iommufd: Add iommufd_access_change_ioas(_id) helpers

The complication of the mutex and refcount will be amplified after we
introduce the replace support for accesses. So, add a preparatory change
of a constitutive helper iommufd_access_change_ioas() and its wrapper
iommufd_access_change_ioas_id(). They can simply take care of existing
iommufd_access_attach() and iommufd_access_detach(), properly sequencing
the refcount puts so that they are truely at the end of the sequence after
we know the IOAS pointer is not required any more.

Link: https://lore.kernel.org/r/da0c462532193b447329c4eb975a596f47e49b70.1690523699.git.nicolinc@nvidia.com
Suggested-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Signed-off-by: Nicolin Chen <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

drivers/iommu/iommufd/device.c

@@ -684,6 +684,70 @@ void iommufd_device_detach(struct iommufd_device *idev)
 }
 EXPORT_SYMBOL_NS_GPL(iommufd_device_detach, IOMMUFD);
 
+/*
+ * On success, it will refcount_inc() at a valid new_ioas and refcount_dec() at
+ * a valid cur_ioas (access->ioas). A caller passing in a valid new_ioas should
+ * call iommufd_put_object() if it does an iommufd_get_object() for a new_ioas.
+ */
+static int iommufd_access_change_ioas(struct iommufd_access *access,
+				      struct iommufd_ioas *new_ioas)
+{
+	u32 iopt_access_list_id = access->iopt_access_list_id;
+	struct iommufd_ioas *cur_ioas = access->ioas;
+	int rc;
+
+	lockdep_assert_held(&access->ioas_lock);
+
+	/* We are racing with a concurrent detach, bail */
+	if (cur_ioas != access->ioas_unpin)
+		return -EBUSY;
+
+	if (cur_ioas == new_ioas)
+		return 0;
+
+	/*
+	 * Set ioas to NULL to block any further iommufd_access_pin_pages().
+	 * iommufd_access_unpin_pages() can continue using access->ioas_unpin.
+	 */
+	access->ioas = NULL;
+
+	if (new_ioas) {
+		rc = iopt_add_access(&new_ioas->iopt, access);
+		if (rc) {
+			access->ioas = cur_ioas;
+			return rc;
+		}
+		refcount_inc(&new_ioas->obj.users);
+	}
+
+	if (cur_ioas) {
+		if (access->ops->unmap) {
+			mutex_unlock(&access->ioas_lock);
+			access->ops->unmap(access->data, 0, ULONG_MAX);
+			mutex_lock(&access->ioas_lock);
+		}
+		iopt_remove_access(&cur_ioas->iopt, access, iopt_access_list_id);
+		refcount_dec(&cur_ioas->obj.users);
+	}
+
+	access->ioas = new_ioas;
+	access->ioas_unpin = new_ioas;
+
+	return 0;
+}
+
+static int iommufd_access_change_ioas_id(struct iommufd_access *access, u32 id)
+{
+	struct iommufd_ioas *ioas = iommufd_get_ioas(access->ictx, id);
+	int rc;
+
+	if (IS_ERR(ioas))
+		return PTR_ERR(ioas);
+	rc = iommufd_access_change_ioas(access, ioas);
+	iommufd_put_object(&ioas->obj);
+	return rc;
+}
+
 void iommufd_access_destroy_object(struct iommufd_object *obj)
 {
 	struct iommufd_access *access =
@@ -761,60 +825,29 @@ EXPORT_SYMBOL_NS_GPL(iommufd_access_destroy, IOMMUFD);
 
 void iommufd_access_detach(struct iommufd_access *access)
 {
-	struct iommufd_ioas *cur_ioas = access->ioas;
-
 	mutex_lock(&access->ioas_lock);
-	if (WARN_ON(!access->ioas))
-		goto out;
-	/*
-	 * Set ioas to NULL to block any further iommufd_access_pin_pages().
-	 * iommufd_access_unpin_pages() can continue using access->ioas_unpin.
-	 */
-	access->ioas = NULL;
-
-	if (access->ops->unmap) {
+	if (WARN_ON(!access->ioas)) {
 		mutex_unlock(&access->ioas_lock);
-		access->ops->unmap(access->data, 0, ULONG_MAX);
-		mutex_lock(&access->ioas_lock);
+		return;
 	}
-	iopt_remove_access(&cur_ioas->iopt, access,
-			   access->iopt_access_list_id);
-	refcount_dec(&cur_ioas->obj.users);
-out:
-	access->ioas_unpin = NULL;
+	WARN_ON(iommufd_access_change_ioas(access, NULL));
 	mutex_unlock(&access->ioas_lock);
 }
 EXPORT_SYMBOL_NS_GPL(iommufd_access_detach, IOMMUFD);
 
 int iommufd_access_attach(struct iommufd_access *access, u32 ioas_id)
 {
-	struct iommufd_ioas *new_ioas;
-	int rc = 0;
+	int rc;
 
 	mutex_lock(&access->ioas_lock);
-	if (WARN_ON(access->ioas || access->ioas_unpin)) {
+	if (WARN_ON(access->ioas)) {
 		mutex_unlock(&access->ioas_lock);
 		return -EINVAL;
 	}
 
-	new_ioas = iommufd_get_ioas(access->ictx, ioas_id);
-	if (IS_ERR(new_ioas)) {
-		mutex_unlock(&access->ioas_lock);
-		return PTR_ERR(new_ioas);
-	}
-
-	rc = iopt_add_access(&new_ioas->iopt, access);
-	if (rc) {
-		mutex_unlock(&access->ioas_lock);
-		iommufd_put_object(&new_ioas->obj);
-		return rc;
-	}
-	iommufd_ref_to_users(&new_ioas->obj);
-
-	access->ioas = new_ioas;
-	access->ioas_unpin = new_ioas;
+	rc = iommufd_access_change_ioas_id(access, ioas_id);
 	mutex_unlock(&access->ioas_lock);
-	return 0;
+	return rc;
 }
 EXPORT_SYMBOL_NS_GPL(iommufd_access_attach, IOMMUFD);