Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles
From: Peter Zijlstra
Date: Tue Feb 17 2015 - 08:05:41 EST
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);
> + /*
> + * 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;
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.
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