Messages in this thread | | | Date | Mon, 14 Sep 2015 14:08:06 -0700 | From | Davidlohr Bueso <> | Subject | Re: [PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics |
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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?
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