[PATCH 3/5] drm/virtio: track whether or not a context has been initiated

[Chia-I Wu]

On Thu, Feb 13, 2020 at 3:18 PM Gurchetan Singh
<gurchetansingh at chromium.org> wrote:
>
> Use an atomic variable to track whether a context has been
> initiated.
>
> v2: Fix possible race (@olv)
>
> Signed-off-by: Gurchetan Singh <gurchetansingh at chromium.org>
> ---
>  drivers/gpu/drm/virtio/virtgpu_drv.h   | 1 +
>  drivers/gpu/drm/virtio/virtgpu_ioctl.c | 3 +++
>  drivers/gpu/drm/virtio/virtgpu_kms.c   | 1 +
>  3 files changed, 5 insertions(+)
>
> diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
> index 72c1d9b59dfe..ca505984f8ab 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_drv.h
> +++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
> @@ -209,6 +209,7 @@ struct virtio_gpu_device {
>
>  struct virtio_gpu_fpriv {
>         uint32_t ctx_id;
> +       atomic_t context_initiated;
>  };
>
>  /* virtio_ioctl.c */
> diff --git a/drivers/gpu/drm/virtio/virtgpu_ioctl.c b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> index 896c3f419a6d..a98884462944 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> @@ -44,6 +44,9 @@ void virtio_gpu_create_context(struct drm_device *dev,
>         if (!vgdev->has_virgl_3d)
>                 return;
>
> +       if (!atomic_add_unless(&vfpriv->context_initiated, 1, 1))
> +               return;
> +
How does this work?  When thread A and B enter this function at the
same time, and thread B returns early, it is possible that thread B
queues other commands before thread A has the chance to queue
virtio_gpu_cmd_context_create.

>         get_task_comm(dbgname, current);
>         virtio_gpu_cmd_context_create(vgdev, vfpriv->ctx_id,
>                                       strlen(dbgname), dbgname);
> diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
> index 282558576527..25248bad3fc4 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
> @@ -263,6 +263,7 @@ int virtio_gpu_driver_open(struct drm_device *dev, struct drm_file *file)
>         }
>
>         vfpriv->ctx_id = handle + 1;
> +       atomic_set(&vfpriv->context_initiated, 0);
>         file->driver_priv = vfpriv;
>         virtio_gpu_create_context(dev, file);
>         return 0;
> --
> 2.25.0.265.gbab2e86ba0-goog
>