linux-kernel - Re: [PATCH 05/10] rust: xarray: use `xas_load` instead of `xa

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Message-ID: <CAJ-ks9mXoTdOO+Zb9F=cpTrwa+XZ2mLb3NDH9pyd_GZhD2TVRQ@mail.gmail.com>
Date: Mon, 29 Dec 2025 11:06:45 -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 05/10] rust: xarray: use `xas_load` instead of `xa_load`
 in `Guard::load`

On Wed, Dec 3, 2025 at 5:27 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> Replace the call to `xa_load` with `xas_load` in `Guard::load`. The
> `xa_load` function takes the XArray lock internally, which would cause
> a double lock since the `Guard` already holds the lock.

This is not correct. `xa_load` takes and releases the RCU lock only,
not the XArray lock.

> The `xas_load`
> function operates on XArray state and assumes the lock is already held,
> which is the correct API to use when holding a `Guard`.
>
> This change also removes the `#[expect(dead_code)]` annotation from
> `XArrayState` and its constructor, as they are now in use.
>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
>  rust/kernel/xarray.rs | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> index f6e610b059625..0f69a523b72bf 100644
> --- a/rust/kernel/xarray.rs
> +++ b/rust/kernel/xarray.rs
> @@ -215,8 +215,10 @@ fn load<F, U>(&self, index: usize, f: F) -> Option<U>
>      where
>          F: FnOnce(NonNull<c_void>) -> U,
>      {
> -        // SAFETY: `self.xa.xa` is always valid by the type invariant.
> -        let ptr = unsafe { bindings::xa_load(self.xa.xa.get(), index) };
> +        let mut state = XArrayState::new(self, index);
> +        // SAFETY: `state.state` is always valid by the type invariant of
> +        // `XArrayState`.
> +        let ptr = unsafe { bindings::xas_load(&raw mut state.state) };

Can this be a method on XArrayState?

>          let ptr = NonNull::new(ptr.cast())?;
>          Some(f(ptr))
>      }
> @@ -327,7 +329,6 @@ pub fn store(
>  /// # 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.
> @@ -336,7 +337,6 @@ pub(crate) struct XArrayState<'a, 'b, T: ForeignOwnable> {
>  }
>
>  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.
>
> --
> 2.51.2
>
>