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Message-ID: <C2202295.BA49%keir@xensource.com> Date: Fri, 16 Mar 2007 10:31:49 +0000 From: Keir Fraser <keir@...source.com> To: Ingo Molnar <mingo@...e.hu>, Keir Fraser <Keir.Fraser@...source.com> CC: <virtualization@...ts.osdl.org>, <xen-devel@...ts.xensource.com>, Chris Wright <chrisw@...s-sol.org>, Andi Kleen <ak@....de>, Andrew Morton <akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org> Subject: Re: [Xen-devel] Re: [patch 21/26] Xen-paravirt_ops: Use the hvc console infrastructure for Xen console On 16/3/07 09:58, "Ingo Molnar" <mingo@...e.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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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