linux-ext4 - Re: [EXT4 set 4][PATCH 1/5] i_version:64 bit inode version

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Message-Id: <1184164086.12154.15.camel@kleikamp.austin.ibm.com>
Date:	Wed, 11 Jul 2007 09:28:06 -0500
From:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
To:	Neil Brown <neilb@...e.de>
Cc:	cmm@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, nfsv4@...ux-nfs.org,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [EXT4 set 4][PATCH 1/5] i_version:64 bit inode version

On Wed, 2007-07-11 at 15:05 +1000, Neil Brown wrote:
> It just occurred to me:
> 
>  If i_version is 64bit, then knfsd would need to be careful when
>  reading it on a 32bit host.  What are the locking rules?

How does knfsd use i_version?  I would think that if all it was doing
was to compare (i_version == previous_version), then locking wouldn't
really matter.  Well, theoretically, previous_version could be
0x100000000, and i_version could be 0x1ffffffff, knfsd checks the high
word, then ext4 updates i_version to 0x200000000, then knfsd checks the
low word, detecting no change.  How likely is this?  (I don't understand
why i_version even needs to be 64 bits in the first place.)

>  Presumably it is only updated under i_mutex protection, but having to
>  get i_mutex to read it would seem a little heavy handed.

How does knfsd protect itself from the inode changing after i_version is
checked?  Is any locking being done otherwise?

>  Should it use a seqlock like i_size?
>  Could we use the same seqlock that i_size uses, or would we need a
>  separate one?
> 
> NeilBrown

-- 
David Kleikamp
IBM Linux Technology Center

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