linux-kernel - [PATCH v1] list: Add a macro to test if entry is pointing to the head

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Date:   Tue, 29 Sep 2020 14:28:56 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Cezary Rojewski <cezary.rojewski@...el.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: [PATCH v1] list: Add a macro to test if entry is pointing to the head

Add a macro to test if entry is pointing to the head of the list
which is useful in cases like:

  list_for_each_entry(pos, &head, member) {
    if (cond)
      break;
  }
  if (list_entry_is_head(pos, &head, member))
    return -ERRNO;

that allows to avoid additional variable to be added to track if loop
has not been stopped in the middle.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
 include/linux/list.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/list.h b/include/linux/list.h
index 796975c3c35c..49efaa8c3403 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -770,6 +770,17 @@ static inline void list_splice_tail_init(struct list_head *list,
 #define list_safe_reset_next(pos, n, member)				\
 	n = list_next_entry(pos, member)
 
+/**
+ * list_entry_is_head - test if the entry points to the head of the list
+ * @pos:	the type * to cursor
+ * @head:	the head for your list.
+ * @member:	the name of the list_head within the struct.
+ *
+ * This macro can be used to check if the loop over the list wasn't stopped.
+ */
+#define list_entry_is_head(pos, head, member)				\
+	(&pos->member == (head))
+
 /*
  * Double linked lists with a single pointer list head.
  * Mostly useful for hash tables where the two pointer list head is
-- 
2.28.0