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 09:58, "Ingo Molnar" <mingo@xxxxxxx> wrote:

>> 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().

It's needed for writing data /after/ reading the consumer index that shows
you have space to write. Looking through xenbus_comms.c I think all the
barriers are correct except there is a spurious extra mb() in xb_read(),
where there is a later rmb() which is sufficient by itself. All the others
have a purpose.

 -- Keir


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