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Subject[34-longterm 141/209] tcp: avoid a possible divide by zero
Date
From: Eric Dumazet <eric.dumazet@gmail.com>

=====================================================================
| This is a commit scheduled for the next v2.6.34 longterm release. |
| If you see a problem with using this for longterm, please comment.|
=====================================================================

commit ad9f4f50fe9288bbe65b7dfd76d8820afac6a24c upstream

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@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
net/ipv4/tcp_output.c | 6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e1819aa..6fdff30 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1514,6 +1514,7 @@ static int tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb)
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 & TCPCB_FLAG_FIN)
goto send_now;
@@ -1545,13 +1546,14 @@ static int tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb)
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 {
--
1.7.4.4


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