Re: [PATCH/RFC] llist: add llist_[add|del_first]_exclusive

["Huang\, Ying" <ying.huang@intel.com>]

Vitaly Wool <vitalywool@gmail.com> writes:

> It sometimes is necessary to be able to be able to use llist in
> the following manner:
> 	if (node_unlisted(node))
> 		llst_add(node, list);
> i. e. only add a node to the list if it's not already on a list.
>
> This is not possible without taking locks because otherwise there's
> an obvious race between the check and the addition. This patch adds
> the routine to add a node only if it is not on any list that is
> lockless and race free as long as there's only one consumer.
>
> That is, llist_add_exclusive would only add a node if it's not
> already on a list, and llist_del_first_exclusive will delete the
> first node off the list and mark it as not being on any list.
>
> Signed-off-by: Vitaly Wool <vitaly.vul@sony.com>
> ---
>  include/linux/llist.h | 25 +++++++++++++++++++++++++
>  lib/llist.c           | 29 +++++++++++++++++++++++++++++
>  2 files changed, 54 insertions(+)
>
> diff --git a/include/linux/llist.h b/include/linux/llist.h
> index 85abc29..5c60e9e 100644
> --- a/include/linux/llist.h
> +++ b/include/linux/llist.h
> @@ -74,6 +74,10 @@ struct llist_node {
>  #define LLIST_HEAD_INIT(name)	{ NULL }
>  #define LLIST_HEAD(name)	struct llist_head name = LLIST_HEAD_INIT(name)
>  
> +#define LLIST_NODE_UNLISTED	((void *)(-1L))
> +#define LLIST_NODE_INIT(name)	{ LLIST_NODE_UNLISTED }
> +#define LLIST_NODE(name)	struct llist_node name = LLIST_NODE_INIT(name)
> +
>  /**
>   * init_llist_head - initialize lock-less list head
>   * @head:	the head for your lock-less list
> @@ -238,4 +242,25 @@ extern struct llist_node *llist_del_first(struct llist_head *head);
>  
>  struct llist_node *llist_reverse_order(struct llist_node *head);
>  
> +/**
> + * llist_del_first_exclusive - delete the first entry of lock-less list
> + * 			       and make sure its ->next is NULL
> + * @head:	the head for your lock-less list
> + *
> + * If list is empty, return NULL, otherwise, return the first entry
> + * deleted, this is the newest added one.
> + *
> + */
> +static inline struct llist_node *llist_del_first_exclusive(
> +				struct llist_head *head)
> +{
> +	struct llist_node *node = llist_del_first(head);
> +
> +	if (READ_ONCE(node))
> +		smp_store_release(&node->next, LLIST_NODE_UNLISTED);
> +	return node;
> +}
> +
> +extern bool llist_add_exclusive(struct llist_node *, struct llist_head *);
> +
>  #endif /* LLIST_H */
> diff --git a/lib/llist.c b/lib/llist.c
> index 7062e93..c85fa6b 100644
> --- a/lib/llist.c
> +++ b/lib/llist.c
> @@ -102,3 +102,32 @@ struct llist_node *llist_reverse_order(struct llist_node *head)
>  	return new_head;
>  }
>  EXPORT_SYMBOL_GPL(llist_reverse_order);
> +
> +/**
> + * llist_add_exclusive - add a node only if its ->next is NULL
> + * @node:	the node to be added
> + * @head:	the head for your lock-less list
> + *
> + * Return true if the node was added, or false otherwise.
> + */
> +bool llist_add_exclusive(struct llist_node *node, struct llist_head *head)
> +{
> +	struct llist_node *next = LLIST_NODE_UNLISTED;
> +	struct llist_node *entry, *new_next;
> +
> +	/*
> +	 * if new_next is next (== LLIST_NODE_UNLISTED) on the first run, we
> +	 * get exclusive access to this node as long as others use
> +	 * llist_add_safe too.
> +	 * Then false can be returned no more and we'll loop until we get the
> +	 * stuff right.
> +	 */
> +	do {
> +		entry = READ_ONCE(head->first);
> +		if ((new_next = cmpxchg(&node->next, next, entry)) != next)
> +			return false;
> +		next = entry;
> +	} while (cmpxchg(&head->first, entry, node) != entry);
> +	return true;
> +}
> +EXPORT_SYMBOL_GPL(llist_add_exclusive);

I think this could be implemented on top of llist, why add it into llist
itself?

Best Regards,
Huang, Ying