vfio: Simplify vfio_create_group()

The vfio.group_lock is now only used to serialize vfio_group creation and
destruction, we don't need a micro-optimization of searching, unlocking,
then allocating and searching again. Just hold the lock the whole time.

Grabbed from:
https://lore.kernel.org/kvm/20220922152338.2a2238fe.alex.williamson@redhat.com/

Link: https://lore.kernel.org/r/20221201145535.589687-2-yi.l.liu@intel.comReviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Lixiao Yang <lixiao.yang@intel.com>
Tested-by: Yu He <yu.he@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>

drivers/vfio/vfio_main.c

@@ -143,10 +143,12 @@ EXPORT_SYMBOL_GPL(vfio_device_set_open_count);
  * Group objects - create, release, get, put, search
  */
 static struct vfio_group *
-__vfio_group_get_from_iommu(struct iommu_group *iommu_group)
+vfio_group_get_from_iommu(struct iommu_group *iommu_group)
 {
 	struct vfio_group *group;
 
+	lockdep_assert_held(&vfio.group_lock);
+
 	/*
 	 * group->iommu_group from the vfio.group_list cannot be NULL
 	 * under the vfio.group_lock.
@@ -160,17 +162,6 @@ __vfio_group_get_from_iommu(struct iommu_group *iommu_group)
 	return NULL;
 }
 
-static struct vfio_group *
-vfio_group_get_from_iommu(struct iommu_group *iommu_group)
-{
-	struct vfio_group *group;
-
-	mutex_lock(&vfio.group_lock);
-	group = __vfio_group_get_from_iommu(iommu_group);
-	mutex_unlock(&vfio.group_lock);
-	return group;
-}
-
 static void vfio_group_release(struct device *dev)
 {
 	struct vfio_group *group = container_of(dev, struct vfio_group, dev);
@@ -225,6 +216,8 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group,
 	struct vfio_group *ret;
 	int err;
 
+	lockdep_assert_held(&vfio.group_lock);
+
 	group = vfio_group_alloc(iommu_group, type);
 	if (IS_ERR(group))
 		return group;
@@ -237,26 +230,16 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group,
 		goto err_put;
 	}
 
-	mutex_lock(&vfio.group_lock);
-
-	/* Did we race creating this group? */
-	ret = __vfio_group_get_from_iommu(iommu_group);
-	if (ret)
-		goto err_unlock;
-
 	err = cdev_device_add(&group->cdev, &group->dev);
 	if (err) {
 		ret = ERR_PTR(err);
-		goto err_unlock;
+		goto err_put;
 	}
 
 	list_add(&group->vfio_next, &vfio.group_list);
 
-	mutex_unlock(&vfio.group_lock);
 	return group;
 
-err_unlock:
-	mutex_unlock(&vfio.group_lock);
 err_put:
 	put_device(&group->dev);
 	return ret;
@@ -467,7 +450,9 @@ static struct vfio_group *vfio_noiommu_group_alloc(struct device *dev,
 	if (ret)
 		goto out_put_group;
 
+	mutex_lock(&vfio.group_lock);
 	group = vfio_create_group(iommu_group, type);
+	mutex_unlock(&vfio.group_lock);
 	if (IS_ERR(group)) {
 		ret = PTR_ERR(group);
 		goto out_remove_device;
@@ -516,9 +501,11 @@ static struct vfio_group *vfio_group_find_or_alloc(struct device *dev)
 		return ERR_PTR(-EINVAL);
 	}
 
+	mutex_lock(&vfio.group_lock);
 	group = vfio_group_get_from_iommu(iommu_group);
 	if (!group)
 		group = vfio_create_group(iommu_group, VFIO_IOMMU);
+	mutex_unlock(&vfio.group_lock);
 
 	/* The vfio_group holds a reference to the iommu_group */
 	iommu_group_put(iommu_group);