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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 02A88C43381 for ; Mon, 1 Apr 2019 15:27:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CD7EE2133D for ; Mon, 1 Apr 2019 15:27:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728670AbfDAP1n convert rfc822-to-8bit (ORCPT ); Mon, 1 Apr 2019 11:27:43 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([146.101.78.151]:55859 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726905AbfDAP1j (ORCPT ); Mon, 1 Apr 2019 11:27:39 -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-FIAlWESBMeai6qXJUmKCAg-1; Mon, 01 Apr 2019 16:27:30 +0100 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b::d117) by AcuMS.aculab.com (fd9f:af1c:a25b::d117) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 1 Apr 2019 16:28:35 +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; Mon, 1 Apr 2019 16:28:35 +0100 From: David Laight To: 'Alan Stern' , Oleg Nesterov CC: "Paul E. McKenney" , Jann Horn , Joel Fernandes , Kees Cook , "Eric W. Biederman" , LKML , Android Kernel Team , Kernel Hardening , Andrew Morton , Matthew Wilcox , "Michal Hocko" , "Reshetova, Elena" Subject: RE: [PATCH] Convert struct pid count to refcount_t Thread-Topic: [PATCH] Convert struct pid count to refcount_t Thread-Index: AQHU5mgbEUS1Lz7T4USOARfdCVSWPqYncFxg Date: Mon, 1 Apr 2019 15:28:35 +0000 Message-ID: <76f3b894980d4a79a2c0ae815425d6a0@AcuMS.aculab.com> References: <20190329173209.GA23683@redhat.com> In-Reply-To: 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-MC-Unique: FIAlWESBMeai6qXJUmKCAg-1 X-Mimecast-Spam-Score: 0 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: Alan Stern > Sent: 29 March 2019 19:45 ... > There is a big difference between WRITE_ONCE() and plain assignment. > Given "WRITE_ONCE(X, 2)", the compiler will emit a simple store > instruction. But given "X = 2", the compiler is allowed to emit > instructions equivalent to: > > if (X != 2) > X = 2; Worse for you, it can also emit: X = 0; X = 2; Many years ago I fell foul of a compiler (not C) that implemented a write to a 2 bit wide bitfield as: X &= ~3 X |= value even when 'value' was a compile time constant of 3. Took a while to find out why the linked list got f*cked. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)