drm: check for minor master before allowing drop master.

When fast user switching a lot eventually we get to the point,
where we were checking for the wrong thing in this function.

Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/gpu/drm/drm_stub.c b/drivers/gpu/drm/drm_stub.c
index 1b3e0ff..ef87861 100644 (file)
--- a/drivers/gpu/drm/drm_stub.c
+++ b/drivers/gpu/drm/drm_stub.c
@@ -185,6 +185,9 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
        if (!file_priv->is_master)
                return -EINVAL;
 
+       if (!file_priv->minor->master)
+               return -EINVAL;
+
        mutex_lock(&dev->struct_mutex);
        drm_master_put(&file_priv->minor->master);
        file_priv->is_master = 0;