tracing: Drop the tr check from the graph tracing path

Each time we save a function entry from the function graph
tracer, we check if the trace array is set, which is wasteful
because it is set anyway before we start the tracer. All we need
is to ensure we have good read and write orderings. When we set
the trace array, we just need to guarantee it to be visible
before starting tracing.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
LKML-Reference: <1263453795-7496-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_functions_graph.c

@@ -212,9 +212,6 @@ int trace_graph_entry(struct ftrace_graph_ent *trace)
 	int cpu;
 	int pc;
 
-	if (unlikely(!tr))
-		return 0;
-
 	if (!ftrace_trace_task(current))
 		return 0;
 
@@ -287,11 +284,20 @@ void trace_graph_return(struct ftrace_graph_ret *trace)
 	local_irq_restore(flags);
 }
 
+void set_graph_array(struct trace_array *tr)
+{
+	graph_array = tr;
+
+	/* Make graph_array visible before we start tracing */
+
+	smp_mb();
+}
+
 static int graph_trace_init(struct trace_array *tr)
 {
 	int ret;
 
-	graph_array = tr;
+	set_graph_array(tr);
 	ret = register_ftrace_graph(&trace_graph_return,
 				    &trace_graph_entry);
 	if (ret)
@@ -301,11 +307,6 @@ static int graph_trace_init(struct trace_array *tr)
 	return 0;
 }
 
-void set_graph_array(struct trace_array *tr)
-{
-	graph_array = tr;
-}
-
 static void graph_trace_reset(struct trace_array *tr)
 {
 	tracing_stop_cmdline_record();