linux-kernel - Re: [PATCH] sched: fix priority leakage in pick_next_highest_task

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Date:	Tue, 17 May 2011 10:28:35 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] sched: fix priority leakage in pick_next_highest_task_rt()

On Mon, May 16, 2011 at 8:55 PM, Hillf Danton <dhillf@...il.com> wrote:
> When picking the second highest RT task for a given runqueue, if no
> task found after scanning the queue of priority == idx, the next idx
> should also be checked even in case that next is already existing, or
> the window of priority leakage could be opened.

I don't see what kind of problem you patch will fix.
And mind explaining how priority leakage could happen?

Thanks,
Yong

>
> Signed-off-by: Hillf Danton <dhillf@...il.com>
> ---
>
> --- a/kernel/sched_rt.c 2011-04-27 11:48:50.000000000 +0800
> +++ b/kernel/sched_rt.c 2011-05-16 19:58:42.000000000 +0800
> @@ -1166,6 +1166,8 @@ static struct task_struct *pick_next_hig
>        int idx;
>
>        for_each_leaf_rt_rq(rt_rq, rq) {
> +               struct task_struct *this;
> +
>                array = &rt_rq->active;
>                idx = sched_find_first_bit(array->bitmap);
>  next_idx:
> @@ -1173,6 +1175,7 @@ next_idx:
>                        continue;
>                if (next && next->prio < idx)
>                        continue;
> +               this = NULL;
>                list_for_each_entry(rt_se, array->queue + idx, run_list) {
>                        struct task_struct *p;
>
> @@ -1181,11 +1184,15 @@ next_idx:
>
>                        p = rt_task_of(rt_se);
>                        if (pick_rt_task(rq, p, cpu)) {
> -                               next = p;
> +                               this = p;
>                                break;
>                        }
>                }
> -               if (!next) {
> +               if (this != NULL)
> +                       next = this;
> +               else {  /*
> +                        * we have to check next idx even if next != NULL
> +                        */
>                        idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
>                        goto next_idx;
>                }
>



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