linux-kernel - Re: [PATCH v2 8/9] sched: Don't reinvent the wheel but use existing llist API

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Message-ID: <20170213100457.GL6515@twins.programming.kicks-ass.net>
Date:   Mon, 13 Feb 2017 11:04:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     mingo@...nel.org, neilb@...e.de, nab@...ux-iscsi.org,
        viro@...iv.linux.org.uk, ying.huang@...el.com, oleg@...hat.com,
        shli@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 8/9] sched: Don't reinvent the wheel but use existing
 llist API

On Mon, Feb 13, 2017 at 04:21:08PM +0900, Byungchul Park wrote:
> Although llist provides proper APIs, they are not used. Make them used.
> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/sched/core.c | 13 ++-----------
>  1 file changed, 2 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d01f9d0..417060b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1783,17 +1783,8 @@ void sched_ttwu_pending(void)
>  	raw_spin_lock_irqsave(&rq->lock, flags);
>  	rq_pin_lock(rq, &rf);
>  
> -	while (llist) {
> -		int wake_flags = 0;
> -
> -		p = llist_entry(llist, struct task_struct, wake_entry);
> -		llist = llist_next(llist);
> -
> -		if (p->sched_remote_wakeup)
> -			wake_flags = WF_MIGRATED;
> -
> -		ttwu_do_activate(rq, p, wake_flags, &rf);
> -	}
> +	llist_for_each_entry(p, llist, wake_entry)
> +		ttwu_do_activate(rq, p, p->sched_remote_wakeup ? WF_MIGRATED : 0, &rf);

I think this suffers the exact same problem the others did. After
ttwu_do_activate() the llist entry can be reused, so doing list_next()
after it is flaky.