linux-kernel - Re: [PATCH v19 09/20] s390/vfio-ap: use proper locking order when setting/clearing KVM pointer

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Date:   Fri, 27 May 2022 08:41:46 -0400
From:   "Jason J. Herne" <jjherne@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     freude@...ux.ibm.com, borntraeger@...ibm.com, cohuck@...hat.com,
        mjrosato@...ux.ibm.com, pasic@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        fiuczy@...ux.ibm.com
Subject: Re: [PATCH v19 09/20] s390/vfio-ap: use proper locking order when
 setting/clearing KVM pointer

On 4/4/22 18:10, Tony Krowiak wrote:
> 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@...ux.ibm.com>
> ---
>   drivers/s390/crypto/vfio_ap_ops.c | 56 +++++++++++++++++++++++++------
>   1 file changed, 46 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index 077b8c9c831b..757bbf449b04 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -30,6 +30,47 @@ 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.
> + */
> +#define get_update_locks_for_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.
> + */
> +#define release_update_locks_for_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
> @@ -1263,13 +1304,11 @@ 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;
>   			}
>   		}
> @@ -1280,8 +1319,7 @@ 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;
> @@ -1332,16 +1370,14 @@ 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);
>   	}
>   }
>   

In isolation... Reviewed-by: Jason J. Herne <jjherne@...ux.ibm.com>

-- 
-- Jason J. Herne (jjherne@...ux.ibm.com)