dccp: annotate data-races in dccp_poll()

We changed tcp_poll() over time, bug never updated dccp.

Note that we also could remove dccp instead of maintaining it.

Fixes: 7c657876 ("[DCCP]: Initial implementation")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230818015820.2701595-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dccp/proto.c

@@ -315,11 +315,15 @@ EXPORT_SYMBOL_GPL(dccp_disconnect);
 __poll_t dccp_poll(struct file *file, struct socket *sock,
 		       poll_table *wait)
 {
-	__poll_t mask;
 	struct sock *sk = sock->sk;
+	__poll_t mask;
+	u8 shutdown;
+	int state;
 
 	sock_poll_wait(file, sock, wait);
-	if (sk->sk_state == DCCP_LISTEN)
+
+	state = inet_sk_state_load(sk);
+	if (state == DCCP_LISTEN)
 		return inet_csk_listen_poll(sk);
 
 	/* Socket is not locked. We are protected from async events
@@ -328,20 +332,21 @@ __poll_t dccp_poll(struct file *file, struct socket *sock,
 	 */
 
 	mask = 0;
-	if (sk->sk_err)
+	if (READ_ONCE(sk->sk_err))
 		mask = EPOLLERR;
+	shutdown = READ_ONCE(sk->sk_shutdown);
 
-	if (sk->sk_shutdown == SHUTDOWN_MASK || sk->sk_state == DCCP_CLOSED)
+	if (shutdown == SHUTDOWN_MASK || state == DCCP_CLOSED)
 		mask |= EPOLLHUP;
-	if (sk->sk_shutdown & RCV_SHUTDOWN)
+	if (shutdown & RCV_SHUTDOWN)
 		mask |= EPOLLIN | EPOLLRDNORM | EPOLLRDHUP;
 
 	/* Connected? */
-	if ((1 << sk->sk_state) & ~(DCCPF_REQUESTING | DCCPF_RESPOND)) {
+	if ((1 << state) & ~(DCCPF_REQUESTING | DCCPF_RESPOND)) {
 		if (atomic_read(&sk->sk_rmem_alloc) > 0)
 			mask |= EPOLLIN | EPOLLRDNORM;
 
-		if (!(sk->sk_shutdown & SEND_SHUTDOWN)) {
+		if (!(shutdown & SEND_SHUTDOWN)) {
 			if (sk_stream_is_writeable(sk)) {
 				mask |= EPOLLOUT | EPOLLWRNORM;
 			} else {  /* send SIGIO later */
@@ -359,7 +364,6 @@ __poll_t dccp_poll(struct file *file, struct socket *sock,
 	}
 	return mask;
 }
-
 EXPORT_SYMBOL_GPL(dccp_poll);
 
 int dccp_ioctl(struct sock *sk, int cmd, int *karg)