Kernel: Some futex improvements

This adds support for FUTEX_WAKE_OP, FUTEX_WAIT_BITSET, FUTEX_WAKE_BITSET,
FUTEX_REQUEUE, and FUTEX_CMP_REQUEUE, as well well as global and private
futex and absolute/relative timeouts against the appropriate clock. This
also changes the implementation so that kernel resources are only used when
a thread is blocked on a futex.

Global futexes are implemented as offsets in VMObjects, so that different
processes can share a futex against the same VMObject despite potentially
being mapped at different virtual addresses.

Kernel/VM/Region.h

@@ -192,6 +192,11 @@ public:
         return m_offset_in_vmobject;
     }
 
+    size_t offset_in_vmobject_from_vaddr(VirtualAddress vaddr) const
+    {
+        return m_offset_in_vmobject + vaddr.get() - this->vaddr().get();
+    }
+
     size_t amount_resident() const;
     size_t amount_shared() const;
     size_t amount_dirty() const;

Kernel/VM/VMObject.cpp

@@ -45,6 +45,13 @@ VMObject::VMObject(size_t size)
 
 VMObject::~VMObject()
 {
+    {
+        ScopedSpinLock lock(m_on_deleted_lock);
+        for (auto& it : m_on_deleted)
+            it->vmobject_deleted(*this);
+        m_on_deleted.clear();
+    }
+
     MM.unregister_vmobject(*this);
     ASSERT(m_regions_count.load(AK::MemoryOrder::memory_order_relaxed) == 0);
 }

Kernel/VM/VMObject.h

@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/HashTable.h>
 #include <AK/InlineLinkedList.h>
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
@@ -38,6 +39,12 @@ namespace Kernel {
 class Inode;
 class PhysicalPage;
 
+class VMObjectDeletedHandler {
+public:
+    virtual ~VMObjectDeletedHandler() { }
+    virtual void vmobject_deleted(VMObject&) = 0;
+};
+
 class VMObject : public RefCounted<VMObject>
     , public Weakable<VMObject>
     , public InlineLinkedListNode<VMObject> {
@@ -71,6 +78,15 @@ public:
     ALWAYS_INLINE void unref_region() { m_regions_count--; }
     ALWAYS_INLINE bool is_shared_by_multiple_regions() const { return m_regions_count > 1; }
 
+    void register_on_deleted_handler(VMObjectDeletedHandler& handler)
+    {
+        m_on_deleted.set(&handler);
+    }
+    void unregister_on_deleted_handler(VMObjectDeletedHandler& handler)
+    {
+        m_on_deleted.remove(&handler);
+    }
+
 protected:
     explicit VMObject(size_t);
     explicit VMObject(const VMObject&);
@@ -89,6 +105,8 @@ private:
     VMObject(VMObject&&) = delete;
 
     Atomic<u32, AK::MemoryOrder::memory_order_relaxed> m_regions_count { 0 };
+    HashTable<VMObjectDeletedHandler*> m_on_deleted;
+    SpinLock<u8> m_on_deleted_lock;
 };
 
 }