ftrace/x86: Add a comment to why we take text_mutex in ftrace_arch_code_modify_prepare()

Taking the text_mutex in ftrace_arch_code_modify_prepare() is to fix a
race against module loading and live kernel patching that might try to
change the text permissions while ftrace has it as read/write. This
really needs to be documented in the code. Add a comment that does such.

Link: http://lkml.kernel.org/r/20190627211819.5a591f52@gandalf.local.homeSuggested-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: NPetr Mladek <pmladek@suse.com>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

arch/x86/kernel/ftrace.c

@@ -36,6 +36,11 @@
 
 int ftrace_arch_code_modify_prepare(void)
 {
+	/*
+	 * Need to grab text_mutex to prevent a race from module loading
+	 * and live kernel patching from changing the text permissions while
+	 * ftrace has it set to "read/write".
+	 */
 	mutex_lock(&text_mutex);
 	set_kernel_text_rw();
 	set_all_modules_text_rw();