Re: [tip:x86/idle] x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Dec 19, 2013 at 09:40:51PM +0100, Ingo Molnar wrote:
> > > +		if (static_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
> > > +			mb();
> > >  			clflush((void *)&current_thread_info()->flags);
> > > +			mb();
> > > +		}
> 
> >  	if (!current_set_polling_and_test()) {
> >  		if (static_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
> > -			mb();
> > +			/*
> > +			 * There's no need for an extra barrier here: current_set_polling_and_test()
> > +			 * already has an smp_mb() after ->flags gets modified, see sched.h.
> > +			 */
> >  			clflush((void *)&current_thread_info()->flags);
> >  			mb();
> >  		}
> 
> So almost, except that smp_mb() will disappear on an UP kernel whereas
> this errata might just always need the MFENCE.

Yeah. Sigh, so I noticed that and even thought through the UP case and 
concluded: on UP the race cannot happen - which is bogus. Yesterday 
was so much fail, maybe today's a better day ;-)

Thanks,

	ngo