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 <[email protected]>
Closes: https://gitlab.freedesktop.org/panfrost/linux/-/work_items/86
Signed-off-by: Boris Brezillon <[email protected]>
---
drivers/gpu/drm/drm_gem.c | 34 ++++++++++++++++++++++++++++------
1 file changed, 28 insertions(+), 6 deletions(-)
diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index fca42949eb2b..0e087c770883 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -1573,11 +1573,31 @@ drm_gem_lru_remove(struct drm_gem_object *obj)
{
struct drm_gem_lru *lru = obj->lru;
+ /*
+ * We do the lru != NULL check without the lru->lock held, which
+ * means we might end up with a stale lru value by the time the
+ * lock is acquired.
+ *
+ * This is deemed safe because:
+ * 1. the LRU is assumed to outlive any GEM object it was attached
+ * (LRUs are usually bound to a drm_device). So even if obj->lru
+ * has become NULL, it still point to a valid object that can
+ * safely be dereferenced to get the lock.
+ *
+ * 2. all LRUs a GEM object might be attached to must share the same
+ * lock (lock that's usually part of the driver-specific device
+ * object), so taking the lock on the 'old' LRU is equivalent
+ * to taking it on the new one (if any)
+ */
if (!lru)
return;
mutex_lock(lru->lock);
- drm_gem_lru_remove_locked(obj);
+ /* Check a second time with the lock held to make sure we're not racing
+ * with another drm_gem_lru_remove[_locked]() call.
+ */
+ if (obj->lru)
+ drm_gem_lru_remove_locked(obj);
mutex_unlock(lru->lock);
}
EXPORT_SYMBOL(drm_gem_lru_remove);
@@ -1660,15 +1680,17 @@ 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)) {
+ 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
