netdev - Re: [PATCH net v2] bonding: fix div by zero while enslaving and transmitting

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Date:	Wed, 17 Sep 2014 13:08:19 +0200
From:	Nikolay Aleksandrov <nikolay@...hat.com>
To:	Ding Tianhong <dingtianhong@...wei.com>, netdev@...r.kernel.org
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Veaceslav Falico <vfalico@...il.com>
Subject: Re: [PATCH net v2] bonding: fix div by zero while enslaving and transmitting

On 17/09/14 08:15, Ding Tianhong wrote:
> On 2014/9/12 23:38, Nikolay Aleksandrov wrote:
>> The problem is that the slave is first linked and slave_cnt is
>> incremented afterwards leading to a div by zero in the modes that use it
>> as a modulus. What happens is that in bond_start_xmit()
>> bond_has_slaves() is used to evaluate further transmission and it becomes
>> true after the slave is linked in, but when slave_cnt is used in the xmit
>> path it is still 0, so fetch it once and transmit based on that. Since
>> it is used only in round-robin and XOR modes, the fix is only for them.
>> Thanks to Eric Dumazet for pointing out the fault in my first try to fix
>> this.
>>
>
> Hi, I think no need to add more checks in the xmit fast path, why not add a barrier to make
> sure the slave_cnt inc to 1 before access it.
>
> +	/* Increment slave_cnt before linking in the slave so we won't end up in
> +	 * bond_start_xmit with bond_has_slaves() true and slave_cnt == 0.
> +	 */
> +	bond->slave_cnt++;
> +	wmb();
>
> I think it looks more efficiency, sorry for reply so late.
>
> Regards
> Ding
>
>

Hi Ding,
You should re-read Eric's comment to my first fix. In my first attempt I moved 
the increment before the slave linking which does rcu_assign_pointer() which 
implies a full memory barrier, IIRC. The issue is that this fixes the writer 
side and makes sure the increment is visible before linking the slave, but I 
missed that on the reader side (bond_start_xmit()) we don't have any barriers, 
so the CPU is free to do whatever it likes with the access to slave_cnt F.e. it 
can fetch it before the slave list.
Now, this fix shouldn't be felt much performance-wise since the likely() hint 
will be correct 99% of the time because the situation where slave_cnt is not in 
sync is only in a very short period of time while enslaving and releasing 
slaves. If you'd like to further remove this one check - you could. You can 
fetch slave_cnt only once in bond_start_xmit() and use that as a check for 
further transmitting instead of empty slave list but you must pass down the 
fetched value to the xmitting functions, that is you should not re-fetch it, so 
it'd probably require you to add additional parameter to all modes' xmit 
functions so you can pass it down from bond_start_xmit(). Since only 2 modes 
actually use slave_cnt I don't think that is necessary.
In any case net should be merged with net-next first.

Cheers,
  Nik

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