netdev - RE: [PATCH net-next 1/9] net/packet: annotate accesses to po->xmit

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Message-ID: <641333d07c6b0_3333c72088e@willemb.c.googlers.com.notmuch>
Date:   Thu, 16 Mar 2023 11:20:48 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, Willem de Bruijn <willemb@...gle.com>,
        eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH net-next 1/9] net/packet: annotate accesses to po->xmit

Eric Dumazet wrote:
> po->xmit can be set from setsockopt(PACKET_QDISC_BYPASS),
> while read locklessly.
> 
> Use READ_ONCE()/WRITE_ONCE() to avoid potential load/store
> tearing issues.
> 
> Fixes: d346a3fae3ff ("packet: introduce PACKET_QDISC_BYPASS socket option")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/packet/af_packet.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index d4e76e2ae153ea3b3c7d73ef6ac2f8c2742f790d..d25dd9f63cc4f11ad8197ab66d60180b6358132f 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -307,7 +307,8 @@ static void packet_cached_dev_reset(struct packet_sock *po)
>  
>  static bool packet_use_direct_xmit(const struct packet_sock *po)
>  {
> -	return po->xmit == packet_direct_xmit;
> +	/* Paired with WRITE_ONCE() in packet_setsockopt() */
> +	return READ_ONCE(po->xmit) == packet_direct_xmit;
>  }
>  
>  static u16 packet_pick_tx_queue(struct sk_buff *skb)
> @@ -2867,7 +2868,8 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
>  		packet_inc_pending(&po->tx_ring);
>  
>  		status = TP_STATUS_SEND_REQUEST;
> -		err = po->xmit(skb);
> +		/* Paired with WRITE_ONCE() in packet_setsockopt() */
> +		err = READ_ONCE(po->xmit)(skb);
>  		if (unlikely(err != 0)) {
>  			if (err > 0)
>  				err = net_xmit_errno(err);
> @@ -3070,7 +3072,8 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
>  		virtio_net_hdr_set_proto(skb, &vnet_hdr);
>  	}
>  
> -	err = po->xmit(skb);
> +	/* Paired with WRITE_ONCE() in packet_setsockopt() */
> +	err = READ_ONCE(po->xmit)(skb);
>  	if (unlikely(err != 0)) {
>  		if (err > 0)
>  			err = net_xmit_errno(err);
> @@ -4007,7 +4010,8 @@ packet_setsockopt(struct socket *sock, int level, int optname, sockptr_t optval,
>  		if (copy_from_sockptr(&val, optval, sizeof(val)))
>  			return -EFAULT;
>  
> -		po->xmit = val ? packet_direct_xmit : dev_queue_xmit;
> +		/* Paired with all lockless reads of po->xmit */
> +		WRITE_ONCE(po->xmit, val ? packet_direct_xmit : dev_queue_xmit);

We could avoid the indirect function call with a branch on
packet_use_direct_xmit() and just store a bit?

>  		return 0;
>  	}
>  	default:
> -- 
> 2.40.0.rc2.332.ga46443480c-goog
>