tracing: Fix to check event_mutex is held while accessing trigger list

Since the check_user_trigger() is called outside of RCU
read lock, this list_for_each_entry_rcu() caused a suspicious
RCU usage warning.

 # echo hist:keys=pid > events/sched/sched_stat_runtime/trigger
 # cat events/sched/sched_stat_runtime/trigger
[   43.167032]
[   43.167418] =============================
[   43.167992] WARNING: suspicious RCU usage
[   43.168567] 5.19.0-rc5-00029-g19ebe4651abf #59 Not tainted
[   43.169283] -----------------------------
[   43.169863] kernel/trace/trace_events_trigger.c:145 RCU-list traversed in non-reader section!!
...

However, this file->triggers list is safe when it is accessed
under event_mutex is held.
To fix this warning, adds a lockdep_is_held check to the
list_for_each_entry_rcu().

Link: https://lkml.kernel.org/r/166226474977.223837.1992182913048377113.stgit@devnote2

Cc: [email protected]
Fixes: 7491e2c ("tracing: Add a probe that attaches to trace events")
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

kernel/trace/trace_events_trigger.c

@@ -142,7 +142,8 @@ static bool check_user_trigger(struct trace_event_file *file)
 {
 	struct event_trigger_data *data;
 
-	list_for_each_entry_rcu(data, &file->triggers, list) {
+	list_for_each_entry_rcu(data, &file->triggers, list,
+				lockdep_is_held(&event_mutex)) {
 		if (data->flags & EVENT_TRIGGER_FL_PROBE)
 			continue;
 		return true;