On Mon, 28 Aug 2017 11:16:48 +1000
Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
> On Sun, 27 Aug 2017 16:12:19 -0700
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index baba290c276b..0b41c8cbeabc 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -986,10 +986,6 @@ static inline int
> > wait_on_page_bit_common(wait_queue_head_t *q,
> >
> > if (likely(test_bit(bit_nr, &page->flags))) {
> > io_schedule();
> > - if (unlikely(signal_pending_state(state, current))) {
> > - ret = -EINTR;
> > - break;
> > - }
> > }
> >
> > if (lock) {
> > @@ -999,6 +995,11 @@ static inline int
> > wait_on_page_bit_common(wait_queue_head_t *q,
> > if (!test_bit(bit_nr, &page->flags))
> > break;
> > }
> > +
> > + if (unlikely(signal_pending_state(state, current))) {
> > + ret = -EINTR;
> > + break;
> > + }
> > }
> >
> > finish_wait(q, wait);
> >
> > but maybe I'm missing something.
> >
> > Nick, comments?
>
> No I don't think you're missing something. We surely could lose our only
> wakeup in this window. So an exclusive waiter has to always make sure
> they propagate the wakeup (regardless of what they do with the contended
> resources itself).
>
> Seems like your fix should solve it. By the look of how wait_on_bit_lock
> is structured, the author probably did think about this case a little
> better than I did :\
BTW. since you are looking at this stuff, one other small problem I remember
with exclusive waiters is that losing to a concurrent locker puts them to
the back of the queue. I think that could be fixed with some small change to
the wait loops (first add to tail, then retries add to head). Thoughts?
Thanks,
Nick
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