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=-9.0 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,INCLUDES_CR_TRAILER,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 06C78C433B4 for ; Sun, 4 Apr 2021 18:01:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D1457611C1 for ; Sun, 4 Apr 2021 18:01:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231341AbhDDSBq (ORCPT ); Sun, 4 Apr 2021 14:01:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58782 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229861AbhDDSBd (ORCPT ); Sun, 4 Apr 2021 14:01:33 -0400 Received: from mail-qk1-x72d.google.com (mail-qk1-x72d.google.com [IPv6:2607:f8b0:4864:20::72d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5885BC0617A7; Sun, 4 Apr 2021 11:01:28 -0700 (PDT) Received: by mail-qk1-x72d.google.com with SMTP id v70so9693778qkb.8; Sun, 04 Apr 2021 11:01:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=vqjTFrhw8is34xFbRLeJBk6zObb/0KU6NDcwEfDwcS0=; b=l2AjGlLTHKa6vPsOcVT9lSv7QAhpNb1GX7xYJ2agOybHNczEtfkCFH8OmOx33qjzej L6xkIjeDjGig1AIJjUxg7Zv9XNbsXefdFb/GnHKcBFfR3lkSLUneOAcvXaHhrYkavZDR l/1VzqmAJTa1mBLECjiapkDCpOVKextkWwD/NeNFYta2KRsWyFlWCydm6JrL9XIX7l/t tEZ3s5KlbhZUintD0sutFq6ELuQLeRTKm3hDFLgIXbbed6LmFHu2E34lM9aqBH+O9c/c oxBGdJ2W2vmqB3jtvaYAUpZ6aOnrYV0B0m1Hlo9F14NlA5Avj4YQEzZTSp16hWhl0z4e QI7Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:from:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to; bh=vqjTFrhw8is34xFbRLeJBk6zObb/0KU6NDcwEfDwcS0=; b=V7VCtEQ9xjw5WfKl9e+nj50qbwLAGyifXZcqWWj8URFCgbBtpHlsdsqQ3zTQ5J9Skb lQgiB3cIWGNoRT9tMujwH2H4uvfh+ArBq43It87SYWc5+EfBeARgG7OhrLbgRrZzGF2f BUpW064n/IYqEtNsAY1sb44YOp5kjakkqQ4kkPqEnh57QEhLjy0QUU9HMNIjRWgM72jz qpfAfUL24xT4Vsqq4spcv2Z5AH5Tis3DMnz44BgsUweAdgPxd2opjqTgzvIp/kg4gxlR b+BLqkCDYoNvDSjeCza+tl1WPuSSI295V6bNMoK9DOVjq4wng4TRiDFwxL2jPK4ItRYq XFVg== X-Gm-Message-State: AOAM5335gWjoXoo/8VRFuWL4oXmWTC/lfPJNGnNcbXOLwyHUWtXCXoMp zpXVAPLYS1WF6jLmpOnfMUqmTdjBrG5E3w== X-Google-Smtp-Source: ABdhPJzLNUaqWgD1Wf964A7EQG9L1i3NH/sayvR0/nfYb5WNXuZR+X12deaHWKWkor/+gKkJqRNq2g== X-Received: by 2002:a05:620a:527:: with SMTP id h7mr21428959qkh.108.1617559287424; Sun, 04 Apr 2021 11:01:27 -0700 (PDT) Received: from localhost (dhcp-6c-ae-f6-dc-d8-61.cpe.echoes.net. [199.96.183.179]) by smtp.gmail.com with ESMTPSA id 79sm12220473qki.37.2021.04.04.11.01.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 04 Apr 2021 11:01:27 -0700 (PDT) Sender: Tejun Heo Date: Sun, 4 Apr 2021 14:01:26 -0400 From: Tejun Heo To: Muchun Song Cc: viro@zeniv.linux.org.uk, axboe@fb.com, willy@infradead.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, duanxiongchun@bytedance.com, Michal Hocko Subject: Re: [PATCH v3] writeback: fix obtain a reference to a freeing memcg css Message-ID: References: <20210402091145.80635-1-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210402091145.80635-1-songmuchun@bytedance.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 02, 2021 at 05:11:45PM +0800, 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 Acked-by: Tejun Heo Thanks. -- tejun