linux-kernel - Re: [PATCH v2 1/3] rust: helpers: Add list helpers for C linked list operations

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Message-Id: <DEI89VUEYXAJ.1IQQPC3QRLITP@nvidia.com>
Date: Wed, 26 Nov 2025 10:16:30 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Joel Fernandes" <joelagnelf@...dia.com>, "Alexandre Courbot"
 <acourbot@...dia.com>
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Subject: Re: [PATCH v2 1/3] rust: helpers: Add list helpers for C linked
 list operations

On Wed Nov 26, 2025 at 3:16 AM JST, Joel Fernandes wrote:
>
>
>> On Nov 25, 2025, at 9:52 AM, Alexandre Courbot <acourbot@...dia.com> wrote:
>> 
>> On Wed Nov 12, 2025 at 2:13 AM JST, Joel Fernandes wrote:
>>> Add Rust helper functions for common C linked list operations
>>> that are implemented as macros or inline functions and thus not
>>> directly accessible from Rust.
>>> 
>>> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
>>> ---
>>> rust/helpers/helpers.c |  1 +
>>> rust/helpers/list.c    | 32 ++++++++++++++++++++++++++++++++
>>> 2 files changed, 33 insertions(+)
>>> create mode 100644 rust/helpers/list.c
>>> 
>>> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
>>> index 79c72762ad9c..634fa2386bbb 100644
>>> --- a/rust/helpers/helpers.c
>>> +++ b/rust/helpers/helpers.c
>>> @@ -32,6 +32,7 @@
>>> #include "io.c"
>>> #include "jump_label.c"
>>> #include "kunit.c"
>>> +#include "list.c"
>>> #include "maple_tree.c"
>>> #include "mm.c"
>>> #include "mutex.c"
>>> diff --git a/rust/helpers/list.c b/rust/helpers/list.c
>>> new file mode 100644
>>> index 000000000000..fea2a18621da
>>> --- /dev/null
>>> +++ b/rust/helpers/list.c
>>> @@ -0,0 +1,32 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +
>>> +/*
>>> + * Helpers for C Circular doubly linked list implementation.
>>> + */
>>> +
>>> +#include <linux/list.h>
>>> +
>>> +bool rust_helper_list_empty(const struct list_head *head)
>>> +{
>>> +    return list_empty(head);
>>> +}
>>> +
>>> +void rust_helper_list_del(struct list_head *entry)
>>> +{
>>> +    list_del(entry);
>>> +}
>>> +
>>> +void rust_helper_INIT_LIST_HEAD(struct list_head *list)
>>> +{
>>> +    INIT_LIST_HEAD(list);
>>> +}
>>> +
>>> +void rust_helper_list_add(struct list_head *new, struct list_head *head)
>>> +{
>>> +    list_add(new, head);
>>> +}
>>> +
>>> +void rust_helper_list_add_tail(struct list_head *new, struct list_head *head)
>>> +{
>>> +    list_add_tail(new, head);
>>> +}
>> 
>> Just noticed, but of these helpers only `INIT_LIST_HEAD` and
>> `list_add_tail` seem to be used (in doccomments).
>
> Correct, but it makes sense to add the most obvious/common ones (also to make it clear that using these are supported).

"It makes sense" is subjective, and in this case I am confident it is
not the right intuition to add dead code just for the sake of it.

Each of these helpers adds a potential breakage point from the C API
should the latter change, so we should only add them if they are indeed
necessary.

Actually, some of these helpers are not used when they could have been -
you have a `is_empty` method that rewrites the C function instead of
calling the helper. The only helpers that are unjustified as of now as
`list_add` and `list_del`, and these are easy to add when they become
necessary.

But this raises an interesting dilemma: these helpers cannot be inlined
and add the overhead of a function call. On the other hand, the
definition of `list_head` is so excessively simple that manipulating it
directly is virtually as intuitive as invoking the helper - and doesn't
bear the overhead. So should we double-down on these helpers, or just
drop them completely and re-implement the list functionality we need for
increased performance?