GR translation: it is the poet singing back the silence, adjuring the white riders

[Mike Jing]

P175.19-28   “Here, don’t you know there’s a war on?” Thus Pointsman
receives, with his cup, a terrible scowl. In truth, he is hoping with
nitwit irrelevancies to discourage Gwenhidwy from going on about his
City Paranoiac. Pointsman would rather talk about the rocket victims
admitted today to the hospital down there. But this is exorcism man,
it is the poet singing back the silence, adjuring the white riders,
and Gwenhidwy knows, as Pointsman cannot, that it’s part of the plan
of the day to sit inside this mean room and cry into just such a
deafness: that Mr. Pointsman is to play exactly himself—stylized,
irritable, uncomprehending. . . .

What is the reference here?  Who are the white riders?