[PATCH v3 -tip 1/4] tracing: add checks if tr->buffer is NULL in tracing_reset{_online_cpus}

[Hiraku Toyooka <hiraku.toyooka.gu@hitachi.com>]

max_tr->buffer could be NULL in the tracing_reset{_online_cpus}. In this
case, a NULL pointer dereference happens, so we should return immediately
from these functions.

Signed-off-by: Hiraku Toyooka <hiraku.toyooka.gu@hitachi.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org
---
 kernel/trace/trace.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c705c7a..a8ce008 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -922,6 +922,9 @@ void tracing_reset(struct trace_array *tr, int cpu)
 {
 	struct ring_buffer *buffer = tr->buffer;
 
+	if (!buffer)
+		return;
+
 	ring_buffer_record_disable(buffer);
 
 	/* Make sure all commits have finished */
@@ -936,6 +939,9 @@ void tracing_reset_online_cpus(struct trace_array *tr)
 	struct ring_buffer *buffer = tr->buffer;
 	int cpu;
 
+	if (!buffer)
+		return;
+
 	ring_buffer_record_disable(buffer);
 
 	/* Make sure all commits have finished */