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 39BF2C38A30 for ; Sun, 19 Apr 2020 21:37:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 17DD921473 for ; Sun, 19 Apr 2020 21:37:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726055AbgDSVhP convert rfc822-to-8bit (ORCPT ); Sun, 19 Apr 2020 17:37:15 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([146.101.78.151]:21884 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725848AbgDSVhP (ORCPT ); Sun, 19 Apr 2020 17:37:15 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-95-zqFcyXnbPHardQKmriLBlQ-1; Sun, 19 Apr 2020 22:37:11 +0100 X-MC-Unique: zqFcyXnbPHardQKmriLBlQ-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, 19 Apr 2020 22:37:10 +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, 19 Apr 2020 22:37:10 +0100 From: David Laight To: 'Petko Manolov' CC: "Paul E . McKenney" , LKML Subject: RE: [RFC] WRITE_ONCE_INC() and friends Thread-Topic: [RFC] WRITE_ONCE_INC() and friends Thread-Index: AQHWFi8qKt3GQH4qWkGP1JyWbckPg6iAvLKQ///3OoCAAELwsA== Date: Sun, 19 Apr 2020 21:37:10 +0000 Message-ID: <8e5a0283ed76465aac19a2b97a27ff15@AcuMS.aculab.com> References: <20200419094439.GA32841@carbon> <491f0b0bc9e4419d93a78974fd7f44c7@AcuMS.aculab.com> <20200419182957.GA36919@carbon> In-Reply-To: <20200419182957.GA36919@carbon> 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: Petko Manolov > Sent: 19 April 2020 19:30 > > On 20-04-19 18:02:50, David Laight wrote: > > From: Petko Manolov > > > Sent: 19 April 2020 10:45 > > > Recently I started reading up on KCSAN and at some point I ran into stuff like: > > > > > > WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1); > > > WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1); > > > > If all the accesses use READ/WRITE_ONCE() why not just mark the structure > > field 'volatile'? > > This is a bit heavy. I guess you've read this one: > > https://lwn.net/Articles/233479/ I remember reading something similar before. I also remember a very old gcc (2.95?) that did a readback after every volatile write on sparc (to flush the store buffer). That broke everything. I suspect there is a lot more code that is attempting to be lockless these days. Ring buffers (one writer and one reader) are a typical example where you don't need locks but do need to use a consistent value. Now you may also need ordering between accesses - which I think needs more than volatile. > And no, i am not sure all accesses are through READ/WRITE_ONCE(). If, for > example, all others are from withing spin_lock/unlock pairs then we _may_ not > need READ/WRITE_ONCE(). The cost of volatile accesses is probably minimal unless the code is written assuming the compiler will only access things once. > I merely proposed the _INC() variant for better readability. More like shorter code lines :-) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)