Re: [PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Sep 14, 2015 at 12:37:23AM -0700, Davidlohr Bueso wrote:
> The barrier parings for wake-queues are very straightforward, and thus
> we can ease the barrier requirements, for archs that support it, for
> wake_q_add by relying on acquire semantics. As such, (i) we keep the
> pairing structure/logic and (ii) users, such as mqueues, can continue to
> rely on a full barrier after the successful [Rmw].

> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
>  kernel/sched/core.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 6ab415a..7567603 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -523,14 +523,14 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
>  	struct wake_q_node *node = &task->wake_q;
>  
>  	/*
> +	 * Atomically grab the task. If ->wake_q is non-nil (failed cmpxchg)
> +	 * then the task is already queued (by us or someone else) and will
> +	 * get the wakeup due to that.
>  	 *
> +	 * Use acquire semantics to add the next pointer, which pairs with the
> +	 * write barrier implied by the wakeup in wake_up_list().
>  	 */
> +	if (cmpxchg_acquire(&node->next, NULL, WAKE_Q_TAIL))
>  		return;
>  
>  	get_task_struct(task);

I'm not seeing a _why_ on the acquire semantics. Not saying the patch is
wrong, just saying I want words on why acquire is correct.