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

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Message-ID: <1388674404.1607.4.camel@bwh-desktop.uk.level5networks.com>
Date:	Thu, 2 Jan 2014 14:53:24 +0000
From:	Ben Hutchings <bhutchings@...arflare.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 Wed, 2014-01-01 at 22:50 -0500, 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?
> 
> > 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.

Then you add them to the sum repeatedly in case of a retry...

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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