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Date: Sun, 18 Jul 2010 15:21:25 +0530
From: Nitin Gupta <ngupta@vflare.org>
Reply-To: ngupta@vflare.org
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Subject: Re: [PATCH 2/8] Basic zcache functionality
References: <1279283870-18549-1-git-send-email-ngupta@vflare.org>	 <1279283870-18549-3-git-send-email-ngupta@vflare.org> <1279442671.2476.34.camel@edumazet-laptop>
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Sender: owner-linux-mm@kvack.org
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Andrew Morton <akpm@linux-foundation.org>, Greg KH <greg@kroah.com>, Dan Magenheimer <dan.magenheimer@oracle.com>, Rik van Riel <riel@redhat.com>, Avi Kivity <avi@redhat.com>, Christoph Hellwig <hch@infradead.org>, Minchan Kim <minchan.kim@gmail.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, linux-mm <linux-mm@kvack.org>, linux-kernel <linux-kernel@vger.kernel.org>
List-ID: <linux-mm.kvack.org>

On 07/18/2010 02:14 PM, Eric Dumazet wrote:
> Le vendredi 16 juillet 2010 A  18:07 +0530, Nitin Gupta a A(C)crit :
> 
>> 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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