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

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Date:	Wed, 7 Mar 2012 14:39:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Arnd Bergmann <arnd@...db.de>, gcc@....gnu.org, 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


* Alex Shi <alex.shi@...el.com> wrote:

> > 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?

Dunno. How many real bugs have you found via this patch?

Thanks,

	Ingo
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