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

[Namhyung Kim <namhyung@kernel.org>]

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?

Thanks,
Namhyung


>  	if (workload_pid < 0 || workload_pid == sample_pid)
>  		return false;
>  
> -- 
> 2.49.0.1238.gf8c92423fb-goog
>