Kernel: Add simplistic work queues

We can't use deferred functions for anything that may require preemption,
such as copying from/to user or accessing the disk. For those purposes
we should use a work queue, which is essentially a kernel thread that
may be preempted or blocked.

Kernel/WorkQueue.h

@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2021, The SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/IntrusiveList.h>
+#include <Kernel/Forward.h>
+
+namespace Kernel {
+
+extern WorkQueue* g_io_work;
+
+class WorkQueue {
+    AK_MAKE_NONCOPYABLE(WorkQueue);
+    AK_MAKE_NONMOVABLE(WorkQueue);
+
+public:
+    static void initialize();
+
+    WorkQueue(const char*);
+
+    void queue(void (*function)(void*), void* data = nullptr, void (*free_data)(void*) = nullptr)
+    {
+        auto* item = new WorkItem; // TODO: use a pool
+        item->function = function;
+        item->data = data;
+        item->free_data = free_data;
+        do_queue(item);
+    }
+
+    template<typename Function>
+    void queue(Function function)
+    {
+        auto* item = new WorkItem; // TODO: use a pool
+        item->function = [](void* f) {
+            (*reinterpret_cast<Function*>(f))();
+        };
+        if constexpr (sizeof(Function) <= sizeof(item->inline_data)) {
+            item->data = new (item->inline_data) Function(move(function));
+            item->free_data = [](void* f) {
+                reinterpret_cast<Function*>(f)->~Function();
+            };
+
+        } else {
+            item->data = new Function(move(function));
+            item->free_data = [](void* f) {
+                delete reinterpret_cast<Function*>(f);
+            };
+        }
+        do_queue(item);
+    }
+
+private:
+    struct WorkItem {
+        IntrusiveListNode m_node;
+        void (*function)(void*);
+        void* data;
+        void (*free_data)(void*);
+        u8 inline_data[4 * sizeof(void*)];
+    };
+
+    void do_queue(WorkItem*);
+
+    const char* const m_name;
+    RefPtr<Thread> m_thread;
+    WaitQueue m_wait_queue;
+    IntrusiveList<WorkItem, &WorkItem::m_node> m_items;
+    SpinLock<u8> m_lock;
+};
+
+}