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

[Boris Brezillon <boris.brezillon@collabora.com>]

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