On Fri, May 8, 2026 at 1:41 AM Boris Brezillon <[email protected]> wrote: > > On Thu, 7 May 2026 14:38:23 -0700 > Rob Clark <[email protected]> wrote: > > > On Thu, May 7, 2026 at 5:46 AM Boris Brezillon > > <[email protected]> wrote: > > > > > > 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? > > > > 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
