Re: [PATCH v13 05/15] s390/vfio-ap: manage link between queue struct and matrix mdev

Halil Pasic
2021-01-14T02:50:59Z

On Wed, 13 Jan 2021 16:41:27 -0500
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> On 1/11/21 2:17 PM, Halil Pasic wrote:
> > On Tue, 22 Dec 2020 20:15:56 -0500
> > Tony Krowiak <akrowiak@linux.ibm.com> wrote:
> >
> >> Let's create links between each queue device bound to the vfio_ap device
> >> driver and the matrix mdev to which the queue's APQN is assigned. The idea
> >> is to facilitate efficient retrieval of the objects representing the queue
> >> devices and matrix mdevs as well as to verify that a queue assigned to
> >> a matrix mdev is bound to the driver.
> >>
> >> The links will be created as follows:
> >>
> >> * When the queue device is probed, if its APQN is assigned to a matrix
> >> mdev, the structures representing the queue device and the matrix mdev
> >> will be linked.
> >>
> >> * When an adapter or domain is assigned to a matrix mdev, for each new
> >> APQN assigned that references a queue device bound to the vfio_ap
> >> device driver, the structures representing the queue device and the
> >> matrix mdev will be linked.
> >>
> >> The links will be removed as follows:
> >>
> >> * When the queue device is removed, if its APQN is assigned to a matrix
> >> mdev, the structures representing the queue device and the matrix mdev
> >> will be unlinked.
> >>
> >> * When an adapter or domain is unassigned from a matrix mdev, for each
> >> APQN unassigned that references a queue device bound to the vfio_ap
> >> device driver, the structures representing the queue device and the
> >> matrix mdev will be unlinked.
> >>
> >> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> > Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
> >

[..]

> >> +
> >> int vfio_ap_mdev_probe_queue(struct ap_device *apdev)
> >> {
> >> struct vfio_ap_queue *q;
> >> @@ -1324,9 +1404,13 @@ int vfio_ap_mdev_probe_queue(struct ap_device *apdev)
> >> q = kzalloc(sizeof(*q), GFP_KERNEL);
> >> if (!q)
> >> return -ENOMEM;
> >> + mutex_lock(&matrix_dev->lock);
> >> dev_set_drvdata(&apdev->device, q);
> >> q->apqn = to_ap_queue(&apdev->device)->qid;
> >> q->saved_isc = VFIO_AP_ISC_INVALID;
> >> + vfio_ap_queue_link_mdev(q);
> >> + mutex_unlock(&matrix_dev->lock);
> >> +
> > Does the critical section have to include more than just
> > vfio_ap_queue_link_mdev()? Did we need the critical section
> > before this patch?
>
> We did not need the critical section before this patch because
> the only function that retrieved the vfio_ap_queue via the queue
> device's drvdata was the remove callback. I included the initialization
> of the vfio_ap_queue object under lock because the
> vfio_ap_find_queue() function retrieves the vfio_ap_queue object from
> the queue device's drvdata so it might be advantageous to initialize
> it under the mdev lock. On the other hand, I can't come up with a good
> argument to change this.
>
>

I was asking out of curiosity, not because I want it changed. I was
also wondering if somebody could see a partially initialized device:
we even first call dev_set_drvdata() and only then finish the
initialization. Before 's390/vfio-ap: use new AP bus interface to search
for queue devices', which is the previous patch, we had the klist code
in between, which uses spinlocks, which I think ensure, that all
effects of probe are seen when we get the queue from
vfio_ap_find_queue(). But with patch 4 in place that is not the case any
more. Or am I wrong?

Regards,
Halil