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Date:   Sat, 21 Jan 2023 11:49:04 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     kernel test robot <oliver.sang@intel.com>, oe-lkp@lists.linux.dev,
        lkp@intel.com, linux-kernel@vger.kernel.org, x86@kernel.org,
        Ingo Molnar <mingo@kernel.org>,
        linux-trace-kernel@vger.kernel.org
Subject: Re: [tip:sched/core] [tracing, hardirq]  9aedeaed6f:
 WARNING:suspicious_RCU_usage
Message-ID: <20230121114904.13d6825d@gandalf.local.home>
In-Reply-To: <Y8pkFhi32XhvCVOo@hirez.programming.kicks-ass.net>
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        <Y8llrdNT6RD/0dbq@hirez.programming.kicks-ass.net>
        <20230119112433.611fa273@gandalf.local.home>
        <Y8pkFhi32XhvCVOo@hirez.programming.kicks-ass.net>
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On Fri, 20 Jan 2023 10:51:18 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> > does trigger the issue, you can then bisect the functions with the script:
> > 
> >   scripts/tracing/ftrace-bisect.sh  
> 
> Pff, that all sounds like actual work :-)
> 
> Instead I did me the below hack and added my early_printk() hacks and
> that got me a usable backtrace.

Well, if you system is still running after the issue, then sure. I created
this when it would cause a triple fault reboot. In which case, there was no
real debugging output to use.

-- Steve


> 
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 5e7ead52cfdb..7defc6e24f8f 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -646,6 +646,9 @@ void prepare_ftrace_return(unsigned long ip, unsigned long *parent,
>  	if (unlikely(atomic_read(&current->tracing_graph_pause)))
>  		return;
>  
> +	if (WARN_ONCE(!rcu_is_watching(), "RCU not on for: %pS\n", (void *)ip))
> +		return;
> +
>  	bit = ftrace_test_recursion_trylock(ip, *parent);
>  	if (bit < 0)
>  		return;