Hi,
On Tue, 20 May 2014 09:53:15 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, May 20, 2014 at 09:08:53AM +0400, Kirill Tkhai wrote:
> >
> >
> > 20.05.2014, 04:00, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>:
> > > On Mon, May 19, 2014 at 11:31:19PM +0400, Kirill Tkhai wrote:
> > >
> > >> @@ -513,9 +513,17 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
> > >> struct sched_dl_entity,
> > >> dl_timer);
> > >> struct task_struct *p = dl_task_of(dl_se);
> > >> - struct rq *rq = task_rq(p);
> > >> + struct rq *rq;
> > >> +again:
> > >> + rq = task_rq(p);
> > >> raw_spin_lock(&rq->lock);
> > >>
> > >> + if (unlikely(rq != task_rq(p))) {
> > >> + /* Task was moved, retrying. */
> > >> + raw_spin_unlock(&rq->lock);
> > >> + goto again;
> > >> + }
> > >> +
> > >
> > > That thing is called: rq = __task_rq_lock(p);
> >
> > But p->pi_lock is not held. The problem is __task_rq_lock() has lockdep assert.
> > Should we change it?
>
> Ok, so now that I'm awake ;-)
>
> So the trivial problem as described by your initial changelog isn't
> right, because we cannot call sched_setaffinity() on deadline tasks, or
> rather we can, but we can't actually change the affinity mask.
>
Well, if we disable AC we can. And I was able to recreate that race in
that case.
> Now I suppose the problem can still actually happen when you change the
> root domain and trigger a effective affinity change that way.
>
Yeah, I think here too.
> That said, no leave it as you proposed, adding a *task_rq_lock() variant
> without lockdep assert in will only confuse things, as normally we
> really should be also taking ->pi_lock.
>
> The only reason we don't strictly need ->pi_lock now is because we're
> guaranteed to have p->state == TASK_RUNNING here and are thus free of
> ttwu races.
Maybe we could add this as part of the comment.
Thanks,
- Juri
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