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

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Message-ID: <20210610210907.hgfnlja3hbmgeqxx@kafai-mbp>
Date:   Thu, 10 Jun 2021 14:09:07 -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 Wed, Jun 09, 2021 at 12:33:13PM +0200, Toke Høiland-Jørgensen wrote:
[ ... ]

> @@ -551,7 +551,8 @@ static void cpu_map_free(struct bpf_map *map)
>  	for (i = 0; i < cmap->map.max_entries; i++) {
>  		struct bpf_cpu_map_entry *rcpu;
>  
> -		rcpu = READ_ONCE(cmap->cpu_map[i]);
> +		rcpu = rcu_dereference_check(cmap->cpu_map[i],
> +					     rcu_read_lock_bh_held());
Is rcu_read_lock_bh_held() true during map_free()?

[ ... ]

> @@ -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?

[ ... ]

>  void xsk_map_try_sock_delete(struct xsk_map *map, struct xdp_sock *xs,
> -			     struct xdp_sock **map_entry)
> +			     struct xdp_sock __rcu **map_entry)
>  {
>  	spin_lock_bh(&map->lock);
> -	if (READ_ONCE(*map_entry) == xs) {
> -		WRITE_ONCE(*map_entry, NULL);
> +	if (rcu_dereference(*map_entry) == xs) {
nit. rcu_access_pointer()?

> +		rcu_assign_pointer(*map_entry, NULL);
>  		xsk_map_sock_delete(xs, map_entry);
>  	}
>  	spin_unlock_bh(&map->lock);
> -- 
> 2.31.1
>