1 files changed, 21 insertions, 1 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index be56b21435f8..9fec1836057a 100644
--- a/fs/eventpoll.c
+++ b/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) {
@@ -1324,6 +1324,26 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
         }
 
         /*
+         * 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.
          */