s390/vfio-ap: use proper locking order when setting/clearing KVM pointer

The group notifier that handles the VFIO_GROUP_NOTIFY_SET_KVM event must
use the required locks in proper locking order to dynamically update the
guest's APCB. The proper locking order is:

       1. matrix_dev->guests_lock: required to use the KVM pointer to
          update a KVM guest's APCB.

       2. matrix_mdev->kvm->lock: required to update a KVM guest's APCB.

       3. matrix_dev->mdevs_lock: required to store or access the data
          stored in a struct ap_matrix_mdev instance.

Two macros are introduced to acquire and release the locks in the proper
order. These macros are now used by the group notifier functions.
Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

drivers/s390/crypto/vfio_ap_ops.c

@@ -34,6 +34,49 @@ static int vfio_ap_mdev_reset_queues(struct ap_matrix_mdev *matrix_mdev);
 static struct vfio_ap_queue *vfio_ap_find_queue(int apqn);
 static const struct vfio_device_ops vfio_ap_matrix_dev_ops;
 
+/**
+ * get_update_locks_for_kvm: Acquire the locks required to dynamically update a
+ *			     KVM guest's APCB in the proper order.
+ *
+ * @kvm: a pointer to a struct kvm object containing the KVM guest's APCB.
+ *
+ * The proper locking order is:
+ * 1. matrix_dev->guests_lock: required to use the KVM pointer to update a KVM
+ *			       guest's APCB.
+ * 2. kvm->lock:	       required to update a guest's APCB
+ * 3. matrix_dev->mdevs_lock:  required to access data stored in a matrix_mdev
+ *
+ * Note: If @kvm is NULL, the KVM lock will not be taken.
+ */
+static inline void get_update_locks_for_kvm(struct kvm *kvm)
+{
+	mutex_lock(&matrix_dev->guests_lock);
+	if (kvm)
+		mutex_lock(&kvm->lock);
+	mutex_lock(&matrix_dev->mdevs_lock);
+}
+
+/**
+ * release_update_locks_for_kvm: Release the locks used to dynamically update a
+ *				 KVM guest's APCB in the proper order.
+ *
+ * @kvm: a pointer to a struct kvm object containing the KVM guest's APCB.
+ *
+ * The proper unlocking order is:
+ * 1. matrix_dev->mdevs_lock
+ * 2. kvm->lock
+ * 3. matrix_dev->guests_lock
+ *
+ * Note: If @kvm is NULL, the KVM lock will not be released.
+ */
+static inline void release_update_locks_for_kvm(struct kvm *kvm)
+{
+	mutex_unlock(&matrix_dev->mdevs_lock);
+	if (kvm)
+		mutex_unlock(&kvm->lock);
+	mutex_unlock(&matrix_dev->guests_lock);
+}
+
 /**
  * vfio_ap_mdev_get_queue - retrieve a queue with a specific APQN from a
  *			    hash table of queues assigned to a matrix mdev
@@ -1266,13 +1309,11 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
 		kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
 		up_write(&kvm->arch.crypto.pqap_hook_rwsem);
 
-		mutex_lock(&kvm->lock);
-		mutex_lock(&matrix_dev->mdevs_lock);
+		get_update_locks_for_kvm(kvm);
 
 		list_for_each_entry(m, &matrix_dev->mdev_list, node) {
 			if (m != matrix_mdev && m->kvm == kvm) {
-				mutex_unlock(&kvm->lock);
-				mutex_unlock(&matrix_dev->mdevs_lock);
+				release_update_locks_for_kvm(kvm);
 				return -EPERM;
 			}
 		}
@@ -1283,8 +1324,7 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
 					  matrix_mdev->shadow_apcb.aqm,
 					  matrix_mdev->shadow_apcb.adm);
 
-		mutex_unlock(&kvm->lock);
-		mutex_unlock(&matrix_dev->mdevs_lock);
+		release_update_locks_for_kvm(kvm);
 	}
 
 	return 0;
@@ -1335,16 +1375,14 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
 		kvm->arch.crypto.pqap_hook = NULL;
 		up_write(&kvm->arch.crypto.pqap_hook_rwsem);
 
-		mutex_lock(&kvm->lock);
-		mutex_lock(&matrix_dev->mdevs_lock);
+		get_update_locks_for_kvm(kvm);
 
 		kvm_arch_crypto_clear_masks(kvm);
 		vfio_ap_mdev_reset_queues(matrix_mdev);
 		kvm_put_kvm(kvm);
 		matrix_mdev->kvm = NULL;
 
-		mutex_unlock(&kvm->lock);
-		mutex_unlock(&matrix_dev->mdevs_lock);
+		release_update_locks_for_kvm(kvm);
 	}
 }