vt: synchronize_rcu() under spinlock is not nice...

commit e8cd8169 upstream.

vcs_poll_data_free() calls unregister_vt_notifier(), which calls
atomic_notifier_chain_unregister(), which calls synchronize_rcu().
Do it *after* we'd dropped ->f_lock.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/tty/vt/vc_screen.c

@@ -93,7 +93,7 @@ vcs_poll_data_free(struct vcs_poll_data *poll)
 static struct vcs_poll_data *
 vcs_poll_data_get(struct file *file)
 {
-	struct vcs_poll_data *poll = file->private_data;
+	struct vcs_poll_data *poll = file->private_data, *kill = NULL;
 
 	if (poll)
 		return poll;
@@ -122,10 +122,12 @@ vcs_poll_data_get(struct file *file)
 		file->private_data = poll;
 	} else {
 		/* someone else raced ahead of us */
-		vcs_poll_data_free(poll);
+		kill = poll;
 		poll = file->private_data;
 	}
 	spin_unlock(&file->f_lock);
+	if (kill)
+		vcs_poll_data_free(kill);
 
 	return poll;
 }