+ llist-avoid-extra-memory-read-in-llist_add_batch.patch added to mm-nonmm-unstable branch

+ llist-avoid-extra-memory-read-in-llist_add_batch.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: llist: avoid extra memory read in llist_add_batch
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     llist-avoid-extra-memory-read-in-llist_add_batch.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/llist-avoid-extra-memory-read-in-llist_add_batch.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Uros Bizjak <ubizjak@gmail.com>
Subject: llist: avoid extra memory read in llist_add_batch
Date: Mon, 17 Oct 2022 16:52:26 +0200

try_cmpxchg implicitly assigns old head->first value to "first" when
cmpxchg fails. There is no need to re-read the value in the loop.

Link: https://lkml.kernel.org/r/20221017145226.4044-1-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/llist.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/lib/llist.c~llist-avoid-extra-memory-read-in-llist_add_batch
+++ a/lib/llist.c
@@ -26,10 +26,10 @@
 bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
 		     struct llist_head *head)
 {
-	struct llist_node *first;
+	struct llist_node *first = READ_ONCE(head->first);
 
 	do {
-		new_last->next = first = READ_ONCE(head->first);
+		new_last->next = first;
 	} while (!try_cmpxchg(&head->first, &first, new_first));
 
 	return !first;
_

Patches currently in -mm which might be from ubizjak@gmail.com are

llist-avoid-extra-memory-read-in-llist_add_batch.patch