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>

1 files changed, 11 insertions, 3 deletions

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);
 }