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

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Submitter Greg KH
Date Jan. 15, 2013, 10:54 p.m.
Message ID <>
Download mbox | patch
Permalink /patch/212350/
State Not Applicable
Delegated to: David Miller
Headers show


Greg KH - Jan. 15, 2013, 10:54 p.m.
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:

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

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

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--- 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;
+	epi-> = event->events; /* need barrier below */
 	epi-> = 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.