Spinlock

Repeatedly test the lock’s availability until lock is obtained

Efficient for short waiting times

Example

mutex_lock(bool *mutex) {
	while (xchg(mutex, true) == true) {}; //test and set
}

mutex_unlock(bool *mutex) {
	*mutex = false;
}

Thread busy-waits in a loop until mutex is acquired

Interrupts are disabled on the thread that holds a spinlock

Thread will only execute a few instructions before releasing spinlock

Implementation in CasterOS

void Spinlock_Lock(Spinlock *lock)
{
	/* Disable Interrupts */
	Critical_Enter();
	while (atomic_swap_uint64(&lock->lock, 1) == 1)
	/* Spin! */;
	lock->cpu = CPU();
}
void Spinlock_Unlock(Spinlock *lock)
{
	ASSERT(lock->cpu == CPU());
	atomic_set_uint64(&lock->lock, 0);
	/* Re-enable Interrupts (if not spinlocks held) */
	Critical_Exit();
}