Subject: Re: [PATCH net v3 04/11] list: Move on_list_rcu() to list.h and add on_list() also

From: Jakub Kicinski <[email-masked]>
Date: Sun, 29 Mar 2026 12:12:08 -0700

On Thu, 26 Mar 2026 13:18:29 +0000 David Howells wrote:
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 00ea8e5fb88b..d224e7210d1b 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -381,6 +381,32 @@ static inline int list_empty(const struct list_head *head)
>  	return READ_ONCE(head->next) == head;
>  }
>  
> +/**
> + * on_list - Test whether an entry is on a list.
> + * @entry: The entry to check
> + *
> + * Test whether an entry is on a list.  Safe to use on an entry initialised
> + * with INIT_LIST_HEAD() or LIST_HEAD() or removed with things like
> + * list_del_init().  Not safe for use with list_del() or list_del_rcu().
> + */
> +static inline bool on_list(const struct list_head *entry)
> +{
> +	return !list_empty(entry);
> +}
> +
> +/**
> + * on_list_rcu - Test whether an entry is on a list (RCU-del safe).
> + * @entry: The entry to check
> + *
> + * Test whether an entry is on a list.  Safe to use on an entry initialised
> + * with INIT_LIST_HEAD() or LIST_HEAD() or removed with things like
> + * list_del_init().  Also safe for use with list_del() or list_del_rcu().
> + */
> +static inline bool on_list_rcu(const struct list_head *entry)
> +{
> +	return !list_empty(entry) && entry->prev != LIST_POISON2;
> +}

Could someone with sufficient weight to their name ack this?

The non-RCU version of on_list() does not sit well with me.
It provides no additional semantics above list_empty() and
the uninit / poison-related gotchas more obvious when typing
!list_empty(&entry->list).

I can believe the RCU version is more useful. It could probably
be used on both RCU and non-RCU entries?

Last minor nit - the list API consistently uses list_ as a prefix.
I have no better name to suggest but it's sad that on_list_rcu()
breaks that.

I think you're missing a READ_ONCE(), BTW.