Re: [PATCH v2 2/3] virtio-balloon: Add locking to prevent possible race when starting hinting

[Peter Maydell]

On Mon, 6 Jul 2020 at 22:15, Alexander Duyck <alexander.duyck@gmail.com> wrote:
>
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>
> There is already locking in place when we are stopping free page hinting
> but there is not similar protections in place when we start. I can only
> assume this was overlooked as in most cases the page hinting should not be
> occurring when we are starting the hinting, however there is still a chance
> we could be processing hints by the time we get back around to restarting
> the hinting so we are better off making sure to protect the state with the
> mutex lock rather than just updating the value with no protections.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
>  hw/virtio/virtio-balloon.c |    4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 0c0fd7114799..b3e96a822b4d 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -593,6 +593,8 @@ static void virtio_balloon_free_page_start(VirtIOBalloon 
> *s)
>          return;
>      }
>
> +    qemu_mutex_lock(&s->free_page_lock);
> +
>      if (s->free_page_report_cmd_id == UINT_MAX) {
>          s->free_page_report_cmd_id =
>                         VIRTIO_BALLOON_FREE_PAGE_REPORT_CMD_ID_MIN;
> @@ -601,6 +603,8 @@ static void virtio_balloon_free_page_start(VirtIOBalloon 
> *s)
>      }
>
>      s->free_page_report_status = FREE_PAGE_REPORT_S_REQUESTED;
> +    qemu_mutex_unlock(&s->free_page_lock);
> +
>      virtio_notify_config(vdev);
>  }

Coverity spotted this lack of locking too: CID 1430269.

thanks
-- PMM