Re: [PATCH RFC v6 01/26] rust: clist: Add support to interface with C linked lists

[Joel Fernandes <joelagnelf@nvidia.com>]

On 1/21/2026 3:36 PM, Gary Guo wrote:
>> [...]
>>>> +
>>>> +/// Initialize a `list_head` object to point to itself.
>>>> +///
>>>> +/// # Safety
>>>> +///
>>>> +/// `list` must be a valid pointer to a `list_head` object.
>>>> +#[inline]
>>>> +pub unsafe fn init_list_head(list: *mut bindings::list_head) {
>>>> +    // SAFETY: Caller guarantees `list` is a valid pointer to a `list_head`.
>>>> +    unsafe {
>>>> +        (*list).next = list;
>>>> +        (*list).prev = list;
>>>
>>> This needs to be an atomic write or it'll depart from the C implementation.
>> I am curious what you mean by atomic write, can you define it?  Does rust
>> compiler have load/store fusing, invented stores, etc, like C does? Sorry I am
>> only familiar with these concepts on C. Could you provide example of a race
>> condition in Rust that can happen?
>
> Oh yes, this would definitely happen. It's down to LLVM to compile anyway. If
> you create a reference, there'll be even more freedom to do these.
>

Ok.

>> Also I did this addition based on feedback from past review:
>> https://lore.kernel.org/all/DEI89VUEYXAJ.1IQQPC3QRLITP@nvidia.com/
>>
>> There was some concerns around pointless function call overhead when the rust
>> implementation is already quite intertwined with internals of the C linked list
>> implementation. I do agree with that point of view too.
>
> Overall our practice is to not duplicate code. Even `ERR_PTR` is calling into
> helpers.
> 
> For performance, it's a valid concern. However Alice and I have series out there
> that enable you to inline the helpers. I'd say unless there's an absolute need,
> we should do the helpers. Especially with caveats like WRITE_ONCE in this case.

Sounds good, so I will then go back to adding a INIT_LIST_HEAD C helper for the
next spin. I agree with the suggestion and now that we are inlining helpers,
there seems little point in adding a separate rust function to do the same.

-- 
Joel Fernandes