Re: [PATCH v9 09/22] s390: vfio-ap: register matrix device with VFIO mdev framework

[Tony Krowiak]

On 08/17/2018 04:43 AM, Cornelia Huck wrote:
> On Thu, 16 Aug 2018 12:24:16 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > On 08/14/2018 07:19 AM, Cornelia Huck wrote:
> > > On Mon, 13 Aug 2018 17:48:06 -0400
> > > Tony Krowiak <akrowiak@xxxxxxxxxxxxxxxxxx> wrote:
> > > > +static int vfio_ap_mdev_create(struct kobject *kobj, struct mdev_device *mdev)
> > > > +{
> > > > +	struct ap_matrix_mdev *matrix_mdev;
> > > > +
> > > > +	matrix_mdev = kzalloc(sizeof(*matrix_mdev), GFP_KERNEL);
> > > > +	if (!matrix_mdev)
> > > > +		return -ENOMEM;
> > > > +
> > > > +	matrix_mdev->name = dev_name(mdev_dev(mdev));
> > > > +	vfio_ap_matrix_init(&matrix_dev.info, &matrix_mdev->matrix);
> > > > +	mdev_set_drvdata(mdev, matrix_mdev);
> > > > +
> > > > +	if (atomic_dec_if_positive(&matrix_dev.available_instances) < 0) {
> > > > +		kfree(matrix_mdev);
> > > > +		return -EPERM;
> > > > +	}
> > > Maybe move this check to the top of the function?
> > Please ignore my previous response to your comment. I can't move the call to
> > atomic_dec_if_positive() to the top of the function because it
> > decrements the
> > available_instances and if the kzalloc() of matrix_mdev fails, then the
> > value
> > would have to then be incremented to remain valid. What I can do is this:
> >
> > 1. Check the value of available_instances using atomic_read() at the top of
> >      the function and if it is zero, return an error.
> >
> > 2. Replace the call to atomic_dec_if_positive() with a call to atomic_dec()
> >      to decrement the available_instances.
> >
> > I agree that it makes sense to return before attempting to allocate the
> > matrix_mdev if available_instances is zero.
> Wouldn't that be racy, though?
>
> I don't think re-incrementing the counter is too bad, and it's
> certainly better than going through allocation/freeing of structures.

I'll make it happen.

> > >   
> > > > +
> > > > +	mutex_lock(&matrix_dev.lock);
> > > > +	list_add(&matrix_mdev->list, &matrix_dev.mdev_list);
> > > > +	mutex_unlock(&matrix_dev.lock);
> > > > +
> > > > +	return 0;
> > > > +}