Date:   Thu, 18 Aug 2022 23:42:02 -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>,
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        llvm@...ts.linux.dev
Subject: Re: [PATCH v1 5/6] perf mutex: Fix thread safety analysis

On Thu, Aug 18, 2022 at 11:17 AM Ian Rogers <irogers@...gle.com> wrote:
>
> On Thu, Aug 18, 2022 at 9:41 AM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Tue, Aug 16, 2022 at 10:39 PM Ian Rogers <irogers@...gle.com> wrote:
> > >
> > > Add annotations to describe lock behavior. Add missing unlocks to
> > > perf_sched__replay. Alter hist_iter__top_callback as the thread-safety
> > > analysis cannot follow pointers through local variables.
> > >
> > > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > > ---
> > >  tools/perf/builtin-sched.c | 8 ++++++++
> > >  tools/perf/builtin-top.c   | 5 +++--
> > >  2 files changed, 11 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
> > > index 0f52f73be896..a8a765ed28ce 100644
> > > --- a/tools/perf/builtin-sched.c
> > > +++ b/tools/perf/builtin-sched.c
> > > @@ -658,6 +658,8 @@ static void *thread_func(void *ctx)
> > >  }
> > >
> > >  static void create_tasks(struct perf_sched *sched)
> > > +       EXCLUSIVE_LOCK_FUNCTION(sched->start_work_mutex)
> > > +       EXCLUSIVE_LOCK_FUNCTION(sched->work_done_wait_mutex)
> > >  {
> > >         struct task_desc *task;
> > >         pthread_attr_t attr;
> > > @@ -687,6 +689,8 @@ static void create_tasks(struct perf_sched *sched)
> > >  }
> > >
> > >  static void wait_for_tasks(struct perf_sched *sched)
> > > +       EXCLUSIVE_LOCKS_REQUIRED(sched->work_done_wait_mutex)
> > > +       EXCLUSIVE_LOCKS_REQUIRED(sched->start_work_mutex)
> > >  {
> > >         u64 cpu_usage_0, cpu_usage_1;
> > >         struct task_desc *task;
> > > @@ -738,6 +742,8 @@ static void wait_for_tasks(struct perf_sched *sched)
> > >  }
> > >
> > >  static void run_one_test(struct perf_sched *sched)
> > > +       EXCLUSIVE_LOCKS_REQUIRED(sched->work_done_wait_mutex)
> > > +       EXCLUSIVE_LOCKS_REQUIRED(sched->start_work_mutex)
> > >  {
> > >         u64 T0, T1, delta, avg_delta, fluct;
> > >
> > > @@ -3314,6 +3320,8 @@ static int perf_sched__replay(struct perf_sched *sched)
> > >         for (i = 0; i < sched->replay_repeat; i++)
> > >                 run_one_test(sched);
> > >
> > > +       mutex_unlock(&sched->start_work_mutex);
> > > +       mutex_unlock(&sched->work_done_wait_mutex);
> >
> > But this would wake up the replay tasks and let them burn cpus unnecessarily.
> > Maybe we can make them exit at the moment.
>
> I think I've stumbled on a can of worms. Why would you spin and not
> use a condition variable? Anyway, I can remove this by just saying
> this function leaves these locked.

I think you can add a boolean variable and set it before unlocking the
mutexes.  In the thread body, it can check the variable and exit.


>
> >
> > >         return 0;
> > >  }
> > >
> > > diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> > > index 3757292bfe86..e832f04e3076 100644
> > > --- a/tools/perf/builtin-top.c
> > > +++ b/tools/perf/builtin-top.c
> > > @@ -196,6 +196,7 @@ static void perf_top__record_precise_ip(struct perf_top *top,
> > >                                         struct hist_entry *he,
> > >                                         struct perf_sample *sample,
> > >                                         struct evsel *evsel, u64 ip)
> > > +       EXCLUSIVE_LOCKS_REQUIRED(he->hists->lock)
> > >  {
> > >         struct annotation *notes;
> > >         struct symbol *sym = he->ms.sym;
> > > @@ -724,13 +725,13 @@ static void *display_thread(void *arg)
> > >  static int hist_iter__top_callback(struct hist_entry_iter *iter,
> > >                                    struct addr_location *al, bool single,
> > >                                    void *arg)
> > > +       EXCLUSIVE_LOCKS_REQUIRED(iter->he->hists->lock)
> > >  {
> > >         struct perf_top *top = arg;
> > > -       struct hist_entry *he = iter->he;
> > >         struct evsel *evsel = iter->evsel;
> > >
> > >         if (perf_hpp_list.sym && single)
> > > -               perf_top__record_precise_ip(top, he, iter->sample, evsel, al->addr);
> > > +               perf_top__record_precise_ip(top, iter->he, iter->sample, evsel, al->addr);
> > >
> > >         hist__account_cycles(iter->sample->branch_stack, al, iter->sample,
> > >                      !(top->record_opts.branch_stack & PERF_SAMPLE_BRANCH_ANY),
> >
> > Looks like a separate change.
>
> This is subtle and relates to how the thread safety pass in clang is
> implemented. I'll waffle but the TL;DR is that without this change we
> can't enable Wthread-safety so I'd say it is part of the same change.
> The waffley bit:
>
> Thread safety checking puts the annotation on to the variable and not
> the type. We know that:
> const char *x = "hi";
> char *y = x;
> will give a compile time error on the assignment to y as const-ness
> was lost. With the thread safety checks you could have:
> char *x PT_GUARDED_BY(lock) = ...;
> char *y = x;
> And if you used x without holding "lock" you'd get an error but you
> wouldn't get the same error from y, even though behind the scenes it
> is the same memory. It is the same case here, on entry we know that
> "iter->he->hists->lock" is held but the assignment to "he" means clang
> doesn't know that "he->hists->lock" is held. This then fails the check
> on perf_top__record_precise_ip that the lock be held as we pass "he"
> rather than "iter->he".

Oh, I mean this perf top change can be separated from perf sched.

Thanks,
Namhyung

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