linux-ext4 - Re: kernel BUG at fs/ext4/fsync.c:LINE!

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Date:   Fri, 15 Sep 2017 02:22:01 +0800
From:   ChunYu Wang <chunwang@...hat.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     GeneBlue <geneblue.mail@...il.com>,
        "Theodore Ts'o" <tytso@....edu>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: kernel BUG at fs/ext4/fsync.c:LINE!

On Fri, Sep 15, 2017 at 12:41 AM, Andreas Dilger <adilger@...ger.ca> wrote:
> I don't think a reproducer is needed.  It looks like the fsync callpath
> is happening from an IRQ context due to IO completion, and then re-entering
> the filesystem while a transaction is already started.  It looks like the
> original IO was submitted with AIO based on the functions on the IRQ stack,
> which is likely why nobody has hit it (AIO isn't very commonly used).
>
> That said, I don't follow the reasoning behind the convoluted series of AIO
> callbacks that has IO _completion_ calling vfs_fsync_range() and re-entering
> the filesystem to flush out more data?

Thanks for analyzing, and I do think the syzkaller reproducer(in fact,
log) may also answer your question and help positioning the precise
issue trigger in-depth. Moreover, for me, I am not professional enough
to analyze such a complex problem with call trace and code only :)

- ChunYu