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From: Badari Pulavarty <pbadari@us.ibm.com>
To: Daniel McNeil <daniel@osdl.org>, Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH 2.6.2-rc3-mm1] DIO read race fix
Date: Wed, 4 Feb 2004 17:54:48 -0800
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Cc: Janet Morgan <janetmor@us.ibm.com>,
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       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Suparna Bhattacharya <suparna@in.ibm.com>
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Hmmm !! You beat me by few hours :)

It sounds convincing. I am not sure about the fix either.
Let me try the fix and see if really fixes the problem.

Thanks,
Badari

On Wednesday 04 February 2004 05:39 pm, Daniel McNeil wrote:
> I have found (finally) the problem causing DIO reads racing with
> buffered writes to see uninitialized data on ext3 file systems
> (which is what I have been testing on).
>
> The problem is caused by the changes to __block_write_page_full()
> and a race with journaling:
>
> journal_commit_transaction() -> ll_rw_block() -> submit_bh()
>
> ll_rw_block() locks the buffer, clears buffer dirty and calls
> submit_bh()
>
> A racing __block_write_full_page() (from ext3_ordered_writepage())
>
> 	would see that buffer_dirty() is not set because the i/o
>         is still in flight, so it would not do a bh_submit()
>
> 	It would SetPageWriteback() and unlock_page() and then
> 	see that no i/o was submitted and call end_page_writeback()
> 	(with the i/o still in flight).
>
> This would allow the DIO code to issue the DIO read while buffer writes
> are still in flight.  The i/o can be reordered by i/o scheduling and
> the DIO can complete BEFORE the writebacks complete.  Thus the DIO
> sees the old uninitialized data.
>
> Here is a quick hack that fixes it, but I am not sure if this the
> proper long term fix.
>
> Thoughts?
>
> Daniel