linux-kernel - RE: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute_

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Date:	Mon, 3 Feb 2014 10:05:14 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Dan Carpenter' <dan.carpenter@...cle.com>,
	Chen Gang <gang.chen.5i5j@...il.com>
CC:	James Hogan <james.hogan@...tec.com>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	"andreas.dilger@...el.com" <andreas.dilger@...el.com>,
	Antonio Quartulli <antonio@...hcoding.com>,
	"Greg KH" <gregkh@...uxfoundation.org>,
	"bergwolf@...il.com" <bergwolf@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	"oleg.drokin@...el.com" <oleg.drokin@...el.com>,
	"jacques-charles.lafoucriere@....fr" 
	<jacques-charles.lafoucriere@....fr>,
	"jinshan.xiong@...el.com" <jinshan.xiong@...el.com>,
	netdev <netdev@...r.kernel.org>,
	"linux-metag@...r.kernel.org" <linux-metag@...r.kernel.org>
Subject: RE: [PATCH] drivers: staging: lustre: lustre: include: add
 "__attribute__((packed))" for the related union

From: Dan Carpenter
> On Sat, Feb 01, 2014 at 09:57:39PM +0800, Chen Gang wrote:
> > It seems, our kernel still stick to treate 'pack' region have effect
> > with both 'align' and 'sizeof'.
> 
> It's not about packed regions.  It's about unions.  It's saying the
> sizeof() a union is a multiple of 4 unless it's packed.
> 
> union foo {
> 	short x;
> 	short y;
> };
> 
> The author intended the sizeof(union foo) to be 2 but on metag arch then
> it is 4.

The same is probably be true of: struct foo { _u16 bar; };

Architectures that define such alignment rules are a right PITA.
You either need to get the size to 2 without using 'packed', or
just not define such structures.
It is worth seeing if adding aligned(2) will change the size - I'm
not sure.

	David



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