[PATCH v2] drm/vkms: Fix race condition around accessing frame number

[Daniel Vetter]

On Tue, Sep 04, 2018 at 12:18:17AM +0300, Haneen Mohammed wrote:
> crtc_state is accessed by both vblank_handle() and the ordered
> work_struct handle vkms_crc_work_handle() to retrieve and or update
> the frame number for computed CRC.
> 
> Since work_struct can fail, add frame_end to account for missing frame
> numbers.
> 
> Use (frame_[start/end]) for synchronization between hrtimer callback
> and ordered work_struct handle.
> 
> This patch passes the following subtests from igt kms_pipe_crc_basic test:
> bad-source, read-crc-pipe-A, read-crc-pipe-A-frame-sequence,
> nonblocking-crc-pipe-A, nonblocking-crc-pipe-A-frame-sequence
> 
> Signed-off-by: Haneen Mohammed <hamohammed.sa at gmail.com>
> ---
> v2:
> - use u64 with spinlock instead of atomic_t
> - add out label for common exit routine @vkms_crc_work_handle()

Yup I think that looks good, thanks for your patch. Applied.

btw, do you still plan to type up a vkms.rst for documentation, so that we
could put a few of the todo items there? Or should I do that? Next
outreachy round starts soon, we need to get this ready.
-Daniel

> 
>  drivers/gpu/drm/vkms/vkms_crc.c  | 36 ++++++++++++++++++++++++++++++--
>  drivers/gpu/drm/vkms/vkms_crtc.c | 16 ++++++++++++--
>  drivers/gpu/drm/vkms/vkms_drv.h  |  8 +++++--
>  3 files changed, 54 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_crc.c b/drivers/gpu/drm/vkms/vkms_crc.c
> index ed47d67cecd6..68db42f15086 100644
> --- a/drivers/gpu/drm/vkms/vkms_crc.c
> +++ b/drivers/gpu/drm/vkms/vkms_crc.c
> @@ -34,6 +34,15 @@ static uint32_t _vkms_get_crc(struct vkms_crc_data *crc_data)
>  	return crc;
>  }
>  
> +/**
> + * vkms_crc_work_handle - ordered work_struct to compute CRC
> + *
> + * @work: work_struct
> + *
> + * Work handler for computing CRCs. work_struct scheduled in
> + * an ordered workqueue that's periodically scheduled to run by
> + * _vblank_handle() and flushed at vkms_atomic_crtc_destroy_state().
> + */
>  void vkms_crc_work_handle(struct work_struct *work)
>  {
>  	struct vkms_crtc_state *crtc_state = container_of(work,
> @@ -45,8 +54,18 @@ void vkms_crc_work_handle(struct work_struct *work)
>  						output);
>  	struct vkms_crc_data *primary_crc = NULL;
>  	struct drm_plane *plane;
> -
>  	u32 crc32 = 0;
> +	u64 frame_start, frame_end;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&out->state_lock, flags);
> +	frame_start = crtc_state->frame_start;
> +	frame_end = crtc_state->frame_end;
> +	spin_unlock_irqrestore(&out->state_lock, flags);
> +
> +	/* _vblank_handle() hasn't updated frame_start yet */
> +	if (!frame_start || frame_start == frame_end)
> +		goto out;
>  
>  	drm_for_each_plane(plane, &vdev->drm) {
>  		struct vkms_plane_state *vplane_state;
> @@ -67,7 +86,20 @@ void vkms_crc_work_handle(struct work_struct *work)
>  	if (primary_crc)
>  		crc32 = _vkms_get_crc(primary_crc);
>  
> -	drm_crtc_add_crc_entry(crtc, true, crtc_state->n_frame, &crc32);
> +	frame_end = drm_crtc_accurate_vblank_count(crtc);
> +
> +	/* queue_work can fail to schedule crc_work; add crc for
> +	 * missing frames
> +	 */
> +	while (frame_start <= frame_end)
> +		drm_crtc_add_crc_entry(crtc, true, frame_start++, &crc32);
> +
> +out:
> +	/* to avoid using the same value for frame number again */
> +	spin_lock_irqsave(&out->state_lock, flags);
> +	crtc_state->frame_end = frame_end;
> +	crtc_state->frame_start = 0;
> +	spin_unlock_irqrestore(&out->state_lock, flags);
>  }
>  
>  static int vkms_crc_parse_source(const char *src_name, bool *enabled)
> diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> index 9d0b1a325a78..177bbcb38306 100644
> --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> @@ -22,8 +22,19 @@ static void _vblank_handle(struct vkms_output *output)
>  		DRM_ERROR("vkms failure on handling vblank");
>  
>  	if (state && output->crc_enabled) {
> -		state->n_frame = drm_crtc_accurate_vblank_count(crtc);
> -		queue_work(output->crc_workq, &state->crc_work);
> +		u64 frame = drm_crtc_accurate_vblank_count(crtc);
> +
> +		/* update frame_start only if a queued vkms_crc_work_handle()
> +		 * has read the data
> +		 */
> +		spin_lock(&output->state_lock);
> +		if (!state->frame_start)
> +			state->frame_start = frame;
> +		spin_unlock(&output->state_lock);
> +
> +		ret = queue_work(output->crc_workq, &state->crc_work);
> +		if (!ret)
> +			DRM_WARN("failed to queue vkms_crc_work_handle");
>  	}
>  
>  	spin_unlock(&output->lock);
> @@ -211,6 +222,7 @@ int vkms_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,
>  	drm_crtc_helper_add(crtc, &vkms_crtc_helper_funcs);
>  
>  	spin_lock_init(&vkms_out->lock);
> +	spin_lock_init(&vkms_out->state_lock);
>  
>  	vkms_out->crc_workq = alloc_ordered_workqueue("vkms_crc_workq", 0);
>  
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> index 2017a2ccc43d..80af6d3a65e7 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.h
> +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> @@ -39,12 +39,14 @@ struct vkms_plane_state {
>   * vkms_crtc_state - Driver specific CRTC state
>   * @base: base CRTC state
>   * @crc_work: work struct to compute and add CRC entries
> - * @n_frame: frame number for computed CRC
> + * @n_frame_start: start frame number for computed CRC
> + * @n_frame_end: end frame number for computed CRC
>   */
>  struct vkms_crtc_state {
>  	struct drm_crtc_state base;
>  	struct work_struct crc_work;
> -	unsigned int n_frame;
> +	u64 frame_start;
> +	u64 frame_end;
>  };
>  
>  struct vkms_output {
> @@ -59,6 +61,8 @@ struct vkms_output {
>  	struct workqueue_struct *crc_workq;
>  	/* protects concurrent access to crc_data */
>  	spinlock_t lock;
> +	/* protects concurrent access to crtc_state */
> +	spinlock_t state_lock;
>  };
>  
>  struct vkms_device {
> -- 
> 2.17.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch