iommufd: Add a pre_destroy() op for objects

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().

Link: https://patch.msgid.link/r/20250716070349.1807226-4-yilun.xu@linux.intel.com
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c
index 71135f0ec72df7d1e33ebbad0b696a8df8e45990..53085d24ce4a9874ad053062877c5063c24165fc 100644 (file)
--- 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)))