先来先服务（FCFS）调度可避免死锁吗？使用该调度能否确保系统无死锁？

Great question—let's unpack this step by step, since it's easy to mix up scheduling algorithms with deadlock prevention strategies.

First, remember that deadlocks occur when four necessary conditions are all met at the same time:

1. Mutual Exclusion: Resources can't be shared (only one process uses them at a time)
2. Hold and Wait: A process holds existing resources while waiting for new ones
3. No Preemption: Resources can't be taken from a process until it voluntarily releases them
4. Circular Wait: A chain of processes each waits for a resource held by the next in the chain

Now, let's look at how FCFS interacts with these conditions:

• FCFS doesn't eliminate mutual exclusion: This is a property of the resources themselves (like a printer or a critical section), not the scheduling algorithm. FCFS doesn't change how resources are accessed exclusively.
• FCFS doesn't fix hold-and-wait: Processes scheduled via FCFS can still hold onto resources they've already acquired while waiting for additional ones. There's nothing in FCFS that prevents this behavior.
• FCFS relies on no preemption (by default): FCFS is a non-preemptive algorithm—once a process starts running, it keeps going until it finishes or voluntarily blocks. This means the "no preemption" condition for deadlocks remains satisfied.
• FCFS alone doesn't break circular wait: The only way FCFS could help here is if you pair it with a resource ordering rule (e.g., all processes must request resources in the same fixed order). But that's an extra strategy, not part of the core FCFS scheduling logic. Without that, processes can still request resources in a way that creates a circular wait chain.

Example of Deadlock with FCFS #

Let's say we have two processes:

• Process 1: Requests Resource A first, then Resource B
• Process 2: Requests Resource B first, then Resource A

With FCFS scheduling, if Process 1 starts first and grabs Resource A, then Process 2 starts and grabs Resource B—both will now wait indefinitely for the other's resource. Deadlock occurs, even with FCFS in place.

Final Takeaway #

FCFS scheduling alone cannot guarantee that a system won't enter a deadlock state. It's a CPU scheduling algorithm that determines the order in which processes run, but it doesn't address the root conditions that cause deadlocks. To avoid deadlocks, you need to combine FCFS with additional strategies like:

• Enforcing a fixed resource request order (to break circular wait)
• Using the Banker's Algorithm (deadlock avoidance)
• Implementing resource preemption (though this contradicts FCFS's non-preemptive nature)

内容的提问来源于stack exchange，提问作者Eduardo Humberto