linux-kernel - Re: [PATCH 1/2] list: Add list_is_null() to check if a list

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Date:   Sat, 27 Feb 2021 12:14:55 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...il.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        linux-renesas-soc@...r.kernel.org,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
Subject: Re: [PATCH 1/2] list: Add list_is_null() to check if a list_head has
 been initialized

Hello!

On 27.02.2021 1:49, Laurent Pinchart wrote:

> From: Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
> 
> 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.

    So zeroed or initialized/added? :-)

> 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@...asonboard.com>
> ---
>   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.

    So zeroed or initialized/added? :-)

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

    Maybe instead:

	return !list->prev && !list->next;

[...]

MBR, Sergei