tracing: Fix initialization failure path in tracing_set_tracer()

If tracer->init() fails, current code will leave current_tracer pointing
to an unusable tracer, which at best makes 'current_tracer' report
inaccurate value.

Fix the issue by pointing current_tracer to nop tracer, and only update
current_tracer with the new one after all the initialization succeeds.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/trace/trace.c

@@ -3172,10 +3172,10 @@ static int tracing_set_tracer(const char *buf)
 	}
 	destroy_trace_option_files(topts);
 
-	current_trace = t;
+	current_trace = &nop_trace;
 
-	topts = create_trace_option_files(current_trace);
-	if (current_trace->use_max_tr) {
+	topts = create_trace_option_files(t);
+	if (t->use_max_tr) {
 		int cpu;
 		/* we need to make per cpu buffer sizes equivalent */
 		for_each_tracing_cpu(cpu) {
@@ -3195,6 +3195,7 @@ static int tracing_set_tracer(const char *buf)
 			goto out;
 	}
 
+	current_trace = t;
 	trace_branch_enable(tr);
  out:
 	mutex_unlock(&trace_types_lock);