[PATCH] rust: drm: device: drop_in_place() the drm::Device in release()

[Danilo Krummrich <dakr@kernel.org>]

In drm::Device::new() we allocate with __drm_dev_alloc() and return an
ARef<drm::Device>.

When the reference count of the drm::Device falls to zero, the C code
automatically calls drm_dev_release(), which eventually frees the memory
allocated in drm::Device::new().

However, due to that, drm::Device::drop() is never called. As a result
the destructor of the user's private data, i.e. drm::Device::data is
never called. Hence, fix this by calling drop_in_place() from the DRM
device's release callback.

Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/drm/device.rs | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 624d7a4c83ea..14c1aa402951 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -66,7 +66,7 @@ impl<T: drm::Driver> Device<T> {
         open: Some(drm::File::<T::File>::open_callback),
         postclose: Some(drm::File::<T::File>::postclose_callback),
         unload: None,
-        release: None,
+        release: Some(Self::release),
         master_set: None,
         master_drop: None,
         debugfs_init: None,
@@ -162,6 +162,16 @@ pub unsafe fn as_ref<'a>(ptr: *const bindings::drm_device) -> &'a Self {
         // SAFETY: `ptr` is valid by the safety requirements of this function.
         unsafe { &*ptr.cast() }
     }
+
+    extern "C" fn release(ptr: *mut bindings::drm_device) {
+        // SAFETY: `ptr` is a valid pointer to a `struct drm_device` and embedded in `Self`.
+        let this = unsafe { Self::from_drm_device(ptr) };
+
+        // SAFETY:
+        // - When `release` runs it is guaranteed that there is no further access to `this`.
+        // - `this` is valid for dropping.
+        unsafe { core::ptr::drop_in_place(this) };
+    }
 }
 
 impl<T: drm::Driver> Deref for Device<T> {

base-commit: 615cc4223fcbe1e0e6f68b8494b26bb6c08d917a
-- 
2.50.0