GCD串行队列同步疑问：是否需NSLock或内存屏障？修改NSMutableArray需注意什么？

1. Do GCD serial queues require NSLock or memory barriers for work synchronization? #

Nope, you don’t need to add explicit NSLock or manual memory barriers when using a GCD serial queue. Here’s why:

• Serial queues guarantee that tasks execute one after another, with no overlap. The next task won’t start until the previous one has fully completed.
• GCD automatically handles memory visibility for you. Any memory changes made by a task in the serial queue are guaranteed to be visible to subsequent tasks in the same queue. This is because GCD inserts implicit memory barriers between task executions, ensuring all writes from the prior task are committed to memory before the next task runs.

In short, the serial queue itself provides all the synchronization and memory ordering you need for tasks submitted to it.

2. What happens when modifying a non-thread-safe NSMutableArray in a serial queue, and do we need @synchronized or memory barriers if tasks run on different threads? #

Let’s break this down clearly:

• First, NSMutableArray is thread-unsafe because it can’t handle concurrent reads/writes. But serial queues eliminate the "concurrent" risk entirely. Even if tasks run on different threads (GCD often reuses or switches threads for serial queue tasks), only one task is executing at any given time. There’s never a scenario where two tasks are modifying or reading the array simultaneously.
• Because each task finishes completely before the next starts, all memory changes to the array from one task are fully visible to the next task—again, thanks to GCD’s implicit memory barriers. You don’t need @synchronized, NSLock, or manual memory barriers here.

The critical caveat: This only holds true if all accesses and modifications to the NSMutableArray go through the same serial queue. If you have code outside this queue reading or modifying the array directly, you’ll still run into thread-safety issues. Stick to routing every interaction with the array through the serial queue, and you’re good to go.

内容的提问来源于stack exchange，提问作者Rudolfs Bundulis