linux-ext4 - Re: [PATCH -v2] ext4: move ext4_update_i_disksize() into mpage_map_and_submit

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Message-ID: <20140415192642.GE4456@thunk.org>
Date:	Tue, 15 Apr 2014 15:26:42 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	pfonseca@...-sws.org, stable@...r.kernel.org
Subject: Re: [PATCH -v2] ext4: move ext4_update_i_disksize() into
 mpage_map_and_submit_extent()

On Tue, Apr 15, 2014 at 06:26:27PM +0200, Jan Kara wrote:
> On Sat 12-04-14 09:45:27, Ted Tso wrote:
> > The function ext4_update_i_disksize() is used in only one place, in
> > the function mpage_map_and_submit_extent().  Move there to simplify
> > the code paths, and also move the call to ext4_mark_inode_dirty() into
> > the i_data_sem's critical region, to be consistent with all of the
> > other places where we update i_disksize.  That way, we also keep the
> > raw_inode's i_disksize protected.
> > 
> > Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> > Cc: stable@...r.kernel.org
>   I agree that it makes sense to have all the places consistent and protect
> raw disk inode i_disksize with i_data_sem. OTOH I don't see a way how this
> can cause any real harm (but I guess you expect there might be something as
> you CCed stable), so can you explain it please?

This was the case I was worried about:

        CPU #1                                 CPU #2

1.      down_write(&i_data_sem)
2.      Modify i_disk_size
4.      up_write(&i_data_sem)
5.                                           down_write(&i_data_sem)
6.                                           Modify i_disk_size
7.                                           Copy i_disk_size to on-disk inode
8.                                           up_write(&i_data_sem)
9.      Copy i_disk_size to on-disk inode


It's the standard data race; it might not be a problem on Intel CPU's,
but in general, cpu #1 might still have a stale copy of i_disk_size in
its cache, and hence it might copying the old, outdated value into the
on-disk inode.

							- Ted
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