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

From: Will Deacon
Date: Wed Nov 11 2015 - 12:44:05 EST

On Wed, Nov 11, 2015 at 12:35:48PM -0500, David Miller wrote:
> From: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
> 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.

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