From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock
Date: Mon, 19 Apr 2010 12:43:56 +0200
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Cc: Glauber Costa <glommer@redhat.com>, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Jeremy Fitzhardinge <jeremy@goop.org>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Zachary Amsden <zamsden@redhat.com>
To: Avi Kivity <avi@redhat.com>
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On Sat, 2010-04-17 at 21:49 +0300, Avi Kivity wrote:
> On 04/17/2010 09:48 PM, Avi Kivity wrote:
> >
> >>
> >> +static u64 last_value = 0;
> >
> > Needs to be atomic64_t.
> >
> >> +
> >>   cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
> >>   {
> >>       struct pvclock_shadow_time shadow;
> >>       unsigned version;
> >>       cycle_t ret, offset;
> >> +    u64 last;
> >>
> >>
> >> +    do {
> >> +        last = last_value;
> >
> > Otherwise, this assignment can see a partial update.
> 
> On a 32-bit guest, that is.

Right, do bear in mind that the x86 implementation of atomic64_read() is
terrifyingly expensive, it is better to not do that read and simply use
the result of the cmpxchg.