Re: [PATCH] Mark list_add and __list_add as __always_inline

[Nick Desaulniers]

On Fri, Jul 31, 2026 at 1:15 PM Jordan R Abrahams-Whitehead
<ajordanr@xxxxxxxxxx> wrote:
>
> This commit resolves an issue where modpost section
> verification fails due to section mismatches between list_add
> and its callers.
>
> At present, list_add (and its internal __list_add) are called from
> both .text and .init code sections. Since inlining can vary per call
> site, list_add can be 4 different states:
>
>   list_add in text with arguments to non-.init.data values
>   list_add in init with arguments to static .init.data values
>   list_add in init with arguments to non-.init.data values
>   list_add in text with arguments to static .init.data values
>
> It is last instance that ends up causing the section mismatch caused by
> constant propagation of the address of static libs inside the `dir_add`

I think you meant `static __initdata LIST_HEADs`? (not `static libs`).
Maybe AKPM could fix that up for you when applying?

> as seen below (with the dir_list being defined statically in
> initramfs.c, resting in .init.data).
>
>   WARNING: modpost: vmlinux.o: section mismatch in reference: __list_add
>   (section: .text.unlikely.) -> dir_list (section: .init.data)
>
> Because of these section matching requirements, semantically,
> __list_add and list_add MUST be inlined. This will then ensure
> callers inside .init will receive a list_add that exists and refers
> to only .init data, and list_add code in .text sections will only refer
> to non-init data.
>
> This issue manifests predominently in AutoFDO with clang, which is very
> hesitant to inline cold functions such as list_add even when marked
> `inline`. Marking them as `__always_inline` therefore matches the
> existing semantic constraints imposed by modpost's section mismatch
> checks.
>

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

We've been seeing this particularly on Android and ChromeOS while
trying to enable AutoFDO, which seems more likely to outline cold
code.  include/linux/init.h defines __init as __cold.

I saw no change in size for x86 defconfig; enabling
CONFIG_LIST_HARDENED resulted in a change of a few bytes (i.e. less
that 0.01%).  I assume we don't care as much about the size if
CONFIG_DEBUG_LIST is set.  Checked with bloaty and
scripts/bloat-o-meter (thanks to Eric for pointing out that tool in
the linked thread).

I never heard back from Kees or Marco about their thoughts on this in
relation to
commit b16c42c8fde8 ("list_debug: Introduce inline wrappers for debug checks")
but I wouldn't mind an Ack (or Nack) from at least one of them.

> Closes: https://github.com/ClangBuiltLinux/linux/issues/2173
> Signed-off-by: Jordan R Abrahams-Whitehead <ajordanr@xxxxxxxxxx>
> Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/CANn89iJVQe=wedLheJmjZjOTJsWHijT0jZs=iRxKssJZbjAxHw@xxxxxxxxxxxxxx/
> ---
>  include/linux/list.h | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 09d979976b3b..59f8aa0905d3 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -150,10 +150,13 @@ static inline bool __list_del_entry_valid(struct list_head *entry)
>   *
>   * This is only for internal list manipulation where we know
>   * the prev/next entries already!
> + *
> + * Must be inlined to ensure it can be safely called
> + * with initdata arguments.
>   */
> -static inline void __list_add(struct list_head *new,
> -                             struct list_head *prev,
> -                             struct list_head *next)
> +static __always_inline void __list_add(struct list_head *new,
> +                                      struct list_head *prev,
> +                                      struct list_head *next)
>  {
>         if (!__list_add_valid(new, prev, next))
>                 return;
> @@ -171,8 +174,12 @@ static inline void __list_add(struct list_head *new,
>   *
>   * Insert a new entry after the specified head.
>   * This is good for implementing stacks.
> + *
> + * Must be inlined to ensure it can be safely called
> + * with initdata arguments.
>   */
> -static inline void list_add(struct list_head *new, struct list_head *head)
> +static __always_inline void list_add(struct list_head *new,
> +                                    struct list_head *head)
>  {
>         __list_add(new, head, head->next);
>  }
>
> ---
> base-commit: fc46aed51f6280801f43a2cf4b5060cc33b572f9
> change-id: 20260730-always-inline-list-add-363639606e26
>
> Best regards,
> --
> Jordan R Abrahams-Whitehead <ajordanr@xxxxxxxxxx>
>


-- 
Thanks,
~Nick Desaulniers