linux-kernel - [PATCH] fs: Use try_cmpxchg() in sb_init_done

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Message-ID: <20250811132326.620521-1-ubizjak@gmail.com>
Date: Mon, 11 Aug 2025 15:23:03 +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>,
	Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>
Subject: [PATCH] fs: Use try_cmpxchg() in sb_init_done_wq()

Use !try_cmpxchg() instead of cmpxchg(*ptr, old, new) != old.

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after CMPXCHG.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>
---
 fs/super.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 7f876f32343a..e91718017701 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -2318,13 +2318,15 @@ int sb_init_dio_done_wq(struct super_block *sb)
 						      sb->s_id);
 	if (!wq)
 		return -ENOMEM;
+
+	old = NULL;
 	/*
 	 * This has to be atomic as more DIOs can race to create the workqueue
 	 */
-	old = cmpxchg(&sb->s_dio_done_wq, NULL, wq);
-	/* Someone created workqueue before us? Free ours... */
-	if (old)
+	if (!try_cmpxchg(&sb->s_dio_done_wq, &old, wq)) {
+		/* Someone created workqueue before us? Free ours... */
 		destroy_workqueue(wq);
+	}
 	return 0;
 }
 EXPORT_SYMBOL_GPL(sb_init_dio_done_wq);
-- 
2.50.1