From: Andrew Perepechko <anserper@yandex.ru>
Subject: Re: quota: dqio_mutex design
Date: Thu, 03 Aug 2017 15:24:05 +0300
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Cc: Shuichi Ihara <sihara@ddn.com>, Wang Shilong <wshilong@ddn.com>,
        Li Xi <lixi@ddn.com>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Jan Kara <jack@suse.cz>, linux-fsdevel@vger.kernel.org
To: Wang Shilong <wangshilong1991@gmail.com>
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> 
> I don't think this is right, as far as i understand, journal quota need go
> together with quota space change update inside same transaction, this will
> break consistency if power off or RO happen.
> 

Hello Wang!

There is no transaction change in this case because all callers of this
function have open handles for the same transaction.

If you enter that DQ_MOD_B check, you are guaranteed to reference
the SAME transaction as the thread that's in between of mark_dirty
and clear_dirty.

Thank you,
Andrew