AK+Kernel: Make automatically locking RefPtr & co a kernel-only thing

Some time ago, automatic locking was added to the AK smart pointers to
paper over various race conditions in the kernel. Until we've actually
solved the issues in the kernel, we're stuck with the locking.

However, we don't need to punish single-threaded userspace programs with
the high cost of locking. This patch moves the thread-safe variants of
RefPtr, NonnullRefPtr, WeakPtr and RefCounted into Kernel/Library/.

Kernel/Library/ThreadSafeRefCounted.h

@@ -0,0 +1,111 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Assertions.h>
+#include <AK/Atomic.h>
+#include <AK/Checked.h>
+#include <AK/Noncopyable.h>
+#include <AK/Platform.h>
+#include <AK/StdLibExtras.h>
+
+namespace AK {
+
+template<class T>
+constexpr auto call_will_be_destroyed_if_present(const T* object) -> decltype(const_cast<T*>(object)->will_be_destroyed(), TrueType {})
+{
+    const_cast<T*>(object)->will_be_destroyed();
+    return {};
+}
+
+constexpr auto call_will_be_destroyed_if_present(...) -> FalseType
+{
+    return {};
+}
+
+template<class T>
+constexpr auto call_one_ref_left_if_present(const T* object) -> decltype(const_cast<T*>(object)->one_ref_left(), TrueType {})
+{
+    const_cast<T*>(object)->one_ref_left();
+    return {};
+}
+
+constexpr auto call_one_ref_left_if_present(...) -> FalseType
+{
+    return {};
+}
+
+class RefCountedBase {
+    AK_MAKE_NONCOPYABLE(RefCountedBase);
+    AK_MAKE_NONMOVABLE(RefCountedBase);
+
+public:
+    using RefCountType = unsigned int;
+    using AllowOwnPtr = FalseType;
+
+    void ref() const
+    {
+        auto old_ref_count = m_ref_count.fetch_add(1, AK::MemoryOrder::memory_order_relaxed);
+        VERIFY(old_ref_count > 0);
+        VERIFY(!Checked<RefCountType>::addition_would_overflow(old_ref_count, 1));
+    }
+
+    [[nodiscard]] bool try_ref() const
+    {
+        RefCountType expected = m_ref_count.load(AK::MemoryOrder::memory_order_relaxed);
+        for (;;) {
+            if (expected == 0)
+                return false;
+            VERIFY(!Checked<RefCountType>::addition_would_overflow(expected, 1));
+            if (m_ref_count.compare_exchange_strong(expected, expected + 1, AK::MemoryOrder::memory_order_acquire))
+                return true;
+        }
+    }
+
+    [[nodiscard]] RefCountType ref_count() const
+    {
+        return m_ref_count.load(AK::MemoryOrder::memory_order_relaxed);
+    }
+
+protected:
+    RefCountedBase() = default;
+    ~RefCountedBase()
+    {
+        VERIFY(m_ref_count.load(AK::MemoryOrder::memory_order_relaxed) == 0);
+    }
+
+    RefCountType deref_base() const
+    {
+        auto old_ref_count = m_ref_count.fetch_sub(1, AK::MemoryOrder::memory_order_acq_rel);
+        VERIFY(old_ref_count > 0);
+        return old_ref_count - 1;
+    }
+
+    mutable Atomic<RefCountType> m_ref_count { 1 };
+};
+
+template<typename T>
+class RefCounted : public RefCountedBase {
+public:
+    bool unref() const
+    {
+        auto new_ref_count = deref_base();
+        if (new_ref_count == 0) {
+            call_will_be_destroyed_if_present(static_cast<const T*>(this));
+            delete static_cast<const T*>(this);
+            return true;
+        } else if (new_ref_count == 1) {
+            call_one_ref_left_if_present(static_cast<const T*>(this));
+        }
+        return false;
+    }
+};
+
+}
+
+using AK::RefCounted;
+using AK::RefCountedBase;