[TCP]: Move sack_ok access to obviously named funcs & cleanup

Previously code had IsReno/IsFack defined as macros that were
local to tcp_input.c though sack_ok field has user elsewhere too
for the same purpose. This changes them to static inlines as
preferred according the current coding style and unifies the
access to sack_ok across multiple files. Magic bitops of sack_ok
for FACK and DSACK are also abstracted to functions with
appropriate names.

Note:
- One sack_ok = 1 remains but that's self explanary, i.e., it
  enables sack
- Couple of !IsReno cases are changed to tcp_is_sack
- There were no users for IsDSack => I dropped it
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -719,6 +719,34 @@ static inline void tcp_ca_event(struct sock *sk, const enum tcp_ca_event event)
 		icsk->icsk_ca_ops->cwnd_event(sk, event);
 }
 
+/* These functions determine how the current flow behaves in respect of SACK
+ * handling. SACK is negotiated with the peer, and therefore it can vary
+ * between different flows.
+ *
+ * tcp_is_sack - SACK enabled
+ * tcp_is_reno - No SACK
+ * tcp_is_fack - FACK enabled, implies SACK enabled
+ */
+static inline int tcp_is_sack(const struct tcp_sock *tp)
+{
+	return tp->rx_opt.sack_ok;
+}
+
+static inline int tcp_is_reno(const struct tcp_sock *tp)
+{
+	return !tcp_is_sack(tp);
+}
+
+static inline int tcp_is_fack(const struct tcp_sock *tp)
+{
+	return tp->rx_opt.sack_ok & 2;
+}
+
+static inline void tcp_enable_fack(struct tcp_sock *tp)
+{
+	tp->rx_opt.sack_ok |= 2;
+}
+
 static inline unsigned int tcp_left_out(const struct tcp_sock *tp)
 {
 	return tp->sacked_out + tp->lost_out;

net/ipv4/tcp.c

@@ -2014,7 +2014,7 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info)
 
 	if (tp->rx_opt.tstamp_ok)
 		info->tcpi_options |= TCPI_OPT_TIMESTAMPS;
-	if (tp->rx_opt.sack_ok)
+	if (tcp_is_sack(tp))
 		info->tcpi_options |= TCPI_OPT_SACK;
 	if (tp->rx_opt.wscale_ok) {
 		info->tcpi_options |= TCPI_OPT_WSCALE;

net/ipv4/tcp_minisocks.c

@@ -445,7 +445,7 @@ struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req,
 		newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
 		if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
 			if (sysctl_tcp_fack)
-				newtp->rx_opt.sack_ok |= 2;
+				tcp_enable_fack(newtp);
 		}
 		newtp->window_clamp = req->window_clamp;
 		newtp->rcv_ssthresh = req->rcv_wnd;

net/ipv4/tcp_output.c

@@ -737,7 +737,7 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len, unsigned int mss
 
 		if (diff > 0) {
 			/* Adjust Reno SACK estimate. */
-			if (!tp->rx_opt.sack_ok) {
+			if (tcp_is_reno(tp)) {
 				tcp_dec_pcount_approx_int(&tp->sacked_out, diff);
 				tcp_verify_left_out(tp);
 			}
@@ -1728,7 +1728,7 @@ static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *skb, int m
 		if (TCP_SKB_CB(next_skb)->sacked&TCPCB_LOST)
 			tp->lost_out -= tcp_skb_pcount(next_skb);
 		/* Reno case is special. Sigh... */
-		if (!tp->rx_opt.sack_ok && tp->sacked_out)
+		if (tcp_is_reno(tp) && tp->sacked_out)
 			tcp_dec_pcount_approx(&tp->sacked_out, next_skb);
 
 		/* Not quite right: it can be > snd.fack, but
@@ -1976,7 +1976,7 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
 		return;
 
 	/* No forward retransmissions in Reno are possible. */
-	if (!tp->rx_opt.sack_ok)
+	if (tcp_is_reno(tp))
 		return;
 
 	/* Yeah, we have to make difficult choice between forward transmission

net/ipv4/tcp_timer.c

@@ -315,7 +315,7 @@ static void tcp_retransmit_timer(struct sock *sk)
 	if (icsk->icsk_retransmits == 0) {
 		if (icsk->icsk_ca_state == TCP_CA_Disorder ||
 		    icsk->icsk_ca_state == TCP_CA_Recovery) {
-			if (tp->rx_opt.sack_ok) {
+			if (tcp_is_sack(tp)) {
 				if (icsk->icsk_ca_state == TCP_CA_Recovery)
 					NET_INC_STATS_BH(LINUX_MIB_TCPSACKRECOVERYFAIL);
 				else