linux-kernel - Re: [PATCH] btrfs: resume qgroup rescan on rw remount

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Date:   Sat, 26 Aug 2017 16:39:02 +1000
From:   Aleksa Sarai <asarai@...e.de>
To:     dsterba@...e.cz, Nikolay Borisov <nborisov@...e.com>,
        Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] btrfs: resume qgroup rescan on rw remount

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: <stable@...r.kernel.org> # v3.11+
>>>> Cc: Jeff Mahoney <jeffm@...e.com>
>>>> Fixes: b382a324b60f ("Btrfs: fix qgroup rescan resume on mount")
>>>> Signed-off-by: Aleksa Sarai <asarai@...e.de>
>>>
>>> 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/