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=-10.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,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 D0C2BC433E8 for ; Mon, 27 Jul 2020 14:14:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A7F372078E for ; Mon, 27 Jul 2020 14:14:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1595859256; bh=qCDLhlhkFegasnlzwVfmv02VbyBzY+ZJq3EAce/cSaM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=nWT/wrRBozlvKIEEqebTcd4kr66yA/Katd6LeNNz/87DufuIDtqMPqj6Hg25pYMUk /eix9DlGq0N5R1XQKhgWh7Gvu5ADH0PbZiJvng3Gc2BmAIDJcv2zsfVFvF/S5FUEGJ WYNioMGBpBJvPrWTsHJlR/VVhgHgf3UaqQvAaiHo= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730483AbgG0OOP (ORCPT ); Mon, 27 Jul 2020 10:14:15 -0400 Received: from mail.kernel.org ([198.145.29.99]:39590 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730490AbgG0OOL (ORCPT ); Mon, 27 Jul 2020 10:14:11 -0400 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 81E962078E; Mon, 27 Jul 2020 14:14:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1595859251; bh=qCDLhlhkFegasnlzwVfmv02VbyBzY+ZJq3EAce/cSaM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=VSQqyDjyku8QDRDbx+V0qsQftUJnL1UaGPBBdgfvTxv7voHT/1ixgCoN0VA14LsG9 XOG/WFMyUiJtkRvWxdZGoM5ow2bc7bS7fnwI3Mw8voCzsSH3GsfP2qgfyIF3wLTSGo 6JCoH+Q8BUrpyRoTEIe/DauTI33BQ1Sff8l7I1rc= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Boris Burkov , David Sterba Subject: [PATCH 5.4 034/138] btrfs: fix mount failure caused by race with umount Date: Mon, 27 Jul 2020 16:03:49 +0200 Message-Id: <20200727134927.080881996@linuxfoundation.org> X-Mailer: git-send-email 2.27.0 In-Reply-To: <20200727134925.228313570@linuxfoundation.org> References: <20200727134925.228313570@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Boris Burkov commit 48cfa61b58a1fee0bc49eef04f8ccf31493b7cdd upstream. It is possible to cause a btrfs mount to fail by racing it with a slow umount. The crux of the sequence is generic_shutdown_super not yet calling sop->put_super before btrfs_mount_root calls btrfs_open_devices. If that occurs, btrfs_open_devices will decide the opened counter is non-zero, increment it, and skip resetting fs_devices->total_rw_bytes to 0. From here, mount will call sget which will result in grab_super trying to take the super block umount semaphore. That semaphore will be held by the slow umount, so mount will block. Before up-ing the semaphore, umount will delete the super block, resulting in mount's sget reliably allocating a new one, which causes the mount path to dutifully fill it out, and increment total_rw_bytes a second time, which causes the mount to fail, as we see double the expected bytes. Here is the sequence laid out in greater detail: CPU0 CPU1 down_write sb->s_umount btrfs_kill_super kill_anon_super(sb) generic_shutdown_super(sb); shrink_dcache_for_umount(sb); sync_filesystem(sb); evict_inodes(sb); // SLOW btrfs_mount_root btrfs_scan_one_device fs_devices = device->fs_devices fs_info->fs_devices = fs_devices // fs_devices-opened makes this a no-op btrfs_open_devices(fs_devices, mode, fs_type) s = sget(fs_type, test, set, flags, fs_info); find sb in s_instances grab_super(sb); down_write(&s->s_umount); // blocks sop->put_super(sb) // sb->fs_devices->opened == 2; no-op spin_lock(&sb_lock); hlist_del_init(&sb->s_instances); spin_unlock(&sb_lock); up_write(&sb->s_umount); return 0; retry lookup don't find sb in s_instances (deleted by CPU0) s = alloc_super return s; btrfs_fill_super(s, fs_devices, data) open_ctree // fs_devices total_rw_bytes improperly set! btrfs_read_chunk_tree read_one_dev // increment total_rw_bytes again!! super_total_bytes < fs_devices->total_rw_bytes // ERROR!!! To fix this, we clear total_rw_bytes from within btrfs_read_chunk_tree before the calls to read_one_dev, while holding the sb umount semaphore and the uuid mutex. To reproduce, it is sufficient to dirty a decent number of inodes, then quickly umount and mount. for i in $(seq 0 500) do dd if=/dev/zero of="/mnt/foo/$i" bs=1M count=1 done umount /mnt/foo& mount /mnt/foo does the trick for me. CC: stable@vger.kernel.org # 4.4+ Signed-off-by: Boris Burkov Reviewed-by: David Sterba Signed-off-by: David Sterba Signed-off-by: Greg Kroah-Hartman --- fs/btrfs/volumes.c | 8 ++++++++ 1 file changed, 8 insertions(+) --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -7270,6 +7270,14 @@ int btrfs_read_chunk_tree(struct btrfs_f mutex_lock(&fs_info->chunk_mutex); /* + * It is possible for mount and umount to race in such a way that + * we execute this code path, but open_fs_devices failed to clear + * total_rw_bytes. We certainly want it cleared before reading the + * device items, so clear it here. + */ + fs_info->fs_devices->total_rw_bytes = 0; + + /* * Read all device items, and then all the chunk items. All * device items are found before any chunk item (their object id * is smaller than the lowest possible object id for a chunk