sctp: fix busy polling

Busy polling while holding the socket lock makes litle sense,
because incoming packets wont reach our receive queue.

Fixes: 8465a5fc ("sctp: add support for busy polling to sctp protocol")
Reported-by: Jacob Moroni <jmoroni@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -2102,6 +2102,10 @@ static int sctp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 	if (unlikely(flags & MSG_ERRQUEUE))
 		return inet_recv_error(sk, msg, len, addr_len);
 
+	if (sk_can_busy_loop(sk) &&
+	    skb_queue_empty_lockless(&sk->sk_receive_queue))
+		sk_busy_loop(sk, flags & MSG_DONTWAIT);
+
 	lock_sock(sk);
 
 	if (sctp_style(sk, TCP) && !sctp_sstate(sk, ESTABLISHED) &&
@@ -9046,12 +9050,6 @@ struct sk_buff *sctp_skb_recv_datagram(struct sock *sk, int flags, int *err)
 		if (sk->sk_shutdown & RCV_SHUTDOWN)
 			break;
 
-		if (sk_can_busy_loop(sk)) {
-			sk_busy_loop(sk, flags & MSG_DONTWAIT);
-
-			if (!skb_queue_empty_lockless(&sk->sk_receive_queue))
-				continue;
-		}
 
 		/* User doesn't want to wait.  */
 		error = -EAGAIN;