On Tue, May 20, 2025 at 08:45:24AM -0700, Rob Clark wrote: > On Tue, May 20, 2025 at 8:21 AM Danilo Krummrich <d...@kernel.org> wrote: > > > > On Tue, May 20, 2025 at 07:57:36AM -0700, Rob Clark wrote: > > > On Tue, May 20, 2025 at 12:23 AM Danilo Krummrich <d...@kernel.org> wrote: > > > > On Mon, May 19, 2025 at 10:51:24AM -0700, Rob Clark wrote: > > > > > diff --git a/drivers/gpu/drm/drm_gpuvm.c b/drivers/gpu/drm/drm_gpuvm.c > > > > > index f9eb56f24bef..1e89a98caad4 100644 > > > > > --- a/drivers/gpu/drm/drm_gpuvm.c > > > > > +++ b/drivers/gpu/drm/drm_gpuvm.c > > > > > @@ -1511,7 +1511,9 @@ drm_gpuvm_bo_destroy(struct kref *kref) > > > > > drm_gpuvm_bo_list_del(vm_bo, extobj, lock); > > > > > drm_gpuvm_bo_list_del(vm_bo, evict, lock); > > > > > > > > > > - drm_gem_gpuva_assert_lock_held(obj); > > > > > + if (kref_read(&obj->refcount) > 0) > > > > > + drm_gem_gpuva_assert_lock_held(obj); > > > > > > > > Again, this is broken. What if the reference count drops to zero right > > > > after > > > > the kref_read() check, but before drm_gem_gpuva_assert_lock_held() is > > > > called? > > > > > > No, it is not. If you find yourself having this race condition, then > > > you already have bigger problems. There are only two valid cases when > > > drm_gpuvm_bo_destroy() is called. Either: > > > > > > 1) You somehow hold a reference to the GEM object, in which case the > > > refcount will be a positive integer. Maybe you race but on either > > > side of the race you have a value that is greater than zero. > > > 2) Or, you are calling this in the GEM object destructor path, in > > > which case no one else should have a reference to the object, so it > > > isn't possible to race > > > > What about: > > > > 3) You destroy the VM_BO, because the VM is destroyed, but someone else > > (e.g. > > another VM) holds a reference of this BO, which is dropped concurrently? > > I mean, that is already broken, so I'm not sure what your point is?
No, it's not. In upstream GPUVM the last thing drm_gpuvm_bo_destroy() does is calling drm_gem_object_put(), because a struct drm_gpuvm_bo holds a reference to the GEM object. The above is only racy with your patch that disables this reference count and leaves this trap for the caller. > > This patch is specifically about the case were VMAs are torn down in > gem->free_object(). > > BR, > -R > > > Please don't tell me "but MSM doesn't do that". This is generic > > infrastructure, > > it is perfectly valid for drivers to do that. > > > > > If the refcount drops to zero after the check, you are about to blow > > > up regardless. > > > > Exactly, that's why the whole approach of removing the reference count a > > VM_BO > > has on the BO, i.e. the proposed DRM_GPUVM_VA_WEAK_REF is broken. > > > > As mentioned, make it DRM_GPUVM_MSM_LEGACY_QUIRK and get an approval from > > Dave / > > Sima for it. > > > > You can't make DRM_GPUVM_VA_WEAK_REF work as a generic thing without > > breaking > > the whole design and lifetimes of GPUVM. > > > > We'd just end up with tons of traps for drivers with lots of WARN_ON() > > paths and > > footguns like the one above if a driver works slightly different than MSM.
