[PATCH 3.16 091/328] tracing: Do not call start/stop() functions when tracing_on does not change

[Ben Hutchings]

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

commit f143641bfef9a4a60c57af30de26c63057e7e695 upstream.

Currently, when one echo's in 1 into tracing_on, the current tracer's
"start()" function is executed, even if tracing_on was already one. This can
lead to strange side effects. One being that if the hwlat tracer is enabled,
and someone does "echo 1 > tracing_on" into tracing_on, the hwlat tracer's
start() function is called again which will recreate another kernel thread,
and make it unable to remove the old one.

Link: http://lkml.kernel.org/r/1533120354-22923-1-git-send-email-erica.bugden@xxxxxxxxxxxxx

Fixes: 2df8f8a6a897e ("tracing: Fix regression with irqsoff tracer and tracing_on file")
Reported-by: Erica Bugden <erica.bugden@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/trace/trace.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6180,7 +6180,9 @@ rb_simple_write(struct file *filp, const
 
 	if (buffer) {
 		mutex_lock(&trace_types_lock);
-		if (val) {
+		if (!!val == tracer_tracing_is_on(tr)) {
+			val = 0; /* do nothing */
+		} else if (val) {
 			tracer_tracing_on(tr);
 			if (tr->current_trace->start)
 				tr->current_trace->start(tr);