netdev - Re: [RCU PATCH 13/14] net: sched: make bstats per cpu and estimator RCU safe

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Date:	Mon, 10 Mar 2014 11:06:14 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	John Fastabend <john.fastabend@...il.com>
Cc:	xiyou.wangcong@...il.com, jhs@...atatu.com, netdev@...r.kernel.org,
	davem@...emloft.net
Subject: Re: [RCU PATCH 13/14] net: sched: make bstats per cpu and estimator
 RCU safe

On Mon, 2014-03-10 at 10:08 -0700, John Fastabend wrote:

> +void
> +__gnet_stats_copy_basic_cpu(struct gnet_stats_basic_packed *bstats,
> +			    struct gnet_stats_basic_cpu *b)
> +{
> +	int i;
> +
> +	for_each_possible_cpu(i) {
> +		struct gnet_stats_basic_cpu *bcpu = per_cpu_ptr(b, i);
> +		unsigned int start;
> +
> +		do {
> +			start = u64_stats_fetch_begin_bh(&bcpu->syncp);
> +			bstats->bytes += bcpu->bstats.bytes;
> +			bstats->packets += bcpu->bstats.packets;
> +		} while (u64_stats_fetch_retry_bh(&bcpu->syncp, start));
> +	}
> +}

This is buggy. You need to use temporary variables, otherwise if the
retry is done, you add bcpu->bstats.bytes/packets multiple times.


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