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

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

On Wed, 6 May 2026 06:21:42 -0700
Rob Clark <rob.clark@oss.qualcomm.com> wrote:

> On Wed, May 6, 2026 at 5:16 AM Boris Brezillon
> <boris.brezillon@collabora.com> 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 <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);  
> 
> if we are iterating a LRU.. and lru->lock is held, shouldn't obj->lru
> always be non-null?

Right, the != NULL check is not needed here.