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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Thu, 21 Oct 2021 16:24:33 -0700
Message-ID: <CAADnVQ+cJLYL-r6S8TixJxH1JEXXaNojVoewB3aKcsi7Y8XPdQ@mail.gmail.com>
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     X86 ML <x86@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        bpf <bpf@vger.kernel.org>, Andrii Nakryiko <andrii@kernel.org>
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On Thu, Oct 21, 2021 at 3:40 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Oct 21, 2021 at 11:03:33AM -0700, Alexei Starovoitov wrote:
>
> > > I nicked it from emit_bpf_tail_call() in the 32bit jit :-) It seemed a
> > > lot more robust than the 64bit one and I couldn't figure out why the
> > > difference.
> >
> > Interesting. Daniel will recognize that trick then :)
>
> > > Is there concurrency on the jit?
> >
> > The JIT of different progs can happen in parallel.
>
> In that case I don't think the patch is safe. I'll see if I can find a
> variant that doesn't use static storage.

The variable can only change from one fixed value to another fixed value.
Different threads will compute the same value. So I think it's safe
as-is. READ_ONCE/WRITE_ONCE won't hurt though.