netdev - Re: [RFC net-next 5/8] nfp: bpf: take a reference on offloaded programs

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Date:   Tue, 20 Jun 2017 01:23:05 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        netdev@...r.kernel.org
CC:     davem@...emloft.net, kafai@...com, netoptimizer@...uer.com,
        oss-drivers@...ronome.com
Subject: Re: [RFC net-next 5/8] nfp: bpf: take a reference on offloaded programs

On 06/17/2017 01:57 AM, Jakub Kicinski wrote:
> The xdp_prog member of the adapter's data path structure is used
> for XDP in driver mode.  In case a XDP program is loaded with in
> HW-only mode, we need to store it somewhere else.  Add a new XDP
> prog pointer in the main structure and use that when we need to
> know whether any XDP program is loaded, not only a driver mode
> one.  Only release our reference on adapter free instead of
> immediately after netdev unregister to allow offload to be disabled
> first.
>
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
[...]
> @@ -3327,6 +3323,10 @@ nfp_net_xdp_setup(struct nfp_net *nn, struct bpf_prog *prog, u32 flags,
>   		return err;
>
>   	nfp_app_xdp_offload(nn->app, nn, offload_prog);
> +
> +	if (nn->xdp_prog)
> +		bpf_prog_put(nn->xdp_prog);
> +	nn->xdp_prog = prog;
>   	nn->xdp_flags = flags;
>
>   	return 0;

Can you elaborate on the extra reference on the prog? So in
nfp_net_xdp_setup(), assuming a prog was already loaded on
driver side: after your set, nfp_net_xdp_setup_drv() will then
do the xchg() on nn->dp.xdp_prog, bpf_prog_put() this one and
later back in nfp_net_xdp_setup() we check nn->xdp_prog and
bpf_prog_put() it if it existed before and update nn->xdp_prog
to the current prog. So you end up with two puts on the same
program, but I don't see where you take the one additional ref
aside from the ref that you already get from dev_change_xdp_fd().
What am I missing?