From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755863Ab3AMRse (ORCPT ); Sun, 13 Jan 2013 12:48:34 -0500 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:35103 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754900Ab3AMRpp (ORCPT ); Sun, 13 Jan 2013 12:45:45 -0500 Message-Id: <20130113174302.452380110@decadent.org.uk> User-Agent: quilt/0.60-1 Date: Sun, 13 Jan 2013 17:43:27 +0000 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Eric Wong , Hans Verkuil , Jiri Olsa , Jonathan Corbet , Al Viro , Davide Libenzi , Hans de Goede , Mauro Carvalho Chehab , David Miller , Eric Dumazet , Andreas Voellmy , "Junchang(Jason) Wang" , Linus Torvalds Subject: [ 32/49] epoll: prevent missed events on EPOLL_CTL_MOD In-Reply-To: <20130113174255.736888844@decadent.org.uk> X-SA-Exim-Connect-IP: 2001:470:1f08:1539:21c:bfff:fe03:f805 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Eric Wong 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 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" Cc: netdev@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Signed-off-by: Linus Torvalds [bwh: Backported to 3.2: adjust context] Signed-off-by: Ben Hutchings --- 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. */