linux-kernel - Re: [PATCH v5 01/50] perf comm: Use regular mutex

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Message-ID: <CAP-5=fUBr60o22P4Op-J=TPkdfnby9vLetHQZ4UqjuX+nvbG9w@mail.gmail.com>
Date:   Thu, 30 Nov 2023 10:27:56 -0800
From:   Ian Rogers <irogers@...gle.com>
To:     Namhyung Kim <namhyung@...nel.org>
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>,
        Nick Terrell <terrelln@...com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kajol Jain <kjain@...ux.ibm.com>,
        Athira Rajeev <atrajeev@...ux.vnet.ibm.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        "Steinar H. Gunderson" <sesse@...gle.com>,
        Liam Howlett <liam.howlett@...cle.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Colin Ian King <colin.i.king@...il.com>,
        Dmitrii Dolgov <9erthalion6@...il.com>,
        Yang Jihong <yangjihong1@...wei.com>,
        Ming Wang <wangming01@...ngson.cn>,
        James Clark <james.clark@....com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Leo Yan <leo.yan@...aro.org>,
        Ravi Bangoria <ravi.bangoria@....com>,
        German Gomez <german.gomez@....com>,
        Changbin Du <changbin.du@...wei.com>,
        Paolo Bonzini <pbonzini@...hat.com>, Li Dong <lidong@...o.com>,
        Sandipan Das <sandipan.das@....com>,
        liuwenyu <liuwenyu7@...wei.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        Guilherme Amadio <amadio@...too.org>
Subject: Re: [PATCH v5 01/50] perf comm: Use regular mutex

On Wed, Nov 29, 2023 at 4:56 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Mon, Nov 27, 2023 at 2:09 PM Ian Rogers <irogers@...gle.com> wrote:
> >
> > The rwsem is only after used for writing so switch to a mutex that has
> > better error checking.
> >
> > Fixes: 7a8f349e9d14 ("perf rwsem: Add debug mode that uses a mutex")
>
> I think we talked about fixing this separately, no?

Sorry, I'm unclear on an action to do. Currently changing the
RWS_ERRORCHECK in tools/perf/util/rwsem.h will break the build without
this change.

> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >  tools/perf/util/comm.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/perf/util/comm.c b/tools/perf/util/comm.c
> > index afb8d4fd2644..4ae7bc2aa9a6 100644
> > --- a/tools/perf/util/comm.c
> > +++ b/tools/perf/util/comm.c
> > @@ -17,7 +17,7 @@ struct comm_str {
> >
> >  /* Should perhaps be moved to struct machine */
> >  static struct rb_root comm_str_root;
> > -static struct rw_semaphore comm_str_lock = {.lock = PTHREAD_RWLOCK_INITIALIZER,};
> > +static struct mutex comm_str_lock = {.lock = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP,};
>
> IIUC it has a problem with musl libc.  Actually I think it's better to
> hide the field and the pthread initializer under some macro like
> MUTEX_INITIALIZER or DEFINE_MUTEX() like in the kernel.

Will there be enough use to justify this? I think ideally we'd not be
having global locks needing global initializers as we run into
problems like we see in metrics needing to mix counting and sampling.

Thanks,
Ian

> Thanks,
> Namhyung
>
> >
> >  static struct comm_str *comm_str__get(struct comm_str *cs)
> >  {
> > @@ -30,9 +30,9 @@ static struct comm_str *comm_str__get(struct comm_str *cs)
> >  static void comm_str__put(struct comm_str *cs)
> >  {
> >         if (cs && refcount_dec_and_test(&cs->refcnt)) {
> > -               down_write(&comm_str_lock);
> > +               mutex_lock(&comm_str_lock);
> >                 rb_erase(&cs->rb_node, &comm_str_root);
> > -               up_write(&comm_str_lock);
> > +               mutex_unlock(&comm_str_lock);
> >                 zfree(&cs->str);
> >                 free(cs);
> >         }
> > @@ -98,9 +98,9 @@ static struct comm_str *comm_str__findnew(const char *str, struct rb_root *root)
> >  {
> >         struct comm_str *cs;
> >
> > -       down_write(&comm_str_lock);
> > +       mutex_lock(&comm_str_lock);
> >         cs = __comm_str__findnew(str, root);
> > -       up_write(&comm_str_lock);
> > +       mutex_unlock(&comm_str_lock);
> >
> >         return cs;
> >  }
> > --
> > 2.43.0.rc1.413.gea7ed67945-goog
> >