Re: [Xen-devel] Re: [patch 21/26] Xen-paravirt_ops: Use the hvc console infrastructure for Xen console

Ingo Molnar
2007-03-16T09:59:49Z

* Keir Fraser <Keir.Fraser@xensource.com> wrote:

> On 16/3/07 08:54, "Ingo Molnar" <mingo@elte.hu> wrote:
>
> >> + prod = intf->in_prod;
> >> + mb();
> >> + BUG_ON((prod - cons) > sizeof(intf->in));
> >
> > such mb()'s are typically a sign of "i have no clear idea what SMP
> > serialization rules apply here, but something is needed because
> > otherwise it breaks" ?
>
> These mb()'s are pretty standard for lock-free producer/consumer
> rings. Write descriptor /then/ write the updated producer. Read the
> producer /then/ read any descriptors revealed by this new producer
> value.

then use rmb()/wmb(). Rarely does a ring protocol truly need mb().

Ingo