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

Message ID 20211111162428.2286605-3-hjl.tools@gmail.com
State Committed
Commit 0b82747dc48d5bf0871bdc6da8cb6eec1256355f
Headers show
Series Optimize CAS [BZ #28537] | expand

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent

Commit Message

H.J. Lu Nov. 11, 2021, 4:24 p.m. UTC 
Replace boolean CAS with value CAS to avoid the extra load.
---
 nptl/pthread_mutex_lock.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

Comments

Szabolcs Nagy Nov. 12, 2021, 4:31 p.m. UTC | #1 
The 11/11/2021 08:24, H.J. Lu via Libc-alpha wrote:
> Replace boolean CAS with value CAS to avoid the extra load.

this looks good in general.

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

> ---
>  nptl/pthread_mutex_lock.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> 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)
>  		{
> -		  oldval = mutex->__data.__lock;
> +		  oldval = val;
>  		  continue;
>  		}
>  	      oldval |= FUTEX_WAITERS;
> -- 
> 2.33.1
>
Andreas Schwab Nov. 12, 2021, 6:50 p.m. UTC | #2 
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.
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Patch

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)
 		{
-		  oldval = mutex->__data.__lock;
+		  oldval = val;
 		  continue;
 		}
 	      oldval |= FUTEX_WAITERS;