[RFC PATCH powerpc ] Protect smp_processor_id() in arch_spin_unlock_wait()

[Li Zhong]

On Thu, 2013-01-10 at 17:02 +1100, Benjamin Herrenschmidt wrote:
> On Mon, 2012-11-19 at 14:16 +0800, Li Zhong wrote:
> > This patch tries to disable preemption for using smp_processor_id() in arch_spin_unlock_wait(), 
> > to avoid following report:
> 
>  .../...
> 
> > diff --git a/arch/powerpc/lib/locks.c b/arch/powerpc/lib/locks.c
> > index bb7cfec..7a7c31b 100644
> > --- a/arch/powerpc/lib/locks.c
> > +++ b/arch/powerpc/lib/locks.c
> > @@ -72,8 +72,10 @@ void arch_spin_unlock_wait(arch_spinlock_t *lock)
> >  {
> >  	while (lock->slock) {
> >  		HMT_low();
> > +		preempt_disable();
> >  		if (SHARED_PROCESSOR)
> >  			__spin_yield(lock);
> > +		preempt_enable();
> >  	}
> 
> I assume what you are protecting is the PACA access in SHARED_PROCESSOR
> or is there more ?

Yes, only the one in SHARED_PROCESSOR.

> 
> In that case I'd say just make it use local_paca-> directly or something
> like that. It doesn't matter if the access is racy, all processors will
> have the same value for that field as far as I can tell.

It also seemed to me that all processors have the same value :). I'll
send an updated version based on your suggestion soon.

Thanks, Zhong

> 
> Cheers,
> Ben.
> 
>