exit: exit_notify() can trust signal->notify_count < 0

signal_struct->count in its current form must die.

- it has no reasons to be atomic_t

- it looks like a reference counter, but it is not

- otoh, we really need to make task->signal refcountable, just look at
  the extremely ugly task_rq_unlock_wait() called from __exit_signals().

- we should change the lifetime rules for task->signal, it should be
  pinned to task_struct.  We have a lot of code which can be simplified
  after that.

- it is not needed!  while the code is correct, any usage of this
  counter is artificial, except fs/proc uses it correctly to show the
  number of threads.

This series removes the usage of sig->count from exit pathes.

This patch:

Now that Veaceslav changed copy_signal() to use zalloc(), exit_notify()
can just check notify_count < 0 to ensure the execing sub-threads needs
the notification from us.  No need to do other checks, notify_count != 0
must always mean ->group_exit_task != NULL is waiting for us.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -856,12 +856,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 
 	tsk->exit_state = signal == DEATH_REAP ? EXIT_DEAD : EXIT_ZOMBIE;
 
-	/* mt-exec, de_thread() is waiting for us */
+	/* mt-exec, de_thread() is waiting for group leader */
-	if (thread_group_leader(tsk) &&
+	if (unlikely(tsk->signal->notify_count < 0))
-	    tsk->signal->group_exit_task &&
-	    tsk->signal->notify_count < 0)
 		wake_up_process(tsk->signal->group_exit_task);
-
 	write_unlock_irq(&tasklist_lock);
 
 	tracehook_report_death(tsk, signal, cookie, group_dead);