Re: [Qemu-devel] [PATCH] nbd/server: attach client channel to the export's AioContext

[Eric Blake <eblake@redhat.com>]

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On 9/11/19 11:15 AM, Sergio Lopez wrote:
> On creation, the export's AioContext is set to the same one as the
> BlockBackend, while the AioContext in the client QIOChannel is left
> untouched.
> 
> As a result, when using data-plane, nbd_client_receive_next_request()
> schedules coroutines in the IOThread AioContext, while the client's
> QIOChannel is serviced from the main_loop, potentially triggering the
> assertion at qio_channel_restart_[read|write].
> 
> To fix this, as soon we have the export corresponding to the client,
> we call qio_channel_attach_aio_context() to attach the QIOChannel
> context to the export's AioContext. This matches with the logic in
> blk_aio_attached().
> 
> RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1748253
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
>  nbd/server.c | 2 ++
>  1 file changed, 2 insertions(+)

I'd like a second opinion from Kevin, but the description makes sense to
me.  I'm happy to queue this through my NBD tree.

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/nbd/server.c b/nbd/server.c
> index 10faedcfc5..51322e2343 100644
> --- a/nbd/server.c
> +++ b/nbd/server.c
> @@ -471,6 +471,7 @@ static int nbd_negotiate_handle_export_name(NBDClient *client,
>      QTAILQ_INSERT_TAIL(&client->exp->clients, client, next);
>      nbd_export_get(client->exp);
>      nbd_check_meta_export(client);
> +    qio_channel_attach_aio_context(client->ioc, client->exp->ctx);
>  
>      return 0;
>  }
> @@ -673,6 +674,7 @@ static int nbd_negotiate_handle_info(NBDClient *client, uint16_t myflags,
>          QTAILQ_INSERT_TAIL(&client->exp->clients, client, next);
>          nbd_export_get(client->exp);
>          nbd_check_meta_export(client);
> +        qio_channel_attach_aio_context(client->ioc, exp->ctx);
>          rc = 1;
>      }
>      return rc;
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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