signals: microoptimize the usage of ->curr_target

Suggested by Roland McGrath.

Initialize signal->curr_target in copy_signal().  This way ->curr_target is
never == NULL, we can kill the check in __group_complete_signal's hot path.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
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/fork.c

@@ -892,7 +892,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 	sig->group_exit_code = 0;
 	sig->group_exit_task = NULL;
 	sig->group_stop_count = 0;
-	sig->curr_target = NULL;
+	sig->curr_target = tsk;
 	init_sigpending(&sig->shared_pending);
 	INIT_LIST_HEAD(&sig->posix_timers);
 

kernel/signal.c

@@ -863,10 +863,6 @@ __group_complete_signal(int sig, struct task_struct *p)
 		 * Otherwise try to find a suitable thread.
 		 */
 		t = signal->curr_target;
-		if (t == NULL)
-			/* restart balancing at this thread */
-			t = signal->curr_target = p;
-
 		while (!wants_signal(sig, t)) {
 			t = next_thread(t);
 			if (t == signal->curr_target)