Cooperative multitasking means that unless a running fiber deliberately yields control, it is not preempted by some other fiber. But a running fiber will deliberately yield when it encounters a “yield point”: a transaction commit, an operating system call, or an explicit «yield» request. Any system call which can block will be performed asynchronously, and any running fiber which must wait for a system call will be preempted, so that another ready-to-run fiber takes its place and becomes the new running fiber.
This model makes all programmatic locks unnecessary: cooperative multitasking ensures that there will be no concurrency around a resource, no race conditions, and no memory consistency issues. The way to achieve this is simple: Use no yields, explicit or implicit in critical sections, and no one can interfere with code execution.
For small requests, such as simple UPDATE or INSERT or DELETE or SELECT, fiber scheduling is fair: it takes little time to process the request, schedule a disk write, and yield to a fiber serving the next client.
However, a function may perform complex calculations or be written in such a way that yields take a long time to occur. This can lead to unfair scheduling when a single client throttles the rest of the system, or to apparent stalls in processing requests. It is the responsibility of the function author to avoid this situation.