linux-kernel - Re: [PATCH v4] kvm: make vcpu life cycle separated from kvm instance

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Date:	Thu, 15 Dec 2011 16:25:51 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Liu ping fan <kernelfans@...il.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	aliguori@...ibm.com, gleb@...hat.com, mtosatti@...hat.com,
	jan.kiszka@....de
Subject: Re: [PATCH v4] kvm: make vcpu life cycle separated from kvm instance

On 12/15/2011 02:53 PM, Liu ping fan wrote:


> 
>>> +struct kvm_vcpu *kvm_vcpu_get(struct kvm_vcpu *vcpu)
>>> +{
>>> +     if (vcpu == NULL)
>>> +             return NULL;
>>> +     if (atomic_add_unless(&vcpu->refcount, 1, 0))
>>
>>
>> Why do not use atomic_inc()?
>> Also, i think a memory barrier is needed after increasing refcount.
>>
> Because when refcout==0, we prepare to destroy vcpu, and do not to
> disturb it by increasing the refcount.


Oh, get it.

> And sorry but I can not figure out the scene why memory barrier needed
> here.  Seems no risks on SMP.
> 


If atomic_add_unless is necessary, memory barrier is not needed here.

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