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

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Message-ID: <aDdua_Kp6Dz91xwm@google.com>
Date: Wed, 28 May 2025 13:13:31 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Jiapeng Chong <jiapeng.chong@...ux.alibaba.com>,
	James Clark <james.clark@...aro.org>,
	Howard Chu <howardchu95@...il.com>,
	Weilin Wang <weilin.wang@...el.com>,
	Stephen Brennan <stephen.s.brennan@...cle.com>,
	Andi Kleen <ak@...ux.intel.com>, Dmitry Vyukov <dvyukov@...gle.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/7] perf intel-tpebs: Avoid race when evlist is being
 deleted

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@...nel.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@...gle.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