linux-kernel - [PATCH] list: always set pos in list_prepare

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Date:   Sun,  6 Mar 2022 17:26:35 +0100
From:   Jakob Koschel <jakobkoschel@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jakob Koschel <jakobkoschel@...il.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Brian Johannesmeyer" <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: [PATCH] list: always set pos in list_prepare_entry()

In order to assign the result from list_prepare_entry() to another
variable, it should also be set when pos != NULL.

This will be useful once the list iterator is no longer used after
the loop.

The original code:

	list_for_each_entry(fl, head, list) {
		if (...)
			break;
	}

	list_for_each_entry_continue(fl, head, list) {

	}

can then just be turned into something like this:


	list_for_each_entry(fl, head, list) {
		if (...) {
			tmp = fl;
			break;
		}
	}

	fl = list_prepare_entry(tmp, head, list);
	list_for_each_entry_continue(fl, head, list) {

	}


The compiler should happily optimize away the
TRUE case if the variable being assigned is the same:

	pos = list_prepare_entry(pos, head, list);

and only do it in the case where the variable to assign
is different:

	pos = list_prepare_entry(tmp, head, list);

Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
---
 include/linux/list.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index dd6c2041d09c..050747fb5ea8 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -659,7 +659,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  * Prepares a pos entry for use as a start point in list_for_each_entry_continue().
  */
 #define list_prepare_entry(pos, head, member) \
-	((pos) ? : list_entry(head, typeof(*pos), member))
+	((pos) ? pos : list_entry(head, typeof(*pos), member))

 /**
  * list_for_each_entry_continue - continue iteration over list of given type

base-commit: 719fce7539cd3e186598e2aed36325fe892150cf
--
2.25.1