ftrace: Avoid needless updates of the ftrace function call

Song Shuai reported:

    The list func (ftrace_ops_list_func) will be patched first
    before the transition between old and new calls are set,
    which fixed the race described in this commit `59338f75`.

    While ftrace_trace_function changes from the list func to a
    ftrace_ops func, like unregistering the klp_ops to leave the only
    global_ops in ftrace_ops_list, the ftrace_[regs]_call will be
    replaced with the list func although it already exists. So there
    should be a condition to avoid this.

And suggested using another variable to keep track of what the ftrace
function is set to. But this could be simplified by using a helper
function that does the same with a static variable.

Link: https://lore.kernel.org/lkml/20221026132039.2236233-1-suagrfillet@gmail.com/
Link: https://lore.kernel.org/linux-trace-kernel/20221122180905.737b6f52@gandalf.local.homeReported-by: Song Shuai <suagrfillet@gmail.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -2763,6 +2763,19 @@ void __weak ftrace_arch_code_modify_post_process(void)
 {
 }
 
+static int update_ftrace_func(ftrace_func_t func)
+{
+	static ftrace_func_t save_func;
+
+	/* Avoid updating if it hasn't changed */
+	if (func == save_func)
+		return 0;
+
+	save_func = func;
+
+	return ftrace_update_ftrace_func(func);
+}
+
 void ftrace_modify_all_code(int command)
 {
 	int update = command & FTRACE_UPDATE_TRACE_FUNC;
@@ -2783,7 +2796,7 @@ void ftrace_modify_all_code(int command)
 	 * traced.
 	 */
 	if (update) {
-		err = ftrace_update_ftrace_func(ftrace_ops_list_func);
+		err = update_ftrace_func(ftrace_ops_list_func);
 		if (FTRACE_WARN_ON(err))
 			return;
 	}
@@ -2799,7 +2812,7 @@ void ftrace_modify_all_code(int command)
 		/* If irqs are disabled, we are in stop machine */
 		if (!irqs_disabled())
 			smp_call_function(ftrace_sync_ipi, NULL, 1);
-		err = ftrace_update_ftrace_func(ftrace_trace_function);
+		err = update_ftrace_func(ftrace_trace_function);
 		if (FTRACE_WARN_ON(err))
 			return;
 	}