Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction

From: Daniel Borkmann <[email-masked]>
Date: Wed, 11 Nov 2015 11:42:11 +0100

On 11/11/2015 11:24 AM, Will Deacon wrote:
> On Wed, Nov 11, 2015 at 09:49:48AM +0100, Arnd Bergmann wrote:
>> On Tuesday 10 November 2015 18:52:45 Z Lim wrote:
>>> On Tue, Nov 10, 2015 at 4:42 PM, Alexei Starovoitov
>>> <[email-masked]> wrote:
>>>> On Tue, Nov 10, 2015 at 04:26:02PM -0800, Shi, Yang wrote:
>>>>> On 11/10/2015 4:08 PM, Eric Dumazet wrote:
>>>>>> On Tue, 2015-11-10 at 14:41 -0800, Yang Shi wrote:
>>>>>>> aarch64 doesn't have native support for XADD instruction, implement it by
>>>>>>> the below instruction sequence:
>>>
>>> aarch64 supports atomic add in ARMv8.1.
>>> For ARMv8(.0), please consider using LDXR/STXR sequence.
>>
>> Is it worth optimizing for the 8.1 case? It would add a bit of complexity
>> to make the code depend on the CPU feature, but it's certainly doable.
>
> What's the atomicity required for? Put another way, what are we racing
> with (I thought bpf was single-threaded)? Do we need to worry about
> memory barriers?
>
> Apologies if these are stupid questions, but all I could find was
> samples/bpf/sock_example.c and it didn't help much :(

The equivalent code more readable in restricted C syntax (that can be
compiled by llvm) can be found in samples/bpf/sockex1_kern.c. So the
built-in __sync_fetch_and_add() will be translated into a BPF_XADD
insn variant.

What you can race against is that an eBPF map can be _shared_ by
multiple eBPF programs that are attached somewhere in the system, and
they could all update a particular entry/counter from the map at the
same time.

Best,
Daniel