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: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 9353609..b2428cb 100644 (file)
--- a/drivers/gpu/drm/drm_irq.c
+++ b/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;