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

[Huang\, Ying]

Vitaly Wool <vitalywool@xxxxxxxxx> writes:

> 2017-12-19 2:35 GMT+01:00 Huang, Ying <ying.huang@xxxxxxxxx>:
>
>> Vitaly Wool <vitalywool@xxxxxxxxx> 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@xxxxxxxx>
>> > ---
>> >  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?
>>
>
>
> Could you please elaborate how this would be implemented "on top"?

struct llist_node *my_del_first_exclusive(struct llist_head *head)
{
        struct llist_node *node = llist_del_first(head);

        if (node)
                node->next = LLIST_NODE_UNLISTED;
}

bool my_add_exclusive(struct llist_node *node, struct llist_head *head)
{
        if (node->next != LLIST_NODE_UNLIST)
                return false;
        if (cmpxchg(&node->next, LLIST_NODE_UNLIST, NULL) !=
            LLIST_NODE_UNLIST)
                return false;
        llist_add(node, head);
        return true;
}

Best Regards,
Huang, Ying