linux-ext4 - Re: DIO process stuck apparently due to dioread

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Date:	Mon, 15 Aug 2011 16:53:34 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Tao Ma <tm@....ma>, linux-ext4@...r.kernel.org, sandeen@...hat.com,
	Jan Kara <jack@...e.cz>
Subject: Re: DIO process stuck apparently due to dioread_nolock (3.0)

Hi Michael,

On Mon, Aug 15, 2011 at 1:56 AM, Michael Tokarev <mjt@....msk.ru> wrote:
> 15.08.2011 12:00, Michael Tokarev wrote:
> [....]
>
> So, it looks like this (starting with cold cache):
>
> 1. rename the redologs and copy them over - this will
>   make a hot copy of redologs
> 2. startup oracle - it will complain that the redologs aren't
>   redologs, the header is corrupt
> 3. shut down oracle, start it up again - it will succeed.
>
> If between 1 and 2 you'll issue sync(1) everything will work.
> When shutting down, oracle calls fsync(), so that's like
> sync(1) again.
>
> If there will be some time between 1. and 2., everything
> will work too.
>
> Without dioread_nolock I can't trigger the problem no matter
> how I tried.
>
>
> A smaller test case.  I used redo1.odf file (one of the
> redologs) as a test file, any will work.
>
>  $ cp -p redo1.odf temp
>  $ dd if=temp of=foo iflag=direct count=20
Isn't this the expected behavior here? When doing
'cp -p redo1.odf temp', data is copied to temp through
buffer write, but there is no guarantee when data will be
actually written to disk. Then with 'dd if=temp of=foo
iflag=direct count=20', data is read directly from disk.
Very likely, the written data hasn't been flushed to disk
yet so ext4 returns zero in this case.

Jiaying
>
> Now, first 512bytes of "foo" will contain all zeros, while
> the beginning of redo1.odf is _not_ zeros.
>
> Again, without aioread_nolock it works as expected.
>
>
> And the most important note: without the patch there's no
> data corruption like that.  But instead, there is the
> lockup... ;)
>
> Thank you,
>
> /mjt
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