Re: [PATCH v4 07/20] block/export: stop using is_external in vhost-user-blk server

[Kevin Wolf <kwolf@redhat.com>]

Am 25.04.2023 um 19:27 hat Stefan Hajnoczi geschrieben:
> vhost-user activity must be suspended during bdrv_drained_begin/end().
> This prevents new requests from interfering with whatever is happening
> in the drained section.
> 
> Previously this was done using aio_set_fd_handler()'s is_external
> argument. In a multi-queue block layer world the aio_disable_external()
> API cannot be used since multiple AioContext may be processing I/O, not
> just one.
> 
> Switch to BlockDevOps->drained_begin/end() callbacks.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/export/vhost-user-blk-server.c | 43 ++++++++++++++--------------
>  util/vhost-user-server.c             | 10 +++----
>  2 files changed, 26 insertions(+), 27 deletions(-)
> 
> diff --git a/block/export/vhost-user-blk-server.c b/block/export/vhost-user-blk-server.c
> index 092b86aae4..d20f69cd74 100644
> --- a/block/export/vhost-user-blk-server.c
> +++ b/block/export/vhost-user-blk-server.c
> @@ -208,22 +208,6 @@ static const VuDevIface vu_blk_iface = {
>      .process_msg           = vu_blk_process_msg,
>  };
>  
> -static void blk_aio_attached(AioContext *ctx, void *opaque)
> -{
> -    VuBlkExport *vexp = opaque;
> -
> -    vexp->export.ctx = ctx;
> -    vhost_user_server_attach_aio_context(&vexp->vu_server, ctx);
> -}
> -
> -static void blk_aio_detach(void *opaque)
> -{
> -    VuBlkExport *vexp = opaque;
> -
> -    vhost_user_server_detach_aio_context(&vexp->vu_server);
> -    vexp->export.ctx = NULL;
> -}

So for changing the AioContext, we now rely on the fact that the node to
be changed is always drained, so the drain callbacks implicitly cover
this case, too?

>  static void
>  vu_blk_initialize_config(BlockDriverState *bs,
>                           struct virtio_blk_config *config,
> @@ -272,6 +256,25 @@ static void vu_blk_exp_resize(void *opaque)
>      vu_config_change_msg(&vexp->vu_server.vu_dev);
>  }
>  
> +/* Called with vexp->export.ctx acquired */
> +static void vu_blk_drained_begin(void *opaque)
> +{
> +    VuBlkExport *vexp = opaque;
> +
> +    vhost_user_server_detach_aio_context(&vexp->vu_server);
> +}

Compared to the old code, we're losing the vexp->export.ctx = NULL. This
is correct at this point because after drained_begin we still keep
processing requests until we arrive at a quiescent state.

However, if we detach the AioContext because we're deleting the
iothread, won't we end up with a dangling pointer in vexp->export.ctx?
Or can we be certain that nothing interesting happens before drained_end
updates it with a new valid pointer again?

Kevin