signals: document CLD_CONTINUED notification mechanics

A couple of small comments about how CLD_CONTINUED notification works.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -633,6 +633,11 @@ static int prepare_signal(int sig, struct task_struct *p)
 			why |= SIGNAL_CLD_STOPPED;
 
 		if (why) {
+			/*
+			 * The first thread which returns from finish_stop()
+			 * will take ->siglock, notice SIGNAL_CLD_MASK, and
+			 * notify its parent. See get_signal_to_deliver().
+			 */
 			signal->flags = why | SIGNAL_STOP_CONTINUED;
 			signal->group_stop_count = 0;
 			signal->group_exit_code = 0;
@@ -1694,7 +1699,11 @@ relock:
 	try_to_freeze();
 
 	spin_lock_irq(&sighand->siglock);
-
+	/*
+	 * Every stopped thread goes here after wakeup. Check to see if
+	 * we should notify the parent, prepare_signal(SIGCONT) encodes
+	 * the CLD_ si_code into SIGNAL_CLD_MASK bits.
+	 */
 	if (unlikely(signal->flags & SIGNAL_CLD_MASK)) {
 		int why = (signal->flags & SIGNAL_STOP_CONTINUED)
 				? CLD_CONTINUED : CLD_STOPPED;