signal: retarget_shared_pending: consider shared/unblocked signals only

exit_signals() checks signal_pending() before retarget_shared_pending() but
this is suboptimal. We can avoid the while_each_thread() loop in case when
there are no shared signals visible to us.

Add the "shared_pending.signal & ~blocked" check. We don't use tsk->blocked
directly but pass ~blocked as an argument, this is needed for the next patch.

Note: we can optimize this more. while_each_thread(t) can check t->blocked
into account and stop after every pending signal has the new target, see the
next patch.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Matt Fleming <matt.fleming@linux.intel.com>
Acked-by: Tejun Heo <tj@kernel.org>

kernel/signal.c

@@ -2203,10 +2203,15 @@ relock:
  * group-wide signal. Another thread should be notified now to take
  * the signal since we will not.
  */
-static void retarget_shared_pending(struct task_struct *tsk)
+static void retarget_shared_pending(struct task_struct *tsk, sigset_t *which)
 {
+	sigset_t retarget;
 	struct task_struct *t;
 
+	sigandsets(&retarget, &tsk->signal->shared_pending.signal, which);
+	if (sigisemptyset(&retarget))
+		return;
+
 	t = tsk;
 	while_each_thread(tsk, t) {
 		if (!signal_pending(t) && !(t->flags & PF_EXITING))
@@ -2217,6 +2222,7 @@ static void retarget_shared_pending(struct task_struct *tsk)
 void exit_signals(struct task_struct *tsk)
 {
 	int group_stop = 0;
+	sigset_t unblocked;
 
 	if (thread_group_empty(tsk) || signal_group_exit(tsk->signal)) {
 		tsk->flags |= PF_EXITING;
@@ -2232,7 +2238,9 @@ void exit_signals(struct task_struct *tsk)
 	if (!signal_pending(tsk))
 		goto out;
 
-	retarget_shared_pending(tsk);
+	unblocked = tsk->blocked;
+	signotset(&unblocked);
+	retarget_shared_pending(tsk, &unblocked);
 
 	if (unlikely(tsk->group_stop & GROUP_STOP_PENDING) &&
 	    task_participate_group_stop(tsk))