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. > > 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.
