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Re: [RFC patch] spindep: add cross cache lines checking
- From: Alex Shi <alex dot shi at intel dot com>
- To: Ingo Molnar <mingo at elte dot hu>
- Cc: Arnd Bergmann <arnd at arndb dot de>, gcc at gcc dot gnu dot org, tglx at linutronix dot de, "mingo at redhat dot com" <mingo at redhat dot com>, hpa at zytor dot com, akpm at linux-foundation dot org, linux-kernel at vger dot kernel dot org, x86 at kernel dot org, andi dot kleen at intel dot com, gcc-help at gcc dot gnu dot org
- Date: Thu, 08 Mar 2012 10:21:02 +0800
- Subject: Re: [RFC patch] spindep: add cross cache lines checking
- References: <1330917630.18835.44.camel@debian> <201203060932.45223.arnd@arndb.de> <1331108607.18835.343.camel@debian> <201203071154.36059.arnd@arndb.de> <4F575F09.3010107@intel.com> <20120307133937.GB12676@elte.hu>
On Wed, 2012-03-07 at 14:39 +0100, Ingo Molnar wrote:
> * Alex Shi <alex.shi@intel.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.
>
> Thanks,
>
> Ingo