tracing: fix deadlock when setting set_ftrace_pid

Impact: fix deadlock while using set_ftrace_pid

Reproducer:

	# cd /sys/kernel/debug/tracing
	# echo $$ > set_ftrace_pid

	then, console becomes hung.

Details:

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 always deadlocks when ftrace_pid_write() is called.

In past days, ftrace_pid_write() used ftrace_start_lock, but
commit e6ea44e9 consolidated
ftrace_start_lock to ftrace_lock.
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Cc: Steven Rostedt <srostedt@redhat.com>
LKML-Reference: <20090306151155.0778.A69D9226@jp.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

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 */