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

[Tony Krowiak]

On 1/13/21 9:50 PM, Halil Pasic wrote:
> On Wed, 13 Jan 2021 16:41:27 -0500
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > On 1/11/21 2:17 PM, Halil Pasic wrote:
> > > On Tue, 22 Dec 2020 20:15:56 -0500
> > > Tony Krowiak <akrowiak@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > Reviewed-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > >   
> [..]
>
> > > > +
> > > >    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?

You are correct insofar as patch 4 replaces the driver_find_device()
function call with a call to AP bus's ap_get_qdev() function which
does not use spinlocks. Without digging deeply into the probe call
chain I do not know whether or not  the use of spinlocks by the klist
code ensures all effects of the probe are seen when we get the
queue from vfio_ap_find_queue(). What I'm sure about is that since
both vfio_ap_find_queue() and the setting of the drvdata in the
probe function are always done under the mdev lock, consistency
should be maintained. What I did decide when thinking about your
previous review comment is that we should probably initialize the
vfio_ap_queue object before setting the drvdata, so I made that change.

> Regards,
> Halil