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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1DF3BCFB43F for ; Sun, 6 Oct 2024 19:53:52 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 6288D6B0171; Sun, 6 Oct 2024 15:53:51 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 5D9726B0172; Sun, 6 Oct 2024 15:53:51 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4C7956B0173; Sun, 6 Oct 2024 15:53:51 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id 2E6AF6B0171 for ; Sun, 6 Oct 2024 15:53:51 -0400 (EDT) Received: from smtpin24.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 9B24740357 for ; Sun, 6 Oct 2024 19:53:50 +0000 (UTC) X-FDA: 82644227820.24.08F9E77 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by imf28.hostedemail.com (Postfix) with ESMTP id 47D58C0018 for ; Sun, 6 Oct 2024 19:53:48 +0000 (UTC) Authentication-Results: imf28.hostedemail.com; dkim=none; spf=pass (imf28.hostedemail.com: domain of david.laight@aculab.com designates 185.58.85.151 as permitted sender) smtp.mailfrom=david.laight@aculab.com; dmarc=pass (policy=none) header.from=aculab.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1728244360; a=rsa-sha256; cv=none; b=aSoRCjUhTuv6z3WxRqg4yRt4mYDb82yI2/IX2qxXebmohEotv3X2Urjfnf7izp+UaMbHBa s8kaUtDhwPa7hQBbqvJmioPNNVFZP+3haIkIQl8/sHMEhIbOh3X8AEdQZ/BwrLKWhupHBK 8qqe7bucs+XZwGLliQkxdBOfO9oB3OQ= ARC-Authentication-Results: i=1; imf28.hostedemail.com; dkim=none; spf=pass (imf28.hostedemail.com: domain of david.laight@aculab.com designates 185.58.85.151 as permitted sender) smtp.mailfrom=david.laight@aculab.com; dmarc=pass (policy=none) header.from=aculab.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1728244360; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=urAlj3YIPI6U4fGcdRwOSd1d81yifTqvt07cFMkvJsU=; b=cY41UYmyKSHWIUSmRzN3794xJRilRQwC3Tv4aIEPsHaAVn1Z56nICLnkqP9yfTu1C/Gwx6 9Tv4Y+AZb+XpgpSffENZNcWIl8AUqoEWNFwh7IZW+xGPyYp2E2MW+/5wQqDqY5MSDou/Ca h8unwnM7v99wDTa03uDAsqpLyHBiDlQ= Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-202-PX6nP_3yOCep-K_4vURKFQ-1; Sun, 06 Oct 2024 20:53:44 +0100 X-MC-Unique: PX6nP_3yOCep-K_4vURKFQ-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Sun, 6 Oct 2024 20:52:50 +0100 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Sun, 6 Oct 2024 20:52:49 +0100 From: David Laight To: 'Mathieu Desnoyers' , Boqun Feng CC: "linux-kernel@vger.kernel.org" , "Linus Torvalds" , Andrew Morton , Peter Zijlstra , "Nicholas Piggin" , Michael Ellerman , "Greg Kroah-Hartman" , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , Alan Stern , John Stultz , Neeraj Upadhyay , "Frederic Weisbecker" , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , "Waiman Long" , Mark Rutland , "Thomas Gleixner" , Vlastimil Babka , "maged.michael@gmail.com" , Mateusz Guzik , Jonas Oberhauser , "rcu@vger.kernel.org" , "linux-mm@kvack.org" , "lkmm@lists.linux.dev" , Gary Guo , Nikita Popov , "llvm@lists.linux.dev" Subject: RE: [RFC PATCH v2 2/4] Documentation: RCU: Refer to ptr_eq() Thread-Topic: [RFC PATCH v2 