Re: [PATCH-queue/locking/core] locking/mutex: Unify yield_to_waiter & waiter_spinning

From: Jason Low
Date: Thu Aug 18 2016 - 21:33:31 EST


On Thu, 2016-08-18 at 21:15 -0400, Waiman Long wrote:
> Both waiter_spinning and yield_to_waiter are used for somewhat similar
> purpose. The waiter_spinning flag is used in CONFIG_MUTEX_SPIN_ON_OWNER
> to make optimistic spinner yield to spinning waiter; whereas the
> yield_to_waiter is used in !CONFIG_MUTEX_SPIN_ON_OWNER to make new
> incoming mutex locker to yield to the sleeping waiter.
>
> This patch unifies these 2 flags into a single yield_to_waiter
> flag that is used in both CONFIG_MUTEX_SPIN_ON_OWNER and
> !CONFIG_MUTEX_SPIN_ON_OWNER codes.
>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
> ---
> include/linux/mutex.h | 8 ++++++--
> kernel/locking/mutex.c | 12 ++++++------
> 2 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> index 988c020..2b3dcdb 100644
> --- a/include/linux/mutex.h
> +++ b/include/linux/mutex.h
> @@ -57,8 +57,12 @@ struct mutex {
> #endif
> #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> struct optimistic_spin_queue osq; /* Spinner MCS lock */
> - int waiter_spinning;
> -#elif defined(CONFIG_SMP)
> +#endif
> +#ifdef CONFIG_SMP
> + /*
> + * Used by both CONFIG_MUTEX_SPIN_ON_OWNER (depends on CONFIG_SMP) &
> + * !CONFIG_MUTEX_SPIN_ON_OWNER codes.
> + */
> int yield_to_waiter;
> #endif
> #ifdef CONFIG_DEBUG_MUTEXES
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index c4261fa..7218835 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -55,8 +55,8 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
> mutex_clear_owner(lock);
> #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> osq_lock_init(&lock->osq);
> - lock->waiter_spinning = false;
> -#elif defined(CONFIG_SMP)
> +#endif
> +#ifdef CONFIG_SMP
> lock->yield_to_waiter = false;
> #endif
>
> @@ -351,7 +351,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
> * Turn on the waiter spinning flag to discourage the spinner
> * from getting the lock.

Might want to update this comment to "Turn on the yield to waiter flag
to discourage optimistic spinners from stealing the lock."

Besides that:

Acked-by: Jason Low <jason.low2@xxxxxxx>