[PATCH -mm, resend] de_thread: fix lockless do_each_thread

[Oleg Nesterov <oleg@tv-sign.ru>]

We should keep the value of old_leader->tasks.next in de_thread,
otherwise we can't do for_each_process/do_each_thread without
tasklist_lock held.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- MM/fs/exec.c~3_RCU	2006-05-14 22:14:42.000000000 +0400
+++ MM/fs/exec.c	2006-05-14 22:32:13.000000000 +0400
@@ -706,7 +706,7 @@ static int de_thread(struct task_struct 
 		attach_pid(current, PIDTYPE_PID,  current->pid);
 		attach_pid(current, PIDTYPE_PGID, current->signal->pgrp);
 		attach_pid(current, PIDTYPE_SID,  current->signal->session);
-		list_add_tail_rcu(&current->tasks, &init_task.tasks);
+		list_replace_rcu(&leader->tasks, &current->tasks);
 
 		current->group_leader = current;
 		leader->group_leader = current;
@@ -714,7 +714,6 @@ static int de_thread(struct task_struct 
 		/* Reduce leader to a thread */
 		detach_pid(leader, PIDTYPE_PGID);
 		detach_pid(leader, PIDTYPE_SID);
-		list_del_init(&leader->tasks);
 
 		current->exit_signal = SIGCHLD;