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

Peter Zijlstra
2010-04-19T14:46:57Z

On Mon, 2010-04-19 at 17:33 +0300, Avi Kivity wrote:
> On 04/19/2010 05:21 PM, Glauber Costa wrote:
> >
> >> Oh yes, just trying to avoid a patch with both atomic64_read() and
> >> ACCESS_ONCE().
> >>
> > you're mixing the private version of the patch you saw with this one.
> > there isn't any atomic reads in here. I'll use a barrier then
> >
>
> This patch writes last_value atomically, but reads it non-atomically. A
> barrier is insufficient.

What avi says! :-)

On a 32bit machine a 64bit read are two 32bit reads, so

last = last_value;

becomes:

last.high = last_value.high;
last.low = last_vlue.low;

(or the reverse of course)

Now imagine a write getting interleaved with that ;-)