Re: [PATCH 04/13] powerpc: Use soft_enabled_set api to update paca->soft_enabled

[Nicholas Piggin <npiggin@gmail.com>]

On Fri, 16 Sep 2016 11:43:13 +0000
David Laight <David.Laight@ACULAB.COM> wrote:

> From: Nicholas Piggin
> > Sent: 16 September 2016 10:53
> > On Thu, 15 Sep 2016 18:31:54 +0530
> > Madhavan Srinivasan <maddy@linux.vnet.ibm.com> wrote:
> >   
> > > Force use of soft_enabled_set() wrapper to update paca-soft_enabled
> > > wherever possisble. Also add a new wrapper function, soft_enabled_set_return(),
> > > added to force the paca->soft_enabled updates.  
> ...
> > > diff --git a/arch/powerpc/include/asm/hw_irq.h b/arch/powerpc/include/asm/hw_irq.h
> > > index 8fad8c24760b..f828b8f8df02 100644
> > > --- a/arch/powerpc/include/asm/hw_irq.h
> > > +++ b/arch/powerpc/include/asm/hw_irq.h
> > > @@ -53,6 +53,20 @@ static inline notrace void soft_enabled_set(unsigned long enable)
> > >  	: : "r" (enable), "i" (offsetof(struct paca_struct, soft_enabled)));
> > >  }
> > >
> > > +static inline notrace unsigned long soft_enabled_set_return(unsigned long enable)
> > > +{
> > > +	unsigned long flags;
> > > +
> > > +	asm volatile(
> > > +		"lbz %0,%1(13); stb %2,%1(13)"
> > > +		: "=r" (flags)
> > > +		: "i" (offsetof(struct paca_struct, soft_enabled)),\
> > > +		  "r" (enable)
> > > +		: "memory");
> > > +
> > > +	return flags;
> > > +}  
> > 
> > Why do you have the "memory" clobber here while soft_enabled_set() does not?  
> 
> I wondered about the missing memory clobber earlier.
> 
> Any 'clobber' ought to be restricted to the referenced memory area.
> If the structure is only referenced by r13 through 'asm volatile' it isn't needed.

Well a clobber (compiler barrier) at some point is needed in irq_disable and
irq_enable paths, so we correctly open and close the critical section vs interrupts.
I just wonder about these helpers. It might be better to take the clobbers out of
there and add barrier(); in callers, which would make it more obvious.