[PATCH] tcp: bug fix in proportional rate reduction.

[Nandita Dukkipati <nanditad@google.com>]

This patch is a fix for a bug triggering newly_acked_sacked < 0
in tcp_ack(.).

The bug is triggered by sacked_out decreasing relative to prior_sacked,
but packets_out remaining the same as pior_packets. This is because the
snapshot of prior_packets is taken after tcp_sacktag_write_queue() while
prior_sacked is captured before tcp_sacktag_write_queue(). The problem
is: tcp_sacktag_write_queue (tcp_match_skb_to_sack() -> tcp_fragment)
adjusts the pcount for packets_out and sacked_out (MSS change or other
reason). As a result, this delta in pcount is reflected in
(prior_sacked - sacked_out) but not in (prior_packets - packets_out).

This patch does the following:
1) initializes prior_packets at the start of tcp_ack() so as to
capture the delta in packets_out created by tcp_fragment.
2) introduces a new "previous_packets_out" variable that snapshots
packets_out right before tcp_clean_rtx_queue, so pkts_acked can be
correctly computed as before.

Signed-off-by: Nandita Dukkipati <nanditad@google.com>
---
 net/ipv4/tcp_input.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index d7d3694..2986e10 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3265,9 +3265,10 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	bool is_dupack = false;
 	u32 prior_in_flight;
 	u32 prior_fackets;
-	int prior_packets;
+	int prior_packets = tp->packets_out;
 	int prior_sacked = tp->sacked_out;
 	int pkts_acked = 0;
+	int previous_packets_out = 0;
 
 	/* If the ack is older than previous acks
 	 * then we can probably ignore it.
@@ -3338,14 +3339,14 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	sk->sk_err_soft = 0;
 	icsk->icsk_probes_out = 0;
 	tp->rcv_tstamp = tcp_time_stamp;
-	prior_packets = tp->packets_out;
 	if (!prior_packets)
 		goto no_queue;
 
 	/* See if we can take anything off of the retransmit queue. */
+	previous_packets_out = tp->packets_out;
 	flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una);
 
-	pkts_acked = prior_packets - tp->packets_out;
+	pkts_acked = previous_packets_out - tp->packets_out;
 
 	if (tcp_ack_is_dubious(sk, flag)) {
 		/* Advance CWND, if state allows this. */
-- 
1.8.2.1