On 11/26/20 10:54 AM, Halil Pasic wrote:
On Tue, 24 Nov 2020 16:40:06 -0500 Tony Krowiak <[email protected]> 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
