From c75c50ecac2967217966762d492c4d9d268e51a3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Pedro=20J=2E=20Est=C3=A9banez?= <pedrojrulez@gmail.com>
Date: Thu, 18 Jul 2024 14:54:58 +0200
Subject: 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.

(cherry picked from commit f4d76853b9d921e3645295f9bebc39eb73661e67)
---
 core/os/safe_binary_mutex.h | 76 ++++++++++++++++++++++-----------------------
 1 file changed, 38 insertions(+), 38 deletions(-)

(limited to 'core/os/safe_binary_mutex.h')

diff --git a/core/os/safe_binary_mutex.h b/core/os/safe_binary_mutex.h
index 1e98cc074c..4ca4b50b02 100644
--- a/core/os/safe_binary_mutex.h
+++ b/core/os/safe_binary_mutex.h
@@ -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
-- 
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