11.04.2019 20:13, Vladimir Sementsov-Ogievskiy wrote:
> 11.04.2019 19:48, Kevin Wolf wrote:
>> 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.
>>
>
> Ah, oops, sorry my English, I read it like "nothing would prevent".
> Understand now, thanks.
Or not again.. We will not return to drain code, as we will loop in
aio_wait_bh_oneshot,
which will not return until BH handled
>
>>>>
>>>>>>
>>>>>> 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
>
>
--
Best regards,
Vladimir
