On Wed, May 6, 2026 at 5:16 AM Boris Brezillon <[email protected]> wrote: > > 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]> > Reviewed-by: Chia-I Wu <[email protected]> > --- > 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);
if we are iterating a LRU.. and lru->lock is held, shouldn't obj->lru always be non-null? BR, -R > + > 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 >
