netdev - Re: [Patch net 1/2] l2tp: convert l2tp_tunnel

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Date:   Mon, 9 Jan 2023 22:49:41 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     netdev@...r.kernel.org, g.nault@...halink.fr,
        Cong Wang <cong.wang@...edance.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net 1/2] l2tp: convert l2tp_tunnel_list to idr

On Sat, Jan 07, 2023 at 08:48:51PM +0100, Guillaume Nault wrote:
> On Thu, Jan 05, 2023 at 11:13:38AM -0800, Cong Wang wrote:
> > +int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id,
> > +		       u32 peer_tunnel_id, struct l2tp_tunnel_cfg *cfg,
> > +		       struct l2tp_tunnel **tunnelp)
> >  {
> >  	struct l2tp_tunnel *tunnel = NULL;
> >  	int err;
> >  	enum l2tp_encap_type encap = L2TP_ENCAPTYPE_UDP;
> > +	struct l2tp_net *pn = l2tp_pernet(net);
> >  
> >  	if (cfg)
> >  		encap = cfg->encap;
> >  
> > +	spin_lock_bh(&pn->l2tp_tunnel_idr_lock);
> > +	err = idr_alloc_u32(&pn->l2tp_tunnel_idr, NULL, &tunnel_id, tunnel_id,
> > +			    GFP_ATOMIC);
> > +	if (err) {
> > +		spin_unlock_bh(&pn->l2tp_tunnel_idr_lock);
> > +		return err;
> > +	}
> > +	spin_unlock_bh(&pn->l2tp_tunnel_idr_lock);
> 
> Why reserving the tunnel_id in l2tp_tunnel_create()? This function is
> supposed to just allocate a structure and pre-initialise some fields.
> The only cleanup required upon error after this call is to kfree() the
> new structure. So I can't see any reason to guarantee the id will be
> accepted by the future l2tp_tunnel_register() call.
> 
> Looks like you could reserve the id at the beginning of
> l2tp_tunnel_register() instead. That'd avoid changing the API and thus
> the side effects on l2tp_{ppp,netlink}.c. Also we wouldn't need create
> l2tp_tunnel_remove().
> 

The idr_replace() is guaranteed to succeed in terms of ID allocation.
So either way could work, but I think you are right that the patch could
be smaller if we do it in l2tp_tunnel_register().

Thanks.