From patchwork Tue Jan 15 22:54:43 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg KH X-Patchwork-Id: 212350 X-Patchwork-Delegate: davem@davemloft.net Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id 9A30C2C007E for ; Wed, 16 Jan 2013 10:04:56 +1100 (EST) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934414Ab3AOXEh (ORCPT ); Tue, 15 Jan 2013 18:04:37 -0500 Received: from mail.kernel.org ([198.145.19.201]:48366 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934355Ab3AOW5N (ORCPT ); Tue, 15 Jan 2013 17:57:13 -0500 Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 62F4E2056F; Tue, 15 Jan 2013 22:57:12 +0000 (UTC) Received: from localhost (unknown [64.168.229.50]) (using TLSv1 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2EDCD20568; Tue, 15 Jan 2013 22:57:11 +0000 (UTC) From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Greg Kroah-Hartman , 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 , Andrew Morton , Andreas Voellmy , "Junchang(Jason) Wang" , Linus Torvalds , Ben Hutchings , netdev@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: [ 46/71] epoll: prevent missed events on EPOLL_CTL_MOD Date: Tue, 15 Jan 2013 14:54:43 -0800 Message-Id: <20130115225343.734388897@linuxfoundation.org> X-Mailer: git-send-email 1.8.1.336.g94702dd In-Reply-To: <20130115225340.367496998@linuxfoundation.org> References: <20130115225340.367496998@linuxfoundation.org> User-Agent: quilt/0.60-1 X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00, UNPARSEABLE_RELAY autolearn=ham version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 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 Signed-off-by: Greg Kroah-Hartman --- fs/eventpoll.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) -- 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 --- 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. */