py/scheduler: Fix race in checking scheduler pending state.

Because the atomic section starts after checking whether the scheduler
state is pending, it's possible it can become a different state by the time
the atomic section starts.

This is especially likely on ports where MICROPY_BEGIN_ATOMIC_SECTION is
implemented with a mutex (i.e. it might block), but the race exists
regardless, i.e. if a context switch occurs between those two lines.

py/scheduler.c

@@ -60,22 +60,27 @@ static inline bool mp_sched_empty(void) {
 void mp_handle_pending(bool raise_exc) {
     if (MP_STATE_VM(sched_state) == MP_SCHED_PENDING) {
         mp_uint_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
-        mp_obj_t obj = MP_STATE_VM(mp_pending_exception);
-        if (obj != MP_OBJ_NULL) {
-            MP_STATE_VM(mp_pending_exception) = MP_OBJ_NULL;
-            if (!mp_sched_num_pending()) {
-                MP_STATE_VM(sched_state) = MP_SCHED_IDLE;
-            }
-            if (raise_exc) {
-                MICROPY_END_ATOMIC_SECTION(atomic_state);
-                nlr_raise(obj);
+        // Re-check state is still pending now that we're in the atomic section.
+        if (MP_STATE_VM(sched_state) == MP_SCHED_PENDING) {
+            mp_obj_t obj = MP_STATE_VM(mp_pending_exception);
+            if (obj != MP_OBJ_NULL) {
+                MP_STATE_VM(mp_pending_exception) = MP_OBJ_NULL;
+                if (!mp_sched_num_pending()) {
+                    MP_STATE_VM(sched_state) = MP_SCHED_IDLE;
+                }
+                if (raise_exc) {
+                    MICROPY_END_ATOMIC_SECTION(atomic_state);
+                    nlr_raise(obj);
+                }
             }
+            mp_handle_pending_tail(atomic_state);
+        } else {
+            MICROPY_END_ATOMIC_SECTION(atomic_state);
         }
-        mp_handle_pending_tail(atomic_state);
     }
 }
 
-// This function should only be called be mp_sched_handle_pending,
+// This function should only be called by mp_handle_pending,
 // or by the VM's inlined version of that function.
 void mp_handle_pending_tail(mp_uint_t atomic_state) {
     MP_STATE_VM(sched_state) = MP_SCHED_LOCKED;

py/vm.c

@@ -1366,18 +1366,23 @@ pending_exception_check:
                 #if MICROPY_ENABLE_SCHEDULER
                 // This is an inlined variant of mp_handle_pending
                 if (MP_STATE_VM(sched_state) == MP_SCHED_PENDING) {
-                    MARK_EXC_IP_SELECTIVE();
                     mp_uint_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
-                    mp_obj_t obj = MP_STATE_VM(mp_pending_exception);
-                    if (obj != MP_OBJ_NULL) {
-                        MP_STATE_VM(mp_pending_exception) = MP_OBJ_NULL;
-                        if (!mp_sched_num_pending()) {
-                            MP_STATE_VM(sched_state) = MP_SCHED_IDLE;
+                    // Re-check state is still pending now that we're in the atomic section.
+                    if (MP_STATE_VM(sched_state) == MP_SCHED_PENDING) {
+                        MARK_EXC_IP_SELECTIVE();
+                        mp_obj_t obj = MP_STATE_VM(mp_pending_exception);
+                        if (obj != MP_OBJ_NULL) {
+                            MP_STATE_VM(mp_pending_exception) = MP_OBJ_NULL;
+                            if (!mp_sched_num_pending()) {
+                                MP_STATE_VM(sched_state) = MP_SCHED_IDLE;
+                            }
+                            MICROPY_END_ATOMIC_SECTION(atomic_state);
+                            RAISE(obj);
                         }
+                        mp_handle_pending_tail(atomic_state);
+                    } else {
                         MICROPY_END_ATOMIC_SECTION(atomic_state);
-                        RAISE(obj);
                     }
-                    mp_handle_pending_tail(atomic_state);
                 }
                 #else
                 // This is an inlined variant of mp_handle_pending