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

[Kirill Tkhai]

Ð ÐÑ, 17/02/2015 Ð 13:12 +0100, Peter Zijlstra ÐÐÑÐÑ:
> On Tue, Feb 17, 2015 at 01:47:01PM +0300, Kirill Tkhai wrote:
> > 
> > We migrate a task using TASK_ON_RQ_MIGRATING state of on_rq:
> > 
> > 	raw_spin_lock(&old_rq->lock);
> > 	deactivate_task(old_rq, p, 0);
> > 	p->on_rq = TASK_ON_RQ_MIGRATING;
> > 	set_task_cpu(p, new_cpu);
> > 	raw_spin_unlock(&rq->lock);
> > 
> > I.e.:
> > 
> > 	write TASK_ON_RQ_MIGRATING
> > 	smp_wmb() (in __set_task_cpu)
> > 	write new_cpu
> > 
> > But {,__}task_rq_lock() don't use smp_rmb(), and they may see
> > the cpu and TASK_ON_RQ_MIGRATING in opposite order. In this case
> > {,__}task_rq_lock() lock new_rq before the task is actually queued
> > on it.
> 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index fc12a1d..a42fb88 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -319,8 +319,12 @@ static struct rq *task_rq_lock(struct task_struct *p, unsigned long *flags)
> >  		raw_spin_lock_irqsave(&p->pi_lock, *flags);
> >  		rq = task_rq(p);
> >  		raw_spin_lock(&rq->lock);
> > -		if (likely(rq == task_rq(p) && !task_on_rq_migrating(p)))
> > -			return rq;
> > +		if (likely(rq == task_rq(p))) {
> > +			/* Pairs with smp_wmb() in __set_task_cpu() */
> 
> That comment really is insufficient; but aside from that:
> 
> If we observe the old cpu value we've just acquired the old rq->lock and
> therefore we must observe the new cpu value and retry -- we don't care
> about the migrate value in this case.
> 
> If we observe the new cpu value, we've acquired the new rq->lock and its
> ACQUIRE will pair with the WMB to ensure we see the migrate value.

Yes, I warried about new_cpu case.

So, spin_lock() implies smp_rmb(). I used to think it does not do
(I was confused by smp_mb__before_spin_lock(), but it's for STORE).

Thanks for the explanation :)


> So I think the current code is correct; albeit it could use a comment.
> 
> > +			smp_rmb();
> > +			if (likely(!task_on_rq_migrating(p)))
> > +				return rq;
> > +		}
> 
> 
> ---
> Subject: sched: Clarify ordering between task_rq_lock() and move_queued_task()
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Tue Feb 17 13:07:38 CET 2015
> 
> There was a wee bit of confusion around the exact ordering here;
> clarify things.
> 
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Reported-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> --- 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);


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