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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Sat, 2 Dec 2023 15:54:59 -0800
Message-ID: <CAM9d7cjO2vxa_bvKFrVpX2PViHqZy_dtyf28EaSACwKv6BEHiw@mail.gmail.com>
Subject: Re: [PATCH v5 01/50] perf comm: Use regular mutex
To:     Ian Rogers <irogers@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Nick Terrell <terrelln@fb.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kajol Jain <kjain@linux.ibm.com>,
        Athira Rajeev <atrajeev@linux.vnet.ibm.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Vincent Whitchurch <vincent.whitchurch@axis.com>,
        "Steinar H. Gunderson" <sesse@google.com>,
        Liam Howlett <liam.howlett@oracle.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Colin Ian King <colin.i.king@gmail.com>,
        Dmitrii Dolgov <9erthalion6@gmail.com>,
        Yang Jihong <yangjihong1@huawei.com>,
        Ming Wang <wangming01@loongson.cn>,
        James Clark <james.clark@arm.com>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        Sean Christopherson <seanjc@google.com>,
        Leo Yan <leo.yan@linaro.org>,
        Ravi Bangoria <ravi.bangoria@amd.com>,
        German Gomez <german.gomez@arm.com>,
        Changbin Du <changbin.du@huawei.com>,
        Paolo Bonzini <pbonzini@redhat.com>, Li Dong <lidong@vivo.com>,
        Sandipan Das <sandipan.das@amd.com>,
        liuwenyu <liuwenyu7@huawei.com>, linux-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org,
        Guilherme Amadio <amadio@gentoo.org>
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On Thu, Nov 30, 2023 at 10:28 AM Ian Rogers <irogers@google.com> wrote:
>
> On Wed, Nov 29, 2023 at 4:56 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Mon, Nov 27, 2023 at 2:09 PM Ian Rogers <irogers@google.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.

Can it be like this?

#ifdef RWS_ERRORCHECK
#define RWSEM_INITIALIZER  { .lock = PTHREAD_MUTEX_INITIALIZER, }
#else
#define RWSEM_INITIALIZER  { .lock = PTHREAD_RWLOCK_INITIALIZER, }
#endif

>
> > > Signed-off-by: Ian Rogers <irogers@google.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.

I don't know but there might be a reason to use global locks.
Then we need to support the initialization and it'd be better
to make it easier to handle internal changes like this.

Thanks,
Namhyung