Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil

[Davidlohr Bueso]

I messed up something such that waiman was not in the thread. Ccing.

> On Thu, 29 Nov 2018, Waiman Long wrote:
>
> > That can be costly for x86 which will now have 2 locked instructions.
>
> Yeah, and when used as an actual queue we should really start to notice.
> Some users just have a single task in the wake_q because avoiding the cost
> of wake_up_process() with locks held is significant.
>
> How about instead of adding the barrier before the cmpxchg, we do it
> in the failed branch, right before we return. This is the uncommon
> path.
>
> Thanks,
> Davidlohr
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 091e089063be..0d844a18a9dc 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -408,8 +408,14 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
> 	 * This cmpxchg() executes a full barrier, which pairs with the full
> 	 * barrier executed by the wakeup in wake_up_q().
> 	 */
> -	if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
> +	if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL)) {
> +		/*
> +		 * Ensure, that when the cmpxchg() fails, the corresponding
> +		 * wake_up_q() will observe our prior state.
> +		 */
> +		smp_mb__after_atomic();
> 		return;
> +	}
>
> 	get_task_struct(task);