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

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Message-ID: <buw7x4axu2vojhr5mtjtrfnbynianng42atvzz7gk2t2tbjvqc@tvfo7bbjkgsx>
Date: Fri, 22 Aug 2025 16:20:57 +0200
From: Jan Kara <jack@...e.cz>
To: Uros Bizjak <ubizjak@...il.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] fs: Use try_cmpxchg() in start_dir_add()

On Mon 11-08-25 14:52:38, 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 (and related
> move instruction in front of CMPXCHG).
> 
> Note that the value from *ptr should be read using READ_ONCE() to
> prevent the compiler from merging, refetching or reordering the read.
> 
> 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>

Looks good. Feel free to add:

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

								Honza

> ---
>  fs/dcache.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 60046ae23d51..336bdb4c4b1f 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -2509,8 +2509,8 @@ static inline unsigned start_dir_add(struct inode *dir)
>  {
>  	preempt_disable_nested();
>  	for (;;) {
> -		unsigned n = dir->i_dir_seq;
> -		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
> +		unsigned n = READ_ONCE(dir->i_dir_seq);
> +		if (!(n & 1) && try_cmpxchg(&dir->i_dir_seq, &n, n + 1))
>  			return n;
>  		cpu_relax();
>  	}
> -- 
> 2.50.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR