[057/152] tcp: avoid a possible divide by zero

[Greg KH]

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

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


From: Eric Dumazet <eric.dumazet@xxxxxxxxx>

[ Upstream commit ad9f4f50fe9288bbe65b7dfd76d8820afac6a24c ]

sysctl_tcp_tso_win_divisor might be set to zero while one cpu runs in
tcp_tso_should_defer(). Make sure we dont allow a divide by zero by
reading sysctl_tcp_tso_win_divisor exactly once.

Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
---
 net/ipv4/tcp_output.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1518,6 +1518,7 @@ static int tcp_tso_should_defer(struct s
 	struct tcp_sock *tp = tcp_sk(sk);
 	const struct inet_connection_sock *icsk = inet_csk(sk);
 	u32 send_win, cong_win, limit, in_flight;
+	int win_divisor;
 
 	if (TCP_SKB_CB(skb)->flags & TCPHDR_FIN)
 		goto send_now;
@@ -1549,13 +1550,14 @@ static int tcp_tso_should_defer(struct s
 	if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
 		goto send_now;
 
-	if (sysctl_tcp_tso_win_divisor) {
+	win_divisor = ACCESS_ONCE(sysctl_tcp_tso_win_divisor);
+	if (win_divisor) {
 		u32 chunk = min(tp->snd_wnd, tp->snd_cwnd * tp->mss_cache);
 
 		/* If at least some fraction of a window is available,
 		 * just use it.
 		 */
-		chunk /= sysctl_tcp_tso_win_divisor;
+		chunk /= win_divisor;
 		if (limit >= chunk)
 			goto send_now;
 	} else {


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