+ /*
+ * The curr_ret_stack is an index to ftrace return stack of
+ * current task. Its value should be in [0, FTRACE_RETFUNC_
+ * DEPTH) when the function graph tracer is used. To support
+ * filtering out specific functions, it makes the index
+ * negative by subtracting huge value (FTRACE_NOTRACE_DEPTH)
+ * so when it sees a negative index the ftrace will ignore
+ * the record. And the index gets recovered when returning
+ * from the filtered function by adding the FTRACE_NOTRACE_
+ * DEPTH and then it'll continue to record functions normally.
+ *
+ * The curr_ret_stack is initialized to -1 and get increased
+ * in this function. So it can be less than -1 only if it was
+ * filtered out via ftrace_graph_notrace_addr() which can be
+ * set from set_graph_notrace file in debugfs by user.
+ */
+ if (current->curr_ret_stack < -1)
+ return -EBUSY;
+