Re: [PATCH v3 2/2] vhost-user-blk: delay vhost_user_blk_disconnect

[Raphael Norwitz]

I'm mostly happy with this. A couple comments.

On Wed, May 20, 2020 at 11:54 AM Dima Stepanov <address@hidden> wrote:
>
> A socket write during vhost-user communication may trigger a disconnect
> event, calling vhost_user_blk_disconnect() and clearing all the
> vhost_dev structures holding data that vhost-user functions expect to
> remain valid to roll back initialization correctly. Delay the cleanup to
> keep vhost_dev structure valid.
> There are two possible states to handle:
> 1. RUN_STATE_PRELAUNCH: skip bh oneshot call and perform disconnect in
> the caller routine.
> 2. RUN_STATE_RUNNING: delay by using bh
>
> BH changes are based on the similar changes for the vhost-user-net
> device:
>   commit e7c83a885f865128ae3cf1946f8cb538b63cbfba
>   "vhost-user: delay vhost_user_stop"
>
I'd also give credit to Li Feng here - he sent a similar patch:

https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg02255.html

> Signed-off-by: Dima Stepanov <address@hidden>
> ---
>  hw/block/vhost-user-blk.c | 49 
> +++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 43 insertions(+), 6 deletions(-)
>
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index 9d8c0b3..447fc9c 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -337,11 +337,6 @@ static void vhost_user_blk_disconnect(DeviceState *dev)
>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>      VHostUserBlk *s = VHOST_USER_BLK(vdev);
>
> -    if (!s->connected) {
> -        return;
> -    }
> -    s->connected = false;
> -
>      if (s->dev.started) {
>          vhost_user_blk_stop(vdev);
>      }
> @@ -349,6 +344,19 @@ static void vhost_user_blk_disconnect(DeviceState *dev)
>      vhost_dev_cleanup(&s->dev);
>  }
>
> +static void vhost_user_blk_event(void *opaque, QEMUChrEvent event);
> +
> +static void vhost_user_blk_chr_closed_bh(void *opaque)
> +{
> +    DeviceState *dev = opaque;
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> +    VHostUserBlk *s = VHOST_USER_BLK(vdev);
> +
> +    vhost_user_blk_disconnect(dev);
> +    qemu_chr_fe_set_handlers(&s->chardev, NULL, NULL, vhost_user_blk_event,
> +            NULL, opaque, NULL, true);
> +}
> +
>  static void vhost_user_blk_event(void *opaque, QEMUChrEvent event)
>  {
>      DeviceState *dev = opaque;
> @@ -363,7 +371,28 @@ static void vhost_user_blk_event(void *opaque, 
> QEMUChrEvent event)
>          }
>          break;
>      case CHR_EVENT_CLOSED:
> -        vhost_user_blk_disconnect(dev);
> +        /*
> +         * A close event may happen during a read/write, but vhost
> +         * code assumes the vhost_dev remains setup, so delay the
> +         * stop & clear. There are two possible paths to hit this
> +         * disconnect event:
> +         * 1. When VM is in the RUN_STATE_PRELAUNCH state. The
> +         * vhost_user_blk_device_realize() is a caller.
> +         * 2. In tha main loop phase after VM start.
> +         *
> +         * For p2 the disconnect event will be delayed. We can't
> +         * do the same for p1, because we are not running the loop
> +         * at this moment. So just skip this step and perform
> +         * disconnect in the caller function.
> +         */
> +        if (s->connected && runstate_is_running()) {
> +            AioContext *ctx = qemu_get_current_aio_context();
> +
> +            qemu_chr_fe_set_handlers(&s->chardev, NULL, NULL, NULL, NULL,
> +                    NULL, NULL, false);
> +            aio_bh_schedule_oneshot(ctx, vhost_user_blk_chr_closed_bh, 
> opaque);
> +        }
> +        s->connected = false;
>          break;
>      case CHR_EVENT_BREAK:
>      case CHR_EVENT_MUX_IN:
> @@ -428,6 +457,14 @@ reconnect:
>
>      ret = vhost_dev_get_config(&s->dev, (uint8_t *)&s->blkcfg,
>                                 sizeof(struct virtio_blk_config));

I find checking s->connected before ret a little confusing. I think we
should also enforce a reconnect if s->connected is false. AFIK if
s->connected is false, ret must also be less than 0, but to be safe
I’d prefer something like:

if (ret < 0 || !s->connected) {
    if (!s->connected) {
        /*
         * Perform disconnect before making reconnect. More detailed
         * comment why it was delayed is in the vhost_user_blk_event()
         * routine.
         */
          vhost_user_blk_disconnect(dev);
    }
    if (ret < 0) {
           error_report(“vhost-user-blk: get block config failed”);
    }
    goto reconnect;
}

> +    if (!s->connected) {
> +        /*
> +         * Perform disconnect before making reconnect. More detailed
> +         * comment why it was delayed is in the vhost_user_blk_event()
> +         * routine.
> +         */
> +        vhost_user_blk_disconnect(dev);
> +    }
>      if (ret < 0) {
>          error_report("vhost-user-blk: get block config failed");
>          goto reconnect;
> --
> 2.7.4
>
>