Re: [PATCH v3 1/7] list: Add mutable iterator variants

[David Laight <david.laight.linux@gmail.com>]

On Mon, 22 Jun 2026 12:05:31 +0800
Kaitao Cheng <kaitao.cheng@linux.dev> wrote:

> From: Kaitao Cheng <chengkaitao@kylinos.cn>
> 
> The list_for_each*_safe() helpers are used when the loop body may
> remove the current entry.  Their API exposes the temporary cursor at
> every call site, even though most users only need it for the iterator
> implementation and never reference it in the loop body.
> 
> Add *_mutable() variants for list and hlist iteration.  The new helpers
> support both forms: callers may keep passing an explicit temporary cursor
> when they need to inspect or reset it, or omit it and let the helper use
> a unique internal cursor.

I'm not really sure 'mutable' means anything either.
It is possible to make it valid for the loop body (or even other threads)
to delete arbitrary list items - but that needs significant extra overheads.

It might be worth doing something that doesn't need the extra variable,
but there is little point doing all the churn just to rename things.

> 
> This makes call sites that only mutate the list through the current entry
> less noisy, while keeping the existing *_safe() helpers available for
> compatibility.
> 
> Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
> ---
>  include/linux/list.h | 269 +++++++++++++++++++++++++++++++++++++------
>  1 file changed, 231 insertions(+), 38 deletions(-)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 09d979976b3b..1081def7cea9 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -7,6 +7,7 @@
>  #include <linux/stddef.h>
>  #include <linux/poison.h>
>  #include <linux/const.h>
> +#include <linux/args.h>
>  
>  #include <asm/barrier.h>
>  
> @@ -763,28 +764,72 @@ static inline void list_splice_tail_init(struct list_head *list,
>  #define list_for_each_prev(pos, head) \
>  	for (pos = (head)->prev; !list_is_head(pos, (head)); pos = pos->prev)
>  
> -/**
> - * list_for_each_safe - iterate over a list safe against removal of list entry
> - * @pos:	the &struct list_head to use as a loop cursor.
> - * @n:		another &struct list_head to use as temporary storage
> - * @head:	the head for your list.
> +/*
> + * list_for_each_safe is an old interface, use list_for_each_mutable instead.
>   */
>  #define list_for_each_safe(pos, n, head) \
>  	for (pos = (head)->next, n = pos->next; \
>  	     !list_is_head(pos, (head)); \
>  	     pos = n, n = pos->next)
>  
> +#define __list_for_each_mutable_internal(pos, tmp, head)		\
> +	for (typeof(pos) tmp = (pos = (head)->next)->next;		\

Use auto

> +	     !list_is_head(pos, (head));				\
> +	     pos = tmp, tmp = pos->next)
> +
> +#define __list_for_each_mutable1(pos, head)				\
> +	__list_for_each_mutable_internal(pos, __UNIQUE_ID(next), head)
> +
> +#define __list_for_each_mutable2(pos, next, head)			\
> +	list_for_each_safe(pos, next, head)
> +
>  /**
> - * list_for_each_prev_safe - iterate over a list backwards safe against removal of list entry
> + * list_for_each_mutable - iterate over a list safe against entry removal
>   * @pos:	the &struct list_head to use as a loop cursor.
> - * @n:		another &struct list_head to use as temporary storage
> - * @head:	the head for your list.
> + * @...:	either (head) or (next, head)
> + *
> + * next:	another &struct list_head to use as optional temporary storage.
> + *		The temporary cursor is internal unless explicitly supplied by
> + *		the caller.
> + * head:	the head for your list.
> + */
> +#define list_for_each_mutable(pos, ...)					\
> +	CONCATENATE(__list_for_each_mutable, COUNT_ARGS(__VA_ARGS__))	\
> +		(pos, __VA_ARGS__)

The variable argument count logic really just slows down compilation.
Maybe there aren't enough copies of this code to make that significant.
But just because you can do it doesn't mean it is a gooD idea.
I'm also not sure it really adds anything to the readability.

And, it you are going to make the middle argument optional there is
no need to change the macro name.

	David