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

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Date:   Tue, 14 Feb 2017 07:59:48 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 11:04:57AM +0100, Peter Zijlstra wrote:
> 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.

Indeed. I thought it's safe since it's within rq locked and cannot be
reused. But I was wrong but it can be unlocked in ttwu_do_activate().
I will fix it. Thank you very much.