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introduce list_get_first/last (Was: [PATCH 3/6] gfs2: simplify current_t

To: Jeff Liu <jeff.liu@xxxxxxxxxx>
Subject: introduce list_get_first/last (Was: [PATCH 3/6] gfs2: simplify current_tail() via list_last_entry_or_null())
From: Oleg Nesterov <oleg@xxxxxxxxxx>
Date: Fri, 15 Nov 2013 18:12:59 +0100
Cc: linux-kernel@xxxxxxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>, cluster-devel@xxxxxxxxxx, linux-mtd@xxxxxxxxxxxxxxxxxxx, jfs-discussion@xxxxxxxxxxxxxxxxxxxxx, jiri@xxxxxxxxxxx, gregkh@xxxxxxxxxxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20131115163957.GA28467@xxxxxxxxxx>
References: <5285A3F4.4040000@xxxxxxxxxx> <20131115163957.GA28467@xxxxxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
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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