basic implementation of a no-block / no-wait mutex for ARM Cortex M3 and compatible models

arm_cortex_m3_mutex.c

/*!
 * Cortex M3 mutex implementation.
 * Ported to gcc by Stuart Longland <stuartl@vrt.com.au>
 *
 * Credit: https://gist.github.com/hdznrrd/4032002
 */

#ifndef _CM3MUTEX_H
#define _CM3MUTEX_H

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

/*
 * This is a very basic mutex implementation.
 * Its aim is to try to acquire access to a critical section or fail.
 * There is no spinlock or event-waiting going on.
 * In case it fails to acquire the lock, it simply returns false.
 *
 * relevant read:
 *
 * Synchronization primitives
 * http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0552a/BABHCIHB.html
 *
 * LDREX and STREX
 * http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0552a/BABFFBJB.html
 *
 * In what situations might I need to insert memory barrier instructions?
 * http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka14041.html
 */

/*!
 * Attempt to acquire a mutex.  This operation is non-blocking.
 *
 * @param[inout]	lock	Mutex lock object
 * @retval		true	Mutex acquired
 * @retval		false	Mutex already acquired
 */
static inline _Bool mutex_acquire_asm(volatile uint8_t* lock) {
	volatile uint8_t success;
	__asm volatile(
		/* Assembly routine template */
		"          MOV      %[success], #1  ;\n" /* initialize success false */
		"          MOV      R4, #1          ;\n" /* initialise our target lock state */
		"          LDREXB   %[success], "
					"[%[lock]]  ;\n" /* Load the lock value */
		"          CMP      %[success], #0  ;\n" /* Is the lock free? */

		"          ITTT     EQ              ;\n" /* IF ( EQUAL ) THEN { STREX, CMP, DMB } */
		"          STREXBEQ %[success], R4, "
					"[%[lock]]  ;\n" /* Try and claim the lock */
		"          CMPEQ    %[success], #0  ;\n" /* Did this succeed? */
		"          DMBEQ                    ;\n" /* data memory barrier before
							    accessing restricted resource */
		/* Outputs */
		: [success] "=r" (success),	/* =r: "we write to this register" */
		  [lock] "+r" (lock)		/* +r: "we write to *and read from* this register */
		/* Inputs */
		:
		/* Clobber list: this stuff is changed by our code */
		: "memory", "r4" );

	return success == 0;
}

/*!
 * Release a mutex.
 *
 * @param[inout]	lock	Mutex lock object
 * @retval		true	Mutex released
 * @retval		false	Mutex already released
 */
static inline _Bool mutex_release_asm(volatile uint8_t* lock) {
	volatile uint8_t success;
	__asm volatile(
		/* Assembly routine template */
		"          MOV      %[success], #1  ;\n" /* initialize success false */
		"          MOV      R4, #0          ;\n" /* initialise our target lock state */
		"          LDREXB   %[success], "
					"[%[lock]]  ;\n" /* Load the lock value */
		"          CMP      %[success], #1  ;\n" /* Is the lock in use? */

		"          ITTT     EQ              ;\n" /* IF ( EQUAL ) THEN { STREX, CMP, DMB } */
		"          STREXBEQ %[success], R4, "
					"[%[lock]]  ;\n" /* Try and release the lock */
		"          CMPEQ    %[success], #0  ;\n" /* Did this succeed? */
		"          DMBEQ                    ;\n" /* data memory barrier before
							    accessing restricted resource */
		/* Outputs */
		: [success] "=r" (success),	/* =r: "we write to this register" */
		  [lock] "+r" (lock)		/* +r: "we write to *and read from* this register */
		/* Inputs */
		:
		/* Clobber list: this stuff is changed by our code */
		: "memory", "r4" );

	return success == 0;
}

#endif