Re: [PATCH v2 3/5] rust: percpu: add a rust per-CPU variable test

["Benno Lossin" <lossin@kernel.org>]

On Sat Jul 12, 2025 at 11:31 PM CEST, Mitchell Levy wrote:
> Add a short exercise for Rust's per-CPU variable API, modelled after
> lib/percpu_test.c
>
> Signed-off-by: Mitchell Levy <levymitchell0@gmail.com>
> ---
>  lib/Kconfig.debug       |   9 ++++
>  lib/Makefile            |   1 +
>  lib/percpu_test_rust.rs | 120 ++++++++++++++++++++++++++++++++++++++++++++++++

I don't know if this is the correct place, the code looks much more like
a sample, so why not place it there instead?

>  rust/helpers/percpu.c   |  11 +++++
>  4 files changed, 141 insertions(+)
> diff --git a/lib/percpu_test_rust.rs b/lib/percpu_test_rust.rs
> new file mode 100644
> index 000000000000..a9652e6ece08
> --- /dev/null
> +++ b/lib/percpu_test_rust.rs
> @@ -0,0 +1,120 @@
> +// SPDX-License-Identifier: GPL-2.0
> +//! A simple self test for the rust per-CPU API.
> +
> +use core::ffi::c_void;
> +
> +use kernel::{
> +    bindings::{on_each_cpu, smp_processor_id},
> +    define_per_cpu,
> +    percpu::{cpu_guard::*, *},
> +    pr_info,
> +    prelude::*,
> +    unsafe_get_per_cpu,
> +};
> +
> +module! {
> +    type: PerCpuTestModule,
> +    name: "percpu_test_rust",
> +    author: "Mitchell Levy",
> +    description: "Test code to exercise the Rust Per CPU variable API",
> +    license: "GPL v2",
> +}
> +
> +struct PerCpuTestModule;
> +
> +define_per_cpu!(PERCPU: i64 = 0);
> +define_per_cpu!(UPERCPU: u64 = 0);
> +
> +impl kernel::Module for PerCpuTestModule {
> +    fn init(_module: &'static ThisModule) -> Result<Self, Error> {
> +        pr_info!("rust percpu test start\n");
> +
> +        let mut native: i64 = 0;
> +        // SAFETY: PERCPU is properly defined
> +        let mut pcpu: StaticPerCpu<i64> = unsafe { unsafe_get_per_cpu!(PERCPU) };

I don't understand why we need unsafe here, can't we just create
something specially in the `define_per_cpu` macro that is then confirmed
by the `get_per_cpu!` macro and thus it can be safe?

> +        // SAFETY: We only have one PerCpu that points at PERCPU
> +        unsafe { pcpu.get(CpuGuard::new()) }.with(|val: &mut i64| {

Hmm I also don't like the unsafe part here...

Can't we use the same API that `thread_local!` in the standard library
has:

    https://doc.rust-lang.org/std/macro.thread_local.html

So in this example you would store a `Cell<i64>` instead.

I'm not familiar with per CPU variables, but if you're usually storing
`Copy` types, then this is much better wrt not having unsafe code
everywhere.

If one also often stores `!Copy` types, then we might be able to get
away with `RefCell`, but that's a small runtime overhead -- which is
probably bad given that per cpu variables are most likely used for
performance reasons? In that case the user might just need to store
`UnsafeCell` and use unsafe regardless. (or we invent something
specifically for that case, eg tokens that are statically known to be
unique etc)

---
Cheers,
Benno

> +            pr_info!("The contents of pcpu are {}\n", val);
> +
> +            native += -1;
> +            *val += -1;
> +            pr_info!("Native: {}, *pcpu: {}\n", native, val);
> +            assert!(native == *val && native == -1);
> +
> +            native += 1;
> +            *val += 1;
> +            pr_info!("Native: {}, *pcpu: {}\n", native, val);
> +            assert!(native == *val && native == 0);
> +        });