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Date:   Wed, 21 Oct 2020 10:12:16 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Zong Li <zong.li@sifive.com>
Cc:     paulmck@kernel.org, josh@joshtriplett.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
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        tglx@linutronix.de, paul.walmsley@sifive.com,
        palmerdabbelt@google.com, guoren@kernel.org, atishp@atishpatra.org,
        mhiramat@kernel.org, greentime.hu@sifive.com,
        colin.king@canonical.com, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org
Subject: Re: [PATCH] stop_machine: Mark functions as notrace
Message-ID: <20201021101216.4d840e15@gandalf.local.home>
In-Reply-To: <20201021073839.43935-1-zong.li@sifive.com>
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On Wed, 21 Oct 2020 15:38:39 +0800
Zong Li <zong.li@sifive.com> wrote:

> Like the commit cb9d7fd51d9f ("watchdog: Mark watchdog touch functions
> as notrace"), some architectures assume that the stopped CPUs don't make
> function calls to traceable functions when they are in the stopped
> state. For example, it causes unexpected kernel crashed when switching
> tracer on RISC-V.
> 
> The following patches added calls to these two functions, fix it by
> adding the notrace annotations.
> 
> Fixes: 4ecf0a43e729 ("processor: get rid of cpu_relax_yield")
> Fixes: 366237e7b083 ("stop_machine: Provide RCU quiescent state in
> multi_cpu_stop()")

I really do not like to add "notrace" to core functions because a single
architecture has issues with it. Why does RISCV have problems with these
functions but no other architecture does?

NACK from me until it is shown that these are issues for a broader set of
architectures.

It sounds to me like you are treating a symptom and not the disease.

-- Steve


> 
> Signed-off-by: Zong Li <zong.li@sifive.com>
> ---
>  kernel/rcu/tree.c     | 2 +-
>  kernel/stop_machine.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 06895ef85d69..2a52f42f64b6 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -409,7 +409,7 @@ bool rcu_eqs_special_set(int cpu)
>   *
>   * The caller must have disabled interrupts and must not be idle.
>   */
> -void rcu_momentary_dyntick_idle(void)
> +notrace void rcu_momentary_dyntick_idle(void)
>  {
>  	int special;
>  
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 865bb0228ab6..890b79cf0e7c 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -178,7 +178,7 @@ static void ack_state(struct multi_stop_data *msdata)
>  		set_state(msdata, msdata->state + 1);
>  }
>  
> -void __weak stop_machine_yield(const struct cpumask *cpumask)
> +notrace void __weak stop_machine_yield(const struct cpumask *cpumask)
>  {
>  	cpu_relax();
>  }