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Message-Id: <20130115225343.734388897@linuxfoundation.org>
Date: Tue, 15 Jan 2013 14:54:43 -0800
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
alan@...rguk.ukuu.org.uk, Eric Wong <normalperson@...t.net>,
Hans Verkuil <hans.verkuil@...co.com>,
Jiri Olsa <jolsa@...hat.com>, Jonathan Corbet <corbet@....net>,
Al Viro <viro@...iv.linux.org.uk>,
Davide Libenzi <davidel@...ilserver.org>,
Hans de Goede <hdegoede@...hat.com>,
Mauro Carvalho Chehab <mchehab@...radead.org>,
David Miller <davem@...emloft.net>,
Eric Dumazet <eric.dumazet@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andreas Voellmy <andreas.voellmy@...e.edu>,
"Junchang(Jason) Wang" <junchang.wang@...e.edu>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Ben Hutchings <ben@...adent.org.uk>, netdev@...r.kernel.org,
linux-fsdevel@...r.kernel.org
Subject: [ 46/71] epoll: prevent missed events on EPOLL_CTL_MOD
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 <normalperson@...t.net>
Cc: Hans Verkuil <hans.verkuil@...co.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Jonathan Corbet <corbet@....net>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Davide Libenzi <davidel@...ilserver.org>
Cc: Hans de Goede <hdegoede@...hat.com>
Cc: Mauro Carvalho Chehab <mchehab@...radead.org>
Cc: David Miller <davem@...emloft.net>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andreas Voellmy <andreas.voellmy@...e.edu>
Tested-by: "Junchang(Jason) Wang" <junchang.wang@...e.edu>
Cc: netdev@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
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.
*/
--
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