On Wed, Sep 07, 2022 at 12:23:18PM +0200, Jan Kara wrote:
> On Tue 06-09-22 15:27:51, Keith Busch wrote:
> > On Wed, Aug 03, 2022 at 08:15:04PM +0800, Yu Kuai wrote:
> > > wait_cnt = atomic_dec_return(&ws->wait_cnt);
> > > - if (wait_cnt <= 0) {
> > > - int ret;
> > > + /*
> > > + * For concurrent callers of this, callers should call this function
> > > + * again to wakeup a new batch on a different 'ws'.
> > > + */
> > > + if (wait_cnt < 0 || !waitqueue_active(&ws->wait))
> > > + return true;
> >
> > If wait_cnt is '0', but the waitqueue_active happens to be false due to racing
> > with add_wait_queue(), this returns true so the caller will retry.
>
> Well, note that sbq_wake_ptr() called to obtain 'ws' did waitqueue_active()
> check. So !waitqueue_active() should really happen only if waiter was woken
> up by someone else or so. Not that it would matter much but I wanted to
> point it out.
>
> > The next atomic_dec will set the current waitstate wait_cnt < 0, which
> > also forces an early return true. When does the wake up happen, or
> > wait_cnt and wait_index get updated in that case?
>
> I guess your concern could be rephrased as: Who's going to ever set
> ws->wait_cnt to value > 0 if we ever exit with wait_cnt == 0 due to
> !waitqueue_active() condition?
>
> And that is a good question and I think that's a bug in this patch. I think
> we need something like:
>
> ...
> /*
> * For concurrent callers of this, callers should call this function
> * again to wakeup a new batch on a different 'ws'.
> */
> if (wait_cnt < 0)
> return true;
> /*
> * If we decremented queue without waiters, retry to avoid lost
> * wakeups.
> */
> if (wait_cnt > 0)
> return !waitqueue_active(&ws->wait);
I'm not sure about this part. We've already decremented, so the freed bit is
accounted for against the batch. Returning true here may double-count the freed
bit, right?
> /*
> * When wait_cnt == 0, we have to be particularly careful as we are
> * responsible to reset wait_cnt regardless whether we've actually
> * woken up anybody. But in case we didn't wakeup anybody, we still
> * need to retry.
> */
> ret = !waitqueue_active(&ws->wait);
> wake_batch = READ_ONCE(sbq->wake_batch);
> /*
> * Wake up first in case that concurrent callers decrease wait_cnt
> * while waitqueue is empty.
> */
> wake_up_nr(&ws->wait, wake_batch);
> ...
>
> return ret;
>
> Does this fix your concern Keith?
Other than the above comment, this does appear to address the concern. Thanks!
