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/*
* Copyright (c) 2018-2022, Andreas Kling <kling@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <AK/Assertions.h>
#include <AK/Atomic.h>
#include <AK/AtomicRefCounted.h>
#include <AK/StdLibExtras.h>
#include <Kernel/Arch/Processor.h>
#include <Kernel/Library/LockRefPtr.h>
#include <Kernel/ScopedCritical.h>
namespace AK {
template<typename T>
class LockWeakable;
template<typename T>
class LockWeakPtr;
class WeakLink final : public AtomicRefCounted<WeakLink> {
template<typename T>
friend class LockWeakable;
template<typename T>
friend class LockWeakPtr;
public:
template<typename T, typename PtrTraits = LockRefPtrTraits<T>>
LockRefPtr<T, PtrTraits> strong_ref() const
requires(IsBaseOf<AtomicRefCountedBase, T>)
{
LockRefPtr<T, PtrTraits> ref;
{
// We don't want to be preempted while we are trying to obtain
// a strong reference
Kernel::ScopedCritical critical;
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_lock_ref(*ptr);
}
m_consumers.fetch_sub(1u << 1, AK::MemoryOrder::memory_order_release);
}
return ref;
}
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);
}
bool is_null() const
{
return unsafe_ptr<void>() == 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) {
Kernel::Processor::wait_check();
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);
}
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
};
template<typename T>
class LockWeakable {
private:
class Link;
public:
template<typename U = T>
ErrorOr<LockWeakPtr<U>> try_make_weak_ptr() const;
protected:
LockWeakable() = default;
~LockWeakable()
{
m_being_destroyed.store(true, AK::MemoryOrder::memory_order_release);
revoke_weak_ptrs();
}
void revoke_weak_ptrs()
{
if (auto link = move(m_link))
link->revoke();
}
private:
mutable LockRefPtr<WeakLink> m_link;
Atomic<bool> m_being_destroyed { false };
};
}
using AK::LockWeakable;
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