[PATCH] de_thread: fix lockless do_each_thread

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>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/exec.c

@@ -707,7 +707,7 @@ static int de_thread(struct task_struct *tsk)
 		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;
@@ -715,7 +715,6 @@ static int de_thread(struct task_struct *tsk)
 		/* Reduce leader to a thread */
 		detach_pid(leader, PIDTYPE_PGID);
 		detach_pid(leader, PIDTYPE_SID);
-		list_del_init(&leader->tasks);
 
 		current->exit_signal = SIGCHLD;