Re: [PATCH v2 2/4] KVM: introduce "xinterface" API for external interaction with guests

[Gregory Haskins <gregory.haskins@gmail.com>]

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Avi Kivity wrote:
> On 10/06/2009 04:22 PM, Gregory Haskins wrote:
>>>>>>> +
>>>>>>> +static inline void
>>>>>>> +_kvm_xinterface_release(struct kref *kref)
>>>>>>> +{
>>>>>>> +    struct kvm_xinterface *intf;
>>>>>>> +    struct module *owner;
>>>>>>> +
>>>>>>> +    intf = container_of(kref, struct kvm_xinterface, kref);
>>>>>>> +
>>>>>>> +    owner = intf->owner;
>>>>>>> +    rmb();
>>>>>>>
>>>>>>>
>>>>>>>              
>>>>>> Why rmb?
>>>>>>
>>>>>>            
>>>>> the intf->ops->release() line may invalidate the intf pointer, so we
>>>>> want to ensure that the read completes before the release() is called.
>>>>>
>>>>> TBH: I'm not 100% its needed, but I was being conservative.
>>>>>
>>>>>          
>>>> rmb()s are only needed if an external agent can issue writes, otherwise
>>>> you'd need one after every statement.
>>>>        
>>> I was following lessons learned here:
>>>
>>> http://lkml.org/lkml/2009/7/7/175
>>>
>>> Perhaps mb() or barrier() are more appropriate than rmb()?  I'm CC'ing
>>> David Howells in case he has more insight.
>>>      
>> BTW: In case it is not clear, the rationale as I understand it is we
>> worry about the case where one cpu reorders the read to be after the
>> ->release(), and another cpu might grab the memory that was kfree()'d
>> within the ->release() and scribble something else on it before the read
>> completes.
>>
>> I know rmb() typically needs to be paired with wmb() to be correct, so
>> you are probably right to say that the rmb() itself is not appropriate.
>>   This problem in general makes my head hurt, which is why I said I am
>> not 100% sure of what is required.  As David mentions, perhaps
>> "smp_mb()" is more appropriate for this application.  I also speculate
>> barrier() may be all that we need.
>>    
> 
> barrier() is the operation for a compiler barrier.  But it's unneeded
> here - unless the compiler can prove that ->release(intf) will not
> modify intf->owner it is not allowed to move the access afterwards.  An
> indirect function call is generally a barrier() since the compiler can't
> assume memory has not been modified.
> 

You're logic seems reasonable to me.  I will defer to David, since he
brought up the issue with the similar logic originally.

Kind Regards,
-Greg


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