[PATCH v3] nptl: Effectively skip CAS in spinlock loop

[H.J. Lu]

On Tue, Dec 14, 2021 at 4:31 AM Jangwoong Kim via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> The commit:
> "Add LLL_MUTEX_READ_LOCK [BZ #28537]"
> SHA1: d672a98a1af106bd68deb15576710cd61363f7a6
>
> introduced LLL_MUTEX_READ_LOCK, to skip CAS in spinlock loop
> if atomic load fails. But, "continue" inside of do-while loop
> does not skip the evaluation of escape expression, thus CAS
> is not skipped.
>
> Replace do-while with while and skip LLL_MUTEX_TRYLOCK if
> LLL_MUTEX_READ_LOCK fails.
> ---
>  nptl/pthread_mutex_lock.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
> index 47b88a6b5b..06274c68e4 100644
> --- a/nptl/pthread_mutex_lock.c
> +++ b/nptl/pthread_mutex_lock.c
> @@ -138,7 +138,7 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
>           int cnt = 0;
>           int max_cnt = MIN (max_adaptive_count (),
>                              mutex->__data.__spins * 2 + 10);
> -         do
> +         while (LLL_MUTEX_READ_LOCK (mutex) != 0 || LLL_MUTEX_TRYLOCK (mutex) != 0)
>             {
>               if (cnt++ >= max_cnt)
>                 {
> @@ -146,10 +146,7 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
>                   break;
>                 }
>               atomic_spin_nop ();
> -             if (LLL_MUTEX_READ_LOCK (mutex) != 0)
> -               continue;
>             }
> -         while (LLL_MUTEX_TRYLOCK (mutex) != 0);
>
>           mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
>         }
> --
> 2.25.1
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.

-- 
H.J.