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WorkerThreadPool (plus friends): Overhaul unlock allowance zones 

This fixes a rare but possible deadlock, maybe due to undefined behavior. The new implementation is safer, at the cost of some added boilerplate.
This commit is contained in:
Pedro J. Estébanez 2024-07-18 14:54:58 +02:00
parent df23858488
commit f4d76853b9
12 changed files with 118 additions and 110 deletions
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2
SConstruct
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@ -861,7 +861,7 @@ else: # GCC, Clang
if cc_version_major >= 11: # Broke on MethodBind templates before GCC 11.
env.Append(CCFLAGS=["-Wlogical-op"])
elif methods.using_clang(env) or methods.using_emcc(env):
env.Append(CCFLAGS=["-Wimplicit-fallthrough"])
env.Append(CCFLAGS=["-Wimplicit-fallthrough", "-Wno-undefined-var-template"])
elif env["warnings"] == "all":
env.Append(CCFLAGS=["-Wall"] + common_warnings)
elif env["warnings"] == "moderate":

6
core/io/resource_loader.cpp
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@ -1357,8 +1357,12 @@ thread_local int ResourceLoader::load_nesting = 0;
thread_local Vector<String> ResourceLoader::load_paths_stack;
thread_local HashMap<int, HashMap<String, Ref<Resource>>> ResourceLoader::res_ref_overrides;
SafeBinaryMutex<ResourceLoader::BINARY_MUTEX_TAG> &_get_res_loader_mutex() {
return ResourceLoader::thread_load_mutex;
}
template <>
thread_local uint32_t SafeBinaryMutex<ResourceLoader::BINARY_MUTEX_TAG>::count = 0;
thread_local SafeBinaryMutex<ResourceLoader::BINARY_MUTEX_TAG>::TLSData SafeBinaryMutex<ResourceLoader::BINARY_MUTEX_TAG>::tls_data(_get_res_loader_mutex());
SafeBinaryMutex<ResourceLoader::BINARY_MUTEX_TAG> ResourceLoader::thread_load_mutex;
HashMap<String, ResourceLoader::ThreadLoadTask> ResourceLoader::thread_load_tasks;
bool ResourceLoader::cleaning_tasks = false;

3
core/io/resource_loader.h
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@ -194,7 +194,10 @@ private:
static thread_local int load_nesting;
static thread_local HashMap<int, HashMap<String, Ref<Resource>>> res_ref_overrides; // Outermost key is nesting level.
static thread_local Vector<String> load_paths_stack;
static SafeBinaryMutex<BINARY_MUTEX_TAG> thread_load_mutex;
friend SafeBinaryMutex<BINARY_MUTEX_TAG> &_get_res_loader_mutex();
static HashMap<String, ThreadLoadTask> thread_load_tasks;
static bool cleaning_tasks;

60
core/object/worker_thread_pool.cpp
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@ -32,6 +32,7 @@
#include "core/object/script_language.h"
#include "core/os/os.h"
#include "core/os/safe_binary_mutex.h"
#include "core/os/thread_safe.h"
WorkerThreadPool::Task *const WorkerThreadPool::ThreadData::YIELDING = (Task *)1;
@ -46,7 +47,7 @@ void WorkerThreadPool::Task::free_template_userdata() {
WorkerThreadPool *WorkerThreadPool::singleton = nullptr;
#ifdef THREADS_ENABLED
thread_local uintptr_t WorkerThreadPool::unlockable_mutexes[MAX_UNLOCKABLE_MUTEXES] = {};
thread_local WorkerThreadPool::UnlockableLocks WorkerThreadPool::unlockable_locks[MAX_UNLOCKABLE_LOCKS];
#endif
void WorkerThreadPool::_process_task(Task *p_task) {
@ -428,13 +429,9 @@ Error WorkerThreadPool::wait_for_task_completion(TaskID p_task_id) {
void WorkerThreadPool::_lock_unlockable_mutexes() {
#ifdef THREADS_ENABLED
for (uint32_t i = 0; i < MAX_UNLOCKABLE_MUTEXES; i++) {
if (unlockable_mutexes[i]) {
if ((((uintptr_t)unlockable_mutexes[i]) & 1) == 0) {
((Mutex *)unlockable_mutexes[i])->lock();
} else {
((BinaryMutex *)(unlockable_mutexes[i] & ~1))->lock();
}
for (uint32_t i = 0; i < MAX_UNLOCKABLE_LOCKS; i++) {
if (unlockable_locks[i].ulock) {
unlockable_locks[i].ulock->lock();
}
}
#endif
@ -442,13 +439,9 @@ void WorkerThreadPool::_lock_unlockable_mutexes() {
void WorkerThreadPool::_unlock_unlockable_mutexes() {
#ifdef THREADS_ENABLED
for (uint32_t i = 0; i < MAX_UNLOCKABLE_MUTEXES; i++) {
if (unlockable_mutexes[i]) {
if ((((uintptr_t)unlockable_mutexes[i]) & 1) == 0) {
((Mutex *)unlockable_mutexes[i])->unlock();
} else {
((BinaryMutex *)(unlockable_mutexes[i] & ~1))->unlock();
}
for (uint32_t i = 0; i < MAX_UNLOCKABLE_LOCKS; i++) {
if (unlockable_locks[i].ulock) {
unlockable_locks[i].ulock->unlock();
}
}
#endif
@ -675,37 +668,28 @@ WorkerThreadPool::TaskID WorkerThreadPool::get_caller_task_id() {
}
#ifdef THREADS_ENABLED
uint32_t WorkerThreadPool::thread_enter_unlock_allowance_zone(Mutex *p_mutex) {
return _thread_enter_unlock_allowance_zone(p_mutex, false);
}
uint32_t WorkerThreadPool::thread_enter_unlock_allowance_zone(BinaryMutex *p_mutex) {
return _thread_enter_unlock_allowance_zone(p_mutex, true);
}
uint32_t WorkerThreadPool::_thread_enter_unlock_allowance_zone(void *p_mutex, bool p_is_binary) {
for (uint32_t i = 0; i < MAX_UNLOCKABLE_MUTEXES; i++) {
if (unlikely((unlockable_mutexes[i] & ~1) == (uintptr_t)p_mutex)) {
uint32_t WorkerThreadPool::_thread_enter_unlock_allowance_zone(THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &p_ulock) {
for (uint32_t i = 0; i < MAX_UNLOCKABLE_LOCKS; i++) {
DEV_ASSERT((bool)unlockable_locks[i].ulock == (bool)unlockable_locks[i].rc);
if (unlockable_locks[i].ulock == &p_ulock) {
// Already registered in the current thread.
return UINT32_MAX;
}
if (!unlockable_mutexes[i]) {
unlockable_mutexes[i] = (uintptr_t)p_mutex;
if (p_is_binary) {
unlockable_mutexes[i] |= 1;
}
unlockable_locks[i].rc++;
return i;
} else if (!unlockable_locks[i].ulock) {
unlockable_locks[i].ulock = &p_ulock;
unlockable_locks[i].rc = 1;
return i;
}
}
ERR_FAIL_V_MSG(UINT32_MAX, "No more unlockable mutex slots available. Engine bug.");
ERR_FAIL_V_MSG(UINT32_MAX, "No more unlockable lock slots available. Engine bug.");
}
void WorkerThreadPool::thread_exit_unlock_allowance_zone(uint32_t p_zone_id) {
if (p_zone_id == UINT32_MAX) {
return;
DEV_ASSERT(unlockable_locks[p_zone_id].ulock && unlockable_locks[p_zone_id].rc);
unlockable_locks[p_zone_id].rc--;
if (unlockable_locks[p_zone_id].rc == 0) {
unlockable_locks[p_zone_id].ulock = nullptr;
}
DEV_ASSERT(unlockable_mutexes[p_zone_id]);
unlockable_mutexes[p_zone_id] = 0;
}
#endif

