diff mbox series

[v3] nptl: Effectively skip CAS in spinlock loop

Message ID 20211214123051.800-1-6812skiii@gmail.com
State Superseded
Headers
Series [v3] nptl: Effectively skip CAS in spinlock loop |

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit success Build for i686

Commit Message

Jangwoong Kim Dec. 14, 2021, 12:30 p.m. UTC 
  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(-)

Comments

H.J. Lu Jan. 20, 2022, 12:53 p.m. UTC | #1 
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.
diff mbox series

Patch

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;
 	}