Re: [PATCH bpf-next v2 1/8] xdp: restructure redirect actions

Björn Töpel [off-list ref] writes:

From: Björn Töpel <redacted>

The XDP_REDIRECT implementations for maps and non-maps are fairly
similar, but obviously need to take different code paths depending on
if the target is using a map or not. Today, the redirect targets for
XDP either uses a map, or is based on ifindex.

Future commits will introduce yet another redirect target via the a
new helper, bpf_redirect_xsk(). To pave the way for that, we introduce
an explicit redirect type to bpf_redirect_info. This makes the code
easier to follow, and makes it easier to add new redirect targets.

Further, using an explicit type in bpf_redirect_info has a slight
positive performance impact by avoiding a pointer indirection for the
map type lookup, and instead use the hot cacheline for
bpf_redirect_info.

The bpf_redirect_info flags member is not used by XDP, and not
read/written any more. The map member is only written to when
required/used, and not unconditionally.

I like the simplification. However, the handling of map clearing becomes
a bit murky with this change:

You're not changing anything in bpf_clear_redirect_map(), and you're
removing most of the reads and writes of ri->map. Instead,
bpf_xdp_redirect_map() will store the bpf_dtab_netdev pointer in
ri->tgt_value, which xdp_do_redirect() will just read and use without
checking. But if the map element (or the entire map) has been freed in
the meantime that will be a dangling pointer. I *think* the RCU callback
in dev_map_delete_elem() and the rcu_barrier() in dev_map_free()
protects against this, but that is by no means obvious. So confirming
this, and explaining it in a comment would be good.

Also, as far as I can tell after this, ri->map is only used for the
tracepoint. So how about just storing the map ID and getting rid of the
READ/WRITE_ONCE() entirely?

(Oh, and related to this I think this patch set will conflict with
Hangbin's multi-redirect series, so maybe you two ought to coordinate? :))

-Toke