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 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

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