Re: [PATCH 1/3] llist: add ability to remove a particular entry from the list

[Chuck Lever <chuck.lever@oracle.com>]

On 1/15/25 6:24 PM, Olga Kornievskaia wrote:
> nfsd stores its network transports in a lwq (which is a lockless list)
> llist has no ability to remove a particular entry which nfsd needs
> to remove a listener thread.

Note that scripts/get_maintainer.pl says that the maintainer of this
file is:

   linux-kernel@vger.kernel.org

so that address should appear on the cc: list of this series. So
should the list of reviewers for NFSD that appear in MAINTAINERS.

ISTR Neil found the same gap in the llist API. I don't think it's
possible to safely remove an item from the middle of an llist. Much
of the complexity of the current svc thread scheduler is due to this
complication.

I think you will need to mark the listener as dead and find some
way of safely dealing with it later.


> Suggested-by: Jeff Layton <jlayton@kernel.org>
> Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
> ---
>   include/linux/llist.h | 36 ++++++++++++++++++++++++++++++++++++
>   1 file changed, 36 insertions(+)
> 
> diff --git a/include/linux/llist.h b/include/linux/llist.h
> index 2c982ff7475a..fe6be21897d9 100644
> --- a/include/linux/llist.h
> +++ b/include/linux/llist.h
> @@ -253,6 +253,42 @@ static inline bool __llist_add(struct llist_node *new, struct llist_head *head)
>   	return __llist_add_batch(new, new, head);
>   }
>   
> +/**
> + * llist_del_entry - remove a particular entry from a lock-less list
> + * @head: head of the list to remove the entry from
> + * @entry: entry to be removed from the list
> + *
> + * Walk the list, find the given entry and remove it from the list.
> + * The caller must ensure that nothing can race in and change the
> + * list while this is running.
> + *
> + * Returns true if the entry was found and removed.
> + */
> +static inline bool llist_del_entry(struct llist_head *head, struct llist_node *entry)
> +{
> +	struct llist_node *pos;
> +
> +	if (!head->first)
> +		return false;
> +
> +	/* Is it the first entry? */
> +	if (head->first == entry) {
> +		head->first = entry->next;
> +		entry->next = entry;
> +		return true;
> +	}
> +
> +	/* Find it in the list */
> +	llist_for_each(head->first, pos) {
> +		if (pos->next == entry) {
> +			pos->next = entry->next;
> +			entry->next = entry;
> +			return true;
> +		}
> +	}
> +	return false;
> +}
> +
>   /**
>    * llist_del_all - delete all entries from lock-less list
>    * @head:	the head of lock-less list to delete all entries


-- 
Chuck Lever