[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.

This fixes the issues identified by Albert Cahalan in thread
http://lkml.org/lkml/2006/12/21/22.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Mike Galbraith <efault@gmx.de>
Cc: Albert Cahalan <acahalan@gmail.com>
Cc: Andrew Morton <akpm@osdl.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Coywolf Qi Hunt <qiyong@fc-cn.com>
Acked-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
[chrisw: fold in 241ceee0, Oleg's fix to restore user visible behaviour]
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

kernel/exit.c

@@ -603,10 +603,6 @@ choose_new_parent(struct task_struct *p, struct task_struct *reaper)
 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);
@@ -626,13 +622,7 @@ reparent_thread(struct task_struct *p, struct task_struct *father, int traced)
 		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
@@ -642,6 +632,23 @@ reparent_thread(struct task_struct *p, struct task_struct *father, int traced)
 		}
 	}
 
+	/* 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 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
 	 * Case ii: Our child is in a different pgrp