Re: [PATCH 2/2] s390/vfio-ap: replace open coded locks for VFIO_GROUP_NOTIFY_SET_KVM notification

[Halil Pasic <pasic@linux.ibm.com>]

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 ..

> +		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.

Can you please explain why can we get away with being more
lax when dealing with matrix_mdev->kvm?

Regards,
Halil

[..]