[PATCH] tcp: do not update snd_una if it is same with ack_seq

[Yafang Shao]

In the slow path, TCP_SKB_SB(skb)->ack_seq may be same with tp->snd_una,
and under this condition we don't need to update the snd_una.

Furthermore, tcp_ack_update_window() is only called in slow path,
so introducing this check won't affect the fast path processing.

By the way, '&' is a little faster than '-', so I replaced after() with
"flag & FLAG_SND_UNA_ADVANCED".

Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
---
 net/ipv4/tcp_input.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2868ef2..db5a6b7 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3376,7 +3376,8 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32
 		}
 	}
 
-	tcp_snd_una_update(tp, ack);
+	if (after(ack, tp->snd_una))
+		tcp_snd_una_update(tp, ack);
 
 	return flag;
 }
@@ -3610,7 +3611,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	if (flag & FLAG_UPDATE_TS_RECENT)
 		tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
 
-	if (!(flag & FLAG_SLOWPATH) && after(ack, prior_snd_una)) {
+	if (!(flag & FLAG_SLOWPATH) && flag & FLAG_SND_UNA_ADVANCED) {
 		/* Window is constant, pure forward advance.
 		 * No more checks are required.
 		 * Note, we use the fact that SND.UNA>=SND.WL2.
-- 
1.8.3.1