[PATCH 176/222] epoll: prevent missed events on EPOLL_CTL_MOD

[Herton Ronaldo Krzesinski]

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

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

From: Eric Wong <normalperson at yhbt.net>

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 <normalperson at yhbt.net>
Cc: Hans Verkuil <hans.verkuil at cisco.com>
Cc: Jiri Olsa <jolsa at redhat.com>
Cc: Jonathan Corbet <corbet at lwn.net>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: Davide Libenzi <davidel at xmailserver.org>
Cc: Hans de Goede <hdegoede at redhat.com>
Cc: Mauro Carvalho Chehab <mchehab at infradead.org>
Cc: David Miller <davem at davemloft.net>
Cc: Eric Dumazet <eric.dumazet at gmail.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Andreas Voellmy <andreas.voellmy at yale.edu>
Tested-by: "Junchang(Jason) Wang" <junchang.wang at yale.edu>
Cc: netdev at vger.kernel.org
Cc: linux-fsdevel at vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
---
 fs/eventpoll.c |   22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 1c8b556..dd1a55d 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1285,7 +1285,7 @@ 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) {
@@ -1296,6 +1296,26 @@ 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.
 	 */
-- 
1.7.9.5