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SubjectRe: [PATCH 2/2] s390/vfio-ap: replace open coded locks for VFIO_GROUP_NOTIFY_SET_KVM notification
On Thu, 22 Jul 2021 09:09:26 -0400
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> On 7/21/21 10:45 AM, Halil Pasic wrote:
> > On Mon, 19 Jul 2021 15:35:03 -0400
> > Tony Krowiak <akrowiak@linux.ibm.com> wrote:
> >
> >> It was pointed out during an unrelated patch review that locks should not
> > [..]
> >
> >> -static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
> >> +static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev,
> >> + struct kvm *kvm)
> >> {
> >> - /*
> >> - * If the KVM pointer is in the process of being set, wait until the
> >> - * process has completed.
> >> - */
> >> - wait_event_cmd(matrix_mdev->wait_for_kvm,
> >> - !matrix_mdev->kvm_busy,
> >> - mutex_unlock(&matrix_dev->lock),
> >> - mutex_lock(&matrix_dev->lock));
> >> -
> >> - if (matrix_mdev->kvm) {
> > We used to check if matrix_mdev->kvm is null, but ...
> >
> >> - matrix_mdev->kvm_busy = true;
> >> - mutex_unlock(&matrix_dev->lock);
> >> -
> >> - if (matrix_mdev->kvm->arch.crypto.crycbd) {
> >> - down_write(&matrix_mdev->kvm->arch.crypto.pqap_hook_rwsem);
> >> - matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> >> - up_write(&matrix_mdev->kvm->arch.crypto.pqap_hook_rwsem);
> >> -
> >> - kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> >> - }
> >> + if (kvm->arch.crypto.crycbd) {
> > ... now we just try to dereference it. And ..
>
> We used to check matrix_mdev->kvm, now the kvm pointer is passed into
> the function; however, having said that, the pointer passed in should be
> checked before de-referencing it.
>
> >
> >> + down_write(&kvm->arch.crypto.pqap_hook_rwsem);
> >> + kvm->arch.crypto.pqap_hook = NULL;
> >> + up_write(&kvm->arch.crypto.pqap_hook_rwsem);
> >>
> >> + mutex_lock(&kvm->lock);
> >> mutex_lock(&matrix_dev->lock);
> >> +
> >> + kvm_arch_crypto_clear_masks(kvm);
> >> vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
> >> - kvm_put_kvm(matrix_mdev->kvm);
> >> + kvm_put_kvm(kvm);
> >> matrix_mdev->kvm = NULL;
> >> - matrix_mdev->kvm_busy = false;
> >> - wake_up_all(&matrix_mdev->wait_for_kvm);
> >> +
> >> + mutex_unlock(&kvm->lock);
> >> + mutex_unlock(&matrix_dev->lock);
> >> }
> >> }
> >>
> > [..]
> >
> >> @@ -1363,14 +1323,11 @@ static void vfio_ap_mdev_release(struct mdev_device *mdev)
> >> {
> >> struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
> >>
> >> - mutex_lock(&matrix_dev->lock);
> >> - vfio_ap_mdev_unset_kvm(matrix_mdev);
> >> - mutex_unlock(&matrix_dev->lock);
> >> -
> > .. before access to the matrix_mdev->kvm used to be protected by
> > the matrix_dev->lock ...
> >
> >> vfio_unregister_notifier(mdev_dev(mdev), VFIO_IOMMU_NOTIFY,
> >> &matrix_mdev->iommu_notifier);
> >> vfio_unregister_notifier(mdev_dev(mdev), VFIO_GROUP_NOTIFY,
> >> &matrix_mdev->group_notifier);
> >> + vfio_ap_mdev_unset_kvm(matrix_mdev, matrix_mdev->kvm);
> > ... but it is not any more. BTW I don't think the code is guaranteed
> > to fetch ->kvm just once.
>
> There are a couple of things to point out here:
> 1. The vfio_ap_mdev_unset_kvm function() is the only place where the
>     matrix_mdev->kvm pointer is cleared. That function is called here
>     as well as by the group notifier callback for VFIO_GROUP_NOTIFY_SET_KVM
>     events. If you notice in the code above, the group notifier is
> unregistered
>     before calling the unset function, so either the notifier will have
> already
>     been invoked and the pointer cleared (which is why you are correct
>     that the KVM pointer passed in needs to get checked in the unset
> function),
>     or will get cleared here.

Hm, vfio_unregister_notifier() indeed seems to guarantee, that by the
time it returns no notifer is running. I didn't know that. But this
blocking notifier chain uses an rwsem. So mutual exclusion with
vfio_ap_mdev_open() is guaranteed, than it is indeed guaranteed. A quick
glance at the code didn't tell me if vfio/mdev guarantees that.

I mean it would make sense to me to make the init and the cleanup
mutually exclusive, but I'm reluctant to just assume it is like that.
Can you please point me into the right direction?


> 2. The release callback is invoked when the mdev fd is closed by userspace.
>     The remove callback is the only place where the matrix_mdev is
> freed. The
>     remove callback is not called until the mdev fd is released, so it
> is guaranteed
>     the matrix_mdev will exist when the release callback is invoked.
> 3. The matrix_dev->lock is then taken in the vfio_ap_mdev_unset_kvm function
>     before doing any operations that modify the matrix_mdev.

Yeah but both the reader, and the writer needs to use the same lock to
have the protected by the lock type of situation. That is why I asked
about the place where you read matrix_mdev members outside the
matrix_dev->lock.

Regards,
Halil

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