netdev - Re: [PATCHv9 bpf-next 2/4] xdp: extend xdp_redirect

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Date:   Thu, 22 Apr 2021 20:02:18 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Hangbin Liu <liuhangbin@...il.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Jiri Benc <jbenc@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Björn Töpel 
        <bjorn.topel@...il.com>, Martin KaFai Lau <kafai@...com>,
        brouer@...hat.com
Subject: Re: [PATCHv9 bpf-next 2/4] xdp: extend xdp_redirect_map with
 broadcast support

Jesper Dangaard Brouer <brouer@...hat.com> writes:

> On Thu, 22 Apr 2021 15:14:52 +0800
> Hangbin Liu <liuhangbin@...il.com> wrote:
>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index cae56d08a670..afec192c3b21 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
> [...]
>>  int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>>  		    struct bpf_prog *xdp_prog)
>>  {
>> @@ -3933,6 +3950,7 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>>  	enum bpf_map_type map_type = ri->map_type;
>>  	void *fwd = ri->tgt_value;
>>  	u32 map_id = ri->map_id;
>> +	struct bpf_map *map;
>>  	int err;
>>  
>>  	ri->map_id = 0; /* Valid map id idr range: [1,INT_MAX[ */
>> @@ -3942,7 +3960,12 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>>  	case BPF_MAP_TYPE_DEVMAP:
>>  		fallthrough;
>>  	case BPF_MAP_TYPE_DEVMAP_HASH:
>> -		err = dev_map_enqueue(fwd, xdp, dev);
>> +		map = xchg(&ri->map, NULL);
>
> Hmm, this looks dangerous for performance to have on this fast-path.
> The xchg call can be expensive, AFAIK this is an atomic operation.

Ugh, you're right. That's my bad, I suggested replacing the
READ_ONCE()/WRITE_ONCE() pair with the xchg() because an exchange is
what it's doing, but I failed to consider the performance implications
of the atomic operation. Sorry about that, Hangbin! I guess this should
be changed to:

+		map = READ_ONCE(ri->map);
+		if (map) {
+			WRITE_ONCE(ri->map, NULL);
+			err = dev_map_enqueue_multi(xdp, dev, map,
+						    ri->flags & BPF_F_EXCLUDE_INGRESS);
+		} else {
+			err = dev_map_enqueue(fwd, xdp, dev);
+		}

(and the same for the generic-XDP path, of course)

-Toke