linux-kernel - Re: [PATCH] perf evlist: Avoid iteration for empty evlist.

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Date:   Wed, 16 Mar 2022 15:08:00 +0100
From:   Jiri Olsa <olsajiri@...il.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf evlist: Avoid iteration for empty evlist.

On Wed, Mar 16, 2022 at 12:10:49AM -0700, Ian Rogers wrote:
> As seen with 'perf stat --null ..' and reported in:
> https://lore.kernel.org/lkml/YjCLcpcX2peeQVCH@kernel.org/
> 
> Fixes: 472832d2c000 ("perf evlist: Refactor evlist__for_each_cpu()")
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
>  tools/perf/util/evlist.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 8134d45e2164..a2dba9e00765 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -354,7 +354,10 @@ struct evlist_cpu_iterator evlist__cpu_begin(struct evlist *evlist, struct affin
>  		.affinity = affinity,
>  	};
>  
> -	if (itr.affinity) {
> +	if (evlist__empty(evlist)) {
> +		/* Ensure the empty list doesn't iterate. */
> +		itr.evlist_cpu_map_idx = itr.evlist_cpu_map_nr;

I can't see the crash anymore, but I'm bit confused with the code

if evlist is empty then itr.evsel is bogus.. and the loop code
__run_perf_stat is just lucky, right?

I think we need to set itr.evsel to NULL and skip the loop in
case evlist is empty

thanks,
jirka

> +	} else if (itr.affinity) {
>  		itr.cpu = perf_cpu_map__cpu(evlist->core.all_cpus, 0);
>  		affinity__set(itr.affinity, itr.cpu.cpu);
>  		itr.cpu_map_idx = perf_cpu_map__idx(itr.evsel->core.cpus, itr.cpu);
> -- 
> 2.35.1.723.g4982287a31-goog
>