[PATCH v6 2/4] Avoid extra load with CAS in __pthread_mutex_lock_full [BZ #28537]

[Sunil Pandey]

On Fri, Nov 12, 2021 at 10:52 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
>
> On Nov 11 2021, H.J. Lu wrote:
>
> > diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
> > index 72058c719c..762059b230 100644
> > --- a/nptl/pthread_mutex_lock.c
> > +++ b/nptl/pthread_mutex_lock.c
> > @@ -304,12 +304,12 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
> >            meantime.  */
> >         if ((oldval & FUTEX_WAITERS) == 0)
> >           {
> > -           if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> > -                                                     oldval | FUTEX_WAITERS,
> > -                                                     oldval)
> > -               != 0)
> > +           int val;
> > +           if ((val = atomic_compare_and_exchange_val_acq
> > +                (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> > +                 oldval)) != oldval)
>
> Please move the assignment out of the condition.
>
> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
> "And now for something completely different."

I would like to backport this patch to release branch 2.33 and 2.34

Any comments/suggestions or objections on this.

commit 49302b8fdf9103b6fc0a398678668a22fa19574c
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Nov 11 06:54:01 2021 -0800

    Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]

    Replace boolean CAS with value CAS to avoid the extra load.

    Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>