->snd_up = tp->write_seq;
-	tp->snd_nxt = tp->write_seq;
+	WRITE_ONCE(tp->snd_nxt, tp->write_seq);
 
 	if (likely(!tp->repair))
 		tp->rcv_nxt = 0;
@@ -3586,11 +3586,11 @@ int tcp_connect(struct sock *sk)
 	/* We change tp->snd_nxt after the tcp_transmit_skb() call
 	 * in order to make this packet get counted in tcpOutSegs.
 	 */
-	tp->snd_nxt = tp->write_seq;
+	WRITE_ONCE(tp->snd_nxt, tp->write_seq);
 	tp->pushed_seq = tp->write_seq;
 	buff = tcp_send_head(sk);
 	if (unlikely(buff)) {
-		tp->snd_nxt	= TCP_SKB_CB(buff)->seq;
+		WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(buff)->seq);
 		tp->pushed_seq	= TCP_SKB_CB(buff)->seq;
 	}
 	TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);