Re: [PATCH v14 5/6] rust: drm: gem/shmem: Add DmaResvGuard helper

[Alice Ryhl <aliceryhl@google.com>]

On Tue, May 26, 2026 at 05:28:56PM -0400, Lyude Paul wrote:
> Just a temporary holdover to make locking/unlocking the dma_resv lock much
> easier.
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Co-authored-by: Alexandre Courbot <acourbot@nvidia.com>

Missing SoB for Alexandre.

> ---
>  rust/kernel/drm/gem/shmem.rs | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
> index 116ed0a13eac2..1b24cb1129a8b 100644
> --- a/rust/kernel/drm/gem/shmem.rs
> +++ b/rust/kernel/drm/gem/shmem.rs
> @@ -27,7 +27,10 @@
>          Deref,
>          DerefMut, //
>      },
> -    ptr::NonNull,
> +    ptr::{
> +        self,
> +        NonNull, //
> +    },
>  };
>  use gem::{
>      BaseObjectPrivate,
> @@ -233,3 +236,29 @@ impl<T: DriverObject> driver::AllocImpl for Object<T> {
>          dumb_map_offset: None,
>      };
>  }
> +
> +/// Private helper-type for holding the `dma_resv` object for a GEM shmem object.
> +///
> +/// When this is dropped, the `dma_resv` lock is dropped as well.
> +///
> +// TODO: This should be replace with a WwMutex equivalent once we have such bindings in the kernel.
> +struct DmaResvGuard<'a, T: DriverObject>(&'a Object<T>);

Is this missing a NotThreadSafe, or is it safe to unlock on a different
thread than where it was locked?

> +impl<'a, T: DriverObject> DmaResvGuard<'a, T> {
> +    #[inline(always)]
> +    #[expect(unused)]
> +    fn new(obj: &'a Object<T>) -> Self {
> +        // SAFETY: This lock is initialized throughout the lifetime of `object`.
> +        unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
> +
> +        Self(obj)
> +    }
> +}
> +
> +impl<'a, T: DriverObject> Drop for DmaResvGuard<'a, T> {
> +    #[inline(always)]
> +    fn drop(&mut self) {
> +        // SAFETY: We are releasing the lock grabbed during the creation of this object.
> +        unsafe { bindings::dma_resv_unlock(self.0.raw_dma_resv()) };
> +    }
> +}
> -- 
> 2.54.0
>