linux-kernel - Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled

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Date:   Mon, 23 Jan 2023 16:53:04 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, will@...nel.org, boqun.feng@...il.com,
        mark.rutland@....com, tglx@...utronix.de, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        seanjc@...gle.com, pbonzini@...hat.com, jgross@...e.com,
        srivatsa@...il.mit.edu, amakhalov@...are.com,
        pv-drivers@...are.com, mhiramat@...nel.org, wanpengli@...cent.com,
        vkuznets@...hat.com, boris.ostrovsky@...cle.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        linux-trace-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU
 is disabled

On Mon, 23 Jan 2023 21:50:12 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> All RCU disabled code should be noinstr and hence we should never get
> here -- when we do, WARN about it and make sure to not actually do
> tracing.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/kernel/ftrace.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -646,6 +646,9 @@ void prepare_ftrace_return(unsigned long
>  	if (unlikely(atomic_read(&current->tracing_graph_pause)))
>  		return;
>  
> +	if (WARN_ONCE(!rcu_is_watching(), "RCU not on for: %pS\n", (void *)ip))
> +		return;
> +

Please add this to after recursion trylock below. Although WARN_ONCE()
should not not have recursion issues, as function tracing can do weird
things, I rather be safe than sorry, and not have the system triple boot
due to some path that might get added in the future.

If rcu_is_watching() is false, it will still get by the below recursion
check and warn. That is, the below check should be done before this
function calls any other function.

>  	bit = ftrace_test_recursion_trylock(ip, *parent);
>  	if (bit < 0)
>  		return;
> 

-- Steve