[PATCH for tip][bugfix] ftrace: fix deadlock when setting set_ftrace_pid

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Impact: fix deadlock bug

reproduce way:
	# cd /sys/kernel/debug/tracing
	# echo $$ > set_ftrace_pid

	then, console become hangup.

detail:
when writing set_ftracepid, kernel callstack is following

	ftrace_pid_write()
		mutex_lock(&ftrace_lock);
		ftrace_update_pid_func()
			mutex_lock(&ftrace_lock);
			mutex_unlock(&ftrace_lock);
		mutex_unlock(&ftrace_lock);

then, system alaways deadlock when ftrace_pid_write() is called.

In past days, ftrace_pid_write() use ftrace_start_lock, but
commit e6ea44e9b4c12325337cd1c06103cd515a1c02b2 consolidate
ftrace_start_lock to ftrace_lock.


Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 5a3a06b..0d3b393 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -218,10 +218,8 @@ static void ftrace_update_pid_func(void)
 {
 	ftrace_func_t func;
 
-	mutex_lock(&ftrace_lock);
-
 	if (ftrace_trace_function == ftrace_stub)
-		goto out;
+		return;
 
 	func = ftrace_trace_function;
 
@@ -238,9 +236,6 @@ static void ftrace_update_pid_func(void)
 #else
 	__ftrace_trace_function = func;
 #endif
-
- out:
-	mutex_unlock(&ftrace_lock);
 }
 
 /* set when tracing only a pid */