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

[Keir Fraser <keir@xxxxxxxxxxxxx>]

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.

All our ring protocols (and we have a few) require memory barriers. Those
that use our standard set of ring macros have the memory barriers hidden
from view, but they're still there.

 -- Keir


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