Messages in this thread | | | Date | Fri, 14 Mar 2008 14:00:07 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH -mm 1/5] list.h: add list_singleton |
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On Fri, 14 Mar 2008 16:40:36 -0400 Masami Hiramatsu <mhiramat@redhat.com> wrote:
> Add list_singleton to check a list has just one entry. > > list_singleton is useful to check whether a list_head which > have been temporarily allocated for listing objects can be > released or not. > > Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com> > --- > include/linux/list.h | 9 +++++++++ > 1 file changed, 9 insertions(+) > > Index: 2.6.25-rc5-mm1/include/linux/list.h > =================================================================== > --- 2.6.25-rc5-mm1.orig/include/linux/list.h > +++ 2.6.25-rc5-mm1/include/linux/list.h > @@ -211,6 +211,15 @@ static inline int list_empty_careful(con > return (next == head) && (next == head->prev); > } > > +/** > + * list_singleton - tests whether a list has just one entry. > + * @head: the list to test. > + */ > +static inline int list_singleton(const struct list_head *head) > +{ > + return !list_empty(head) && (head->next == head->prev); > +} > +
This hurts my brain.
If your usage pattern is:
struct foo { ... struct list_head bar_list; /* A list of `struct bar's */ };
struct bar { struct list_head list; /* Attached to foo.bar_list */ ... };
then yes, list_singleton() makes sense.
But in other usage patterns it does not:
struct foo { struct bar *bar_list; ... };
struct bar { struct list_head list; /* All the other bars go here */ ... };
In the second case, emptiness is signified by foo.bar_list==NULL. And in this case, code which does
if (foo->bar_list && list_singleton(&foo->bar_list->list))
will fail if there is a single item on the list!
The second usage pattern is uncommon and list_empty() also returns misleading answers when list_heads are used this way.
So I guess we can proceed with your list_singleton(), but I'd just like to flag this possible confusion, see what people think..
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