introduce list_get_first/last (Was: [PATCH 3/6] gfs2: simplify current_tail() via list_last_entry_or_null())

[Jeff Liu]

Hi Oleg,

Sorry for my late response!

On 11/16/2013 01:12 AM, Oleg Nesterov wrote:

> On 11/15, Oleg Nesterov wrote:
>>
>> But probably list_last_entry() makes sense in the "else" branch,
>> athough this is minor.
>>
>>
>> Off-topic. Not sure this really makes sense, but I was thinking about
>>
>> 	list_get_first(pos, head, member)	\
>> 		((pos) = list_first_entry(head, typeof(*pos), member))
>>
>> and list_get_first() last of course. The obvious advantage is that
>> compared to
>>
>> 	tr = list_last_entry(sdp->sd_ail1_list, struct gfs2_trans, tr_list);
>>
>> above you do not need to type "struct gfs2_trans",
>>
>> 	list_get_last(tr, sdp->sd_ail1_list, tr_list);
>>
>> looks a bit better.
> 
> And list.h can use it too.
> 
> So how about the patch below? I can't decide whether it cleanups or
> uglfies the code, I do not trust my taste ;) But to me it looks a
> bit better this way.

I personally think this is better for GFS2 case if I understood correctly,
and it can make the existing list helper a bit neat per my taste :)

The current list_first_entry() works fine consider a common pattern below:
while (!list_empty(ptr)) {
	struct dummy *p = list_first_entry(ptr, struct dummy, member);

	....
}

But I'm not sure if we can find out more external use cases can get
benefits with list_get_first/last...

Thanks,
-Jeff

> 
> Oleg.
> --
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index ef95941..f52aba8 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -372,6 +372,12 @@ static inline void list_splice_tail_init(struct list_head *list,
>  #define list_last_entry(ptr, type, member) \
>  	list_entry((ptr)->prev, type, member)
>  
> +#define list_get_first(pos, head, member)	\
> +	((pos) = list_first_entry(head, typeof(*(pos)), member))
> +
> +#define list_get_last(pos, head, member)	\
> +	((pos) = list_last_entry(head, typeof(*(pos)), member))
> +
>  /**
>   * list_first_entry_or_null - get the first element from a list
>   * @ptr:	the list head to take the element from.
> @@ -443,7 +449,7 @@ static inline void list_splice_tail_init(struct list_head *list,
>   * @member:	the name of the list_struct within the struct.
>   */
>  #define list_for_each_entry(pos, head, member)				\
> -	for (pos = list_first_entry(head, typeof(*pos), member);	\
> +	for (list_get_first(pos, head, member);				\
>  	     &pos->member != (head);					\
>  	     pos = list_next_entry(pos, member))
>  
> @@ -454,7 +460,7 @@ static inline void list_splice_tail_init(struct list_head *list,
>   * @member:	the name of the list_struct within the struct.
>   */
>  #define list_for_each_entry_reverse(pos, head, member)			\
> -	for (pos = list_last_entry(head, typeof(*pos), member);		\
> +	for (list_get_last(pos, head, member);				\
>  	     &pos->member != (head); 					\
>  	     pos = list_prev_entry(pos, member))
>  
> @@ -517,7 +523,7 @@ static inline void list_splice_tail_init(struct list_head *list,
>   * @member:	the name of the list_struct within the struct.
>   */
>  #define list_for_each_entry_safe(pos, n, head, member)			\
> -	for (pos = list_first_entry(head, typeof(*pos), member),	\
> +	for (list_get_first(pos, head, member),				\
>  		n = list_next_entry(pos, member);			\
>  	     &pos->member != (head); 					\
>  	     pos = n, n = list_next_entry(n, member))
> @@ -564,7 +570,7 @@ static inline void list_splice_tail_init(struct list_head *list,
>   * of list entry.
>   */
>  #define list_for_each_entry_safe_reverse(pos, n, head, member)		\
> -	for (pos = list_last_entry(head, typeof(*pos), member),		\
> +	for (list_get_last(pos, head, member),				\
>  		n = list_prev_entry(pos, member);			\
>  	     &pos->member != (head); 					\
>  	     pos = n, n = list_prev_entry(n, member))
> 
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