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Date: Fri, 20 Dec 2013 09:47:10 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: hpa@zytor.com, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        tglx@linutronix.de, hpa@linux.intel.com, len.brown@intel.com,
        linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/idle] x86, idle: Use static_cpu_has() for CLFLUSH
 workaround, add barriers
Message-ID: <20131220084710.GA21133@gmail.com>
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* 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