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

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

On Tue Jul 15, 2025 at 11:34 PM CEST, Boqun Feng wrote:
> On Tue, Jul 15, 2025 at 07:44:01PM +0200, Benno Lossin wrote:
> [...]
>> >> >
>> >> > First of all, `thread_local!` has to be implemented by some sys-specific
>> >> > unsafe mechanism, right? For example on unix, I think it's using
>> >> > pthread_key_t:
>> >> >
>> >> > 	https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_key_create.html
>> >> >
>> >> > what we are implementing (or wrapping) is the very basic unsafe
>> >> > mechanism for percpu here. Surely we can explore the design for a safe
>> >> > API, but the unsafe mechanism is probably necessary to look into at
>> >> > first.
>> >> 
>> >> But this is intended to be used by drivers, right? If so, then we should
>> >
>> > Not necessarily only for drivers, we can also use it for implementing
>> > other safe abstraction (e.g. hazard pointers, percpu counters etc)
>> 
>> That's fair, but then it should be `pub(crate)`.
>> 
>
> Fine by me, but please see below.
>
>> >> do our usual due diligence and work out a safe abstraction. Only fall
>> >> back to unsafe if it isn't possible.
>> >> 
>> >
>> > All I'm saying is instead of figuring out a safe abstraction at first,
>> > we should probably focus on identifying how to implement it and which
>> > part is really unsafe and the safety requirement for that.
>> 
>> Yeah. But then we should do that before merging :)
>> 
>
> Well, who's talknig about merging? ;-) I thought we just began reviewing
> here ;-)

I understand [PATCH] emails as "I want to merge this" and [RFC PATCH] as
"I want to talk about merging this". It might be that I haven't seen the
RFC patch series, because I often mute those.

>> >> I'm not familiar with percpu, but from the name I assumed that it's
>> >> "just a variable for each cpu" so similar to `thread_local!`, but it's
>> >> bound to the specific cpu instead of the thread.
>> >> 
>> >> That in my mind should be rather easy to support in Rust at least with
>> >> the thread_local-style API. You just need to ensure that no reference
>> >> can escape the cpu, so we can make it `!Send` & `!Sync` + rely on klint
>> >
>> > Not really, in kernel, we have plenty of use cases that we read the
>> > other CPU's percpu variables. For example, each CPU keeps it's own
>> > counter and we sum them other in another CPU.
>> 
>> But then you need some sort of synchronization?
>> 
>
> Right, but the synchronization can exist either in the percpu operations
> themselves or outside the percpu operations. Some cases, the data types
> are small enough to fit in atomic data types, and operations are just
> load/store/cmpxchg etc, then operations on the current cpu and remote
> read will be naturally synchronized. Sometimes extra synchronization is
> needed.

Sure, so we probably want direct atomics support. What about "extra
synchronization"? Is that using locks or RCU or what else?

> Keyword find all these cases are `per_cpu_ptr()`:
>
> 	https://elixir.bootlin.com/linux/v6.15.6/A/ident/per_cpu_ptr

Could you explain to me how to find them? I can either click on one of
the files with horrible C preprocessor macros or the auto-completion in
the search bar. But that one only shows 3 suggestions `_hyp_sym`,
`_nvhe_sym` and `_to_phys` which doesn't really mean much to me.

---
Cheers,
Benno