linux-kernel - Re: [PATCH RFC 3/3] rust: percpu: add a rust per-CPU variable test

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Message-ID: <5374de79-0ee6-e817-0f87-c800a6fbb733@gentwo.org>
Date: Fri, 20 Dec 2024 09:56:54 -0800 (PST)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
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>, 
    Benno Lossin <benno.lossin@...ton.me>, 
    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>, linux-kernel@...r.kernel.org, 
    rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH RFC 3/3] rust: percpu: add a rust per-CPU variable test

On Thu, 19 Dec 2024, Mitchell Levy wrote:

> +        let mut native: i64 = 0;
> +        let mut pcpu: PerCpuRef<i64> = unsafe { unsafe_get_per_cpu_ref!(PERCPU, CpuGuard::new()) };

A bit complex.

> +        native += -1;
> +        *pcpu += -1;
> +        assert!(native == *pcpu && native == -1);
> +
> +        native += 1;
> +        *pcpu += 1;
> +        assert!(native == *pcpu && native == 0);
> +

That's pretty straightforward..... But is there no symbolic access to the
per cpu namespace? How would you access the kernel per cpu variables
defined in C?

How do you go about using per cpu atomics like

this_cpu_inc(nr_dentry_unused);