linux-kernel - introduce list_get_first/last (Was: [PATCH 3/6] gfs2: simplify current_tail() via list_last_entry_or

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Message-ID: <20131115171259.GA10687@redhat.com>
Date:	Fri, 15 Nov 2013 18:12:59 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jeff Liu <jeff.liu@...cle.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	"xfs@....sgi.com" <xfs@....sgi.com>, cluster-devel@...hat.com,
	linux-mtd@...ts.infradead.org,
	jfs-discussion@...ts.sourceforge.net, jiri@...nulli.us,
	gregkh@...uxfoundation.org
Subject: introduce list_get_first/last (Was: [PATCH 3/6] gfs2: simplify
	current_tail() via list_last_entry_or_null())

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.

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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