linux-kernel - Re: [PATCH 1/2] aio: make kiocb->private NUll in init_sync

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Date:	Fri, 01 Jun 2012 09:41:52 +0800
From:	Junxiao Bi <junxiao.bi@...cle.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	ocfs2-devel@....oracle.com, linux-aio@...ck.org, mfasheh@...e.com,
	jlbec@...lplan.org, bcrl@...ck.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, joe.jin@...cle.com
Subject: Re: [PATCH 1/2] aio: make kiocb->private NUll in init_sync_kiocb()

On 05/31/2012 10:08 PM, Jeff Moyer wrote:
> Junxiao Bi <junxiao.bi@...cle.com> writes:
>
>> Ocfs2 uses kiocb.*private as a flag of unsigned long size. In
>> commit a11f7e6 ocfs2: serialize unaligned aio, the unaligned
>> io flag is involved in it to serialize the unaligned aio. As
>> *private is not initialized in init_sync_kiocb() of do_sync_write(),
>> this unaligned io flag may be unexpectly set in an aligned dio.
>> And this will cause OCFS2_I(inode)->ip_unaligned_aio decreased
>> to -1 in ocfs2_dio_end_io(), thus the following unaligned dio
>> will hang forever at ocfs2_aiodio_wait() in ocfs2_file_write_iter().
>> We can't initialized this flag in ocfs2_file_write_iter() since
>> it may be invoked several times by do_sync_write(). So we initialize
>> it in init_sync_kiocb(), it's also useful for other similiar use of
>> it in the future.
> I don't see any ocfs2_file_write_iter in the upstream kernel.
> ocfs2_file_aio_write most certainly could set ->private to 0, it
> will only be called once for a given kiocb.
>From sys_io_submit->..->io_submit_one->aio_run_iocb->aio_rw_vect_retry, 
it seems that aio_write could be called two times. See the following
scenario.
1. There is a file opened with direct io flag, in aio_rw_vect_retry,
aio_write is called first time. If the direct io can
not be completed, it will fall back into buffer io, see line 2329 in
aio_write.
2. If the very buffer io is a partial write, then it will return back
to  aio_rw_vect_retry and issue the second aio_write.
>
> That point aside, I have no issues with setting private to NULL in
> init_sync_kiocb.  If you fix up the comment to reflect reality
> w.r.t. the upstream kernel source, I'll ack the patch.
OK, I will fix the comment.
>
> Cheers,
> Jeff

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