Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 17, 2015 at 01:12:58PM +0100, Peter Zijlstra wrote:
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -341,6 +341,22 @@ static struct rq *task_rq_lock(struct ta
>  		raw_spin_lock_irqsave(&p->pi_lock, *flags);
>  		rq = task_rq(p);
>  		raw_spin_lock(&rq->lock);
> +		/*
> +		 *	move_queued_task()		task_rq_lock()
> +		 *
> +		 *	ACQUIRE (rq->lock)
> +		 *	[S] ->on_rq = MIGRATING		[L] rq = task_rq()
> +		 *	WMB (__set_task_cpu())		ACQUIRE (rq->lock);
> +		 *	[S] ->cpu = new_cpu		[L] task_rq()
> +		 *					[L] ->on_rq
> +		 *	RELEASE (rq->lock)
> +		 *
> +		 * If we observe the old cpu in task_rq_lock, the acquire of
> +		 * the old rq->lock will fully serialize against the stores.
> +		 *
> +		 * If we observe the new cpu in task_rq_lock, the acquire will
> +		 * pair with the WMB to ensure we must then also see migrating.
> +		 */
>  		if (likely(rq == task_rq(p) && !task_on_rq_migrating(p)))
>  			return rq;
>  		raw_spin_unlock(&rq->lock);


Hey Paul, remember this: https://lkml.org/lkml/2014/7/16/310

I just used a creative one :-)

BTW, should we attempt to include that table in memory-barriers.txt like
Mathieu said? As a cheat sheet with references to longer explanations
for the 'interesting' ones?

FWIW, we should probably update that table to include control
dependencies too; we didn't (formally) have those back then I think.

The blob under SMP BARRIER PAIRING does not mention pairing with control
dependencies; and I'm rather sure I've done so.