From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752103AbdHZGjM (ORCPT ); Sat, 26 Aug 2017 02:39:12 -0400 Received: from mx2.suse.de ([195.135.220.15]:40436 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750737AbdHZGjL (ORCPT ); Sat, 26 Aug 2017 02:39:11 -0400 Subject: Re: [PATCH] btrfs: resume qgroup rescan on rw remount To: dsterba@suse.cz, Nikolay Borisov , Chris Mason , Josef Bacik , David Sterba , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20170704114906.8419-1-asarai@suse.de> <725eb058-4fb4-a167-9ba4-a062de718555@suse.com> <532a6e98-1745-4181-260a-38f4d5015857@suse.com> <20170711170335.GV2866@twin.jikos.cz> From: Aleksa Sarai Message-ID: Date: Sat, 26 Aug 2017 16:39:02 +1000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170711170335.GV2866@twin.jikos.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/12/2017 03:03 AM, David Sterba wrote: > On Mon, Jul 10, 2017 at 04:56:36PM +0300, Nikolay Borisov wrote: >> On 10.07.2017 16:12, Nikolay Borisov wrote: >>> On 4.07.2017 14:49, Aleksa Sarai wrote: >>>> Several distributions mount the "proper root" as ro during initrd and >>>> then remount it as rw before pivot_root(2). Thus, if a rescan had been >>>> aborted by a previous shutdown, the rescan would never be resumed. >>>> >>>> This issue would manifest itself as several btrfs ioctl(2)s causing the >>>> entire machine to hang when btrfs_qgroup_wait_for_completion was hit >>>> (due to the fs_info->qgroup_rescan_running flag being set but the rescan >>>> itself not being resumed). Notably, Docker's btrfs storage driver makes >>>> regular use of BTRFS_QUOTA_CTL_DISABLE and BTRFS_IOC_QUOTA_RESCAN_WAIT >>>> (causing this problem to be manifested on boot for some machines). >>>> >>>> Cc: # v3.11+ >>>> Cc: Jeff Mahoney >>>> Fixes: b382a324b60f ("Btrfs: fix qgroup rescan resume on mount") >>>> Signed-off-by: Aleksa Sarai >>> >>> Indeed, looking at the code it seems that b382a324b60f ("Btrfs: fix >>> qgroup rescan resume on mount") missed adding the qgroup_rescan_resume >>> in the remount path. One thing which I couldn't verify though is whether >>> reading fs_info->qgroup_flags without any locking is safe from remount >>> context. >>> >>> During remount I don't see any locks taken that prevent operations which >>> can modify qgroup_flags. >> >> Further inspection reveals that the access rules to qgroup_flags are >> somewhat broken so this patch doesn't really make things any worse than >> they are. > > The usage follows a pattern for a bitfield, updated by set_bit/clear_bit > etc. The updates to the state or inconsistency is not safe, so some > updates could get lost under some circumstances. > > Patch added to devel queue, possibly will be submitted to 4.13 so stable > can pick it. Looks like it wasn't merged in the 4.13 window (so stable hasn't picked it), will this be submitted for 4.14? Thanks. -- Aleksa Sarai Software Engineer (Containers) SUSE Linux GmbH https://www.cyphar.com/