tracing/osnoise: Improve comments about barrier need for NMI callbacks

trace_osnoise_callback_enabled is used by ftrace_nmi_enter/exit()
to know when to call the NMI callback. The barrier is used to
avoid having callbacks enabled before the resetting date during
the start or to touch the values after stopping the tracer.

Link: https://lkml.kernel.org/r/a413b8f14aa9312fbd1ba99f96225a8aed831053.1635702894.git.bristot@kernel.org

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Tom Zanussi <zanussi@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: linux-rt-users@vger.kernel.org
Cc: linux-trace-devel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_osnoise.c

@@ -1930,8 +1930,10 @@ static int __osnoise_tracer_start(struct trace_array *tr)
 	retval = osnoise_hook_events();
 	if (retval)
 		return retval;
+
 	/*
-	 * Make sure NMIs see reseted values.
+	 * Make sure that ftrace_nmi_enter/exit() see reset values
+	 * before enabling trace_osnoise_callback_enabled.
 	 */
 	barrier();
 	trace_osnoise_callback_enabled = true;
@@ -1966,6 +1968,10 @@ static void osnoise_tracer_stop(struct trace_array *tr)
 		return;
 
 	trace_osnoise_callback_enabled = false;
+	/*
+	 * Make sure that ftrace_nmi_enter/exit() see
+	 * trace_osnoise_callback_enabled as false before continuing.
+	 */
 	barrier();
 
 	stop_per_cpu_kthreads();