drm: Fix race between drm_vblank_off() and drm_queue_vblank_event()

Currently it's possible that the following will happen:
1. drm_wait_vblank() calls drm_vblank_get()
2. drm_vblank_off() gets called
3. drm_wait_vblank() calls drm_queue_vblank_event() which
   adds the event to the queue event though vblank interrupts
   are currently disabled (and may not be re-enabled ever again).

To fix the problem, add another vblank->enabled check into
drm_queue_vblank_event().

drm_vblank_off() holds event_lock around the vblank disable,
so no further locking needs to be added to drm_queue_vblank_event().
vblank disable from another source is not possible since
drm_wait_vblank() already holds a vblank reference.
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_irq.c

@@ -1270,6 +1270,7 @@ static int drm_queue_vblank_event(struct drm_device *dev, int pipe,
 				  union drm_wait_vblank *vblwait,
 				  struct drm_file *file_priv)
 {
+	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
 	struct drm_pending_vblank_event *e;
 	struct timeval now;
 	unsigned long flags;
@@ -1293,6 +1294,18 @@ static int drm_queue_vblank_event(struct drm_device *dev, int pipe,
 
 	spin_lock_irqsave(&dev->event_lock, flags);
 
+	/*
+	 * drm_vblank_off() might have been called after we called
+	 * drm_vblank_get(). drm_vblank_off() holds event_lock
+	 * around the vblank disable, so no need for further locking.
+	 * The reference from drm_vblank_get() protects against
+	 * vblank disable from another source.
+	 */
+	if (!vblank->enabled) {
+		ret = -EINVAL;
+		goto err_unlock;
+	}
+
 	if (file_priv->event_space < sizeof e->event) {
 		ret = -EBUSY;
 		goto err_unlock;