Untested mutex implementation on bare-metal Cortex M4 (and maybe other Cortexes with LDREX/STREX)

mutex.c

#include "mutex.h"

/**
 * Simple mutex implemented using LDREX/STREX, as described in
 * https://developer.arm.com/documentation/dht0008/a/arm-synchronization-primitives/practical-uses/implementing-a-mutex.
 */

#include "stm32l476xx.h"

static const mutex_t MUTEX_UNLOCKED = 0;
static const mutex_t MUTEX_LOCKED = 1;

bool mutex_try_lock(volatile mutex_t *mutex) {
	if (__LDREXB(mutex) == MUTEX_LOCKED)
		return false;

	/* Try to change mutex value to locked */
	int strex_status = __STREXB(MUTEX_LOCKED, mutex);

	__DMB();

	/* strex_status == 0 when the store is successful, 1 otherwise */
	return !strex_status;
}

void mutex_unlock(volatile mutex_t *mutex) {
	__DMB();

	*mutex = MUTEX_UNLOCKED;
}

mutex.h

#ifndef MUTEX_H
#define MUTEX_H

#include <stdbool.h>
#include <stdint.h>

typedef uint8_t mutex_t;

/**
 * Attempt to acquire (or lock) a mutex.
 * @param mutex the mutex to acquire.
 * @return true if the mutex is successfully acquired, false otherwise
 *         (for example, if the mutex is already acquired)
 */
bool mutex_try_lock(volatile mutex_t *mutex);

/**
 * Unlock a mutex. This will always unlock a mutex, even if the mutex is not locked.
 * @param mutex the mutex to unlock.
 */
void mutex_unlock(volatile mutex_t *mutex);

#endif