[PATCH 2/3] drm/gem: Fix a race between drm_gem_lru_scan() and drm_gem_object_release()

Boris Brezillon
2026-05-06T12:16:40Z

The following race can currently happen:

| Thread 0 in `drm_gem_lru_scan` | Thread 1 in `drm_gem_object_release` |
| - | - |
| move obj1 with refcount==0 to `still_in_lru` | |
| move obj2 with refcount!=0 to `still_in_lru` | |
| mutex_unlock | |
| shrink obj2 | |
| | lru = obj1->lru; // `still_in_lru` |
| mutex_lock | |
| move obj1 back to the original lru | |
| mutex_unlock | |
| return | |
| | dereference `still_in_lru` |

Move the drm_gem_lru_move_tail_locked() after the
kref_get_unless_zero() check so that we don't end up with a
vanishing LRU when we hit drm_gem_object_release(). We also need to
remove the skipped object from its LRU, otherwise we'll keep hitting
it on subsequent loop iterations until it's actually removed from the
list in the drm_gem_release().

Fixes: e7c2af13f811 ("drm/gem: Add LRU/shrinker helper")
Reported-by: Chia-I Wu <olvaffe@gmail.com>
Closes: https://gitlab.freedesktop.org/panfrost/linux/-/work_items/86
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
---
drivers/gpu/drm/drm_gem.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index fca42949eb2b..97cf63de0112 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -1660,15 +1660,19 @@ drm_gem_lru_scan(struct drm_gem_lru *lru,
if (!obj)
break;

- drm_gem_lru_move_tail_locked(&still_in_lru, obj);
-
/*
* If it's in the process of being freed, gem_object->free()
- * may be blocked on lock waiting to remove it. So just
- * skip it.
+ * may be blocked on lock waiting to remove it. So just remove
+ * it from its current LRU and skip it.
*/
- if (!kref_get_unless_zero(&obj->refcount))
+ if (!kref_get_unless_zero(&obj->refcount)) {
+ if (obj->lru)
+ drm_gem_lru_remove_locked(obj);
+
continue;
+ }
+
+ drm_gem_lru_move_tail_locked(&still_in_lru, obj);

/*
* Now that we own a reference, we can drop the lock for the

--
2.54.0