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

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Date:	Fri, 16 Mar 2007 03:41:23 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Keir Fraser <keir@...source.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Keir Fraser <Keir.Fraser@...source.com>,
	<virtualization@...ts.osdl.org>, <xen-devel@...ts.xensource.com>,
	Chris Wright <chrisw@...s-sol.org>, Andi Kleen <ak@....de>,
	<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 Fri, 16 Mar 2007 10:31:49 +0000 Keir Fraser <keir@...source.com> wrote:

> 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.
> 

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.
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