Subject: Re: [RFC] [patch 4/6 -rt] powerpc 2.6.20-rt8: fix a runtime warnings for xmon

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 07 Mar 2007 11:10:59 +0100

On Wed, 2007-03-07 at 10:16 +0100, Ingo Molnar wrote:
> * Tsutomu OWA <[email-masked]> wrote:
> 
> > @@ -342,6 +342,7 @@ static int xmon_core(struct pt_regs *reg
> >  
> >  	msr = mfmsr();
> >  	mtmsr(msr & ~MSR_EE);	/* disable interrupts */
> > +	preempt_disable();
> 
> i'm not an xmon expert, but maybe it might make more sense to first 
> disable preemption, then interrupts - otherwise you could be preempted 
> right after having disabled these interrupts (and be scheduled to 
> another CPU, etc.). What is the difference between local_irq_save() and 
> the above 'disable interrupts' sequence? If it's not the same and 
> xmon_core() relied on having hardirqs disabled then it might make sense 
> to do a local_irq_save() there, instead of a preempt_disable().

powerpc 64 bits nowadays does lazy HW masking, so local_irq_disable()
will not actually switch MSR_EE off. However, xmon needs that to happen
(though we have a nicer accessor to do it, I suspect some bitrot need
fixing in there, possibly already fixed in .21)

I agree that preempt_disable() should be put before the MSR tweaking
though.

Ben.