taskENTER_CRITICAL() and priority higher interrupt in FreeRTOS application

Embedded Linux and RTOS
1

In my below FreeRTOS code, the global variable command_buffer is protected from altering by using critical API ( taskENTER_CRITICAL() and taskEXIT_CRITICAL() API). Is it possible that any interrupts with a priority, say 4 can still access this global variable?

void command_handling_task(void *params) {
      
	while(1) {
 
		xTaskNotifyWait(0,0,NULL,portMAX_DELAY);
 
		taskENTER_CRITICAL();
		new_cmd = parseCMD(command_buffer);
		taskEXIT_CRITICAL();
 
		xQueueSendToFront(command_queue_handle, &new_cmd, portMAX_DELAY);
	}

}
2

taskENTER_CRITICAL implementation varies depending on the port. In ARM_CM4 port, it raises BASEPRI (does not disable all interrupts). Interrupts below (interrupt priority is numerically opposite in ARM devices) configMAX_SYSCALL_INTERRUPT_PRIORITY (BASEPRI value) will be in a pending state until you come out of the critical section and all high priority interrupts have been executed.

interrupt-priorities-interrupt-nesting

1 Like
3

Lets assume configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY = 5, and still any interrupt with higest priority (NVIC_SetPriority(USART2_IRQn, 0)) can access any global variable in between taskENTER_CRITICAL() and taskEXIT_CRITICAL(). Am I correct??

1 Like
4

Yes, you are correct. FreeRTOS does not disable all interrupts to avoid increasing interrupt latency.

You can keep a breakpoint at the critical section, and another inside an interrupt to analyse this yourself.

2 Likes
5

Now I get it. Thank you.