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Date:	Wed, 30 Apr 2014 10:53:42 -0400
From:	Neal Cardwell <>
To:	Eric Dumazet <>
Cc:	Liu Yu <>,
	Stephen Hemminger <>,
	"David S. Miller" <>,
	Netdev <>
Subject: Re: [PATCH] tcp_cubic: fix the range of delayed_ack

On Wed, Apr 30, 2014 at 10:18 AM, Eric Dumazet <> wrote:
> On Wed, 2014-04-30 at 17:34 +0800, Liu Yu wrote:
>> From: Liu Yu <>
>> 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 <>
>> Signed-off-by: Liu Yu <>
>> ---
>>  net/ipv4/tcp_cubic.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
> Signed-off-by: Eric Dumazet <>

Acked-by: Neal Cardwell <>

Indeed, thanks!

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