Re: [PATCH 3/4] nbd: make client_close synchronous

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

25.06.2020 17:25, Vladimir Sementsov-Ogievskiy wrote:
> client_close doesn't guarantee that client is closed: nbd_trip() keeps
> reference to it. Let's wait for nbd_trip to finish.
> 
> Without this fix, the following crash is possible:
> 
> - export bitmap through unternal Qemu NBD server
> - connect a client
> - shutdown Qemu
> 
> On shutdown nbd_export_close_all is called, but it actually don't wait
> for nbd_trip() to finish and to release its references. So, export is
> not release, and exported bitmap remains busy, and on try to remove the
> bitmap (which is part of bdrv_close()) the assertion fairs:
> 
> bdrv_release_dirty_bitmap_locked: Assertion `!bdrv_dirty_bitmap_busy(bitmap)' failed
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   nbd/server.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/nbd/server.c b/nbd/server.c
> index 20754e9ebc..5e27a8d31a 100644
> --- a/nbd/server.c
> +++ b/nbd/server.c
> @@ -1419,6 +1419,8 @@ static void client_close(NBDClient *client, bool negotiated)
>       qio_channel_shutdown(client->ioc, QIO_CHANNEL_SHUTDOWN_BOTH,
>                            NULL);
>   
> +    AIO_WAIT_WHILE(client->exp->ctx, client->recv_coroutine);
> +
>       /* Also tell the client, so that they release their reference.  */
>       if (client->close_fn) {
>           client->close_fn(client, negotiated);
> @@ -2450,6 +2452,7 @@ static coroutine_fn void nbd_trip(void *opaque)
>   
>       trace_nbd_trip();
>       if (client->closing) {
> +        client->recv_coroutine = NULL;
>           nbd_client_put(client);
>           return;
>       }
> 

I have a doubt, isn't it possible to hang in AIO_WAIT_WHILE() after this patch?

Do we need aio_wait_kick() in the nbd_trip()? Or may be, something like
aio_wait_kick(), but to wake exactly client->exp->ctx aio context?

Also, why in block/io.c we kick the main context, but not bs->aio_context?


-- 
Best regards,
Vladimir