Re: [PATCH 1/2] list: Add list_is_null() to check if a list_head has been initialized

[Sakari Ailus]

Hi Laurent,

Thanks for the patch.

On Sat, Feb 27, 2021 at 12:49:37AM +0200, Laurent Pinchart wrote:
> From: Laurent Pinchart <laurent.pinchart+renesas@xxxxxxxxxxxxxxxx>
> 
> The new function checks if the list_head prev and next pointers are
> NULL, in order to see if a list_head that has been zeroed when allocated
> has been initialized with INIT_LIST_HEAD() or added to a list.
> 
> This can be used in cleanup functions that want to support being safely
> called when an object has not been initialized, to return immediately.
> In most cases other fields of the object can be checked for this
> purpose, but in some cases a list_head field is the only option.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@xxxxxxxxxxxxxxxx>
> ---
>  include/linux/list.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 85c92555e31f..e4fc6954de3b 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -29,6 +29,19 @@ static inline void INIT_LIST_HEAD(struct list_head *list)
>  	list->prev = list;
>  }
>  
> +/**
> + * list_is_null - check if a list_head has been initialized
> + * @list: the list
> + *
> + * Check if the list_head prev and next pointers are NULL. This is useful to
> + * see if a list_head that has been zeroed when allocated has been initialized
> + * with INIT_LIST_HEAD() or added to a list.

How this should work with an entry that has been removed from a list with
list_del()? The values will be LIST_POISON[12] and so this function will
return true. Should it return false instead?

> + */
> +static inline bool list_is_null(struct list_head *list)
> +{
> +	return list->prev == NULL && list->next == NULL;

What would you think of issuing a warning if one is NULL but the other one
isn't? That could happen if the memory is uninitialised by the caller. It
should return true in that case, too.

> +}
> +
>  #ifdef CONFIG_DEBUG_LIST
>  extern bool __list_add_valid(struct list_head *new,
>  			      struct list_head *prev,

-- 
Kind regards,

Sakari Ailus