netdev - Re: [PATCH net-next] ipv6: fix the use of pcpu_tstats in ip6

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Date:	Thu, 2 Jan 2014 12:41:02 +0800
From:	Rongqing Li <rongqing.li@...driver.com>
To:	David Miller <davem@...emloft.net>
CC:	<roy.qing.li@...il.com>, <netdev@...r.kernel.org>,
	<edumazet@...gle.com>
Subject: Re: [PATCH net-next] ipv6: fix the use of pcpu_tstats in ip6_tunnel



On 01/02/2014 11:50 AM, David Miller wrote:
> From: roy.qing.li@...il.com
> Date: Thu,  2 Jan 2014 08:49:40 +0800
>
>> Eric Dumazet <edumazet@...gle.com>
>
> What is this?

Sorry, it should CC to Eric Dumazet

>
>> Signed-off-by: Li RongQing <roy.qing.li@...il.com>
>   ...
>>   {
>> -	struct pcpu_tstats sum = { 0 };
>> +	struct pcpu_tstats tmp, sum = { 0 };
>>   	int i;
>>
>> -	for_each_possible_cpu(i) {
>> -		const struct pcpu_tstats *tstats = per_cpu_ptr(dev->tstats, i);
>> +	BUILD_BUG_ON(offsetof(typeof(tmp), syncp) !=
>> +		     sizeof(tmp) - sizeof(tmp.syncp));
>>
>> -		sum.rx_packets += tstats->rx_packets;
>> -		sum.rx_bytes   += tstats->rx_bytes;
>> -		sum.tx_packets += tstats->tx_packets;
>> -		sum.tx_bytes   += tstats->tx_bytes;
>> +	for_each_possible_cpu(i) {
>> +		unsigned int start;
>> +		const struct pcpu_tstats *stat = per_cpu_ptr(dev->tstats, i);
>> +
>> +		do {
>> +			start = u64_stats_fetch_begin_bh(&stat->syncp);
>> +			memcpy(&tmp, stat, sizeof(tmp) - sizeof(stat->syncp));
>> +		} while (u64_stats_fetch_retry_bh(&stat->syncp, start));
>
> It's entirely wasteful and non-canonical to use an on-stack copy for this,
> just surround the entire set of statistic increments within the u64 stats
> updata sequence.  Then you don't need the on-stack thing.
>

Ok, I see.

Thanks

-Roy


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-- 
Best Reagrds,
Roy | RongQing Li
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