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

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Date:   Thu, 17 Mar 2022 09:57:49 +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 01:16:30PM -0700, Ian Rogers wrote:
> On Wed, Mar 16, 2022 at 7:08 AM Jiri Olsa <olsajiri@...il.com> wrote:
> >
> > 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?
> 
> The itr.evsel is the list head, so bogus.
> 
> > I think we need to set itr.evsel to NULL and skip the loop in
> > case evlist is empty
> 
> So that's the effect of this change except that the evsel is the list
> head. I'm not sure it is worth adding the condition to setting the
> evsel to capture that given that with the end condition having been
> met it would be invalid to read it. There's a similar problem for the
> other fields of the iterator.

ah ok, I got it.. but it seems to be just evsel that's wrong,
just as a precaution, could we set it only when it's valid?
with something like below

thanks,
jirka


---
diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index a2dba9e00765..76a6c70c3c40 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -346,7 +346,7 @@ struct evlist_cpu_iterator evlist__cpu_begin(struct evlist *evlist, struct affin
 {
 	struct evlist_cpu_iterator itr = {
 		.container = evlist,
-		.evsel = evlist__first(evlist),
+		.evsel = evlist__empty(evlist) ? NULL : evlist__first(evlist),
 		.cpu_map_idx = 0,
 		.evlist_cpu_map_idx = 0,
 		.evlist_cpu_map_nr = perf_cpu_map__nr(evlist->core.all_cpus),