rcu/nocb: Wake up gp thread when flushing

We notice that rcu_barrier() can take a really long time. It appears
that this can happen when all CBs are lazy and the timer does not fire
yet. So after flushing, nothing wakes up GP thread. This patch forces
GP thread to wake when bypass flushing happens, this fixes the
rcu_barrier() delays with lazy CBs.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

kernel/rcu/tree_nocb.h

@@ -325,6 +325,8 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 	rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
 	WRITE_ONCE(rdp->nocb_bypass_first, j);
 	rcu_nocb_bypass_unlock(rdp);
+
+	wake_nocb_gp(rdp, true);
 	return true;
 }