Re: [Qemu-devel] [PULL 16/71] nbd: Increase bs->in_flight during AioContext switch

[Kevin Wolf <kwolf@redhat.com>]

Am 11.04.2019 um 15:40 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 25.02.2019 18:19, Kevin Wolf wrote:
> > bdrv_drain() must not leave connection_co scheduled, so bs->in_flight
> > needs to be increased while the coroutine is waiting to be scheduled
> > in the new AioContext after nbd_client_attach_aio_context().
> 
> Hi!
> 
> I have some questions, could you explain, please?
> 
> "bdrv_drain() must not leave connection_co scheduled" - it's because we want to be
> sure that connection_co yielded from nbd_read_eof, yes?
> 
> But it is guaranteed by aio_wait_bh_oneshot.. Why do we need additioinally inc/dec
> bs->in_flight ?

Without incrementing bs->in_flight, nothing would guarantee that
aio_poll() is called and the BH is actually executed before bdrv_drain()
returns.

Kevin

> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >   block/nbd-client.c | 20 ++++++++++++++++++--
> >   1 file changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/block/nbd-client.c b/block/nbd-client.c
> > index 60f38f0320..bfbaf7ebe9 100644
> > --- a/block/nbd-client.c
> > +++ b/block/nbd-client.c
> > @@ -977,14 +977,30 @@ void nbd_client_detach_aio_context(BlockDriverState *bs)
> >       qio_channel_detach_aio_context(QIO_CHANNEL(client->ioc));
> >   }
> >   
> > +static void nbd_client_attach_aio_context_bh(void *opaque)
> > +{
> > +    BlockDriverState *bs = opaque;
> > +    NBDClientSession *client = nbd_get_client_session(bs);
> > +
> > +    /* The node is still drained, so we know the coroutine has yielded in
> > +     * nbd_read_eof(), the only place where bs->in_flight can reach 0, or it is
> > +     * entered for the first time. Both places are safe for entering the
> > +     * coroutine.*/
> > +    qemu_aio_coroutine_enter(bs->aio_context, client->connection_co);
> > +    bdrv_dec_in_flight(bs);
> > +}
> > +
> >   void nbd_client_attach_aio_context(BlockDriverState *bs,
> >                                      AioContext *new_context)
> >   {
> >       NBDClientSession *client = nbd_get_client_session(bs);
> >       qio_channel_attach_aio_context(QIO_CHANNEL(client->ioc), new_context);
> >   
> > -    /* FIXME Really need a bdrv_inc_in_flight() here */
> > -    aio_co_schedule(new_context, client->connection_co);
> > +    bdrv_inc_in_flight(bs);
> > +
> > +    /* Need to wait here for the BH to run because the BH must run while the
> > +     * node is still drained. */
> > +    aio_wait_bh_oneshot(new_context, nbd_client_attach_aio_context_bh, bs);
> >   }
> >   
> >   void nbd_client_close(BlockDriverState *bs)
> > 
> 
> 
> -- 
> Best regards,
> Vladimir