linux-kernel - Re: [PATCH] namespace: Use atomic64_inc_return() in alloc_mnt

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Message-ID: <20241007-unklar-wurzel-ce2d0693dfc8@brauner>
Date: Mon, 7 Oct 2024 16:56:51 +0200
From: Christian Brauner <brauner@...nel.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Uros Bizjak <ubizjak@...il.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] namespace: Use atomic64_inc_return() in alloc_mnt_ns()

On Mon, Oct 07, 2024 at 03:50:34PM GMT, Al Viro wrote:
> On Mon, Oct 07, 2024 at 10:52:37AM +0200, Uros Bizjak wrote:
> > Use atomic64_inc_return(&ref) instead of atomic64_add_return(1, &ref)
> > to use optimized implementation and ease register pressure around
> > the primitive for targets that implement optimized variant.
> > 
> > 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/namespace.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index 93c377816d75..9a3c251d033d 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -3901,7 +3901,7 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns, bool a
> >  	}
> >  	new_ns->ns.ops = &mntns_operations;
> >  	if (!anon)
> > -		new_ns->seq = atomic64_add_return(1, &mnt_ns_seq);
> > +		new_ns->seq = atomic64_inc_return(&mnt_ns_seq);
> 
> On which load do you see that path hot enough for the change to
> make any difference???

I don't think that's really an issue. Imho, *inc_return() is just
straightforward compared to the add variant. That can easily be
reflected in the commit message when I push out.