Re: [PATCH] block: simplify write-threshold and drop write notifiers

[Vladimir Sementsov-Ogievskiy]

05.05.2021 13:10, Stefan Hajnoczi wrote:
> On Thu, Apr 22, 2021 at 01:09:50AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > @@ -1981,8 +1985,15 @@ bdrv_co_write_req_prepare(BdrvChild *child, int64_t 
> > offset, int64_t bytes,
> >           } else {
> >               assert(child->perm & BLK_PERM_WRITE);
> >           }
> > -        return notifier_with_return_list_notify(&bs->before_write_notifiers,
> > -                                                req);
> > +        write_threshold = qatomic_read(&bs->write_threshold_offset);
> > +        if (write_threshold > 0 && offset + bytes > write_threshold) {
> > +            qapi_event_send_block_write_threshold(
> > +                bs->node_name,
> > +                offset + bytes - write_threshold,
> > +                write_threshold);
> > +            qatomic_set(&bs->write_threshold_offset, 0);
>
> It's safer to reset the threshold before emitting the event. That way
> there is no race with the QMP client setting a new threshold via
> bdrv_write_threshold_is_set(). I guess the race is possible since
> qatomic is used and there is no lock.
>
> I like the idea of dropping write notifiers:
> Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

Sorry, this is an outdated patch, I've sent a v2:

 [PATCH v2 0/9] block: refactor write threshold

--
Best regards,
Vladimir