Read/Write lock

rwlock.c

struct rwlock {
  volatile uint32_t state;
};

static void __rwlock_init (struct rwlock *lock) {
  lock->state = 0;
}

static void __read_lock (struct rwlock *lock) {
  unsigned int new_state;
  unsigned int expected;
  do {
    expected = lock->state & 0xffff;  /* I expect no write lock */
    new_state = expected + 1;         /* Add me as reader */
  } while (!__sync_bool_compare_and_swap(&(lock->state), expected, new_state));
}

static void __read_unlock (struct rwlock *lock) {
  unsigned int new_state;
  unsigned int expected;
  do {
    expected = lock->state;     /* I expect a write lock and other readers */
    new_state = expected - 1;   /* Drop me as reader */
  } while (!__sync_bool_compare_and_swap(&(lock->state), expected, new_state));
}

static void __write_lock (struct rwlock *lock) {
  unsigned int new_state;
  unsigned int expected;
  do {
    expected = lock->state & 0xffff;  /* I expect some 0+ readers */
    new_state = (1 << 16) | expected; /* I want to lock the other writers */
  } while (!__sync_bool_compare_and_swap(&(lock->state), expected, new_state));

  /* Wait pending reads */
  while ((lock->state & 0xffff) > 0) /* cpu_relax(); */;
}

static void __write_unlock (struct rwlock *lock) {
  unsigned int new_state;
  unsigned int expected;
  do {
    expected = 1 << 16; /* I expect to be the only writer */
    new_state = 0;      /* reset: no writers/no readers */
  } while (!__sync_bool_compare_and_swap(&(lock->state), expected, new_state));
}