Re: [PATCH 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks

[Tony Krowiak]

On 2/10/21 10:32 AM, Halil Pasic wrote:
> On Wed, 10 Feb 2021 16:24:29 +0100
> Halil Pasic <pasic@xxxxxxxxxxxxx> wrote:
>
> > > Maybe you could
> > > - grab a reference to kvm while holding the lock
> > > - call the mask handling functions with that kvm reference
> > > - lock again, drop the reference, and do the rest of the processing?
> > I agree, matrix_mdev->kvm can go NULL any time and we are risking
> > a null pointer dereference here.
> >
> > Another idea would be to do
> >
> >
> > static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
> > {
> >          struct kvm *kvm;
> >                                                          
> >          mutex_lock(&matrix_dev->lock);
> >          if (matrix_mdev->kvm) {
> >                  kvm = matrix_mdev->kvm;
> >                  matrix_mdev->kvm = NULL;
> >                  mutex_unlock(&matrix_dev->lock);
> >                  kvm_arch_crypto_clear_masks(kvm);
> >                  mutex_lock(&matrix_dev->lock);
> >                  matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> s/matrix_mdev->kvm/kvm
> >                  vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
> >                  kvm_put_kvm(kvm);
> >          }
> >          mutex_unlock(&matrix_dev->lock);
> > }
> >
> > That way only one unset would actually do the unset and cleanup
> > and every other invocation would bail out with only checking
> > matrix_mdev->kvm.
> But the problem with that is that we enable the the assign/unassign
> prematurely, which could interfere wit reset_queues(). Forget about
> it.

Not sure what you mean by this.