linux-kernel - Re: [PATCH V2] arch: patch_text: Fixup last cpu should be master

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Date:   Fri, 18 Mar 2022 00:03:06 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Max Filippov <jcmvbkbc@...il.com>,
        Will Deacon <will@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Chris Zankel <chris@...kel.net>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH V2] arch: patch_text: Fixup last cpu should be master

On Wed, Mar 16, 2022 at 10:38 PM Catalin Marinas
<catalin.marinas@....com> wrote:
>
> On Sun, Mar 13, 2022 at 09:22:21AM +0800, guoren@...nel.org wrote:
> > From: Guo Ren <guoren@...ux.alibaba.com>
> >
> > These patch_text implementations are using stop_machine_cpuslocked
> > infrastructure with atomic cpu_count. The original idea: When the
> > master CPU patch_text, the others should wait for it.
>
> I couldn't find the original intent in the commit logs (at least not in
> the arm64 logs). Maybe the intention was for the CPUs to wait for the
> text patching to complete rather than the master CPU to wait for the
> others to enter the cpu_relax() loop before patching.
>
> I think your patch makes sense anyway, the master CPU would wait for all
> the others to enter the cpu_relax() loop before patching and releasing
> them with another increment. You probably wouldn't see any issues in
> practice unless you insert probes in the multi_stop_cpu() function (or
> we could mark this function as __kprobes and get rid of the extra loops
> entirely).
That could depend on micro-arch, trigger other harts' IPI is not
guaranteed by hw.

>
> > --- a/arch/arm64/kernel/patching.c
> > +++ b/arch/arm64/kernel/patching.c
> > @@ -117,8 +117,8 @@ static int __kprobes aarch64_insn_patch_text_cb(void *arg)
> >       int i, ret = 0;
> >       struct aarch64_insn_patch *pp = arg;
> >
> > -     /* The first CPU becomes master */
> > -     if (atomic_inc_return(&pp->cpu_count) == 1) {
> > +     /* The last CPU becomes master */
> > +     if (atomic_inc_return(&pp->cpu_count) == num_online_cpus()) {
> >               for (i = 0; ret == 0 && i < pp->insn_cnt; i++)
> >                       ret = aarch64_insn_patch_text_nosync(pp->text_addrs[i],
> >                                                            pp->new_insns[i]);
>
> For arm64:
>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
Thx

>
> --
> Catalin



-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/