Re: [PATCH] namespace: Use atomic64_inc_return() in alloc_mnt_ns()

[Christian Brauner]

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@xxxxxxxxx>
> > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > ---
> >  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.