[PATCH 3/3] drm/gem: Stop exposing the racy/unsafe drm_gem_lru_remove() helper

[Boris Brezillon]

The only place where it's safe to call drm_gem_lru_remove() is when
we know the drm_gem_object::lru field can't be concurrently updated,
which we know is the case when the drm_gem_object is destroyed.

Rather than trying to make that safe, let's kill the function and inline
its content in drm_gem_object_release().

Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
Reviewed-by: Chia-I Wu <olvaffe@xxxxxxxxx>
---
 drivers/gpu/drm/drm_gem.c | 49 +++++++++++++++++------------------------------
 include/drm/drm_gem.h     |  1 -
 2 files changed, 18 insertions(+), 32 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 97cf63de0112..b6df4f62f7b5 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -1108,6 +1108,15 @@ drm_gem_release(struct drm_device *dev, struct drm_file *file_private)
 	idr_destroy(&file_private->object_idr);
 }
 
+static void
+drm_gem_lru_remove_locked(struct drm_gem_object *obj)
+{
+	obj->lru->count -= obj->size >> PAGE_SHIFT;
+	WARN_ON(obj->lru->count < 0);
+	list_del(&obj->lru_node);
+	obj->lru = NULL;
+}
+
 /**
  * drm_gem_object_release - release GEM buffer object resources
  * @obj: GEM buffer object
@@ -1124,7 +1133,15 @@ drm_gem_object_release(struct drm_gem_object *obj)
 	drm_gem_private_object_fini(obj);
 
 	drm_gem_free_mmap_offset(obj);
-	drm_gem_lru_remove(obj);
+
+	/* If the object refcount drops to zero, it means no one can change
+	 * the LRU it's inserted into, so it's safe to dereference
+	 * drm_gem_object::lru without the drm_gem_lru::lock held.
+	 */
+	if (obj->lru) {
+		guard(mutex)(obj->lru->lock);
+		drm_gem_lru_remove_locked(obj);
+	}
 }
 EXPORT_SYMBOL(drm_gem_object_release);
 
@@ -1552,36 +1569,6 @@ drm_gem_lru_init(struct drm_gem_lru *lru, struct mutex *lock)
 }
 EXPORT_SYMBOL(drm_gem_lru_init);
 
-static void
-drm_gem_lru_remove_locked(struct drm_gem_object *obj)
-{
-	obj->lru->count -= obj->size >> PAGE_SHIFT;
-	WARN_ON(obj->lru->count < 0);
-	list_del(&obj->lru_node);
-	obj->lru = NULL;
-}
-
-/**
- * drm_gem_lru_remove - remove object from whatever LRU it is in
- *
- * If the object is currently in any LRU, remove it.
- *
- * @obj: The GEM object to remove from current LRU
- */
-void
-drm_gem_lru_remove(struct drm_gem_object *obj)
-{
-	struct drm_gem_lru *lru = obj->lru;
-
-	if (!lru)
-		return;
-
-	mutex_lock(lru->lock);
-	drm_gem_lru_remove_locked(obj);
-	mutex_unlock(lru->lock);
-}
-EXPORT_SYMBOL(drm_gem_lru_remove);
-
 /**
  * drm_gem_lru_move_tail_locked - move the object to the tail of the LRU
  *
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 86f5846154f7..d527df98d142 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -611,7 +611,6 @@ int drm_gem_dumb_map_offset(struct drm_file *file, struct drm_device *dev,
 			    u32 handle, u64 *offset);
 
 void drm_gem_lru_init(struct drm_gem_lru *lru, struct mutex *lock);
-void drm_gem_lru_remove(struct drm_gem_object *obj);
 void drm_gem_lru_move_tail_locked(struct drm_gem_lru *lru, struct drm_gem_object *obj);
 void drm_gem_lru_move_tail(struct drm_gem_lru *lru, struct drm_gem_object *obj);
 unsigned long

-- 
2.54.0