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

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Message-ID: <20260125015127.GA66441@joelbox2>
Date: Sat, 24 Jan 2026 20:51:27 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: Gary Guo <gary@...yguo.net>
Cc: linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH RFC v6 01/26] rust: clist: Add support to interface with
 C linked lists

On Wed, Jan 21, 2026 at 08:36:05PM +0000, Gary Guo wrote:
>>> Why is this callback necessary? The user can just create the list head and
>>> then reference it later? I don't see what this specifically gains over just
>>> doing
>>> 
>>>     fn new() -> impl PinInit<Self>;
>>> 
>>> and have user-side
>>> 
>>>     list <- CListHead::new(),
>>>     _: {
>>>         do_want_ever(&list)
>>>     }
>>
>> The list initialization can fail, see the GPU buddy patch:
>>
>>         // Create pin-initializer that initializes list and allocates blocks.
>>         let init = try_pin_init!(AllocatedBlocks {
>>             list <- CListHead::try_init(|list| {
>>                 // Lock while allocating to serialize with concurrent frees.
>>                 let guard = buddy_arc.lock();
>>
>>                 // SAFETY: guard provides exclusive access, list is initialized.
>>                 to_result(unsafe {
>>                     bindings::gpu_buddy_alloc_blocks(
>>                         guard.as_raw(),
>>                         params.start_range_address,
>>                         params.end_range_address,
>>                         params.size_bytes,
>>                         params.min_block_size_bytes,
>>                         list.as_raw(),
>>                         params.buddy_flags.as_raw(),
>>                     )
>>                 })
>>             }),
>>             buddy: Arc::clone(&buddy_arc),
>>             flags: params.buddy_flags,
>>         });
> 
> The list initialization doesn't fail? It's the subsequent action you did that
> failed.
> 
> You can put failing things in the `_: { ... }` block too.

This worked out well, thanks for the suggestion! I've updated the code
to use `CListHead::new()` with the failable allocation in a `_: { ... }` block:

        let init = try_pin_init!(AllocatedBlocks {
            buddy: Arc::clone(&buddy_arc),
            list <- CListHead::new(),
            flags: params.buddy_flags,
            _: {
                // Lock while allocating to serialize with concurrent frees.
                let guard = buddy.lock();

                // SAFETY: `guard` provides exclusive access to the buddy allocator.
                to_result(unsafe {
                    bindings::gpu_buddy_alloc_blocks(
                        guard.as_raw(),
                        params.start_range_address,
                        params.end_range_address,
                        params.size_bytes,
                        params.min_block_size_bytes,
                        list.as_raw(),
                        params.buddy_flags.as_raw(),
                    )
                })?
            }
        });

I'll remove the try_init() method from CListHead since new() is sufficient.

-- 
Joel Fernandes