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Subject: Re: [PATCH -rt 4/5] use migrate_disable for __local_begin
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>, Oleg Nesterov <oleg@tv-sign.ru>,
       Steven Rostedt <rostedt@goodmis.org>,
       Christoph Lameter <clameter@sgi.com>
In-Reply-To: <20070714171644.GC6975@Krystal>
References: <20070714175733.194012000@chello.nl>
	 <20070714175840.153584000@chello.nl>  <20070714171644.GC6975@Krystal>
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Date: Sat, 14 Jul 2007 19:32:22 +0200
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On Sat, 2007-07-14 at 13:16 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (a.p.zijlstra@chello.nl) wrote:
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > ---
> >  include/asm-i386/local.h   |    7 ++++---
> >  include/asm-x86_64/local.h |    7 ++++---
> >  2 files changed, 8 insertions(+), 6 deletions(-)
> > 
> > Index: linux-2.6/include/asm-i386/local.h
> > ===================================================================
> > --- linux-2.6.orig/include/asm-i386/local.h
> > +++ linux-2.6/include/asm-i386/local.h
> > @@ -197,11 +197,12 @@ static __inline__ long local_sub_return(
> >  #define __local_begin(__flags)					\
> >  {								\
> >  	(__flags) = 0;						\
> > -	preempt_disable();					\
> > +	migrate_disable();					\
> 
> Brrrr. That's wrong. Your non atomic __local*() updates only makes sense
> when preempt_disable/enable() protects them from concurrent threads on
> the same CPU, which is not the case of migrate_disable/enable(). This is
> why I suggest that you use local_begin/end() mapped to
> migrate_disable/enable() for normal local variables, and, if you really
> want a __local_begin/end(), then it should be mapped to
> preempt_disable/enable() and should state that it provides no protection
> against interrupts.

Sure, but on -rt it does suffice, this part of the patch is rather WIP.