Subject: [ 46/71] epoll: prevent missed events on EPOLL_CTL_MOD

From: Greg Kroah-Hartman <[email-masked]>
Date: Tue, 15 Jan 2013 14:54:43 -0800

3.0-stable review patch.  If anyone has any objections, please let me know.

------------------

commit 128dd1759d96ad36c379240f8b9463e8acfd37a1 upstream.

EPOLL_CTL_MOD sets the interest mask before calling f_op->poll() to
ensure events are not missed.  Since the modifications to the interest
mask are not protected by the same lock as ep_poll_callback, we need to
ensure the change is visible to other CPUs calling ep_poll_callback.

We also need to ensure f_op->poll() has an up-to-date view of past
events which occured before we modified the interest mask.  So this
barrier also pairs with the barrier in wq_has_sleeper().

This should guarantee either ep_poll_callback or f_op->poll() (or both)
will notice the readiness of a recently-ready/modified item.

This issue was encountered by Andreas Voellmy and Junchang(Jason) Wang in:
http://thread.gmane.org/gmane.linux.kernel/1408782/

Signed-off-by: Eric Wong <[email-masked]>
Cc: Hans Verkuil <[email-masked]>
Cc: Jiri Olsa <[email-masked]>
Cc: Jonathan Corbet <[email-masked]>
Cc: Al Viro <[email-masked]>
Cc: Davide Libenzi <[email-masked]>
Cc: Hans de Goede <[email-masked]>
Cc: Mauro Carvalho Chehab <[email-masked]>
Cc: David Miller <[email-masked]>
Cc: Eric Dumazet <[email-masked]>
Cc: Andrew Morton <[email-masked]>
Cc: Andreas Voellmy <[email-masked]>
Tested-by: "Junchang(Jason) Wang" <[email-masked]>
Cc: [email-masked]
Cc: [email-masked]
Signed-off-by: Linus Torvalds <[email-masked]>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <[email-masked]>
Signed-off-by: Greg Kroah-Hartman <[email-masked]>

---
 fs/eventpoll.c |   22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -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.
+	 */
+	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.
 	 */