Convert to CV and spinup counter

Author: BMagnu

code/utils/threading.cpp

@@ -17,10 +17,19 @@
 
 namespace threading {
 	static size_t num_threads = 1;
+
 	static std::condition_variable wait_for_task;
 	static std::mutex wait_for_task_mutex;
 	static bool wait_for_task_condition;
-	static std::atomic_uint32_t wait_for_spindown_tasks;
+
+	static std::condition_variable wait_for_spindown_tasks;
+	static std::mutex wait_for_spindown_task_mutex;
+	static size_t wait_for_spindown_tasks_counter;
+
+	static std::condition_variable wait_for_spinup_tasks;
+	static std::mutex wait_for_spinup_task_mutex;
+	static size_t wait_for_spinup_tasks_counter;
+
 	static std::atomic<WorkerThreadTask> worker_task;
 
 	static SCP_vector<std::thread> worker_threads;
@@ -29,11 +38,21 @@ namespace threading {
 	static void mp_worker_thread_main(size_t threadIdx) {
 		while(true) {
 			{
-				//We're waiting for a new task, so spindown was successful
-				wait_for_spindown_tasks.fetch_add(1, std::memory_order_release);
+				std::scoped_lock lock {wait_for_spindown_task_mutex};
+				++wait_for_spindown_tasks_counter;
+				wait_for_spindown_tasks.notify_all();
+			}
+			//We're waiting for a new task, so spindown was successful
+			{
 				std::unique_lock<std::mutex> lk(wait_for_task_mutex);
 				wait_for_task.wait(lk, []() { return wait_for_task_condition; });
 			}
+			//Notify that we passed the wait and can now start processing. This is necessary, as slow thread wakeups in very low workloads could cause the wait_for_task_condition to be false, never waking up the thread, locking on spindown wait
+			{
+				std::scoped_lock lock {wait_for_spinup_task_mutex};
+				++wait_for_spinup_tasks_counter;
+				wait_for_spinup_tasks.notify_all();
+			}
 
 			switch (worker_task.load(std::memory_order_acquire)) {
 				case WorkerThreadTask::EXIT:
@@ -148,7 +167,11 @@ namespace threading {
 	//External Functions
 
 	void spin_up_threaded_task(WorkerThreadTask task) {
-		wait_for_spindown_tasks.store(0, std::memory_order_release);
+		{
+			std::scoped_lock lock {wait_for_spindown_task_mutex};
+			wait_for_spindown_tasks_counter = 0;
+			//No notify here cause we only ever lock, never unlock the wait here.
+		}
 		worker_task.store(task);
 		{
 			std::scoped_lock lock {wait_for_task_mutex};
@@ -158,13 +181,24 @@ namespace threading {
 	}
 
 	void spin_down_threaded_task() {
-		std::scoped_lock lock {wait_for_task_mutex};
-		wait_for_task_condition = false;
+		//Make sure all threads are actually running before stopping the pool again
+		{
+			std::unique_lock<std::mutex> lk(wait_for_spinup_task_mutex);
+			wait_for_spinup_tasks.wait(lk, []() { return wait_for_spinup_tasks_counter >= num_threads; });
+			wait_for_spinup_tasks_counter = 0;
+		}
+		{
+			std::scoped_lock lock {wait_for_task_mutex};
+			wait_for_task_condition = false;
+		}
 	}
 
 	void spin_down_wait_complete() {
 		//Technically, spindowns should only occur when the actual code is confirmed to be complete. So busy-waiting here is not an issue.
-		while (wait_for_spindown_tasks.load(std::memory_order_acquire) < num_threads);
+		{
+			std::unique_lock<std::mutex> lk(wait_for_spindown_task_mutex);
+			wait_for_spindown_tasks.wait(lk, []() { return wait_for_spindown_tasks_counter >= num_threads; });
+		};
 	}
 
 	void init_task_pool() {