linux-kernel - Re: [PATCH 04/10] rust: xarray: add `XArrayState`

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Message-ID: <CAJ-ks9koBuQabEc7VKAPt1kSLbLyv21NuaNkBOb0FGvZ2hBLPA@mail.gmail.com>
Date: Mon, 29 Dec 2025 11:04:31 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Daniel Gomez <da.gomez@...nel.org>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/10] rust: xarray: add `XArrayState`

On Wed, Dec 3, 2025 at 5:27 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> Add `XArrayState` as internal state for XArray iteration and entry
> operations. This struct wraps the C `xa_state` structure and holds a
> reference to a `Guard` to ensure exclusive access to the XArray for the
> lifetime of the state object.
>
> The `XAS_RESTART` constant is also exposed through the bindings helper
> to properly initialize the `xa_node` field.
>
> The struct and its constructor are marked with `#[expect(dead_code)]` as
> there are no users yet. We will remove this annotation in a later patch.
>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
>  rust/bindings/bindings_helper.h |  1 +
>  rust/kernel/xarray.rs           | 41 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 41 insertions(+), 1 deletion(-)
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 2e43c66635a2c..86bca946faff0 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -109,6 +109,7 @@ const xa_mark_t RUST_CONST_HELPER_XA_PRESENT = XA_PRESENT;
>  const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC = XA_FLAGS_ALLOC;
>  const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC1 = XA_FLAGS_ALLOC1;
>  const vm_flags_t RUST_CONST_HELPER_VM_MERGEABLE = VM_MERGEABLE;
> +const size_t RUST_CONST_HELPER_XAS_RESTART = (size_t)XAS_RESTART;

Can we avoid casting it here? It's quite distant from the site of use
which makes it difficult to understand why it's necessary. It also
suffers from C casting rules being quite lax, which means a change in
the type on the C side would likely go unnoticed.

>
>  #if IS_ENABLED(CONFIG_ANDROID_BINDER_IPC_RUST)
>  #include "../../drivers/android/binder/rust_binder.h"
> diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> index ede48b5e1dba3..f6e610b059625 100644
> --- a/rust/kernel/xarray.rs
> +++ b/rust/kernel/xarray.rs
> @@ -8,7 +8,10 @@
>      iter,
>      marker::PhantomData,
>      pin::Pin,
> -    ptr::NonNull, //
> +    ptr::{
> +        null_mut,
> +        NonNull, //
> +    },
>  };
>  use kernel::{
>      alloc,
> @@ -319,6 +322,42 @@ pub fn store(
>      }
>  }
>
> +/// Internal state for XArray iteration and entry operations.
> +///
> +/// # Invariants
> +///
> +/// - `state` is always a valid `bindings::xa_state`.
> +#[expect(dead_code)]
> +pub(crate) struct XArrayState<'a, 'b, T: ForeignOwnable> {
> +    /// Holds a reference to the lock guard to ensure the lock is not dropped
> +    /// while `Self` is live.
> +    _access: &'b Guard<'a, T>,

Can this comment explain why this isn't a PhantomData? IIUC we just
need the lifetime, rather than the actual guard.

> +    state: bindings::xa_state,
> +}
> +
> +impl<'a, 'b, T: ForeignOwnable> XArrayState<'a, 'b, T> {
> +    #[expect(dead_code)]
> +    fn new(access: &'b Guard<'a, T>, index: usize) -> Self {
> +        let ptr = access.xa.xa.get();
> +        // INVARIANT: We initialize `self.state` to a valid value below.
> +        Self {
> +            _access: access,
> +            state: bindings::xa_state {
> +                xa: ptr,
> +                xa_index: index,
> +                xa_shift: 0,
> +                xa_sibs: 0,
> +                xa_offset: 0,
> +                xa_pad: 0,
> +                xa_node: bindings::XAS_RESTART as *mut bindings::xa_node,

Odd that we're casting it back to a pointer here.

> +                xa_alloc: null_mut(),
> +                xa_update: None,
> +                xa_lru: null_mut(),
> +            },
> +        }
> +    }
> +}
> +
>  // SAFETY: `XArray<T>` has no shared mutable state so it is `Send` iff `T` is `Send`.
>  unsafe impl<T: ForeignOwnable + Send> Send for XArray<T> {}
>
>
> --
> 2.51.2
>
>