Re: [PATCH v4 1/6] kvm: add device control API

Gleb Natapov
2013-04-26T09:53:18Z

On Thu, Apr 25, 2013 at 01:59:20PM -0500, Scott Wood wrote:
> On 04/25/2013 01:22:04 PM, Gleb Natapov wrote:
> >On Thu, Apr 25, 2013 at 11:51:08AM -0500, Scott Wood wrote:
> >> On 04/25/2013 05:47:39 AM, Alexander Graf wrote:
> >> >
> >> >On 25.04.2013, at 11:43, Gleb Natapov wrote:
> >> >
> >> >>> +void kvm_device_put(struct kvm_device *dev)
> >> >>> +{
> >> >>> + if (atomic_dec_and_test(&dev->users))
> >> >>> + dev->ops->destroy(dev);
> >> >>> +}
> >> >>> +
> >> >>> +static int kvm_device_release(struct inode *inode, struct file
> >> >*filp)
> >> >>> +{
> >> >>> + struct kvm_device *dev = filp->private_data;
> >> >>> + struct kvm *kvm = dev->kvm;
> >> >>> +
> >> >>> + kvm_device_put(dev);
> >> >>> + kvm_put_kvm(kvm);
> >> >> We may put kvm only if users goes to zero, otherwise kvm can be
> >> >> freed while something holds a reference to a device. Why not make
> >> >> kvm_device_put() do it?
> >> >
> >> >Nice catch. I'll change the patch so it does the kvm_put_kvm
> >> >inside kvm_device_put's destroy branch.
> >>
> >> No, please don't. The KVM reference being "put" here is associated
> >> with the file descriptor, not with the MPIC object.
> >Is it so? Device holds a pointer to kvm, so it increments kvm
> >reference
> >to make sure the pointer is valid. What prevents kvm from been
> >destroyed
> >while device is still in use in current code?
>
> Where will that kvm pointer be used, after all the file descriptors
> go away and the vcpus stop running? mmio_mapped guards against
> unmapping the MMIO if it's already been unmapped due to KVM
> destruction. We don't have any timers or other delayed work.
>
MPIC does not, but timer device will have one.

> Well, I do see one place, that Alex added -- the NULLing out of
> dev->kvm->arch.mpic, which didn't exist in my patchset.
>
> >> that change I think you'll have circular references and thus a
> >> memory leak, because the vcpus can hold a reference to the MPIC
> >> object.
> >>
> >How circular reference can be created?
>
> MPIC holds reference on KVM, vcpu holds reference on MPIC, and vcpu
> is not destroyed until KVM is destroyed.
>
Yes, you are right. So we need to think about how to fix it in a
different way. What about holding all devices in kvm->devices[] array
and destroy them during kvm destruction, like we do for vcpus?

--
Gleb.