vfio: Use a refcount_t instead of a kref in the vfio_group

The next patch adds a struct device to the struct vfio_group, and it is
confusing/bad practice to have two krefs in the same struct. This kref is
controlling the period when the vfio_group is registered in sysfs, and
visible in the internal lookup. Switch it to a refcount_t instead.

The refcount_dec_and_mutex_lock() is still required because we need
atomicity of the list searches and sysfs presence.
Reviewed-by: Liu Yi L <yi.l.liu@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/4-v3-2fdfe4ca2cc6+18c-vfio_group_cdev_jgg@nvidia.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio.c

@@ -69,7 +69,7 @@ struct vfio_unbound_dev {
 };
 
 struct vfio_group {
-	struct kref			kref;
+	refcount_t			users;
 	int				minor;
 	atomic_t			container_users;
 	struct iommu_group		*iommu_group;
@@ -377,7 +377,7 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group,
 	if (!group)
 		return ERR_PTR(-ENOMEM);
 
-	kref_init(&group->kref);
+	refcount_set(&group->users, 1);
 	INIT_LIST_HEAD(&group->device_list);
 	mutex_init(&group->device_lock);
 	INIT_LIST_HEAD(&group->unbound_list);
@@ -433,10 +433,10 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group,
 	return group;
 }
 
-/* called with vfio.group_lock held */
-static void vfio_group_release(struct kref *kref)
+static void vfio_group_put(struct vfio_group *group)
 {
-	struct vfio_group *group = container_of(kref, struct vfio_group, kref);
+	if (!refcount_dec_and_mutex_lock(&group->users, &vfio.group_lock))
+		return;
 
 	/*
 	 * These data structures all have paired operations that can only be
@@ -454,15 +454,9 @@ static void vfio_group_release(struct kref *kref)
 	vfio_group_unlock_and_free(group);
 }
 
-static void vfio_group_put(struct vfio_group *group)
-{
-	kref_put_mutex(&group->kref, vfio_group_release, &vfio.group_lock);
-}
-
-/* Assume group_lock or group reference is held */
 static void vfio_group_get(struct vfio_group *group)
 {
-	kref_get(&group->kref);
+	refcount_inc(&group->users);
 }
 
 static struct vfio_group *vfio_group_get_from_minor(int minor)
@@ -1657,6 +1651,9 @@ struct vfio_group *vfio_group_get_external_user(struct file *filep)
 	if (ret)
 		return ERR_PTR(ret);
 
+	/*
+	 * Since the caller holds the fget on the file group->users must be >= 1
+	 */
 	vfio_group_get(group);
 
 	return group;