Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin()

[Davidlohr Bueso]

On Fri, 12 Feb 2016, Waiman Long wrote:

> This patch adds a new waiter parameter to the mutex_optimistic_spin()
> function to prepare it to be used by a waiter-spinner that doesn't
> need to go into the OSQ as there can only be one waiter-spinner which
> is the head of the waiting queue.
>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> ---
> kernel/locking/mutex.c |   55 ++++++++++++++++++++++++++++++++---------------
> 1 files changed, 37 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 0551c21..3c41448 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -273,11 +273,15 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
>
> /*
>  * Atomically try to take the lock when it is available
> + *
> + * For waiter-spinner, the count needs to be set to -1 first which will be
> + * cleared to 0 later on if the list becomes empty. For regular spinner,
> + * the count will be set to 0.
>  */
> -static inline bool mutex_try_to_acquire(struct mutex *lock)
> +static inline bool mutex_try_to_acquire(struct mutex *lock, int waiter)
> {
> 	return !mutex_is_locked(lock) &&
> -		(atomic_cmpxchg_acquire(&lock->count, 1, 0) == 1);
> +		(atomic_cmpxchg_acquire(&lock->count, 1, waiter ? -1 : 0) == 1);
> }

This can be a really hot path, could we get rid of the waiter check and just
introduce mutex_tro_to_acquire_waiter() or such and set the counter to -1 there?

Thanks,
Davidlohr