tracing/events: reuse trace event ids after overflow

With modules being able to add trace events, and the max trace event
counter is 16 bits (65536) we can overflow the counter easily
with a simple while loop adding and removing modules that contain
trace events.

This patch links together the registered trace events and on overflow
searches for available trace event ids. It will still fail if
over 65536 events are registered, but considering that a typical
kernel only has 22000 functions, 65000 events should be sufficient.

Reported-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

include/linux/ftrace_event.h

@@ -56,6 +56,7 @@ typedef enum print_line_t (*trace_print_func)(struct trace_iterator *iter,
 					      int flags);
 struct trace_event {
 	struct hlist_node	node;
+	struct list_head	list;
 	int			type;
 	trace_print_func	trace;
 	trace_print_func	raw;

kernel/trace/trace_output.c

@@ -483,6 +483,36 @@ struct trace_event *ftrace_find_event(int type)
 	return NULL;
 }
 
+static LIST_HEAD(ftrace_event_list);
+
+static int trace_search_list(struct list_head **list)
+{
+	struct trace_event *e;
+	int last = __TRACE_LAST_TYPE;
+
+	if (list_empty(&ftrace_event_list)) {
+		*list = &ftrace_event_list;
+		return last + 1;
+	}
+
+	/*
+	 * We used up all possible max events,
+	 * lets see if somebody freed one.
+	 */
+	list_for_each_entry(e, &ftrace_event_list, list) {
+		if (e->type != last + 1)
+			break;
+		last++;
+	}
+
+	/* Did we used up all 65 thousand events??? */
+	if ((last + 1) > FTRACE_MAX_EVENT)
+		return 0;
+
+	*list = &e->list;
+	return last + 1;
+}
+
 /**
  * register_ftrace_event - register output for an event type
  * @event: the event type to register
@@ -505,20 +535,40 @@ int register_ftrace_event(struct trace_event *event)
 
 	mutex_lock(&trace_event_mutex);
 
-	if (!event) {
-		ret = next_event_type++;
+	if (WARN_ON(!event))
 		goto out;
-	}
 
-	if (!event->type)
-		event->type = next_event_type++;
-	else if (event->type > __TRACE_LAST_TYPE) {
+	INIT_LIST_HEAD(&event->list);
+
+	if (!event->type) {
+		struct list_head *list;
+
+		if (next_event_type > FTRACE_MAX_EVENT) {
+
+			event->type = trace_search_list(&list);
+			if (!event->type)
+				goto out;
+
+		} else {
+
+			event->type = next_event_type++;
+			list = &ftrace_event_list;
+		}
+
+		if (WARN_ON(ftrace_find_event(event->type)))
+			goto out;
+
+		list_add_tail(&event->list, list);
+
+	} else if (event->type > __TRACE_LAST_TYPE) {
 		printk(KERN_WARNING "Need to add type to trace.h\n");
 		WARN_ON(1);
-	}
-
-	if (ftrace_find_event(event->type))
 		goto out;
+	} else {
+		/* Is this event already used */
+		if (ftrace_find_event(event->type))
+			goto out;
+	}
 
 	if (event->trace == NULL)
 		event->trace = trace_nop_print;
@@ -537,8 +587,6 @@ int register_ftrace_event(struct trace_event *event)
  out:
 	mutex_unlock(&trace_event_mutex);
 
-	WARN_ON_ONCE(next_event_type > FTRACE_MAX_EVENT);
-
 	return ret;
 }
 EXPORT_SYMBOL_GPL(register_ftrace_event);
@@ -551,6 +599,7 @@ int unregister_ftrace_event(struct trace_event *event)
 {
 	mutex_lock(&trace_event_mutex);
 	hlist_del(&event->node);
+	list_del(&event->list);
 	mutex_unlock(&trace_event_mutex);
 
 	return 0;