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Date: Mon, 12 Feb 2024 13:53:20 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Mete Durlu <meted@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>, Masami Hiramatsu
 <mhiramat@kernel.org>, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
 linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH] tracing: use ring_buffer_record_is_set_on() in
 tracer_tracing_is_on()
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On Thu, 8 Feb 2024 11:25:50 +0100
Mete Durlu <meted@linux.ibm.com> wrote:

> I have been only able to reliably reproduce this issue when the system
> is under load from stressors. But I am not sure if it can be considered
> as *really stressed*.
> 
> system : 8 cpus (4 physical cores)
> load   : stress-ng --fanotify 1 (or --context 2)
> result : ~5/10 test fails
> 
> of course as load increases test starts to fail more often, but a
> single stressor doesn't seem like much to me for a 4 core machine.
> 
> after adding synchronize_rcu() + patch from Sven, I am no longer seeing
> failures with the setup above. So it seems like synchronize_rcu() did
> the trick(or at least it helps a lot) for the case described on the
> previous mail. I couldn't trigger the failure yet, not even with
> increased load(but now the test case takes > 5mins to finish :) ).

Right, it will definitely force the race window to go away.

Can you still trigger this issue with just Sven's patch and not this change?

-- Steve

> 
> Here is the diff:
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> @@ -9328,10 +9328,12 @@ rb_simple_write(struct file *filp, const char 
> __user *ubuf,
>                          val = 0; /* do nothing */
>                  } else if (val) {
>                          tracer_tracing_on(tr);
> +                       synchronize_rcu();
>                          if (tr->current_trace->start)
>                                  tr->current_trace->start(tr);
>                  } else {
>                          tracer_tracing_off(tr);
> +                       synchronize_rcu();
>                          if (tr->current_trace->stop)
>                                  tr->current_trace->stop(tr);
> 
> Not 100% sure if these were the correct places to add them.