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

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Message-ID: <Y9jz+zUDebQ4VLlF@andrea>
Date:   Tue, 31 Jan 2023 11:56:59 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     Guo Ren <guoren@...nel.org>
Cc:     Björn Töpel <bjorn@...nel.org>,
        "liaochang (A)" <liaochang1@...wei.com>, palmer@...belt.com,
        paul.walmsley@...ive.com, mhiramat@...nel.org,
        conor.dooley@...rochip.com, penberg@...nel.org,
        mark.rutland@....com, 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

> > It's the concurrent modification that I was referring to (removing
> > stop_machine()). You're saying "it'll always work", I'm saying "I'm not
> > so sure". :-) E.g., writing c.ebreak on an 32b insn. Can you say that
> Software must ensure write c.ebreak on the head of an 32b insn.
> 
> That means IFU only see:
>  - c.ebreak + broken/illegal insn.
> or
>  - origin insn
> 
> Even in the worst case, such as IFU fetches instructions one by one:
> If the IFU gets the origin insn, it will skip the broken/illegal insn.
> If the IFU gets the c.ebreak + broken/illegal insn, then an ebreak
> exception is raised.
> 
> Because c.ebreak would raise an exception, I don't see any problem.

That's the problem, this discussion is:

Reviewer: "I'm not sure, that's not written in our spec"
Submitter: "I said it, it's called -accurate atomicity-"

  Andrea