Add a pre_destroy() op which gives objects a chance to clear their
short term users references before destruction. This op is intended for
external driver created objects (e.g. idev) which does deterministic
destruction.

In order to manage the lifecycle of interrelated objects as well as the
deterministic destruction (e.g. vdev can't outlive idev, and idev
destruction can't fail), short term users references are allowed to
live out of an ioctl execution. An immediate use case is, vdev holds
idev's short term user reference until vdev destruction completes, idev
leverages existing wait_shortterm mechanism to ensure it is destroyed
after vdev.

This extended usage makes the referenced object unable to just wait for
its reference gone. It needs to actively trigger the reference removal,
as well as prevent new references before wait. Should implement these
work in pre_destroy().

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>
---
 drivers/iommu/iommufd/main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c
index 71135f0ec72d..53085d24ce4a 100644
--- a/drivers/iommu/iommufd/main.c
+++ b/drivers/iommu/iommufd/main.c
@@ -23,6 +23,7 @@
 #include "iommufd_test.h"
 
 struct iommufd_object_ops {
+	void (*pre_destroy)(struct iommufd_object *obj);
 	void (*destroy)(struct iommufd_object *obj);
 	void (*abort)(struct iommufd_object *obj);
 };
@@ -160,6 +161,9 @@ static int iommufd_object_dec_wait_shortterm(struct iommufd_ctx *ictx,
 	if (refcount_dec_and_test(&to_destroy->shortterm_users))
 		return 0;
 
+	if (iommufd_object_ops[to_destroy->type].pre_destroy)
+		iommufd_object_ops[to_destroy->type].pre_destroy(to_destroy);
+
 	if (wait_event_timeout(ictx->destroy_wait,
 			       refcount_read(&to_destroy->shortterm_users) == 0,
 			       msecs_to_jiffies(60000)))
-- 
2.25.1

On Tue, Jul 15, 2025 at 02:32:40PM +0800, Xu Yilun wrote:
> Add a pre_destroy() op which gives objects a chance to clear their
> short term users references before destruction. This op is intended for
> external driver created objects (e.g. idev) which does deterministic
> destruction.
> 
> In order to manage the lifecycle of interrelated objects as well as the
> deterministic destruction (e.g. vdev can't outlive idev, and idev
> destruction can't fail), short term users references are allowed to
> live out of an ioctl execution. An immediate use case is, vdev holds
> idev's short term user reference until vdev destruction completes, idev
> leverages existing wait_shortterm mechanism to ensure it is destroyed
> after vdev.
> 
> This extended usage makes the referenced object unable to just wait for
> its reference gone. It needs to actively trigger the reference removal,
> as well as prevent new references before wait. Should implement these
> work in pre_destroy().
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
> Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>
> ---
>  drivers/iommu/iommufd/main.c | 4 ++++
>  1 file changed, 4 insertions(+)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

>  struct iommufd_object_ops {
> +	void (*pre_destroy)(struct iommufd_object *obj);

I would capture the key points of the commit message in a comment
right here though..

Jason