netdev - Re: [PATCH bpf-next] bpf/devmap: convert remaining READ_ONCE() to rcu

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Message-ID: <20210628235559.sgq7txsjmz3s2zeb@kafai-mbp.dhcp.thefacebook.com>
Date:   Mon, 28 Jun 2021 16:55:59 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH bpf-next] bpf/devmap: convert remaining READ_ONCE() to
 rcu_dereference()

On Tue, Jun 29, 2021 at 01:00:51AM +0200, Toke Høiland-Jørgensen wrote:
> There were a couple of READ_ONCE()-invocations left-over by the devmap RCU
> conversion. Convert these to rcu_dereference() as well to avoid complaints
> from sparse.
> 
> Fixes: 782347b6bcad ("xdp: Add proper __rcu annotations to redirect map entries")
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  kernel/bpf/devmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 2f6bd75cd682..7a0c008f751b 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -558,7 +558,7 @@ int dev_map_enqueue_multi(struct xdp_buff *xdp, struct net_device *dev_rx,
>  
>  	if (map->map_type == BPF_MAP_TYPE_DEVMAP) {
>  		for (i = 0; i < map->max_entries; i++) {
> -			dst = READ_ONCE(dtab->netdev_map[i]);
> +			dst = rcu_dereference(dtab->netdev_map[i]);
__dev_map_lookup_elem() uses rcu_dereference_check(dtab->netdev_map[key], rcu_read_lock_bh_held()).
It is not needed here?