Re: [PATCH 2/8] Basic zcache functionality

Eric Dumazet
2010-07-18T08:44:37Z

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;
}

...
}