Re: [PATCH 1/5] Add a global synchronization point for pvclock

Peter Zijlstra
2010-04-19T11:05:10Z

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.