From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joseph Qi <joseph.qi@huawei.com>
Date: Tue, 14 Apr 2015 11:52:23 +0800
Subject: [Ocfs2-devel] Question about commit cf1b5ea1c5cd
In-Reply-To: <20150414034009.GV889@ZenIV.linux.org.uk>
References: <552C8440.2000106@huawei.com>
	<20150414034009.GV889@ZenIV.linux.org.uk>
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To: ocfs2-devel@oss.oracle.com

On 2015/4/14 11:40, Al Viro wrote:
> On Tue, Apr 14, 2015 at 11:06:40AM +0800, Joseph Qi wrote:
>> Hi Viro,
>> I have questions about your commit cf1b5ea1c5cd
>> ("[regression] ocfs2: do *not* increment ->ki_pos twice").
>> Yes, ->ki_pos is increased in generic_file_direct_write(). But
>> *ppos doesn't. So I increase it here for further use in
>> generic_perform_write.
>> After this, ->ki_pos and *ppos are equal.
> 
> They are equal all along, for a very simple reason: ppos *points* *to*
> iocb->ki_pos.  So yes, you do increase it twice, once via one alias,
> once via another.
> 
> Check and you'll see - ppos is initialized with &iocb->ki_pos and never
> reassigned.  What happens is an equivalent of
> 
> int x = 0;
> int *p = &x;
> 
> x += 10;
> *p += 10;
> 
> which obviously ends with x increased by 20, not by 10...
> 
> 
I see. My mistake of ignoring this.
Thanks very much for spotting it out.