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:

quoted

On 2021-03-01 17:10, Toke Høiland-Jørgensen wrote:

quoted

Björn Töpel [off-list ref] writes:

quoted

From: Björn Töpel <redacted>

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

quoted

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