Re: [PATCH v4 01/40] drm/gpuvm: Don't require obj lock in destructor path

[Danilo Krummrich <dakr@kernel.org>]

On Thu, May 15, 2025 at 10:35:21AM -0700, Rob Clark wrote:
> On Thu, May 15, 2025 at 2:06 AM Danilo Krummrich <dakr@kernel.org> wrote:
> >
> > On Thu, May 15, 2025 at 10:54:27AM +0200, Danilo Krummrich wrote:
> > > Hi Rob,
> > >
> > > Can you please CC me on patches for GPUVM?
> > >
> > > On Wed, May 14, 2025 at 10:53:15AM -0700, Rob Clark wrote:
> > > > From: Rob Clark <robdclark@chromium.org>
> > > >
> > > > See commit a414fe3a2129 ("drm/msm/gem: Drop obj lock in
> > > > msm_gem_free_object()") for justification.
> > >
> > > Please write a proper commit message that explains the problem and the solution.
> > > Please don't just refer to another commit and leave it to the reviewer of the
> > > patch to figure this out.
> > >
> > > > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > > > ---
> > > >  drivers/gpu/drm/drm_gpuvm.c | 7 +++++--
> > > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > >
> > >
> > > > 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);
> > > > +
> > > >     list_del(&vm_bo->list.entry.gem);
> > >
> > > This seems wrong.
> > >
> > > A VM_BO object keeps a reference of the underlying GEM object, so this should
> > > never happen.
> > >
> > > This function calls drm_gem_object_put() before it returns.
> >
> > I noticed your subsequent patch that allows VM_BO structures to have weak
> > references to GEM objects.
> >
> > However, even with that this seems wrong. If the reference count of the GEM
> > object is zero when drm_gpuvm_bo_destroy() is called it means that the GEM
> > object is dead. However, until drm_gpuvm_bo_destroy() is called the GEM object
> > potentially remains to be on the extobj and eviced list, which means that other
> > code paths might fetch it from those lists and consider it to be a valid GEM
> > object.
> 
> We only iterate extobj or evicted in VM_BIND mode, where we aren't
> using WEAK_REF.  I suppose some WARN_ON()s or BUG_ON()s could make
> this more clear.

There is also the GEM object's list of VM_BOs, are you using that?

Anyways, I don't agree with that. Even if you can tweak your driver to not run
into trouble with this, we can't introduce a mode that violates GOUVM's internal
lifetimes and subsequently fix it up with WARN_ON() or BUG_ON().

I still don't see a real technical reason why msm can't be reworked to follow
those lifetime rules.