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Date:	Wed, 30 Apr 2014 07:18:18 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Liu Yu <allanliuyu@...il.com>
Cc:	ncardwell@...gle.com, shemminger@...tta.com,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH]  tcp_cubic: fix the range of delayed_ack

On Wed, 2014-04-30 at 17:34 +0800, Liu Yu wrote:
> From: Liu Yu <allanyuliu@...cent.com>
> 
> commit b9f47a3aaeab (tcp_cubic: limit delayed_ack ratio to prevent
> divide error) try to prevent divide error, but there is still a little
> chance that delayed_ack can reach zero. In case the param cnt get
> negative value, then ratio+cnt would overflow and may happen to be zero.
> As a result, min(ratio, ACK_RATIO_LIMIT) will calculate to be zero.
> 
> In some old kernels, such as 2.6.32, there is a bug that would
> pass negative param, which then ultimately leads to this divide error.
> 
> commit 5b35e1e6e9c (tcp: fix tcp_trim_head() to adjust segment count
> with skb MSS) fixed the negative param issue. However, 
> it's safe that we fix the range of delayed_ack as well,
> to make sure we do not hit a divide by zero.
> 
> CC: Stephen Hemminger <shemminger@...tta.com>
> Signed-off-by: Liu Yu <allanyuliu@...cent.com>
> ---
>  net/ipv4/tcp_cubic.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)

Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Thanks !


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