linux-kernel - [PATCH] llist: Avoid extra memory read in llist_add

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Message-Id: <20221017145226.4044-1-ubizjak@gmail.com>
Date:   Mon, 17 Oct 2022 16:52:26 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] llist: Avoid extra memory read in llist_add_batch

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.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 lib/llist.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/llist.c b/lib/llist.c
index 7d78b736e8af..6e668fa5a2c6 100644
--- a/lib/llist.c
+++ b/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;
-- 
2.37.3