Re: [Qemu-block] [PATCH 07/12] nbd: Increase bs->in_flight during AioContext switch

[Kevin Wolf]

Am 18.02.2019 um 18:22 hat Paolo Bonzini geschrieben:
> On 18/02/19 17:18, Kevin Wolf wrote:
> > +            /* aio_ctx_switch is only supposed to be set if we're sitting 
> > in
> > +             * the qio_channel_yield() below. */
> > +            assert(!*aio_ctx_switch);
> >              bdrv_dec_in_flight(bs);
> >              qio_channel_yield(ioc, G_IO_IN);
> > -            bdrv_inc_in_flight(bs);
> > +            if (*aio_ctx_switch) {
> > +                /* nbd_client_attach_aio_context() already increased 
> > in_flight
> > +                 * when scheduling this coroutine for reentry */
> > +                *aio_ctx_switch = false;
> > +            } else {
> > +                bdrv_inc_in_flight(bs);
> > +            }
> 
> Hmm, my first thought would have been to do the bdrv_inc_in_flight(bs);
> unconditionally here, and in nbd_connection_entry do the opposite, like
> 
>       if (s->aio_ctx_switch) {
>           s->aio_ctx_switch = false;
>           bdrv_dec_in_flight(bs);
>       }
> 
> but I guess the problem is that then bdrv_drain could hang.
> 
> So my question is:
> 
> 1) is there a testcase that shows the problem with this "obvious"
> refactoring;
> 
> 2) maybe instead of aio_co_schedul-ing client->connection_co and having
> the s->aio_ctx_switch flag, you could go through a bottom half that does
> the bdrv_inc_in_flight and then enters client->connection_co?

Actually, this is going to become a bit ugly, too. I can't just schedule
the BH and return because then the node isn't drained any more when the
BH actually runs - and when it's not drained, we don't know where the
coroutine is, so we can't reenter it.

With an AIO_WAIT_WHILE() in the old thread, it should work, though...

Kevin