From patchwork Wed Jul 14 09:30:49 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Su Yue X-Patchwork-Id: 12376391 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=ham 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 C535CC07E9A for ; Wed, 14 Jul 2021 09:31:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9D4606120A for ; Wed, 14 Jul 2021 09:31:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238362AbhGNJdx (ORCPT ); Wed, 14 Jul 2021 05:33:53 -0400 Received: from eu-shark2.inbox.eu ([195.216.236.82]:49788 "EHLO eu-shark2.inbox.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238189AbhGNJdx (ORCPT ); Wed, 14 Jul 2021 05:33:53 -0400 Received: from eu-shark2.inbox.eu (localhost [127.0.0.1]) by eu-shark2-out.inbox.eu (Postfix) with ESMTP id F30B71E0051C; Wed, 14 Jul 2021 12:31:00 +0300 (EEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=inbox.eu; s=20140211; t=1626255060; bh=OrHCNZxX90vrgyl+yyhQBt65j3Tt5Fz0vepwWkA1Ilg=; h=From:To:Cc:Subject:Date; b=UHAL6mRU9raLK3NHep3jHsHYwDecMTXFoqES853II1wjRwmBujkGc6zTuLCcZcq9V 1N472N8AYCQCPnhYiD9Y/H8HEDDvUhIZ2M71jeMIxdgP21XfHGB9SUOwDk6PTm6HfN 9cZKlAk5QaBiP0hSeuW8VlFdkjQpEMHXVujfU3Gc= Received: from localhost (localhost [127.0.0.1]) by eu-shark2-in.inbox.eu (Postfix) with ESMTP id EABC71E004B3; Wed, 14 Jul 2021 12:31:00 +0300 (EEST) Received: from eu-shark2.inbox.eu ([127.0.0.1]) by localhost (eu-shark2.inbox.eu [127.0.0.1]) (spamfilter, port 35) with ESMTP id 3DM9JJlFaMMq; Wed, 14 Jul 2021 12:31:00 +0300 (EEST) Received: from mail.inbox.eu (eu-pop1 [127.0.0.1]) by eu-shark2-in.inbox.eu (Postfix) with ESMTP id 899351E004F7; Wed, 14 Jul 2021 12:31:00 +0300 (EEST) Received: from localhost.localdomain (unknown [211.106.132.202]) (Authenticated sender: l@damenly.su) by mail.inbox.eu (Postfix) with ESMTPA id 29F451BE01CB; Wed, 14 Jul 2021 12:30:58 +0300 (EEST) From: Su Yue To: linux-btrfs@vger.kernel.org Cc: l@damenly.su Subject: [RFC PATCH] btrfs: do not warn if fs_devices has no device when call btrfs_show_devname Date: Wed, 14 Jul 2021 17:30:49 +0800 Message-Id: <20210714093049.303978-1-l@damenly.su> X-Mailer: git-send-email 2.30.1 MIME-Version: 1.0 X-Virus-Scanned: OK X-ESPOL: 885mlotPBD+kjECgQHnABwY1s0g9Uezj++a42B5YmH3mU12JfFR+Ix3M/3AFM3H44X8X Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org while running btrfs/238 in my test box, the following warning occurs in high chance: ------------[ cut here ]------------ WARNING: CPU: 3 PID: 481 at fs/btrfs/super.c:2509 btrfs_show_devname+0x104/0x1e8 [btrfs] CPU: 2 PID: 1 Comm: systemd Tainted: G W O 5.14.0-rc1-custom #72 Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015 Call trace: btrfs_show_devname+0x108/0x1b4 [btrfs] show_mountinfo+0x234/0x2c4 m_show+0x28/0x34 seq_read_iter+0x12c/0x3c4 vfs_read+0x29c/0x2c8 ksys_read+0x80/0xec __arm64_sys_read+0x28/0x34 invoke_syscall+0x50/0xf8 do_el0_svc+0x88/0x138 el0_svc+0x2c/0x8c el0t_64_sync_handler+0x84/0xe4 el0t_64_sync+0x198/0x19c ---[ end trace 3efd7e5950b8af05 ]--- It's also reproducible by creating a sprout filesystem and reading /proc/self/mounts in parallel. The warning is produced if btrfs_show_devname() can't find any available device in fs_info->fs_devices->devices which is protected by RCU. The warning is desirable to exercise there is at least one device in the mounted filesystem. However, it's not always true for a sprouting fs. While a new device is being added into fs to be sprouted, call stack is: btrfs_ioctl_add_dev btrfs_init_new_device btrfs_prepare_sprout list_splice_init_rcu(&fs_devices->devices, &seed_devices->devices, synchronize_rcu); list_add_rcu(&device->dev_list, &fs_devices->devices); Looking at btrfs_prepare_sprout(), every new RCU reader will read a empty fs_devices->devices once synchronize_rcu() is called. After commit 4faf55b03823 ("btrfs: don't traverse into the seed devices in show_devname"), btrfs_show_devname() won't looking into fs_devices->seed_list even there is no device in fs_devices->devices. And Since commit 88c14590cdd6 ("btrfs: use RCU in btrfs_show_devname for device list traversal"), btrfs_show_devname() only uses RCU no heavy mutex lock for device list traversal. It read an empty fs_devices->devices and found no device in the list then triggers the warning. The commit just enlarged the window that the fs device list could be empty. Even btrfs_show_devname() uses mutex_lock(), there is a tiny chance of reading an empty devices list between mutex_unlock() in btrfs_prepare_sprout() and next mutex_lock() in btrfs_init_new_device(). Just remove the WARN_ON(1) if there is no valid device since the least one device check is not suitable for the one short period of sprouting filesystem. Signed-off-by: Su Yue --- Make it RFC since I wonder if there is any better solution not dropping the sanity check for normal fs. --- fs/btrfs/super.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index d07b18b2b250..34f9b1c930f8 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -2503,8 +2503,6 @@ static int btrfs_show_devname(struct seq_file *m, struct dentry *root) if (first_dev) seq_escape(m, rcu_str_deref(first_dev->name), " \t\n\\"); - else - WARN_ON(1); rcu_read_unlock(); return 0; }