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=-16.8 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,URIBL_BLOCKED,USER_AGENT_GIT 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 B6945C47082 for ; Sun, 6 Jun 2021 03:08:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8F03E61420 for ; Sun, 6 Jun 2021 03:08:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230134AbhFFDH0 (ORCPT ); Sat, 5 Jun 2021 23:07:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43984 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230090AbhFFDHY (ORCPT ); Sat, 5 Jun 2021 23:07:24 -0400 Received: from mail-pg1-x52c.google.com (mail-pg1-x52c.google.com [IPv6:2607:f8b0:4864:20::52c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4CE5BC061766 for ; Sat, 5 Jun 2021 20:05:21 -0700 (PDT) Received: by mail-pg1-x52c.google.com with SMTP id i5so11163283pgm.0 for ; Sat, 05 Jun 2021 20:05:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20150623.gappssmtp.com; s=20150623; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=TmNq4peZOAOyDPklFXMeKeYFKClQGReaGV/U+xkvXQ0=; b=w7tT8XJpLxNPgu+o/5mF+K2jB/4Nuo2lE+4B0epXqNpufBtNPiER3R7Tss6U6gkmar IQFrIZ/Whw8TqMtbCCbBq9F41ZBiIFybwAsEhPcN1iA/+JWsOXjkgVDJbHA5L8GN7YyR xpbYvCGtDVd7Ayl7jcqd1/V3RElnhQAGivpppSn6L8IY9AAD98EfNpPb8WOM8DkVAsLf LqlNRVZwIUtCi6ilNm9ot8vxwa/cOCRT5+fWgFVvMJRXbGk4W1OhaliyolRCJfUNCTlk fiA2NbJSPmbLubBh/TfW8UECkjmQwTLQ5UT8pXaTluyg/X6VZvW6Df4/BrbJ3UkY8eXw boIw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=TmNq4peZOAOyDPklFXMeKeYFKClQGReaGV/U+xkvXQ0=; b=ZXofe9xtziQfzWJQlRhyYe8f86CaY8H55nJ/mQl1K+wGz6pRbYc+eM21tPT98ttWh0 AGuYPpadB1qaa5GyqHm6sXFMuBgKNzK1/aRL8N7DMKbQ+rz9QjOL+5UmQ+USCqEYmHNM +2yEsxFHTcd87Gp23MsJcySbzZqUm5exjUof3QnRaHFHYAVk+H/IrjkQ4TBaq1ZlG1H1 ieY3aq2isWruloyzTEvL425/gdLgWQMgYyCs/Lelxu7XLwba/vlLSzFySFgbogjCCskQ oayK4fjfdg7Bio0CSqbSzLe9JzamjUbz3XRJIfa/SRAolcZAhkCpx4zRAVS/z8glls2/ jhxg== X-Gm-Message-State: AOAM532r/kGuP32W7PsPbqHDqF78QTEe/WzIp9ZlnEEBnorgJrNojSgD TGKht0MQw1x9z1ZHuaC4bSTCfA== X-Google-Smtp-Source: ABdhPJwupnG0F7AOML64hOYrBSXuTXNGc5Kpa7E8SpvMxYSBytsEswYDa/ueeM2osUYRANIxrxCxqA== X-Received: by 2002:aa7:8588:0:b029:28e:dfa1:e31a with SMTP id w8-20020aa785880000b029028edfa1e31amr11432836pfn.77.1622948720555; Sat, 05 Jun 2021 20:05:20 -0700 (PDT) Received: from localhost.localdomain ([139.177.225.249]) by smtp.gmail.com with ESMTPSA id q23sm5435219pgj.61.2021.06.05.20.05.16 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Sat, 05 Jun 2021 20:05:20 -0700 (PDT) From: Muchun Song To: viro@zeniv.linux.org.uk, tj@kernel.org, axboe@fb.com, willy@infradead.org Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Muchun Song , Michal Hocko , stable@vger.kernel.org Subject: [PATCH RESEND v3] writeback: fix obtain a reference to a freeing memcg css Date: Sun, 6 Jun 2021 11:02:26 +0800 Message-Id: <20210606030226.66667-1-songmuchun@bytedance.com> X-Mailer: git-send-email 2.21.0 (Apple Git-122) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Cc: --- Since this patch has not been merged to the linux-next tree, just resend it. 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