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

[Charalampos Mitrodimas <charmitro@posteo.net>]

"Christoph Lameter (Ampere)" <cl@gentwo.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())
  }

>
>> +        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);