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Re: [RFC patch] spindep: add cross cache lines checking
- From: Ingo Molnar <mingo at elte dot hu>
- To: Alex Shi <alex dot shi at intel dot com>
- 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, 8 Mar 2012 08:13:14 +0100
- 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> <1331173262.18835.347.camel@debian>
* Alex Shi <alex.shi@intel.com> wrote:
> 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.
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