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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:
    >>
    >> 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.

    --
    error compiling committee.c: too many arguments to function



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