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

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Date:	Sun, 18 Jul 2010 15:21:25 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
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

On 07/18/2010 02:14 PM, Eric Dumazet wrote:
> 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;
> 	}
> 
> ...
> }
> 
> 

Oh, my bad. Thanks for the fix!

On a side note: u64_stats_* should probably be renamed to stats64_* since
they are equally applicable for s64.


Thanks,
Nitin
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