On Tue, 29 Aug 2023 11:44:13 +0200 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx> wrote: > On Tue, 29 Aug 2023 10:52:03 +0200 > Christian König <christian.koenig@xxxxxxx> wrote: > > > Am 29.08.23 um 09:29 schrieb Boris Brezillon: > > > On Tue, 29 Aug 2023 05:34:23 +0300 > > > Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote: > > > > > >> On 8/28/23 13:12, Boris Brezillon wrote: > > >>> On Sun, 27 Aug 2023 20:54:43 +0300 > > >>> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote: > > >>> > > >>>> In a preparation of adding drm-shmem memory shrinker, move all reservation > > >>>> locking lockdep checks to use new drm_gem_shmem_resv_assert_held() that > > >>>> will resolve spurious lockdep warning about wrong locking order vs > > >>>> fs_reclam code paths during freeing of shmem GEM, where lockdep isn't > > >>>> aware that it's impossible to have locking contention with the fs_reclam > > >>>> at this special time. > > >>>> > > >>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> > > >>>> --- > > >>>> drivers/gpu/drm/drm_gem_shmem_helper.c | 37 +++++++++++++++++--------- > > >>>> 1 file changed, 25 insertions(+), 12 deletions(-) > > >>>> > > >>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c > > >>>> index d96fee3d6166..ca5da976aafa 100644 > > >>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c > > >>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c > > >>>> @@ -128,6 +128,23 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t > > >>>> } > > >>>> EXPORT_SYMBOL_GPL(drm_gem_shmem_create); > > >>>> > > >>>> +static void drm_gem_shmem_resv_assert_held(struct drm_gem_shmem_object *shmem) > > >>>> +{ > > >>>> + /* > > >>>> + * Destroying the object is a special case.. drm_gem_shmem_free() > > >>>> + * calls many things that WARN_ON if the obj lock is not held. But > > >>>> + * acquiring the obj lock in drm_gem_shmem_free() can cause a locking > > >>>> + * order inversion between reservation_ww_class_mutex and fs_reclaim. > > >>>> + * > > >>>> + * This deadlock is not actually possible, because no one should > > >>>> + * be already holding the lock when drm_gem_shmem_free() is called. > > >>>> + * Unfortunately lockdep is not aware of this detail. So when the > > >>>> + * refcount drops to zero, we pretend it is already locked. > > >>>> + */ > > >>>> + if (kref_read(&shmem->base.refcount)) > > >>>> + drm_gem_shmem_resv_assert_held(shmem); > > >>>> +} > > >>>> + > > >>>> /** > > >>>> * drm_gem_shmem_free - Free resources associated with a shmem GEM object > > >>>> * @shmem: shmem GEM object to free > > >>>> @@ -142,8 +159,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem) > > >>>> if (obj->import_attach) { > > >>>> drm_prime_gem_destroy(obj, shmem->sgt); > > >>>> } else if (!shmem->imported_sgt) { > > >>>> - dma_resv_lock(shmem->base.resv, NULL); > > >>>> - > > >>>> drm_WARN_ON(obj->dev, kref_read(&shmem->vmap_use_count)); > > >>>> > > >>>> if (shmem->sgt) { > > >>>> @@ -156,8 +171,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem) > > >>>> drm_gem_shmem_put_pages_locked(shmem); > > >>> AFAICT, drm_gem_shmem_put_pages_locked() is the only function that's > > >>> called in the free path and would complain about resv-lock not being > > >>> held. I think I'd feel more comfortable if we were adding a > > >>> drm_gem_shmem_free_pages() function that did everything > > >>> drm_gem_shmem_put_pages_locked() does except for the lock_held() check > > >>> and the refcount dec, and have it called here (and in > > >>> drm_gem_shmem_put_pages_locked()). This way we can keep using > > >>> dma_resv_assert_held() instead of having our own variant. > > >> It's not only drm_gem_shmem_free_pages(), but any drm-shmem function > > >> that drivers may use in the GEM's freeing callback. > > >> > > >> For example, panfrost_gem_free_object() may unpin shmem BO and then do > > >> drm_gem_shmem_free(). > > > Is this really a valid use case? > > > > I haven't followed the whole discussion, but I think it isn't a valid > > use case. > > > > That page_use_count is none zero while the GEM object is about to be > > destroyed can only happen is someone managed to grab a reference to the > > page without referencing the GEM object. > > Actually, drm_gem_shmem_object is a bit special (weird?) in this regard. > drm_gem_shmem_get_pages_sgt_locked() creates the sgt and takes a > pages ref (pages_use_count++). The sgt itself is cached (next call to > drm_gem_shmem_get_pages_sgt_locked() will return the existing sgt) but > not refcounted, which means it will stay around until the GEM object is > destroyed or its pages are purged (GEM eviction). Because of that, > shmem->pages_use_count == 1 in drm_gem_shmem_free_pages() is valid iff > shmem->sgt != NULL. pages_use_count > 1 is invalid though, as should be > pages_pin_count after Dmitry's patches. > > If we want to 'fix' that (not convinced this is a bug, more a design > choice), we need to refcount the sgt users and add > drm_gem_shmem_put_pages_sgt[_locked](), so drivers can reflect when > they're done using the sgt. Or we simply create the sgt in drm_gem_shmem_get_pages_locked(), and make drm_gem_shmem_get_pages_sgt() a dummy wrapper returning shmem->sgt, which will force callers to explicitly call drm_gem_shmem_{get,pin}_pages[_locked]() if they want a non-NULL sgt. By doing that, we avoid adding yet another level of refcounting and we keep drivers responsible for pages_{use,pin}_count balancing. The only downside would be the unconditional creation of the sg_table, but I suspect all current users of drm_gem_shmem_object want this sg_table anyway.