tcp: avoid a possible divide by zero

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>

1 files changed, 4 insertions, 2 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 3c59ab42df2b..0d4a3cebfb46 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1512,6 +1512,7 @@ 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 & TCPHDR_FIN)
                 goto send_now;
@@ -1543,13 +1544,14 @@ 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 {