Messages in this thread | | | Date | Fri, 16 Mar 2007 10:31:49 +0000 | Subject | Re: [Xen-devel] Re: [patch 21/26] Xen-paravirt_ops: Use the hvc console infrastructure for Xen console | From | Keir Fraser <> |
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On 16/3/07 09:58, "Ingo Molnar" <mingo@elte.hu> 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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