linux-kernel - Re: [PATCH v2 07/18] perf record: Update use of pthread mutex

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Message-ID: <CAP-5=fWZaN94M1OS2xRYo6M012mi-cRP_boeBi=p6VPnhbOS7g@mail.gmail.com>
Date:   Wed, 24 Aug 2022 08:04:53 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Adrian Hunter <adrian.hunter@...el.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>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        André Almeida <andrealmeid@...lia.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
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        Athira Rajeev <atrajeev@...ux.vnet.ibm.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
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        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Remi Bernon <rbernon@...eweavers.com>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        bpf@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v2 07/18] perf record: Update use of pthread mutex

On Wed, Aug 24, 2022 at 3:15 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
>
> On 24/08/22 01:09, Ian Rogers wrote:
> > Switch to the use of mutex wrappers that provide better error checking
> > for synth_lock.
>
> It would be better to distinguish patches that make drop-in
> replacements from patches like this that change logic.

The only change here is PTHREAD_MUTEX_INITIALIZER to mutex_init
because PTHREAD_MUTEX_INITIALIZER doesn't have error checking. The two
are morally equivalent and so no logic change is intended - although
one may inadvertently happen by the moving initialization from compile
time to runtime.

> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >  tools/perf/builtin-record.c | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> >
> > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > index 4713f0f3a6cf..02eb85677e99 100644
> > --- a/tools/perf/builtin-record.c
> > +++ b/tools/perf/builtin-record.c
> > @@ -21,6 +21,7 @@
> >  #include "util/evsel.h"
> >  #include "util/debug.h"
> >  #include "util/mmap.h"
> > +#include "util/mutex.h"
> >  #include "util/target.h"
> >  #include "util/session.h"
> >  #include "util/tool.h"
> > @@ -608,17 +609,18 @@ static int process_synthesized_event(struct perf_tool *tool,
> >       return record__write(rec, NULL, event, event->header.size);
> >  }
> >
> > +static struct mutex synth_lock;
> > +
> >  static int process_locked_synthesized_event(struct perf_tool *tool,
> >                                    union perf_event *event,
> >                                    struct perf_sample *sample __maybe_unused,
> >                                    struct machine *machine __maybe_unused)
> >  {
> > -     static pthread_mutex_t synth_lock = PTHREAD_MUTEX_INITIALIZER;
> >       int ret;
> >
> > -     pthread_mutex_lock(&synth_lock);
> > +     mutex_lock(&synth_lock);
> >       ret = process_synthesized_event(tool, event, sample, machine);
> > -     pthread_mutex_unlock(&synth_lock);
> > +     mutex_unlock(&synth_lock);
> >       return ret;
> >  }
> >
> > @@ -1917,6 +1919,7 @@ static int record__synthesize(struct record *rec, bool tail)
> >       }
> >
> >       if (rec->opts.nr_threads_synthesize > 1) {
> > +             mutex_init(&synth_lock, /*pshared=*/false);
>
> It would be better to have mutex_init() and mutex_init_shared()
> since /*pshared=*/true is rarely used.

Will change in v3.

Thanks,
Ian

> >               perf_set_multithreaded();
> >               f = process_locked_synthesized_event;
> >       }
> > @@ -1930,8 +1933,10 @@ static int record__synthesize(struct record *rec, bool tail)
> >                                                   rec->opts.nr_threads_synthesize);
> >       }
> >
> > -     if (rec->opts.nr_threads_synthesize > 1)
> > +     if (rec->opts.nr_threads_synthesize > 1) {
> >               perf_set_singlethreaded();
> > +             mutex_destroy(&synth_lock);
> > +     }
> >
> >  out:
> >       return err;
>