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Date:	Tue, 10 Nov 2015 16:42:09 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	"Shi, Yang" <yang.shi@...aro.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, ast@...nel.org,
	daniel@...earbox.net, catalin.marinas@....com, will.deacon@....com,
	zlim.lnx@...il.com, xi.wang@...il.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction

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:
> >>
> >>Load (dst + off) to a register
> >>Add src to it
> >>Store it back to (dst + off)
> >
> >Not really what is needed ?
> >
> >See this BPF_XADD as an atomic_add() equivalent.
> 
> I see. Thanks. The documentation doesn't say too much about "exclusive" add.
> If so it should need load-acquire/store-release.

I think doc is clear enough, but it can always be improved. Pls suggest a patch.
It's quite hard to write a test for atomicity in test_bpf framework, so
code review is the key. Eric, thanks for catching it!

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