Fix race condition in resource loader when a load task is reused

core/io/resource_loader.cpp

@@ -33,6 +33,7 @@
 #include "core/config/project_settings.h"
 #include "core/io/file_access.h"
 #include "core/io/resource_importer.h"
+#include "core/os/condition_variable.h"
 #include "core/os/os.h"
 #include "core/string/print_string.h"
 #include "core/string/translation.h"
@@ -233,7 +234,7 @@ void ResourceLoader::_thread_load_function(void *p_userdata) {
 	ThreadLoadTask &load_task = *(ThreadLoadTask *)p_userdata;
 	load_task.loader_id = Thread::get_caller_id();
 
-	if (load_task.semaphore) {
+	if (load_task.cond_var) {
 		//this is an actual thread, so wait for Ok from semaphore
 		thread_load_semaphore->wait(); //wait until its ok to start loading
 	}
@@ -247,7 +248,7 @@ void ResourceLoader::_thread_load_function(void *p_userdata) {
 	} else {
 		load_task.status = THREAD_LOAD_LOADED;
 	}
-	if (load_task.semaphore) {
+	if (load_task.cond_var) {
 		if (load_task.start_next && thread_waiting_count > 0) {
 			thread_waiting_count--;
 			//thread loading count remains constant, this ends but another one begins
@@ -258,11 +259,9 @@ void ResourceLoader::_thread_load_function(void *p_userdata) {
 
 		print_lt("END: load count: " + itos(thread_loading_count) + " / wait count: " + itos(thread_waiting_count) + " / suspended count: " + itos(thread_suspended_count) + " / active: " + itos(thread_loading_count - thread_suspended_count));
 
-		for (int i = 0; i < load_task.poll_requests; i++) {
-			load_task.semaphore->post();
-		}
-		memdelete(load_task.semaphore);
-		load_task.semaphore = nullptr;
+		load_task.cond_var->notify_all();
+		memdelete(load_task.cond_var);
+		load_task.cond_var = nullptr;
 	}
 
 	if (load_task.resource.is_valid()) {
@@ -373,7 +372,7 @@ Error ResourceLoader::load_threaded_request(const String &p_path, const String &
 
 	if (load_task.resource.is_null()) { //needs to be loaded in thread
 
-		load_task.semaphore = memnew(Semaphore);
+		load_task.cond_var = memnew(ConditionVariable);
 		if (thread_loading_count < thread_load_max) {
 			thread_loading_count++;
 			thread_load_semaphore->post(); //we have free threads, so allow one
@@ -438,9 +437,8 @@ ResourceLoader::ThreadLoadStatus ResourceLoader::load_threaded_get_status(const
 Ref<Resource> ResourceLoader::load_threaded_get(const String &p_path, Error *r_error) {
 	String local_path = _validate_local_path(p_path);
 
-	thread_load_mutex->lock();
+	MutexLock thread_load_lock(*thread_load_mutex);
 	if (!thread_load_tasks.has(local_path)) {
-		thread_load_mutex->unlock();
 		if (r_error) {
 			*r_error = ERR_INVALID_PARAMETER;
 		}
@@ -449,13 +447,10 @@ Ref<Resource> ResourceLoader::load_threaded_get(const String &p_path, Error *r_e
 
 	ThreadLoadTask &load_task = thread_load_tasks[local_path];
 
-	//semaphore still exists, meaning it's still loading, request poll
-	Semaphore *semaphore = load_task.semaphore;
-	if (semaphore) {
-		load_task.poll_requests++;
-
+	//cond var still exists, meaning it's still loading, request poll
+	if (load_task.cond_var) {
 		{
-			// As we got a semaphore, this means we are going to have to wait
+			// As we got a cond var, this means we are going to have to wait
 			// until the sub-resource is done loading
 			//
 			// As this thread will become 'blocked' we should "exchange" its
@@ -477,14 +472,13 @@ Ref<Resource> ResourceLoader::load_threaded_get(const String &p_path, Error *r_e
 			print_lt("GET: load count: " + itos(thread_loading_count) + " / wait count: " + itos(thread_waiting_count) + " / suspended count: " + itos(thread_suspended_count) + " / active: " + itos(thread_loading_count - thread_suspended_count));
 		}
 
