linux-kernel - RE: [patch] clarify AIO_EVENTS

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Date:	Fri, 13 Oct 2006 13:38:01 -0700
From:	"Chen, Kenneth W" <kenneth.w.chen@...el.com>
To:	"'Benjamin LaHaise'" <bcrl@...ck.org>
Cc:	"'Zach Brown'" <zach.brown@...cle.com>,
	"'Suparna Bhattacharya'" <suparna@...ibm.com>,
	"Lahaise, Benjamin C" <benjamin.c.lahaise@...el.com>,
	<linux-kernel@...r.kernel.org>, "'linux-aio'" <linux-aio@...ck.org>
Subject: RE: [patch] clarify AIO_EVENTS_OFFSET constant

Benjamin LaHaise wrote on Friday, October 13, 2006 7:23 AM
> On Thu, Oct 12, 2006 at 05:00:24PM -0700, Chen, Kenneth W wrote:
> > A clean up patch: I think it is a lot easier to read AIO_EVENTS_OFFSET
> > as an offset because of aio_ring at the beginning of a head page, instead
> > of doing arithmetic of (event on 2nd page - event on 1st page).
> 
> Nak.  Your change fails if aio_ring is not an exact multiple of the 
> io_event size due to rounding errors, while the original code rounds 
> correctly.


Yeah, neither form is bullet proof, even the original one will break down
when io_event size is not in power of 2.  The good news is that no where
near in sight that these two data structures size will change.

To make it a real bullet proof, it should be something like the following:

#define AIO_EVENTS_OFFSET       ((sizeof(struct aio_ring) +             \
                                  sizeof(struct io_event) - 1 ) /       \
                                  sizeof(struct io_event))

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