Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes:
> On Thu, 22 Apr 2021 15:14:52 +0800
> Hangbin Liu <liuhangbin@xxxxxxxxx> 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
