@@ -1197,10 +1197,30 @@ static int ep_modify(struct eventpoll *e
* 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 */
epi->event.data = event->data; /* protected by mtx */
+ * 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.
* Get current event bits. We can safely use the file* here because
* its usage count has been increased by the caller of this function.