linux-kernel - Re: [RFC -v7 PATCH 4/7] Add yield_to(task, preempt) functionality.

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Date:	Mon, 31 Jan 2011 12:49:02 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Rik van Riel <riel@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Avi Kiviti <avi@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Chris Wright <chrisw@...s-sol.org>, ttracy@...hat.com,
	"Nakajima, Jun" <jun.nakajima@...el.com>
Subject: Re: [RFC -v7 PATCH 4/7] Add yield_to(task, preempt) functionality.

On Wed, 2011-01-26 at 17:21 -0500, Rik van Riel wrote:
> +bool __sched yield_to(struct task_struct *p, bool preempt)
> +{
> +       struct task_struct *curr = current;
> +       struct rq *rq, *p_rq;
> +       unsigned long flags;
> +       bool yielded = 0;
> +
> +       local_irq_save(flags);
> +       rq = this_rq();
> +
> +again:
> +       p_rq = task_rq(p);
> +       double_rq_lock(rq, p_rq);
> +       while (task_rq(p) != p_rq) {
> +               double_rq_unlock(rq, p_rq);
> +               goto again;
> +       }
> +
> +       if (!curr->sched_class->yield_to_task)
> +               goto out;
> +
> +       if (curr->sched_class != p->sched_class)
> +               goto out;
> +
> +       if (task_running(p_rq, p) || p->state)
> +               goto out;
> +
> +       yielded = curr->sched_class->yield_to_task(rq, p, preempt);
> +
> +out:
> +       double_rq_unlock(rq, p_rq);
> +       local_irq_restore(flags);
> +
> +       if (yielded)
> +               yield();
> +
> +       return yielded;
> +}
> +EXPORT_SYMBOL_GPL(yield_to); 

yield() will again acquire rq->lock.. not not simply have
->yield_to_task() do everything required and make that an unconditional
schedule()?
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