linux-kernel - Re: [PATCH v4] riscv: patch: Fixup lockdep warning in stop

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Date:   Wed, 15 Feb 2023 12:01:17 +0100
From:   Björn Töpel <bjorn@...nel.org>
To:     Changbin Du <changbin.du@...wei.com>, Guo Ren <guoren@...nel.org>
Cc:     Conor Dooley <conor@...nel.org>,
        Changbin Du <changbin.du@...wei.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Hui Wang <hw.huiwang@...wei.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Conor Dooley <conor.dooley@...rochip.com>,
        Zong Li <zong.li@...ive.com>
Subject: Re: [PATCH v4] riscv: patch: Fixup lockdep warning in stop_machine

Changbin Du <changbin.du@...wei.com> writes:

> On Wed, Feb 15, 2023 at 09:24:33AM +0800, Guo Ren wrote:
>> On Wed, Feb 15, 2023 at 8:22 AM Conor Dooley <conor@...nel.org> wrote:
>> >
>> > Hey Changbin,
>> >
>> > On Thu, Feb 02, 2023 at 07:41:16PM +0800, Changbin Du wrote:
>> > > The task of ftrace_arch_code_modify(_post)_prepare() caller is
>> > > stop_machine, whose caller and work thread are of different tasks. The
>> > > lockdep checker needs the same task context, or it's wrong. That means
>> > > it's a bug here to use lockdep_assert_held because we don't guarantee
>> > > the same task context.
>> I'm trying to delete all stop_machine in riscv, from ftrace to kprobe.
>> When I have done, we needn't this patch.
>>
> Which approch would you use? I looked through the riscv-spec, but didn't find any
> description abount concurrent modification and execution.

CMODX is not specified for RISC-V yet, unfortunately.

This has been discussed here [1]. Maybe we can start with stating for
which implementations Guo's approach work?

[1] https://lore.kernel.org/all/CAJF2gTS0s4X_uwLaEeSqKAyRmxCR2vxRuHhz7-SP2w4bBqzr+Q@mail.gmail.com/