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

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Message-ID: <20210303153932.GB19247@willie-the-truck>
Date:   Wed, 3 Mar 2021 15:39:32 +0000
From:   Will Deacon <will@...nel.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     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,
        magnus.karlsson@...el.com, jonathan.lemon@...il.com,
        maximmi@...dia.com, andrii@...nel.org
Subject: Re: [PATCH bpf-next 2/2] libbpf, xsk: add libbpf_smp_store_release
 libbpf_smp_load_acquire

On Tue, Mar 02, 2021 at 10:13:21AM +0100, 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.
> 
> Would also be great to get Will's ACK on that when you have a v2. :)

Please stick me on CC for that and I'll take a look as I've forgotten pretty
much everything about this since last time :)

Will