tracing/ftrace: don't insert TRACE_PRINT during selftests

Impact: fix tracer selfstests false results

After setting a ftrace_printk somewhere in th kernel, I saw the
Function tracer selftest failing.

When a selftest occurs, the ring buffer is lurked to see if
some entries were inserted. But concurrent insertion such as
ftrace_printk could occured at the same time and could give
false positive or negative results.

This patch prevent prevent from TRACE_PRINT entries insertion
during selftests.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace.c

@@ -44,6 +44,14 @@
 unsigned long __read_mostly	tracing_max_latency = (cycle_t)ULONG_MAX;
 unsigned long __read_mostly	tracing_thresh;
 
+/* We need to change this state when a selftest is running.
+ * A selftest will lurk into the ring-buffer to count the
+ * entries inserted during the selftest although some concurrent
+ * insertions into the ring-buffer such as ftrace_printk could occurred
+ * at the same time, giving false positive or negative results.
+ */
+static atomic_t tracing_selftest_running = ATOMIC_INIT(0);
+
 /* For tracers that don't implement custom flags */
 static struct tracer_opt dummy_tracer_opt[] = {
 	{ }
@@ -589,6 +597,8 @@ int register_tracer(struct tracer *type)
 		struct tracer *saved_tracer = current_trace;
 		struct trace_array *tr = &global_trace;
 		int i;
+
+		atomic_set(&tracing_selftest_running, 1);
 		/*
 		 * Run a selftest on this tracer.
 		 * Here we reset the trace buffer, and set the current
@@ -603,6 +613,7 @@ int register_tracer(struct tracer *type)
 		/* the test is responsible for initializing and enabling */
 		pr_info("Testing tracer %s: ", type->name);
 		ret = type->selftest(type, tr);
+		atomic_set(&tracing_selftest_running, 0);
 		/* the test is responsible for resetting too */
 		current_trace = saved_tracer;
 		if (ret) {
@@ -3594,7 +3605,7 @@ int trace_vprintk(unsigned long ip, int depth, const char *fmt, va_list args)
 	unsigned long flags, irq_flags;
 	int cpu, len = 0, size, pc;
 
-	if (tracing_disabled)
+	if (tracing_disabled || atomic_read(&tracing_selftest_running))
 		return 0;
 
 	pc = preempt_count();