linux-ext4 - Re: [PATCH 6/8] fs: always maintain i_dio

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Date:	Mon, 20 Jun 2011 18:23:24 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	viro@...iv.linux.org.uk, tglx@...utronix.de,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-btrfs@...r.kernel.org, hirofumi@...l.parknet.co.jp,
	mfasheh@...e.com
Subject: Re: [PATCH 6/8] fs: always maintain i_dio_count

On Mon, Jun 20, 2011 at 02:29:24PM -0700, Joel Becker wrote:
> 	Oh god you're making the world scary.  Are you guaranteeing that
> all allocation changes are locked out by the time we get into
> file_aio_write() and file_aio_read()?  This is not obvious to me.

I have no idea how ocfs2's internal allocator locking works, but this
patch doesn't change it.  What this patch touches is exclusion between
truncate and pending direct I/O requests, and even there only the
implementation and not the semantics.

The old and new semantics are that you may have either

	1 ongoing truncate

OR

	n (>= 0; <= ATOMIC_T_MAX) ongoing direct I/O reads or writes

before that was enforced using the i_alloc_sem rw_semaphore, including
non-owner releases of it from AIO code, in the new code it's done using
a combination of i_mutex which was already taken in the truncate path,
and when starting new direct I/O requests, and the new i_dio_count
counter.

> 
> Joel
> 
> -- 
> 
> "Glory is fleeting, but obscurity is forever."  
>          - Napoleon Bonaparte
> 
> 			http://www.jlbec.org/
> 			jlbec@...lplan.org
---end quoted text---
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