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

[Rob Clark]

On Fri, May 8, 2026 at 1:41 AM Boris Brezillon
<boris.brezillon@xxxxxxxxxxxxx> wrote:
>
> On Thu, 7 May 2026 14:38:23 -0700
> Rob Clark <rob.clark@xxxxxxxxxxxxxxxx> wrote:
>
> > On Thu, May 7, 2026 at 5:46 AM Boris Brezillon
> > <boris.brezillon@xxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, 06 May 2026 14:16:27 +0200
> > > Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx> 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@xxxxxxxxx>
> > > > Closes: https://gitlab.freedesktop.org/panfrost/linux/-/work_items/86
> > > > Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> > > > Reviewed-by: Chia-I Wu <olvaffe@xxxxxxxxx>
> > > > ---
> > > >  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?
> >
> > I don't think there is a race, because of the kref_get_unless_zero().
> > Other than lru_scan, there shouldn't be cases where someone is moving
> > an obj between LRUs racing with drm_gem_object_release(), because that
> > means they don't own a reference on the obj they are manipulating.
>
> Yeah, but the race I'm talking about is drm_gem_object_release()
> vs drm_gem_lru_scan(), so at this point refcount is zero, and this
> patch only moves the needle, but doesn't fix the problem entirely:
>
>
> | Thread 0 in `drm_gem_lru_scan`               | Thread 1 in `drm_gem_object_release` |
> | -                                            | -                                    |
> |                                              | drm_gem_lru_remove()                 |
> |                                              |    lru = obj->lru                    |
> |                                              |    if (!lru) return;                 |
> | lock(still_in_lru.lock)                      |                                      |
> |    if (refcount == 0)                        |                                      |
> |       drm_gem_lru_remove_locked(obj)         |                                      |
> |         obj->lru = NULL                      |                                      |
> |    .....                                     |                                      |
> | unlock(still_in_lru.lock)                    |                                      |
> |                                              |    lock(lru->lock)                   |
> |                                              |       drm_gem_lru_remove_locked(obj) |
> |                                              |         obj->lru==NULL => NULL deref |
> |                                              |    unlock(lru->lock)                 |
>
> We can of course add an extra
>
>         if (!obj->lru) return;
>
> in drm_gem_lru_remove_locked() to cover for this race, and add a
> READ_ONCE in drm_gem_lru_remove() to make sure the compiler doesn't
> do crazy things like dereferencing obj->lru twice instead of having
> the LRU pointer stored in a register. That still assumes that the lru
> we assigned to our local variable is valid even after the
> drm_gem_lru_remove_locked(obj) call, which is true at least for MSM and
> and panthor because they have their LRUs attached to the drm_device,
> which outlives any GEMs attached to it. But it's not something the API
> enforce or document as a requirement.

Ahh, right.. yeah drm_gem_lru_remove() should READ_ONCE(), and check
obj->lru after acquiring the lock.

And yeah, that is assuming there aren't lifetime issues with the lock itself

> >
> > That said, I can't really think of a sensible thing to do with more
> > than a single LRU lock per device.  And it does make things easier to
> > reason about.
>
> Okay, I'll give it a try then.

sounds good

BR,
-R