Re: Warning from swake_up_all in 4.14.15-rt13 non-RT

[Corey Minyard]

On 03/09/2018 11:46 AM, Peter Zijlstra wrote:
> On Fri, Mar 09, 2018 at 12:04:18PM +0100, Sebastian Andrzej Siewior wrote:
> > +void swake_add_all_wq(struct swait_queue_head *q, struct wake_q_head *wq)
> >   {
> >   	struct swait_queue *curr;
> >   
> >   	while (!list_empty(&q->task_list)) {
> >   
> >   		curr = list_first_entry(&q->task_list, typeof(*curr),
> >   					task_list);
> >   		list_del_init(&curr->task_list);
> > +		wake_q_add(wq, curr->task);
> >   	}
> >   }
> > +EXPORT_SYMBOL(swake_add_all_wq);
> >   
> >   void swake_up(struct swait_queue_head *q)
> >   {
> > @@ -66,25 +62,14 @@ EXPORT_SYMBOL(swake_up);
> >    */
> >   void swake_up_all(struct swait_queue_head *q)
> >   {
> > +	unsigned long flags;
> > +	DEFINE_WAKE_Q(wq);
> >   
> > +	raw_spin_lock_irqsave(&q->lock, flags);
> > +	swake_add_all_wq(q, &wq);
> > +	raw_spin_unlock_irqrestore(&q->lock, flags);
> >   
> > +	wake_up_q(&wq);
> >   }
> >   EXPORT_SYMBOL(swake_up_all);
> This is fundamentally wrong. The whole point of wake_up_all() is that
> _all_ is unbounded and should not ever land in a single critical
> section, be it IRQ or PREEMPT disabled. The above does both.

It seems to me to be better than what was there, certainly more efficient.

And if I understand this correctly it is unbounded when !RT, but it is 
bounded
on RT.

And I'm biased, because it should fix my problem :).

> Yes, wake_up_all() is crap, it is also fundamentally incompatible with
> in-*irq usage. Nothing to be done about that.
>
> So NAK on this.

So what would you suggest? At this point getting rid of all the users of
wake_up_all() from interrupt context is not really an option, though as
an eventual goal it would be good.

-corey