From:   =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
To:     Guo Ren <guoren@kernel.org>, Mark Rutland <mark.rutland@arm.com>
Cc:     "liaochang (A)" <liaochang1@huawei.com>, palmer@dabbelt.com,
        paul.walmsley@sifive.com, mhiramat@kernel.org,
        conor.dooley@microchip.com, penberg@kernel.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        Guo Ren <guoren@linux.alibaba.com>
Subject: Re: [PATCH] riscv: kprobe: Optimize kprobe with accurate atomicity
In-Reply-To: <CAJF2gTRgze_owuWvJjnrPpBNs8+GY-km7wvHU4EuJzarQc+BPQ@mail.gmail.com>
References: <20230126161559.1467374-1-guoren@kernel.org>
 <0abbbdd4-6b85-9659-03ee-97c56a5b77c1@huawei.com>
 <CAJF2gTS0s4X_uwLaEeSqKAyRmxCR2vxRuHhz7-SP2w4bBqzr+Q@mail.gmail.com>
 <87r0vc9h4g.fsf@all.your.base.are.belong.to.us>
 <Y9fm+6LPXgtDSma/@FVFF77S0Q05N>
 <CAJF2gTRgze_owuWvJjnrPpBNs8+GY-km7wvHU4EuJzarQc+BPQ@mail.gmail.com>
Date:   Tue, 31 Jan 2023 08:12:22 +0100
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Guo Ren <guoren@kernel.org> writes:

> On Mon, Jan 30, 2023 at 11:49 PM Mark Rutland <mark.rutland@arm.com> wrot=
e:
>>
>> Hi Bjorn,
>>
>> On Mon, Jan 30, 2023 at 04:28:15PM +0100, Bj=C3=B6rn T=C3=B6pel wrote:
>> > Guo Ren <guoren@kernel.org> writes:
>> >
>> > >> In the serie of RISCV OPTPROBES [1], it patches a long-jump instruc=
tions pair
>> > >> AUIPC/JALR in kernel text, so in order to ensure other CPUs does no=
t execute
>> > >> in the instructions that will be modified, it is still need to stop=
 other CPUs
>> > >> via patch_text API, or you have any better solution to achieve the =
purpose?
>> > >  - The stop_machine is an expensive way all architectures should
>> > > avoid, and you could keep that in your OPTPROBES implementation files
>> > > with static functions.
>> > >  - The stop_machine couldn't work with PREEMPTION, so your
>> > > implementation needs to work with !PREEMPTION.
>> >
>> > ...and stop_machine() with !PREEMPTION is broken as well, when you're
>> > replacing multiple instructions (see Mark's post at [1]). The
>> > stop_machine() dance might work when you're replacing *one* instructio=
n,
>> > not multiple as in the RISC-V case. I'll expand on this in a comment in
>> > the OPTPROBES v6 series.
>>
>> Just to clarify, my comments in [1] were assuming that stop_machine() wa=
s not
>> used, in which case there is a problem with or without PREEMPTION.
>>
>> I believe that when using stop_machine(), the !PREEMPTION case is fine, =
since
>> stop_machine() schedules work rather than running work in IRQ context on=
 the
>> back of an IPI, so no CPUs should be mid-sequnce during the patching, an=
d it's
>> not possible for there to be threads which are preempted mid-sequence.
>>
>> That all said, IIUC optprobes is going to disappear once fprobe is ready
>> everywhere, so that might be moot.
> The optprobes could be in the middle of a function, but fprobe must be
> the entry of a function, right?
>
> Does your fprobe here mean: ?
>
> The Linux kernel configuration item CONFIG_FPROBE:
>
> prompt: Kernel Function Probe (fprobe)
> type: bool
> depends on: ( CONFIG_FUNCTION_TRACER ) && (
> CONFIG_DYNAMIC_FTRACE_WITH_REGS ) && ( CONFIG_HAVE_RETHOOK )
> defined in kernel/trace/Kconfig

See the cover of [1]. It's about direct calls for BPF tracing (and more)
on Arm, and you're completly right, that it's *not* related to optprobes
at all.

[1] https://lore.kernel.org/all/20221108220651.24492-1-revest@chromium.org/