netdev - Re: [PATCH bpf-next 2/2] libbpf, xsk: add libbpf_smp_store_release libbpf_smp_load

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Date:   Tue, 2 Mar 2021 10:13:21 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, netdev@...r.kernel.org, bpf@...r.kernel.org
Cc:     magnus.karlsson@...el.com, jonathan.lemon@...il.com,
        maximmi@...dia.com, andrii@...nel.org, will@...nel.org
Subject: Re: [PATCH bpf-next 2/2] libbpf, xsk: add libbpf_smp_store_release
 libbpf_smp_load_acquire

On 3/2/21 9:05 AM, Björn Töpel wrote:
> On 2021-03-01 17:10, Toke Høiland-Jørgensen wrote:
>> Björn Töpel <bjorn.topel@...il.com> writes:
>>> From: Björn Töpel <bjorn.topel@...el.com>
>>>
>>> Now that the AF_XDP rings have load-acquire/store-release semantics,
>>> move libbpf to that as well.
>>>
>>> The library-internal libbpf_smp_{load_acquire,store_release} are only
>>> valid for 32-bit words on ARM64.
>>>
>>> Also, remove the barriers that are no longer in use.
>>
>> So what happens if an updated libbpf is paired with an older kernel (or
>> vice versa)?
> 
> "This is fine." ;-) This was briefly discussed in [1], outlined by the
> previous commit!
> 
> ...even on POWER.

Could you put a summary or quote of that discussion on 'why it is okay and does not
cause /forward or backward/ compat issues with user space' directly into patch 1's
commit message?

I feel just referring to a link is probably less suitable in this case as it should
rather be part of the commit message that contains the justification on why it is
waterproof - at least it feels that specific area may be a bit under-documented, so
having it as direct part certainly doesn't hurt.

Would also be great to get Will's ACK on that when you have a v2. :)

Thanks,
Daniel

> [1] https://lore.kernel.org/bpf/20200316184423.GA14143@willie-the-truck/