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Date: Wed, 11 Nov 2015 13:58:07 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Will Deacon <will.deacon@....com>
Cc: Daniel Borkmann <daniel@...earbox.net>,
Arnd Bergmann <arnd@...db.de>,
"Shi, Yang" <yang.shi@...aro.org>, linaro-kernel@...ts.linaro.org,
Eric Dumazet <eric.dumazet@...il.com>,
Z Lim <zlim.lnx@...il.com>,
Alexei Starovoitov <ast@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>,
Xi Wang <xi.wang@...il.com>,
Catalin Marinas <catalin.marinas@....com>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction
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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