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

[Danilo Krummrich <dakr@kernel.org>]

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.

>  
>  	if (ops && ops->vm_bo_free)
> @@ -1871,7 +1873,8 @@ drm_gpuva_unlink(struct drm_gpuva *va)
>  	if (unlikely(!obj))
>  		return;
>  
> -	drm_gem_gpuva_assert_lock_held(obj);
> +	if (kref_read(&obj->refcount) > 0)
> +		drm_gem_gpuva_assert_lock_held(obj);
>  	list_del_init(&va->gem.entry);
>  
>  	va->vm_bo = NULL;
> -- 
> 2.49.0
>