linux-kernel - Re: [PATCH 2/8] Basic zcache functionality

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Date:	Sun, 18 Jul 2010 10:44:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Rik van Riel <riel@...hat.com>, Avi Kivity <avi@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/8] Basic zcache functionality

Le vendredi 16 juillet 2010 à 18:07 +0530, Nitin Gupta a écrit :

> This particular patch implemets basic functionality only:
> +static u64 zcache_get_stat(struct zcache_pool *zpool,
> +		enum zcache_pool_stats_index idx)
> +{
> +	int cpu;
> +	s64 val = 0;
> +
> +	for_each_possible_cpu(cpu) {
> +		unsigned int start;
> +		struct zcache_pool_stats_cpu *stats;
> +
> +		stats = per_cpu_ptr(zpool->stats, cpu);
> +		do {
> +			start = u64_stats_fetch_begin(&stats->syncp);
> +			val += stats->count[idx];
> +		} while (u64_stats_fetch_retry(&stats->syncp, start));
> +	}
> +
> +	BUG_ON(val < 0);
> +	return val;
> +}

Sorry this is wrong.

Inside the fetch/retry block you should not do the addition to val, only
a read of value to a temporary variable, since this might be done
several times.

You want something like :

static u64 zcache_get_stat(struct zcache_pool *zpool,
			   enum zcache_pool_stats_index idx)
{
	int cpu;
	s64 temp, val = 0;

	for_each_possible_cpu(cpu) {
		unsigned int start;
		struct zcache_pool_stats_cpu *stats;

		stats = per_cpu_ptr(zpool->stats, cpu);
		do {
			start = u64_stats_fetch_begin(&stats->syncp);
			temp = stats->count[idx];
		} while (u64_stats_fetch_retry(&stats->syncp, start));
		val += temp;
	}

...
}



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