From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:51590 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751059AbeEDF7W (ORCPT ); Fri, 4 May 2018 01:59:22 -0400 Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id E3674AEBB for ; Fri, 4 May 2018 05:59:20 +0000 (UTC) Subject: Re: [PATCH v3 0/3] btrfs: qgroup rescan races (part 1) To: Jeff Mahoney , dsterba@suse.com, linux-btrfs@vger.kernel.org References: <20180502211156.9460-1-jeffm@suse.com> From: Nikolay Borisov Message-ID: <14832abb-4d2c-c643-07e4-d81dc6ab8209@suse.com> Date: Fri, 4 May 2018 08:59:19 +0300 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 4.05.2018 01:27, Jeff Mahoney wrote: > On 5/3/18 2:23 AM, Nikolay Borisov wrote: >> >> >> On 3.05.2018 00:11, jeffm@suse.com wrote: >>> From: Jeff Mahoney >>> >>> Hi Dave - >>> >>> Here's the updated patchset for the rescan races. This fixes the issue >>> where we'd try to start multiple workers. It introduces a new "ready" >>> bool that we set during initialization and clear while queuing the worker. >>> The queuer is also now responsible for most of the initialization. >>> >>> I have a separate patch set start that gets rid of the racy mess surrounding >>> the rescan worker startup. We can handle it in btrfs_run_qgroups and >>> just set a flag to start it everywhere else. >> I'd be interested in seeing those patches. Some time ago I did send a >> patch which cleaned up the way qgroup rescan was initiated. It was done >> from "btrfs_run_qgroups" and I think this is messy. Whatever we do we >> ought to really have well-defined semantics when qgroups rescan are run, >> preferably we shouldn't be conflating rescan + run (unless there is >> _really_ good reason to do). In the past the rescan from scan was used >> only during qgroup enabling. > > I think btrfs_run_qgroups is the place to do it. Here's why: > > 2773 int > 2774 btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info) > 2775 { > 2776 int ret = 0; > 2777 struct btrfs_trans_handle *trans; > 2778 > 2779 ret = qgroup_rescan_init(fs_info, 0, 1); > 2780 if (ret) > 2781 return ret; > 2782 > 2783 /* > 2784 * We have set the rescan_progress to 0, which means no more > 2785 * delayed refs will be accounted by btrfs_qgroup_account_ref. > 2786 * However, btrfs_qgroup_account_ref may be right after its call > 2787 * to btrfs_find_all_roots, in which case it would still do the > 2788 * accounting. > 2789 * To solve this, we're committing the transaction, which will > 2790 * ensure we run all delayed refs and only after that, we are > 2791 * going to clear all tracking information for a clean start. > 2792 */ > 2793 > 2794 trans = btrfs_join_transaction(fs_info->fs_root); > 2795 if (IS_ERR(trans)) { > 2796 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN; > 2797 return PTR_ERR(trans); > 2798 } > 2799 ret = btrfs_commit_transaction(trans); > 2800 if (ret) { > 2801 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN; > 2802 return ret; > 2803 } > 2804 > 2805 qgroup_rescan_zero_tracking(fs_info); > 2806 > 2807 queue_rescan_worker(fs_info); > 2808 return 0; > 2809 } > > The delayed ref race should exist anywhere we initiate a rescan outside of > initially enabling qgroups. We already zero the tracking and queue the rescan > worker in btrfs_run_qgroups for when we enable qgroups. Why not just always > queue the worker there so the initialization and execution has a clear starting point? This is no longer true in upstream as of commit 5d23515be669 ("btrfs: Move qgroup rescan on quota enable to btrfs_quota_enable"). Hence my asking about this. I guess if we make it unconditional it won't increase the complexity, but the original code which was only run during qgroup enable was rather iffy I Just don't want to repeat this. > There are a few other races I'd like to fix as well. We call btrfs_run_qgroups > directly from btrfs_ioctl_qgroup_assign, which is buggy since > btrfs_add_qgroup_relation only checks to see if the quota_root exists. It will > exist as soon as btrfs_quota_enable runs but we won't have committed the > transaction yet. The call will end up enabling quotas in the middle of a transaction. > > -Jeff >