iommufd/access: Add internal APIs for HW queue to use

The new HW queue object, as an internal iommufd object, wants to reuse the
struct iommufd_access to pin some iova range in the iopt.

However, an access generally takes the refcount of an ictx. So, in such an
internal case, a deadlock could happen when the release of the ictx has to
wait for the release of the access first when releasing a hw_queue object,
which could wait for the release of the ictx that is refcounted:
    ictx --releases--> hw_queue --releases--> access
      ^                                         |
      |_________________releases________________v

To address this, add a set of lightweight internal APIs to unlink the ictx
and the access, i.e. no ictx refcounting by the access:
    ictx --releases--> hw_queue --releases--> access

Then, there's no point in setting the access->ictx. So simply define !ictx
as an flag for an internal use and add an inline helper.

Link: https://patch.msgid.link/r/d8d84bf99cbebec56034b57b966a3d431385b90d.1752126748.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/iommu/iommufd/device.c

@@ -1084,9 +1084,41 @@ void iommufd_access_destroy_object(struct iommufd_object *obj)
 	if (access->ioas)
 		WARN_ON(iommufd_access_change_ioas(access, NULL));
 	mutex_unlock(&access->ioas_lock);
+	if (!iommufd_access_is_internal(access))
 		iommufd_ctx_put(access->ictx);
 }
 
+static struct iommufd_access *__iommufd_access_create(struct iommufd_ctx *ictx)
+{
+	struct iommufd_access *access;
+
+	/*
+	 * There is no uAPI for the access object, but to keep things symmetric
+	 * use the object infrastructure anyhow.
+	 */
+	access = iommufd_object_alloc(ictx, access, IOMMUFD_OBJ_ACCESS);
+	if (IS_ERR(access))
+		return access;
+
+	/* The calling driver is a user until iommufd_access_destroy() */
+	refcount_inc(&access->obj.users);
+	mutex_init(&access->ioas_lock);
+	return access;
+}
+
+struct iommufd_access *iommufd_access_create_internal(struct iommufd_ctx *ictx)
+{
+	struct iommufd_access *access;
+
+	access = __iommufd_access_create(ictx);
+	if (IS_ERR(access))
+		return access;
+	access->iova_alignment = PAGE_SIZE;
+
+	iommufd_object_finalize(ictx, &access->obj);
+	return access;
+}
+
 /**
  * iommufd_access_create - Create an iommufd_access
  * @ictx: iommufd file descriptor
@@ -1106,11 +1138,7 @@ iommufd_access_create(struct iommufd_ctx *ictx,
 {
 	struct iommufd_access *access;
 
-	/*
+	access = __iommufd_access_create(ictx);
-	 * There is no uAPI for the access object, but to keep things symmetric
-	 * use the object infrastructure anyhow.
-	 */
-	access = iommufd_object_alloc(ictx, access, IOMMUFD_OBJ_ACCESS);
 	if (IS_ERR(access))
 		return access;
 
@@ -1122,13 +1150,10 @@ iommufd_access_create(struct iommufd_ctx *ictx,
 	else
 		access->iova_alignment = 1;
 
-	/* The calling driver is a user until iommufd_access_destroy() */
-	refcount_inc(&access->obj.users);
 	access->ictx = ictx;
 	iommufd_ctx_get(ictx);
 	iommufd_object_finalize(ictx, &access->obj);
 	*id = access->obj.id;
-	mutex_init(&access->ioas_lock);
 	return access;
 }
 EXPORT_SYMBOL_NS_GPL(iommufd_access_create, "IOMMUFD");
@@ -1173,6 +1198,22 @@ int iommufd_access_attach(struct iommufd_access *access, u32 ioas_id)
 }
 EXPORT_SYMBOL_NS_GPL(iommufd_access_attach, "IOMMUFD");
 
+int iommufd_access_attach_internal(struct iommufd_access *access,
+				   struct iommufd_ioas *ioas)
+{
+	int rc;
+
+	mutex_lock(&access->ioas_lock);
+	if (WARN_ON(access->ioas)) {
+		mutex_unlock(&access->ioas_lock);
+		return -EINVAL;
+	}
+
+	rc = iommufd_access_change_ioas(access, ioas);
+	mutex_unlock(&access->ioas_lock);
+	return rc;
+}
+
 int iommufd_access_replace(struct iommufd_access *access, u32 ioas_id)
 {
 	int rc;

drivers/iommu/iommufd/iommufd_private.h

@@ -484,6 +484,29 @@ void iopt_remove_access(struct io_pagetable *iopt,
 			struct iommufd_access *access, u32 iopt_access_list_id);
 void iommufd_access_destroy_object(struct iommufd_object *obj);
 
+/* iommufd_access for internal use */
+static inline bool iommufd_access_is_internal(struct iommufd_access *access)
+{
+	return !access->ictx;
+}
+
+struct iommufd_access *iommufd_access_create_internal(struct iommufd_ctx *ictx);
+
+static inline void
+iommufd_access_destroy_internal(struct iommufd_ctx *ictx,
+				struct iommufd_access *access)
+{
+	iommufd_object_destroy_user(ictx, &access->obj);
+}
+
+int iommufd_access_attach_internal(struct iommufd_access *access,
+				   struct iommufd_ioas *ioas);
+
+static inline void iommufd_access_detach_internal(struct iommufd_access *access)
+{
+	iommufd_access_detach(access);
+}
+
 struct iommufd_eventq {
 	struct iommufd_object obj;
 	struct iommufd_ctx *ictx;