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

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      Last update: 2009-10-06 19:07    [W:0.047 / U:80.208 seconds]
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