Re: [PATCH 1/2] list: introduce a new cutting helper

[Nilay Shroff]

On 6/12/24 21:21, Keith Busch wrote:
> From: Keith Busch <kbusch@xxxxxxxxxx>
> 
> Provide a helper to remove elements from a list to the end, and place
> those elements in a new list.
> 
> Signed-off-by: Keith Busch <kbusch@xxxxxxxxxx>
> ---
>  include/linux/list.h | 20 ++++++++++++++++++++
>  lib/list-test.c      | 29 +++++++++++++++++++++++++++++
>  2 files changed, 49 insertions(+)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 5f4b0a39cf46a..f22850e854820 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -520,6 +520,26 @@ static inline void list_cut_before(struct list_head *list,
>  	entry->prev = head;
>  }
>  
> +/**
> + * list_cut - cut a list into two from the entry
> + * @list: a new list to add all removed entries
> + * @head: a list with entries
> + * @entry: an entry within head, could be the head itself
> + *
> + * This helper removes elements from @head starting at @entry until the end,
> + * and appends them to @lists.
> + */
> +static inline void list_cut(struct list_head *list,
> +		struct list_head *head, struct list_head *entry)
> +{
> +	list->next = entry;
> +	list->prev = head->prev;
> +	head->prev = entry->prev;
> +	entry->prev->next = head;
> +	entry->prev = list;
> +	list->prev->next = list;
> +}
I am wondering whether we really need the _rcu version of list_cut here?
I think that @head could point to an _rcu protected list and that's true 
for this patch. So there might be concurrent readers accessing @head using
_rcu list-traversal primitives, such as list_for_each_entry_rcu().

An _rcu version of list_cut():

static inline void list_cut_rcu(struct list_head *list,
		struct list_head *head, struct list_head *entry)
{
	list->next = entry;
	list->prev = head->prev;
	head->prev = entry->prev;
	rcu_assign_pointer(list_next_rcu(entry->prev), head);
	entry->prev = list;
	list->prev->next = list;
}

Thanks,
--Nilay