[PATCH] tracing: Drop the tr check from the graph tracing path

[Frederic Weisbecker <fweisbec@gmail.com>]

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>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/trace/trace_functions_graph.c |   19 ++++++++++---------
 1 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index b1342c5..f225229 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/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();
-- 
1.6.2.3