On Thu, 22 Apr 2021 20:02:18 +0200
Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
> 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);
> + }
This is highly sensitive fast-path code, as you saw Bjørn have been
hunting nanosec in this area. The above code implicitly have "map" as
the likely option, which I don't think it is.
> (and the same for the generic-XDP path, of course)
>
> -Toke
>
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
