USB: core: document purpose of interface parent reference

USB interfaces hold a reference to their parent USB device to make sure
they do not outlive the parent.

Add a comment explaining that this reference is specifically needed for
asynchronous device resets and the device type need_parent_lock feature
(to allow driver core to lock the parent when only holding a reference
to the interface).

See commits:

  524134d422 ("USB: don't cancel queued resets when unbinding drivers")
  0f554506a3fc ("driver core: fix parent lock use-after-free on USB disconnect")

for further details.

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20261015074831.1418677-1-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/message.c

@@ -2208,6 +2208,11 @@ free_interfaces:
 		pm_runtime_no_callbacks(&intf->dev);
 		dev_set_name(&intf->dev, "%d-%s:%d.%d", dev->bus->busnum,
 				dev->devpath, configuration, ifnum);
+		/*
+		 * Make sure the interface does not outlive its parent (e.g.
+		 * as needed for usb_queue_reset_device() and
+		 * usb_if_device_type->need_parent_lock).
+		 */
 		usb_get_dev(dev);
 	}
 	kfree(new_interfaces);