Date: Wed, 11 Nov 2015 13:58:07 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>, Arnd Bergmann <arnd@arndb.de>,
        "Shi, Yang" <yang.shi@linaro.org>, linaro-kernel@lists.linaro.org,
        Eric Dumazet <eric.dumazet@gmail.com>, Z Lim <zlim.lnx@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        Xi Wang <xi.wang@gmail.com>, Catalin Marinas <catalin.marinas@arm.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction
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On Wed, Nov 11, 2015 at 12:38:31PM +0000, Will Deacon wrote:
> > Hmm, gcc doesn't have an eBPF compiler backend, so this won't work on
> > gcc at all. The eBPF backend in LLVM recognizes the __sync_fetch_and_add()
> > keyword and maps that to a BPF_XADD version (BPF_W or BPF_DW). In the
> > interpreter (__bpf_prog_run()), as Eric mentioned, this maps to atomic_add()
> > and atomic64_add(), respectively. So the struct bpf_insn prog[] you saw
> > from sock_example.c can be regarded as one possible equivalent program
> > section output from the compiler.
> 
> Ok, so if I understand you correctly, then __sync_fetch_and_add() has
> different semantics depending on the backend target. That seems counter
> to the LLVM atomics Documentation:
> 
>   http://llvm.org/docs/Atomics.html
> 
> which specifically calls out the __sync_* primitives as being
> sequentially-consistent and requiring barriers on ARM (which isn't the
> case for atomic[64]_add in the kernel).
> 
> If we re-use the __sync_* naming scheme in the source language, I don't
> think we can overlay our own semantics in the backend. The
> __sync_fetch_and_add primitive is also expected to return the old value,
> which doesn't appear to be the case for BPF_XADD.

Yikes. That's double fail. Please don't do this.

If you use the __sync stuff (and I agree with Will, you should not) it
really _SHOULD_ be sequentially consistent, which means full barriers
all over the place.

And if you name something XADD (exchange and add, or fetch-add) then it
had better return the previous value.

atomic*_add() does neither.
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