From ad98cb4e292f67409c4baba443cf899a2dd240c1 Mon Sep 17 00:00:00 2001
From: riastradh <riastradh@NetBSD.org>
Date: Thu, 3 Mar 2022 05:50:31 +0000
Subject: [PATCH] usbnet: Print diagnostic about refcnt stragglers.

I don't think there can be any, but this message, if printed, would
falsify my hypothesis!
---
 sys/dev/usb/usbnet.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/sys/dev/usb/usbnet.c b/sys/dev/usb/usbnet.c
index 71f650ed9802..4d3af0ac54a0 100644
--- a/sys/dev/usb/usbnet.c
+++ b/sys/dev/usb/usbnet.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: usbnet.c,v 1.71 2022/03/03 05:50:22 riastradh Exp $	*/
+/*	$NetBSD: usbnet.c,v 1.72 2022/03/03 05:50:31 riastradh Exp $	*/
 
 /*
  * Copyright (c) 2019 Matthew R. Green
@@ -31,7 +31,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: usbnet.c,v 1.71 2022/03/03 05:50:22 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: usbnet.c,v 1.72 2022/03/03 05:50:31 riastradh Exp $");
 
 #include <sys/param.h>
 #include <sys/kernel.h>
@@ -1671,6 +1671,10 @@ usbnet_detach(device_t self, int flags)
 
 	mutex_enter(&unp->unp_core_lock);
 	unp->unp_refcnt--;
+	if (unp->unp_refcnt >= 0) {
+		aprint_error_dev(un->un_dev, "%d stragglers\n",
+		    unp->unp_refcnt + 1);
+	}
 	while (unp->unp_refcnt >= 0) {
 		/* Wait for processes to go away */
 		cv_wait(&unp->unp_detachcv, &unp->unp_core_lock);