ftrace: allow archs to preform pre and post process for code modification

This patch creates the weak functions: ftrace_arch_code_modify_prepare
and ftrace_arch_code_modify_post_process that are called before and
after the stop machine is called to modify the kernel text.

If the arch needs to do pre or post processing, it only needs to define
these functions.

[ Update: Ingo Molnar suggested using the name ftrace_arch_code_modify_*
          over using ftrace_arch_modify_* ]
Signed-off-by: Steven Rostedt <srostedt@redhat.com>

include/linux/ftrace.h

@@ -99,6 +99,9 @@ stack_trace_sysctl(struct ctl_table *table, int write,
 /* asm/ftrace.h must be defined for archs supporting dynamic ftrace */
 #include <asm/ftrace.h>
 
+int ftrace_arch_code_modify_prepare(void);
+int ftrace_arch_code_modify_post_process(void);
+
 enum {
 	FTRACE_FL_FREE		= (1 << 0),
 	FTRACE_FL_FAILED	= (1 << 1),

kernel/trace/ftrace.c

@@ -585,6 +585,24 @@ ftrace_code_disable(struct module *mod, struct dyn_ftrace *rec)
 	return 1;
 }
 
+/*
+ * archs can override this function if they must do something
+ * before the modifying code is performed.
+ */
+int __weak ftrace_arch_code_modify_prepare(void)
+{
+	return 0;
+}
+
+/*
+ * archs can override this function if they must do something
+ * after the modifying code is performed.
+ */
+int __weak ftrace_arch_code_modify_post_process(void)
+{
+	return 0;
+}
+
 static int __ftrace_modify_code(void *data)
 {
 	int *command = data;
@@ -607,7 +625,17 @@ static int __ftrace_modify_code(void *data)
 
 static void ftrace_run_update_code(int command)
 {
+	int ret;
+
+	ret = ftrace_arch_code_modify_prepare();
+	FTRACE_WARN_ON(ret);
+	if (ret)
+		return;
+
 	stop_machine(__ftrace_modify_code, &command, NULL);
+
+	ret = ftrace_arch_code_modify_post_process();
+	FTRACE_WARN_ON(ret);
 }
 
 static ftrace_func_t saved_ftrace_func;