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

[Ingo Molnar]

* Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx> wrote:

> On 16/3/07 08:54, "Ingo Molnar" <mingo@xxxxxxx> 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
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