linux-kernel - Re: [PATCH] rcu: ftrace: avoid tracing a few functions executed in multi_cpu

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Message-ID: <20220420114454.69ab108c@gandalf.local.home>
Date:   Wed, 20 Apr 2022 11:44:54 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     patrick wang <patrick.wang.shcn@...il.com>
Cc:     paulmck@...nel.org, frederic@...nel.org, quic_neeraju@...cinc.com,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, joel@...lfernandes.org,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: ftrace: avoid tracing a few functions executed in
 multi_cpu_stop()

On Wed, 20 Apr 2022 18:34:34 +0800
patrick wang <patrick.wang.shcn@...il.com> wrote:

[ I had a power outage yesterday, just catching up now ]

> Sorry for the format. Need get used to gmail.
> 
> These functions are running within stop machine and ftrace modify
> code by using stop machine to ensure the safety on some
> architectures(e.g. RISC-V). These functions' instructions will be
> modified during ftrace modifying code. When instructions are being
> modified, they shouldn't be executed typically. Or the executor
> may behave unpredictably.

Interesting. On x86 when we used stop machine[*] it was not an issue to
modify the code that is being executed in stop machine. I'm curious to
exactly what the issue is if something does get traced in the stop machine
processing. Why does it crash?

-- Steve


[*] You really should come up with a better way than stop machine, because
the stop machine method is really disruptive, can you not use the break
point method for updates?