linux-kernel - Re: [RFC patch] spindep: add cross cache lines checking

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Date:	Wed, 07 Mar 2012 21:13:45 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	gcc@....gnu.org, Ingo Molnar <mingo@...e.hu>, tglx@...utronix.de,
	"mingo@...hat.com" <mingo@...hat.com>, hpa@...or.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, andi.kleen@...el.com, gcc-help@....gnu.org
Subject: Re: [RFC patch] spindep: add cross cache lines checking


> I think the check should be (__alignof__(lock) < __alignof__(rwlock_t)),
> otherwise it will still pass when you have structure with attribute((packed,aligned(2)))


reasonable!

> 
>> 1, it is alignof bug for default gcc on my fc15 and Ubuntu 11.10 etc?
>>
>> struct sub {
>>         int  raw_lock;
>>         char a;
>> };
>> struct foo {
>>         struct sub z;
>>         int slk;
>>         char y;
>> }__attribute__((packed));
>>
>> struct foo f1;
>>
>> __alignof__(f1.z.raw_lock) is 4, but its address actually can align on
>> one byte. 
> 
> That looks like correct behavior, because the alignment of raw_lock inside of
> struct sub is still 4. But it does mean that there can be cases where the
> compile-time check is not sufficient, so we might want the run-time check
> as well, at least under some config option.


what's your opinion of this, Ingo?
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