---

yaml
---
r: 348096
b: refs/heads/master
c: 128dd17
h: refs/heads/master
v: v3

[refs]

@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 4a490b78cb7e0e5efa44425df72a9fedc1c36366
+refs/heads/master: 128dd1759d96ad36c379240f8b9463e8acfd37a1

trunk/fs/eventpoll.c

@@ -1313,7 +1313,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
 	 * otherwise we might miss an event that happens between the
 	 * f_op->poll() call and the new event set registering.
 	 */
-	epi->event.events = event->events;
+	epi->event.events = event->events; /* need barrier below */
 	pt._key = event->events;
 	epi->event.data = event->data; /* protected by mtx */
 	if (epi->event.events & EPOLLWAKEUP) {
@@ -1323,6 +1323,26 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
 		ep_destroy_wakeup_source(epi);
 	}
 
+	/*
+	 * The following barrier has two effects:
+	 *
+	 * 1) Flush epi changes above to other CPUs.  This ensures
+	 *    we do not miss events from ep_poll_callback if an
+	 *    event occurs immediately after we call f_op->poll().
+	 *    We need this because we did not take ep->lock while
+	 *    changing epi above (but ep_poll_callback does take
+	 *    ep->lock).
+	 *
+	 * 2) We also need to ensure we do not miss _past_ events
+	 *    when calling f_op->poll().  This barrier also
+	 *    pairs with the barrier in wq_has_sleeper (see
+	 *    comments for wq_has_sleeper).
+	 *
+	 * This barrier will now guarantee ep_poll_callback or f_op->poll
+	 * (or both) will notice the readiness of an item.
+	 */
+	smp_mb();
+
 	/*
 	 * Get current event bits. We can safely use the file* here because
 	 * its usage count has been increased by the caller of this function.