On Fri, 16 Mar 2007 10:31:49 +0000 Keir Fraser <keir@xxxxxxxxxxxxx> wrote: > 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. > If Ingo couldn't work this out from reading the code then nobody else can, and we have a maintainability problem which can only be solved with adequate commenting. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxx https://lists.osdl.org/mailman/listinfo/virtualization
