@@ -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.
*/
This is a note to let you know that I have just added a patch titled epoll: prevent missed events on EPOLL_CTL_MOD to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree which can be found at: http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue If you, or anyone else, feels it should not be added to this tree, please reply to this email. For more information about the 3.5.y.z tree, see https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable Thanks. -Herton ------ From 318ea9d60ee27838448a04a898ef67249f2e79bb Mon Sep 17 00:00:00 2001 From: Eric Wong <normalperson@yhbt.net> Date: Tue, 1 Jan 2013 21:20:27 +0000 Subject: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD 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@yhbt.net> Cc: Hans Verkuil <hans.verkuil@cisco.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Davide Libenzi <davidel@xmailserver.org> Cc: Hans de Goede <hdegoede@redhat.com> Cc: Mauro Carvalho Chehab <mchehab@infradead.org> Cc: David Miller <davem@davemloft.net> Cc: Eric Dumazet <eric.dumazet@gmail.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andreas Voellmy <andreas.voellmy@yale.edu> Tested-by: "Junchang(Jason) Wang" <junchang.wang@yale.edu> Cc: netdev@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com> --- fs/eventpoll.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) -- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html