linux-kernel - Re: [PATCH-queue/locking/core] locking/mutex: Unify yield_to_waiter & waiter

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Date:   Thu, 18 Aug 2016 18:28:49 -0700
From:   Jason Low <jason.low2@....com>
To:     Waiman Long <Waiman.Long@....com>
Cc:     jason.low2@....com, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Imre Deak <imre.deak@...el.com>, jason.low2@...com
Subject: Re: [PATCH-queue/locking/core] locking/mutex: Unify yield_to_waiter
 & waiter_spinning

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@....com>
> ---
>  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@....com>