19
core/object/worker_thread_pool.h
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@ -162,8 +162,12 @@ private:
static WorkerThreadPool *singleton;
#ifdef THREADS_ENABLED
static const uint32_t MAX_UNLOCKABLE_MUTEXES = 2;
static thread_local uintptr_t unlockable_mutexes[MAX_UNLOCKABLE_MUTEXES];
static const uint32_t MAX_UNLOCKABLE_LOCKS = 2;
struct UnlockableLocks {
THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> *ulock = nullptr;
uint32_t rc = 0;
};
static thread_local UnlockableLocks unlockable_locks[MAX_UNLOCKABLE_LOCKS];
#endif
TaskID _add_task(const Callable &p_callable, void (*p_func)(void *), void *p_userdata, BaseTemplateUserdata *p_template_userdata, bool p_high_priority, const String &p_description);
@ -192,7 +196,7 @@ private:
void _wait_collaboratively(ThreadData *p_caller_pool_thread, Task *p_task);
#ifdef THREADS_ENABLED
static uint32_t _thread_enter_unlock_allowance_zone(void *p_mutex, bool p_is_binary);
static uint32_t _thread_enter_unlock_allowance_zone(THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &p_ulock);
#endif
void _lock_unlockable_mutexes();
@ -242,11 +246,14 @@ public:
static TaskID get_caller_task_id();
#ifdef THREADS_ENABLED
static uint32_t thread_enter_unlock_allowance_zone(Mutex *p_mutex);
static uint32_t thread_enter_unlock_allowance_zone(BinaryMutex *p_mutex);
_ALWAYS_INLINE_ static uint32_t thread_enter_unlock_allowance_zone(const MutexLock<BinaryMutex> &p_lock) { return _thread_enter_unlock_allowance_zone(p_lock._get_lock()); }
template <int Tag>
_ALWAYS_INLINE_ static uint32_t thread_enter_unlock_allowance_zone(const SafeBinaryMutex<Tag> &p_mutex) { return _thread_enter_unlock_allowance_zone(p_mutex._get_lock()); }
static void thread_exit_unlock_allowance_zone(uint32_t p_zone_id);
#else
static uint32_t thread_enter_unlock_allowance_zone(void *p_mutex) { return UINT32_MAX; }
static uint32_t thread_enter_unlock_allowance_zone(const MutexLock<BinaryMutex> &p_lock) { return UINT32_MAX; }
template <int Tag>
static uint32_t thread_enter_unlock_allowance_zone(const SafeBinaryMutex<Tag> &p_mutex) { return UINT32_MAX; }
static void thread_exit_unlock_allowance_zone(uint32_t p_zone_id) {}
#endif

8
core/os/condition_variable.h
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@ -32,6 +32,7 @@
#define CONDITION_VARIABLE_H
#include "core/os/mutex.h"
#include "core/os/safe_binary_mutex.h"
#ifdef THREADS_ENABLED
@ -56,7 +57,12 @@ class ConditionVariable {
public:
template <typename BinaryMutexT>
_ALWAYS_INLINE_ void wait(const MutexLock<BinaryMutexT> &p_lock) const {
condition.wait(const_cast<THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &>(p_lock.lock));
condition.wait(const_cast<THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &>(p_lock._get_lock()));
}
template <int Tag>
_ALWAYS_INLINE_ void wait(const MutexLock<SafeBinaryMutex<Tag>> &p_lock) const {
condition.wait(const_cast<THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &>(p_lock.mutex._get_lock()));
}
_ALWAYS_INLINE_ void notify_one() const {

9
core/os/mutex.h
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@ -72,13 +72,18 @@ public:
template <typename MutexT>
class MutexLock {
friend class ConditionVariable;
THREADING_NAMESPACE::unique_lock<typename MutexT::StdMutexType> lock;
public:
explicit MutexLock(const MutexT &p_mutex) :
lock(p_mutex.mutex) {}
// Clarification: all the funny syntax is needed so this function exists only for binary mutexes.
template <typename T = MutexT>
_ALWAYS_INLINE_ THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &_get_lock(
typename std::enable_if<std::is_same<T, THREADING_NAMESPACE::mutex>::value> * = nullptr) const {
return const_cast<THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &>(lock);
}
};
using Mutex = MutexImpl<THREADING_NAMESPACE::recursive_mutex>; // Recursive, for general use

