tcp: avoid a possible divide by zero

[ 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@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index ea3983835872a165fea290cbf0e54e5f3637e8a4..e1fafbbe91aeb78b6925b0f1071f95ff3ae9bea3 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1518,6 +1518,7 @@ 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;
@@ -1549,13 +1550,14 @@ 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 {