[PATCH] simplify exec from init's subthread

I think it is enough to take tasklist_lock for reading while changing
child_reaper:

	Reparenting needs write_lock(tasklist_lock)

	Only one thread in a thread group can do exec()

	sighand->siglock garantees that get_signal_to_deliver()
	will not see a stale value of child_reaper.

This means that we can change child_reaper earlier, without calling
zap_other_threads() twice.

"child_reaper = current" is a NOOP when init does exec from main thread, we
don't care.
Signed-off-by: NOleg Nesterov <oleg@tv-sign.ru>
Acked-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

fs/exec.c

@@ -616,6 +616,15 @@ static int de_thread(struct task_struct *tsk)
 		kmem_cache_free(sighand_cachep, newsighand);
 		return -EAGAIN;
 	}
+
+	/*
+	 * child_reaper ignores SIGKILL, change it now.
+	 * Reparenting needs write_lock on tasklist_lock,
+	 * so it is safe to do it under read_lock.
+	 */
+	if (unlikely(current->group_leader == child_reaper))
+		child_reaper = current;
+
 	zap_other_threads(current);
 	read_unlock(&tasklist_lock);
 
@@ -660,23 +669,12 @@ static int de_thread(struct task_struct *tsk)
 		struct dentry *proc_dentry1, *proc_dentry2;
 		unsigned long ptrace;
 
-		leader = current->group_leader;
-		/*
-		 * If our leader is the child_reaper become
-		 * the child_reaper and resend SIGKILL signal.
-		 */
-		if (unlikely(leader == child_reaper)) {
-			write_lock(&tasklist_lock);
-			child_reaper = current;
-			zap_other_threads(current);
-			write_unlock(&tasklist_lock);
-		}
-
 		/*
 		 * Wait for the thread group leader to be a zombie.
 		 * It should already be zombie at this point, most
 		 * of the time.
 		 */
+		leader = current->group_leader;
 		while (leader->exit_state != EXIT_ZOMBIE)
 			yield();