Re: [PATCH v2 4/7] perf intel-tpebs: Avoid race when evlist is being deleted

[Namhyung Kim <namhyung@kernel.org>]

On Wed, May 28, 2025 at 11:02:44AM -0700, Ian Rogers wrote:
> On Wed, May 28, 2025 at 10:53 AM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > Hi Ian,
> >
> > On Tue, May 27, 2025 at 08:26:34PM -0700, Ian Rogers wrote:
> > > Reading through the evsel->evlist may seg fault if a sample arrives
> > > when the evlist is being deleted. Detect this case and ignore samples
> > > arriving when the evlist is being deleted.
> > >
> > > Fixes: bcfab08db7fb ("perf intel-tpebs: Filter non-workload samples")
> > > Signed-off-by: Ian Rogers <irogers@google.com>
> > > ---
> > >  tools/perf/util/intel-tpebs.c | 12 ++++++++++--
> > >  1 file changed, 10 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/tools/perf/util/intel-tpebs.c b/tools/perf/util/intel-tpebs.c
> > > index 4ad4bc118ea5..3b92ebf5c112 100644
> > > --- a/tools/perf/util/intel-tpebs.c
> > > +++ b/tools/perf/util/intel-tpebs.c
> > > @@ -162,9 +162,17 @@ static bool is_child_pid(pid_t parent, pid_t child)
> > >
> > >  static bool should_ignore_sample(const struct perf_sample *sample, const struct tpebs_retire_lat *t)
> > >  {
> > > -     pid_t workload_pid = t->evsel->evlist->workload.pid;
> > > -     pid_t sample_pid = sample->pid;
> > > +     pid_t workload_pid, sample_pid = sample->pid;
> > >
> > > +     /*
> > > +      * During evlist__purge the evlist will be removed prior to the
> > > +      * evsel__exit calling evsel__tpebs_close and taking the
> > > +      * tpebs_mtx. Avoid a segfault by ignoring samples in this case.
> > > +      */
> > > +     if (t->evsel->evlist == NULL)
> > > +             return true;
> > > +
> > > +     workload_pid = t->evsel->evlist->workload.pid;
> >
> > I'm curious if there's a chance of TOCTOU race.  It'd certainly help
> > the segfault but would this code prevent it completely?
> 
> Good point. I think the race is already small as it doesn't happen
> without sanitizers for me.
> Thinking about the evlist problem. When a destructor (evlist__delete)
> it is generally assumed the code is being single threaded and in C++
> clang's -Wthread-safety will ignore destructors for this reason
> (annoying imo as it hides bugs). I don't see a good way to solve that
> for the evlist and evsel for the TPEBS case without using reference
> counting. Adding reference counts to evlist and evsel would be do-able
> as we could use reference count checking, but it would be a large and
> invasive change. Wdyt?

Would it be possible to kill the TPEBS thread before deleting evlist?

Thanks,
Namhyung