linux-kernel - Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch

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Message-ID: <20201202111902.GC9710@google.com>
Date:   Wed, 2 Dec 2020 11:19:02 +0000
From:   Brendan Jackman <jackmanb@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Yonghong Song <yhs@...com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for
 atomic[64]_[fetch_]sub

On Tue, Dec 01, 2020 at 09:55:22PM -0800, Alexei Starovoitov wrote:
> On Tue, Dec 1, 2020 at 4:38 AM Brendan Jackman <jackmanb@...gle.com> wrote:
> >
> > I guess it's also worth remembering other archs might have an atomic
> > subtract.
> 
> which one?
> arm64 LSE implements atomic_fetch_sub as neg+ldadd.
> imo x64 and arm64 example outweighs choices by other archs if there are such.
> Even without LSE it will be neg+llsc loop.
> The reason I proposed bpf xsub insn earlier is that I thought that llvm
> won't be able to emit it so easily and JIT/verifier would struggle.

Ack, I'll drop the atomic subtract instruction.