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

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Message-ID: <CAM9d7cjCO8e7nbrtrcy4rsbexemQ94=XK+b5byMFFTDDgoJ2eg@mail.gmail.com>
Date:   Wed, 29 Nov 2023 16:55:52 -0800
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>,
        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 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?


> 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.

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
>