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

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Date:   Fri, 11 Jun 2021 01:41:17 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Martin KaFai Lau <kafai@...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

Martin KaFai Lau <kafai@...com> writes:

> 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));

Right, that's what I had in mind as well :)

-Toke