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

[Ingo Molnar]

* Peter Zijlstra <pet...@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
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