diff mbox series

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

Message ID 20210226224938.18166-1-laurent.pinchart@ideasonboard.com (mailing list archive)
State New
Headers show
Series [1/2] list: Add list_is_null() to check if a list_head has been initialized | expand

Commit Message

Laurent Pinchart Feb. 26, 2021, 10:49 p.m. UTC
From: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.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.

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@ideasonboard.com>
---
 include/linux/list.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

Comments

Sergei Shtylyov Feb. 27, 2021, 9:14 a.m. UTC | #1
Hello!

On 27.02.2021 1:49, Laurent Pinchart wrote:

> From: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.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@ideasonboard.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
Sakari Ailus March 1, 2021, 8:31 a.m. UTC | #2
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@ideasonboard.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.
> 
> 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@ideasonboard.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.

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,
diff mbox series

Patch

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.
+ */
+static inline bool list_is_null(struct list_head *list)
+{
+	return list->prev == NULL && list->next == NULL;
+}
+
 #ifdef CONFIG_DEBUG_LIST
 extern bool __list_add_valid(struct list_head *new,
 			      struct list_head *prev,