Dispatch Queues and Thread Safety

Dispatch Queues and Thread Safety

It might seem odd to talk about thread safety in the context of dispatch queues, but thread safety is still a relevant topic. Any time you are implementing concurrency in your application, there are a few things you should know: 

• Dispatch queues themselves are thread safe. In other words, you can submit tasks to a dispatch queue from any thread on the system without first taking a lock or synchronizing access to the queue.

• Do not call the dispatch_sync function from a task that is executing on the same queue that you pass to your function call. Doing so will deadlock the queue. If you need to dispatch to the current queue, do so asynchronously using the dispatch_async function. 

• Avoid taking locks from the tasks you submit to a dispatch queue. Although it is safe to use locks from your tasks, when you acquire the lock, you risk blocking a serial queue entirely if that lock is unavailable. Similarly, for concurrent queues, waiting on a lock might prevent other tasks from executing instead. If you need to synchronize parts of your code, use a serial dispatch queue instead of a lock.

• Although you can obtain information about the underlying thread running a task, it is better to avoid doing so. For more information about the compatibility of dispatch queues with threads, see Compatibility with POSIX Threads.

https://developer.apple.com/library/archive/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW28