FAILED: patch "[PATCH] tracing/samples: Fix creation and deletion of" failed to apply to 4.9-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 6575257c60e1a26a5319ccf2b5ce5b6449001017 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Date: Tue, 17 Oct 2017 14:55:24 -0400
Subject: [PATCH] tracing/samples: Fix creation and deletion of
 simple_thread_fn creation

Commit 7496946a8 ("tracing: Add samples of DECLARE_EVENT_CLASS() and
DEFINE_EVENT()") added template examples for all the events. It created a
DEFINE_EVENT_FN() example which reused the foo_bar_reg and foo_bar_unreg
functions.

Enabling both the TRACE_EVENT_FN() and DEFINE_EVENT_FN() example trace
events caused the foo_bar_reg to be called twice, creating the test thread
twice. The foo_bar_unreg would remove it only once, even if it was called
multiple times, leaving a thread existing when the module is unloaded,
causing an oops.

Add a ref count and allow foo_bar_reg() and foo_bar_unreg() be called by
multiple trace events.

Cc: stable@vger.kernel.org
Fixes: 7496946a8 ("tracing: Add samples of DECLARE_EVENT_CLASS() and DEFINE_EVENT()")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/samples/trace_events/trace-events-sample.c b/samples/trace_events/trace-events-sample.c
index bc7fcf010a5b..446beb7ac48d 100644
--- a/samples/trace_events/trace-events-sample.c
+++ b/samples/trace_events/trace-events-sample.c
@@ -78,29 +78,37 @@ static int simple_thread_fn(void *arg)
 }
 
 static DEFINE_MUTEX(thread_mutex);
+static bool simple_thread_cnt;
 
 int foo_bar_reg(void)
 {
+	mutex_lock(&thread_mutex);
+	if (simple_thread_cnt++)
+		goto out;
+
 	pr_info("Starting thread for foo_bar_fn\n");
 	/*
 	 * We shouldn't be able to start a trace when the module is
 	 * unloading (there's other locks to prevent that). But
 	 * for consistency sake, we still take the thread_mutex.
 	 */
-	mutex_lock(&thread_mutex);
 	simple_tsk_fn = kthread_run(simple_thread_fn, NULL, "event-sample-fn");
+ out:
 	mutex_unlock(&thread_mutex);
 	return 0;
 }
 
 void foo_bar_unreg(void)
 {
-	pr_info("Killing thread for foo_bar_fn\n");
-	/* protect against module unloading */
 	mutex_lock(&thread_mutex);
+	if (--simple_thread_cnt)
+		goto out;
+
+	pr_info("Killing thread for foo_bar_fn\n");
 	if (simple_tsk_fn)
 		kthread_stop(simple_tsk_fn);
 	simple_tsk_fn = NULL;
+ out:
 	mutex_unlock(&thread_mutex);
 }