hack to fix 3986

just a proof to show that issue analysis is correct

Author: hathach

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) {

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);
 }