-		thread_load_mutex->unlock();
-		semaphore->wait();
-		thread_load_mutex->lock();
+		do {
+			load_task.cond_var->wait(thread_load_lock);
+		} while (load_task.cond_var); // In case of spurious wakeup.
 
 		thread_suspended_count--;
 
 		if (!thread_load_tasks.has(local_path)) { //may have been erased during unlock and this was always an invalid call
-			thread_load_mutex->unlock();
 			if (r_error) {
 				*r_error = ERR_INVALID_PARAMETER;
 			}
@@ -507,8 +501,6 @@ Ref<Resource> ResourceLoader::load_threaded_get(const String &p_path, Error *r_e
 		thread_load_tasks.erase(local_path);
 	}
 
-	thread_load_mutex->unlock();
-
 	return resource;
 }
 
@@ -1067,7 +1059,7 @@ void ResourceLoader::remove_custom_loaders() {
 }
 
 void ResourceLoader::initialize() {
-	thread_load_mutex = memnew(Mutex);
+	thread_load_mutex = memnew(SafeBinaryMutex<BINARY_MUTEX_TAG>);
 	thread_load_max = OS::get_singleton()->get_processor_count();
 	thread_loading_count = 0;
 	thread_waiting_count = 0;
@@ -1090,7 +1082,9 @@ bool ResourceLoader::create_missing_resources_if_class_unavailable = false;
 bool ResourceLoader::abort_on_missing_resource = true;
 bool ResourceLoader::timestamp_on_load = false;
 
-Mutex *ResourceLoader::thread_load_mutex = nullptr;
+template <>
+thread_local uint32_t SafeBinaryMutex<ResourceLoader::BINARY_MUTEX_TAG>::count = 0;
+SafeBinaryMutex<ResourceLoader::BINARY_MUTEX_TAG> *ResourceLoader::thread_load_mutex = nullptr;
 HashMap<String, ResourceLoader::ThreadLoadTask> ResourceLoader::thread_load_tasks;
 Semaphore *ResourceLoader::thread_load_semaphore = nullptr;
 

core/io/resource_loader.h

@@ -37,6 +37,8 @@
 #include "core/os/semaphore.h"
 #include "core/os/thread.h"
 
+class ConditionVariable;
+
 class ResourceFormatLoader : public RefCounted {
 	GDCLASS(ResourceFormatLoader, RefCounted);
 
@@ -105,6 +107,8 @@ public:
 		THREAD_LOAD_LOADED
 	};
 
+	static const int BINARY_MUTEX_TAG = 1;
+
 private:
 	static Ref<ResourceFormatLoader> loader[MAX_LOADERS];
 	static int loader_count;
@@ -136,7 +140,7 @@ private:
 	struct ThreadLoadTask {
 		Thread *thread = nullptr;
 		Thread::ID loader_id = 0;
-		Semaphore *semaphore = nullptr;
+		ConditionVariable *cond_var = nullptr;
 		String local_path;
 		String remapped_path;
 		String type_hint;
@@ -149,12 +153,11 @@ private:
 		bool use_sub_threads = false;
 		bool start_next = true;
 		int requests = 0;
-		int poll_requests = 0;
 		HashSet<String> sub_tasks;
 	};
 
 	static void _thread_load_function(void *p_userdata);
-	static Mutex *thread_load_mutex;
+	static SafeBinaryMutex<BINARY_MUTEX_TAG> *thread_load_mutex;
 	static HashMap<String, ThreadLoadTask> thread_load_tasks;
 	static Semaphore *thread_load_semaphore;
 	static int thread_waiting_count;