mutex.c

#include <linux/futex.h>
#include <syscall.h>
#include <time.h>
#include <unistd.h>

#include "mutex.h"

#define unlikely(x) __builtin_expect(!!(x), 0)

static int sys_futex(void *uaddr, int op, int val1, struct timespec *timeout, void *uaddr2,
                     int val3) {
    return syscall(SYS_futex, uaddr, op, val1, timeout, uaddr2, val3);
}

bool mutex_init(mutex *m) {
    *m = 0;
    return false;
}

bool mutex_trylock(mutex *m) {
    int expected = 0;
    return !atomic_compare_exchange_strong_explicit(m, &expected, 1, memory_order_acquire,
                                                    memory_order_relaxed);
}

void mutex_lock(mutex *m) {
    int expected = 0;
    if (unlikely(!atomic_compare_exchange_strong_explicit(m, &expected, 1, memory_order_acquire,
                                                          memory_order_relaxed))) {
        if (expected != 2) {
            expected = atomic_exchange_explicit(m, 2, memory_order_acquire);
        }
        while (expected) {
            sys_futex(m, FUTEX_WAIT_PRIVATE, 2, NULL, NULL, 0);
            expected = atomic_exchange_explicit(m, 2, memory_order_acquire);
        }
    }
}

void mutex_unlock(mutex *m) {
    if (unlikely(atomic_fetch_sub_explicit(m, 1, memory_order_release) != 1)) {
        atomic_store_explicit(m, 0, memory_order_release);
        sys_futex(m, FUTEX_WAKE_PRIVATE, 1, NULL, NULL, 0);
    }
}

mutex.h

#include <stdatomic.h>
#include <stdbool.h>

#define MUTEX_INITIALIZER 0
typedef atomic_int mutex;

bool mutex_init(mutex *m);
bool mutex_trylock(mutex *m);
void mutex_lock(mutex *m);
void mutex_unlock(mutex *m);