Date: Wed, 11 Nov 2015 17:44:01 +0000
From: Will Deacon <will.deacon@arm.com>
To: David Miller <davem@davemloft.net>
Cc: alexei.starovoitov@gmail.com, daniel@iogearbox.net, peterz@infradead.org,
        arnd@arndb.de, yang.shi@linaro.org, linaro-kernel@lists.linaro.org,
        eric.dumazet@gmail.com, zlim.lnx@gmail.com, ast@kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        xi.wang@gmail.com, catalin.marinas@arm.com,
        linux-arm-kernel@lists.infradead.org, yhs@plumgrid.com,
        bblanco@plumgrid.com
Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction
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On Wed, Nov 11, 2015 at 12:35:48PM -0500, David Miller wrote:
> From: Alexei Starovoitov <alexei.starovoitov@gmail.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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