fs: Use try_cmpxchg() in sb_init_done_wq()

[PATCH] fs: Use try_cmpxchg() in sb_init_done_wq()

Posted by Uros Bizjak 1 month, 3 weeks ago

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@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.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

Re: [PATCH] fs: Use try_cmpxchg() in sb_init_done_wq()

Posted by Christian Brauner 1 month, 1 week ago

On Mon, 11 Aug 2025 15:23:03 +0200, Uros Bizjak wrote:
> 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.
> 
> [...]

Applied to the vfs-6.18.misc branch of the vfs/vfs.git tree.
Patches in the vfs-6.18.misc branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs-6.18.misc

[1/1] fs: Use try_cmpxchg() in sb_init_done_wq()
      https://git.kernel.org/vfs/vfs/c/ec6f613ef376

Re: [PATCH] fs: Use try_cmpxchg() in sb_init_done_wq()

Posted by Jan Kara 1 month, 1 week ago

On Mon 11-08-25 15:23:03, Uros Bizjak wrote:
> 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@gmail.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Jan Kara <jack@suse.cz>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  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
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR