rp2/mpthreadport: Fix atomic section.

dpgeorge · dpgeorge · commit d062a6f2dfae · 2024-01-02T01:04:03.000+11:00
Two fixes:
- Use a recursive mutex to prevent deadlocks on the same thread, if an IRQ
  races against the thread level when acquiring the lock (mutex may be
  taken but IRQs not yet disabled).
- Unlock the mutex if it was locked, not if core1_entry is still non-null,
  to fix the case where the second core finishes while the first core is
  in the middle of an atomic operation.

Signed-off-by: Damien George &lt;damien@micropython.org&gt;
diff --git a/ports/rp2/mpthreadport.c b/ports/rp2/mpthreadport.c
@@ -45,13 +45,15 @@ STATIC uint32_t *core1_stack = NULL;
 STATIC size_t core1_stack_num_words = 0;
 
 // Thread mutex.
-STATIC mp_thread_mutex_t atomic_mutex;
+STATIC recursive_mutex_t atomic_mutex;
+STATIC unsigned int atomic_taken;
 
 uint32_t mp_thread_begin_atomic_section(void) {
     if (core1_entry) {
         // When both cores are executing, we also need to provide
         // full mutual exclusion.
-        mp_thread_mutex_lock(&atomic_mutex, 1);
+        recursive_mutex_enter_blocking(&atomic_mutex);
+        ++atomic_taken;
     }
 
     return save_and_disable_interrupts();
@@ -60,16 +62,17 @@ uint32_t mp_thread_begin_atomic_section(void) {
 void mp_thread_end_atomic_section(uint32_t state) {
     restore_interrupts(state);
 
-    if (core1_entry) {
-        mp_thread_mutex_unlock(&atomic_mutex);
+    if (atomic_taken) {
+        --atomic_taken;
+        recursive_mutex_exit(&atomic_mutex);
     }
 }
 
 // Initialise threading support.
 void mp_thread_init(void) {
     assert(get_core_num() == 0);
 
-    mp_thread_mutex_init(&atomic_mutex);
+    recursive_mutex_init(&atomic_mutex);
 
     // Allow MICROPY_BEGIN_ATOMIC_SECTION to be invoked from core1.
     multicore_lockout_victim_init();
