fs/exec.c:de_thread: move notify_count write under lock

We set sig->notify_count = -1 between RELEASE and ACQUIRE operations:

	spin_unlock_irq(lock);
	...
	if (!thread_group_leader(tsk)) {
		...
                for (;;) {
			sig->notify_count = -1;
                        write_lock_irq(&tasklist_lock);

There are no restriction on it so other processors may see this STORE
mixed with other STOREs in both areas limited by the spinlocks.

Probably, it may be reordered with the above

	sig->group_exit_task = tsk;
	sig->notify_count = zap_other_threads(tsk);

in some way.

Set it under tasklist_lock locked to be sure nothing will be reordered.
Signed-off-by: NKirill Tkhai <ktkhai@parallels.com>
Acked-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -926,10 +926,14 @@ static int de_thread(struct task_struct *tsk)
 	if (!thread_group_leader(tsk)) {
 		struct task_struct *leader = tsk->group_leader;
 
-		sig->notify_count = -1;	/* for exit_notify() */
 		for (;;) {
 			threadgroup_change_begin(tsk);
 			write_lock_irq(&tasklist_lock);
+			/*
+			 * Do this under tasklist_lock to ensure that
+			 * exit_notify() can't miss ->group_exit_task
+			 */
+			sig->notify_count = -1;
 			if (likely(leader->exit_state))
 				break;
 			__set_current_state(TASK_KILLABLE);