76
core/os/safe_binary_mutex.h
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@ -47,76 +47,76 @@
// Also, don't forget to declare the thread_local variable on each use.
template <int Tag>
class SafeBinaryMutex {
friend class MutexLock<SafeBinaryMutex>;
friend class MutexLock<SafeBinaryMutex<Tag>>;
using StdMutexType = THREADING_NAMESPACE::mutex;
mutable THREADING_NAMESPACE::mutex mutex;
static thread_local uint32_t count;
struct TLSData {
mutable THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> lock;
uint32_t count = 0;
TLSData(SafeBinaryMutex<Tag> &p_mutex) :
lock(p_mutex.mutex, THREADING_NAMESPACE::defer_lock) {}
};
static thread_local TLSData tls_data;
public:
_ALWAYS_INLINE_ void lock() const {
if (++count == 1) {
mutex.lock();
if (++tls_data.count == 1) {
tls_data.lock.lock();
}
}
_ALWAYS_INLINE_ void unlock() const {
DEV_ASSERT(count);
if (--count == 0) {
mutex.unlock();
DEV_ASSERT(tls_data.count);
if (--tls_data.count == 0) {
tls_data.lock.unlock();
}
}
_ALWAYS_INLINE_ bool try_lock() const {
if (count) {
count++;
return true;
} else {
if (mutex.try_lock()) {
count++;
return true;
} else {
return false;
}
}
_ALWAYS_INLINE_ THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &_get_lock() const {
return const_cast<THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> &>(tls_data.lock);
}
~SafeBinaryMutex() {
DEV_ASSERT(!count);
_ALWAYS_INLINE_ SafeBinaryMutex() {
}
_ALWAYS_INLINE_ ~SafeBinaryMutex() {
DEV_ASSERT(!tls_data.count);
}
};
// This specialization is needed so manual locking and MutexLock can be used
// at the same time on a SafeBinaryMutex.
template <int Tag>
class MutexLock<SafeBinaryMutex<Tag>> {
friend class ConditionVariable;
THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> lock;
const SafeBinaryMutex<Tag> &mutex;
public:
_ALWAYS_INLINE_ explicit MutexLock(const SafeBinaryMutex<Tag> &p_mutex) :
lock(p_mutex.mutex) {
SafeBinaryMutex<Tag>::count++;
};
_ALWAYS_INLINE_ ~MutexLock() {
SafeBinaryMutex<Tag>::count--;
};
explicit MutexLock(const SafeBinaryMutex<Tag> &p_mutex) :
mutex(p_mutex) {
mutex.lock();
}
~MutexLock() {
mutex.unlock();
}
};
#else // No threads.
template <int Tag>
class SafeBinaryMutex : public MutexImpl {
static thread_local uint32_t count;
};
class SafeBinaryMutex {
struct TLSData {
TLSData(SafeBinaryMutex<Tag> &p_mutex) {}
};
static thread_local TLSData tls_data;
template <int Tag>
class MutexLock<SafeBinaryMutex<Tag>> {
public:
MutexLock(const SafeBinaryMutex<Tag> &p_mutex) {}
~MutexLock() {}
void lock() const {}
void unlock() const {}
};
#endif // THREADS_ENABLED

8
core/templates/command_queue_mt.cpp
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@ -33,14 +33,6 @@
#include "core/config/project_settings.h"
#include "core/os/os.h"
void CommandQueueMT::lock() {
mutex.lock();
}
void CommandQueueMT::unlock() {
mutex.unlock();
}
CommandQueueMT::CommandQueueMT() {
command_mem.reserve(DEFAULT_COMMAND_MEM_SIZE_KB * 1024);
}

