Re: [PATCH v5 01/40] drm/gpuvm: Don't require obj lock in destructor path

[Danilo Krummrich <dakr@kernel.org>]

On Tue, May 20, 2025 at 07:57:36AM -0700, Rob Clark wrote:
> On Tue, May 20, 2025 at 12:23 AM Danilo Krummrich <dakr@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?

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.