Subject: Re: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union

From: James Hogan <[email-masked]>
Date: Mon, 3 Feb 2014 10:22:04 +0000

On 03/02/14 10:05, David Laight wrote:
> 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; };

Yes indeed.

> 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.

__aligned(2) alone doesn't seem to have any effect on sizeof() or
__alignof__() unless it is accompanied by __packed. x86_64 is similar in
that respect (it just packs sanely in the first place).

Combining __packed with __aligned(2) does the trick though (__packed
alone sets __aligned(1) which is obviously going to be suboptimal).

Cheers
James