rust: usb: don't retain device context for the interface parent

When deriving the parent USB device (struct usb_device) from a USB
interface (struct usb_interface), do not retain the device context.

For the Bound context, as pointed out by Alan in [1], it is not
guaranteed that the parent USB device is always bound when the interface
is bound.

The bigger problem, however, is that we can't infer the Core context,
since eventually it indicates that the device lock is held. However,
there is no guarantee that if the device lock of the interface is held,
also the device lock of the parent USB device is held.

Hence, fix this by not inferring any device context information; while
at it, fix up the (affected) safety comments.

Link: https://lore.kernel.org/all/[email protected]/ [1]
Fixes: e7e2296 ("rust: usb: add basic USB abstractions")
Signed-off-by: Danilo Krummrich <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Danilo Krummrich

rust/kernel/usb.rs

@@ -340,14 +340,13 @@ impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Interface<Ctx> {
     }
 }
 
-impl<Ctx: device::DeviceContext> AsRef<Device<Ctx>> for Interface<Ctx> {
-    fn as_ref(&self) -> &Device<Ctx> {
-        // SAFETY: `self.as_raw()` is valid by the type invariants. For a valid interface,
-        // the helper should always return a valid USB device pointer.
+impl<Ctx: device::DeviceContext> AsRef<Device> for Interface<Ctx> {
+    fn as_ref(&self) -> &Device {
+        // SAFETY: `self.as_raw()` is valid by the type invariants.
         let usb_dev = unsafe { bindings::interface_to_usbdev(self.as_raw()) };
 
-        // SAFETY: The helper returns a valid interface pointer that shares the
-        // same `DeviceContext`.
+        // SAFETY: For a valid `struct usb_interface` pointer, the above call to
+        // `interface_to_usbdev()` guarantees to return a valid pointer to a `struct usb_device`.
         unsafe { &*(usb_dev.cast()) }
     }
 }