2/4] Documentation: RCU: Refer to ptr_eq() Thread-Index: AQHbFpQfGMiCXlKVq0GFl3C1s2BlXbJ6I7mg Date: Sun, 6 Oct 2024 19:52:49 +0000 Message-ID: <72af935f4a2a4e23b68845d2f6855103@AcuMS.aculab.com> References: <20241004182734.1761555-1-mathieu.desnoyers@efficios.com> <20241004182734.1761555-3-mathieu.desnoyers@efficios.com> In-Reply-To: <20241004182734.1761555-3-mathieu.desnoyers@efficios.com> Accept-Language: en-GB, 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-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Stat-Signature: 7354ebpm444i5g7wkgdt8pekoqj4cfs7 X-Rspamd-Queue-Id: 47D58C0018 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1728244428-812201 X-HE-Meta: U2FsdGVkX180gGFN4ujHQXvrCeAg38Y7oaVLXg9acotWEfw0uUG07LJ3sFkv9Cd+3wQUERxXF4qd2NnpYWGj0jR/wKOJToBCH4ubUANUQpC8G6MSnWJK79uzObZJXxK1FEizvohyHbg19ReuJCQLPdlhoAkVn4f1eLaecp8on41mooLQRWTFp8skwGeQHWob4EapErun1+8BIDVC2FkfvC7AIt1E5jY0WPYzn+vtY//5ioFzo1HFCeqgFR0SoLWOpbo39OAYznkpsUoOA4tZ9AcFCMFTNd7R9D7M0LY5h2hyJ0FT8I57s8ZK7ZG4ZIDkZ5OtWQ4pl0Hc09Tybk3dF2m1MISZ/IGnQARX9m/+rAEizlv6NkmZ2LzncgonYtPvCPGJ14gAvGfw+sJvhJIySCAFdoPZaSQDysqhToJxQMmDvd4d8KNyJP2wgVPlmkgsGSS7AUizn/W6FdLINpUw6KCinnUyTsV/FPDXboFJ5b7Vg5opwA9yoUGdc0dXus1fas016TzNauDv6mvzYj35aS9kG2eEA77t7JcQJduGmBqLR/JtIElRVpikthripOccX8L9k3NA7IpYhs+2x6rLkmIgymt0TOVBzamJGTPcEkBjnRTIsEHCUvZUCwY/6iWRwB62pQEy5e2dljHd9/t3cOeLIkiiLiKN3/0G32pVnaCA74VZtp1wdlnMechGyIq29ozF2BZ6LasFG2WcTXaSQ1qP9B8DM+wXDadKkhrKpVo3XA1gaUVCfc3DkjSi/3cwiwTq/zzbLkflYyJ+S9Nu4zrDfmR9RDQlH7bNPEAnEvDkjiFxrpVB0gFjJfjT6kAlESMFhsKaaw6Uwhct5Z2GhlLoBLzt9zDsGJLX9qPHcly6sb1UNEfGm1BTYZy8YculBMZ+CXi7/K4SQ0O3uAE3O3cKbJEkqDioBHlsw6wEL8QGDMIlgjz5RPOWYgt73wEoLiUbUjbSi0UaB5I/OKd 3njwfTfY NxFoWsTh2mGvcIoa84SW78CKtTKt9+i1BIxBehHorXaErrT/0VEDrtg2hMwOb0Zby+aqGHoFFMeKb+JRgoFoSJmSXfBcepjvEWYxVkTu69NX8JaOdT3o/gkSje3LnbMVbaQsr8kBVJ5HiHnoQesEAeOdyCKdt+pZr6H2D0JMHP3+TB6vci0D81XcdH7uVQqCK1iFGxsfIerTRtaQcfUHWYiqjWKRapPR+NNTpV+/tlM5fCkrINaUZw1jWPxGykU90Xn1dhULfrbB6kL/DnKZDd5p7ajF/xsBSGrta5dtP3HC8IXXJ+xsAMhseLcvR36XCOiwkjWgyFgRAY6dypwMl2vlV31tbiUAlPdGpxV4tx1s0876GoO1s+oR28cgAJD1iE8Szbq0EgTbWs6+nEbLaMxY5lZU0jrjRvuhFEJDYW9z573uXRfW/7J0VDQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Mathieu Desnoyers > Sent: 04 October 2024 19:28 >=20 > Refer to ptr_eq() in the rcu_dereference() documentation. >=20 > ptr_eq() is a mechanism that preserves address dependencies when > comparing pointers, and should be favored when comparing a pointer > obtained from rcu_dereference() against another pointer. Why does this ever really matter for rcu? The check just ensure that any speculative load uses a specific one of the pointers when they are different. This can only matter if you care about the side effects of the speculative load. But rcu is all about (things like) lockless list following. So you need to wait until it is impossible for another execution context to have a reference to some memory before actually completely invalidating it (ie kfree()). And that 50 line comment is pointless. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)