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

[Ian Rogers]

On Wed, Nov 29, 2023 at 4:56 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> On Mon, Nov 27, 2023 at 2:09 PM Ian Rogers <irogers@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >  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
> >