[patch 19/28] TCP: Fix TCP handling of SACK in bidirectional flows.

[Greg KH]

-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxx>

It's possible that new SACK blocks that should trigger new LOST
markings arrive with new data (which previously made is_dupack
false). In addition, I think this fixes a case where we get
a cumulative ACK with enough SACK blocks to trigger the fast
recovery (is_dupack would be false there too).

I'm not completely pleased with this solution because readability
of the code is somewhat questionable as 'is_dupack' in SACK case
is no longer about dupacks only but would mean something like
'lost_marker_work_todo' too... But because of Eifel stuff done
in CA_Recovery, the FLAG_DATA_SACKED check cannot be placed to
the if statement which seems attractive solution. Nevertheless,
I didn't like adding another variable just for that either... :-)

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 net/ipv4/tcp_input.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2112,7 +2112,10 @@ tcp_fastretrans_alert(struct sock *sk, u
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
-	int is_dupack = (tp->snd_una == prior_snd_una && !(flag&FLAG_NOT_DUP));
+	int is_dupack = (tp->snd_una == prior_snd_una &&
+			 (!(flag&FLAG_NOT_DUP) ||
+			  ((flag&FLAG_DATA_SACKED) &&
+			   (tp->fackets_out > tp->reordering))));
 
 	/* Some technical things:
 	 * 1. Reno does not count dupacks (sacked_out) automatically. */

-- 
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