From e699a598905c95eb8249e89e436030376a518239 Mon Sep 17 00:00:00 2001
From: hathach <thach@tinyusb.org>
Date: Wed, 3 Feb 2021 11:01:33 +0700
Subject: [PATCH] hack to fix 3986

just a proof to show that issue analysis is correct
---
 supervisor/shared/background_callback.c | 8 ++++++--
 supervisor/shared/usb/usb.c             | 6 +++---
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/supervisor/shared/background_callback.c b/supervisor/shared/background_callback.c
index 288c9a4df2..44709b60b5 100644
--- a/supervisor/shared/background_callback.c
+++ b/supervisor/shared/background_callback.c
@@ -33,7 +33,7 @@
 #include "supervisor/shared/tick.h"
 #include "shared-bindings/microcontroller/__init__.h"
 
-STATIC volatile background_callback_t *callback_head, *callback_tail;
+STATIC volatile background_callback_t * volatile callback_head, * volatile callback_tail;
 
 #define CALLBACK_CRITICAL_BEGIN (common_hal_mcu_disable_interrupts())
 #define CALLBACK_CRITICAL_END (common_hal_mcu_enable_interrupts())
@@ -50,7 +50,6 @@ void background_callback_add_core(background_callback_t *cb) {
     cb->prev = (background_callback_t*)callback_tail;
     if (callback_tail) {
         callback_tail->next = cb;
-        cb->prev = (background_callback_t*)callback_tail;
     }
     if (!callback_head) {
         callback_head = cb;
@@ -106,6 +105,9 @@ void background_callback_end_critical_section() {
     CALLBACK_CRITICAL_END;
 }
 
+extern background_callback_t usb_callback;
+extern void usb_background_do(void* unused);
+
 void background_callback_reset() {
     CALLBACK_CRITICAL_BEGIN;
     background_callback_t *cb = (background_callback_t*)callback_head;
@@ -118,6 +120,8 @@ void background_callback_reset() {
     callback_tail = NULL;
     in_background_callback = false;
     CALLBACK_CRITICAL_END;
+
+    background_callback_add(&usb_callback, usb_background_do, NULL);
 }
 
 void background_callback_gc_collect(void) {
diff --git a/supervisor/shared/usb/usb.c b/supervisor/shared/usb/usb.c
index 07c6aee6c1..af34f25987 100644
--- a/supervisor/shared/usb/usb.c
+++ b/supervisor/shared/usb/usb.c
@@ -93,13 +93,13 @@ void usb_background(void) {
     }
 }
 
-static background_callback_t usb_callback;
-static void usb_background_do(void* unused) {
+/*static*/ background_callback_t usb_callback;
+/*static*/ void usb_background_do(void* unused) {
     usb_background();
 }
 
 void usb_irq_handler(void) {
-    tud_int_handler(0);
+    tud_int_handler(0); \
     background_callback_add(&usb_callback, usb_background_do, NULL);
 }