From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:28512 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727919AbhALBVD (ORCPT ); Mon, 11 Jan 2021 20:21:03 -0500 Date: Tue, 12 Jan 2021 02:20:12 +0100 From: Halil Pasic Subject: Re: [PATCH v13 11/15] s390/vfio-ap: implement in-use callback for vfio_ap driver Message-ID: <20210112022012.4bad464f.pasic@linux.ibm.com> In-Reply-To: <20201223011606.5265-12-akrowiak@linux.ibm.com> References: <20201223011606.5265-1-akrowiak@linux.ibm.com> <20201223011606.5265-12-akrowiak@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit List-ID: To: Tony Krowiak , mjrosato@linux.ibm.com Cc: linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, freude@linux.ibm.com, borntraeger@de.ibm.com, cohuck@redhat.com, alex.williamson@redhat.com, kwankhede@nvidia.com, fiuczy@linux.ibm.com, frankja@linux.ibm.com, david@redhat.com, hca@linux.ibm.com, gor@linux.ibm.com On Tue, 22 Dec 2020 20:16:02 -0500 Tony Krowiak 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 > --- > 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_ */