netdev - Re: [PATCH bpf-next 04/17] xdp: add proper __rcu annotations to redirect map entries

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Message-ID: <20210610233250.pef2dwo2r5atluwt@kafai-mbp>
Date:   Thu, 10 Jun 2021 16:32:50 -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>
Subject: Re: [PATCH bpf-next 04/17] xdp: add proper __rcu annotations to
 redirect map entries

On Fri, Jun 11, 2021 at 01:19:04AM +0200, Toke Høiland-Jørgensen wrote:
> >> @@ -149,7 +152,8 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
> >>  			       u64 map_flags)
> >>  {
> >>  	struct xsk_map *m = container_of(map, struct xsk_map, map);
> >> -	struct xdp_sock *xs, *old_xs, **map_entry;
> >> +	struct xdp_sock __rcu **map_entry;
> >> +	struct xdp_sock *xs, *old_xs;
> >>  	u32 i = *(u32 *)key, fd = *(u32 *)value;
> >>  	struct xsk_map_node *node;
> >>  	struct socket *sock;
> >> @@ -179,7 +183,7 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
> >>  	}
> >>  
> >>  	spin_lock_bh(&m->lock);
> >> -	old_xs = READ_ONCE(*map_entry);
> >> +	old_xs = rcu_dereference_check(*map_entry, rcu_read_lock_bh_held());
> > Is it actually protected by the m->lock at this point?
> 
> True, can just add that to the check.
this should be enough
rcu_dereference_protected(*map_entry, lockdep_is_held(&m->lock));