tracing: Use the ring-buffer nesting to allow synthetic events to be traced

Synthetic events can be done within the recording of other events. Notify
the ring buffer via ring_buffer_nest_start() and ring_buffer_nest_end() that
this is intended and not to block it due to its recursion protection.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -640,6 +640,7 @@ static notrace void trace_event_raw_event_synth(void *__data,
 	struct trace_event_file *trace_file = __data;
 	struct synth_trace_event *entry;
 	struct trace_event_buffer fbuffer;
+	struct ring_buffer *buffer;
 	struct synth_event *event;
 	unsigned int i, n_u64;
 	int fields_size = 0;
@@ -651,10 +652,17 @@ static notrace void trace_event_raw_event_synth(void *__data,
 
 	fields_size = event->n_u64 * sizeof(u64);
 
+	/*
+	 * Avoid ring buffer recursion detection, as this event
+	 * is being performed within another event.
+	 */
+	buffer = trace_file->tr->trace_buffer.buffer;
+	ring_buffer_nest_start(buffer);
+
 	entry = trace_event_buffer_reserve(&fbuffer, trace_file,
 					   sizeof(*entry) + fields_size);
 	if (!entry)
-		return;
+		goto out;
 
 	for (i = 0, n_u64 = 0; i < event->n_fields; i++) {
 		if (event->fields[i]->is_string) {
@@ -670,6 +678,8 @@ static notrace void trace_event_raw_event_synth(void *__data,
 	}
 
 	trace_event_buffer_commit(&fbuffer);
+out:
+	ring_buffer_nest_end(buffer);
 }
 
 static void free_synth_event_print_fmt(struct trace_event_call *call)