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


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