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=-7.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 C6D56C35246 for ; Sun, 26 Jan 2020 17:15:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9E35520702 for ; Sun, 26 Jan 2020 17:15:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727250AbgAZRPI (ORCPT ); Sun, 26 Jan 2020 12:15:08 -0500 Received: from jabberwock.ucw.cz ([46.255.230.98]:45202 "EHLO jabberwock.ucw.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725944AbgAZRPI (ORCPT ); Sun, 26 Jan 2020 12:15:08 -0500 Received: by jabberwock.ucw.cz (Postfix, from userid 1017) id 46C341C036D; Sun, 26 Jan 2020 18:15:06 +0100 (CET) Date: Sun, 26 Jan 2020 18:15:05 +0100 From: Pavel Machek To: Greg Kroah-Hartman Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, Eric Dumazet , Jakub Kicinski , Sasha Levin Subject: Re: [PATCH 4.19 599/639] net: avoid possible false sharing in sk_leave_memory_pressure() Message-ID: <20200126171505.GD19082@duo.ucw.cz> References: <20200124093047.008739095@linuxfoundation.org> <20200124093204.391643194@linuxfoundation.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="XvKFcGCOAo53UbWW" Content-Disposition: inline In-Reply-To: <20200124093204.391643194@linuxfoundation.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --XvKFcGCOAo53UbWW Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri 2020-01-24 10:32:49, Greg Kroah-Hartman wrote: > From: Eric Dumazet >=20 > [ Upstream commit 503978aca46124cd714703e180b9c8292ba50ba7 ] >=20 > As mentioned in https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_= ONCE#it-may-improve-performance > a C compiler can legally transform : >=20 > if (memory_pressure && *memory_pressure) > *memory_pressure =3D 0; >=20 > to : >=20 > if (memory_pressure) > *memory_pressure =3D 0; Well, C compiler can do a lot of stuff, and we rely on C compiler being "sane" -- that is gcc. Even if compiler did the transformation, that will only result in slightly slower performance, right? Is there any evidence this is problem in practice? Should this be in stable? Best regards, Pavel > +++ b/net/core/sock.c > @@ -2179,8 +2179,8 @@ static void sk_leave_memory_pressure(struct sock *s= k) > } else { > unsigned long *memory_pressure =3D sk->sk_prot->memory_pressure; > =20 > - if (memory_pressure && *memory_pressure) > - *memory_pressure =3D 0; > + if (memory_pressure && READ_ONCE(*memory_pressure)) > + WRITE_ONCE(*memory_pressure, 0); > } > } --=20 (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo= g.html --XvKFcGCOAo53UbWW Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iF0EABECAB0WIQRPfPO7r0eAhk010v0w5/Bqldv68gUCXi3JGQAKCRAw5/Bqldv6 8tXqAJ9xRik6QC4lKIxZJnGR9pEUzJKgpwCgt5fiRlxg/KfBmn58c/mwksuZ8Ms= =ep4Y -----END PGP SIGNATURE----- --XvKFcGCOAo53UbWW--