netdev - Re: [PATCH bpf] xsk: do not call synchronize

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Date:   Mon, 8 Oct 2018 17:30:39 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, eric.dumazet@...il.com,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>
Subject: Re: [PATCH bpf] xsk: do not call synchronize_net() under RCU read lock

On Mon, Oct 8, 2018 at 10:41 AM Björn Töpel <bjorn.topel@...il.com> wrote:
>
> From: Björn Töpel <bjorn.topel@...el.com>
>
> The XSKMAP update and delete functions called synchronize_net(), which
> can sleep. It is not allowed to sleep during an RCU read section.
>
> Instead we need to make sure that the sock sk_destruct (xsk_destruct)
> function is asynchronously called after an RCU grace period. Setting
> the SOCK_RCU_FREE flag for XDP sockets takes care of this.
>
> Fixes: fbfc504a24f5 ("bpf: introduce new bpf AF_XDP map type BPF_MAP_TYPE_XSKMAP")
> Reported-by: Eric Dumazet <eric.dumazet@...il.com>
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
Acked-by: Song Liu <songliubraving@...com>

> ---
>  kernel/bpf/xskmap.c | 10 ++--------
>  net/xdp/xsk.c       |  2 ++
>  2 files changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/bpf/xskmap.c b/kernel/bpf/xskmap.c
> index 9f8463afda9c..47147c9e184d 100644
> --- a/kernel/bpf/xskmap.c
> +++ b/kernel/bpf/xskmap.c
> @@ -192,11 +192,8 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
>         sock_hold(sock->sk);
>
>         old_xs = xchg(&m->xsk_map[i], xs);
> -       if (old_xs) {
> -               /* Make sure we've flushed everything. */
> -               synchronize_net();
> +       if (old_xs)
>                 sock_put((struct sock *)old_xs);
> -       }
>
>         sockfd_put(sock);
>         return 0;
> @@ -212,11 +209,8 @@ static int xsk_map_delete_elem(struct bpf_map *map, void *key)
>                 return -EINVAL;
>
>         old_xs = xchg(&m->xsk_map[k], NULL);
> -       if (old_xs) {
> -               /* Make sure we've flushed everything. */
> -               synchronize_net();
> +       if (old_xs)
>                 sock_put((struct sock *)old_xs);
> -       }
>
>         return 0;
>  }
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 0577cd49aa72..07156f43d295 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -754,6 +754,8 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
>         sk->sk_destruct = xsk_destruct;
>         sk_refcnt_debug_inc(sk);
>
> +       sock_set_flag(sk, SOCK_RCU_FREE);
> +
>         xs = xdp_sk(sk);
>         mutex_init(&xs->mutex);
>         spin_lock_init(&xs->tx_completion_lock);
> --
> 2.17.1
>