linux-kernel - Re: [PATCH] perf cs-etm: Use swap() instead of open coding it

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Message-ID: <20220620172438.GB1458883@p14s>
Date:   Mon, 20 Jun 2022 11:24:38 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Jiapeng Chong <jiapeng.chong@...ux.alibaba.com>
Cc:     john.garry@...wei.com, will@...nel.org, leo.yan@...aro.org,
        suzuki.poulose@....com, mike.leach@...aro.org,
        peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...nel.org, namhyung@...nel.org,
        linux-arm-kernel@...ts.infradead.org, coresight@...ts.linaro.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Abaci Robot <abaci@...ux.alibaba.com>, james.clark@....com
Subject: Re: [PATCH] perf cs-etm: Use swap() instead of open coding it

[Adding James]

Hi Jiapeng,

On Fri, May 06, 2022 at 05:17:18PM +0800, Jiapeng Chong wrote:
> Clean the following coccicheck warning:
> 
> ./tools/perf/util/cs-etm.c:418:34-35: WARNING opportunity for swap().
> 
> Reported-by: Abaci Robot <abaci@...ux.alibaba.com>
> Signed-off-by: Jiapeng Chong <jiapeng.chong@...ux.alibaba.com>
> ---
>  tools/perf/util/cs-etm.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 8b95fb3c4d7b..0cb555cc766f 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -406,18 +406,13 @@ struct cs_etm_packet_queue
>  static void cs_etm__packet_swap(struct cs_etm_auxtrace *etm,
>  				struct cs_etm_traceid_queue *tidq)
>  {
> -	struct cs_etm_packet *tmp;
> -
>  	if (etm->synth_opts.branches || etm->synth_opts.last_branch ||
> -	    etm->synth_opts.instructions) {
> +	    etm->synth_opts.instructions)
>  		/*
>  		 * Swap PACKET with PREV_PACKET: PACKET becomes PREV_PACKET for
>  		 * the next incoming packet.
>  		 */
> -		tmp = tidq->packet;
> -		tidq->packet = tidq->prev_packet;
> -		tidq->prev_packet = tmp;

Those 3 lines have burned a lot of eyes...  As far as I can remember the idea is
simply to make sure that after that point, ->prev_packet is now set to ->packet
in preparation for the next iteration.  There is no point in setting ->packet to
->prev_packet.  As such the following would work just fine:

                tidq->prev_packet = tidq->packet;

... but I will let James and Leo have a final say on that.

> -	}
> +		swap(tidq->packet, tidq->prev_packet);

If we absolutely need to keep swapping the packets please add the header file
where swap() is found.

Thanks,
Mathieu

>  }
>  
>  static void cs_etm__packet_dump(const char *pkt_string)
> -- 
> 2.20.1.7.g153144c
>