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=-13.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 A997CC433B4 for ; Thu, 20 May 2021 03:46:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 89C12611AD for ; Thu, 20 May 2021 03:46:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230312AbhETDrW (ORCPT ); Wed, 19 May 2021 23:47:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52114 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230298AbhETDrV (ORCPT ); Wed, 19 May 2021 23:47:21 -0400 Received: from mail-pl1-x635.google.com (mail-pl1-x635.google.com [IPv6:2607:f8b0:4864:20::635]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 75DCFC061574 for ; Wed, 19 May 2021 20:45:59 -0700 (PDT) Received: by mail-pl1-x635.google.com with SMTP id t21so8240675plo.2 for ; Wed, 19 May 2021 20:45:59 -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=dsx7U7xFEMYwysEjpdGkhCMavbrgfZJA5H4Xahpf5cI=; b=K1wI5foB/OFwa+YxtQyl0ye+iraAQYmdC/sZ87b1fWKNRpvWVu/f99q9N6jwfkD5yl Iy31CmZi4cuch3FS144/63jLe29jilr2SE5Cy2AF9AOZfdQIScIY5ZP9nXWDwWtTMEgN RMqF0Go5He+ZYyWIp9r8jNUgQ8G+JYIYhokz8jyk35r583XRegVPp637VbVUfIJnrlU+ zNGq4IW/pZhPe35JHlrpAmKgZibJaqDyj27wEeYRemTsvGzIhAr2i/OD4rxFGAfPfbNk ucGbH9k5n05eqo3ZYZv24VUePrzJ/nE6ETGEJNwJFr1A2NGW+JGSXS6Xlnm7PgA9TtRZ Q85Q== 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=dsx7U7xFEMYwysEjpdGkhCMavbrgfZJA5H4Xahpf5cI=; b=EjGm0cxd6qXsVjwjLsOK2QDP4vPAafkbRtgTsAb1mvHhm6YVaVA0DlqA9wXm6WXgQe uyaVxN1V4ZqH/2+XMPKHvePRO3XfyVS0ZzH4vFtryFqeuTGfG7WIsdcP5mOAF4qfNux8 OFS8lXwGKrQHgFUDbFCrwj5xZKifOqe0vwJGViPBavAOmoMaIfHvjHEeHc7H+Yz6pVf9 sHpU54J8Akokfl/TzqB7tBDJ2hFsENwCeSMQ14pJN2C2+TILuiXbTlRd9EYJIm5WSy20 e7u+UZkyo6IwJb/77vRiLVuEw8Kvt9AGw7GSIaSzD1GwDvpfVoWBT2Rz2XhzDUz5kQnU 0ndg== X-Gm-Message-State: AOAM532Iixh74BB1XJaDD4T0f1oXvSbEpuhbN8GKCnl3d+LcbVoX8MYZ a3PdTWgJoK8L15cYtdixRRkiHBUr2iiuw1RqbNSk9w== X-Google-Smtp-Source: ABdhPJzsTfKJ0VkQK6fxhnPimvNAb8jp7vq5SsLJdXpFeRYgoELgi5Ucv3Tv8vbqJdubvqR7ufMgTbx+6ERjqnj1yjM= X-Received: by 2002:a17:90a:6d43:: with SMTP id z61mr2855552pjj.147.1621482359005; Wed, 19 May 2021 20:45:59 -0700 (PDT) MIME-Version: 1.0 References: <20210402091145.80635-1-songmuchun@bytedance.com> In-Reply-To: <20210402091145.80635-1-songmuchun@bytedance.com> From: Muchun Song Date: Thu, 20 May 2021 11:45:23 +0800 Message-ID: Subject: Re: [PATCH v3] writeback: fix obtain a reference to a freeing memcg css To: Alexander Viro , Tejun Heo , axboe@fb.com, Matthew Wilcox Cc: linux-fsdevel , LKML , Xiongchun duan , Michal Hocko Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Hi, It seems like this patch has not been added to the linux-next tree. Can anyone help with this? Thanks. On Fri, Apr 2, 2021 at 5:13 PM Muchun Song wrote: > > The caller of wb_get_create() should pin the memcg, because > wb_get_create() relies on this guarantee. The rcu read lock > only can guarantee that the memcg css returned by css_from_id() > cannot be released, but the reference of the memcg can be zero. > > rcu_read_lock() > memcg_css = css_from_id() > wb_get_create(memcg_css) > cgwb_create(memcg_css) > // css_get can change the ref counter from 0 back to 1 > css_get(memcg_css) > rcu_read_unlock() > > Fix it by holding a reference to the css before calling > wb_get_create(). This is not a problem I encountered in the > real world. Just the result of a code review. > > Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates") > Signed-off-by: Muchun Song > Acked-by: Michal Hocko > --- > Changelog in v3: > 1. Do not change GFP_ATOMIC. > 2. Update commit log. > > Thanks for Michal's review and suggestions. > > Changelog in v2: > 1. Replace GFP_ATOMIC with GFP_NOIO suggested by Matthew. > > > fs/fs-writeback.c | 9 +++++++-- > 1 file changed, 7 insertions(+), 2 deletions(-) > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > index 3ac002561327..dedde99da40d 100644 > --- a/fs/fs-writeback.c > +++ b/fs/fs-writeback.c > @@ -506,9 +506,14 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) > /* find and pin the new wb */ > rcu_read_lock(); > memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys); > - if (memcg_css) > - isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC); > + if (memcg_css && !css_tryget(memcg_css)) > + memcg_css = NULL; > rcu_read_unlock(); > + if (!memcg_css) > + goto out_free; > + > + isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC); > + css_put(memcg_css); > if (!isw->new_wb) > goto out_free; > > -- > 2.11.0 >