ftrace: disable tracing on resize

Impact: fix for bug on resize

This patch addresses the bug found here:

 http://bugzilla.kernel.org/show_bug.cgi?id=11996

When ftrace converted to the new unified trace buffer, the resizing of
the buffer was not protected as much as it was originally. If tracing
is performed while the resize occurs, then the buffer can be corrupted.

This patch disables all ftrace buffer modifications before a resize
takes place.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

kernel/trace/trace.c

@@ -2676,7 +2676,7 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
 {
 	unsigned long val;
 	char buf[64];
-	int ret;
+	int ret, cpu;
 	struct trace_array *tr = filp->private_data;
 
 	if (cnt >= sizeof(buf))
@@ -2704,6 +2704,14 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
 		goto out;
 	}
 
+	/* disable all cpu buffers */
+	for_each_tracing_cpu(cpu) {
+		if (global_trace.data[cpu])
+			atomic_inc(&global_trace.data[cpu]->disabled);
+		if (max_tr.data[cpu])
+			atomic_inc(&max_tr.data[cpu]->disabled);
+	}
+
 	if (val != global_trace.entries) {
 		ret = ring_buffer_resize(global_trace.buffer, val);
 		if (ret < 0) {
@@ -2735,6 +2743,13 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
 	if (tracing_disabled)
 		cnt = -ENOMEM;
  out:
+	for_each_tracing_cpu(cpu) {
+		if (global_trace.data[cpu])
+			atomic_dec(&global_trace.data[cpu]->disabled);
+		if (max_tr.data[cpu])
+			atomic_dec(&max_tr.data[cpu]->disabled);
+	}
+
 	max_tr.entries = global_trace.entries;
 	mutex_unlock(&trace_types_lock);