ftrace/x86: Let dynamic trampolines call ops->func even for dynamic fops

Dynamically allocated trampolines call ftrace_ops_get_func to get the
function which they should call. For dynamic fops (FTRACE_OPS_FL_DYNAMIC
flag is set) ftrace_ops_list_func is always returned. This is reasonable
for static trampolines but goes against the main advantage of dynamic
ones, that is avoidance of going through the list of all registered
callbacks for functions that are only being traced by a single callback.

We can fix it by returning ops->func (or recursion safe version) from
ftrace_ops_get_func whenever it is possible for dynamic trampolines.

Note that dynamic trampolines are not allowed for dynamic fops if
CONFIG_PREEMPT=y.

Link: http://lkml.kernel.org/r/[e-mail]
Link: http://lkml.kernel.org/r/[e-mail]

Reported-by: Miroslav Benes <[e-mail]>
Signed-off-by: Steven Rostedt <[e-mail]>

kernel/trace/ftrace.c

···
 static inline void update_function_graph_func(void) { }
 #endif
 
+
+static ftrace_func_t ftrace_ops_get_list_func(struct ftrace_ops *ops)
+{
+	/*
+	 * If this is a dynamic ops or we force list func,
+	 * then it needs to call the list anyway.
+	 */
+	if (ops->flags & FTRACE_OPS_FL_DYNAMIC || FTRACE_FORCE_LIST_FUNC)
+		return ftrace_ops_list_func;
+
+	return ftrace_ops_get_func(ops);
+}
+
 static void update_ftrace_function(void)
 {
 	ftrace_func_t func;
···
 	 * then have the mcount trampoline call the function directly.
 	 */
 	} else if (ftrace_ops_list->next == &ftrace_list_end) {
-		func = ftrace_ops_get_func(ftrace_ops_list);
+		func = ftrace_ops_get_list_func(ftrace_ops_list);
 
 	} else {
 		/* Just use the default ftrace_ops */
···
  */
 ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops)
 {
-	/*
-	 * If this is a dynamic ops or we force list func,
-	 * then it needs to call the list anyway.
-	 */
-	if (ops->flags & FTRACE_OPS_FL_DYNAMIC || FTRACE_FORCE_LIST_FUNC)
-		return ftrace_ops_list_func;
-
 	/*
 	 * If the func handles its own recursion, call it directly.
 	 * Otherwise call the recursion protected function that