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=-3.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,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 6A937C4320A for ; Thu, 19 Aug 2021 02:04:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 48D57610CB for ; Thu, 19 Aug 2021 02:04:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235736AbhHSCFX (ORCPT ); Wed, 18 Aug 2021 22:05:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41108 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235668AbhHSCFU (ORCPT ); Wed, 18 Aug 2021 22:05:20 -0400 Received: from mail-pj1-x102b.google.com (mail-pj1-x102b.google.com [IPv6:2607:f8b0:4864:20::102b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C07ACC061764 for ; Wed, 18 Aug 2021 19:04:44 -0700 (PDT) Received: by mail-pj1-x102b.google.com with SMTP id oc2-20020a17090b1c0200b00179e56772d6so374170pjb.4 for ; Wed, 18 Aug 2021 19:04:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=P8pSq2ahTDuGoiVpRUZKFPdbsfAusrUrIIUu2Ni7e/s=; b=MwyQKUFxuRCfpT7aSdwhgTpvG5iUx1xgrIXYt3rxB7MHSmzkskfDLFcbxeXvm+FoKM ocgF37XHI8qFhpJONlkRaoLK7yTqzWaJM4ZwtgzcdPvKbjFItjbiDvIFispOfR9w7nTI Ha1VyaEWTKRQrbtabTSg3aiHf++IfgVW6B9UZCAKnxuFLzhbOXcvjVwQWEdMj5/YtvOX y21AQozEUgxJl7t8+BKEGuTIIBkxxvKkt1Jb2VjvfIux1gW0lzNyvIK/h3cmdSqbAOvq huqmoj0KHFnAis97/PsQ2ysH82VXin9Oc7MN1dD+z0Vtx5WVHd6aiu8JT4ygUwq5szei LAoQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=P8pSq2ahTDuGoiVpRUZKFPdbsfAusrUrIIUu2Ni7e/s=; b=NZFS/LAX/bGHqPmTMYZO8JqQlt/qGFsCmNIr/t5baCddE5Jnyo7/bnR+7tTg0JxfzT lIu+OI7f34HkYT/hSxjJtRr+7ZlERctIVggXWNQO/q5+fYFdTEDdkGd/2/BwXFlBnXCB aQHfH94EVmPZYZpiZBqiV30cPxRqEgo//0Ed/PlGlo4d7/ZfLY7bKeeN1h/lnhzzdWNz l1RSBhXPN8+X9se8bHwh7Ywh20M516/KQ9fsfkgeo3H+Eq7OM/uIDX2NtXTRrALhDTRY InX6gwq1lrlgnvNjrkEOcbmOtcty/tbyolSU29+3EdPKvTBHi2TI8DVP3RrStXKyzxRK 10nw== X-Gm-Message-State: AOAM5338NSy8U38kH7rIg51CBfsccHrZ5+tVyMOrg58E9ysM7jLefx2i d8e99k9ujMNA01L8zDptP8CEht9oGV5zh4mQHvvXgQ== X-Google-Smtp-Source: ABdhPJwEss61R+YGuKuQ/7eeNweeErVCe+4SBPDaGMiRjtIhpwl7hLMuELKSaujA/O01JKE9PkmjK0uXJc8fTLRHebY= X-Received: by 2002:a17:902:d48e:b0:12d:7aa5:de2e with SMTP id c14-20020a170902d48e00b0012d7aa5de2emr9670291plg.34.1629338684303; Wed, 18 Aug 2021 19:04:44 -0700 (PDT) MIME-Version: 1.0 References: <20210815061034.84309-1-yanghui.def@bytedance.com> <20210816175952.3c0d1eee821cd2d9ed7c3879@linux-foundation.org> In-Reply-To: From: Muchun Song Date: Thu, 19 Aug 2021 10:04:06 +0800 Message-ID: Subject: Re: [PATCH v3] mm/mempolicy: fix a race between offset_il_node and mpol_rebind_task To: Matthew Wilcox Cc: Andrew Morton , yanghui , Linux Memory Management List , LKML Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 18, 2021 at 11:07 PM Matthew Wilcox wrote: > > On Wed, Aug 18, 2021 at 10:02:46PM +0800, Muchun Song wrote: > > On Tue, Aug 17, 2021 at 9:43 AM Matthew Wilcox wrote: > > > > > + unsigned int target, nnodes; > > > > > int i; > > > > > int nid; > > > > > + /* > > > > > + * The barrier will stabilize the nodemask in a register or on > > > > > + * the stack so that it will stop changing under the code. > > > > > + * > > > > > + * Between first_node() and next_node(), pol->nodes could be changed > > > > > + * by other threads. So we put pol->nodes in a local stack. > > > > > + */ > > > > > + barrier(); > > > > > > I think this could be an smp_rmb()? > > > > Hi Matthew, > > > > I have a question. Why is barrier() not enough? > > I think barrier() may be more than is necessary. We don't need a > barrier on non-SMP systems (or do we?) And we only need to order reads, > not writes. Here barrier() is just a compiler barrier, which is cheaper than smp_rmb() which usually equals to memory barrier instruction plus barrier(). So I think barrier() , which will stabilize the nodemask in a register or on the stack, is more appropriate here. Thanks.