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SubjectRe: [PATCH v2 2/4] KVM: introduce "xinterface" API for external interaction with guests
Gregory Haskins wrote:
> Gregory Haskins wrote:
>> 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
>
> gak. or "your logic" even.
>
>> seems reasonable to me. I will defer to David, since he
>> brought up the issue with the similar logic originally.
>>
>> Kind Regards,
>> -Greg
>>
>
>

Thinking about this some more over lunch, I think we (Avi and I) might
both be wrong (and David is right). Avi is right that we don't need
rmb() or barrier() for the reasons already stated, but I think David is
right that we need an smp_mb() to ensure the cpu doesn't do the
reordering. Otherwise a different cpu could invalidate the memory if it
reuses the freed memory in the meantime, iiuc. IOW: its not a compiler
issue but a cpu issue.

Or am I still confused?

-Greg

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