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

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Date:	Wed, 11 Nov 2015 17:44:01 +0000
From:	Will Deacon <will.deacon@....com>
To:	David Miller <davem@...emloft.net>
Cc:	alexei.starovoitov@...il.com, daniel@...earbox.net,
	peterz@...radead.org, arnd@...db.de, yang.shi@...aro.org,
	linaro-kernel@...ts.linaro.org, eric.dumazet@...il.com,
	zlim.lnx@...il.com, ast@...nel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, xi.wang@...il.com, catalin.marinas@....com,
	linux-arm-kernel@...ts.infradead.org, yhs@...mgrid.com,
	bblanco@...mgrid.com
Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction

On Wed, Nov 11, 2015 at 12:35:48PM -0500, David Miller wrote:
> From: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Date: Wed, 11 Nov 2015 09:27:00 -0800
> 
> > BPF_XADD == atomic_add() in kernel. period.
> > we are not going to deprecate it or introduce something else.
> 
> Agreed, it makes no sense to try and tie C99 or whatever atomic
> semantics to something that is already clearly defined to have
> exactly kernel atomic_add() semantics.

... and which is emitted by LLVM when asked to compile __sync_fetch_and_add,
which has clearly defined (yet conflicting) semantics.

If the discrepancy is in LLVM (and it sounds like it is), then I'll raise
a bug over there instead.

Will
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