ftrace: comment arch ftrace code

Add comments to explain what is happening in the x86 arch ftrace code.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/ftrace.c

@@ -66,18 +66,23 @@ ftrace_modify_code(unsigned long ip, unsigned char *old_code,
 	/*
 	 * Note: Due to modules and __init, code can
 	 *  disappear and change, we need to protect against faulting
-	 *  as well as code changing.
+	 *  as well as code changing. We do this by using the
+	 *  __copy_*_user functions.
 	 *
 	 * No real locking needed, this code is run through
 	 * kstop_machine, or before SMP starts.
 	 */
+
+	/* read the text we want to modify */
 	if (__copy_from_user_inatomic(replaced, (char __user *)ip,
 				      MCOUNT_INSN_SIZE))
 		return -EFAULT;
 
+	/* Make sure it is what we expect it to be */
 	if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0)
 		return -EINVAL;
 
+	/* replace the text with the new text */
 	if (__copy_to_user_inatomic((char __user *)ip, new_code,
 				    MCOUNT_INSN_SIZE))
 		return -EPERM;