Re: [PATCH] drm/virtio: defer hotplug event from dequeue worker to avoid deadlock

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

Hi,

On 6/30/26 12:16, Ryosuke Yasuoka wrote:
> A probe-time deadlock can occur between the dequeue worker and
> drm_client_register(). During probe, drm_client_register() holds
> clientlist_mutex and calls the fbdev hotplug callback, which triggers an
> atomic commit that ends up sleeping in virtio_gpu_queue_ctrl_sgs()
> waiting for virtqueue space. The dequeue worker that would free that
> space calls virtio_gpu_cmd_get_display_info_cb(), which invokes
> drm_kms_helper_hotplug_event() -> drm_client_dev_hotplug(), attempting
> to acquire the same clientlist_mutex. Since wake_up() is only called
> after the resp_cb loop, the probe thread is never woken and both threads
> deadlock.
> 
> Fix this by deferring the hotplug notification from
> virtio_gpu_cmd_get_display_info_cb() to a separate work item. The
> display data (outputs[i].info) is still updated synchronously in the
> callback, and the deferred work only triggers a re-probe notification to
> DRM clients.
> 
> Fixes: 27655b9bb9f0 ("drm/client: Send hotplug event after registering a client")
> Closes: https://syzkaller.appspot.com/bug?id=d6dd6f86d3aaf7eebe7406e45c1c6e549453f224
> Closes: https://syzkaller.appspot.com/bug?id=908bd910da5dd79b88de4cf7baf376cc873a922e
> Signed-off-by: Ryosuke Yasuoka <ryasuoka@redhat.com>
> ---
>  drivers/gpu/drm/virtio/virtgpu_drv.h |  3 +++
>  drivers/gpu/drm/virtio/virtgpu_kms.c |  3 +++
>  drivers/gpu/drm/virtio/virtgpu_vq.c  | 12 ++++++++++--
>  3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
> index 7449907754a4..27ffa4697ae9 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_drv.h
> +++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
> @@ -264,6 +264,8 @@ struct virtio_gpu_device {
>  
>  	struct work_struct config_changed_work;
>  
> +	struct work_struct hotplug_work;
> +
>  	struct work_struct obj_free_work;
>  	spinlock_t obj_free_lock;
>  	struct list_head obj_free_list;
> @@ -350,6 +352,7 @@ void virtio_gpu_cmd_transfer_to_host_2d(struct virtio_gpu_device *vgdev,
>  					uint32_t x, uint32_t y,
>  					struct virtio_gpu_object_array *objs,
>  					struct virtio_gpu_fence *fence);
> +void virtio_gpu_hotplug_work_func(struct work_struct *work);
>  void virtio_gpu_panic_cmd_resource_flush(struct virtio_gpu_device *vgdev,
>  					 uint32_t resource_id,
>  					 uint32_t x, uint32_t y,
> diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
> index cfde9f573df6..cfb532ba43a4 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
> @@ -154,6 +154,8 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
>  	INIT_WORK(&vgdev->config_changed_work,
>  		  virtio_gpu_config_changed_work_func);
>  
> +	INIT_WORK(&vgdev->hotplug_work, virtio_gpu_hotplug_work_func);
> +
>  	INIT_WORK(&vgdev->obj_free_work,
>  		  virtio_gpu_array_put_free_work);
>  	INIT_LIST_HEAD(&vgdev->obj_free_list);
> @@ -293,6 +295,7 @@ void virtio_gpu_deinit(struct drm_device *dev)
>  	flush_work(&vgdev->obj_free_work);
>  	flush_work(&vgdev->ctrlq.dequeue_work);
>  	flush_work(&vgdev->cursorq.dequeue_work);
> +	flush_work(&vgdev->hotplug_work);
>  	flush_work(&vgdev->config_changed_work);
>  	virtio_reset_device(vgdev->vdev);
>  	vgdev->vdev->config->del_vqs(vgdev->vdev);
> diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
> index 67865810a2e7..084d98f5dc7b 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_vq.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
> @@ -816,6 +816,15 @@ virtio_gpu_cmd_resource_detach_backing(struct virtio_gpu_device *vgdev,
>  	virtio_gpu_queue_fenced_ctrl_buffer(vgdev, vbuf, fence);
>  }
>  
> +void virtio_gpu_hotplug_work_func(struct work_struct *work)
> +{
> +	struct virtio_gpu_device *vgdev =
> +		container_of(work, struct virtio_gpu_device, hotplug_work);
> +
> +	if (!drm_helper_hpd_irq_event(vgdev->ddev))
> +		drm_kms_helper_hotplug_event(vgdev->ddev);
> +}
> +
>  static void virtio_gpu_cmd_get_display_info_cb(struct virtio_gpu_device *vgdev,
>  					       struct virtio_gpu_vbuffer *vbuf)
>  {
> @@ -841,8 +850,7 @@ static void virtio_gpu_cmd_get_display_info_cb(struct virtio_gpu_device *vgdev,
>  	spin_unlock(&vgdev->display_info_lock);
>  	wake_up(&vgdev->resp_wq);
>  
> -	if (!drm_helper_hpd_irq_event(vgdev->ddev))
> -		drm_kms_helper_hotplug_event(vgdev->ddev);
> +	schedule_work(&vgdev->hotplug_work);
>  }
>  
>  static void virtio_gpu_cmd_get_capset_info_cb(struct virtio_gpu_device *vgdev,

Could you please move drm_kms_helper_hotplug_event() to virtio_gpu_init(), placing it after wait_event_timeout(display_info_pending)? This will avoid additional work_struct that otherwise needs to be cancelled in virtio_gpu_init() on the timeout.

-- 
Best regards,
Dmitry