linux-kernel - Re: [PATCH 1/5] Add a global synchronization point for pvclock

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Message-ID: <4BCA02D1.2020608@redhat.com>
Date:	Sat, 17 Apr 2010 21:49:53 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

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.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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