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

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 14 Sep 2015, Peter Zijlstra wrote:

>On Mon, Sep 14, 2015 at 12:37:23AM -0700, Davidlohr Bueso wrote:
>>	/*
>> +	 * 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.

Well, I was just taking advantage of removing the upper barrier. Considering
that the formal semantics, you are right that we need not actual acquire per-se
(ie for node->next) but instead merely ensure a barrier in wake_q_add(). This is
kind of why I had hinted of going full _relaxed(). We could also rephrase the
comment, something like:

      * Use ACQUIRE semantics to add the next pointer, such that
      * wake_q_add() implies a full barrier. This pairs with the
      * write barrier implied by the wakeup in wake_up_list().
      */

What do you think?