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

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Message-ID: <m2septp9dz.fsf@posteo.net>
Date: Wed, 08 Jan 2025 16:18:00 +0000
From: Charalampos Mitrodimas <charmitro@...teo.net>
To: Mitchell Levy <levymitchell0@...il.com>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,  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

Mitchell Levy <levymitchell0@...il.com> writes:

> On Sun, Jan 05, 2025 at 01:01:43PM +0000, Charalampos Mitrodimas wrote:
>> "Christoph Lameter (Ampere)" <cl@...two.org> writes:
>>
>> > 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.
>>
>> I agree with this, maybe a helper function would suffise? Something in
>> terms of,
>>   unsafe fn get_per_cpu<T>(var: &PerCpuVariable<T>) -> PerCpuRef<T> {
>> 	  unsafe_get_per_cpu_ref!(var, CpuGuard::new())
>>   }
>
> I'm certainly open to adding such a helper. Is the main concern here the
> unwieldy name? Generally, I prefer to keep modifications to global state
> (disabling preemption via CpuGuard::new()) as explicit as possible, but
> if there's consensus to the contrary, I'm happy to roll it into the
> macro/a helper function.

Yes, in my opinion, the macro name is indeed complex. You're right about
keeping modifications as explicit as possible. A helper wouldn’t be
necessary if the macro name were simpler.

Is adding "unsafe_" to a macro that is unsafe a standard practice? Maybe
we can remove it from the macro name. Documentation is enough IMO.

>
>> >
>> >> +        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);