move the related code from exit_notify() to exit_signals()

The previous bugfix was not optimal, we shouldn't care about group stop
when we are the only thread or the group stop is in progress.  In that case
nothing special is needed, just set PF_EXITING and return.

Also, take the related "TIF_SIGPENDING re-targeting" code from exit_notify().

So, from the performance POV the only difference is that we don't trust
!signal_pending() until we take ->siglock.  But this in fact fixes another
___pure___ theoretical minor race.  __group_complete_signal() finds the
task without PF_EXITING and chooses it as the target for signal_wake_up().
But nothing prevents this task from exiting in between without noticing the
pending signal and thus unpredictably delaying the actual delivery.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -745,24 +745,6 @@ static void exit_notify(struct task_struct *tsk)
 	struct task_struct *t;
 	struct pid *pgrp;
 
-	if (signal_pending(tsk) && !(tsk->signal->flags & SIGNAL_GROUP_EXIT)
-	    && !thread_group_empty(tsk)) {
-		/*
-		 * This occurs when there was a race between our exit
-		 * syscall and a group signal choosing us as the one to
-		 * wake up.  It could be that we are the only thread
-		 * alerted to check for pending signals, but another thread
-		 * should be woken now to take the signal since we will not.
-		 * Now we'll wake all the threads in the group just to make
-		 * sure someone gets all the pending signals.
-		 */
-		spin_lock_irq(&tsk->sighand->siglock);
-		for (t = next_thread(tsk); t != tsk; t = next_thread(t))
-			if (!signal_pending(t) && !(t->flags & PF_EXITING))
-				recalc_sigpending_and_wake(t);
-		spin_unlock_irq(&tsk->sighand->siglock);
-	}
-
 	/*
 	 * This does two things:
 	 *