Messages in this thread | | | Subject | Re: [PATCH v12 07/17] s390/vfio-ap: implement in-use callback for vfio_ap driver | From | Tony Krowiak <> | Date | Mon, 14 Dec 2020 22:52:17 -0500 |
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On 11/26/20 10:54 AM, Halil Pasic wrote: > On Tue, 24 Nov 2020 16:40:06 -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. > 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 > > >
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