tracing/samples: Fix creation and deletion of simple_thread_fn creation

commit 6575257c60e1a26a5319ccf2b5ce5b6449001017 upstream.

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.

Fixes: 7496946a8 ("tracing: Add samples of DECLARE_EVENT_CLASS() and DEFINE_EVENT()")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/samples/trace_events/trace-events-sample.c b/samples/trace_events/trace-events-sample.c
index 880a7d1d27d24025c9ceb8703075f81dab80c35d..d69715d99e4d0d89c142195a8bf3fad336997c4d 100644 (file)
--- a/samples/trace_events/trace-events-sample.c
+++ b/samples/trace_events/trace-events-sample.c
@@ -78,28 +78,36 @@ static int simple_thread_fn(void *arg)
 }
 
 static DEFINE_MUTEX(thread_mutex);
+static bool simple_thread_cnt;
 
 void 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);
 }
 
 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);
 }