Re: [PATCH v3 net-next 1/1] net_sched: Introduce skbmod action

[Eric Dumazet]

On Mon, 2016-08-29 at 07:35 -0400, Jamal Hadi Salim wrote:
>        flags = READ_ONCE(d->flags);
>         rcu_read_lock();
>         if (flags & SKBMOD_F_DMAC)
>                 ether_addr_copy(eth_hdr(skb)->h_dest, d->eth_dst);
>         if (flags & SKBMOD_F_SMAC)
>                 ether_addr_copy(eth_hdr(skb)->h_source, d->eth_src);
>         if (flags & SKBMOD_F_ETYPE)
>                 eth_hdr(skb)->h_proto = d->eth_type;
>         if (flags & SKBMOD_F_SWAPMAC) {
>                 u8 tmpaddr[ETH_ALEN];
>                 /*XXX: I am sure we can come up with something more
> efficient */
>                 ether_addr_copy(tmpaddr, eth_hdr(skb)->h_dest);
>                 ether_addr_copy(eth_hdr(skb)->h_dest,
> eth_hdr(skb)->h_source);
>                 ether_addr_copy(eth_hdr(skb)->h_source, tmpaddr);
>         }
>         rcu_read_unlock();

You would need to store everything in an object, managed by rcu.

struct my_rcu_safe_struct {
     u32 flags;
     u8   eth_dst[ETH_ALEN];
     u8   eth_src[ETH_ALEN];
     __be16 eth_type;
};

And then allocate a new one when you need to update the infos (from
tcf_skbmod_init(())

RCU : Read Copy Update.

Then in the reader you would use 

rcu_read_lock();
myptr = rcu_dereference(d->ptr);
if (myptr) {
        if (myptr->flags & SKBMOD_F_DMAC)
                 ether_addr_copy(eth_hdr(skb)->h_dest, myptr->eth_dst);
         if (myptr->flags & SKBMOD_F_SMAC)
                 ether_addr_copy(eth_hdr(skb)->h_source, myptr->eth_src);
         if (myptr->flags & SKBMOD_F_ETYPE)
                 eth_hdr(skb)->h_proto = myptr->eth_type;
}
rcu_read_unlock();