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

From: ChunYu Wang
Date: Thu Sep 14 2017 - 14:22:13 EST

On Fri, Sep 15, 2017 at 12:41 AM, Andreas Dilger <adilger@xxxxxxxxx> 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