linux-kernel - [PATCH] epoll: Use try_cmpxchg in list_add_tail

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Date:   Thu, 14 Jul 2022 19:32:55 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: [PATCH] epoll: Use try_cmpxchg in list_add_tail_lockless

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old
in list_add_tail_lockless. x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after
cmpxchg (and related move instruction in front of cmpxchg).

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
---
 fs/eventpoll.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index e2daa940ebce..6705cb965fbe 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1065,7 +1065,7 @@ static inline bool list_add_tail_lockless(struct list_head *new,
 	 * added to the list from another CPU: the winner observes
 	 * new->next == new.
 	 */
-	if (cmpxchg(&new->next, new, head) != new)
+	if (!try_cmpxchg(&new->next, &new, head))
 		return false;
 
 	/*
-- 
2.35.3