AK: Make Weakable non-atomic

Let's not punish single-threaded workloads with the performance cost of
atomic weakables. The kernel keeps using LockWeakable.

Author: awesomekling

AK/Weakable.h

@@ -7,11 +7,9 @@
 #pragma once
 
 #include <AK/Assertions.h>
-#include <AK/Atomic.h>
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
 #include <AK/StdLibExtras.h>
-#include <sched.h>
 
 namespace AK {
 
@@ -31,59 +29,27 @@ class WeakLink : public RefCounted<WeakLink> {
     RefPtr<T> strong_ref() const
         requires(IsBaseOf<RefCountedBase, T>)
     {
-        RefPtr<T> ref;
-
-        {
-            if (!(m_consumers.fetch_add(1u << 1, AK::MemoryOrder::memory_order_acquire) & 1u)) {
-                T* ptr = (T*)m_ptr.load(AK::MemoryOrder::memory_order_acquire);
-                if (ptr && ptr->try_ref())
-                    ref = adopt_ref(*ptr);
-            }
-            m_consumers.fetch_sub(1u << 1, AK::MemoryOrder::memory_order_release);
-        }
-
-        return ref;
+        return static_cast<T*>(m_ptr);
     }
 
     template<typename T>
     T* unsafe_ptr() const
     {
-        if (m_consumers.load(AK::MemoryOrder::memory_order_relaxed) & 1u)
-            return nullptr;
-        // NOTE: This may return a non-null pointer even if revocation
-        // has been triggered as there is a possible race! But it's "unsafe"
-        // anyway because we return a raw pointer without ensuring a
-        // reference...
-        return (T*)m_ptr.load(AK::MemoryOrder::memory_order_acquire);
+        return static_cast<T*>(m_ptr);
     }
 
-    bool is_null() const
-    {
-        return unsafe_ptr<void>() == nullptr;
-    }
+    bool is_null() const { return m_ptr == nullptr; }
 
-    void revoke()
-    {
-        auto current_consumers = m_consumers.fetch_or(1u, AK::MemoryOrder::memory_order_relaxed);
-        VERIFY(!(current_consumers & 1u));
-        // We flagged revocation, now wait until everyone trying to obtain
-        // a strong reference is done
-        while (current_consumers > 0) {
-            sched_yield();
-            current_consumers = m_consumers.load(AK::MemoryOrder::memory_order_acquire) & ~1u;
-        }
-        // No one is trying to use it (anymore)
-        m_ptr.store(nullptr, AK::MemoryOrder::memory_order_release);
-    }
+    void revoke() { m_ptr = nullptr; }
 
 private:
     template<typename T>
     explicit WeakLink(T& weakable)
         : m_ptr(&weakable)
     {
     }
-    mutable Atomic<void*> m_ptr;
-    mutable Atomic<unsigned> m_consumers; // LSB indicates revocation in progress
+
+    mutable void* m_ptr { nullptr };
 };
 
 template<typename T>