Re: wait_on_page_bit_common(TASK_KILLABLE, EXCLUSIVE) can miss wakeup?

[Oleg Nesterov <oleg@redhat.com>]

On 06/30, Nicholas Piggin wrote:
> Excerpts from Oleg Nesterov's message of June 30, 2020 12:02 am:
> > On 06/29, Nicholas Piggin wrote:
> >>
> >> prepare_to_wait_event() has a pretty good pattern (and comment), I would
> >> favour using that (test the signal when inserting on the waitqueue).
> >>
> >> @@ -1133,6 +1133,15 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
> >>         for (;;) {
> >>                 spin_lock_irq(&q->lock);
> >>
> >> +               if (signal_pending_state(state, current)) {
> >> +                       /* Must not lose an exclusive wake up, see
> >> +                        * prepare_to_wait_event comment */
> >> +                       list_del_init(&wait->entry);
> >> +                       spin_unlock_irq(&q->lock);
> >> +                       ret = -EINTR;
> >
> > Basically this is what my patch in the 1st email does. But note that we can't
> > just set "ret = -EINTR" here, we will need to clear "ret" if test_and_set_bit()
> > below succeeds. That is why I used another "int intr" variable.
>
> You snipped off one more important line of context. No such games are
> required AFAIKS.

		for (;;) {
			spin_lock_irq(&q->lock);
	 
	+               if (signal_pending_state(state, current)) {
	+                       /* Must not lose an exclusive wake up, see
	+                        * prepare_to_wait_event comment */
	+                       list_del_init(&wait->entry);
	+                       spin_unlock_irq(&q->lock);
	+                       ret = -EINTR;
	+                       break;
	+               }


so wait_on_page_bit_common() just returns -EINTR if signal_pending_state() == T.
And this is wrong if "current" was already woken up by unlock_page().

That is why ___wait_event() checks the condition even if prepare_to_wait_event()
returns -EINTR. The comment in prepare_to_wait_event() tries to explain this.

Oleg.