drm/vkms: Fix race condition around accessing frame number

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@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180903211743.GA2773@haneenDRM

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)

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);
 

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 {