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

error compiling committee.c: too many arguments to function

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