Kernel: Implement contended, ref-counted resource framework

This is some syntaxic sugar to use a ContendedResource object with
reference counting. This effectively dissociates merely holding a
reference to an object and actually using the object in a way that
requires locking it against concurrent use.

Kernel/Locking/NonnullRefContendedPtr.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <Kernel/Locking/RefCountedContended.h>
+
+namespace Kernel {
+
+template<typename T>
+using NonnullRefContendedPtr = NonnullRefPtr<RefCountedContended<T>>;
+
+}

Kernel/Locking/NonnullRefContendedPtrVector.h

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+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullPtrVector.h>
+#include <Kernel/Locking/NonnullRefContendedPtr.h>
+
+namespace Kernel {
+
+template<typename T, size_t inline_capacity = 0>
+using NonnullRefContendedPtrVector = AK::NonnullPtrVector<NonnullRefContendedPtr<T>, inline_capacity>;
+
+}

Kernel/Locking/RefContendedPtr.h

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+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefPtr.h>
+#include <Kernel/Locking/RefCountedContended.h>
+
+namespace Kernel {
+
+template<typename T>
+using RefContendedPtr = RefPtr<RefCountedContended<T>>;
+
+}

Kernel/Locking/RefCountedContended.h

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+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <Kernel/Locking/ContendedResource.h>
+#include <Kernel/Locking/Mutex.h>
+
+namespace Kernel {
+
+template<typename T>
+class RefCountedContended : public ContendedResource
+    , public AK::RefCountedBase {
+    AK_MAKE_NONCOPYABLE(RefCountedContended);
+    AK_MAKE_NONMOVABLE(RefCountedContended);
+
+protected:
+    using LockedShared = LockedResource<T const, LockMode::Shared>;
+    using LockedExclusive = LockedResource<T, LockMode::Exclusive>;
+
+    LockedShared lock_shared() const { return LockedShared(static_cast<T const*>(this), this->ContendedResource::m_mutex); }
+    LockedExclusive lock_exclusive() { return LockedExclusive(static_cast<T*>(this), this->ContendedResource::m_mutex); }
+
+public:
+    RefCountedContended() = default;
+
+    bool unref() const
+    {
+        auto new_ref_count = deref_base();
+        if (new_ref_count == 0) {
+            AK::call_will_be_destroyed_if_present(static_cast<T*>(const_cast<RefCountedContended*>(this)->lock_exclusive().get()));
+            delete static_cast<const T*>(this);
+            return true;
+        } else if (new_ref_count == 1) {
+            AK::call_one_ref_left_if_present(static_cast<T*>(const_cast<RefCountedContended*>(this)->lock_exclusive().get()));
+        }
+        return false;
+    }
+
+    template<typename Callback>
+    decltype(auto) with_shared(Callback callback) const
+    {
+        auto lock = lock_shared();
+        return callback(*lock);
+    }
+
+    template<typename Callback>
+    decltype(auto) with_exclusive(Callback callback)
+    {
+        auto lock = lock_exclusive();
+        return callback(*lock);
+    }
+
+    template<typename Callback>
+    void for_each_shared(Callback callback) const
+    {
+        with_shared([&](const auto& value) {
+            for (auto& item : value)
+                callback(item);
+        });
+    }
+
+    template<typename Callback>
+    void for_each_exclusive(Callback callback)
+    {
+        with_exclusive([&](auto& value) {
+            for (auto& item : value)
+                callback(item);
+        });
+    }
+};
+
+}