On Wed, May 06, 2026 at 02:16:26PM +0200, Boris Brezillon wrote: > drm_gem_lru_remove() dereference stores drm_gem_object::lru in a local > variable that's then dereferenced to acquire the LRU lock. Because this > assignment in done without the LRU lock held, it can race with > drm_gem_lru_scan() where drm_gem_object::lru is temporarily assigned > a stack-allcated LRU that goes away when leaving the function. By > the time we dereference this local lru variable, the object might already > be gone. > > It feels like drm_gem_lru_move_tail() was never meant to be used this > way, because there's no easy way we can avoid this race unless we defer > the locking to the caller. Let's add an explicit LRU for unreclaimable > BOs instead, and have all BOs added to this LRU at creation time.
I would argue that drm_gem_lru_scan() is broken by design. If you're going to release the LRU lock in the middle of a loop you can expect that someone will get hold of your stack-allocated LRU and end up picking the pieces. This patch is fine in itself by trying to avoid stepping into the fight, but I think we should also add a warning in drm_gem_lru_scan() for future users to be aware of the dangers. Reviewed-by: Liviu Dudau <[email protected]> Best regards, Liviu > > Fixes: fb42964e2a76 ("drm/panthor: Add a GEM shrinker") > 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/panthor/panthor_device.h | 10 ++++++++++ > drivers/gpu/drm/panthor/panthor_gem.c | 5 ++++- > 2 files changed, 14 insertions(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/panthor/panthor_device.h > b/drivers/gpu/drm/panthor/panthor_device.h > index 4e4607bca7cc..45b71546f83c 100644 > --- a/drivers/gpu/drm/panthor/panthor_device.h > +++ b/drivers/gpu/drm/panthor/panthor_device.h > @@ -190,6 +190,16 @@ struct panthor_device { > /** @reclaim.lock: Lock protecting all LRUs */ > struct mutex lock; > > + /** > + * @reclaim.unreclaimable: unreclaimable BOs > + * > + * Either the BO is unreclaimable because it has no pages > allocated, > + * or it's unreclaimable because pages are pinned. > + * > + * All BOs start in that list at creation time. > + */ > + struct drm_gem_lru unreclaimable; > + > /** > * @reclaim.unused: BOs with unused pages > * > diff --git a/drivers/gpu/drm/panthor/panthor_gem.c > b/drivers/gpu/drm/panthor/panthor_gem.c > index 13295d7a593d..8e31740126e7 100644 > --- a/drivers/gpu/drm/panthor/panthor_gem.c > +++ b/drivers/gpu/drm/panthor/panthor_gem.c > @@ -204,7 +204,7 @@ void panthor_gem_update_reclaim_state_locked(struct > panthor_gem_object *bo, > drm_gem_lru_move_tail(&ptdev->reclaim.gpu_mapped_shared, > &bo->base); > break; > case PANTHOR_GEM_UNRECLAIMABLE: > - drm_gem_lru_remove(&bo->base); > + drm_gem_lru_move_tail(&ptdev->reclaim.unreclaimable, &bo->base); > break; > default: > drm_WARN(&ptdev->base, true, "invalid GEM reclaim state > (%d)\n", new_state); > @@ -994,6 +994,7 @@ static struct panthor_gem_object * > panthor_gem_create(struct drm_device *dev, size_t size, uint32_t flags, > struct panthor_vm *exclusive_vm, u32 usage_flags) > { > + struct panthor_device *ptdev = container_of(dev, struct panthor_device, > base); > struct panthor_gem_object *bo; > int ret; > > @@ -1026,6 +1027,7 @@ panthor_gem_create(struct drm_device *dev, size_t size, > uint32_t flags, > } > > panthor_gem_debugfs_set_usage_flags(bo, usage_flags); > + drm_gem_lru_move_tail(&ptdev->reclaim.unreclaimable, &bo->base); > return bo; > > err_put: > @@ -1551,6 +1553,7 @@ int panthor_gem_shrinker_init(struct panthor_device > *ptdev) > return ret; > > INIT_LIST_HEAD(&ptdev->reclaim.vms); > + drm_gem_lru_init(&ptdev->reclaim.unreclaimable, &ptdev->reclaim.lock); > drm_gem_lru_init(&ptdev->reclaim.unused, &ptdev->reclaim.lock); > drm_gem_lru_init(&ptdev->reclaim.mmapped, &ptdev->reclaim.lock); > drm_gem_lru_init(&ptdev->reclaim.gpu_mapped_shared, > &ptdev->reclaim.lock); > > -- > 2.54.0 > -- ==================== | I would like to | | fix the world, | | but they're not | | giving me the | \ source code! / --------------- ¯\_(ツ)_/¯
