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

Peter Zijlstra
2010-04-19T10:43:59Z

On Sat, 2010-04-17 at 21:49 +0300, Avi Kivity wrote:
> On 04/17/2010 09:48 PM, Avi Kivity wrote:
> >
> >>
> >> +static u64 last_value = 0;
> >
> > Needs to be atomic64_t.
> >
> >> +
> >> cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
> >> {
> >> struct pvclock_shadow_time shadow;
> >> unsigned version;
> >> cycle_t ret, offset;
> >> + u64 last;
> >>
> >>
> >> + do {
> >> + last = last_value;
> >
> > Otherwise, this assignment can see a partial update.
>
> On a 32-bit guest, that is.

Right, do bear in mind that the x86 implementation of atomic64_read() is
terrifyingly expensive, it is better to not do that read and simply use
the result of the cmpxchg.