Kernel: Use the Function class for deferred_call_queue()

This avoids allocations for deferred_call_queue().

Kernel/Arch/i386/CPU.cpp

@@ -2234,6 +2234,7 @@ UNMAP_AFTER_INIT void Processor::deferred_call_pool_init()
     for (size_t i = 0; i < pool_count; i++) {
         auto& entry = m_deferred_call_pool[i];
         entry.next = i < pool_count - 1 ? &m_deferred_call_pool[i + 1] : nullptr;
+        new (entry.handler_storage) DeferredCallEntry::HandlerFunction;
         entry.was_allocated = false;
     }
     m_pending_deferred_calls = nullptr;
@@ -2245,6 +2246,8 @@ void Processor::deferred_call_return_to_pool(DeferredCallEntry* entry)
     VERIFY(m_in_critical);
     VERIFY(!entry->was_allocated);
 
+    entry->handler_value() = {};
+
     entry->next = m_free_deferred_call_pool_entry;
     m_free_deferred_call_pool_entry = entry;
 }
@@ -2262,6 +2265,7 @@ DeferredCallEntry* Processor::deferred_call_get_free()
     }
 
     auto* entry = new DeferredCallEntry;
+    new (entry->handler_storage) DeferredCallEntry::HandlerFunction;
     entry->was_allocated = true;
     return entry;
 }
@@ -2290,20 +2294,14 @@ void Processor::deferred_call_execute_pending()
     pending_list = reverse_list(pending_list);
 
     do {
-        // Call the appropriate callback handler
-        if (pending_list->have_data) {
-            pending_list->callback_with_data.handler(pending_list->callback_with_data.data);
-            if (pending_list->callback_with_data.free)
-                pending_list->callback_with_data.free(pending_list->callback_with_data.data);
-        } else {
-            pending_list->callback.handler();
-        }
+        pending_list->invoke_handler();
 
         // Return the entry back to the pool, or free it
         auto* next = pending_list->next;
-        if (pending_list->was_allocated)
+        if (pending_list->was_allocated) {
+            pending_list->handler_value().~Function();
             delete pending_list;
-        else
+        } else
             deferred_call_return_to_pool(pending_list);
         pending_list = next;
     } while (pending_list);
@@ -2316,21 +2314,7 @@ void Processor::deferred_call_queue_entry(DeferredCallEntry* entry)
     m_pending_deferred_calls = entry;
 }
 
-void Processor::deferred_call_queue(void (*callback)())
-{
-    // NOTE: If we are called outside of a critical section and outside
-    // of an irq handler, the function will be executed before we return!
-    ScopedCritical critical;
-    auto& cur_proc = Processor::current();
-
-    auto* entry = cur_proc.deferred_call_get_free();
-    entry->have_data = false;
-    entry->callback.handler = callback;
-
-    cur_proc.deferred_call_queue_entry(entry);
-}
-
-void Processor::deferred_call_queue(void (*callback)(void*), void* data, void (*free_data)(void*))
+void Processor::deferred_call_queue(Function<void()> callback)
 {
     // NOTE: If we are called outside of a critical section and outside
     // of an irq handler, the function will be executed before we return!
@@ -2338,10 +2322,7 @@ void Processor::deferred_call_queue(void (*callback)(void*), void* data, void (*
     auto& cur_proc = Processor::current();
 
     auto* entry = cur_proc.deferred_call_get_free();
-    entry->have_data = true;
-    entry->callback_with_data.handler = callback;
-    entry->callback_with_data.data = data;
-    entry->callback_with_data.free = free_data;
+    entry->handler_value() = move(callback);
 
     cur_proc.deferred_call_queue_entry(entry);
 }

Kernel/Arch/x86/CPU.h

@@ -9,6 +9,7 @@
 #include <AK/Atomic.h>
 #include <AK/Badge.h>
 #include <AK/Concepts.h>
+#include <AK/Function.h>
 #include <AK/Noncopyable.h>
 #include <AK/Vector.h>
 
@@ -608,19 +609,21 @@ struct ProcessorMessageEntry {
 };
 
 struct DeferredCallEntry {
+    using HandlerFunction = Function<void()>;
+
     DeferredCallEntry* next;
-    union {
-        struct {
-            void (*handler)();
-        } callback;
-        struct {
-            void* data;
-            void (*handler)(void*);
-            void (*free)(void*);
-        } callback_with_data;
-    };
-    bool have_data;
+    alignas(HandlerFunction) u8 handler_storage[sizeof(HandlerFunction)];
     bool was_allocated;
+
+    HandlerFunction& handler_value()
+    {
+        return *bit_cast<HandlerFunction*>(&handler_storage);
+    }
+
+    void invoke_handler()
+    {
+        handler_value()();
+    }
 };
 
 class Processor;
@@ -975,21 +978,7 @@ public:
     static void smp_broadcast_flush_tlb(const PageDirectory*, VirtualAddress, size_t);
     static u32 smp_wake_n_idle_processors(u32 wake_count);
 
-    template<typename Callback>
-    static void deferred_call_queue(Callback callback)
-    {
-        auto* data = new Callback(move(callback));
-        deferred_call_queue(
-            [](void* data) {
-                (*reinterpret_cast<Callback*>(data))();
-            },
-            data,
-            [](void* data) {
-                delete reinterpret_cast<Callback*>(data);
-            });
-    }
-    static void deferred_call_queue(void (*callback)());
-    static void deferred_call_queue(void (*callback)(void*), void* data, void (*free_data)(void*));
+    static void deferred_call_queue(Function<void()> callback);
 
     ALWAYS_INLINE bool has_feature(CPUFeature f) const
     {