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

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Date:   Wed, 20 Jan 2021 14:40:12 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, jonathan.lemon@...il.com, maximmi@...dia.com,
        davem@...emloft.net, hawk@...nel.org, john.fastabend@...il.com,
        ciara.loftus@...el.com, weqaar.a.janjua@...el.com
Subject: Re: [PATCH bpf-next v2 1/8] xdp: restructure redirect actions



On 2021-01-20 13:44, Toke Høiland-Jørgensen wrote:
> Björn Töpel <bjorn.topel@...il.com> writes:
> 
>> From: Björn Töpel <bjorn.topel@...el.com>
>>
>> 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.
>

Yes, *most* of the READ_ONCE(ri->map) are removed, it's pretty much only 
the bpf_redirect_map(), and as you write, the tracepoints.

The content/element of the map is RCU protected, and actually even the
map will be around until the XDP processing is complete. Note the
synchronize_rcu() followed after all bpf_clear_redirect_map() calls.

I'll try to make it clearer in the commit message! Thanks for pointing 
that out!

> 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?
>

...and the bpf_redirect_map() helper. Don't you think the current
READ_ONCE(ri->map) scheme is more obvious/clear?


> (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? :))
>

Yeah, good idea! I would guess Hangbin's would go in before this, so I
would need to adapt.


Thanks for taking of look at the series, Toke! Much appreciated!


Björn