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SubjectRe: [PATCH 2/4] list.h: add list_cut_position()
On Tue, Aug 05, 2008 at 01:52:22AM -0700, Jochen Voß wrote:
> Hi,
>
> 2008/8/5 Luis R. Rodriguez <lrodriguez@atheros.com>:
> > +static inline void __list_cut_position(struct list_head *list,
> > + struct list_head *head, struct list_head *entry)
> > +{
> > + struct list_head *new_first =
> > + (entry->next != head) ? entry->next : head;
>
> Isn't this just an over-complicated way of writing "new_first = entry->next"?

Sorry, yes.

> > + list->next = head->next;
> > + list->next->prev = list;
> > + list->prev = entry;
> > + entry->next = list;
> > + head->next = new_first;
> > + new_first->prev = head;
> > +}
> > +
> > +/**
> > + * list_cut_position - cut a list into two
> > + * @list: a new list to add all removed entries
> > + * @head: a list with entries
> > + * @entry: an entry within head, could be the head itself
> > + * and if so we won't cut the list
> > + */
>
> I think it would be helpful if the comment explained what the function
> actually does, i.e. that it moves the inital part of 'head' (up to and
> including 'entry' from 'head' to 'list'.

Will do.

> > +static inline void list_cut_position(struct list_head *list,
> > + struct list_head *head, struct list_head *entry)
> > +{
> > + BUG_ON(list_empty(head));
> > + if (list_is_singular(head))
> > + BUG_ON(head->next != entry && head != entry);
>
> No other list function in "list.h" has BUG_ONs. Why this one?

Alright, I'll just make some small note on the documentation and exit early
on these.

> > + if (entry == head)
> > + INIT_LIST_HEAD(list);
>
> If there was data in 'list' before the call, it will be lost now. Is
> this intended behaviour?

Yes, the user of this call should not care if we wipe @list for them as
it is just a reference. This lets you, for example, use @list on
list_cut_position() it in a loop. I'll extend the documentation to
clarify this.

> > + else
> > + __list_cut_position(list, head, entry);
> > +}
> > +
>
> I hope this helps,

It certainly has, thanks for reviewing. How's this:

This adds list_cut_position() which lets you cut a list into
two lists given a pivot in the list.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>

diff --git a/include/linux/list.h b/include/linux/list.h
index 453916b..6c02a83 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -214,6 +214,46 @@ static inline int list_is_singular(const struct list_head *head)
return !list_empty(head) && (head->next == head->prev);
}

+static inline void __list_cut_position(struct list_head *list,
+ struct list_head *head, struct list_head *entry)
+{
+ struct list_head *new_first = entry->next;
+ list->next = head->next;
+ list->next->prev = list;
+ list->prev = entry;
+ entry->next = list;
+ head->next = new_first;
+ new_first->prev = head;
+}
+
+/**
+ * list_cut_position - cut a list into two
+ *
+ * This helper moves the initial part of @head, up to and
+ * including @entry, from @head to @list. You should
+ * pass on @entry an element you know is on @head. @list
+ * should be an empty list or a list you do not care about
+ * losing its data.
+ *
+ * @list: a new list to add all removed entries
+ * @head: a list with entries
+ * @entry: an entry within head, could be the head itself
+ * and if so we won't cut the list
+ */
+static inline void list_cut_position(struct list_head *list,
+ struct list_head *head, struct list_head *entry)
+{
+ if (list_empty(head))
+ return;
+ if (list_is_singular(head) &&
+ (head->next != entry && head != entry))
+ return;
+ if (entry == head)
+ INIT_LIST_HEAD(list);
+ else
+ __list_cut_position(list, head, entry);
+}
+
static inline void __list_splice(const struct list_head *list,
struct list_head *head)
{
--
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