From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:37568 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725850AbgLODxE (ORCPT ); Mon, 14 Dec 2020 22:53:04 -0500 Subject: Re: [PATCH v12 07/17] s390/vfio-ap: implement in-use callback for vfio_ap driver References: <20201124214016.3013-1-akrowiak@linux.ibm.com> <20201124214016.3013-8-akrowiak@linux.ibm.com> <20201126165431.6ef1457a.pasic@linux.ibm.com> From: Tony Krowiak Message-ID: <56ffa0c6-0518-907e-2635-ff3d7cf1f395@linux.ibm.com> Date: Mon, 14 Dec 2020 22:52:17 -0500 MIME-Version: 1.0 In-Reply-To: <20201126165431.6ef1457a.pasic@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US List-ID: To: Halil Pasic 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 11/26/20 10:54 AM, Halil Pasic wrote: > 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. You're missing an interim step between 5 and 6 where the apmask_store() function executes the device_reprobe() function which results in queues to be taken from vfio_ap getting unbound. In this case, the vfio_ap_mdev_remove_queue() function gets called to remove the queues resulting in unplugging > > 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. Reordering the patches is not a trivial task, I perfer not to do it. > > Regards, > Halil > > >