From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 931ABC28CBC for ; Sun, 3 May 2020 13:05:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6F5342073E for ; Sun, 3 May 2020 13:05:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728612AbgECNF3 convert rfc822-to-8bit (ORCPT ); Sun, 3 May 2020 09:05:29 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([146.101.78.151]:20475 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728579AbgECNF3 (ORCPT ); Sun, 3 May 2020 09:05:29 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mtapsc-1-PiwzYAMxPhqDKD10rKMWKg-1; Sun, 03 May 2020 14:05:25 +0100 X-MC-Unique: PiwzYAMxPhqDKD10rKMWKg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Sun, 3 May 2020 14:05:24 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Sun, 3 May 2020 14:05:24 +0100 From: David Laight To: 'Jason Baron' , "akpm@linux-foundation.org" CC: "linux-kernel@vger.kernel.org" , "Alexander Viro" , Heiher , Roman Penyaev , Khazhismel Kumykov , Davidlohr Bueso , "stable@vger.kernel.org" Subject: RE: [PATCH] epoll: ensure ep_poll() doesn't miss wakeup events Thread-Topic: [PATCH] epoll: ensure ep_poll() doesn't miss wakeup events Thread-Index: AQHWH+3GezEky6u5mk6GywAVvMmIMKiWVkJw Date: Sun, 3 May 2020 13:05:24 +0000 Message-ID: References: <1588360533-11828-1-git-send-email-jbaron@akamai.com> In-Reply-To: <1588360533-11828-1-git-send-email-jbaron@akamai.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Jason Baron > Sent: 01 May 2020 20:16 > > Now that the ep_events_available() check is done in a lockless way, and > we no longer perform wakeups from ep_scan_ready_list(), we need to ensure > that either ep->rdllist has items or the overflow list is active. Prior to: > commit 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested > epoll"), we did wake_up(&ep->wq) after manipulating the ep->rdllist and the > overflow list. Thus, any waiters would observe the correct state. However, > with that wake_up() now removed we need to be more careful to ensure that > condition. I'm wondering how much all this affects the (probably) more common case of a process reading events from a lot of sockets in 'level' mode. Even the change to a rwlock() may have had an adverse effect on such programs. In 'level' mode it doesn't make any sense to have multiple readers of the event queue. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)