Kernel: Fix deadlock in ext2 filesystem

If multiple threads were waiting for more block buffers without anyone
releasing them, they ended up in a deadlock.

Now we store 6 blocks for 8 threads. If a thread already has a block
buffer, it will not have to wait for a new one. Only if there are more
than 8 threads using blocks, will it block until there are free slots
for a thread available.

kernel/include/kernel/FS/Ext2/FileSystem.h

@@ -28,36 +28,28 @@ namespace Kernel
 			BAN_NON_COPYABLE(BlockBufferWrapper);
 
 		public:
-			BlockBufferWrapper(BAN::Span<uint8_t> buffer, bool* used, Mutex* mutex, ThreadBlocker* blocker)
+			BlockBufferWrapper(BAN::Span<uint8_t> buffer, void (*callback)(void*, const uint8_t*), void* argument)
 				: m_buffer(buffer)
-				, m_used(used)
-				, m_mutex(mutex)
-				, m_blocker(blocker)
-			{
-				ASSERT(m_used && *m_used);
-			}
+				, m_callback(callback)
+				, m_argument(argument)
+			{ }
 			BlockBufferWrapper(BlockBufferWrapper&& other) { *this = BAN::move(other); }
 			~BlockBufferWrapper()
 			{
-				if (m_used == nullptr)
+				if (m_callback == nullptr)
 					return;
-				m_mutex->lock();
-				*m_used = false;
-				m_blocker->unblock();
-				m_mutex->unlock();
+				m_callback(m_argument, m_buffer.data());
 			}
 
 			BlockBufferWrapper& operator=(BlockBufferWrapper&& other)
 			{
 				this->m_buffer = other.m_buffer;
-				this->m_used = other.m_used;
-				this->m_mutex = other.m_mutex;
-				this->m_blocker = other.m_blocker;
+				this->m_callback = other.m_callback;
+				this->m_argument = other.m_argument;
 
 				other.m_buffer = {};
-				other.m_used = nullptr;
-				other.m_mutex = nullptr;
-				other.m_blocker = nullptr;
+				other.m_callback = nullptr;
+				other.m_argument = nullptr;
 
 				return *this;
 			}
@@ -75,9 +67,8 @@ namespace Kernel
 
 		private:
 			BAN::Span<uint8_t> m_buffer;
-			bool* m_used;
-			Mutex* m_mutex;
-			ThreadBlocker* m_blocker;
+			void (*m_callback)(void*, const uint8_t*);
+			void* m_argument;
 		};
 
 	public:
@@ -130,6 +121,9 @@ namespace Kernel
 
 			BAN::ErrorOr<void> initialize(size_t block_size);
 
+		private:
+			void destroy_callback(const uint8_t* buffer_ptr);
+
 		private:
 			struct BlockBuffer
 			{
@@ -137,10 +131,20 @@ namespace Kernel
 				bool used { false };
 			};
 
+			struct ThreadInfo
+			{
+				pid_t tid { 0 };
+				size_t buffers { 0 };
+			};
+
 		private:
+			static constexpr size_t max_threads = 8;
+			static constexpr size_t max_buffers_per_thread = 6;
+
 			Mutex m_buffer_mutex;
 			ThreadBlocker m_buffer_blocker;
-			BAN::Array<BlockBuffer, 16> m_buffers;
+			BAN::Array<BlockBuffer, max_threads * max_buffers_per_thread> m_buffers;
+			BAN::Array<ThreadInfo, max_threads> m_thread_infos;
 		};
 
 	private:

kernel/kernel/FS/Ext2/FileSystem.cpp

@@ -532,22 +532,88 @@ namespace Kernel
 		};
 	}
 
-	BAN::ErrorOr<Ext2FS::BlockBufferWrapper> Ext2FS::BlockBufferManager::get_buffer()
+	void Ext2FS::BlockBufferManager::destroy_callback(const uint8_t* buffer_ptr)
 	{
+		const auto tid = Thread::current_tid();
+
 		LockGuard _(m_buffer_mutex);
 
-		for (;;)
+		for (auto& buffer : m_buffers)
 		{
-			for (auto& buffer : m_buffers)
+			if (buffer.buffer.data() != buffer_ptr)
+				continue;
+			ASSERT(buffer.used);
+			buffer.used = false;
+			break;
+		}
+
+		for (auto& info : m_thread_infos)
+		{
+			if (info.tid != tid)
+				continue;
+			ASSERT(info.buffers > 0);
+			info.buffers--;
+			if (info.buffers != 0)
+				break;
+			info.tid = 0;
+			m_buffer_blocker.unblock();
+			break;
+		}
+	}
+
+	BAN::ErrorOr<Ext2FS::BlockBufferWrapper> Ext2FS::BlockBufferManager::get_buffer()
+	{
+		const auto tid = Thread::current_tid();
+		ASSERT(tid);
+
+		LockGuard _(m_buffer_mutex);
+
+		ThreadInfo* thread_info = nullptr;
+
+		for (auto& info : m_thread_infos)
+		{
+			if (info.tid != tid)
+				continue;
+			thread_info = &info;
+			break;
+		}
+
+		while (thread_info == nullptr)
+		{
+			for (auto& info : m_thread_infos)
 			{
-				if (buffer.used)
+				if (info.tid != 0)
 					continue;
-				buffer.used = true;
-				return Ext2FS::BlockBufferWrapper(buffer.buffer.span(), &buffer.used, &m_buffer_mutex, &m_buffer_blocker);
+				thread_info = &info;
+				break;
+			}
+
+			if (thread_info)
+			{
+				thread_info->tid = tid;
+				thread_info->buffers = 0;
+				break;
 			}
 
 			TRY(Thread::current().block_or_eintr_indefinite(m_buffer_blocker, &m_buffer_mutex));
 		}
+
+		ASSERT(thread_info->buffers < max_buffers_per_thread);
+		thread_info->buffers++;
+
+		for (auto& buffer : m_buffers)
+		{
+			if (buffer.used)
+				continue;
+			buffer.used = true;
+			return Ext2FS::BlockBufferWrapper {
+				buffer.buffer.span(),
+				[](void* self, const uint8_t* buffer) { static_cast<BlockBufferManager*>(self)->destroy_callback(buffer); },
+				this
+			};
+		}
+
+		ASSERT_NOT_REACHED();
 	}
 
 	BAN::ErrorOr<void> Ext2FS::BlockBufferManager::initialize(size_t block_size)