drm/gem: add mutex lock when using drm_gem_mmap_obj

The drm_gem_mmap_obj() has to be protected with dev->struct_mutex,
but some caller functions do not. So it adds mutex lock to missing
callers and adds assertion to check whether drm_gem_mmap_obj() is
called with mutex lock or not.

Signed-off-by: YoungJun Cho <[e-mail]>
Signed-off-by: Seung-Woo Kim <[e-mail]>
Signed-off-by: Kyungmin Park <[e-mail]>
Reviewed-by: Maarten Lankhorst <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Reviewed-by: Rob Clark <[e-mail]>
Reviewed-by: Maarten Lankhorst <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/drm_gem.c

···
  * the GEM object is not looked up based on its fake offset. To implement the
  * DRM mmap operation, drivers should use the drm_gem_mmap() function.
  *
+ * NOTE: This function has to be protected with dev->struct_mutex
+ *
  * Return 0 or success or -EINVAL if the object size is smaller than the VMA
  * size, or if no gem_vm_ops are provided.
  */
···
 		     struct vm_area_struct *vma)
 {
 	struct drm_device *dev = obj->dev;
+
+	lockdep_assert_held(&dev->struct_mutex);
 
 	/* Check for valid size. */
 	if (obj_size < vma->vm_end - vma->vm_start)

drivers/gpu/drm/drm_gem_cma_helper.c

···
 {
 	struct drm_gem_cma_object *cma_obj = dmabuf->priv;
 	struct drm_gem_object *gem_obj = &cma_obj->base;
+	struct drm_device *dev = gem_obj->dev;
 	int ret;
 
+	mutex_lock(&dev->struct_mutex);
 	ret = drm_gem_mmap_obj(gem_obj, gem_obj->size, vma);
+	mutex_unlock(&dev->struct_mutex);
 	if (ret < 0)
 		return ret;
 

drivers/gpu/drm/omapdrm/omap_gem_dmabuf.c

···
 		struct vm_area_struct *vma)
 {
 	struct drm_gem_object *obj = buffer->priv;
+	struct drm_device *dev = obj->dev;
 	int ret = 0;
 
 	if (WARN_ON(!obj->filp))
 		return -EINVAL;
 
+	mutex_lock(&dev->struct_mutex);
 	ret = drm_gem_mmap_obj(obj, omap_gem_mmap_size(obj), vma);
+	mutex_unlock(&dev->struct_mutex);
 	if (ret < 0)
 		return ret;