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

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [patch 21/26] Xen-paravirt_ops: Use the hvc console infrastructure for Xen console
From: Ingo Molnar <mingo@xxxxxxx>
Date: Fri, 16 Mar 2007 10:58:51 +0100
Cc: Zachary Amsden <zach@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Andi Kleen <ak@xxxxxx>, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
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* Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx> wrote:

> On 16/3/07 08:54, "Ingo Molnar" <mingo@xxxxxxx> 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().


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