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 10:25:20 +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,
        paulmck@...nel.org
Subject: Re: [PATCH bpf-next 2/2] libbpf, xsk: add libbpf_smp_store_release
 libbpf_smp_load_acquire

On 3/2/21 10:16 AM, Björn Töpel wrote:
> On 2021-03-02 10:13, Daniel Borkmann wrote:
>> 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.
> 
> I agree; It's enough in the weed as it is already.
> 
> I wonder if it's possible to cook a LKMM litmus test for this...?

That would be amazing! :-)

(Another option which can be done independently could be to update [0] with outlining a
  pairing scenario as we have here describing the forward/backward compatibility on the
  barriers used, I think that would be quite useful as well.)

   [0] Documentation/memory-barriers.txt

>> Would also be great to get Will's ACK on that when you have a v2. :)
> 
> Yup! :-)
> 
> 
> Björn
> 
> 
>> Thanks,
>> Daniel
>>
>>> [1] https://lore.kernel.org/bpf/20200316184423.GA14143@willie-the-truck/