linux-ext4 - Re: loop: it looks like REQ_OP_FLUSH could return before IO completion.

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Date:   Fri, 15 Apr 2022 22:18:36 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Eric Wheeler <linux-block@...ts.ewheeler.net>,
        linux-block@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: loop: it looks like REQ_OP_FLUSH could return before IO
 completion.

On Fri, Apr 15, 2022 at 10:29:34PM +0800, Ming Lei wrote:
> If ext4 expects the following order, it is ext4's responsibility to
> maintain the order, and block layer may re-order all these IOs at will,
> so do not expect IOs are issued to device in submission order

Yes, and it has been so since REQ_FLUSH (which later became
REQ_OP_FLUSH) replaced REQ_BARRIER 12 years ago:

commit 28e7d1845216538303bb95d679d8fd4de50e2f1a
Author: Tejun Heo <tj@...nel.org>
Date:   Fri Sep 3 11:56:16 2010 +0200

block: drop barrier ordering by queue draining
    
    Filesystems will take all the responsibilities for ordering requests
    around commit writes and will only indicate how the commit writes
    themselves should be handled by block layers.  This patch drops
    barrier ordering by queue draining from block layer.