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Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock
From: Peter Zijlstra <peterz@infradead.org>
To: Avi Kivity <avi@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Glauber Costa <glommer@redhat.com>,
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Date: Mon, 19 Apr 2010 13:05:00 +0200
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On Mon, 2010-04-19 at 13:50 +0300, Avi Kivity wrote:
> On 04/19/2010 01:39 PM, Peter Zijlstra wrote:
> > On Fri, 2010-04-16 at 13:36 -0700, Jeremy Fitzhardinge wrote:
> >    
> >>> +     do {
> >>> +             last = last_value;
> >>>
> >>>        
> >> Does this need a barrier() to prevent the compiler from re-reading
> >> last_value for the subsequent lines?  Otherwise "(ret<  last)" and
> >> "return last" could execute with different values for "last".
>     
> > ACCESS_ONCE() is your friend.
> >    
> 
> I think it's implied with atomic64_read().

Yes it would be. I was merely trying to point out that

  last = ACCESS_ONCE(last_value);

Is a narrower way of writing:

  last = last_value;
  barrier();

In that it need not clobber all memory locations and makes it instantly
clear what we want the barrier for.