Re: [PATCH v2 04/14] vfio: Provide better generic support for open/release vfio_device_ops

[Christoph Hellwig <hch@lst.de>]

> +int vfio_assign_device_set(struct vfio_device *device, void *set_id)
> +{
> +	struct vfio_device_set *alloc_dev_set = NULL;
> +	struct vfio_device_set *dev_set;
> +
> +	if (WARN_ON(!set_id))
> +		return -EINVAL;
> +
> +	/*
> +	 * Atomically acquire a singleton object in the xarray for this set_id
> +	 */
> +again:
> +	xa_lock(&vfio_device_set_xa);
> +	if (alloc_dev_set) {
> +		dev_set = __xa_cmpxchg(&vfio_device_set_xa,
> +				       (unsigned long)set_id, NULL,
> +				       alloc_dev_set, GFP_KERNEL);
> +		if (xa_is_err(dev_set)) {
> +			xa_unlock(&vfio_device_set_xa);
> +			kfree(alloc_dev_set);
> +			return xa_err(dev_set);
> +		}
> +		if (!dev_set)
> +			dev_set = alloc_dev_set;
> +	} else {
> +		dev_set = xa_load(&vfio_device_set_xa, (unsigned long)set_id);
> +	}
> +
> +	if (dev_set) {
> +		dev_set->device_count++;
> +		xa_unlock(&vfio_device_set_xa);
> +		device->dev_set = dev_set;
> +		if (dev_set != alloc_dev_set)
> +			kfree(alloc_dev_set);
> +		return 0;
> +	}
> +	xa_unlock(&vfio_device_set_xa);
> +
> +	if (WARN_ON(alloc_dev_set))
> +		return -EINVAL;
> +
> +	alloc_dev_set = kzalloc(sizeof(*alloc_dev_set), GFP_KERNEL);
> +	if (!alloc_dev_set)
> +		return -ENOMEM;
> +	mutex_init(&alloc_dev_set->lock);
> +	alloc_dev_set->set_id = set_id;
> +	goto again;
> +}
> +EXPORT_SYMBOL_GPL(vfio_assign_device_set);

This looks unessecarily complicated.  We can just try to load first
and then store it under the same lock, e.g.:

int vfio_assign_device_set(struct vfio_device *device, void *set_id)
{
	unsigned long idx = (unsigned long)set_id;
	struct vfio_device_set *set, *new;
	int err;

	if (WARN_ON(!set_id))
		return -EINVAL;

	xa_lock(&vfio_device_set_xa);
	set = xa_load(&vfio_device_set_xa, idx);
	if (set)
		goto found;
	xa_unlock(&vfio_device_set_xa);

	new = kzalloc(sizeof(*new), GFP_KERNEL);
	if (!new)
		return -ENOMEM;
	mutex_init(&new->lock);
	alloc_dev_set->set_id = set_id;

	xa_lock(&vfio_device_set_xa);
	set = xa_load(&vfio_device_set_xa, idx);
	if (set) {
		kfree(new);
		goto found;
	}
	err = xa_err(__xa_store(&vfio_device_set_xa, idx, new, GFP_KERNEL));
	xa_unlock(&vfio_device_set_xa);
	if (err)
		kfree(new);
	return err;

found:
	set->device_count++;
	xa_unlock(&vfio_device_set_xa);

	device->dev_set = set;
	return 0;
}

> +static void vfio_release_device_set(struct vfio_device *device)
> +{
> +	struct vfio_device_set *dev_set = device->dev_set;
> +
> +	if (!dev_set)
> +		return;
> +
> +	xa_lock(&vfio_device_set_xa);
> +	dev_set->device_count--;
> +	if (!dev_set->device_count) {

Nit, by I'd find

	if (!--dev_set->device_count) {

easier to follow as it clearly documents the dec_and_test pattern.