How Concurrent GCD Queues Enable Real Parallelism and Data Races in Swift

In Apple's Grand Central Dispatch (GCD), combining a concurrent queue with async dispatch allows multiple tasks to run simultaneously on separate threads, with no guaranteed execution order. While the queue delivers tasks in FIFO order, the operating system scheduler determines when each thread actually starts, making task sequencing unpredictable. Nesting an async call inside a running closure on the same concurrent queue is safe and deadlock-free, since async never blocks the caller. However, when multiple independent tasks access shared mutable state concurrently without synchronization, data races can occur — for example, incrementing a shared counter 100 times may not yield a final value of 100. GCD tools such as DispatchBarrier and DispatchSemaphore are designed to address these race conditions and will be covered in follow-up articles.