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SubjectRe: [PATCH -mm 1/5] list.h: add list_singleton
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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