[PATCH] ARM: fix cpu_relax() in case of doing dmb

[Shawn Guo]

On Thu, Aug 23, 2012 at 11:43:56AM +0100, Will Deacon wrote:
> On Wed, Aug 22, 2012 at 03:52:18PM +0100, Shawn Guo wrote:
> > diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
> > index 99afa74..7cc67ce 100644
> > --- a/arch/arm/include/asm/processor.h
> > +++ b/arch/arm/include/asm/processor.h
> > @@ -80,7 +80,14 @@ extern void release_thread(struct task_struct *);
> >  unsigned long get_wchan(struct task_struct *p);
> >  
> >  #if __LINUX_ARM_ARCH__ == 6 || defined(CONFIG_ARM_ERRATA_754327)
> > -#define cpu_relax()			smp_mb()
> > +#define cpu_relax()		do {					\
> > +					asm("nop");			\
> > +					asm("nop");			\
> > +					asm("nop");			\
> > +					asm("nop");			\
> > +					asm("nop");			\
> 
> Can you use nop() instead of the explicit asm?

Yes.  I just tried, and it works too.

> Also, I think we should try
> and use some methodology on deciding the number of nops to insert. Without
> having a full handle on the problem at the moment, it would seem that we
> need at least NR_CPUS worth (since the number of spinning secondaries is
> NR_CPUS-1 and they may execute their barriers in lock-step).
> 
I'm not sure we get something like that.  In my testing here, I need
at least 5 nops to get rid of the issue.

-- 
Regards,
Shawn