linux-kernel - Re: [PATCH v4 1/9] rust: cpumask: Add a `Cpumask` iterator

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Message-ID: <aQ08d2NUVsIf5ukH@yury>
Date: Thu, 6 Nov 2025 19:25:27 -0500
From: Yury Norov <yury.norov@...il.com>
To: Mitchell Levy <levymitchell0@...il.com>
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>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux.com>,
	Danilo Krummrich <dakr@...nel.org>,
	Benno Lossin <lossin@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Tyler Hicks <code@...icks.com>,
	Allen Pais <apais@...ux.microsoft.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v4 1/9] rust: cpumask: Add a `Cpumask` iterator

On Wed, Nov 05, 2025 at 03:01:13PM -0800, Mitchell Levy wrote:
> Add an iterator for `Cpumask` making use of C's `cpumask_next`.
> 
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> Signed-off-by: Mitchell Levy <levymitchell0@...il.com>
> ---
>  rust/helpers/cpumask.c |  5 +++++
>  rust/kernel/cpumask.rs | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 52 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/helpers/cpumask.c b/rust/helpers/cpumask.c
> index eb10598a0242..d95bfa111191 100644
> --- a/rust/helpers/cpumask.c
> +++ b/rust/helpers/cpumask.c
> @@ -42,6 +42,11 @@ bool rust_helper_cpumask_full(struct cpumask *srcp)
>  	return cpumask_full(srcp);
>  }
>  
> +unsigned int rust_helper_cpumask_next(int n, struct cpumask *srcp)
> +{
> +	return cpumask_next(n, srcp);
> +}
> +
>  unsigned int rust_helper_cpumask_weight(struct cpumask *srcp)
>  {
>  	return cpumask_weight(srcp);
> diff --git a/rust/kernel/cpumask.rs b/rust/kernel/cpumask.rs
> index 3fcbff438670..b7401848f59e 100644
> --- a/rust/kernel/cpumask.rs
> +++ b/rust/kernel/cpumask.rs
> @@ -6,7 +6,7 @@
>  
>  use crate::{
>      alloc::{AllocError, Flags},
> -    cpu::CpuId,
> +    cpu::{self, CpuId},
>      prelude::*,
>      types::Opaque,
>  };
> @@ -161,6 +161,52 @@ pub fn copy(&self, dstp: &mut Self) {
>      }
>  }
>  
> +/// Iterator for a `Cpumask`.
> +pub struct CpumaskIter<'a> {
> +    mask: &'a Cpumask,
> +    last: Option<u32>,

This is not the last, it's a current CPU.

> +}
> +
> +impl<'a> CpumaskIter<'a> {
> +    /// Creates a new `CpumaskIter` for the given `Cpumask`.
> +    fn new(mask: &'a Cpumask) -> CpumaskIter<'a> {
> +        Self { mask, last: None }
> +    }
> +}
> +
> +impl<'a> Iterator for CpumaskIter<'a> {
> +    type Item = CpuId;
> +
> +    fn next(&mut self) -> Option<Self::Item> {
> +        // SAFETY: By the type invariant, `self.mask.as_raw` is a `struct cpumask *`.
> +        let next = unsafe {
> +            bindings::cpumask_next(
> +                if let Some(last) = self.last {
> +                    last.try_into().unwrap()
> +                } else {
> +                    -1
> +                },
> +                self.mask.as_raw(),
> +            )
> +        };
> +
> +        if next == cpu::nr_cpu_ids() {
> +            None

Please:    if next >= cpu::nr_cpu_ids() {

> +        } else {
> +            self.last = Some(next);
> +            // SAFETY: `cpumask_next` returns either `nr_cpu_ids` or a valid CPU ID.

Now that you've handled the no-found case in the previous block, the
comment doesn't look correct. Can you either move it on top of the
if-else, or just drop entirely?

> +            unsafe { Some(CpuId::from_u32_unchecked(next)) }
> +        }
> +    }
> +}
> +
> +impl Cpumask {
> +    /// Returns an iterator over the set bits in the cpumask.
> +    pub fn iter(&self) -> CpumaskIter<'_> {
> +        CpumaskIter::new(self)
> +    }
> +}
> +
>  /// A CPU Mask pointer.
>  ///
>  /// Rust abstraction for the C `struct cpumask_var_t`.
> 
> -- 
> 2.34.1