18
core/templates/command_queue_mt.h
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@ -362,23 +362,24 @@ class CommandQueueMT {
return;
}
lock();
MutexLock lock(mutex);
uint32_t allowance_id = WorkerThreadPool::thread_enter_unlock_allowance_zone(&mutex);
while (flush_read_ptr < command_mem.size()) {
uint64_t size = *(uint64_t *)&command_mem[flush_read_ptr];
flush_read_ptr += 8;
CommandBase *cmd = reinterpret_cast<CommandBase *>(&command_mem[flush_read_ptr]);
uint32_t allowance_id = WorkerThreadPool::thread_enter_unlock_allowance_zone(lock);
cmd->call();
WorkerThreadPool::thread_exit_unlock_allowance_zone(allowance_id);
// Handle potential realloc due to the command and unlock allowance.
cmd = reinterpret_cast<CommandBase *>(&command_mem[flush_read_ptr]);
if (unlikely(cmd->sync)) {
sync_head++;
unlock(); // Give an opportunity to awaiters right away.
lock.~MutexLock(); // Give an opportunity to awaiters right away.
sync_cond_var.notify_all();
lock();
new (&lock) MutexLock(mutex);
// Handle potential realloc happened during unlock.
cmd = reinterpret_cast<CommandBase *>(&command_mem[flush_read_ptr]);
}
@ -387,14 +388,11 @@ class CommandQueueMT {
flush_read_ptr += size;
}
WorkerThreadPool::thread_exit_unlock_allowance_zone(allowance_id);
command_mem.clear();
flush_read_ptr = 0;
_prevent_sync_wraparound();
unlock();
}
_FORCE_INLINE_ void _wait_for_sync(MutexLock<BinaryMutex> &p_lock) {
@ -410,9 +408,6 @@ class CommandQueueMT {
void _no_op() {}
public:
void lock();
void unlock();
/* NORMAL PUSH COMMANDS */
DECL_PUSH(0)
SPACE_SEP_LIST(DECL_PUSH, 15)
@ -446,9 +441,8 @@ public:
}
void set_pump_task_id(WorkerThreadPool::TaskID p_task_id) {
lock();
MutexLock lock(mutex);
pump_task_id = p_task_id;
unlock();
}
CommandQueueMT();

10
modules/gdscript/gdscript_cache.cpp
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@ -144,6 +144,14 @@ GDScriptParserRef::~GDScriptParserRef() {
GDScriptCache *GDScriptCache::singleton = nullptr;
SafeBinaryMutex<GDScriptCache::BINARY_MUTEX_TAG> &_get_gdscript_cache_mutex() {
return GDScriptCache::mutex;
}
template <>
thread_local SafeBinaryMutex<GDScriptCache::BINARY_MUTEX_TAG>::TLSData SafeBinaryMutex<GDScriptCache::BINARY_MUTEX_TAG>::tls_data(_get_gdscript_cache_mutex());
SafeBinaryMutex<GDScriptCache::BINARY_MUTEX_TAG> GDScriptCache::mutex;
void GDScriptCache::move_script(const String &p_from, const String &p_to) {
if (singleton == nullptr || p_from == p_to) {
return;
@ -369,7 +377,7 @@ Ref<GDScript> GDScriptCache::get_full_script(const String &p_path, Error &r_erro
// Allowing lifting the lock might cause a script to be reloaded multiple times,
// which, as a last resort deadlock prevention strategy, is a good tradeoff.
uint32_t allowance_id = WorkerThreadPool::thread_enter_unlock_allowance_zone(&singleton->mutex);
uint32_t allowance_id = WorkerThreadPool::thread_enter_unlock_allowance_zone(singleton->mutex);
r_error = script->reload(true);
WorkerThreadPool::thread_exit_unlock_allowance_zone(allowance_id);
if (r_error) {

9
modules/gdscript/gdscript_cache.h
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@ -34,7 +34,7 @@
#include "gdscript.h"
#include "core/object/ref_counted.h"
#include "core/os/mutex.h"
#include "core/os/safe_binary_mutex.h"
#include "core/templates/hash_map.h"
#include "core/templates/hash_set.h"
@ -95,7 +95,12 @@ class GDScriptCache {
bool cleared = false;
Mutex mutex;
public:
static const int BINARY_MUTEX_TAG = 2;
private:
static SafeBinaryMutex<BINARY_MUTEX_TAG> mutex;
friend SafeBinaryMutex<BINARY_MUTEX_TAG> &_get_gdscript_cache_mutex();
public:
static void move_script(const String &p_from, const String &p_to);