ESP32 Forum

The priority of a task that ‘takes’ a mutex will be temporarily raised if another task of higher priority attempts to obtain the same mutex. The task that owns the mutex ‘inherits’ the priority of the task attempting to ‘take’ the same mutex. This means the mutex must always be ‘given’ back – otherwise the higher priority task will never be able to obtain the mutex, and the lower priority task will never ‘disinherit’ the priority.

#include <stdio.h>

#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "freertos/semphr.h"

static SemaphoreHandle_t sem;

static void take(void* pv) {
	while (true) {
		printf("take %d\n", xSemaphoreTake(sem, portMAX_DELAY) == pdTRUE);
		vTaskDelay(pdMS_TO_TICKS(1000));
	}
}

static void give(void* pv) {
	while (true) {
		printf("give %d\n", xSemaphoreGive(sem) == pdTRUE);
		vTaskDelay(pdMS_TO_TICKS(1000));
	}
}

void app_main() {
	sem = xSemaphoreCreateMutex();
	xTaskCreate(take, "take", 7 * 1024, NULL, tskIDLE_PRIORITY, NULL);
	xTaskCreate(give, "give", 7 * 1024, NULL, tskIDLE_PRIORITY + 1, NULL);
}

I (237) cpu_start: Starting scheduler on PRO CPU.
give 0
take 1
give 1
take 1
give 1
take 1
give 1

WiFive wrote: ↑

Tue Dec 24, 2019 9:28 pm

https://github.com/espressif/esp-idf/bl ... ead.c#L699

ESP_Sprite wrote: ↑

Wed Dec 25, 2019 12:59 am

I think the 'contract' is more a logical result of the way you should use mutexes rather than something that is hardcoded. Mutexes are intended to protect resource access, so normally, a task would take a mutex, do the resource access, then give it back. As a give is always preceded by a take, and by definition two tasks cannot be in the code between a give and take at the same time, the give is always executed on the same task as the take.

ResourceLock CANNOT be a mutex. In FreeRTOS, as in most OS's, a thread is not allowed to unlock another thread's mutex. But the very nature of a Reader Lock allows an arbitrary ordering of unlocks when multiple threads hold the reader lock. Semaphores are not subject to this constraint.