Re: [PATCHv14 bpf-next 3/6] xdp: add a new helper for dev map multicast support

Toke Høiland-Jørgensen
2021-01-18T10:54:33Z

Hangbin Liu <liuhangbin@gmail.com> writes:

> Hi John,
>
> Thanks for the reviewing.
>
> On Sun, Jan 17, 2021 at 04:10:40PM -0800, John Fastabend wrote:
>> > + * The forwarding *map* could be either BPF_MAP_TYPE_DEVMAP or
>> > + * BPF_MAP_TYPE_DEVMAP_HASH. But the *ex_map* must be
>> > + * BPF_MAP_TYPE_DEVMAP_HASH to get better performance.
>>
>> Would be good to add a note ex_map _must_ be keyed by ifindex for the
>> helper to work. Its the obvious way to key a hashmap, but not required
>> iirc.
>
> OK, I will.
>> > + if (!next_obj)
>> > + last_one = true;
>> > +
>> > + if (last_one) {
>> > + bq_enqueue(obj->dev, xdpf, dev_rx, obj->xdp_prog);
>> > + return 0;
>> > + }
>>
>> Just collapse above to
>>
>> if (!next_obj) {
>> bq_enqueue()
>> return
>> }
>>
>> 'last_one' is a bit pointless here.
>
> Yes, thanks.
>
>> > @@ -3986,12 +3993,14 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>> > {
>> > struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>> > struct bpf_map *map = READ_ONCE(ri->map);
>> > + struct bpf_map *ex_map = ri->ex_map;
>>
>> READ_ONCE(ri->ex_map)?
>>
>> > u32 index = ri->tgt_index;
>> > void *fwd = ri->tgt_value;
>> > int err;
>> >
>> > ri->tgt_index = 0;
>> > ri->tgt_value = NULL;
>> > + ri->ex_map = NULL;
>>
>> WRITE_ONCE(ri->ex_map)?
>>
>> > WRITE_ONCE(ri->map, NULL);
>>
>> So we needed write_once, read_once pairs for ri->map do we also need them in
>> the ex_map case?
>
> Toke said this is no need for this read/write_once as there is already one.
>
> https://lore.kernel.org/bpf/87r1wd2bqu.fsf@toke.dk/

And then I corrected that after I figured out the real reason :)

https://lore.kernel.org/bpf/878si2h3sb.fsf@toke.dk/ - Quote:

> The READ_ONCE() is not needed because the ex_map field is only ever read
> from or written to by the CPU owning the per-cpu pointer. Whereas the
> 'map' field is manipulated by remote CPUs in bpf_clear_redirect_map().
> So you need neither READ_ONCE() nor WRITE_ONCE() on ex_map, just like
> there are none on tgt_index and tgt_value.

-Toke