tcp: refine tcp_write_queue_empty() implementation

Due to how tcp_sendmsg() is implemented, we can have an empty
skb at the tail of the write queue.

Most [1] tcp_write_queue_empty() callers want to know if there is
anything to send (payload and/or FIN)

Instead of checking if the sk_write_queue is empty, we need
to test if tp->write_seq == tp->snd_nxt

[1] tcp_send_fin() was the only caller that expected to
 see if an skb was in the write queue, I have changed the code
 to reuse the tcp_write_queue_tail() result.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

include/net/tcp.h

@@ -1766,9 +1766,18 @@ static inline bool tcp_skb_is_last(const struct sock *sk,
 	return skb_queue_is_last(&sk->sk_write_queue, skb);
 }
 
+/**
+ * tcp_write_queue_empty - test if any payload (or FIN) is available in write queue
+ * @sk: socket
+ *
+ * Since the write queue can have a temporary empty skb in it,
+ * we must not use "return skb_queue_empty(&sk->sk_write_queue)"
+ */
 static inline bool tcp_write_queue_empty(const struct sock *sk)
 {
-	return skb_queue_empty(&sk->sk_write_queue);
+	const struct tcp_sock *tp = tcp_sk(sk);
+
+	return tp->write_seq == tp->snd_nxt;
 }
 
 static inline bool tcp_rtx_queue_empty(const struct sock *sk)

net/ipv4/tcp_output.c

@@ -3129,7 +3129,7 @@ void sk_forced_mem_schedule(struct sock *sk, int size)
  */
 void tcp_send_fin(struct sock *sk)
 {
-	struct sk_buff *skb, *tskb = tcp_write_queue_tail(sk);
+	struct sk_buff *skb, *tskb, *tail = tcp_write_queue_tail(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	/* Optimization, tack on the FIN if we have one skb in write queue and
@@ -3137,6 +3137,7 @@ void tcp_send_fin(struct sock *sk)
 	 * Note: in the latter case, FIN packet will be sent after a timeout,
 	 * as TCP stack thinks it has already been transmitted.
 	 */
+	tskb = tail;
 	if (!tskb && tcp_under_memory_pressure(sk))
 		tskb = skb_rb_last(&sk->tcp_rtx_queue);
 
@@ -3144,7 +3145,7 @@ void tcp_send_fin(struct sock *sk)
 		TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;
 		TCP_SKB_CB(tskb)->end_seq++;
 		tp->write_seq++;
-		if (tcp_write_queue_empty(sk)) {
+		if (!tail) {
 			/* This means tskb was already sent.
 			 * Pretend we included the FIN on previous transmit.
 			 * We need to set tp->snd_nxt to the value it would have