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[Xen-devel] Re: Linux questions

To: Andi Kleen <andi@xxxxxxxxxxxxxx>
Subject: [Xen-devel] Re: Linux questions
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Tue, 04 Dec 2007 11:27:02 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Jan Beulich <jbeulich@xxxxxxxxxx>, Samuel Thibault <samuel.thibault@xxxxxxxxxxxxx>
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On 4/12/07 11:13, "Andi Kleen" <andi@xxxxxxxxxxxxxx> wrote:

>>> I have to disagree: At least the uses of barrier() in monotonic_clock()
>>> appear
>>> to be in places where in reality (and from a theoretical standpoint) rmb()
>>> ought to be used.
>> 
>> We're sync'ing against concurrent updates of a this_cpu variable. We can
>> only race updates in a local ISR, and hence barrier() suffices.
> 
> Not if you use RDTSC inside the loop.

I must disagree! And I *know* that RDTSC is not a serialising instruction...

If we race, then there was an interrupt. Interrupt delivery is a
serialisation point for the interrupted instruction stream.

 -- Keir



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