On Wed, 06 May 2026 14:16:27 +0200
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);
> +
Actually, this thing is still racy, because obj->lru is dereferenced
without the lru->lock held in drm_gem_object_release(). At this point
I'm wondering if we should expose a drm_gem_lru_remove() taking the LRU
lock as an argument as suggested by Steve, and delegate the
responsibility to call drm_gem_lru_remove() to the driver. Either that,
or we make it so the LRU lock is attached to the drm_device instead of
the GEM (both MSM and panthor assume a device-wide lock for LRU
manipulation).
Rob, what's your take on this matter?
