linux-kernel - Re: [PATCH] riscv: kprobe: Optimize kprobe with accurate atomicity

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Message-ID: <87edrc2c6s.fsf@all.your.base.are.belong.to.us>
Date:   Mon, 30 Jan 2023 17:56:43 +0100
From:   Björn Töpel <bjorn@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Guo Ren <guoren@...nel.org>,
        "liaochang (A)" <liaochang1@...wei.com>, palmer@...belt.com,
        paul.walmsley@...ive.com, mhiramat@...nel.org,
        conor.dooley@...rochip.com, penberg@...nel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH] riscv: kprobe: Optimize kprobe with accurate atomicity

Mark Rutland <mark.rutland@....com> writes:

>> ...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* instruction,
>> 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() was 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, and it's
> not possible for there to be threads which are preempted mid-sequence.

TIL! stop_cpus() highlights that very nicely. Thanks for clearing that
out! That's good news; That means that this fix [4] should go in.

> That all said, IIUC optprobes is going to disappear once fprobe is ready
> everywhere, so that might be moot.

Yes (However, the stop_machine()/!PREEMPTION issue was with ftrace).


Björn

[4] https://lore.kernel.org/lkml/20230107133549.4192639-2-guoren@kernel.org/