On 2018-03-09 18:46:05 [+0100], 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.
Is it just about the irqsave() usage or something else? I doubt it is
the list walk. It is still unbound if not called from irq-off region.
But it is now possible, I agree. The wake_q usage should be cheaper
compared to IRQ off+on in each loop. And we wanted to do the wake ups
with enabled interrupts - there is still the list_splice() from that
attempt. Now it can be.
> Yes, wake_up_all() is crap, it is also fundamentally incompatible with
> in-*irq usage. Nothing to be done about that.
I still have (or need) completions which are swait based and do
complete_all(). There are complete_all() caller which wake more than one
waiter (that is PM and crypto from the reports I got once I added the
WARN_ON())).
The in-IRQ usage is !RT only and was there before.
> So NAK on this.
So I need completions to be swait based and do complete_all() from IRQ
(on !RT, not RT). I have this one call which breaks the usage on !RT and
has wake_up_all() in it in vanilla which needs an swait equivalent since
it calls its callback from an rcu-sched section.
Sebastian
