From patchwork Wed Aug 18 04:19:44 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Su Yue X-Patchwork-Id: 12442591 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 717ACC4338F for ; Wed, 18 Aug 2021 04:19:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 51F0E60EB5 for ; Wed, 18 Aug 2021 04:19:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229834AbhHREUb (ORCPT ); Wed, 18 Aug 2021 00:20:31 -0400 Received: from eu-shark2.inbox.eu ([195.216.236.82]:41240 "EHLO eu-shark2.inbox.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229448AbhHREUb (ORCPT ); Wed, 18 Aug 2021 00:20:31 -0400 Received: from eu-shark2.inbox.eu (localhost [127.0.0.1]) by eu-shark2-out.inbox.eu (Postfix) with ESMTP id 2EB3E1E006CF; Wed, 18 Aug 2021 07:19:56 +0300 (EEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=inbox.eu; s=20140211; t=1629260396; bh=2KCYXFz6I17gf0KwnG4Mdmi/Ecz6hPEYcGx8PPgwtQY=; h=From:To:Cc:Subject:Date; b=VpBOBZB7KluWsGki8EuvxgImqHOdIwvFlpd6UDdPK68Jlipmo0+LJg0gPxSz7ASQK GjAQIg9hlScbFAx62KmQ1bWG4CYXFAWkJNLfRXU1OwfQbnc8BVBSBSAAE5zh8I4mBi SfPHlDPx+SAdrTC3ejCRxaMUrfr7QnyfR3L5EXzo= Received: from localhost (localhost [127.0.0.1]) by eu-shark2-in.inbox.eu (Postfix) with ESMTP id 22A601E006D6; Wed, 18 Aug 2021 07:19:56 +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 93nhyMVN1grQ; Wed, 18 Aug 2021 07:19:55 +0300 (EEST) Received: from mail.inbox.eu (eu-pop1 [127.0.0.1]) by eu-shark2-in.inbox.eu (Postfix) with ESMTP id A3C4C1E006CF; Wed, 18 Aug 2021 07:19:55 +0300 (EEST) Received: from mini.lan (unknown [49.65.73.48]) (Authenticated sender: l@damenly.su) by mail.inbox.eu (Postfix) with ESMTPA id 214411BE00FE; Wed, 18 Aug 2021 07:19:51 +0300 (EEST) From: Su Yue To: linux-btrfs@vger.kernel.org Cc: l@damenly.su, anand.jain@oracle.com Subject: [PATCH V2] btrfs: traverse seed devices if fs_devices::devices is empty in show_devname Date: Wed, 18 Aug 2021 12:19:44 +0800 Message-Id: <20210818041944.5793-1-l@damenly.su> X-Mailer: git-send-email 2.30.1 (Apple Git-130) MIME-Version: 1.0 X-Virus-Scanned: OK X-ESPOL: +dBm1NUOBlzXh129MAjTdngr1kpEWOT7/eKk1x5HmHPmU1qJf04NURK/nm1yS2A= 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 without mutex lock taken for device list traversal. The function 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 to read an empty devices list between mutex_unlock() in btrfs_prepare_sprout() and next mutex_lock() in btrfs_init_new_device(). So take device_list_mutex then traverse fs_devices->seed_list to seek for a seed device if no device was found in fs_devices->devices. Since a normal fs always has devices in fs_device->devices and the window is small enough, the mutex lock is not too heavy. Signed-off-by: Su Yue --- Changelog: v2: Try to traverse fs_devices->seed_list instead of removing the WARN_ON(). Change the subject. Add description of fix. --- fs/btrfs/super.c | 41 ++++++++++++++++++++++++++++++++++++++--- 1 file changed, 38 insertions(+), 3 deletions(-) diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index d07b18b2b250..31e723eb2ccf 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -2482,7 +2482,9 @@ static int btrfs_unfreeze(struct super_block *sb) static int btrfs_show_devname(struct seq_file *m, struct dentry *root) { struct btrfs_fs_info *fs_info = btrfs_sb(root->d_sb); + struct btrfs_fs_devices *fs_devices = fs_info->fs_devices; struct btrfs_device *dev, *first_dev = NULL; + struct btrfs_fs_devices *seed_devices; /* * Lightweight locking of the devices. We should not need @@ -2492,7 +2494,7 @@ static int btrfs_show_devname(struct seq_file *m, struct dentry *root) * least until the rcu_read_unlock. */ rcu_read_lock(); - list_for_each_entry_rcu(dev, &fs_info->fs_devices->devices, dev_list) { + list_for_each_entry_rcu(dev, &fs_devices->devices, dev_list) { if (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state)) continue; if (!dev->name) @@ -2503,9 +2505,42 @@ 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(); + + if (first_dev) + return 0; + + /* + * While the fs is sprouting, above fs_devices->devices could be empty + * if the RCU read happened in the window between when + * fs_devices->devices was spliced into seed_devices->devices in + * btrfs_prepare_sprout() and new device is not added to + * fs_devices->devices in btrfs_init_new_device(). + * + * Take device_list_mutex to make sure seed_devices has been added into + * fs_devices->seed_list then we can traverse it. + */ + mutex_lock(&fs_devices->device_list_mutex); + list_for_each_entry(seed_devices, &fs_devices->seed_list, seed_list) { + list_for_each_entry(dev, &seed_devices->devices, dev_list) { + if (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state)) + continue; + if (!dev->name) + continue; + if (!first_dev || dev->devid < first_dev->devid) + first_dev = dev; + } + } + + if (first_dev) { + rcu_read_lock(); + seq_escape(m, rcu_str_deref(first_dev->name), " \t\n\\"); + rcu_read_unlock(); + } else { + WARN_ON(1); + } + mutex_unlock(&fs_devices->device_list_mutex); + return 0; }