linux-ext4 - Re: [PATCH 4/7] ext4: fsync should wait for DIO writers

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Date:	Wed, 12 Sep 2012 13:40:01 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Dmitry Monakhov <dmonakhov@...nvz.org>, linux-ext4@...r.kernel.org,
	tytso@....edu, wenqing.lz@...bao.com
Subject: Re: [PATCH 4/7] ext4: fsync should wait for DIO writers

On Mon, Sep 10, 2012 at 11:51:35AM +0200, Jan Kara wrote:
> On Sun 09-09-12 21:27:11, Dmitry Monakhov wrote:
> > fsync and punch_hole are the places where we have to wait for all
> > existing writers (writeback, aio, dio), but currently we simply
> > flush pended end_io request which is not sufficient.
>   Why not? I guess you mean the fact that there can be DIO in flight for
> which end_io() was not called so it is not queued in the queue? But that is
> OK - we have not yet called aio_complete() for that IO so for userspace the
> write has not happened yet. Thus there's no need to flush it to disk -
> fsync() does not say anything about writes in progress while fsync is
> called.
> 
> > Even more i_mutex is not holded while punch_hole which obviously
> > result in dangerous data corruption due to write-after-free.
>   Yes, that's a bug. I also noticed that but didn't get to fixing it (I'm
> actually working on a more long term fix using range locking but that's
> more of a research project so having somehow fixed at least the most
> blatant locking problems is good).

Hi Jan,

Could you please share more detailed information about range locking
with me?  Actually, the goal of extent status tree is to implement a
range locking in ext4 [1], and I am working on it.  So I think that
you have some good ideas to share with me. :-)

1. http://www.spinics.net/lists/linux-ext4/msg32661.html

If you have some problems, please let me know.  Thanks!

Regards,
Zheng
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