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

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Date:	Thu, 8 Mar 2012 08:13:14 +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:

> On Wed, 2012-03-07 at 14:39 +0100, Ingo Molnar wrote:
> > * 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?
> 
> None. Guess stupid code was shot in lkml reviewing. But if the 
> patch in, it is helpful to block stupid code in developing.

The question is, if in the last 10 years not a single such case 
made it through to today's 15 million lines of kernel code, why 
should we add the check now?

If it was a simple build time check then maybe, but judging by 
the discussion it does not seem so simple, does it?

Thanks,

	Ingo
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