Re: [PATCH v2 1/3] drm_gem: add mutex to drm_gem_object.gpuva

[Boris Brezillon <boris.brezillon@collabora.com>]

On Fri, 22 Aug 2025 09:28:24 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> There are two main ways that GPUVM might be used:
> 
> * staged mode, where VM_BIND ioctls update the GPUVM immediately so that
>   the GPUVM reflects the state of the VM *including* staged changes that
>   are not yet applied to the GPU's virtual address space.
> * immediate mode, where the GPUVM state is updated during run_job(),
>   i.e., in the DMA fence signalling critical path, to ensure that the
>   GPUVM and the GPU's virtual address space has the same state at all
>   times.
> 
> Currently, only Panthor uses GPUVM in immediate mode, but the Rust
> drivers Tyr and Nova will also use GPUVM in immediate mode, so it is
> worth to support both staged and immediate mode well in GPUVM. To use
> immediate mode, the GEMs gpuva list must be modified during the fence
> signalling path, which means that it must be protected by a lock that is
> fence signalling safe.
> 
> For this reason, a mutex is added to struct drm_gem_object that is
> intended to achieve this purpose. Adding it directly in the GEM object
> both makes it easier to use GPUVM in immediate mode, but also makes it
> possible to take the gpuva lock from core drm code.
> 
> As a follow-up, another change that should probably be made to support
> immediate mode is a mechanism to postpone cleanup of vm_bo objects, as
> dropping a vm_bo object in the fence signalling path is problematic for
> two reasons:
> 
> * When using DRM_GPUVM_RESV_PROTECTED, you cannot remove the vm_bo from
>   the extobj/evicted lists during the fence signalling path.
> * Dropping a vm_bo could lead to the GEM object getting destroyed.
>   The requirement that GEM object cleanup is fence signalling safe is
>   dubious and likely to be violated in practice.
> 
> Panthor already has its own custom implementation of postponing vm_bo
> cleanup.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

One minor thing below.

> ---
>  drivers/gpu/drm/drm_gem.c | 2 ++
>  include/drm/drm_gem.h     | 4 +++-
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 4a89b6acb6af39720451ac24033b89e144d282dc..8d25cc65707d5b44d931beb0207c9d08a3e2de5a 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -187,6 +187,7 @@ void drm_gem_private_object_init(struct drm_device *dev,
>  	kref_init(&obj->refcount);
>  	obj->handle_count = 0;
>  	obj->size = size;
> +	mutex_init(&obj->gpuva.lock);
>  	dma_resv_init(&obj->_resv);
>  	if (!obj->resv)
>  		obj->resv = &obj->_resv;
> @@ -210,6 +211,7 @@ void drm_gem_private_object_fini(struct drm_gem_object *obj)
>  	WARN_ON(obj->dma_buf);
>  
>  	dma_resv_fini(&obj->_resv);
> +	mutex_destroy(&obj->gpuva.lock);
>  }
>  EXPORT_SYMBOL(drm_gem_private_object_fini);
>  
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index d3a7b43e2c637b164eba5af7cc2fc8ef09d4f0a4..5934d8dc267a65aaf62d2d025869221cd110b325 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -403,11 +403,13 @@ struct drm_gem_object {
>  	 * Provides the list of GPU VAs attached to this GEM object.
>  	 *
>  	 * Drivers should lock list accesses with the GEMs &dma_resv lock
> -	 * (&drm_gem_object.resv) or a custom lock if one is provided.
> +	 * (&drm_gem_object.resv) or a custom lock if one is provided. The
> +	 * mutex inside this struct may be used as the custom lock.
>  	 */
>  	struct {
>  		struct list_head list;
>  
> +		struct mutex lock;

Maybe it's time we start moving some bits of the gpuva field docs next
to the fields they describe:

	/**
	 * @gpuva: Fields used by GPUVM to manage mappings pointing to this GEM object.
	 */
	struct {
		/**
		 * @gpuva.list: list of GPU VAs attached to this GEM object.
		 *
		 * Drivers should lock list accesses with the GEMs &dma_resv lock
		 * (&drm_gem_object.resv) or &drm_gem_object.gpuva.lock if the
		 * list is being updated in places where the resv lock can't be
		 * acquired (fence signalling path).
		 */
		struct list_head list;

		/**
		 * @gpuva.lock: lock protecting access to &drm_gem_object.gpuva.list
		 * when the resv lock can't be used.
		 *
		 * Should only be used when the VM is being modified in a fence
		 * signalling path, otherwise you should use &drm_gem_object.resv to
		 * protect accesses to &drm_gem_object.gpuva.list.
		 */
		struct mutex lock;

		...
	};

>  #ifdef CONFIG_LOCKDEP
>  		struct lockdep_map *lock_dep_map;
>  #endif
>