From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:63454 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2391421AbgKZPyo (ORCPT ); Thu, 26 Nov 2020 10:54:44 -0500 Date: Thu, 26 Nov 2020 16:54:31 +0100 From: Halil Pasic Subject: Re: [PATCH v12 07/17] s390/vfio-ap: implement in-use callback for vfio_ap driver Message-ID: <20201126165431.6ef1457a.pasic@linux.ibm.com> In-Reply-To: <20201124214016.3013-8-akrowiak@linux.ibm.com> References: <20201124214016.3013-1-akrowiak@linux.ibm.com> <20201124214016.3013-8-akrowiak@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit List-ID: To: Tony Krowiak 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, mjrosato@linux.ibm.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, 24 Nov 2020 16:40:06 -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. Good news is: the final product is OK with regards to in_use(). Bad news is: this patch does not do enough. At this stage we are still racy. The problem is that the assign operations don't bother to take the ap_perms_mutex lock under the matrix_dev->lock. The scenario is the following: 1) apmask_store() takes ap_perms_mutex 2) apmask_store() calls vfio_ap_mdev_resource_in_use() which takes matrix_dev->lock 3) vfio_ap_mdev_resource_in_use() releases matrix_dev->lock and returns 0 4) assign_adapter_store() takes matrix_dev->lock does the assign (the queues are still bound to vfio_ap) and releases matrix_dev->lock 5) apmask_store() carries on, does the update to apask and releases ap_perms_mutex 6) The queues get 'stolen' from vfio ap while used. This gets fixed with "s390/vfio-ap: allow assignment of unavailable AP queues to mdev device". Maybe we can reorder these patches. I didn't look into that. We could also just ignore the problem, because it is just for a couple of commits, but I would prefer it gone. Regards, Halil