[Ocfs2-devel] Question about commit cf1b5ea1c5cd

[Al Viro <viro@ZenIV.linux.org.uk>]

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...