[PATCH] Fix reparenting to the same thread group. (take 2)

This patch fixes the case when we reparent to a different thread in the
same thread group. This modifies the code so that we do not send
signals and do not change the signal to send to SIGCHLD unless we have
change the thread group of our parents. It also suppresses sending
pdeath_sig in this cas as well since the result of geppid doesn't
change.

Thanks to Oleg for spotting my bug of only fixing this for non-ptraced
tasks.

Signed-off-by: Eric W. Biederman <[e-mail]>
Cc: Mike Galbraith <[e-mail]>
Cc: Albert Cahalan <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Roland McGrath <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Coywolf Qi Hunt <[e-mail]>
Acked-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/exit.c

···
 static void
 reparent_thread(struct task_struct *p, struct task_struct *father, int traced)
 {
-	/* We don't want people slaying init.  */
-	if (p->exit_signal != -1)
-		p->exit_signal = SIGCHLD;
-
-	if (p->pdeath_signal)
-		/* We already hold the tasklist_lock here.  */
-		group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
-
 	/* Move the child from its dying parent to the new one.  */
 	if (unlikely(traced)) {
 		/* Preserve ptrace links if someone else is tracing this child.  */
···
 		p->parent = p->real_parent;
 		add_parent(p);
 
-		/* If we'd notified the old parent about this child's death,
-		 * also notify the new parent.
-		 */
-		if (p->exit_state == EXIT_ZOMBIE && p->exit_signal != -1 &&
-		    thread_group_empty(p))
-			do_notify_parent(p, p->exit_signal);
-		else if (p->state == TASK_TRACED) {
+		if (p->state == TASK_TRACED) {
 			/*
 			 * If it was at a trace stop, turn it into
 			 * a normal stop since it's no longer being
···
 			ptrace_untrace(p);
 		}
 	}
+
+	/* If this is a threaded reparent there is no need to
+	 * notify anyone anything has happened.
+	 */
+	if (p->real_parent->group_leader == father->group_leader)
+		return;
+
+	/* We don't want people slaying init.  */
+	if (p->exit_signal != -1)
+		p->exit_signal = SIGCHLD;
+		
+	if (p->pdeath_signal)
+		/* We already hold the tasklist_lock here.  */
+		group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
+
+	/* If we'd notified the old parent about this child's death,
+	 * also notify the new parent.
+	 */
+	if (!traced && p->exit_state == EXIT_ZOMBIE &&
+	    p->exit_signal != -1 && thread_group_empty(p))
+		do_notify_parent(p, p->exit_signal);
 
 	/*
 	 * process group orphan check