netdev - [PATCH v3 net 3/5] tcp: move false FR condition into tcp_false_fast_retrans

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Date:   Tue, 13 Mar 2018 12:25:09 +0200
From:   Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
To:     netdev@...r.kernel.org
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Subject: [PATCH v3 net 3/5] tcp: move false FR condition into tcp_false_fast_retrans_possible()

No functional changes. This change simplifies the next change
slightly.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
---
 net/ipv4/tcp_input.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c60745c..72ecfbb 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2211,6 +2211,17 @@ static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
 	}
 }
 
+/* False fast retransmits may occur when SACK is not in use under certain
+ * conditions (RFC6582). The sender MUST hold old state until something
+ * *above* high_seq is ACKed to prevent triggering such false fast
+ * retransmits. SACK TCP is safe.
+ */
+static bool tcp_false_fast_retrans_possible(const struct tcp_sock *tp,
+					    const u32 snd_una)
+{
+	return tcp_is_reno(tp) && (snd_una == tp->high_seq);
+}
+
 static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when)
 {
 	return tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
@@ -2350,10 +2361,10 @@ static bool tcp_try_undo_recovery(struct sock *sk)
 	} else if (tp->rack.reo_wnd_persist) {
 		tp->rack.reo_wnd_persist--;
 	}
-	if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
-		/* Hold old state until something *above* high_seq
-		 * is ACKed. For Reno it is MUST to prevent false
-		 * fast retransmits (RFC2582). SACK TCP is safe. */
+	if (tcp_false_fast_retrans_possible(tp, tp->snd_una)) {
+		/* Hold old state until something *above* high_seq is ACKed
+		 * if false fast retransmit is possible.
+		 */
 		if (!tcp_any_retrans_done(sk))
 			tp->retrans_stamp = 0;
 		return true;
-- 
2.7.4