[Ocfs2-devel] Question about commit cf1b5ea1c5cd

[Joseph Qi <joseph.qi@huawei.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.