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 10:16:52 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        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 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...?


> 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/