Am 11.04.2019 um 16:48 hat Vladimir Sementsov-Ogievskiy geschrieben: > 11.04.2019 17:15, Kevin Wolf wrote: > > 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. > > Don't follow.. Don't we want exactly this, we want BH to be executed while > node is still > drained, as you write in comment?
Yes, exactly. But if bs->in_flight == 0, the AIO_WAIT_WHILE() condition in the drain code could become false, so aio_poll() would not be called again and drain would return even if the BH is still pending. Kevin > > > >>> > >>> Signed-off-by: Kevin Wolf <kw...@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 > > > -- > Best regards, > Vladimir