Re: [PATCH v13 11/15] s390/vfio-ap: implement in-use callback for vfio_ap driver

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

On Tue, 22 Dec 2020 20:16:02 -0500
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> Let's implement the callback to indicate when an APQN
> is in use by the vfio_ap device driver. The callback is
> invoked whenever a change to the apmask or aqmask would
> result in one or more queue devices being removed from the driver. The
> vfio_ap device driver will indicate a resource is in use
> if the APQN of any of the queue devices to be removed are assigned to
> any of the matrix mdevs under the driver's control.
> 
> There is potential for a deadlock condition between the matrix_dev->lock
> used to lock the matrix device during assignment of adapters and domains
> and the ap_perms_mutex locked by the AP bus when changes are made to the
> sysfs apmask/aqmask attributes.
> 
> Consider following scenario (courtesy of Halil Pasic):
> 1) apmask_store() takes ap_perms_mutex
> 2) assign_adapter_store() takes matrix_dev->lock
> 3) apmask_store() calls vfio_ap_mdev_resource_in_use() which tries
>    to take matrix_dev->lock
> 4) assign_adapter_store() calls ap_apqn_in_matrix_owned_by_def_drv
>    which tries to take ap_perms_mutex
> 
> BANG!
> 
> To resolve this issue, instead of using the mutex_lock(&matrix_dev->lock)
> function to lock the matrix device during assignment of an adapter or
> domain to a matrix_mdev as well as during the in_use callback, the
> mutex_trylock(&matrix_dev->lock) function will be used. If the lock is not
> obtained, then the assignment and in_use functions will terminate with
> -EBUSY.
> 
> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> ---
>  drivers/s390/crypto/vfio_ap_drv.c     |  1 +
>  drivers/s390/crypto/vfio_ap_ops.c     | 21 ++++++++++++++++++---
>  drivers/s390/crypto/vfio_ap_private.h |  2 ++
>  3 files changed, 21 insertions(+), 3 deletions(-)
> 
[..]
>  }
> +
> +int vfio_ap_mdev_resource_in_use(unsigned long *apm, unsigned long *aqm)
> +{
> +	int ret;
> +
> +	if (!mutex_trylock(&matrix_dev->lock))
> +		return -EBUSY;
> +	ret = vfio_ap_mdev_verify_no_sharing(NULL, apm, aqm);

If we detect that resources are in use, then we spit warnings to the
message log, right?

@Matt: Is your userspace tooling going to guarantee that this will never
happen?

> +	mutex_unlock(&matrix_dev->lock);
> +
> +	return ret;
> +}
> diff --git a/drivers/s390/crypto/vfio_ap_private.h b/drivers/s390/crypto/vfio_ap_private.h
> index d2d26ba18602..15b7cd74843b 100644
> --- a/drivers/s390/crypto/vfio_ap_private.h
> +++ b/drivers/s390/crypto/vfio_ap_private.h
> @@ -107,4 +107,6 @@ struct vfio_ap_queue {
>  int vfio_ap_mdev_probe_queue(struct ap_device *queue);
>  void vfio_ap_mdev_remove_queue(struct ap_device *queue);
>  
> +int vfio_ap_mdev_resource_in_use(unsigned long *apm, unsigned long *aqm);
> +
>  #endif /* _VFIO_AP_PRIVATE_H_ */