From bb5fbf19cf98e44d2c7d43a42ab9025d996e0ac9 Mon Sep 17 00:00:00 2001
From: Ulrich Mohr <mohr@irs.uni-stuttgart.de>
Date: Tue, 12 Aug 2025 15:10:56 +0200
Subject: [PATCH] once without global mutex

---
 mission/freeRTOS_rust_helper.c | 61 +++++++++++++---------------------
 1 file changed, 24 insertions(+), 37 deletions(-)

diff --git a/mission/freeRTOS_rust_helper.c b/mission/freeRTOS_rust_helper.c
index 08bd8d3..96db555 100644
--- a/mission/freeRTOS_rust_helper.c
+++ b/mission/freeRTOS_rust_helper.c
@@ -9,23 +9,11 @@
 // TODO panic if able, but not in runtime calls
 
 
-StaticSemaphore_t global_once_mutex_data;
-SemaphoreHandle_t global_once_mutex = NULL;
-
-void freertos_init_once() {
-  global_once_mutex =
-      xSemaphoreCreateRecursiveMutexStatic(&global_once_mutex_data);
-  if (global_once_mutex == NULL) {
-    // TODO panic
-  }
-}
-
-
 // Wraps xTaskGetCurrentTaskHandle and returns NULL if no Task is running
 //
 // xTaskGetCurrentTaskHandle() will return a handle even if no task is
 // running as long as one has been created already.
-void *freertos_task_current() {
+void *freertos_task_current(void) {
   // If scheduler is not running, xTaskGetCurrentTaskHandle() might return a
   // valid handle of a already created task, so we check for Scheduler state
   // before calling it
@@ -42,7 +30,6 @@ StaticTask_t init_task_data;
 StackType_t init_task_stack[configMINIMAL_STACK_SIZE * 10];
 
 void freertos_init_and_start_scheduling(TaskFunction_t init_task) {
-  freertos_init_once();
   // TaskHandle_t handle =
       xTaskCreateStatic(init_task, "c_init", configMINIMAL_STACK_SIZE * 10,
                         NULL,  configMAX_PRIORITIES - 1, init_task_stack, &init_task_data);
@@ -166,8 +153,8 @@ typedef struct {
 /**
  * mimics pthread_once()
  *
- * uses a global mutex to threadsafely check if the passed local mutex
- * was created already, creating it if not.
+ * uses a mutex to guard the state, to allow threads to block waiting
+ * until function has been called.
  *
  * After that, the local mutex guards once_data, which is used to only
  * call the function once. All calls sharing the local_mutex and
@@ -181,6 +168,9 @@ typedef struct {
  */
 uint8_t freertos_once(char *once_data_in, uint32_t once_data_len,
                       void *function(void)) {
+  if (once_data_in == NULL) {
+    return 0;
+  }
   if (once_data_len < sizeof(StaticOnceData_t)) {
     // TODO panic
     // printf("freertos_once: data needs to be %zu long\n",
@@ -192,21 +182,22 @@ uint8_t freertos_once(char *once_data_in, uint32_t once_data_len,
   // TODO assert global_once_mutex != NULL
 
   // first, we need to check if the local mutex was already created
-  // this needs to be protected by a mutex to be threadsafe
-  if (xSemaphoreTakeRecursive(global_once_mutex, portMAX_DELAY) != pdTRUE) {
-    // TODO panic
-  }
+  // this needs to be protected to be threadsafe
+  // Documentation says that we must not call APIs from within a critical
+  // Section but I think we are safe in the case of creating a mutex
+  // which basically only intializes the fields
+  // TODO verify?
+  taskENTER_CRITICAL();
   if (once_data->once_state == UNINIT) {
     once_data->local_mutex =
         xSemaphoreCreateRecursiveMutexStatic(&once_data->local_mutex_data);
-    if (once_data->local_mutex == NULL) {
-      // TODO panic
-    }
+    // if (once_data->local_mutex == NULL) {
+    //   // will not happen, xSemaphoreCreateRecursiveMutexStatic returns its parameter
+    //   // which we checked above to be != NULL
+    // }
     once_data->once_state = INIT;
   }
-  if (xSemaphoreGiveRecursive(global_once_mutex) != pdTRUE) {
-    // TODO panic
-  }
+  taskEXIT_CRITICAL();
 
   // Now, we know local mutex is valid, so we use it to guard access to
   // the once_state
@@ -241,24 +232,20 @@ uint8_t freertos_simple_once(uint8_t *once_data) {
   // TODO assert global_once_mutex != NULL
 
   uint8_t result = 0;
-  // Todo: Replace Mutex with critical section
-  // This function is basically a flag stored in once_data, protected by the
-  // global_once_mutex
-  if (xSemaphoreTakeRecursive(global_once_mutex, portMAX_DELAY) != pdTRUE) {
-    // TODO panic
-  }
+  // This function is basically a flag stored in once_data, protected by
+  // a critical section
+  // Critical section is ok, because simple arithmetic is bounded in execution time
+  taskENTER_CRITICAL();
   if (*once_data == 0) {
     *once_data = 1;
     result = 1;
   }
-  if (xSemaphoreGiveRecursive(global_once_mutex) != pdTRUE) {
-    // TODO panic
-  }
+  taskEXIT_CRITICAL();
 
   return result;
 }
 
-uint32_t freertos_task_priority_max() {
+uint32_t freertos_task_priority_max(void) {
   // -1 is the max to be used per documentation
   return configMAX_PRIORITIES - 1;
 }
@@ -301,4 +288,4 @@ void *freertos_mutex_create_static(char *mutex_data, uint32_t mutex_data_len) {
     return NULL;
   }
   return xSemaphoreCreateRecursiveMutexStatic((StaticSemaphore_t *)mutex_data);
-}
\ No newline at end of file
+}