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 10:58:51 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Keir Fraser <Keir.Fraser@...source.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Zachary Amsden <zach@...are.com>,
	xen-devel@...ts.xensource.com, virtualization@...ts.osdl.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, Chris Wright <chrisw@...s-sol.org>,
	Andi Kleen <ak@....de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Xen-devel] Re: [patch 21/26] Xen-paravirt_ops: Use the hvc console infrastructure for Xen console


* Keir Fraser <Keir.Fraser@...source.com> wrote:

> On 16/3/07 08:54, "Ingo Molnar" <mingo@...e.hu> wrote:
> 
> >> + prod = intf->in_prod;
> >> + mb();
> >> + BUG_ON((prod - cons) > sizeof(intf->in));
> > 
> > such mb()'s are typically a sign of "i have no clear idea what SMP
> > serialization rules apply here, but something is needed because
> > otherwise it breaks" ?
> 
> 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().

	Ingo
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