From: Jan Kara <jack@suse.cz>
Subject: Re: a question for i_inode's i_size in ext2
Date: Sat, 20 Sep 2008 05:25:22 +0200
Message-ID: <20080920032522.GB9633@atrey.karlin.mff.cuni.cz>
References: <2014bcab0809190021qbca7bect601c891c2db197cf@mail.gmail.com>
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Cc: linux-ext4@vger.kernel.org
To: =?utf-8?B?7ZmN7Iug?= shin hong <hongshin@gmail.com>
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  Dear Shin Hong,

>   I have a question of inode's i_size. I found that it is hard to find
> any consistent synchronization mechanism that protects inode's i_size
> field.
>   Is there any lock or synchronization mechanism that consistently
> protects i_size fields of inode objects to avoid data race?
>   In inode's definition in /include/linux/fs.h, there is comment that
> i_lock protects i_size but it is not clear.
  As Ted said, i_size changes use i_mutex to guard them (actually,
the comment you are probably refering to speaks about i_mutex - i_lock is
something different). Reading i_size is a different matter - see
i_size_read function. We use seqlock for that - essentially we use an
atomic counter which is incremented on every change of i_size and check
that it's value before we started reading i_size and after we have
finished reading it has not changed. I hope this helps.

								Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs